NON-MYELOABLATIVE TOLEROGENIC TREATMENT

Abstract: The present invention features a method of inducing bilateral transplantation tolerance; host specific tolerance of the graft is especially considered. Tolerogenic treatments of the present invention may be administered to a host prior to transplantation of donor-derived materials. The tolerogenic treatment involves (1) administering an immunosuppressive agent to a host mammal in a non-myeloablative regimen sufficient to decrease, but not necessarily to eliminate, the host mammal's functional T lymphocyte population; (2) infusing donor antigens from a non-syngeneic donor into the host mammal; (3) eliminating those host T lymphocytes responding to the infused donor antigens using a non-myeloablative dose of lymphocytotoxic or tolerizing agent; and (4) administering donor hematopoietic cells to the host mammal. Donor lymphoid cells used for cell therapy of a host mammal can be depleted of host specific immunological reactivity by methods essentially similar to those used for tolerizing a host mammal prior to transplantation.

Description: NON-MYELOABLATIVE TOLEROGENIC TREATMENT This application claims priority of International Application No. US98/10575, filed May 22, 1998, and U.S. Application No. 08/862,550, filed May 23, 1997. Background of the Invention Transplantation of organs, hematopoietic cells and somatic cells has been a crucial therapeutic regimen for patients suffering from a variety of maladies. Although the techniques necessary for transplants are quite straight -forward, the great stumbling block for successful transplantation has been the immune system. A fundamental problem has been the great vigor with which the host immune system reacts against introduction of antigens found in donor tissues or cells. Transplantation of allogeneic donor (i.e., the same species but not genetically identical to the host patient) or xenogeneic donor (i.e., a species other than that of the host) grafts has posed particularly great difficulties. The continued functioning of any donor graft depends upon continued functioning of the donor cells that make up that graft. The cells of donor grafts, however, can elicit an immune reaction on the part of the host that, if unchecked, may lead to destruction of the graft . One method of alleviating the reaction by the host against a graft has been administration of immunosuppressive treatment to the host. Unfortunately, despite the availability of new and very effective immunosuppressive drugs, recurrent episodes of acute and chronic graft rejection remain common, frequently causing loss of graft function. Moreover, the long-term success of transplantation is often limited by complications resulting from drug-related toxicity and from long-term immunosuppression (e.g. infections and secondary malignancies) . In addition, transplantation of bone marrow cells (BMC) or small intestine, which are rich in immunocompetent lymphocytes, frequently is associated with a potential life-threatening complication due to graft versus host disease (GVHD) . It has been shown that a full hematopoietic chimera, i.e., a patient whose own BMC have been 100% replaced by permanently engrafted BMC from another individual (donor) , can permanently accept donor-derived allografts with no need for maintenance immunosuppressive therapy. However, induction of full hematopoietic chimerism has been difficult to accomplish. First, substantially complete destruction of the host's immunohematopoietic compartment ("lethal" conditioning) is usually required for engraftment of matched and especially mismatched BMC. With lethal conditioning of the host, GNHD consistently causes morbidity or mortality. In such cases, T cell depletion of the graft hematopoietic material represents the only approach for effective prevention of GVHD. T cell depletion in turn is associated with an increased incidence of graft rejection. To overcome the problem of graft rejection, recipients of T cell depleted marrow allografts may require particularly strong conditioning or, alternatively, very high numbers of T cell depleted BMC. Subjecting patients to aggressive rejection-prevention protocols, such as total body irradiation (TBI) alone or TBI in combination with a short course of immunosuppressive drugs is unlikely to be accepted by clinicians treating patients in need of organ allografts. It has been proposed that true bilateral tolerance associated with mixed donor/recipient hematopoietic chimerism, i.e., the condition in which a patient possesses both recipient (host) and donor hematopoietic stem cells, rather than with full chimerism, would be preferable in clinical organ transplantation. Several experimental protocols have been designed to induce transplantation tolerance leading to mixed chimerism. Conditioning has required the use of high dose TBI followed by infusion with a mixture of T cell depleted donor and recipient BMC (Sachs et al . , Ann. Thorac . Surg . , 56:1221 (1993); Ildstad et al . , Nature, 307:168 (1984)) or inoculation with donor BMC after lower dose TBI and infusion of a mixture of antibodies against CD4 + T cells, CD8 + T cells and NK cells leading to general pancytopenia . Tomita et al . , J. Immunol . , 153:1087 (1994); Tomita et al . , Transplantation, 61:469 (1996). An alternative approach has been developed recently involving irradiation with a sublethal dose of TBI and inoculation with a very high number of T cell depleted donor-derived hematopoietic cells. Reisner et al . , Immunol. Today, 16:437 (1995); Bachar-Lustig et al . , Nature Medicine, 12:1268 (1986). Tolerogenic treatments using cyclophosphamide (hereinafter also referred to as "Cytoxan" or "Cy") in combination with TBI have also been described. Total lymphoid irradiation (TLI) has been employed successfully as the sole preparatory regimen prior to infusion with donor BMC, to induce mixed hematopoietic chimerism and bilateral transplantation tolerance. Slavin S., Immunol. Today, 3:88 (1987); Slavin et al . , Isr. J. Med. Sci . , 22:264 (1986). TLI is non- myeloablative and routinely given safely on an outpatient basis to transplant recipients and patients with Hodgkin's disease. Unfortunately, consistent induction of chimerism using TLI has required very high cumulative doses of radiation (3,400-4,400 cGy) that again would not be desirable for transplant recipients. TLI has significant advantages over TBI, especially in the clinical setting. TLI, which involves selective irradiation of the lymphoid compartment without exposing the whole body to ionizing irradiation, is well tolerated. In addition, TLI preserves intact a significant portion of the host's immunohematopoietic system, with resultant retained memory to recall antigens including infective agents. However, long courses of TLI can be time consuming and may be associated with short and long-term side effects that may not be suitable for routine clinical application. Summary of the Invention The invention provides a new method for treating a host mammal to induce transplantation tolerance to cell, tissue and organ allografts and xenografts. Such transplants can provide replacement therapy for enzyme or metabolic disorders and adoptive immunotherapy for cancer and life-threatening infections in humans. The method also can be used to provide new animal models for tolerance induction toward allogeneic and xenogeneic cells. The invention also provides a new method of non- syngeneic cell therapy in which the cell population used for therapy is substantially depleted of responsiveness to host antigens prior to administration to the host. In general, the invention features a method of treating a host mammal, including (a) administering donor antigens from a non-syngeneic donor to the host mammal; (b) administering a non-myeloablative dose of lymphocytotoxic agent (e.g., cyclophosphamide) or tolerizing agent to the host mammal to selectively eliminate the host mammal's lymphocytes responding to the donor antigens; and (c) administering a preparation of hematopoietic stem cells from the non-syngeneic donor to the host mammal . Prior to step (a) , the host mammal can be administered an immunosuppressive agent in a non- myeloablative regimen sufficient to decrease the host mammal's functional T lymphocyte population. The immunosuppressive agent can include one or more of an immunosuppressive drug, an alkylating agent, ionizing radiation, or anti- leukocyte or anti- leukocyte function antibodies. It is particularly advantageous to use a short course of TLI (sTLI) as the immunosuppressive agent, for example 1-12, frequently 1-6, doses of 200 cGy/dose . The donor antigens administered to the host mammal can include non-cellular antigens, cells, tissues and/or organs. For example, the donor antigens can include hematopoietic stem cells or other viable cells. If the donor antigens include viable cells such as hematopoietic stem cells, then the immunosuppressive regimen referenced above should decrease the T lymphocyte population of the host to a level permitting at least transient survival of the donor's cells. For example the T lymphocyte population of the host can be decreased by 90%, 95% or 99% The host mammal can be an animal or a human, for example a human cancer patient. The donor can be allogeneic or xenogeneic to the host mammal. Following performance of the method, the host mammal's blood can contain 20% or more donor cells. After administering the preparation of donor hematopoietic stem cells, with resultant engraftment of such cells in the host, the host can be treated with allogeneic cell therapy. This involves infusing allogeneic lymphocytes from the donor into the host mammal. Alternatively, the host can receive transplanted cells, tissues or organs from the donor, with the transplants becoming engrafted in the host due to the donor-specific tolerance induced in the host mammal . In another aspect, the invention features a hosu- derived hematopoietic cell composition, including host- originating and donor-originating hematopoietic cells, with the composition being depleted of donor-specific, host- originating lymphocytes. The hematopoietic cell composition can be made by treating a host mammal as described above, then isolating the hematopoietic cell composition from the host mammal . In a further aspect, the invention features a method of making a non-human mammal/human chimera. This involves performing the methods described above, with the host mammal being a non-human mammal and the donor being a human being. The host mammal can be, for example, a rodent or pig. The result is a rodent, pig or other non- human mammal stably engrafted with human hematopoietic stem cells. As such, the non-human mammal host constitutes a hematopoietic mixed chimera. The invention also encompasses a composition of cells containing a cell population from a first individual mammal . The cell population contains lymphocytes and is depleted of responsiveness to antigens of a second individual mammal that is non-syngeneic (i.e., allogeneic or xenogeneic) with the first individual mammal. The depletion of responsiveness is by a method involving the following sequential steps: (a) administering an antigen source expressed by the second individual mammal to the first individual mammal; (b) administering a non-myeloablative dose of a lymphocytotoxic or tolerizing agent to the first individual mammal; (c) administering a preparation of hemopoietic cells from the second individual mammal to the first individual mammal; and (d) isolating the cell population from the first individual mammal. In the composition of the invention, cells endogenous to the first individual mammal are 50% to 100% of the cells of the population. The antigen used can be cancer cells and the first individual mammal and the second individual mammal can both be humans. Alternatively, the first individual mammal can be a non-human primate, and said second individual mammal can be a human, or the first individual mammal can be a pig and said second individual mammal can a human. The invention also features a method of treating a mammal with non-syngeneic cell therapy. The method involves infusing a population of cells from a donor mammal into a host mammal, with the donor mammal and the host mammal being non-syngeneic with each other. The cell population can contain lymphocytes, and prior to infusing, the cell population can be depleted of responsiveness to antigens expressed by the host mammal. The depletion of responsiveness can be by substantially eliminating T cells from the cell population. Elimination of T cells can be by exposing the cell population to an immunosuppressive agent in a non- myeloablative regimen or by contacting the cell with mafosphamide . These eliminations can be performed in vi tro or in vivo . Alternatively, the depletion of the cell population can be accomplished by contacting the lymphocyte population with a composition comprising antigens expressed by said host mammal and the contacting can be in vi tro or by administering the antigens to the first individual mammal. The method can further include the step of, after the contacting with the antigen composition, delivering a non-myeloablative dose of a lymphocytotoxic or tolerizing agent to the lymphocyte population. This delivering can be in vi tro or by administering the non-myeloablative dose to the donor mammal . The method can also optionally include the steps of: (a) after the delivering, administering a preparation of hemopoietic stem cells from the host mammal to the donor mammal; and/or (b) prior to the contacting with antigen, exposing the lymphocyte population to an immunosuppressive agent in a non-myeloablative regimen sufficient to decrease the number of functional T lymphocytes in the lymphocyte population. In (b) the exposing can be in vi tro or by administering the immunosuppressive agent to the donor mammal. The antigen composition can contain one or more antigen sources, e.g., cells, organs, tissues, and non-cellular antigens. For example, the antigen can include hemopoietic cells or cancer cells expressing major histocompatibility complex molecules of the host mammal . The cancer cells can, for example, be from the host mammal. Also within the invention is an article of manufacture that includes packaging material and a biological cell container within the packaging material. The cell container can contain a composition that includes hematopoietic stem cells and the packaging material can contain a label or package insert indicating that the hematopoietic stem cells are to be used in step (a) or step (c) in a method of inducing non-syngeneic donor-specific tolerance in a host mammal. The method includes the steps of: (a) administering donor antigens from a non-syngeneic donor to the host mammal; (b) administering a non-myeloablative dose of lymphocytotoxic or tolerizing agent to the host mammal to selectively eliminate the host mammal's lymphocytes responding to the donor antigens; and (c) administering a preparation of hematopoietic stem cells from the non-syngeneic donor to the host mammal . Another article of manufacture encompassed by the invention is one that includes packaging material, a biological cell container within said packaging material, with the cell container containing any of the cell compositions of the invention described above. The packaging material contains a label or package insert indicating that the composition is to be used in a method of treatment including administering the composition to a second individual mammal that is in need of the composition. The invention also features a method of inducing tolerance in a host mammal to a graft from a non- syngeneic host mammal . The method includes the following steps: (a) administering donor antigens from a non- syngeneic donor to the host mammal; (b) administering an immunosuppressive agent to the host mammal in a non- myeloablative regimen sufficient to decrease the host mammal's functional T lymphocyte population; (c) transplanting cells, a tissue, or an organ from the donor into the host animal, (d) administering a non- myeloablative dose of lymphocytotoxic or tolerizing agent to the host mammal to selectively eliminate the host mammal's lymphocytes responding to the donor antigens; and (e) administering a preparation of hematopoietic stem cells from the non-syngeneic donor to the host mammal. Steps (a) , (b) , and (c) of the method are performed on the same day and prior to steps (d) and (e) . The term "non-myeloablative" as used herein includes any therapy that does not eliminate substantially all hematopoietic cells of host origin. "Transplantation" as used herein refers to transplantation of any donor-derived material including cells, tissues and organs. The cells may be hematopoietic or non-hematopoietic . "Donor antigens" as used herein refers to any donor-derived material that elicits a host immune response, including non-cellular antigens, cells, tissues or organs. Stem cells are particularly useful as donor antigens. A "lymphocytotoxic agent" is an agent that kills T cells or paralyzes T cell function. A "tolerizing agent" is an agent that energizes or "vetos" T cells by preventing development of normal T cell -dependent responses. The term "cancer" as used herein includes all pathological conditions involving malignant cells; this can include "solid" tumors arising m solid tissues or organs as well as hematopoietic tumors such as leukemias and lymphomas . The term "donor-specific tolerance" as used herein refers to tolerance of the host to donor-derived material "Non-syngeneic" as used herein can be allogeneic or xenogeneic. "Depletion of responsiveness" m a particular cell population, as used herein, means either a decrease m the number of responsive cells, a decrease m the responsiveness of responsive cells, or both. Where cells are herein said to be "endogenous" to an individual mammal, it is understood that the cells themselves, or their precursors, were m that individual mammal prior to any administration of cells from another individual mammal . Induction of donor-specific tolerance across strong major histocompatibility complex MHC and minor histocompatibility loci (MiHL) barriers, as well as across species barriers (xenogeneic tolerance) may be achieved m mammalian hosts using the tolerogenic treatment described herein. Induction of donor-specific transplantation tolerance while avoiding the need for maintenance immunosuppressive treatment is a highly desirable goal m clinical transplantation. The non-myeloablative tolerogenic treatment described herein induces a state of long-lasting donor- specific tolerance to a wide variety of donor-derived material. Such an approach is attractive for allogeneic and xenogeneic transplantation of cells, tissues and organs m clinical settings, since all the steps of the protocol are well tolerated and relatively safe. Since there is no need to eradicate the entire host lmmunohematopoietic system during the course of the procedure, the recipients retain immune memory and are m a better position to resist graft-versus-host disease on the one hand and infectious complications on the other. This can be of crucial importance m clinical practice. The protocols for inducing donor-specific tolerance may be delivered, at least m part, as outpatient procedures. The methods of non-syngeneic cell therapy provided herein can be especially useful m conditions m which cell, tissue, or organ failure or misfunction occurs. They can therefore be useful m, for example, metabolic deficiencies (including genetic metabolic deficiencies) , autoimmune diseases, and cancer. The methods are therefore useful m passively transferring, from a donor to a host, immunity to one more infectious agents. They can be used without prior treatment of the host or subsequent to tolerization of the host to donor antigens by one of the tolerization methods of the invention. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill m the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference m their entirety. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. Brief Description of the Drawings Fig. 1. Effect of T cell depletion from a second BMC infusion on survival of tolerant mice. Fig. 2. GVHD- free survival (upper panel) and donor-type skin allograft survival (lower panel) of BALB/c mice irradiated with various number of daily TLI fractions . Fig. 3. Xenograft survival of Lewis rat skin in B6 mouse recipients following a non-myeloablative tolerogenic protocol based on the use of sTLI , donor BMC and Cy . Fig. 4. Survival of host FI mice after injection of BCLl tumor cells and lymphoid cells from either immunized or non- immunized B6 donor mice incompatible with the host mice at both the MHC and MiHL. Fig. 5. Survival of secondary BALB/c host mice after transfer of spleen cells from primary FI host mice injected with BCLl tumor cells and lymphoid cells from either immunized or non-immunized B6 donor mice incompatible with the host mice at both the MHC and MiHL. Fig. 6. Body weight changes in FI host mice after injection of BCLl tumor cells and lymphoid cells from either immunized or non- immunized B6 donor mice incompatible with the host mice at both the MHC and MiHL. Fig. 7. Survival of host BALB/c mice after injection of BCLl tumor cells and lymphoid cells from either immunized or non-immunized B10.D2 donor mice incompatible with the host mice at MiHL only. Fig. 8. Survival of secondary BALB/c host mice after transfer of spleen cells from primary BALB/c host mice injected with BCLl tumor cells and lymphoid cells from either immunized or non-immunized B10.D2 donor mice incompatible with the primary BALB/c host mice at MiHL only. Fig. 9. Body weight changes in BALB/c host mice after injection of BCLl tumor cells and lymphoid cells from either immunized or non-immunized B10.D2 donor mice incompatible with the host mice at MiHL only. Fig. 10. Production of interferon-γ (IFN-γ) by spleen cells from C57BL/6 mice immunized with either BCLl tumor cells or BALB/c spleen cells and by spleen cells from unimmunized C57BL/6 mice. Fig. 11. Production of interleukin-2 (IL-2) by spleen cells from C57BL/6 mice immunized with either BCLl tumor cells or BALB/c spleen cells and by spleen cells from unimmunized C57BL/6 mice. Fig. 12. Production of interleukin-10 (IL-10) by spleen cells from C57BL/6 mice immunized with either BCLl tumor cells or BALB/c spleen cells and by spleen cells from unimmunized C57BL/6 mice. Fig. 13. Production of tumor necrosis factor-α (TNF-α.) by spleen cells from FI mice injected with the three spleen cell populations represented in Figs. 10-12. Fig. 14. Production of IL-2 by spleen cells from FI mice injected with the three spleen cell populations represented in Figs. 10-12. Fig. 15. Production of IFN-γ by spleen cells from FI mice injected with the three spleen cell populations represented in Figs. 10-12. Fig. 16. Production of interleukin-4 (IL-4) by spleen cells from FI mice injected with the three spleen cell populations represented in Figs. 10-12. Fig. 17. Survival of BALB/c recipient mice and allografts (bone marrow stroma or hearts) from B6 mice after tolerization and transplantation using a protocol in which the sTLI , the first B6 bone marrow infusion, and allograft implantation were all performed on day O. Fig. 18. Lysis of murine YAC-1 tumor targets by killer cells generated by in vi tro activation with IL-2 of BALB/c bone marrow (BM) or spleen (SP) cells after either no treatment or treatment with ASTA-Z and subsequent culture with IL-2. Fig. 19. Lysis of murine P815 tumor target cells by killer cells generated by in vi tro activation with IL- 2 of BALB/c bone marrow (BM) or spleen (SP) cells after either no treatment or treatment with ASTA-Z and subsequent culture with IL-2. Fig. 20. Survival of lethally irradiated SJL/J mice after infusion of either untreated or ASTA-Z treated bone marrow cells from B6 mice. Survival data obtained with control lethally irradiated, unreconstituted SJL/J mice are also shown. Fig. 21. Survival of sub-lethally irradiated BALB/c mice after infusion of either untreated or ASTA-Z treated bone marrow cells from B6 mice. DETAILED DESCRIPTION A. Tolerance Protocols The present inventor has employed novel, non- myeloablative tolerogenic protocols to induce stable and donor-specific tolerance to non-syngeneic transplants (i.e., transplants of cells, tissues or organs not genetically identical to the host) . A protocol for the tolerogenic treatment can be summarized as follows: Step 1 : Administer an immunosuppressive agent to a host mammal in a non-myeloablative regimen sufficient to decrease, but not eliminate, the host mammal's functional T lymphocyte population. Step 2: Infuse donor antigens, preferably viable hematopoietic cells, from a non-syngeneic donor into the host mammal . Step 3 : Eliminate those host T lymphocytes responding to the infused donor antigens using a non- myeloablative dose of lymphocytotoxic or tolerizing agent . Step 4 : Administer a preparation of donor hematopoietic stem cells to the host mammal . This non-myeloablative, donor-specific tolerogenic treatment results in conversion of a host to a hematopoietic mixed chimera with high levels of donor hematopoietic cells. Typically, the mammalian hosts are human patients, although a recipient of the tolerogenic treatment may be any mammal. Non-syngeneic transplantation can include allogeneic as well as xenogeneic transplantation of organs, tissues or cells. Hence, hematopoietic stem cells and other donor antigens used in steps 2 and 4 may be derived from allogeneic or xenogeneic sources. Human patients for which the tolerogenic treatment is appropriate include without limitation those with loss of organ or tissue function including loss of metabolic function such as in diabetes; patients with enzyme deficiencies caused by inborn genetic diseases such as Gaucher's disease, metachromatic leukodystrophy and Hurler's Syndrome; patients with autoimmune disorders such as lupus erythematosus and rheumatoid arthritis; and cancer patients. Patients suffering from heart, liver or kidney failure, for example, are excellent candidates for conditioning with the tolerogenic treatment prior to transplantation with the appropriate organ. Patients requiring a skin or bone graft may also be subjected to the tolerogenic treatment prior to grafting. Cancer patients receiving the tolerogenic treatment can include patients suffering from any malignancy, either solid tumors such as breast cancer or hematopoietic malignancies including acute and chronic leukemia, lymphoma, and myelodysplastic and myeloproliferative disorders . In accordance with this invention, a significant number of the host mammal's functional T lymphocyte population remains in the host after the non- myeloablative regimen of Step 1. Nevertheless, engraftment of donor cells can occur because (a) donor- reactive host T lymphocytes are eliminated in step 3, and (b) donor-derived T lymphocytes and/or stem cells present in the subsequent infusion or infusions (Step 4) may act as "veto" cells to produce a veto effect. Veto cells, as used herein, include T lymphocytes, especially CD8 + T cells, that result in down regulation, rather than stimulation, of other T lymphocytes. Veto effects may be induced by other proliferating hematopoietic cells including T cell-depleted stem cells that are poorly immunogenic but that can veto host T cells. In the veto effect, host -originating T lymphocytes are down-regulated by donor-derived veto cells, including stem cells and/or lymphocytes. Other replicating donor-derived cells, or even non-cellular antigens, can also veto host allo- or xeno-reactive T cells if provided repeatedly and in relatively high concentrations. Similarly, immunocompetent T cells present in the donor infusion may be down-regulated by veto cells of host origin. Thus, tolerance of graft vs host and host vs graft may occur simultaneously due to a balanced equilibrium between veto cells of host and donor origin on the one hand and the degree of immunogenici y and alloreactivity of the graft on the other. (i) Step 1 Examples of immunosuppressive agents useful in Step 1 include without limitation immunosuppressive drugs such as methotrexate and fludarabine (FLU) ; alkylating agents such as Cy, melphalan, thiotepa and busulfan; polyclonal and monoclonal anti-thymocyte globulin (ATG) and anti -lymphocyte globulin (ALG) ; and ionizing radiation such as TLI and TBI. Due to its non-selective effects on all of the host's hematopoietic cells and its severe immediate and long-term side effects, TBI is not preferred. If TBI is used, it should be at a dose level that causes no severe or irreversible pancytopenia . The non-myeloablative regimen advantageously is a short and well-tolerated course of TLI (sTLI) which may cause a major reduction in the number and/or function of host T lymphocytes in all lymphoid organs. As discussed below, it has been discovered that sTLI can effectively induce unresponsiveness to donor antigens at relatively low cumulative radiation doses. The sTLI immunosuppressive regimen may comprise, for example, 1 to 12 daily fractions of 200 cGy/each depending on the host-versus-graft potential and the T lymphocyte content in the stem cell preparation administered in Step 4. Stem cell preparations rich in T lymphocytes may require only 1-3 sTLI fractions, or may not require immunosuppression at all (zero sTLI fractions) . Transplantation of T cell-depleted stem cell preparations or stem cell preparations with low levels of T lymphocytes, however, may require the use of 4-12 fractions. The sTLI regimen causes only a transient reduction in the number of host T lymphocytes and is clinically feasible on an outpatient basis. There are no anticipated severe side effects since a routine cumulative dose of TLI used clinically for lymphoma patients consists of 4,400 cGy. Preferably, the immunosuppressive agent transiently decreases the host functional T lymphocyte population by at least about 90%. More preferably, the non-myeloablative regimen transiently decreases the host functional T lymphocyte population by at least about 95%, and most preferably, by at least about 99%. Reductions of less than 90% of the lymphocytes are also within the scope of this invention, provided that transient survival of donor antigens, provided in Step 2, is possible. In some donor/recipient combinations, tolerance to donor antigens may be inducible without the necessity of performing Step 1. In this case, the preparation of donor hematopoietic stem cells administered in Step 4 must contain a sufficient number of T cells to provide a protective veto function against residual host T cells escaping the effects of Step 3. (ii) Step 2 In Step 2 of the tolerogenic treatment, antigens from a non-syngeneic donor are administered to the host mammal in order to stimulate and cause proliferation of donor-specific T lymphocytes of the host. The stimulated sub-population of donor-specific host T lymphocytes is then eliminated or tolerized in Step 3. The donor antigens may be administered (Step 2) to the host after the non-myeloablative immunosuppressive regimen (Step 1) described above. Alternatively, the donor antigens may be administered to a non-immunosuppressed host (if Step 1 is excluded as described above) . The donor antigens administered in step 2 can include, without limitation, non-cellular antigens, cells, organs, tissues or tissue extracts, or even anti- idiotypic antibodies that mimic donor antigens. In general, any donor antigens that elicit an immune response in the host are within the scope of this invention. Any source of donor antigens from a non- syngeneic donor can be used, and the non-syngeneic donor can be allogeneic or xenogeneic to the host . The infusion of donor antigens should comprise donor antigenic determinants for which tolerance is desired. For example, if it is desired to transplant into the host donor-derived material bearing only class I histocompatibility antigens, it may be necessary to eliminate only class I -reactive host T lymphocytes in Step 3. This could be accomplished by infusing, in Step 2, donor antigens bearing only class I antigenic determinants. On the other hand, additional donor antigenic determinants may be present in the infusion of Step 2 even though host tolerance to these additional antigenic determinants may not be necessary. Thus, elimination of class I- and class II-reactive host T lymphocytes by infusion of donor antigens bearing class I and class II antigenic determinants may be performed even if the later transplanted donor material bears only Class I antigenic determinants. The donor antigens infused in Step 2 can be viable hematopoietic stem cells from a non-syngeneic donor. The donor hematopoietic stem cells generally are not T cell depleted, although use of T cell depleted donor hematopoietic stem cells in Step 2 is also within the scope of this invention. Donor hematopoietic stem cells for use in Steps 2 and/or 4 may be obtained, for example, by direct extraction from the bone marrow or from the peripheral circulation following mobilization from the bone marrow. The latter can be accomplished by treatment of the donor with granulocyte colony stimulating factor (G-CSF) or other appropriate factors that induce mobilization of stem cells from the bone marrow into the peripheral circulation. The mobilized stem cells can be collected from peripheral blood by any appropriate cell pheresis technique, for example through use of a commercially available blood collection device as exemplified by the CS3000 Plus blood cell collection device marketed by the Fenwal Division of Baxter Healthcare Corporation. Methods for performing apheresis with the CS 3000 Plus machine are described m Williams et al . , Bone Marrow Transplantation 5. 129-133 (1990) and Hillyer et al . , Transfusion 33: 316-321 (1993). Alternative sources of stem cells include neonatal stem cells (e.g., cord blood stem cells) and fetal stem cells (e.g., fetal liver of yolk sac cells) . Stem cells that have been expanded m vi tro with a mixture of hematopoietic cytokmes also may be used. Other useful stem cell preparations include stem cells that have been transduced with genes encoding donor-type MHC class I or class II molecules, as well as stem cell preparations containing stem cells and/or T cells transduced with herpes simplex thymidme k ase or other "suicide" genes to render the mature T cells sensitive to ganciclovir or other appropriate drugs the event of severe GVHD. (m) Step 3 With respect to Step 3, "elimination" of the proliferating donor-specific host T lymphocytes as used herein includes host T lymphocyte mactivation or tolerization as well as host T lymphocyte death. Examples of lymphocytotoxic agents useful m Step 3 include Cy, melphalan and methotrexate . Cy, for example, is a short acting cytotoxic drug known for its ability to kill lymphocytes, especially cells that proliferate m response to antigenic stimulation (Bach JF, Amsterdam: North-Holland (1975); Aisenberg et al . , Nature , 213:498 (1967) ; Paul WE, Fundamental Immunology. New York: Raven, (1984) ) . Cy can also facilitate activation of antigen- specific T cell suppressors responsible for maintenance of the tolerant state . Chernyakhovskaya et al . , Transplantation, 38:267 (1984); Maeda et al . , Transplantation, 57:461 (1994) Other agents known to eliminate proliferating T cells in response to donor antigenic stimulation may also be used, including monoclonal antibodies against activation markers of T lymphocytes such as anti-CD25, anti-DC69 and anti-Ia/DR antibodies. Alloreactive host T cells may be tolerized, rather than killed, by using agents that block co- stimulation in conjunction with activation, since T cell engagement with antigen without a second signal provided by co-stimulation results in tolerance. Such tolerizing agents include without limitation CTLA4-Ig, anti-B7.1 or anti-B7.2, anti-CD28, and antibodies against adhesion molecules such as anti-LFAl, anti-CD44 and similar agents. If tolerizing agents are used, steps 2 and 3 can be performed simultaneously. (iv) Step 4 In order to ensure an acceptable state of stable, mixed chimerism with relatively high numbers of circulating donor cells, donor hematopoietic stem cells are administered to the host following performance of Step 3. This infusion of donor stem cells (Step 4) is derived from the same donor, or from a donor genetically identical to that providing the antigens for Step 2. Hematopoietic stem cells from bone marrow, from mobilized peripheral blood populations, or other stem cell preparations as described above (e.g., cord blood stem cells) , may be used. The number of stem cells administered in Step 4 can vary depending on the T cell content of the stem cell preparation. If the preparation is not T cell-depleted, then relatively small numbers of stem cells generally are administered. If the stem cell preparation is T cell -depleted, then larger numbers of stem cells can be administered since there is no risk of GVHD. The donor hematopoietic stem cells of the second infusion may or may not be T cell depleted, depending on the lmmunologic disparity between the donor and recipient, the intensity of immunosuppression given m Step 1 and the degree of chimerism desirable view of the immunogenicity of the graft. When higher fractions of sTLI (4-12), or other immunosuppressive agents providing equivalent immunosuppression, are used m the immunosuppressive regimen of Step 1, the second infusion comprising donor hematopoietic stem cells typically is T cell depleted to control for GVHD. When Step 1 involves little immunosuppression (for example, 1-3 fractions of sTLI) , or when Step 1 is eliminated altogether, the infusion of donor hematopoietic stem cells m Step 4 typically is not T cell depleted. If not T cell depleted, the donor stem cells provided m Step 4 can be infused m graded increments over a period of weeks or several months, while monitoring for signs of GVHD. In mouse experiments reported below, the mice received sTLI of 0-6 fractions of 200 cGy/fraction (Step 1) . The donor-reactive T cells of the host were activated (Step 2) by injecting non-T cell depleted donor BMC (3xl0 7 cells) . The activated host T cells were subsequently eliminated (Step 3) by a non-myeloablative dose (200 mg/kg or 3 doses of 60 mg/kg) of Cy. Mixed chimeras with low levels (e.g., 7%-20%) of donor cells the blood were predominant after the Cy treatment . Induction of higher levels of hematopoietic mixed chimerism (e.g., >20% of donor cells m blood) was achieved by administering (Step 4) a second infusion comprising donor hematopoietic stem cells, allowing lifelong survival of donor skin allografts. In mice treated with 6 doses of TLI, Step 4 was required to achieve a level of tolerance permitting acceptance of full thickness skin allografts. It is well known that full -thickness skin presents the most stringent test for donor-specific tolerance. Skin allograft acceptance can be accomplished only in stable chimeras (Maeda et al . , J. Immunol . , 150:753 (1993)) and success of skin acceptance may depend on the level of donor-derived cells in the host's blood. In further experiments reported below, deletion of all host-derived, donor-reactive T lymphocytes following Step 3 permitted rapid engraftment of even low numbers of donor stem cells (2-3 x 10 6 /mouse) administered in Step 4, which normally would not be sufficient for induction of stable mixed chimerism. In parallel with this, the full anti -donor unresponsiveness induced following Step 3 also resulted in exquisite sensitivity to donor T lymphocytes, leading to lethal GVHD. Hence, whenever donor-reactive host T cells are effectively depleted, elimination of immunocompetent T lymphocytes from the hematopoietic stem cell preparation administered in Step 4, or use of lifespan-limited lymphocytes (e.g., carrying suicide genes) , is crucial for prevention of GVHD. Due to the selective deletion of donor-reactive host lymphocytes, even 2-3xl0 6 BMC (T cell-depleted) , administered following Step 3, engrafted and converted host mice into stable mixed chimeras with relatively high levels (20% - 50%) of donor-derived hematopoietic cells in the blood. T cell depletion of donor stem cell preparations has been known to increase the risk of graft rejection. Thus, inoculation with extremely large numbers of donor stem cells has been mandatory for engraftment of T cell depleted BMC, especially in recipients conditioned with previous non-myeloablative protocols. Truitt et al . , Blood 77, 2515-2523 (1991); Reisner et al . , Immunol . Today 16, 437-440 (1995); Bachar-Lustig et al . , Nat. Med. 12, 1268-1273 (1995). The ability to induce engraftment using low numbers of donor hematopoietic stem cells (T lymphocyte depleted) is a significant advantage of the present protocols. This is due to the improved acceptance of donor hematopoietic stem cells on the one hand combined with a reduced risk of GVHD that would otherwise follow from the use of higher stem cell inocula, on the other. Notably, animals non-specifically immunosuppressed by sTLI and Cy (without infusion of donor-antigens prior to Cy) were shown to reject T lymphocyte depleted BMC. Thus, the present data clearly show the advantage of donor-specific tolerogenic conditioning in comparison with non-specific immunosuppression approaches, while avoiding potentially hazardous high doses of TBI . GVHD can also be controlled using the tolerogenic agents described above for use in step 3. In addition, antibodies to CD52 , CD40 ligand, CD40, IL-2 receptors (e.g., CD25) can be used to modulate GVHD. It is understood that the term antibody applies both to native antibodies, or as functional fragments of antibodies (e.g., Fab, F(ab') 2 , or Fv fragments) . Furthermore, they can be used as immunotoxins, e.g., conjugated with toxins such as Pseudomonas toxin or diphtheria toxin, ricin, or a radionuclide, e.g., 125 I or 131 I. Interestingly, life-long tolerance to full thickness donor-derived skin grafts was also accomplished in recipients who were not subjected to Step 1 and in recipients in which Step 1 involved only a single fraction of TLI (200 cGy) . Thus, a balance exists between the intensity of the conditioning of the host and the susceptibility of the host to GVHD induced by the presence of donor-derived T cells: recipients of a single fraction of sTLI could resist GVHD induction by a large inoculum of non-T cell depleted donor BMC whereas the sensitivity of the hosts to GVHD was increased in recipients conditioned with 6 fractions of sTLI . Hence, a second infusion (Step 4) comprising non-T cell depleted donor BMC could be used relatively safely in recipients of 1 dose of sTLI whereas T cell depletion of the second infusion was mandatory in recipients of 6 fractions of sTLI . The sensitivity of the 6 x sTLI recipients to develop GVHD was likely due to inability to veto the host-reactive donor cells due to effective depletion of all host T cells. Tolerant mixed hematopoietic chimeras generated by the tolerogenic treatment described herein remain immunocompetent to third party grafts. In experiments described below, all tolerant B6→BALB/c chimeras that accepted B6 skin allografts rejected non-relevant CBA skin grafts within 16-20 days (n = 11) . Thus, tolerance induction neither eliminated nor impaired normal reactivity by the host immune system retained in the mixed chimera. This is an important advantage of the method, since recipients are not immunocompromised due to transient loss of all host-derived immune cells, which is otherwise unavoidable when chimeras are comprised of 100% donor cells following TBI. A patient who retains a host- derived immune apparatus with memory cells is in a better position to resist primary and secondary infections. This retained resistance to intercurrent infections, particularly to viral agents infecting host target cells, is of crucial importance. This is because the donor hematopoietic cells may be MHC disparate and, therefore, incapable of providing immune protection against virally- infected host tissues. The above-described tolerogenic treatment may be employed to induce transplantation tolerance across xenogeneic barriers. Xenogeneic skin transplantation may be considered the most stringent test for donor-specific tolerance. As described below, the present inventor has succeeded in inducing permanent tolerance in mouse-to-rat skin grafts. The same donor-specific tolerance induction protocol presented herewith can be applied to xenogeneic transplantation in humans. The xenogeneic graft (e.g., pancreatic islets) may be taken from non-human mammals and transplanted into humans. A tolerogenic treatment for xenogeneic transplantation may be performed as follows. sTLI (Step 1) is carried out, followed by an infusion (Step 2) of xenogeneic donor antigens, for example, BMC. Subsequently, at least one non-myeloablative dose of lymphocytotoxic or tolerizing agent is administered (Step 3) . If necessary, the lymphocytotoxic agent can be administered in multiple low doses over several days. Administration of the lymphocytotoxic agent is followed by a infusion of a preparation comprising T cell -depleted donor hematopoietic stem cells (Step 4) . The stem cells may be obtained from the blood or bone marrow of an adult donor. Alternatively, partially immunocompetent cord blood cells may be used, or even fetal stem cells obtained from the liver or yolk sac of embryos. Stem cells that have been expanded in vi tro with a mixture of hematopoietic cytokines also may be used. Administration of stem cells in Step 4 leads to engraftment of the xenogeneic donor stem cells and permanent transplantation tolerance of the host to donor derived organs. In an alternative embodiment, xenogeneic transplantation may be performed without administration of a non-myeloablative regimen (Step 1 eliminated) and with the second infusion (Step 4) comprising non T cell depleted donor hematopoietic stem cells. (v) Short protocol for induction of transplantation tolerance In regard to use of the tolerogenic method of the invention in non-syngeneic organ, tissue or cell transplantation, the results of the experiments described in Example 14 are of great importance. In these experiments, it was found that allogeneic grafts (bone marrow stroma or hearts) could be implanted without significant rejection on the same day (day 0) as a single dose of TLI (as step 1) and donor bone marrow (as step 2) , followed on day 1 by Cy (as step 3) and on day 2 by a second dose of bone marrow (as step 4) . This method was designated the "short protocol" in order to differentiate it from the longer protocols used in some other experiments. Thus, in such longer protocols, heart or skin grafts were, for example, implanted 20 days after cyclophosphamide treatment (step 3) (Example 5) and 19 days after the second bone marrow injection (step 4) (Examples 2, 3, 4, 6, 7, and 9) . Furthermore, in another long protocol, sTLI, as step 1, was given over a period of 6 days (e.g., Example 3) . It is hypothesized that, in the short protocol, the grafted organ acts together, and possibly in synergy, with the bone marrow given on day 0, as antigen. It is possible, in addition, that the bone marrow given on day 0 may not be necessary for establishment of tolerance. This method of the invention is however not limited by a particular mechanism of action. The success of the short protocol broadens the applicability of the tolerogenic approach of the invention for human transplantation. For example, in the case of cadaveric transplantation, donor bone marrow (or any other tissue that can be used for step 2) is not normally available significantly in advance of the availability of the relevant organ (e.g., kidney, heart, liver, or lung), tissue (e.g., skin, bone, muscle, or cartilage), or cells (e.g., hepatocytes or pancreatic islet cells) . Using the short protocol, bone marrow can be harvested from the donor at the same as the organ and both can be given to the recipient on the same day as non-myeloablative conditioning (e.g., TLI) (day 0). Simultaneously, bone marrow cells can be frozen and stored for use in step 4, either on day 2 and/or on subsequent days. In addition to broadening the applicability of the tolerogenic methodology of the invention for cadaveric allo-transplantation, the short protocol can also, for the same reasons, greatly simplify the logistics of living-related donor or xenogeneic donor transplantation. (vi) Articles of manufacture Also included in the invention are articles of manufacture including packaging material (e.g., a cardboard box) containing a biological cell container ( e.g., a blood bag such as a semipermeable blood bag that can be used for culturing cells) . The biological container can contain a composition that includes hemopoietic stem cells and the packaging material can include a package insert or a label indicating that the composition can be used as the antigen in step 1 and/or as the source of hematopoietic stem cells for step 4 of the tolerogenic protocol . (vii) Chimeras In another embodiment, the invention involves a method of making a non-human mammal/human hematopoietic chimera. The method comprises making a non-human mammal tolerant to antigens originating from a human donor, using the non-myeloablative tolerogenic treatment described herein. That is, the non-human mammal functions as the "host mammal" in the protocols described above, and a human being is the "donor." For example, a rodent can be tolerized to human cells, tissues and organs by employing Steps 1-4 of the disclosed tolerogenic protocol to produce a mixed chimera rodent permanently engrafted with human hematopoietic cells. It is known that such hematopoietic engraftment is possible even between disparate species. For example, it has been demonstrated that human hematopoietic cells can engraft in mice. See, for example, Marcus et al . , Blood 86: 398- 406 (1995) . In those cases where survival and functioning of human hematopoietic cells is less than optimal in non-human mammalian hosts, it is possible to provide the host mammal with human hematopoietic cytokines in order to ensure engraftment of the human cells . There are numerous uses for such chimeric animals. For example, since the host mammals have been tolerized to the human donor, it is possible for human tissues, e.g., tumors or HIV-infected hematopoietic cells, to be transplanted into and accepted by these rodents in order to produce rodent models of human disease. Thus, these non-human mammal/human chimeras may be used to study biological phenomena related to human disease, including testing of new drugs. Production of non-human mammal/human hematopoietic mixed chimeras is of even greater significance for those non-human mammalian species targeted as potential sources of cells, tissues and organs for transplantation into human patients. For example, it is widely recognized that pigs are a potential useful source of tissues and organs for transplantation into humans . Such porcine materials are subject to an immediate, "hyperacute" rejection response when transplanted into human patients, as well as to longer-term immune-mediated rejection by the human host. Pigs are being genetically engineered or otherwise treated to protect tissues and organs of such pigs from being hyperacutely rejected when transplanted into a human patient. This can be accomplished, for example, by providing the pigs human genes encoding human complement regulatory proteins, or by "knocking out" the genes responsible for production of pig antigens recognized by preformed xenoantibodies present in all humans. See, for example, PCT/US96/15255 and PCT/IB95/00088. A "two-way" variation of the present tolerogenic protocols can be applied to such genetically engineered pigs as well as to other donor mammals to allow for ready transplantation of xenogeneic donor cells, tissues and organs into humans. For example, in a preliminary tolerization procedure, a human patient can function as an initial "donor" to provide antigens and hematopoietic stem cells to a "host" pig in the 4 -Step protocol described above. As a result, the pig is transformed into a pig/human hematopoietic mixed chimera, with the pig's hematopoietic cells being tolerized to the human patient's cells, tissues and organs. Following this, the roles of the human patient and pig are reversed, with the pig becoming the donor and the human patient becoming the host in the 4-Step protocol. That is, the pig's hematopoietic cells, with T cells tolerant of the human patient, may be used in the 4-Step protocol for transformation of the human patient into a human/pig hematopoietic mixed chimera. The human patient is then able to accept cells, tissues and organs from the pig, for the reasons discussed above. The crucial advantage is that all of this can be accomplished while avoiding the risk of xenogeneic GVHD engendered by immunocompetent T cells of the pig, since the pig's T cells were made tolerant to the patient in the preliminary tolerization procedure. Thus, assuming the hyperacute rejection response can be overcome in other ways (e.g., genetic engineering of the animal providing the transplanted material) , the present invention allows for xenogeneic transplantation of cells, tissues and organs into humans without the need for long-term immunosuppression. As an alternative to tolerizing the pig to the human patient's antigens, the pig's cells can be isolated and the tolerization can be performed in vi tro, using standard tissue culture techniques familiar to those in the art. The human antigens can be cellular or non-cellular, as in step 2. They can be, for example, any of the hematopoietic cells described herein. Furthermore, the procedure, whether in vivo or in vi tro, can be carried out using allogeneic host/donor mammalian (e.g., human, non-human primate, pig, rat, mouse, rabbit, or guinea pig) combinations as well as xenogeneic combinations. In addition, tolerization of donor cells need not be antigen-specific. It can involve depletion of, for example, substantially all T cells in the bone marrow to be used in step 4, with the retention of stem cells and, optionally, other functional cell subsets (e.g., NK cells) that can have therapeutic benefit for a host subject suffering from a relevant disease (e.g., cancer) . Methods for non-specifically depleting T cells from a hematopoietic cell sample include those described above for steps 1 and steps 3 of the tolerization protocol or those described below for steps A and C of the protocol used to deplete cells to be used for cell therapy. Experiments using such a non-specific in vi tro depleting protocol are described in Example 15. (viii) Cell compositions Another aspect of the invention is a hematopoietic cell composition derived from a host treated with the donor-specific tolerogenic treatment described above. The cell composition comprises host-originating lymphocytes and donor-originating lymphocytes. The proportion of donor-originating lymphocytes may vary. Preferably, the donor-originating lymphocytes comprise about 5% to about 50% of the lymphocyte composition. Most preferably, the donor-originating lymphocytes comprise about 20% to about 50% of the lymphocyte composition. The hematopoietic cell composition is specifically depleted of donor-specific, host-originating lymphocytes. "Depleted" in this context refers to reduction in numbers of T lymphocytes or reduction in T lymphocyte function sufficient to eliminate or significantly reduce anti -donor responses in the host and thus to reduce the risk of GVHD if the composition is administered to the donor. In a related aspect, the invention involves a method of making the above described hematopoietic cell composition. The method includes subjecting a host mammal to the tolerogenic treatment described above using an allogeneic or xenogeneic donor. After tolerogenic treatment, the method involves isolating a composition of hematopoietic cells from the host . This cell composition contains host and donor- originating hematopoietic cells, but is depleted of donor-specific host-derived T lymphocytes. B . Cell Therapy Protocols Completion of the tolerogenic treatment protocols can provide a platform for subsequent allogeneic cell therapy with donor lymphocyte infusions in cancer patients and in other patients with malignant and non- malignant diseases requiring bone marrow transplantation, since donor cells accepted by a tolerant host may induce graft-versus-leukemia (GVL) or graft -versus-tumor (GVT) effects. Such non-malignant diseases include without limitation aplastic anemia, genetic diseases resulting in enzyme deficiencies, and diseases caused by deficiencies in well-defined products of hematopoietic stem cells, such as osteoclast deficiency in infantile osteopetrosis and deficiencies in B cells and T cells in congenital and acquired immune-deficiency syndromes. Donors of cells for use in allogeneic cell therapy can be MHC (e.g., HLA in humans) incompatible or MHC identical with the host. Where MHC incompatible, the donor and host can share no MHC class I or class II alleles. Alternatively, they can share 1 or more (e.g., 2, 3, 4, or 5) MHC class I and/or class II alleles. Where donor and host are MHC identical, they can be incompatible at MiHL. They will thus preferably not be monozygotic twins. Allogeneic cell therapy is described, for example, in PCT publication no's. WO 95/24910 and WO 96/37208. In allogeneic cell therapy, an anti-tumor or other anti-host hematopoietic cell effect is achieved by administering allogeneic peripheral blood lymphocytes to the host, either alone or in combination with a T cell activator. Alternatively, allogeneic peripheral blood lymphocytes are "pre-activated" in vi tro by a T cell activator such as interleukin-2 (IL-2) and then administered either alone or in combination with the same or different T cell activator. Preferably, one or more infusions of about 10 5 to about 10 9 cells/kg of allogeneic peripheral blood lymphocytes, including well-defined lymphocyte subsets, are administered. When preceded by the tolerogenic treatment described herein, these infusions of allogeneic lymphocytes are carried out with a much reduced chance of rejection of the anti-cancer effector cells, which need to become engrafted in the host. In addition, the risk of GVHD is reduced or eliminated by residual hematopoietic cells of the host and, if necessary, relatively late infusion of donor lymphocytes . Allogeneic cell therapy following the tolerogenic treatment protocols described herein can be valuable not only in the context of cancer and other diseases, but also when it is desired to adoptively transfer immunity to infectious agents from the donor to the host. Thus, if a donor used the tolerogenic protocols described herein is immune to an infectious agent (e.g., hepatitis B; see Ilan et al . , Hepatology 18: 246-52 (1993)), th s immunity can be transferred to a host by infusing lymphocytes from the donor to the host following completion of the tolerogenic protocols. Alternatively, the stem cell preparation infused m Step 4 of the tolerogenic protocols can itself provide the adoptive transfer of immunity, since stem cell preparations may contain immunocompetent lymphocytes. From the above descriptions of depletion-mediated allogeneic and xenogeneic tolerance induction, particularly the discussion of methods for depleting hemopoietic cells (e.g., pig hemopoietic cells) of host- specific activity prior to their use is step 4 of the tolerizing protocol, it is clear that cell therapy can be performed using cells from individuals that are xenogeneic as well allogeneic to the host (i.e., recipient of cell therapy) . In xenogeneic cell therapy, the same compositions of lymphocytes, with or without one or more T cell activators, e.g., IL-2, mterferon-γ (IFN- γ) , granulocyte-macrophage colony stimulating factor (GM- CSF) , or mterleukm-12 (IL-12), described above for allogeneic cell therapy, can be used. Moreover, the cells can be pre-activated, either in vi tro or m vivo, prior to administration to the host. The cells to be used for cell therapy can be obtained from the blood, spleen, lymph nodes, or any other source of lymphoid cells, or they can obtained from a hematopoietic cell source, e.g. bone marrow or fetal liver. (i) Depletion of cells to be used for cell therapy Cell therapy can be given by administering unselected lymphoid cells. In general, however, prior to administration, the lymphoid cells will be depleted of T cells reactive to the recipients antigens or of responsiveness of T cells to the host's antigens. In the following description, the word "deplete" (or "depletion") refers to decreasing the numbers of T cells with responsiveness to antigens of the second individual and/or decreasing the responsiveness of such T cells. Depletion can be by decreasing the number of responsive lymphocytes or the responsiveness of the lymphocytes by 20%, 30%, 40%, 50%, 80%, 90%, 95%, 98%, 99% or even 100%. In the above description of depleting donor hemopoietic cells of host-specific activity, the host/donor terminology was reversed. In the following discussion of methods to deplete lymphoid cells to be used for cell therapy, in order to avoid confusion as to the "host" and the "donor", the individual that is the source of the cells to be used for cell therapy will be referred to as the "first individual" and the individual that is to receive the cell therapy (e.g., a human cancer patient) will be referred to as the "second individual". Furthermore, the phrase "non-syngeneic" will be understood to cover both allogeneic and xenogeneic. Methods of depleting lymphoid cells prior to their use for cell therapy basically involve the use of one or more of steps 1-4 described above for tolerizing a host against antigens of a donor. In order, however, to differentiate the steps used for depleting lymphoid cells to be used for cell therapy from those used for tolerizing a host mammal prior to transplantation, the steps used for depletion of lymphoid cells to be used for cell therapy are designated "steps A-D" : Step A: Expose lymphoid cells of a first individual to a non-myeloablative regimen sufficient to decrease, but not eliminate, the first individual's functional T lymphocytes. Step B: Contact the lymphoid cells of the first individual with antigens of a non-syngeneic second individual . Step C: Substantially eliminate T lymphocytes of the first individual that are responsive to the antigens of the second individual by delivering a non- myeloablative dose of a lymphocytotoxic or tolerizing agent to the lymphoid cells. Step D : Administer a preparation of hemopoietic stem cells obtained from the second individual to the first individual . Essentially the same methods for performing steps 1-4 described above can be used to carry out steps A-D, except that some or all of the steps can be carried out in vi tro using tissue culture methods familiar those in the art. Naturally, step D is always in vi vo and can only be included where the entire depleting procedure is performed in vivo . The lymphoid cells can be depleted using either (i) step A alone, (ii) step B alone, (iii) steps A and B, (iv) steps B and C, (v) steps B, C, and D, (vi) steps A, B, and C, or (vii) steps A, B, C, and D. A method of depleting lymphoid cells of a first individual of reactivity against a second individual can involve, for example, harvesting lymphoid cells from the first individual and carrying out steps A-C in vi tro . Alternatively, after carrying out step A in the first individual, the lymphoid cells can be isolated from the first individual and steps B and C carried out in vi tro . In another embodiment, steps A and B can be carried out in the first individual, the lymphoid cells can be isolated from the first individual, and step C carried out in vi tro . In addition, all the steps can be carried out in vivo, optionally including step D. (ii) Step A Step A decreases the total number of T cells in the cells to be used for therapy and is optionally used. Steps B and C are used to deplete only those cells or that T cell reactivity specific to donor antigens. As indicated by the experiments in Examples 10-13 below, step C, in some cases (e.g., where cancer cells are used as antigens) , can be excluded since step B, can be sufficiently tolerogenic. (iii) Step B The same sources of antigen used in step 2 can be used in step B and they can be obtained by the same methods. In addition, however, cancer cells can be used. As indicated by the experiments in Examples 10-13 below, lymphoid cells depleted using cancer cells for step B can be particularly useful for non-syngeneic cell therapy of a mammal with cancer, e.g., a human cancer patient. The experiments, which were carried out using mouse strains (X and Y) which differed either at both MHC and Minor Histocompatibility Loci (MiHL) or at MiHL only, indicate that lymphoid cells from a mouse of strain X exposed to cancer cells of strain Y, while being substantially depleted of graft-versus-host activity against antigens of strain Y, have enhanced anti -cancer therapeutic efficacy when transferred together with cancer cells to mice of strain Y. The therapeutic activity of the strain X lymphoid cells exposed to strain Y cancer cells was significantly greater than that of strain X lymphoid cells exposed to strain Y lymphoid cells, which, at least in the case of a MiHL incompatibility alone, was significantly greater than that of normal, unexposed, strain X lymphoid cells. Furthermore, in a mouse combination in which strains X and Y differ at both the MHC and MiHL, while lymphoid cells from normal, unexposed strain X mice displayed lethal GVH activity when transferred to host animals expressing strain Y antigens, lymphoid cells from strain X animals exposed either to cancer cells of strain Y or to lymphoid cells of strain Y were substantially depleted of the GVH activity. On the other hand, in a mouse combination in which strains X and Y differ at the MiHL only, exposure of strain X animals to cancer cells of strain Y depleted the strain X lymphoid cells of strain Y-specific GVH activity relative to lymphoid cells from normal, unexposed strain X animals but exposure of strain X animals to lymphoid cells from strain Y animals did not have an effect on the GVH activity of strain X lymphoid cells. (iv) Use of tumor cells for step B These experiments point to a novel form of non- syngeneic cell therapy of mammalian, and in particular human, cancer, in which the protocol used to deplete the lymphoid cells to be used for therapy of GVHD activity also results in enhanced anti-cancer therapeutic efficacy in the lymphoid cells. A subject to be given cell therapy with lymphoid cells of another histoincompatible subject could be treated by any or all of steps 1-4 above in order to establish non-reactivity (tolerance) to the antigens of the other subject. Alternatively, the cell therapy could be given without any of these steps. It could be given, for example, to a patient that for unrelated reasons (e.g., HIV AIDS, genetic immunodeficiency, chemotherapy, or radiotherapy) is significantly immunologically depleted (i.e., with a functional T cell population depleted by greater than 80%, 90%, 95%, or 99%) . The lymphoid cells used for the therapy could be from an unrelated MHC compatible or incompatible human, a related MHC compatible or MHC incompatible human, or an individual of another species, e.g., a pig or a non-human primate. The lymphoid cells will preferably not be from a syngeneic individual (e.g., a monozygotic twin) . The cancer cells that can be use for step B are preferably of the type present in the subject to be given the cell therapy, e.g., leukemia, lymphoma, breast cancer, lung cancer, gastrointestinal cancer, melanoma, or genitourinary cancer, and thus will express the same antigens or cross-reactive antigens. However, it remains possible that the enhanced anti-cancer therapeutic efficacy of non-syngeneic lymphoid cells depleted by exposure to tumor cells is not antigen specific and that cells depleted by exposure to a wide variety of tumors, will be have anti-tumor effects against the tumor harbored by a given subject. Thus step B can also be performed using, as a depleting agent, tumor cells of a type unrelated to that of the subject. Cancer cells to be used for Step B will preferably express MHC molecules (at least one MHC class I or MHC class I molecule) of the patient. More preferably, they will be derived from the patient. Tumor cells from all subjects, including human patients, will be obtained by standard methods known to those in the art . In the case of non-solid, hematological cancers, the cells can be isolated or enriched from the blood, bone marrow, spleen, lymph nodes or other lymphoid tissue of the subject. In the case of solid malignancies, tumor cell suspensions can be obtained by disruption of solid tumor tissue removed by surgical excision of either the primary tumor or metastases. Where tumor cells of the patient are not available or are not available in sufficient quantity, it is possible that established tumor cell lines can be used. Such tumor cell lines can endogenously express the MHC molecules of the patient or they can have been stably transfected with and express genes encoding such molecules. If necessary, a plurality of such transfected lines, each expressing, either endogenously or due to stable transfection, the MHC molecules of the patient. Subcellular fractions (e.g., cell lysates or membrane fractions) of any of the above tumor cell preparations can also be used as antigen for step B. Furthermore, subcellular fractions, or isolated tumor antigens can be presented to donor T cells in association with antigen presenting cells (e.g., dendritic cells, macrophages, monocytes, or B lymphocytes), which can be freshly prepared or precultured. The antigen presenting cells can express MHC molecules of the patient, and preferably will be derived from the patient. Such antigen presenting cells can, for example, be pulsed with tumor cell extracts or peptides derived from tumor-associated polypeptide antigens prior to their use in step B. Alternatively, the antigen presenting cells can be transduced or stably transfected with expression vectors encoding full-length tumor-associated polypeptide antigens or peptides corresponding to subregions of such antigens. In addition, cell hybrids formed by fusion of tumor cells and antigen presenting cells can be used as the activating antigen for step B of the method. It is envisaged that, where the second individual (i.e., the recipient of cell therapy) is a human patient and step B is to be carried out in vivo, the first individual (i.e., the source of the non-syngeneic cells) will likely either be a relative of the patient or an individual of another species. However, whether in vivo, in vi tro, in a relative, or in an individual of another species, proliferation of the tumor cells used as antigen can be substantially eliminated by prior exposure of the tumor cells to a sufficient dose of an anti-proliferative agent, e.g., ionizing radiation or mitomycin-C. Alternatively, the above-mentioned subcellular fractions can be used. (v) Non-antigen-specific methods of depleting cells Protocols for non-specifically depleting hematopoeitic cells of responsive T cells can involve the use of any of the methods described above for step A without any of steps B-D. In addition, any of a wide variety of methods known in the art (e.g., use of magnetic beads, lytic complement, or flurorescence activated cell sorting) employing single or combinations of antibodies (monoclonal or polyclonal) that bind to T cells (or cells other than T cells) can be used to remove T cells. Such protocols can employ single or multiple (e.g., 2,3,4,5,6,8,10 or 12) treatments and one or more of the methods can be used. Furthermore, they can be performed either in vivo or in vi tro . Such protocols will result in elimination of substantially all (e.g., greater than 80%, preferably greater than 90%, more preferably greater than 95%, and most preferably greater than 99%) of the T cells in the hematopoietic cell population. It is noted, as described above, that such protocols can also be used to deplete donor cells of reactivity to the host prior to use in step 4 of the tolerization method of the invention. An example of such protocol is provided in Example 15. (vi) Cell compositions The instant invention also encompasses cell compositions containing lymphocytes obtained from a first individual that have been depleted of reactivity to the antigens of a second individual. The depletion will preferably have been performed by one of the methods described above involving the indicated possible combinations of steps A-D. Such methods include the use of both cellular and non-cellular antigens for step B and the optional use of step D (i.e., administering bone marrow cells derived from the second individual to the first individual) . Thus the compositions can contain 20%-100%, 30%-100%, 50%-100%, 70%-100%, or 90-100% of cells derived from (i.e., endogenous to) the first individual. The cells of the compositions will be suspended in a physiological solution, e.g., a saline solution, and provided in an appropriate container, e.g. a blood bag, a semipermeable blood bag, a tissue culture receptacle such as a tissue culture flask, or a bioreactor. The lymphocyte source (e.g., blood, bone marrow, spleen, or lymph nodes) can be obtained from the first individual, at the appropriate stage of depletion, and purified or enriched by methods familiar to those in the art. It is envisaged that "batches" of such cell compositions can be made and stored by a supplier using lymphocytes from appropriate human or non-human (e.g., pigs and non-human primates) donors. The lymphocytes can be depleted, using as step B, for example, single or combinations of tumor cell lines stably transfected with and expressing combinations of all known MHC genes. In this way, batches of lymphocytes would be derived, each being tolerant to a different set of HLA antigens. These custom pretolerized batches of lymphocytes can then be supplied to a practitioner (e.g., an oncologist) whose patient is in need of the therapy after communication of the patient's HLA haplotype to the supplier. If it is proposed to carry out the tolerogenic method of the invention prior to non-syngeneic cell therapy, the practitioner can be supplied with a source of antigenic material (e.g., hemopoietic cells) for use in step 2 of the tolerogenic protocol . This antigenic material can be derived from the individual from which the lymphocytes used for cell therapy were obtained, or from an individual syngeneic with the individual . Alternatively, either cells (e.g., non-malignant hemopoietic cells) stably transfected with appropriate HLA genes or cell extracts of either malignant or non- malignant cells similarly transfected can be used. (vii) Methods of cell therapy Also within the scope of the invention are methods of treatment that include administration of the above cell compositions to a subject, preferably a human patient. As indicated above, such subjects may optionally have been subjected to one of the described tolerogenic regimes. Patients to which the treatment can be given include cancer patients (e.g., those with the above-listed tumors) , patients suffering from infectious diseases such as HIV AIDS or hepatitis B or C, genetic diseases associated with protein (e.g., hemoglobin or an enzyme) deficiencies or abnormalities, aplastic anemia, congenital immunodeficiencies, and autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, insulin-dependent diabetes mellitus, lupus erythematosus, and myasthenia gravis. (viii) Articles of manufacture Also encompassed by the invention are articles of manufacture including packaging material (e.g., a cardboard box) containing a biological cell container such as those listed above. The biological container contains any of the cell compositions described above and the packaging material can include a package insert or a label indicating that the composition is to be used in a method of treatment comprising administering of the composition to a mammal in need of the composition (e.g., patients with any of the diseases listed above) . The invention will be further understood with reference to the following illustrative embodiments, which are purely exemplary, and should not be taken as limiting the true scope of the present invention as described in the claims. EXAMPLE 1 Materials and Methods Animals Inbred BALB/c (H-2 d ) , C57BL/6 (B6) (H-2 b ) , DBA/2 (H-2 d , CBA (H-2 k ) , B10.D2 (H-2 d ) , SJL (H-2 S ) , and (BALB/c x C57BL/6)F 1 (FI) (H-2 d/b ) mice and Lewis rats were purchased from the Hebrew University Hadassah Medical School Animal Facility in Jerusalem, Israel, with breeding pairs originating from Harlan-Olack, Bicester, UK. Two to three month-old mice were used for the study. Mice were kept under standard conditions with food and water provided ad lib. Most of the experiments were carried out in B6 → BALB/c strain combination. Total Lymphoid Irradiation (TLI) Mice were anesthetized and then positioned in an apparatus designed to expose the major lymph nodes, thymus, and spleen to ionizing irradiation, while shielding most of the skull, ribs, lungs, hind limbs and tail with lead, as previously described. Slavin et al . , J . Exp . Med . , 146:34 (1977). Radiation was delivered by a Phillips X -ray unit (250 kv, 20 mA) at a rate of 70 cGy/min, using a Cu 0.2-mm filter. The source-to-skin distance was 40 cm. Tolerogenic Treatment The basic protocol for conditioning prior to transplantation included sTLI (1-6 daily exposures of 200 cGy) to a total dose of up to 1,200 cGy, followed by intravenous inoculation with 3xl0 7 BMC on the day after the last TLI dose. Some mice (see Example 9) were not administered any sTLI . One day after BMC infusion, experimental mice were injected with 200 mg/kg Cy (Taro, Israel) intraperitoneally. Cy was freshly dissolved in sterile phosphate-buffered saline prior to injection. Modifications of the Cy protocol to induce tolerance to xenografts are described in Example 8. A second infusion of donor BMC, after Cy, was also administered in some of the mice. Preparation of Bone Marrow and Spleen Cells Single cell suspensions of BMC and spleen cells were prepared in PBS or RPMI 1640 medium supplemented with 100 μg/ml streptomycin and lOOU/ml penicillin (Biological Industries, Beit Haemek, Israel) . BMC were infused into the lateral tail vein in a total volume of 0.5 ml. Preparation of Blood Cells for Infusion Pooled fresh blood was collected into heparin- containing tubes (preservative-free) . Each recipient was infused with 0.5 ml into the lateral tail vein. T cell Depletion of BMC With Monoclonal Antibodies Monoclonal rat anti -mouse Thyl antibodies (YTS 148.3, IgM and YTS 154.7, IgG2b) were obtained from Dr. H. Waldmann (Oxford University, UK) . BMC (10 7 /ml) were incubated with YTS 148.3 antibody at a final dilution of 1:200 for 40 min, washed and incubated with Low-Tox rabbit complement (Cedarlane, Canada) at a final dilution of 1:10 for an additional 60 min at 37° C, washed and injected intravenously into recipients. YTS 154.7 antibody was used for depletion of T cells from BMC in vivo; BMC (3xl0 6 /ml) were incubated with 750 μg, 150 μg or 30μg of the antibody for 60 min at 4° C and the mixture was injected intravenously into recipients. Skin Grafting Skin grafting was carried out 20 days after completion of the tolerogenic treatment. A full- thickness skin graft measuring 1cm x 1cm was adjusted to the graft bed by 4 Thomas surgery clips (Thomas Scientific, USA ) . The panniculus carnosus was kept intact in the graft bed. The graft was considered to be accepted when hair of donor color grew on the soft flexible underlying skin, and rejected when donor epithelium was lost. Implantation of Bone Marrow Plugs The femora of B6 mice were freed of muscle and irradiated with 400 cGy in vi tro to eliminate most of the hematopoietic cells. Marrow plugs were mechanically pressed out of the femur canal with a mandrin and 2 plugs were implanted under the left kidney capsule of each recipient, as described in Chertkov et al . , Rad. Res. 79, 177-186 (1979) . Heterotopic Heart Grafting Hearts of 1-2 day old B6 mice were transplanted into the ear skin pocket 20 days after tolerogenic treatment, according to the methods of Chernyakhovskaya et al . , Transplantation, 29:409 (1980). An ECG was first recorded two weeks after grafting and thereafter at weekly intervals. Polymerase Chain Reaction (PCR) PCR was carried out on material derived from blood samples as described previously. Pugatsch et al . , Leukemia Res. 17, 999-1002 (1993) . Briefly, blood samples were lysed in distilled water and centrifuged at 12,000 x g. Supernatants were discarded and 50 μl of 0.05 M NaOH were added to the cell pellets. Samples were boiled for 10 min., then 6 μl of 1 M Tris (pH 7.2) were added. Samples were then centrifuged at 12,000 x g and supernatants were used for assay. The 5'- and 3'- oligonucleotide primers chosen for amplification and the PCR reaction conditions are described in Pugatsch et al . Reaction products were visualized on 1.6% agarose gels (Sigma, USA) containing 0.05 μg/ml ethidium bromide. Murine BCLl BCLl, a B-cell leukemia/lymphoma of BALB/c origin (Slavin, S. and Strober, S. Nature, 272:624 (1978); Slavin, S. et al . Cancer Res . , 41:4162 (1981)), was maintained in vivo by serial passage in BALB/c mice. Inoculation of 10 to 100 BCLl cells in BALB/c mice results in typical B-cell leukemia/lymphoma characterized by splenomegaly, extreme peripheral blood lymphocytosis (up to 500,000 lymphocytes/ml) and death of 100% of recipients. BCLl also causes leukemia in FI recipients, but development takes longer than in BALB/c recipients (Slavin et al . (1981), supra) . Immunization of donor mice Donor C57BL/6 (H2- b ) mice were immunized against alloantigens of both the MHC and Minor Histocompatibility Loci (MiHL) by injection with either spleen cells obtained from BCLl-bearing BALB/c (H-2 d ) mice (30 x 10 6 cells per mouse per immunization) or spleen cells obtained from normal BALB/c mice (30 x 10 δ per mouse) . Donor B10.D2 (H-2 d ) mice were immunized against MiHL alloantigens only by injection with the same cell populations used for immunization of the C57BL/6 mice. Immunizations were by intraperitoneal injection on days - 20 and -10 prior to sacrifice, harvesting of spleens, and transfer of isolated spleen cells to FI or BALB/c host animals . Total body irradiation (TBI) Mice were conditioned by TBI with a single dose of 400cGy delivered by linear accelerator (Varian Climac 6 X) at a source to skin distance of 80cm, at a dose rate of 170cGy/min. Transplantation of spleen cells for immunotherapy of BCLl Spleens from C57BL/6 or B10.D2 donors immunized using the protocols described above were teased into single cell suspensions using nylon meshes, washed twice with 10% bovine calf serum in RPMI 1640 medium (Biological Industries, Beit Haemek, Israel), and injected intravenously. GVL effects against BCLl Assays for assessing induction of GVL were performed as follows. For testing GVL across an incompatibility involving both MHC and MiHL alloantigens, total of 10 4 fresh BCLl cells and 30x10 s spleen cells obtained from the immunized or control unimmunized C57BL/6 donors were infused into FI recipients 24h after TBI. For testing GVL across an incompatibility involving only MiHL alloantigens, a total of 2xl0 3 BCLl and 30xl0 6 spleen cells obtained from immunized or control unimmunized B10.D2 donors were infused into BALB/c recipients 24h after TBI. Administration of a known tumor cell number allowed quantitative measurement of GVL effects . Assay for chimerism Chimerization of BALB/c or FI recipients by C57BL/6 spleen cells was measured by testing the percentage of host or donor-type cells in spleen or blood samples using an in vi tro complement -dependent microcytotoxicity assay with specific alloantisera and rabbit complement. Morecki et al . J . Exp . Med . , 165:1468 (1987) . Specific alloantisera ("BALB/c anti-C57BL/6 " and "C57BL/6 anti-BALB/c") were prepared by cross- immunization of the relevant mice with full -thickness skin allografts followed by 6 intraperitoneal injections of 30x10 s donor spleen cells at intervals of 1-2 weeks. Mice were bled and sera stored at -70°C. Spleen or peripheral blood cells from the BALB/c or FI recipients were incubated with both alloantisera and rabbit complement. Host BALB/c cells were lysed with anti-BALB/c and not anti-C57BL/6 antisera, host FI cells were lysed with both anti-BALB/c and anti-C57BL/6 antisera, and cells in the BALB/c FI mice derived from the C57BL/6 donor mice were lysed by only anti-C57BL/6 antiserum. Chimerism was expressed as % donor type (C57BL/6) cells which was calculated as follows: % donor type (C57BL/6) cells = % cells lysed with anti-C57BL/6 antiserum-% cells lysed with anti-BALB/c antiserum-% cells lysed with complement alone. Serial PBL counts for monitoring development of BCL1- leukemia Peripheral blood samples (20μl) were obtained by weekly venipuncture using heparinized glass capillaries. Peripheral blood leukocyte (PBL) counts were determined using a hemocytometer after lysis of red blood cells in 2% acetic acid. Detection of residual BCLl cells by adoptive transfer An in vivo assay was used for detection of residual BCLl cells in treated experimental mice. Aliquots of 10 s lymphocytes obtained from a pool of spleen cells from treated mice with no evidence of disease were adoptively transferred to normal secondary BALB/c recipients (10 mice per group) . The development of leukemia was determined by weekly peripheral blood lymphocyte (PBL) counts, monitoring spleen enlargement, and survival . Monitoring of GVHD Recipients were monitored for survival and clinical signs of GVHD such as ruffled fur, diarrhea, and measurable weight loss. At the time of adoptive transfer, samples of liver and lung sections were obtained form each mouse to test for histological evidence of GVHD. Histological preparations were analyzed by an independent pathologist on a double blind basis . Cytokine assays The levels of interferon-γ (IFN-γ) , tumor necrosis factor-α (TNF-α.) , interleukin-2 (IL-2) , interleukin-4 (IL-4) and interleukin-10 (IL-10) were measured in supernatants of donor (C57BL/6) spleen cells at the time of harvesting for transplantation to host (recipient) animals. Recipient (FI) spleen cell supernatant cytokine levels were measured at 3 weeks after transplantation of donor C57BL/6 spleen cells. Cytokines were measured by "sandwich" ELISAs using murine cytokine-specific monoclonal antibodies for capture using standard methods familiar to those in the art. Antibodies that bind to cytokine determinants that do not overlap with determinants that bind the capture antibodies and alkaline phosphatase conjugated anti-Ig antibodies were used for detection. Standard curves were generated with standard samples of each cytokine and the concentrations of the cytokines in unknown samples were determined from the standard curves . In vi tro T cell proliferation assays Mitogen-induced T cell proliferative responses were measured by culturing spleen cells in the presence of the T cell mitogens concanavalin A (ConA, 10 μg/ml) or phytohemagglutinin (PHA, 1 μg/ml) for three days. [ 3 H] - thymidine was added to the cultures for the last 6 hours of culture and T cell proliferation was measured in terms of counts per minute (cpm) of [ 3 H] -thymidine incorporated into the DNA of the cells using cell harvesting and radioactivity techniques known to those in the art. In vi tro cell-mediated lysis assays Human peripheral blood mononuclear cell (PBMC) and murine spleen or bone marrow populations were tested for their ability to lyse NK-sensitive target cells (human K562 and murine YAC-1 cells) and NK-resistant , activated NK-sensitive target cells (human Daudi and murine P815 cells) using standard 51 Cr-release assays. Briefly, 51 Cr- labeled target cells were incubated at 37°C with PBMC effector cells at various target cell to effector cell ratios for 4 hours. At the end of the incubation, the cells were pelleted by centrifugation and equal aliquots of supernatant were removed from each assay culture well and counted for radioactivity. Percent lysis was calculated by the following formula: Percent lysis = cpm(exp.) - cpmtcont.) x 100 cpm (max.) - cpm(cont.) where: cpm(exp.) are the cpm detected in the supernatant from an experimental culture well containing target cells and effector cells; cpm(cont.) is the mean value of the cpm detected in supernatants from control culture wells containing target cells but no effector cells; and cpm (max.) is the mean value of the cpm detected in supernatants from culture wells containing target cells and a detergent (e.g., sodium dodecyl sulfate) or acid (e.g., IN HC1) and represent the maximum amount or radioactivity releasable from the target cells by lysis. Statistical analyses The statistical significance of the results comparing treated and control mice was calculated by the independent t-test. EXAMPLE 2 Nonspecific immunosuppression of mice treated with sTLI alone or with sTLI and Cy In the first set of experiments (Table 1) , BALB/c mice were given 6, 8 or 12 (experimental groups) or 17 (control groups) fractions of TLI at 200 cGy/fraction . After TLI, 3 x 10 7 BMC from B6 donors were administered one day after the last TLI. Skin allografts from B6 donors were transplanted 20 days after transfer of the BMC. In a second set of experiments (Table 2) , 5 groups of BALB/c recipients were administered sTLI of 6 fractions of 200 cGy/day, followed a day later with 200 mg/kg Cy intraperitoneally. After the above conditioning, the five groups were treated as described below. Group 1 received 3 x 10 7 BMC, whereas group 2 received 3 x 10 7 BMC and 5 x 10 6 spleen cells. Group 3 received 0.3 ml of whole blood. Group 4 received 3 x 10 s BMC, whereas group 5 received 3 x 10 6 T cell depleted BMC. T cell depletion was performed in vi tro with monoclonal antibody Thy 1 and rabbit complement, as described in Example 1. For all of the above five groups, cells and whole blood were from B6 donors and were infused intravenously. Skin allografts were transplanted 20 days after BMC or blood transfer. RESULTS A short course of TLI (sTLI) , in contrast to a long course of TLI (17 fractions, 200cGy each) , was insufficient for acceptance of stem cells from allogeneic BMC or blood. Table 1 shows that none of the BALB/c mice receiving 3xl0 7 fully mismatched BMC from B6 donors became hematopoietic cell chimeras after 6 fractions of TLI, while consistent acceptance of allogeneic BMC was obtained after 17 fractions of TLI. As shown in Table 1, after treatment with sTLI and allogeneic BMC or allogeneic blood cells, BALB/c recipients stayed alive, none developed GVHD and all rejected B6 skin allografts. Thus, after sTLI alone, sufficient numbers of immunocompetent cells remain in the host to reject a donor allograft. TABLE 1. Incidence of allogeneic BMC and skin graft acceptance after fractionated total lymphoid irradiation. No of TLI % of donor cells Skm allograft survival Fractions m blood 100 days >100 days c after cell transfer 3 0 (10) b 0/10 0(3), 56(1) 1/4 12 0 (3) , 50, 90 2/5 17 82, 85, 90(2 ' 93 5/5 a BMC (3xl0 7 ) from B6 donors were given to BALB/c recipients one day after the last TLI . b Number of mice with the same level of chimerism is given m parentheses . c Skm allografts from B6 donor were transplanted 20 days after cell transfer. Table 2. Skin allograft survival and GVHD related death after transplantation of allogeneic cells and skin graft following sTLI in combination with a single injection of Cy. a Cells or whole blood from B6 donors were transferred intravenously to BALB/c recipients after sTLI and Cy. °T cell depletion was performed in vi tro with mAb anti Thy 1 and rabbit complement . c Donor skin allografts were transplanted 20 days after cell transfer. NT - not tested. In sharp contrast, a single injection of Cy (200 mg/kg) given one day after the last fraction of sTLI (6 fractions, 200cGy each), increased the non-specific immunosuppression and allowed the hosts to accept BMC, spleen and blood cells. However, all recipients developed typical acute GVHD which was lethal in most cases (Table 2) . The survival time of the recipients with GVHD appeared to be a function of the number of mature immunocompetent T cells present in the inoculum. Mean survival time of mice inoculated with 3xl0 7 BMC was four times longer as compared with recipients of an equal number of BMC mixed with 5xl0 6 B6 spleen cells (Table 2, groups 1 & 2) . Transfer of fewer BMC (3xl0 δ instead of 3xl0 7 ) did not prolong survival significantly (Table 2, groups 1 & 4) . GVHD was successfully prevented in the mice of group 5 (Table 2), who received T cell depleted BMC after sTLI and Cy. Although none of these experimental mice developed GVHD and all of them remained alive, all but one rejected donor BMC. Accordingly, prolonged skin allograft survival was observed in only 1/7 recipients. EXAMPLE 3 Antigen-specific elimination of residual donor-alloreactive host immunocompetent T cells Three groups of BALB/c recipient mice were administered sTLI in 6 fractions of 200 cGy/day. Non T cell depleted BMC (day 0) from B6 donors were transferred intravenously to the BALB/c recipients in all three groups. The next day all of the recipients were administered 200 mg/kg of Cy. One group (Table 3, Group 1) received a skin graft from B6 donors at day 20. The second group (Table 3, Group 2) received 3 x 10 7 non T cell depleted BMC from B6 donors on day 2 followed by a skin graft at day 20. The third group (Table 3, Group 3) received 3 x 10 s non T cell depleted BMC from B6 donors on day 2 followed by a skin graft at day 20. Levels of donor cells in blood were assayed in all surviving mice at day 100 according to the protocol of Example 1. Table 3. BM and skin allograft acceptance in experimental mice a BM cells from B6 donors were transferred intravenously to BALB/c recipients . b T cell depletion was performed in vi tro with mAb YTS 148.3 and rabbit complement. A dose of 2xl0 6 cells/recipient resulted in similar tolerance (data not shown) . C T cell depletion was performed in vivo with mAb YTS 154.7. d Number of mice is given in parentheses. e Mean + SD survival time of mice. NT - not tested. RESULTS Most of the mice converted to mixed chimeras with a relatively low number (7%-20%) of donor hematopoietic cells in the blood (Table 3, group 1) . Donor non T cell depleted BMC transplanted one day after the Cy engrafted successfully but induced GVHD (Table 3, groups 2, 3) . EXAMPLE 4 Establishment of stable GVHD-free mixed chimeras by transfer of low dose T cell depleted donor BMC Two groups of BALB/c recipient mice were treated as described in Example 3 except that T cell depleted BMC were administered on day 2 instead of non T cell depleted BMC. One group (Table 3, Group 4) was administered 3 x 10 6 of in vi tro T cell depleted BMC from B6 donors. A total of 2 x 10 6 T cell-depleted BMC was sufficient, as demonstrated in one additional experiment (data not shown) . The other group (Table 3, Group 5) was administered 3 x 10 6 of in vivo T cell depleted BMC from B6 donors. T cell depletion was performed as described in Example 1. RESULTS Elimination of immunocompetent T cells from allogeneic donor BMC was crucial for prevention of GVHD (Fig. 1) . In mildly immunosuppressed recipients, after in vi tro depletion of T cells from donor BMC, all treated mice converted to stable mixed chimeras with 20%-50% of donor cells in the blood. The stable mixed chimeric mice accepted full-thickness B6 skin allografts and survived for 152-290 days without clinical signs of GVHD (Table 3, group 4) . Similar results were obtained using an identical protocol for BALB/c → B6 chimeras with permanent (>150 days) survival of BALB/c skin allografts (data not shown) . Depletion of T cells in vi vo from donor BMC was less successful than T cell depletion in vi tro (Table 3, group 5) . Of the mice that received in vivo T cell depleted BMC, only half remained free of GVHD and survived for >250 days (Table 3, group 5) . These animals were all confirmed to be stable mixed chimeras and all accepted donor skin allografts. EXAMPLE 5 Tolerance to allocrrafts of donor BM stroma and neonatal heart Five groups of BALB/c recipient mice were conditioned with sTLI by administration of 6 fractions of 200 cGy/day. All of these groups then received 3 x 10 7 non T cell depleted BMC of B6 donors intravenously one day after the last TLI fraction. A dose of Cy (200 mg/kg) was given one day after the BMC transfer but before allograft transplantation. Twenty- four hours after the Cy, Group 1 (Table 4) was transplanted with non T cell depleted BMC whereas Group 2 (Table 4) was transplanted with in vi tro T cell depleted BMC. Group 3 (Table 4) was transplanted with BMC stroma one day after Cy, Group 4 (Table 4) with heart 20 days after Cy and Group 5 with skin 20 days after Cy. All of the allografts were from B6 donors. Table 4. Acceptance of various cell and tissue allografts m hosts conditioned with sTLI, donor-derived bone marrow cells and Cytoxan. Recipients in groups 1-5 were conditioned prior to transplantation with sTLI (6 daily exposures of 200 cGy) , 3xl0 7 BMC intravenously one day after the last TLI fraction and 200 mg/kg Cy intraperitoneally one day after cell transfer One group of recipients (group 6) were inoculated with a second graft consisting of 3xl0 6 T cell depleted (m vitro) BMC from B6 donors given one day after Cy BM plugs were grafted on day after Cy d Heart muscle or skm allografts were grafted 20 days after Cy e Ectopιc bone formation under the kidney capsule was confirmed by X-ray analysis £ Vιabιlιty and regular pulsatile activity of heart muscle allografts was confirmed by ECG 9 Twenty recipients in group 1 died from GVHD 37 45 days after cell transfer h Number of mice surviving with allografts is indicated in parentheses RESULTS Mice that were given a second infusion of unmanipulated donor BMC had graft acceptance but 20 of the 22 mice died from GVHD 37-45 days after cell transfer. Mice transplanted with T cell depleted BMC in the second infusion had graft acceptance and much higher graft survival . Implantation of two femoral plugs from B6 donors under the kidney capsule of BALB/c recipients one day after Cy without a second inoculum of T cell depleted donor BMC resulted in formation of fully developed ectopic bone confirmed by X-ray analysis and subsequently by autopsy. This bone supported both donor and recipient hematopoiesis (Table 4, Group 3) . Fragments of the ectopic osteo-hematopoietic site, when retransplanted under the kidney capsule of normal mice, formed bones and ectopic hematopoietic sites in secondary recipients of donor origin (9/9 successful allografts in B6 recipients) but not in BALB/c mice (0/9) . These data indicate that ectopic osteo-hematopoietic sites in these mice were of donor origin. The same treatment was also sufficient for acceptance of heterotopically transplanted neonatal heart grafts obtained from 1-2 day old B6 donors. Results show that the heart muscle transplanted into an ear skin pocket of BALB/c recipients 20 days after the tolerogenic treatment were ECG positive for > 80 days (Table 4, Group 4) . In all mice that accepted donor-derived neonatal heart grafts, contractions of the heart muscle could also be detected visually. Mice that received only sTLI and Cy rejected both femoral plugs and neonatal heart grafts from B6 donors within 30 days (Data not shown) . Most of the recipients that received sTLI , BMC and Cy accepted donor BMC, BM stromal precursor cells and neonatal heart allografts. However, the conditioning was not sufficient to ensure survival of skin allografts obtained from the same donor (Tables 3 & 4 ) . EXAMPLE 6 Conditions required for stable donor skin allocrraft acceptance in mice A group of mice were administered sTLI , BMC and Cy as described in Example 5. This group (Table 4, Group 6) also received a second inoculation of 3 x 10 6 of B6 in vi tro T cell depleted B6 donor BMC one day after Cy but prior to skin allograft. RESULTS Mixed hematopoietic cell chimerism was documented among all experimental animals tested, those that accepted as well as those that rejected donor-type skin allograft. However, the level of chimerism was clearly higher in the mice that, after administration of Cy, received donor BMC in suspension or within a BM femoral plug (20%-50% donor cells) as compared with mice that received no second infusion with BMC (<20% donor cells) . These data indicate that skin allograft acceptance, which is a strong immunogen, is dependent on the level of hematopoietic cell chimerism in recipients. Mixed chimeras with 20% or more donor cells in their blood accepted donor skin allografts for >270 days without any additional treatment (Table 4, group 6). Most of the mice that did not receive the second inoculum of donor BMC had less than 20% donor cells in the blood and rejected donor skin allografts (Table 4, group 5) . This same group of mice nonetheless accepted other donor- derived tissues. These data demonstrate that although a relatively low number of donor cells in the blood (e.g., less than about 20%) may be sufficient for successful engraftment of marrow-derived stromal cells and heart grafts, consistent acceptance of skin allografts derived from the same donor across strong MHC barriers requires a higher level of hematopoietic cell chimerism. EXAMPLE 7 Specificity of transplantation tolerance induced by the tolerogenic treatment Tolerant BALB/c recipients with intact B6 skin allografts for 150 days rejected (11/11) , within 18-20 days, a second skin allograft obtained from a third party (CBA) donor while keeping intact the original B6 skin allograft. This indicates that donor-type specific transplantation tolerance was induced and maintained in recipients capable of generating normal immune responses with full expression of alloreactivity to non-relevant transplantation antigens. Acceptance of donor skin allografts was observed in all strain combinations investigated including, DBA→BALB/c (n=10), Bδ→CBA (n=3), B6→BALB/c (n=21) and BALB/c→B6 (n=9) . EXAMPLE 8 Application of tolerogenic treatment for induction of transplantation tolerance to skin xenografts in rat→mouse combination Two groups of mice were administered sTLI of 6 fractions of 200 cGy/day. After sTLI conditioning, both groups were administered 30xl0 6 non-T cell depleted Lewis rat BMC intravenously. The first group was given a single dose of 200mg/kg Cy the next day. Another 3xl0 7 non T cell depleted rat BMC were administered the following day. In the second group, a dose of 60 mg/kg Cy was given daily for 3 days in contrast to the single 200 mg/kg dose given the first group. The first dose of Cy was given 10 hours after the first rat BMC inoculation, the second dose at 24 hours and the third dose at 48 hours (see below) . After administration of three doses of 60 mg/kg Cy, a second inoculation of 3xl0 7 non T cell depleted rat BMC were administered. RESULTS The first group of mice treated as described above (Table 5, group 1) accepted the second inoculum of 3xl0 7 non T cell depleted BMC from Lewis rats. Lethal GVHD was induced, however, in most of the recipients, suggesting fast engraftment of donor cells despite the relatively mild and non-myeloablative immunosuppressive conditioning (Table 5, group 1) . Interestingly, mice that developed GVHD, indicating acceptance of donor cells, were still capable of rejecting donor-derived skin grafts prior to succumbing to the disease. These results confirm the observation that residual donor-reactive host T cells mediating host vs graft reaction may survive the immunosuppressive/tolerogenic treatment and cause rejection of highly immunogenic donor-derived skin xenografts. Table 5. Tolerance m mice to Lewis rat bone marrow cells and skin xenografts. a 3xl0 7 Lewis BMC intravenously. b lst injection with Cy 10 h after the 1st rat BMC inoculation. c 3xl0 7 Lewis BMC intravenously. d In parentheses number of mice keeping rat skm graft for indicated period. e 5/8 mice died from GVHD with rat skm xenograft accepted. Seven of 15 mice with intact sk allografts developed no acute GVHD. The results improved, in the second group, when Cy was divided into three equal doses of 60 mg/kg and injected 10 h, 24 h and 48 h after the first infusion of non-T cell depleted rat BMC. Under these conditions, 15 of 20 B6 recipients accepted full thickness Lewis rat skin xenografts (Table 2, group 2) . By modifying the Cy administration protocol, host xenoreactive cells may have been more effectively controlled. Normal donor hair growth was observed in all 15 recipients, 5 of which developed lethal GVHD. The surviving mice did not develop clinical signs of GVHD although they were transplanted with non T cell depleted xenogeneic BMC suggesting that residual host-type hematopoietic cells may down-regulate donor-derived immunocompetent T cells through a veto effect . EXAMPLE 9 Effect of varying doses of TLI on GVHD-free and donor-type skin allograft survival BALB/c mice were treated with 0 to 6 doses of sTLI , with each dose being 200 cGy. After administration of sTLI , one group of mice received Cy (200mg/kg) , followed, one day later, by 3 x 10 7 or 3 x 10 6 BMC (non-T cell -depleted) obtained from B6 donors. A second group of mice received, one day after sTLI , 3 x 10 7 BMC (non-T cell -depleted) from B6 donors. These mice, 24 hours later, were administered Cy (200mg/kg) . One day after the Cy, these mice again received 3 x 10 7 or 3 x 10 δ non-T cell -depleted BMC also obtained from B6 donors. A third group of mice received, after sTLI , 0.3 ml of blood from B6 donors and, 24 hours later, Cy (200mg/kg) . Once again, one day after Cy, 3 x 10 7 or 3 x 10 6 non-T cell- depleted BMC, obtained from B6 donors, were administered to these mice. Twenty days later, donor B6 skin was grafted into surviving mice of all the groups. Another set of experiments was conducted to correlate donor cell levels and skin allograft acceptance. The tolerogenic treatment included varying numbers of sTLI doses, followed, a day later, by 3 x 10 7 BMC and, 24 hours later, Cy (200mg/kg) . A day after administration of Cy, a second infusion of 3 x 10 7 BMC was administered. Donor skin was grafted 20 days later. Percentages of donor blood cells in host mice were evaluated at 100-130 days after skin grafting. In another set of experiments, sTLI -treated BALB/c mice were tolerized as indicated in Table 6 followed, 20 days later, with donor skin allografts. At either day 100 or day 120, donor cell chimerism was assayed and mixed lymphocyte reaction (MLR) tests were performed. The T cells were then enriched by lysing red blood cells with ammonium chloride, followed by passage through a nylon wool column and reconstituting in RPMI medium supplemented with 10% of AB Human serum, 0.09 mM nonessential amino acids, 1 mM sodium pyruvate, 2 mM glutamine, 100 μg/ml streptomycin and lOOU/ml penicillin (Biological Industries, Beit Haemek, Israel) and 0.05 mM 3 -mercaptoethanol (Sigma, USA) . 10 s responding T cells were incubated with 10 δ stimulating T cells (3000 cGy irradiated) in flat bottom microplates (Costar, USA) at 37°C, 5% C0 2 for 3 days. The cells were pulsed on the third day with 1 μCi [ 3 H] thymidine and harvested on the fourth day. Table 6. MLR reactivity of BALB/c mice that accepted or rejected donor-type skin allografts . a BALB/c mice that received 3xl0 7 BMC from B6 donors and 24 h later, 200 mg/kg Cy were reconstituted with 3xl0 7 BMC (mice 1-3) or with 3xl0 6 BMC (mice 4-7) from B6 donors one day after injection of Cy. b Skin allografts from B6 mice were transplanted 20 days after treatment. Percentage of donor cell chimerism was assayed on the same day as MLR. RESULTS In the absence of donor-specific tolerization (Fig. 2A, Group A), 0, 1, or 2 doses of sTLI appeared to be sufficient for a high probability of GVHD-free survival when 3 x 10 7 BMC or 3 x 10 6 BMC were administered subsequent to the Cy treatment. Administration of these low doses of sTLI to a host results in retention of relatively high numbers of host functional T cells. Consequently, the relatively high rate of GVHD-free survival at low sTLI doses may be due to a veto effect in which the levels of host- and donor-derived veto cells are in balanced equilibrium. Donor skin graft acceptance was low at 0 and 1 sTLI dose but increased at 2 , 3, and 6 sTLI doses (data not available for 4 and 5 sTLI fractions) . However, the GVHD-free survival rate decreased at these doses of sTLI . (Fig. 2A) . Thus, in the absence of donor-specific tolerization, none of the regimens led to a high percentage of GVHD-free survival and a high percentage of donor skin graft acceptance. Donor-specific tolerization in the second group of mice (Fig. 2A, Group B) resulted in high GVHD-free survival at low doses of sTLI , and even with no TLI treatment. The donor-specific tolerization also resulted in a high probability of donor skin allograft acceptance regardless of the number of sTLI doses. Without any sTLI fractions, a second dose of 3 x 10 7 BMC appears to be necessary. The use of higher numbers of sTLI fractions resulted in higher GVHD morbidity, although skin allograft acceptance remained high. With regards to GVHD, the higher numbers of sTLI fractions seem to be more successful with a lower dose (3 x 10 6 ) of BMC. This may be due to a veto effect in which lower numbers of host veto cells, due to administration of higher numbers of sTLI fractions, are in balanced equilibrium with a lower dose of BMC. The infusion of 0.3 ml of blood, without any TLI, in the third group of mice (Fig. 2C, group C) resulted in a high probability of GVHD-free survival but low donor skin allograft acceptance. This third group of mice had high GVHD-free survival when 0, 1, or 2 fractions of sTLI were administered and the mice received a large dose of (3 x 10 7 ) BMC following Cy. However, higher numbers of sTLI fractions were more successful with a smaller dose of (3 x 10 6 ) BMC following Cy. These results may be due to a veto effect as discussed above for the second group of mice . The correlation between the percentages of donor blood cells in a recipient and donor-skin allograft acceptance is illustrated in Fig. 3. These experiments indicated that the percentage of donor blood cells in the recipient was critical for skin allograft acceptance. Recipients with less than 20-25% donor blood cells did not accept donor-skin allografts (solid symbols) . In contrast, recipients having greater than 20-25% donor blood cells accepted donor-skin allografts (empty symbols) . Furthermore, recipients were able to obtain greater that 20-25% of donor blood cells even when conditioned with 0, 1, 2, or 3 sTLI fractions in the tolerogenic treatment . MLR reactivity data (Table 6) indicated that mice with low levels of chimerism (mice 4-7) were not completely tolerized. The responder T lymphocytes proliferated in the presence of stimulators from autologous, BALB/c and B6 sources. The response to B6 stimulators was especially high. In contrast, mice with high levels of chimerism (mice 1-3) did not respond to stimulators from any of the sources. EXAMPLE 10 Immunized donor spleen cells induce stronger GVL effects across an MHC barrier than unimmunized spleen cells In order to measure GVL activity across an incompatibility involving both MHC and MiHL alloantigens, FI mice (pre-conditioned on day 0 with TBI (400cGY) ) were injected on day 1 with 10 4 BCLl cells and 30xl0 6 immunized or normal C57BL/6 spleen cells. There were 4 test groups with 10 mice in each: Group 1 : FI recipients given BCLl cells and spleen cells from C57BL/6 mice immunized with spleen cells from overtly leukemic BCLl-bearing mice. Group 2 : FI recipients given BCLl cells and spleen cells from C57BL/6 mice immunized with normal BALB/c spleen cells. Group 3 : FI recipients given BCLl cells and normal C57BL/6 spleen cells. Group 4 : Untreated FI recipients given BCLl cells only and thereby serving as a control group. RESULTS Fig. 4 shows cumulative data from 4 similar experiments. 75% of the mice in the 1st group and 48% of the mice in the 2nd group were alive after 120 days with no evidence of leukemia. The data described in the following paragraph and shown in Fig. 5 indicate that the 25% of mice in group 1 that died before 120 days, died of GVHD rather than leukemia. In contrast, 79% of the mice in the 3rd group died of GVHD within 75 (median 56) days. All mice in group 4 died of leukemia within 36 (median 24) days. The therapeutic advantage of BCLl -immunized C57BL/6 spleen cells over BALB/c spleen cell-immunized C57BL/6 spleen cells and BCLl -immunized spleen cells over normal, unimmunized C57BL/6 spleen cells was significant (p = 0.001 and 0.002 respectively) . Although FI mice given spleen cells from BALB/c spleen cell -immunized C57BL/6 mice apparently had a higher survival rate compared with FI mice treated with normal, unimmunized C57BL/6 spleen cells (Fig. 4), the advantage was not significant (p=0.083) . To measure the efficacy of allogeneic cell therapy in eradicating residual BCLl cells, spleen cells obtained 3 weeks post transplantation were pooled from each experimental group and 10 5 cells were adoptively transferred to secondary BALB/c recipients (10 mice/group) (Fig. 5) . The results were consistent with those obtained in the primary FI recipient mice. The GVL effect of BCLl -immunized spleen cells was more potent than that of both BALB/c spleen cell -immunized C57BL/6 spleen cells (p=0.001) and normal, unimmunized C57BL/6 spleen cells (p=0.004). The BALB/c spleen cell -immunized C57BL/6 spleen cells showed no greater anti-tumor activity than normal, unimmunized C57BL/6 spleen cells (p=0.5) . All the secondary recipients inoculated with spleen cells obtained from the 1st experimental group remained leukemia-free for >120 days. 25 of 40 secondary BALB/c recipients inoculated with spleen cells obtained from the 2nd experimental group remained leukemia- free for >120 days, and 27 of 40 secondary BALB/c recipients inoculated with spleen cells obtained from the 3rd experimental group remained leukemia free for >150 days. In contrast, all 28 secondary BALB/c recipients of spleen cells obtained from the control group died of leukemia within less than 30 days. PBL taken from FI recipients showed 46-56% donor-type cells, confirming engraftment (Table 7) . Table 7. Chimerism in FI recipients transplanted with C57BL/6 spleen cells. FI Mice Injected with Spleen Cells From: Donor Type Cells (•' C57BL/6 mice immunized with BCLl 50±12 C57BL/6 mice immunized with BALB/c spleen cells 46±14 Normal C57BL/6 mice 56±16 Body weight studies of the mice in each group, as an objective measure of the degree of GVHD, are shown in Fig. 6. All mice had severe GVHD in the first 2 weeks after transplantation and a proportion of the animals in each group did not survive. However, 75% of the mice in the 1st group and 48% of the 2nd group remained active and well with no evidence of GVHD at 120 days. All animals in the 3rd group died within 75 (median 56) days with typical signs of acute GVHD. FI mice that were given normal C57BL/6 spleen cells showed typical clinical signs of GVHD manifested by a continuous weight loss, whereas transient reduction in body weight for only one week was observed for FI mice receiving C57BL/6 spleen cells obtained from donors immunized against either BCLl or normal BALB/c spleen cells. EXAMPLE 11 Immunized donor spleen cells induce stronger GVL effects across an MiHL barrier than unimmunized spleen cells In order to determine the GVL effects across MiHL incompatibility, BALB/c recipients (conditioned on day 0 with TBI (400cGy) ) , were injected intravenously on day 1 with 2xl0 3 BCLl cells and 30x10 s spleen cells from either B10.D2 mice pretreated in one of three ways or no B10.D2 spleen cells. There were 4 experimental groups with 10 mice in each: Group 1 : BALB/c recipients given BCLl cells and spleen cells from B10.D2 immunized with BCLl cells obtained from BALB/c. Group 2 : BALB/c recipients given BCLl cells and spleen cells from B10.D2 mice immunized with normal BALB/c spleen cells. Group 3 : BALB/c recipients given BCLl cells and normal B10.D2 spleen cells. Group 4 : Untreated BALB/c recipients given BCLl cells only and thereby serving as a control group. RESULTS As summarized in Fig. 7, 84% of the mice in the 1st group were alive with neither signs of clinical GVHD nor leukemia for >120 days. Similarly, 47% of the mice in the 2nd group were alive with no sign of GVHD or leukemia for >120 days, whereas 16 of 30 died of leukemia. All mice in the third and fourth groups died of leukemia. There was a significant therapeutic advantage of treating the recipient BALB/c mice with spleen cells from BCLl -immunized B10.D2 mice over treatment with either spleen cells of BALB/c spleen cell- immunized B10.D2 mice (p=0.03) or spleen cells from naive, unimmunized B10.D2 mice (p<0.002) (Fig. 7) . There was also a significant therapeutic advantage in administering spleen cells from BALB/c spleen cell -immunized B10.D2 animals over administering spleen cells from naive, unimmunized B10.D2 mice (p=0.0001). Spleen cells from BCLl -immunized B10.D2 mice displayed greater GVL-mediated therapeutic efficacy than did those from BALB/c spleen cell -immunized B10.D2 mice. The efficiency of MiHL incompatible spleen cells in eradicating residual BCLl cells was also assayed by adoptive transfer into secondary BALB/c recipients (10 mice in each group) of 10 5 spleen cells from separate pools of spleen cells made from the animals in each experimental group (at 3 weeks post transplantation) (Fig. 8) . Interestingly, 95% of secondary BALB/c recipients inoculated with spleen cells obtained from the 1st group remained leukemia- free for >120 days. In contrast, only 41% of secondary BALB/c recipients inoculated with spleen cells obtained from the 2nd group were alive and leukemia-free for >120 days. All secondary BALB/c recipients of cells obtained from groups 3 and 4 developed leukemia within 45 (median 38) days. The anti-leukemic effects induced with BCLl-immunized B10.D2 spleen cells over both normal, unimmunized B10.D2 spleen cells and BALB/c spleen cell -immunized B10.D2 spleen cells were significant (p<0.001 and p=0.027, respectively) . Unlike GVL effects induced across an incompatibility involving the MHC and MiHL, BALB/c spleen cell -immunized B10.D2 spleen cells induced stronger GVL effects compared with normal, unimmunized B10.D2 spleen cells (p<0.0001) . Fig. 9 shows that BALB/c mice inoculated with MiHL-incompatible, BCLl-immune B10.D2 spleen cells, after a minor bout of GVHD manifested by transient weight loss in the first week post grafting, were fully resistant to GVHD. In contrast, BALB/c mice inoculated with either spleen cells from MiHL-incompatible normal, unimmunized B10.D2 mice or spleen cells from MiHL-incompatible BALB/c spleen cell-immunized mice developed lethal GVHD. Nevertheless, recipients given BALB/c spleen cell- im unized B10.D2 spleen cells survived longer than control animals, indicating that some GVL effects may have been induced but were masked by GVHD. EXAMPLE 12 Histological findings in mice with induced GVHD/GVL As indicated above, mice treated with immunized lymphocytes resisted leukemia development more effectively than recipients of non-immunized cells, and yet developed less GVHD across MHC and MiHL and MiHL only barriers. Of 3 mice that received therapy with normal, unimmunized spleen cells, 2 had major histological abnormalities in the liver compatible with acute GVHD. Similar liver lesions were demonstrated in only 1 of 5 mice treated with donor cells from mice immunized with host spleen cells. None of 4 mice treated with spleen cells from BCLl -immunized animals had any histological abnormalities in the liver. No leukemic infiltrates were demonstrable in any of the mice treated across an incompatibility involving both MHC and MiHL. In mice challenged across MiHL barriers only, injection of normal B10.D2 cells did not cause any histological changes consistent with GVHD in the BALB/c recipients, whereas mice injected with B10.D2 spleen cells obtained from BCLl -immunized or BALB/c spleen cells-immunized B10.D2 mice had infiltrations in the lung and in the liver. However, as indicated above, these infiltrations were not associated with increased GVHD. Leukemia cells were found in the lung and liver of BALB/c mice injected with normal, unimmunized donor spleen cells and in BALB/c recipients of spleen cells from BALB/c spleen cell- immunized B10.D2 mice. In contrast, no leukemic infiltrates were found in mice treated with spleen cells obtained from BCL-immunized mice. EXAMPLE 13 Cytokine production correlates with GVHD and GVL potential The cytokine profiles of (a) spleen cells from C57BL/6 mice immunized across an incompatibility involving both MHC and MiHL alloantigens by injection of either BCLl cells or normal BALB/c spleen cells and (b) spleen cells from FI mice inoculated with BCLl cells and the spleen cell populations recited in (a) , were determined. RESULTS As shown in Fig. 10, the level of IFN-γ in the supernatant of spleen cells of BCLl -immunized C57BL/6 mice was 3 times higher than that of spleen cells from BALB/c spleen cell-immunized mice and 6 times higher than that in normal, unimmunized spleen cells. The level of IL-2 in the supernatant of spleen cells of normal, unimmunized C57BL/6 mice was 4 to 5 times higher than the level of IL-2 in the supernatant of spleen cells obtained from donors immunized with either BCLl or with BALB/c spleen cells (Fig. 11) . The level of IL-10 in supernatant of spleen cells from C57BL/6 mice immunized with BCLl cells was 2 times higher than that in the supernatant of spleen cells from C57BL/6 mice immunized with BALB/c spleen cells and of spleen cells from normal, unimmunized C57BL/6 cells (Fig. 12) . No differences in the supernatant levels of TNF-α were detected in spleen cells from normal, unimmunized C57BL/6 mice, BCL1- immunized C57BL/6 mice, and BALB/c spleen cell -immunized C57BL/6 mice. Cytokine levels were also measured in supernatants of cultures of spleen cells isolated 3 weeks after cell therapy from FI recipients of the above C57BL/6 spleen cells. TNF-α levels in supernatants of spleen cells from FI mice treated with normal, unimmunized C57BL/6 spleen cells were 4-10 times higher than in supernatants of spleen cells from FI mice given spleen cells from BCLl immunized or BALB/c spleen cell -immunized mice C57BL/6 (Fig. 13) . The level of IL-2 in the supernatants of spleen cells of FI mice inoculated with spleen cells from BALB/c spleen cell -immunized C57BL/6 mice or with spleen cells from normal, unimmunized C57BL/6 mice were 3 to 4 times higher than in the supernatant of spleen cells from FI mice given spleen cells from BCLl-immunized C57BL/6 mice (Fig. 14) . The level of IFN-γ in the supernatant of spleen cells from FI mice given spleen cells from BCL1- immunized C57BL/6 mice was 10 times lower than that in the supernatant of spleen cells of FI mice that received normal, unimmunized C57BL/6 cells (Fig. 15) . No differences were detected in the supernatant levels of IL-10 in spleen cells from FI mice inoculated with the three types of C57BL/6 cells. The level of IL-4 was about five times higher in the supernatant of spleen cells of FI recipients of spleen cells from BCL1- immunized C57BL/6 mice than that in the supernatants of spleen cells from FI mice inoculated with spleen cells from either BALB/c spleen cell -immunized C57BL/6 mice or normal, unimmunized C57BL/6 mice (Fig. 16) . The potent allospecific (MHC and MiHL) tolerizing activity of BCLl tumor cells, and hence the GVHD suppressive effect of immunization with them, could be due to their presentation of the relevant alloantigens without the participation of co-stimulatory molecules (e.g., B7) , a mode of antigen presentation known to induce tolerance. In addition, the cytokine profiles observed in the experiments described above suggest a possible basis for the seemingly paradoxical findings of (a) enhanced anti-tumor efficacy and (b) concomitant decreased GVH activity in lymphoid cells from donor mice pre-exposed to host tumor cells compared to lymphoid cells from mice pre-exposed to host hemopoietic (spleen) cells. It is possible that the Thl/Th2 balance, both in the lymphoid cells of the immunized donor animals and in those of the host animals to which the immunized lymphoid cells are transferred, is differentially affected by the type of immunization given to the donor animal. Thus, for example, production of IL-10 (a prototypical Th2 cytokine) was up-regulated whereas production of IL-2 (a prototypical Thl cytokine) was decreased in C57BL/6 spleen cells obtained from mice immunized across an MHC barrier with tumor cells compared to spleen cells from C57BL/6 mice immunized across the same MHC barrier, but with normal spleen cells (Figs. 11 and 12) . In addition, an increased level of IL-4 (another prototypical Th2 cytokine) (Fig. 16) accompanied by reduced IL-2 (Fig. 14), TNF-α (Fig. 13) and IFN-γ (Fig. 15) (all prototypical Thl cytokines) levels in supernatants of spleen cells from FI host mice injected with BCL1- immunized C57BL/6 spleen cells, relative to the levels in supernatants of spleen cells from FI host mice injected with spleen cells from BALB/c spleen cell -immunized C57BL/6 donor mice, suggest that the Th2 shifted C57BL/6 donor cells transferred the Th2 bias (i.e., higher level of Th2 effects compared to Thl effects) to the FI hosts. Furthermore, it is known that Thl -type cytokines are generally associated with cellular immune or delayed-type hypersensitivity responses, while Th2 cytokines are generally associated with humoral (antibody) responses and that GVHD is largely due to cell -mediated immunological effects. Thus, in summary, immunization of the donor mice with tumor cells could result in a bias towards Th2 cytokine production and hence diminished GVHD disease potential in the T cells of the donor mice. It is also possible that, while the Thl cytokine levels are decreased sufficiently to minimize GVHD, there may be adequate levels for an effective cellular anti- tumor (GVL) response. Alternatively, the mechanism of the anti-tumor response could differ in some aspects from the GVHD response and, as such, could be facilitated by Th2 cytokines rather than by Thl cytokines. In addition, not only are Th2 cytokines generally considered not to be "helper" cytokines for cellular immune responses, they have been shown to actively suppress cellular immune responses. Thus, the Th2 bias transferred to the FI host mice by cells from BCLl tumor-immunized donor mice may actually act to actively suppress GVHD. The above-described discordancy in GVL and GVHD activity in donor cells from animals immunized with tumor cells versus normal spleen cells could also be due to one or a combination of the following effects: (a) tumor antigen specific effector T cells being relatively resistant to tolerance induction compared to alloantigen (MHC or MiHL) specific T cells; (b) the anti -tumor activity being at least partially mediated by non-T cells (e.g., NK cells) that are activated rather than tolerized by tumor immunization and/or Th2 cytokines; (c) tumor cells being relatively more sensitive to allo-specific effector cells; and (d) tumor cells being relatively poor inducers of activation induced apoptosis in allospecific and/or tumor specific effector cells. While any one or a combination of the above- mechanisms may explain the above-described dissociation between GVL and GVHD activity, the invention is not limited by any particular mechanism of action. EXAMPLE 14 Steps 1-3 of the Tolerization Protocol Can Be Performed on The Same Day as Transplantation ("Short Protocol") BALB/c recipient mice were administered a single dose of sTLI (200 cGy) and an injection of non T cell depleted BMC from B6 donors on day 0. One group of BALB/c mice treated in this way received B6 BM stromal grafts and a second group received B6 heart grafts. All grafts were also performed on day 0. All mice received an injection of Cy (200 mg/kg) on day 1 and a second injection of B6 BMC on day 2. In light of the survival of 100% of the mice in both groups (Fig. 17) , GVHD was prevented by the described protocols. In addition, 100% of the BM stromal grafts and approximately 80% of the heart grafts survived. This experiment indicated that it is possible to successfully transplant an allograft into a subject at the same time as initiating the tolerogenic method of the invention. It is expected that by simply altering doses of, for example, the TLI, BM and/or Cy, as well as the frequency of subsequent administrations of these agents, it will also be possible to apply such a "short" protocol to xenogeneic recipient-donor combinations. EXAMPLE 15 In Vi tro Non-specific Depletion of GVHD Activity The ability of the drug mafosphamide (ASTA-Z) , which is identical to 4-hydroxyperoxycyclophosphamide (4HC) , to deplete BALB/c mouse spleen cells of T cell responsiveness was tested under in vi tro conditions known to result in preservation of hemopoeitic stem cell activity after treatment of hemopoietic cells from humans as well experimental animals (e.g., mice) . BALB/c spleen cells were cultured for 30 minutes at 37°C in tissue culture medium containing 100 μg/ml of ASTA-Z and then tested for in vi tro T cell responses. The exposure to ASTA-Z resulted in the elimination of in vi tro proliferative responses to the T cell mitogens concanavalin A (ConA) and phytohemagglutinin (PHA) (Table 8). Moreover, addition of IL-2 (1,000 IU/ml) to the cultures of spleen cells and ASTA-Z did not overcome inhibition of responsiveness to the mitogens by the ASTA- Z. Table 8. Treatment of murine spleen cells with ASTA-Z results in elimination of responsiveness to T cell mitogens . Cell Proliferation 3 Treatment No Mitogens Con-A PHA Untreated 702 107,277 63, 604 ASTA-Z 219 2,980 5, 118 rIL-2 31, 090 52,287 39,390 ASTA-Z 624 4,767 9,453 +rIL-2 a Cell proliferation was measured as counts per minute (cpm) of [ 3 H] -thymidine incorporated into the cells. On the other hand, ASTA-Z treatment of BALB/c spleen cells (SP) or bone marrow (BM) cells did not decrease the generation of killer cells capable of killing murine YAC- 1 and P815 tumor target cells by culturing of the SP and BM cells in IL-2 (Figs. 18 and 19) . After treatment of the SP or BM cells with ASTA-Z, as described above, excess ASTA-Z was removed and the cells were cultured with human rIL-2 (6,000 IU/ml) for 4 days. They were then harvested and tested for cytolytic activity in standard 51 Cr-release assays. While YAC-1 cells are sensitive to lysis by both NK and activated NK cells, P815 cells are not sensitive to lysis by NK cells but are sensitive to activated NK cells. Thus it appears that the cytolytic activity detected in the assays was, largely at least, due to the action of activated NK cells. Furthermore, the same treatment of human PBMC with ASTA-Z did not decrease the generation of killer cells capable of killing human Daudi and K562 tumor target cells by culturing of the PBMC with human rIL-2 under the same conditions described above (Table 9) . While K562 cells are sensitive to lysis by both NK and activated NK cells, Daudi cells are not sensitive to NK cells but are sensitive to activated NK cells. Thus, as in the murine system described above, the cytotoxic activity was probably due, largely at least, to NK cells activated by IL-2. Table 9. Treatment of human PBMC with ASTA-Z does not decrease NK cell activity Lysis of Target Cells 3 Daudi target cells b K562 target cells' 3 Untreated ASTA-Z Untreated ASTA- Z Experiment PBMC treated PBMC PBMC treated PBMC 1 39 71 35 48 2 40 42 44 52 a Lysis of target cells was measured as the percentage of 51 Cr released from 51 Cr- labeled target cells after incubation with effector cells (at a target cell to effector cell ratio of 1 : 100 ) obtained from cultures containing IL-2 ( 6 , 000 IU/ml ) and either untreated or ASTA-Z treated PBMC . b K562 cells are sensitive to lysis by both activated and unactivated NK cells and Daudi cells are sensitive to lysis by activated but not unactivated NK cells . In parallel murine experiments , culture of B6 bone marrow for 30 minutes with ASTA- Z ( 100 μg/ml ) prior to inj ection ( 25 x 10 s per mouse ) into lethally irradiated ( 1 , 100 cGy of TBI) SJL/J mice reduced the capacity of the bone marrow cells to induce GVHD (Fig. 20) . Similarly, culture of B6 spleen cells for 30 minutes with ASTA-Z (100 μg/ml) prior to injection (25 x 10 6 per mouse) into sublethally irradiated BALB/c mice (600 cGy) reduced the capacity of the spleen cells to induce GVHD (Fig. 21) . Thus, ASTA-Z can be used to deplete hemopoietic cells to be used for either bone marrow transplantation (e.g., for step 4 of the tolerance protocol described herein) . It can also be used to deplete cells to be used for cell therapy of the allospecific (or xenospecific) T cell reactivity that leads to GVHD while retaining or even being enriched for graft-versus-tumor (e.g., leukemia) activity which could, at least in part, be due to the action of NK cells. EXAMPLE 16 Non-Myeloablative , Donor-Specific Tolerogenic Treatment in a Human Patient Patient No. 1 Prior to non-myeloablative conditioning, donor-specific tolerance induction, and allogeneic bone marrow transplantation (ABMT) , this male patient underwent autologous stem cell transplantation (ASCT) almost 38 months after diagnosis of Hodgkin's Disease stage III B. The patient had failed MOPP/ABVD alternative treatment (8 cycles) , radiation therapy, subsequent treatments with velban, adriamycin, bleomycin and DTIC, and repeated cycles of additional chemotherapy including, following his first overt relapse 2 years after diagnosis, MOPP (4 cycles) and VP16, cisplatin, ifosfamide, and uromitexan (5 cycles) . Relapse was noted again 2 months after ASCT and the clinical picture of fever without obvious infectious etiology suggested persistence of the Hodgkin's Disease. Allogeneic bone marrow transplantation (ABMT) , following non-myeloablative conditioning and donor-specific tolerization was offered to the patient as a possible method of treatment. It was considered that this treatment could overcome long-lasting hypoplasia and could antagonize the persisting Hodgkin's Disease by inducing graft vs. Hodgkin's Disease tumor cell responses, Tissue typing data revealed a phenotypic mismatch in HLA class I (serological testing) between the patient and the available donor, his father: Patient: A28 A19 B41 B5 Donor (father) : A28 A30 B41 B51 Typing of HLA class II revealed: Patient: DRB1*1104 DRB1*0404 DQB1*0301 DQB1*0402 Donor (father) : DRB1*1101 DRB1*0404 DQB1*0301 DQB1*0402 Starting on day 0, the patient was conditioned non- myeloablatively with Fludarabine (30mg/kg/day) for 3 consecutive days. One day later, the patient received an infusion of G-CSF mobilized peripheral blood cells ("first allograft") collected form his father (2.98 x 10 8 nucleated cells/kg) as a source of donor-specific antigens, followed by 3 daily non-myeloablative period doses of cytoxan 60mg/kg (4,500 mg daily) to eliminate donor-specific alloreactive T cells. An infusion of unselected paternal bone marrow cells (9.6 x 10 8 nucleated cells/kg) was carried out ("second allograft") one day after termination of the last dose of cytoxan. It was decided to use unmodified bone marrow cells with no further T cell depletion for the second allograft in order to maximize the chance of stem cell engraftment on the one hand as well as GVT effects on the other. Fever up to almost 40°C developed in the first week after the second allograft and the patient required frequent single donor platelet infusions for prevention of bleeding. The patient also received antibiotic therapy with amikacin, tazocin, preventive therapy against fungal infection with diflucan and acyclovir therapy against cytomegalovirus infection. Since fever did not respond completely to antibiotic therapy, amphotericin B (lmg/kg) was given every other day. Fever persisted throughout hospitalization . The patient's white blood cell count (WBC) rose to 1.0 x 10 9 /L on day +14 and his absolute neutrophil count (ANC) reached > 0.5 x 10 9 /L on day +14 and > . 1.0 x 10 9 /L on day +28. The WBC rose gradually to a maximal level of 5.1 x 10 9 /L with 75% granulocytes . However, thrombocytopenia persisted. Engraftment was confirmed by rising counts and by detection of donor DNA by the Variable Number of Tandem Repeats - Polymerase Chain Reaction (VNTR-PCR) , a technique known to those in the art . On day +10, the patient experienced a grand mal seizure which responded to valium infusion. No focal neurologic findings were found except that the Babinski's sign was positive bilaterally. Cyclosporine A was administered as a prophylactic treatment for GVHD. Overt skin rash typical of GVHD appeared on day +12. Liver manifestations developed subsequently. Despite combination therapy with solumedroi (2mg/kg) daily and cyclosporine, with continuation of the antibiotic and anti-fungal therapy, the patient's condition deteriorated gradually, with diarrhea up to 12 times a day, starting on day +16, a symptom indicative of stage IV GVHD. Despite intensive treatment of both GVHD and potential infections, spikes of fever continued with dyspnea that developed in parallel with pulmonary bleeding and bilateral interstitial infiltration in the lungs on day +28. The patient was intubated on day +29. Large volumes of secretion were aspirated through the tube. The secretions included blood but lavage did not reveal any infectious agent . Despite intensive therapy including dopamine drip and careful maintenance of pulmonary system, the blood pressure dropped gradually and the patient expired on day +29. In conclusion, the successful engraftment of the patient by his father' s bone marrow used for the second allograft indicated that HLA mismatched stem cells can be accepted following selective depletion of host cells with the capacity to reject donor alloantigenic tissue and without myeloablative conditioning. Developments in the patient suggest that, due to pancytopenia following ASCT and the failure to establish a high level of protective mixed chimerism, he may have been more susceptible to GVHD. Non T cell depleted bone marrow was used for the second allograft and this unfortunately resulted in GVHD. Nevertheless, the above-described ' findings demonstrated that HLA mismatched cells can be accepted and engrafted without myeloablative conditioning using the described tolerogenic protocol. These data considered in light of murine experiments indicate that it will be possible to obtain engraftment in human patients without GVHD if the donor bone marrow used for the second is allograft is either (a) depleted of T- cells prior to infusion or (b) is used undepleted but a transient stage of mixed chimerism in the recipient is achieved. Furthermore, the combined findings of this clinical study and the murine experiments indicate that, in human patients, it will be possible to prevent rejection of allografts if the recipient is depleted of donor-specific T cells prior to the allograft. EXAMPLE 17 Effective treatment of human chronic myelogenous leukemia (CML) with allogeneic lymphocytes pre-exposed to alloantigens of the patient Allogeneic cell therapy using donor lymphocytes pre- exposed to alloantigens of the patient was given to a female patient with Philadelphia chromosome-positive (Ph+) CML who had relapsed 9 months after allogeneic bone marrow transplantation. The bone marrow cells used for the transplant were from a HLA-A, -B, -C and -DR identical 6- month old brother (the "donor") . The patient had failed to respond to several rounds of allogeneic cell therapy (given subsequent to the bone marrow transplant) using donor PBMC that, in some of the treatments, were activated with IL-2. The patient's bone marrow contained approximately 95% Ph+ cells prior to this allogeneic cell therapy which consisted of the following sequential procedures. (a) 10 7 donor PBMC per kg were administered i.v. 24 hours after a low dose of Cy (500 mg/m 2 ) . No remission was obtained. (b) 10 7 donor PBMC (activated in vi tro with IL-2) per kg were administered i.v.. Beginning on the day of cell infusion, rIL-2 (6 x 10 6 IU/m 2 /day) was administered subcutaneously for 3 days. The whole procedure was performed twice, approximately one month apart, and resulted in a transient decrease to about 67% in the proportion of Ph+ cells in the patient's bone marrow. (c) 6 x 10 s paternal PBMC (activated in vi tro with IL-2) were administered i.v., resulting in a transient decrease to about 60% in the proportion of Ph+ cells in the patient's bone marrow. This decrease was followed by a gradual increase to 94% Ph+ bone marrow cells. At this time it was decided to treat the patient with donor PBMC activated in vi tro against alloantigens of the patient. The patient was infused with donor PBMC that had been exposed twice in vi tro for 1 day to irradiated (3,000 cGy) PBMC from both parents of the patient (and donor) . The donor PBMC were thus activated to parental MHC antigens not expressed by the patient and MiHL antigens expressed by the patient but not the donor, the patient and donor being HLA identical children of the parents. The patient was then given rIL-2 (6 x 10 s IU/m 2 /day) for 3 days, starting on the day of cell infusion. The whole procedure was carried out twice, approximately one month apart. Interferon-α (1.5 x 10 s ) was administered 3 times a week for 4 years. The patient has now been in remission for greater than five years. She has no detectable leukemia cells (both by karyotype analysis and RT-PCR to detect mRNA transcripts derived from a bcr/abl hybrid DNA sequence produced by the Philadelphia t (9 ; 22) (q34 ,-qll) chromosomal translocation) , 100% of both her blood and her bone marrow cells are donor- derived, and she has no clinical signs of GVHD. This study indicates that the efficacy of allogeneic cell therapy of human cancer can be enhanced by pre-exposure of the donor cells to be used for therapy to alloantigens expressed by the patient. In light of the above-described experiments in mice, such pre-exposed donor cells are also likely to display decreased GVH activity compared to unexposed cells. Other Embodiments It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims

Claims: What is claimed is: 1. A composition of cells comprising a cell population endogenous to a first individual mammal, said cell population comprising lymphocytes, said population being depleted of responsiveness to antigens of a second individual mammal, said second individual mammal being non- syngeneic with said first individual mammal, wherein said population of cells endogenous to said first individual mammal is 50% to 100% of the cells of said composition. 2. The composition of claim 1, wherein said depletion of responsiveness comprises contacting said lymphocytes with a source of said antigens of said second individual mammal . 3. The composition of claim 2, wherein said source of antigens comprises cancer cells. 4. The composition of claim 1, wherein said first individual mammal and said second individual mammal are humans . 5. The composition of claim 1, wherein said first individual mammal is a non-human primate, and said second individual mammal is a human. 6. The composition of claim 1, wherein said first individual mammal is a pig and said second individual mammal is a human . 7. A method of treating a host mammal with non-syngeneic cell therapy, said method comprising infusing a cell population from a donor mammal into said host mammal, said cell population comprising lymphocytes, said donor mammal and said host mammal being non-syngeneic with each other, wherein, prior to said infusing, said population is depleted of responsiveness to antigens of said host mammal . 8. The method of claim 7, wherein said depletion comprises contacting said population with a composition comprising antigens expressed by said host mammal . 9. The method of claim 8, wherein said contacting is in vi tro . 10. The method of claim 8, wherein said contacting comprises administering said antigens to said first individual mammal . 11. The method of claim 8, further comprising, after said contacting, delivering a non-myeloablative dose of a lymphocytotoxic or tolerizing agent to said lymphocyte population. 12. The method of claim 11, wherein said delivering is in vi tro . 13. The method of claim 8, wherein said contacting comprises administering of said antigens to said donor mammal and said delivering comprises administering said non- myeloablative dose to donor mammal. 14. The method of claim 11, further comprising, after said delivering, administering a preparation of hemopoietic stem cells from said host mammal to said donor mammal . 15. The method of claim 8, further comprising, prior to said contacting, exposing said population to an immunosuppressive agent in a non-myeloablative regimen sufficient to decrease the number of functional T lymphocytes in said lymphocyte population. 16. The method of claim 15, wherein said exposing is in vi tro . 17. The method of claim 15, wherein said exposing comprises administering said immunosuppressive agent to said donor mammal. 18. The method of claim 8, wherein said composition comprises one or more antigen sources selected from the group consisting of cells, organs, tissues, and non-cellular antigens . 19. The method of claim 8, wherein said antigen comprises hemopoietic cells. 20. The method of claim 8, wherein said antigen comprises cancer cells expressing major histocompatibility complex molecules of said host mammal . 21. The method of claim 20, wherein said cancer cells are from said host mammal . 22. The method of claim 7, further comprising, prior to said infusing, inducing tolerance to antigens of said donor mammal in said host mammal, said tolerance induction comprising: (a) administering donor antigens to said host mammal; (b) administering a non-myeloablative dose of lymphocytotoxic or tolerizing agent to said host mammal to selectively eliminate said host mammal's lymphocytes responding to said donor antigens; and (c) administering a preparation of hematopoietic stem cells from said non-syngeneic donor to said host mammal. 23. The method of claim 22 further comprising, prior to step (a) , administering an immunosuppressive agent to said host mammal in a non-myeloablative regimen sufficient to decrease said host mammal's functional T lymphocyte population. 24. The method of claim 7, wherein said depletion of responsiveness is by substantially eliminating T cells from said population. 25. The method of claim 24, wherein said elimination is by exposing said population to an immunosuppressive agent in a non-myeloablative regimen. 26. The method of claim 24, wherein said elimination is by contacting said population with mafosphamide . 27. The method of claim 24, wherein said elimination is in vi tro. 28. An article of manufacture comprising packaging material and a biological cell container within said packaging material, said cell container having a composition comprising hematopoietic stem cells therein, wherein said packaging material contains a label or package insert indicating that said hematopoietic stem cells are to be used in step (a) or step (c) in a method of inducing non- syngeneic donor-specific tolerance in a host mammal, comprising (a) administering donor antigens from a non-syngeneic donor to said host mammal; (b) administering a non-myeloablative dose of lymphocytotoxic or tolerizing agent to said host mammal to selectively eliminate said host mammal's lymphocytes responding to said donor antigens; and (c) administering a preparation of hematopoietic stem cells from said non-syngeneic donor to said host mammal. 29. An article of manufacture comprising packaging material and a biological cell container within said packaging material, said cell container having the composition of cells of any of claims 1-5 therein, wherein said packaging material contains a label or package insert indicating that said composition is to be used in a method of treatment comprising administering said composition to said second individual mammal, wherein said second individual mammal is in need of said composition. 30. A method of inducing tolerance in a host mammal to a graft from a non-syngeneic host mammal, said method comprising : (a) administering donor antigens from a non-syngeneic donor to said host mammal; (b) administering an immunosuppressive agent to said host mammal in a non-myeloablative regimen sufficient to decrease said host mammal's functional T lymphocyte population; (c) transplanting cells, a tissue, or an organ from said donor into said host animal, (d) administering a non-myeloablative dose of lymphocytotoxic or tolerizing agent to said host mammal to selectively eliminate said host mammal's lymphocytes responding to said donor antigens; and (e) administering a preparation of hematopoietic stem cells from said non-syngeneic donor to said host mammal, wherein steps (a) , (b) , and (c) are performed on the same day and prior to steps (d) and (e) .

Inventors: PRIGOZHINA TATYANA, SLAVIN SHIMON, PRIGOZHINA, TATYANA, SLAVIN, SHIMON
Applicants: BAXTER INT, HADASIT MED RES SERVICE, BAXTER INTERNATIONAL INC., HADASIT MEDICAL RESEARCH SERVICES AND DEVELOPENT LTD.
Application year: 1999
Main classification: C12N5
IPC codes: A01K67, A61K35, A61K39, A61P35, A61P37, C12N5