Abstract:
The present invention relates to a monoclonal antibody which is broadly reactive with all normal human peripheral blood mononuclear cells and granulocyte, and to the hybridoma cell line which produces this monoclonal antibody. This monoclonal antibody is designated WM-65 and the hybridoma cell line is designated F56-1D5 (ECACC 89033001). This monoclonal antibody reacts with a previously unrecognized human leucocyte surface membrane antigen. The relative molecular mass of the antigen recognized by WM-65 is approximately 40-50 Kilodaltons.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a new hybridoma cell line and to the monoclonal antibody produced by this hybridoma cell line This monoclonal antibody reacts with a novel human leucocyte surface membrane antigen with wide distribution within the haemopoietic system. 
     BACKGROUND OF THE INVENTION 
     Since it was shown by Kohler &amp; Milstein (Nature Vol. 256, 495-497, 1975) that it was possible to fuse mouse myeloma cells with spleen cells from immunized mice and thereby product a continuous cell line which produces a homogeneous (monoclonal) antibody, extensive attention has been focused on the production of these hybrid cell lines (hybridomas) and the monoclonal antibodies (Mabs) produced. 
     The development of hybridoma technology has led to a dramatically improved understanding of the antigenic molecules on the surface of human leucocytes, and over the past decade, many murine monoclonal antibodies reacting with haemopoietic cell surface antigens have been described. Following the three International Workshops on Human Leucocyte Differentiation Antigen, the majority of these antibodies have been grouped into Clusters of Differentiation (CD), and have been shown to react with restricted differentiation-lineage or maturation stage membrane antigens on lymphoid or bone marrow-derived cells (Interim Report of the 3rd Workshop Nomenclature Committee (1987) in A.J. McMichael, Ed. Leucocyte Typing III. White cell differentiation antigen. Oxford University Press, PP 949-950). 
     A few antibodies have been shown to have broader haemopoietic cellular reactivity. The best characterised are the antibodies fitting into CD 45 or CD 45R, which identify a family of antigens with molecular weights around 200 Kilodaltons expressed on virtually all human leucocytes (Pizzolo et al., Cancer, 1980, 46, 2640-2647; Dalchau et al., 1980, European Journal of Immunology, 10, 737-744). Other unclustered Mabs with non-lineage reactivity include PHM-1, CAMPATH-1, HuLyM3, which appear to identify separate and unique antigens (Becker et al., 1981, Pathology, 13, 669-680; Hale et al., 1983, Blood, 62, 873-882; Vaughan et al., Transplantation, 36, 446-450). 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention consists in a mouse monoclonal antibody of class IgG 1  produced by a hybridoma formed by a fusion of cells from a mouse myeloma line and spleen cells from a mouse previously immunized with the T cell line HSB-2 and T-CLL cells, the monoclonal antibody being characterised in that it reacts with a leucocyte surface membrane antigen of a relative molecular weight of approximately 40-50 Kilodaltons which is expressed on over 90% of normal human peripheral blood mononuclear cells and granulocytes, but not on normal platelets or erythrocytes. 
     It is preferred that the monoclonal antibody is further characterised in that it: 
     a) reacts with approximately 90% of normal human thymocytes; 
     b) reacts with human B cell leukemia lines RAji, Daudi and Bristol 8; 
     c) reacts with human T cell leukemia lines T-All, MOLT-4, CEM, HSBO2; 
     d) reacts with the Myeloid cell line K562; 
     e) reacts with pre-B cell lines NALM-6, Reh, KM3; 
     f) reacts with approximately 95% of mitogen activated human lymphocytes; 
     g) is unreactive with the cell lines IPMI-8402, U937, Rc2a, and H160; and 
     h) is unreactive with platelets or erythrocytes. 
     In a second aspect the present invention consists in an IgG 1  monoclonal antibody-producing hybridoma cell line formed by fusion of cells from a mouse myeloma line and spleen cells from a mouse previously immunized with the T cell line HSB-2 and T-CLL cells, the monoclonal antibody produced being characterised in that it reacts with a leucocyte surface membrane antigen of a relative molecular weight of approximately 40-50 Kilodaltons which is expressed on over 90% of normal human peripheral blood mononuclear cells and granulocytes, but not on platelets or erythrocytes. 
     It is preferred that the monoclonal antibody produced is further characterised in that it: 
     a) reacts with approximately 90% of normal human thymocytes; 
     b) reacts with human B cell leukemia lines RAji, Daudi and Bristol.8; 
     c) reacts with human T cell leukemia lines T-All, MOLT-4, CEM, HSB.2; 
     d) reacts with the Myeloid cell line K562; 
     e) reacts with pre-B cell lines NALM-6, Reh, KM3; 
     f) reacts with approximately 95% of mitogen activated human lymphocytes; 
     g) is unreactive with the cell lines IPMI-8402, U937, Rc2a, and H160; and 
     h) is unreactive with platelets or erythrocytes. 
     The monoclonal antibody of the first aspect of the present invention has been designated WM-65, and the hybridoma cell line of the second aspect of the present invention has been designated F56-1D5. This hybridoma cell line was deposited with the European Collection of Animal Cell Cultures (ECACC), PHLS Centre for Applied Microbiology and Research, Porton Down, Salisbury, Wilts., United Kingdom on Mar. 30, 1989 and was accorded accession number 89033001. 
     The disclosure of this deposit is herein incorporated by way of cross-reference. The monoclonal antibody of the first aspect of the present invention will hereafter be referred to as WM-65 and the hybridoma cell line of the second aspect of the present invention will hereafter be referred to as F56-1D5. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The hybridoma cell line F56-1D5 was produced using the technique described in detail below, and the monoclonal antibody WM-65 produced by this hybridoma cell line was tested using the techniques described below. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In order that the nature of the present invention may be more clearly understood preferred forms thereof will be described with reference to the accompanying drawings in which: 
     FIG. 1 shows the flow cytometric evaluation of WM-65 reactivity with normal peripheral blood mononuclear cells. Log green fluorescence is indicated on the horizontal scale and log red fluorescence on the vertical scale. In FIG. 1A, cells were labelled with negative control Mabs directly conjugated with FITC or PE. In FIGS. 1B, 1C and 1D cells were labelled with WM-65 directly conjugated with FITC, and CD-19 Mab (B4-PE; FIG. 1B), CD-3 Mab (OKT-3-PE; FIG. 1C), or CD-11c Mab (Leu M5.PE; FIG. 1D) directly conjugated with Phycoerythrin. The results illustrate that virtually all CD-19 + , CD-3 + , and CD-11c +  cells are labelled by WM-65 (indicated in quadrant II in each FIG.). 
     FIG. 2 shows representative immunofluorescence reactivity patterns of WM-65 with cases of AML(FIG. 2A), and common ALL(FIG. 2B). Staining was evaluated in a flow cytometer with log fluorescence intensity displayed on the horizontal axis and cell numbers of the vertical axis. Heavy tracing indicates the fluorescence IgG 1  negative control MAB, and the light tracing is WM-65 staining 
     FIG. 3 shows the autoradiagraph of immunoprecipitation and SDS-PAGE experiment on  125  I-labelled leucocytes. Lane A, IgG 1  negative control Mab; Lane B, WM-65 immunoprecipitation. Position of molecular weight markers are indicated on the left. A broad band in the range of 40-50 Kilodaltons is seen in lane B. 
    
    
     MATERIALS AND METHODS 
     Immunization and Hybridoma Production 
     An 8 week old female BALB/c mouse was injected intraperitoneally with 10 7  cells of the human T-leukemia cell line HSB-2. Five days later 5×10 6  peripheral blood leucocytes from a patient with a T cell form of chronic lymphocytic leukemia (T-CLL) were injected intraperitoneally. Twenty days later a further 8×10 6  T-CLL lines were injected Three days later the animal was sacrificed and the spleen removed. Splenic lymphocytes were fused with the mouse myeloma cell line P3-NS1-Ag3, using a modification of the method described by Kohler and Milstein (Nature, 1975, 26, 495-497; and Fazekas de St. Groth et al, 1980, Journal of Immunological Methods, 35, 1-21). Following fusion, cells were plated out in DULBECCO&#39;S MINIMAL ESSENTIAL MEDIUM (DMEM) a tissue culture medium containing foetal calf serum (FCS: Flow Laboratories), L-glutamine, sodium pyruvate, antibiotics and hypoxanthine-aminopterin-thymidine (HAT; Flow Labs.), into 96 well microtitre plates (Linbro). 
     Culture supernatants from wells containing hybridomas were tested for reactivity on peripheral blood mononuclear cells. One hybridoma, F56-1D5, was selected, and cloned three times by the limiting dilution method Monoclonal antibody was produced by injecting 10 7  hybridoma cells intraperitoneally into pristane-primed mice, and collecting ascitic fluid. 
     Preparation of Cells for Antibody Characterization 
     Mononuclear cells were isolated from heparinized peripheral blood obtained from healthy volunteers by centrifugation on FICOLL-HYPAQUE (Pharmacia), a centrifugation medium, while granulocytes were prepared from the same source using MONO-POLY (Pharmacia), a centrifugation medium. Tonsil and thymic lymphocytes were obtained from fresh surgical samples. The tissue was cut and teased in medium to produce a single cell suspension. The cells were washed and tested immediately, or cryopreserved until required. 
     Leukaemic cells were obtained from heparinized bone marrow or peripheral blood diagnostic samples, and separated on FICOLL-HYPAQUE. Cells were either used fresh or cryopreserved until required. Leukaemic cell lines were grown in RPMI 1640 (Flow Labs.) containing FCS, L-glutamine and antibiotics. 
     Immunofluorescence Staining and Flow Cytometry 
     Peripheral blood leucocytes were prepared for immunofluorescent staining as described previously (Bradstock et al, 1985, Pathology, 17, 392-399). Briefly, 1-2×10 6  cells were reacted at 20° C. with a saturating concentration of Mab for 10 minutes, then washed in phosphate buffered saline containing 0.1% sodium azide (PBSA), followed by incubation with sheep anti-mouse antiserum conjugated to fluorescein isothiocyanate (SAM-FITC; Silenus) for a further 10 minutes at room temperature. For characterization of subsets of normal peripheral blood mononuclear cells, two colour direct immunofluorescence was used. Leucocytes were stained with WM-65 directly conjugated to FITC, and with Mabs to CD-3 (OKT-3 PE), CD-11c (Leu M5-PE), or CD-19 (B4 PE) all directly conjugated with Phycoerythrin. After completion of immunofluorescence staining, cells were then washed again, and reactivity determined using a FACS 440 cytometer (Becton Dickinson, Calif.). An isotype-matched Mab unreactive with human cells was used as a negative control. 
     Complement Mediated Cytoxicity 
     The lytic ability of the Mab was tested in a complement-mediated cytoxicity assay as previously described. Briefly, 10 6  peripheral blood mononuclear cells were incubated with a saturating concentration of Mab for 15 minutes at room temperature. This was followed by the addition of an equal volume of rabbit serum (Pelfreez), and further incubation at 37° C. for 45 minutes. A Mab (W6-32; Sera Labs.) reactive with human Class 1 MHC antigens was used as a positive control Cell viability was determined by Trypan blue exclusion using an inverted microscope. 
     Reactivity with Bone Marrow Progenitor Cells 
     Normal bone marrow was obtained with the informed consent of normal volunteers undergoing harvest for allogeneic bone marrow transplantation. Mononuclear cells were separated on FICOLL-HYPAQUE, washed, then reacted with a saturating solution of Mab or with the appropriate negative control for 10 minutes at 20° C. After washing with PBS the cells were incubated with SAM-FITC, for a further 10 minutes at 20° C., washed again, and then sorted into fluorescence positive and negative populations in a FACS440 cell sorter. Leucocytes from each population were plated at 1×10 5  viable cells in quadruplicate in 0.3% agar containing 25% FCS in 35 mm Petri dishes, with a 0.5% agar underlayer containing 1×10 6  irradiated normal peripheral blood mononuclear cells as a feeder layer. Plates were incubated in 5% CO 2  in air at 37° C., and examined at day 12 using an inverted microscope. Colonies were scored as groups of more than 40 cells. 
     Immunoprecipitation and Electrophoresis 
     Antigenic molecular weight determination was performed as previously described (Bradstock et al, 1985,  Pathology, 17, 392-399). Peripheral blood mononuclear cells were surface-labelled with  125  I using the lactoperoxidase method Cells were disrupted using TRITON-X 100, a detergents and centrifuged to remove cytoskeletal material The radiolabelled lysate was precleared overnight using PANSORBIN (Pharmacia), an absorption medium. The cell lysate was reacted with Mab for 2 hours at 4° C. followed by a further 30 minutes incubation with goat antimouse IgG (Cappell). The complex was absorbed onto PABNSORBIN and resuspended in non-reducing electrophoresis buffer. Samples were reduced by the addition of 15 ul mercaptoethanol Samples, together with molecular weight markers (Biorad), were run on a 5-15% gradient polyacrylamide gel overnight. The gels were stained with Coomassie Brilliant Blue, dried and autoradiographs performed using HYPERFILM(Amersham), an X-ray film at -70° C. 
     Tissue Section Staining 
     Fresh tissue biopsies were snap frozen in isopentane and stored in liquid nitrogen Cryostat sections 6 to 8 microns thick were air dried onto poly-L-Lysine (Sigma) treated slides, fixed in acetone at -10° C. for 5 minutes, and washed with 3% hydrogen peroxide in 0.05 M Tris-HCl/phosphate buffered saline, pH 7.6, for 5 minutes to block endogenous peroxidase. Tissue sections were then washed in Tris-buffered saline for 15 minutes. Prior to immunohistochemical staining, sections were incubated with diluted normal horse serum (Vectastain ABC kit, No. PK4002) for 5 minutes to reduce non-specific binding of the secondary antibody. Mab (or the isotype-matched negative control) in the form of 1:10 dilution of culture supernatant was then added to the sections, and incubated for one hour in a humidified chamber on a platform rocker. The slides were washed in Tris-PBS and incubated with an appropriate dilution of biotinylated horse anti-mouse antibody (Vectastain) for 20 minutes. After further washing in Tris-PBS, sections were incubated for 30 minutes with avidin-horseradish peroxidase complex (Vectastain), washed again, then developed for 4 minutes in DAB solution (0.03% 3,3&#39; diamino benzidine tetra-hydrochloride, Fluka, Switzerland) and 0.2 mM imidazole (Sigma) in PBSA. After further washing in water for 5 minutes, sections were counterstained with Mayer&#39;s haematoxylin and blueing solution, before dehydration in absolute alcohol, clearing in xylol and mounting. 
     RESULTS 
     Cellular Reactivity of WM-65 
     The hybridoma selected, F56-1D5, secreted a murine IgG 1  Mab which was designated WM-65. Its reactivity with both normal and leukaemic haemopoietic cells is detailed in Tables 1 &amp; 2. WM-65 reacted with over 90% of peripheral blood mononuclear cells (FIG. 1) and granulocytes, but not with normal platelets or erythrocytes (Table 1). It reacted with virtually all thymocytes and tonsil lymphocytes, as well as with the majority of normal bone marrow mononuclear cells. However, in myeloid progenitor assays, WM-65 reacted with only a minority (mean value 13.6%, range 2-34%, 5 experiments) of normal CFU GM  (Table 3). The reactivity of WM-65 with PHA-stimulated T lymphoblasts was equivalent to that seen on resting normal T lymphocytes. Based on dye exclusion studies, WM-65 was incapable of lysing mononuclear cells in the presence of rabbit serum. 
     The reactivity of WM-65 with cell lines and leukaemic cells is also shown in Table 2. WM-65 reacted with all pre-B and B cell lines tested as well as 4/5 T cell lines, but with only 1/4 myeloid cell lines. WM-65 showed extensive reactivity with leukaemic cells, including 8/8 cases of common acute lymphoblastic leukaemia (c-ALL), 4/4 B-chronic lymphatic leukaemia (B-CLL) and 2/2 hairy cell leukaemia (HCL), together with 13/13 acute myeloid 
     
                       TABLE 1______________________________________REACTIVITY OF WM-65 WITHNORMAL HAEMOPOIETIC CELLSCELL TYPE         PERCENTAGE POSITIVE.sup.a______________________________________Peripheral blood mononuclearcellsTotal             96.6 +/- 2.8                         (n = 7)T lymphocytes (CD-3.sup.+)             96.6 +/- 4.0                         (n = 4)B lymphocytes (CD-20.sup.+)             97.5 +/- 0.9                         (n = 4)monocytes (CD-11c.sup.+)             97.5 +/- 0.5                         (n = 4)Granulocytes      96.2 +/- 1.5                         (n = 5)Platelets          1          (n = 5)Erythrocytes       1          (n = 5)Thymocytes        91.5 +/- 0.5                         (n = 2)Tonsil lymphocytes             90.5 +/- 5.5                         (n = 2)Bone marrow mononuclear cells             63.2 +/- 10.6                         (n = 5)______________________________________ Footnotes .sup.a Percentage of cells +/- 1 standard deviation positive above negative control by immunofluorescence. 
    
     
                       TABLE 2______________________________________REACTIVITY OF WM-65 WITHLEUKAEMIC CELLS AND CELL LINES                 PERCENTAGECELL TYPE             POSITIVE______________________________________(A)  Leukaemias.sup.aC-ALL                .sup. 8/8.sup.bT-ALL                2/5AML                  13/13CML                  5/6B-CLL                4/4T-CLL                1/1Hairy cell leukaemia 2/2PLL                  1/1(B)  Leukaemic cell linesB cellDaudi                .sup. 98.sup.cRaji                 98Bristol 8            98T cellT-ALL-1              90MOLT-4               98RPMI-8402             1CEM                  98HSB-2                98Pre-BNALM-6               98Reh                  72KM-3                 55MyeloidK-562                98HL-60                 5RC-2A                 5U937                  5(C)  Non-haemopoietic tumor cell linesMelanoma             0/1Squamous cell carcinoma                     0/2Fallopian carcinoma  0/1Ovarian carcinoma    0/4Cervical carcinoma   0/2Neuroblastoma        0/1Breast carcinoma     0/1______________________________________ Footnotes .sup.a Abbreviations used: CALL, common form of acute lymphoblastic leukaemia; TALL, T cell form of ALL; AML, acute myeloid leukaemia; CML, chronic myeloid leukaemia; BCLL, B cell form of chronic lymphatic leukaemia; TCLL, T cell form of CLL; PLL, prolymphocytic leukaemia. .sup.b Number of cases considered to be positive with WM65, with 20% of cells fluorescent above negative control, over total number tested. .sup.c Percentage of cells fluorescent above negative control for each cell line. 
    
     
                       TABLE 3______________________________________REACTIVITY OF WM-65 WITH NORMAL BONEMARROW MYELOID PROGENITOR CELLS.sup.a      NUMBER OF CFU.sub.GMFRACTION     EXPERIMENT 1 EXPERIMENT 2______________________________________UNSEPARATED  125 (100)    266 (100)WM-65.sup.+  9 (5)        174 (34)WM-65.sup.-  164 (95)     332 (66)______________________________________ Footnote .sup.a Bone marrow cells incubated with WM65 and SAMFITC, and passed through a FACS 440 cell sorter, either unsorted, or sorted into fluorescencepositive and negative populations. Two representative experiments are shown CFU.sub.GM values were adjusted to 10.sup.5 cells per plate, with assays being performed in quadruplicate, and mean values tabulated above. Values in parentheses indicate the proportion of CFU.sub.GM in each population. Proportion of bone marrow cells positive for WM65 was 47% in Experiment 1, and 60% in Experiment 2. 
    
     leukaemia (AML) and 5/6 chronic myeloid leukaemia (CML) (FIG. 2). The percentage of cells positively stained by WM-65 in each case of acute leukaemia was generally high, with a mean of 71.7% of AML blasts (range 35.3-96.5) labelled, and mean 75.6% (range 21.8-90.7) ALL cells positive for WM-65. All non-haemopoietic cell lines tested were negative (Table 2). 
     Immunochemical Characterization of the Antigen Recognised by WM-65 
     SDS-polyacrylamide gel electrophoresis of  125  I-surface-labelled peripheral blood mononuclear cells revealed that WM-65 immunoprecipitated a broad band with an apparent molecular weight between 40-50KD under both reduced and non-reduced conditions (FIG. 3). 
     Non-Haemopoietic Tissue Reactivity of WM-65 
     Frozen sections of normal tissues were evaluated for WM-65 reactivity by immunoperoxidase using an avidin-biotin-peroxidase complex. WM-65 was reactive with virtually all lymphoid cells and macrophages in both lymph node and tonsil, but showed no staining of salivary gland, lung, kidney, skeletal muscle and heart, apart from occasional lymphoid cells and macrophages. 
     WM-65 reacts with a novel human leucocyte differentiation antigen with wide distribution within the haemopoietic system. On immunoprecipitation and polyacrylamide gel electrophoresis, the antigen can be identified as a single broad band of molecular weight in the range of 40-50 Kilodaltons, indicating that it is likely to be a single polypeptide with heavy but variable glycosylation. This antigen is expressed on virtually all nucleated bone marrow-derived cells and lymphocytes, but was not detectable on non-haemopoietic cells. 
     It is interesting to note that although a high proportion of bone marrow mononuclear cells were reactive with WM-65, these did not include the majority of myeloid progenitors as judged by CFU GM  assays, indicating that the antigen detected by WM-65 may be maturation linked, and expressed only as lymphocytes mature from earlier marrow stem cells. However, WM-65 did react with immature malignant haemopoietic cells, including leukaemic cell lines as well as cases of both ALL and AML. 
     Overall the data indicate that WM-65 reacts with an antigen not previously described. A number of leucocyte antigens with broad non-lineage specific distribution have been described previously (Pizzolo et al., 1980, Cancer, 46 2640-2647; Dalchau et al., 1980, European Journal of Immunology, 10, 737-744; Becker et al., 1981, Pathology, 13, 669-680; Hale et al., 1983, Blood, 62, 873-882; Vaughan et al., Transplantation, 36, 446-450). Differences in both molecular weight and cellular distribution distinguish these previously described antigens from the antigen recognised by WM-65 (Table 4). Class I MHC antigens have a molecular weight of 47KD, and are associated with the cell membrane of B 2  -microglobulin (12KD) (Strominger, 1980, In Progress in Immunology Vol. 4, London Academic Press, p 539). Although the HLA heavy chain is closely similar to that of the WM-65 antigen, no molecular weight band comparable to the B-2M was seen on immunoprecipitation with WM-65. In addition, class I antigens are present on platelets but not on the B cell line Daudi, a pattern of expression opposite to that seen with WM-65 (Barnstable et al., 1978, Cell, 14, 9-20; Brown et al. 1979, European Journal of Immunology, 9, 272-275). The T200 or leucocyte common antigens (CD-45) have a molecular weight in the range of 180-220KD (Cobbold et al., 1987, In Leucocyte Typing III. White Cell Differentiation Antigens Oxford University Press, P788-803). Although the cellular reactivity patterns of CD-45 Mabs and WM-65 are very similar (Table 3), the molecular weights of the antigens recognised are 
     
                                           TABLE 4__________________________________________________________________________COMPARISON OF WM-65 WITH OTHER NON-LINEAGESPECIFIC ANTIBODIES.sup.aCELL LINE  WM-65      CD45 CLASS II                 PHM-1                     HuLyM3                          CAMPATH__________________________________________________________________________PBL    .sup. +.sup.b      +    +     +   +    +Granulocytes  +   +    +     +   +    +/-Patelets  -   -    +     -   -    -HSB-2  +   +    +     +   +    +/-U937   -   +    +     +   +    -KM3    +   +    +     +   +    +K562   +   +    +     +   -    +Reh    +   +    +     +   +    +MW (K.D).sup.c  40-50      220-180           47,12 180,62                     47   23-30__________________________________________________________________________ Footnotes .sup.a All cellular reactivities determined in the inventors&#39; laboratory. .sup.b +: reactivity 20%, +/-: reactivity 15-20%, -: reactivity 15% by indirect immunofluorescence and flow cytometry.  .sup.c Antigen molecular weights quoted from original references. 
    
     clearly different. 
     Three other Mabs with similar reactivity have been described. PHM 1  reacts with lymphocytes, granulocytes and several cell lines, and immunoprecipitates an antigen of molecular weight 180/62KD (Becker et al., 1981, Pathology, 13, 669-680). Aside from this report of molecular weight, the major property distinguishing PHM-1 from WM.65 is that the former reacts with the U937 cell line, which is unreactive with WM-65. The pan leucocyte Mab HuLyM3 immunoprecipitates an antigen of 47KD molecular weight, with a cellular distribution closely similar to that of WM-65 (Vaughan et al., Transplantation, 36, 446-450). However, HuLyM3 does not react with the myeloid cell line K562, which is moderately strongly labelled by WM.65, indicating that the two Mabs identify different antigens. Finally, CAMPATH-1 can be distinguished from all the above in that it does not react with granulocytes to any significant degree, and precipitates a broadly glycoslated band of 23-30 KD (Hale et al., 1983, Blood, 62, 873-882; Cobbald et al.,  1987, In Leucocyte Typing III. White Cell Differentiation Antigens. Oxford University Press, P788-803). 
     The above data indicates that the 40-50 Kilodalton molecule identified by WM-65 Mab is a newly recognised leucocyte surface membrane antigen. Although its restricted distribution to the haemopoietic family of cells suggests some functional significance, no physiological role for the molecule has yet been investigated. The major practical feature of interest with this Mab is its extensive reactivity with acute and chronic leukaemias of both myeloid and lymphoid types. This is an unusual property for a monoclonal antibody, and raises the interesting possibility that the antibody may be useful in the treatment of haematological malignancies. This is heightened by the fact that WM-65  has comparatively little reactivity with bone marrow progenitor cells, and might therefore allow therapy to be selectively targeted to malignant haemopoietic cells. Although the antibody is not complement-fixing, it may have clinical application if it was conjugated to a toxin, drug or radioisotope. Preliminary studies to link the biological toxin ricin to WM-65 are underway to explore this possibility.