Abstract:
The invention provides an article of manufacture comprising a substantially non-immunogenic articular cartilage xenograft for implantation into humans. The invention further provides a method for preparing an articular cartilage xenograft by removing at least a portion of an articular cartilage from a non-human animal to provide a xenograft; washing the xenograft in saline and alcohol; subjecting the xenograft to at least one treatment selected from the group consisting of exposure to ultraviolet radiation, immersion in alcohol, ozonation, freeze/thaw cycling, and optionally to chemical crosslinking. In addition to or in lieu of the above treatments, the methods include a cellular disruption treatment and glycosidase digestion of carbohydrate moieties of the xenograft followed by treatment of carbohydrate moieties of the xenograft with capping molecules. The invention also provides articles of manufacture produced by one or more of the above-identified methods of the invention. The invention further provides an articular cartilage xenograft for implantation into a human including a portion of an articular cartilage from a non-human animal, wherein the portion includes extracellular matrix and substantially only dead cells. The matrix and dead cells have substantially no surface α-galactosyl moieties and have capping molecules linked to at least a portion of surface carbohydrate moieties. Each of the xenografts of the invention is substantially non-immunogenic and has substantially the same mechanical properties as the respective native articular cartilage.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/779,280, filed Jan. 6, 1997, now U.S. Pat. No. 5,782,915, which is a continuation of prior application Ser. No. 08/529,200, filed Sep. 15, 1995 and now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of treatment of injured human joints, and in particular, to replacement and repair of a damaged human joint articular cartilage using a substantially immunologically compatible articular cartilage from a non-human animal. 
     BACKGROUND OF THE INVENTION 
     Articular cartilage covers the ends of all bones that form articulating joints in humans and animals. The cartilage is made of cells called fibrochondrocytes and an extracellular matrix of collagen fibers as well as a variety of proteoglycans. The cartilage acts in the joint as a mechanism for force distribution and as a lubricant in the area of contact between the bones. Without articular cartilage, stress concentration and friction would occur to the degree that the joint would not permit ease of motion. Loss of the articular cartilage usually leads to painful arthritis and decreased joint motion. 
     It is known that fibrochondrocytes, the cells that produce articular cartilage, have the ability to migrate into a defect filled with a fibrin clot and form tissue substantially similar to natural cartilage. Additionally, it has been shown that fibrochondrocytes in tissue culture are capable of cell division and matrix synthesis (Webber et al. (1985) J Ortho. Res. 3(l):36). However, the amount of cartilage formed by these procedures is generally not adequate to replace severely damaged joint surfaces in vivo. 
     Since joint cartilage in adults does not naturally regenerate to a significant degree once it is destroyed, damaged adult articular cartilage has historically been treated by a variety of surgical interventions including repair, replacement, or by excision. With repair or excision, regeneration of tissue may occur, although the tissue is usually temporary and inadequate to withstand the normal joint forces. 
     Replacement of articular cartilage usually has been by allografting (Sengupta et al. (1974) J Bone Suro. 56B(1): 167-177; Rodrigo et al. (1978) Clin Orthoo. 134:342-349) by periosteal grafts (see, e.g., Engkvist (1979) Scan J Plast. Reconstr. Suro. 13:361-369; Rubak (1982) Acta Orthop. Scan. 53:181-186) or with metal and/or plastic components (Rubash et al., eds. (1991) Clin. Orth. Rel. Res. 271:2-96). Allografting dead cartilage tissue has been tried for years with minimal success. This approach has been only partially successful over the long term due to the host&#39;s immunologic response to the graft, failures in the cryopreservation process, and failures of the attachment sites. Replacement of an entire joint surface with metal and plastic components has met excellent success for the older, more sedentary patients, but is generally considered insufficient for tolerating the impact of athletic activities, and has not been shown to restore normal joint mechanics. 
     In alternative prior art approaches, articular cartilage has been replaced with prostheses composed of bone and/or artificial materials. For example, U.S. Pat. No. 4,627,853 describes the use of demineralized allogenic or xenogenic bone segments as replacements. The proper functioning of these replacements depends on the differential demineralization of the bone segments. U.S. Pat. No. 4,846,835 describes a grafting technique for transplantation of fibrochondrocytes to promote healing lesions in articular cartilage. U.S. Pat. No. 4,642,120 describes the use of gel-like compositions containing embryonal fibrochondrocytes. U.S. Pat. No. 5,306,311 describes a prosthetic articular cartilage which includes a dry, porous volume matrix adapted to have in vivo an outer contour substantially the same as that of natural articular cartilage. 
     Despite these developments, the replacement of cartilage tissue with structures consisting of permanent artificial materials generally has been less than satisfactory, and a structure suitable as articular cartilage and constructed from natural resorbable materials, or analogs thereof, has not been developed. Because the opposing articular cartilage of mammalian joints is so fragile, it will not withstand abrasive interfaces nor compliance variances from normal which eventually result from the implantation of prior art artificial cartilage. Additionally, joint forces are multiples of body weight which, in the case of the knee and hip, are typically encountered over a million cycles per year. Thus far, prior art permanent artificial cartilages have not been composed of materials having natural articular cartilage properties, nor have they been able to be positioned securely enough to withstand such routine forces. 
     Much of the structure and many of the properties of original tissues may be retained in transplants through use of xenogeneic or xenograft materials, that is, tissue from a different species than the graft recipient. For example, tendons or ligaments from cows or other animals are covered with a synthetic mesh and transplanted into a heterologous host in U.S. Pat. No. 4,400,833. Flat tissues such as pig pericardia are also disclosed as being suitable for heterologous transplantation in U.S. Pat. No. 4,400,833. Bovine peritoneum fabricated into a biomaterial suitable for prosthetic heart valves, vascular grafts, burn and other wound dressings is disclosed in U.S. Pat. No. 4,755,593. Bovine, ovine, or porcine blood vessel xenografts are disclosed in WO 84/03036. However, none of these disclosures describe the use of a xenograft for articular cartilage replacement. 
     Once implanted in an individual, a xenograft provokes immunogenic reactions such as chronic and hyperacute rejection of the xenograft. The term &#34;chronic rejection&#34;, as used herein refers to an immunological reaction in an individual against a xenograft being implanted into the individual. Typically, chronic rejection is mediated by the interaction of IgG natural antibodies in the serum of the individual receiving the xenograft and carbohydrate moieties expressed on cells, and/or cellular and/or extracellular matrices of the xenograft. For example, transplantation of cartilage xenografts from nonprimate mammals (e.g., porcine or bovine origin) into humans is primarily prevented by the interaction between the IgG natural anti-Gal antibody present in the serum of humans with the carbohydrate structure Galα1-3Galβ1-4GlcNAc-R (α-galactosyl or α-gal epitope) expressed in the xenograft. K. R. Stone et al., Porcine and bovine cartilage transplants in cynomolgus monkey: I. A model for chronic xenograft rejection, 63 Transplantation 640-645 (1997); U. Galili et al., Porcine and bovine cartilage transplants in cynomolgus monkey: II. Changes in anti-Gal response during chronic rejection, 63 Transplantation 646-651 (1997). In chronic rejection, the immune system typically responds within one to two weeks of implantation of the xenograft. 
     In contrast with &#34;chronic rejection&#34;, &#34;hyper acute rejection&#34; as used herein, refers to the immunological reaction in an individual against a xenograft being implanted into the individual, where the rejection is typically mediated by the interaction of IgM natural antibodies in the serum of the individual receiving the xenograft and carbohydrate moieties expressed on cells. This interaction activates the complement system causing lysis of the vascular bed and stoppage of blood flow in the receiving individual within minutes to two to three hours. 
     The term &#34;extracellular matrix or matrices&#34;, as used herein, refer to collagen and elastic fibers as well a variety of proteoglycans, which are secreted by fibrochondrocytes during cartilage growth and which undergo slow turn-over. 
     Xenograft materials may be chemically treated to reduce immunogenicity prior to implantation into a recipient. For example, glutaraldehyde is used to cross-link or &#34;tan&#34; xenograft tissue in order to reduce its antigenicity, as described in detail in U.S. Pat. No. 4,755,593. Other agents such as aliphatic and aromatic diamine compounds may provide additional crosslinking through the side chain carboxyl groups of aspartic and glutamic acid residues of the collagen polypeptide. Glutaraldehyde and diamine tanning also increases the stability of the xenograft tissue. 
     Xenograft tissues may also be subjected to various physical treatments in preparation for implantation. For example, U.S. Pat. No. 4,755,593 discloses subjecting xenograft tissue to mechanical strain by stretching to produce a thinner and stiffer biomaterial for grafting. Tissue for allograft transplantation is commonly cryopreserved to optimize cell viability during storage, as disclosed, for example, in U.S. Pat. Nos. 5,071,741; 5,131,850; 5,160,313; and 5,171,660. 5,071,741 discloses that freezing tissues causes mechanical injuries to cells therein because of extracellular or intracellular ice crystal formation and osmotic dehydration. 
     A need for an improved replacement for articular cartilage which is biocompatible, soft, lubricating, and durable continues to exist. 
     SUMMARY OF THE INVENTION 
     The present invention provides a substantially non-immunogenic articular cartilage xenograft for implantation into a human in need of articular cartilage repair or replacement. The invention further provides methods for processing xenogeneic articular cartilage with reduced immunogenicity but with substantially native elasticity and load-bearing capabilities for xenografting into humans. 
     As described herein, the term &#34;xenograft&#34; is synonymous with the term &#34;heterograft&#34; and refers to a graft transferred from an animal of one species to one of another species. Stedman&#39;s Medical Dictionary, Williams &amp; Wilkins, Baltimore, Md. (1995). 
     As described herein, the term &#34;xenogeneic&#34;, as in xenogeneic graft, cartilage, etc., refers to a graft, cartilage, etc., transferred from an animal of one species on one of another species. Id. 
     The methods of the invention, include, alone or in combination, treatment with radiation, one or more cycles of freezing and thawing, treatment with a chemical cross-linking agent, treatment with alcohol, or ozonation. In addition to or in lieu of these methods, the methods of the invention include a cellular disruption treatment and glycosidase digestion of carbohydrate moieties of the xenograft followed by treatment of carbohydrate moieties of the xenograft with capping molecules. After one or more of the above-described processing steps, the methods of the invention provide a xenograft having substantially the same mechanical properties as a native articular cartilage. 
     As described herein, the term &#34;cellular disruption&#34; as in, for example, cellular disruption treatment, refers to a treatment for killing cells. 
     As described herein, the term &#34;capping molecules&#34;, refers to molecules which link with carbohydrate chains such that the xenograft is no longer recognized as foreign by the subject&#39;s immune system. 
     In one embodiment, the invention provides an article of manufacture comprising a substantially non-immunogenic articular cartilage xenograft for implantation into a human. 
     In another embodiment, the invention provides a method of preparing an articular cartilage xenograft for implantation into a human, which includes removing at least a portion of an articular cartilage from a joint of a non-human animal to provide a xenograft; washing the xenograft in water and alcohol; and subjecting the xenograft to at least one treatment selected from the group consisting of exposure to ultraviolet radiation, immersion in alcohol, ozonation, and freeze/thaw cycling, whereby the xenograft has substantially the same mechanical properties as a corresponding portion of a native articular cartilage. 
     As described herein, the term &#34;portion&#34;, as in, for example, a portion of articular cartilage or a portion of surface carbohydrate moieties, refers to all or less than all of the respective articular cartilage or surface carbohydrate moieties. 
     In yet another embodiment, the invention provides a method of implanting an articular cartilage xenograft on a surface of an articular joint in a human, comprising the steps of preparing said surface, and affixing a substantially non-immunogenic articular cartilage xenograft to said surface. 
     In still another embodiment, the invention provides a method of preparing an articular xenograft for implantation into a human, which includes removing at least a portion of an articular cartilage from ajoint of a non-human animal to provide a xenograft; washing the xenograft in water and alcohol; subjecting the xenograft to a cellular disruption treatment; digesting the xenograft with a glycosidase to remove substantially first surface carbohydrate moieties from the xenograft; and treating second surface carbohydrate moieties on the xenograft with capping molecules to cap at least a portion of the second surface carbohydrate moieties, whereby the xenograft is substantially non-immunogenic and has substantially the same mechanical properties as a corresponding portion of a native articular cartilage. 
     As described herein, the terms &#34;to cap&#34; or &#34;capping&#34;, refer to linking a capping molecule such as a carbohydrate unit to the end of a carbohydrate chain, as in, for example, linking a carbohydrate unit to surface carbohydrate moieties on the xenograft. 
     In further embodiments, the invention provides articles of manufacture including substantially non-immunogenic articular cartilage xenografts for implantation into humans produced by one or more of the above-identified methods of the invention. 
     In still a further embodiment, the invention provides an articular cartilage xenograft for implantation into a human which includes a portion of an articular cartilage from a joint of a non-human animal, wherein the portion includes an extracellular matrix and substantially only dead cells, the extracellular matrix and the dead cells having substantially no surface a-galactosyl moieties and having capping molecules linked to at least a portion of surface carbohydrate moieties. The xenograft is substantially non-immunogenic and has substantially the same mechanical properties as a corresponding portion of a native articular cartilage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The various features of the invention may be more fully understood from the following description when read together with the accompanying drawing. 
     FIG. 1 shows a simplified diagrammatic representation of a human knee joint 3, showing the normal positioning of articular cartilage 7 on the articulating end of femur 2 and articular cartilage 8 on the articulating end of tibia 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed against the chronic rejection of xenografts for implantation into humans. Accordingly, the articular cartilage xenograft produced in accordance with the method of the invention is substantially non-immunogenic, while generally maintaining the mechanical properties of native articular cartilage. While the articular cartilage may undergo some shrinkage during processing, an articular cartilage xenograft prepared in accordance with the invention will have the general appearance of a native articular cartilage. The articular cartilage xenograft may also be cut into segments, each of which may be implanted into ajoint of a recipient as set forth below. 
     The invention provides, in one embodiment, a method for preparing or processing a xenogeneic articular cartilage for engraftment into humans. The articular cartilage may be harvested from any non-human animal to prepare the xenografts of the invention. Articular cartilage from transgenic non-human animals, or from genetically altered non-human animals may also be used as xenografts in accordance with the present invention. Preferably, bovine, ovine, or porcine knee joints serve as sources of the articular cartilage used to prepare the xenografts. More preferably, immature pig, calf or lamb knee joints are the sources of the articular cartilage, since the cartilage of younger animals may be inherently more elastic and engraftable than that of older animals. Most preferably, the age of the source animal is between six and eighteen months at time of slaughter. 
     In the first step of the method of the invention, an intact articular cartilage is removed from a joint of a non-human animal. Any joint may serve as the source of articular cartilage. Preferably articular cartilage from a corresponding donor joint is used to make the articular cartilage xenograft of the invention. For example, articular cartilage from a femuro-tibial (stifle) joint is used to make an articular cartilage xenograft for implantation into a knee. Similarly, articular cartilage from a donor animal&#39;s hip joint is used to make an articular cartilage xenograft for a human hip joint. The joint which serves as the source of the articular cartilage should be collected from freshly killed animals and preferably immediately placed in a suitable sterile isotonic or other tissue preserving solution. Harvesting of the joints should occur as soon as possible after slaughter of the animal and preferably should be performed in the cold, i.e., in the approximate range about 5° C. to about 20° C., to minimize enzymatic degradation of the articular cartilage tissue. The articular cartilage is harvested from the joints in the cold, under strict sterile technique. 
     In accordance with the invention, a fine peel of articular cartilage with a small layer of subchondral bone is shaved from the donorjoint to form the xenograft. The xenograft is then washed in about ten volumes of sterile cold water to remove residual blood proteins and water soluble materials. The xenograft is then immersed in alcohol at room temperature for about five minutes, to sterilize the tissue and to remove non-collagenous materials. In a preferred form of the invention, a xenograft appears as a hyaline tissue supported on a bone substrate, having generally a spherical-shaped principal surface on the top side (the &#34;superior surface&#34;), with the under surface of bone (the &#34;inferior surface&#34;) being rough. 
     After alcohol immersion, the xenograft may be directly implanted a prepared site at an articular surface of a human patient. Alternatively the xenograft may be subjected to at least one of the following treatments: radiation treatment, treatment with alcohol, ozonation, one or more cycles of freezing and thawing and/or treatment with a chemical cross-linking agent,. When more than one of these treatments is applied to the xenograft, the treatments may occur in any order. 
     In one embodiment of the method of the invention, the xenograft may be treated by exposure to ultraviolet radiation for about fifteen minutes or gamma radiation in an amount of about 0.5 to 3 MegaRad. 
     In another embodiment, the xenograft may be treated by again being placed in an alcohol solution. Any alcohol solution may be used to perform this treatment. Preferably, the xenograft is placed in a 70% solution of isopropanol at room temperature. 
     In still another embodiment, the xenograft may be subjected to ozonation. 
     In a further embodiment of the method of the invention, the xenograft may be treated by freeze/thaw cycling. For example, the xenograft may be frozen using any method of freezing, so long as the xenograft is completely frozen, i.e., no interior warm spots remain which contain unfrozen tissue. Preferably, the xenograft is dipped into liquid nitrogen for about five minutes to perform this step of the method. More preferably, the xenograft is frozen slowly by placing it in a freezer. In the next step of the freeze/thaw cycling treatment, the xenograft is thawed by immersion in an isotonic saline bath at room temperature (about 25° C.) for about ten minutes. No external heat or radiation source is used, in order to minimize fiber degradation. 
     In yet a farther embodiment, the xenograft may optionally be exposed to a chemical agent to tan or crosslink the proteins within the extracellular matrix, to further diminish or reduce the immunogenic determinants present in the xenograft. Any tanning or crosslinking agent may be used for this treatment, and more than one crosslinking step may be performed or more than one crosslinking agent may be used in order to ensure complete crosslinking and thus optimally reduce the immunogenicity of the xenograft. For example, aldehydes such as glutaraldehyde, formaldehyde, adipic dialdehyde, and the like, may be used to crosslink the collagen within the extracellular matrix of the xenograft in accordance wvith the method of the invention. Other suitable crosslinking agents include aliphatic and aromatic diamines, carbodiimides, diisocyanates, and the like. When glutaraldehyde is used as the crosslinking agent, for example, the xenograft may be placed in a buffered solution containing about 0.05 to about 5.0% glutaraldehyde and having a pH of about 7.4. Any suitable buffer may be used, such as phosphate buffered saline or trishydroxymethylaminomethane, and the like, so long as it is possible to maintain control over the pH of the solution for the duration of the crosslinking reaction, which may be from one to fourteen days, and preferably from three to five days. Alternatively, the xenograft can be exposed to a crosslinking agent in a vapor form, including, but not limited to, a vaporized aldehyde crosslinking agent, such as, for example, vaporized formaldehyde. The vaporized crosslinking agent can have a concentration and a pH and the xenograft can be exposed to the vaporized crosslinking agent for a period of time suitable to permit the crosslinking reaction to occur. For example, the xenograft can be exposed to vaporized crosslinking agent having a concentration of about 0.05 to about 5.0% and a pH of about 7.4, for a period of time which can be from one to fourteen days, and preferably from three to five days. Exposure to vaporized crosslinking agent can result in reduced residual chemicals in the xenograft from the crosslinking agent exposure. The crosslinking reaction should continue until the immunogenic determinants are substantially removed from the xenogeneic tissue, but the reaction should be terminated prior to significant alterations of the mechanical properties of the xenograft. When diamines are also used as crosslinking agents, the glutaraldehyde crosslinking should occur after the diamine crosslinking, so that any unreacted diarnines are capped. After the crosslinking reactions have proceeded to completion as described above, the xenograft should be rinsed to remove residual chemicals, and 0.01-0.05 M glycine may be added to cap any unreacted aldehyde groups which remain. 
     In addition to or in lieu of the above treatments, the xenograft can be subjected to a cellular disruption treatment to kill the xenograft&#39;s fibrochondrocytes, which precedes or follows digestion of the xenograft with glycosidases to remove surface carbohydrate moieties from the xenograft. The glycosidase digestion in turn can be followed by linkage with capping molecules to cap surface N-acetyllactosamine ends of carbohydrate chains of the xenograft. 
     In an embodiment of this method of the invention, the xenograft is subjected to a treatment to kill the fibrochondrocytes of the articular cartilage prior to in vitro digestion of the xenograft with glycosidases. Typically after surface carbohydrate moieties have been removed from nucleated cells and the extracellular matrix, nucleated, i.e., living cells reexpress the surface carbohydrate moieties. Reexpression of antigenic moieties of a xenograft can provoke continued immunogenic rejection of the xenograft. In contrast, non-nucleated, i.e., dead cells, are unable to reexpress surface carbohydrate moieties. Removal of antigenic surface carbohydrate moieties from the non-nucleated cells and extracellular matrix of a xenograft substantially permanently eliminates antigenic surface carbohydrate moieties as a source of immunogenic rejection of the xenograft. 
     Accordingly, in the above-identified embodiment, the xenograft of the present invention is subjected to freeze/thaw cycling as discussed above to disrupt, i.e., to kill the fibrochondrocytes of the articular cartilage. Alternatively, the xenograft of the present invention is treated with gamma radiation having an amount of 0.2 MegaRad up to about 3 MegaRad. Such radiation kills the fibrochondrocytes and sterilizes the xenograft. Once killed, the fibrochondrocytes are no longer able to reexpress antigenic surface carbohydrate moieties such α-gal epitopes which are factors in the immunogenic rejection of the transplanted xenografts. 
     Either before or after the fibrochondrocytes are killed, the xenograft is subjected to in vitro digestion of the xenograft with glycosidases, and specifically galactosidases, such as α-galactosidase, to enzymatically eliminate antigenic surface carbohydrate moieties. In particular, α-gal epitopes are eliminated by enzymatic treatment with α-galactosidases, as shown in the following reaction: ##STR1## The N-acetyllactosamine residues are epitopes that are normally expressed on human and mammalian cells and thus are not immunogenic. The in vitro digestion of the xenograft with glycosidases is accomplished by various methods. For example, the xenograft can be soaked or incubated in a buffer solution containing glycosidase. In addition, the xenograft can be pierced to increase permeability, as further described below. Alternatively, a buffer solution containing the glycosidase can be forced under pressure into the xenograft via a pulsatile lavage process. 
     Elimination of the α-gal epitopes from the xenograft diminishes the immune response against the xenograft. The α-gal epitope is expressed as 1×10 6  -35×10 6  epitopes per cell, as well as on macromolecules such as proteoglycans of the extracellular matrix, in nonprimate mammals and in New World monkeys (monkeys of South America). U. Galili et al., Man, apes, and Old World monkeys differ from other mammals in the expression of igalactosyl epitopes on nucleated cells, 263 J. Biol. Chem. 17755 (1988). This epitope is absent in Old World primates (monkeys of Asia and Africa and apes) and humans, however. Id. Anti-Gal is produced in humans and primates as a result of an immune response to α-gal epitope carbohydrate structures on gastrointestinal bacteria. U. Galili et al., Interaction between human natural anti-α-galactosyl immunoglobulin G and bacteria of the human flora, 56 Infect. Immun. 1730 (1988); R. M. Hamadeh et al., Human natural anti-Gal IgG regulates alternative complement pathway activation on bacterial surfaces, 89 J. Clin. Invest. 1223 (1992). Since nonprimate mammals produce a-gal epitopes, xenotransplantation of xenografts from these mammals into primates results in rejection because of primate anti-Gal binding to these epitopes on the xenograft. The binding results in the destruction of the xenograft by complement fixation and by antibody dependent cell cytotoxicity. U. Galili et al., Interaction of the natural anti-Gal antibody with α-galactosyl epitopes: A major obstacle for xenotransplantation in humans, 14 Immunology Today 480 (1993); M. Sandrin et al., Anti-pig IgM antibodies in human serum react predominantly with Gala1-3Gal epitopes, 90 Proc. Natl. Acad. Sci. USA 11391 (1993); H. Good et al., Identification of carbohydrate structures which bind human anti-porcine antibodies: implications for discordant grafting in man. 24 Transplant. Proc. 559 (1992); B. H. Collins et al., Cardiac xenografts between primate species provide evidence for the importance of the α-galactosyl determinant in hyperacute rejection, 154 J. Immunol. 5500 (1995). Furthermore, xenotransplantation results in major activation of the immune system to produce increased amounts of high affinity anti-Gal. 
     The cartilage xenografts of the present invention are particularly well suited to in vitro enzymatic elimination of the (x-gal epitopes, however. In contrast to organs and other tissues, the cartilage extracellular matrix undergoes extremely slow turnover. Moreover, once the fibrochondrocytes are killed, these non-nucleated cells are prevented from reexpressing the α-gal epitopes, as discussed above. Accordingly, the substantial elimination of α-gal epitopes from cells and the extracellular matrix, and the prevention of reexpression of cellular α-gal epitopes can diminish the immune response against the xenograft associated with anti-Gal antibody binding with α-gal epitopes. 
     Following treatment with glycosidase, the remaining carbohydrate chains (e.g., glycosaminoglycans) of the xenograft are optionally treated with capping molecules to cap at least a portion of the remaining carbohydrate chains. Treatment with capping molecules is applicable to both glycosidase-treated and non-glycosidase-treated xenografts, however. For example, xenografts from knock out animals which may lack α-gal epitopes may be treated with capping molecules to cap carbohydrate moieties on the xenograft, thereby reducing the xenograft&#39;s immunogenicity. Examples of capping molecules used in the present invention include fucosyl and n-acetyl glucosamine. 
     Prior to treatment, the outer surface of the xenograft may optionally be pierced to increase permeability to agents used to render the xenograft substantially non-immunogenic. A sterile surgical needle such as an 18 gauge needle may be used to perform this piercing step, or, alternatively a comb-like apparatus containing a plurality of needles may be used. The piercing may be performed with various patterns, and with various pierce-to-pierce spacings, in order to establish a desired access to the interior of the xenograft. Piercing may also be performed with a laser. In one form of the invention, one or more straight lines of punctures about three millimeters apart are established in the outer surface of the xenograft. 
     Prior to implantation, the articular cartilage xenograft of the invention may be treated with limited digestion by proteolytic enzymes such as ficin or trypsin to increase tissue flexibility or coated with anticalcification agents, antithrombotic coatings, antibiotics, growth factors, or other drugs which may enhance the incorporation of the xenograft into the recipient knee joint. The articular cartilage xenograft of the invention may be further sterilized using known methods, for example, with additional glutaraldehyde or formaldehyde treatment, ethylene oxide sterilization, propylene oxide sterilization, or the like. The xenograft may be stored frozen until required for use. 
     The articular cartilage xenograft of the invention, or a segment thereof, may be implanted into damaged human joints by those of skill in the art using known arthroscopic surgical techniques. Specific instruments for performing arthroscopic techniques are known to those of skill in the art, which ensure accurate and reproducible placement of articular cartilage implants. 
     The underlying bone bed of the recipient joint is prepared with a bone burr to produce a cancellous bleeding bed. Grafting can involve either the entire articular surface or a portion of the articular surface. The substantially non-immunogenic articular cartilage xenograft of the invention is applied to the recipient joint as a cover, which is held in place by one or more suture anchors, absorbable pins, screws, staples, and the like. A fibrin clot may also be used to hold the substantially non-immunogenic articular cartilage xenograft in place. 
     This invention is further illustrated by the following Examples which should not be construed as limiting. The contents of all references and published patents and patent applications cited throughout the application are hereby incorporated by reference. 
     EXAMPLE 1 
     Assessment Of Primate Response To Implanted Porcine Cartilage Treated With α-Galactosidase 
     In this example, porcine cartilage implants are treated with α-galactosidase to eliminate α-galactosyl epitopes, the implants are transplanted into cynomolgus monkeys, and the primate response to the cartilage implants is assessed. 
     Porcine stifle joints are sterilely prepared and the articular cartilage and surrounding attached soft tissues surgically removed. The cartilage specimens are washed for at least five minutes with an alcohol, such as ethanol or isopropanol, to remove synovial fluid and lipid soluble contaminants. 
     The cartilage specimens are frozen at a temperature ranging from about -35° C. to about -90° C., and preferably at a temperature up to about -70° C., to disrupt, that, is to kill, the specimens&#39; fibrochondrocytes. 
     The cartilage is cut into two portions. The first portion is immersed in a buffer solution containing α-galactosidase at a predetermined concentration. The specimens are allowed to incubate in the buffer solution for a predetermined time period at a predetermined temperature. The second cartilage portion is incubated under similar conditions as the first cartilage portion in a buffer solution in the absence of α-galactosidase and serves as the control. 
     At the end of the incubation, the cartilage is washed under conditions which allow the enzyme to diffuse out. Assays are performed to confirm the complete removal of the α-gal epitopes. 
     Each cartilage sample is implanted in the supra patellar pouch of six cynomolgus monkeys. With the animals under general inhalation anesthesia, an incision of about 1 cm is made directly into the supra patellar pouch at the superior medial border of the patella extending proximally. A piece of the porcine cartilage of about 0.5 cm to about 1 cm in length is placed into the pouch with a single 3-0 nylon stitch as a marking tag. The procedure is performed under sterile surgical technique, and the wounds are closed with 3-0 vicryl or a suitable equivalent known to those of ordinary skill in the art. The animals are permitted unrestricted cage activity and monitored for any sign of discomfort, swelling, infection, or rejection. Blood samples (e.g., 2 ml) are drawn periodically (e.g., every two weeks) for monitoring of antibodies. 
     The occurrence of an immune response against the xenograft is assessed by determining anti-Gal and non-anti-Gal anti-cartilage antibodies (i.e., antibodies binding to cartilage antigens other than the α-gal epitopes) in serum samples from the transplanted monkeys. At least two ml blood samples are drawn from the transplanted monkeys on the day of implant surgery and at periodic (e.g., two week) intervals post-transplantation. The blood samples are centrifuged and the serum samples are frozen and evaluated for the anti-Gal and other non-anti-Gal anti-cartilage antibody activity. 
     Anti-Gal activity is determined in the serum samples in ELISA with α-gal-BSA as solid phase antigen, according to methods known in the prior art, such as, for example, the methods described in Galili et al., Porcine and bovine cartilage transplants in cynomolgus monkey: II. Changes in anti-Gal response during chronic rejection, 63 Transplantation 645-651 (1997). 
     Assays are conducted to determine whether α-galactosidase treated xenografts induce the formation of anti-cartilage antibodies. For measuring anti-cartilage antibody activity, an ELISA assay is performed according to methods known in the prior art, such as, for example, the methods described in K. R. Stone et al., Porcine and bovine cartilage transplants in cynomolgus monkey: I. A model for chronic xenograft rejection, 63 Transplantation 640-645 (1997). 
     The cartilage is optionally explanted at one to two months post-transplantation, sectioned and stained for histological evaluation of inflammatory infiltrates. Post-transplantation changes in anti-Gal and other anti-cartilage antibody activities are correlated with the inflammatory histologic characteristics (i.e., granulocytes or mononuclear cell infiltrates) within the explanted cartilage, one to two months post-transplantation, using methods known in the art, as, for example, the methods described in K. R. Stone et al., Porcine and bovine cartilage transplants in cynomolgus monkey: I. A model for chronic xenograft rejection, 63 Transplantation 640-645 (1997). 
     Where the cartilage is explanted, the cartilage xenograft is aseptically harvested, using anesthetic procedure, surgical exposure of the joint, removal of the implant and closure of the soft tissue. At the time of the xenograft removal, joint fluid, if present in amounts sufficient to aspirate, is collected from the stifle joints for possible immunologic testing if the gross and histopathologic evaluation of the transplants indicate good performance of the transplanted cartilage. Subsequently, the animals are allowed to recover and are monitored closely until the incisions have healed and the gait is normal. The xenograft samples are collected, processed, and examined microscopically. A portion of the implant and surrounding tissue is frozen in an embedding medium for frozen tissue specimens in embedding molds for immunohistochemistry evaluation according to the methods known in the prior art. &#34;TISSUE-TEK®&#34; O.C.T. compound which includes 10.24% w/w polyvinyl alcohol, 4.26% w/w polyethylene glycol, and 86.60% w/w nonreactive ingredients, and is manufactured by Sakura FinTek, Torrence, Calif., is a non-limiting example of a possible embedding medium for use with the present invention. Other embedding mediums known to those of ordinary skill in the art may also be used. The remaining implant and surrounding tissue is collected in 10% neutral buffered formalin for histopathologic examination. 
     EXAMPLE 2 
     Assessment Of Primate Response To Implanted Cartilage Treated With α-Galactosidase, Fucosyl and Fucosyltransferase 
     In this example, porcine cartilage implants are treated with α-galactosidase to eliminate α-gal epitopes, as described in Example 1. The implants are further treated with fucosyl and fucosyl transferase to cap carbohydrate chains with fucosyl. Fucosyltransferase facilitates the transfer of fucosyl to the xenograft. The fucosyl links to and thus caps the carbohydrate chains. Capping with fucosyl interferes with the ability of the subject&#39;s immune system to recognize the xenograft as foreign. The implants are transplanted into cynomolgus monkeys, and the primate response to the cartilage implants is assessed. 
     Porcine cartilage stifle joints are prepared as described in Example 1 including the α-galactosidase treatment. Prior to implantation into the monkeys, however, the implants are further treated with a predetermined amount of fucosyl and facosyltransferase, at specified concentrations for a predetermined time and at a predetermined temperature, to cap carbohydrate chains with fucosyl. For example, the sample is immersed in a buffer solution at predetermined concentrations of fucosyl and fucosyl transferase. The sample is incubated for a predetermined time period at a temperature. 
     Other molecules, such as n-acetyl glucosamine in combination with the corresponding glycosyltransferase, can also be used for capping the carbohydrate chains of the implants. 
     Subsequently, the samples are washed to remove the enzyme and implanted into the monkeys, and the occurrence of an immune response against the xenograft is assessed as described above in Example 1. 
     Those of skill in the art will recognize that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently described embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all variations of the invention which are encompassed within the meaning and range of equivalency of the claims are therefor intended to be embraced therein.