CD6 dependent interactions of T cells and keratinocytes: functional evidence for a second CD6 ligand on gamma-interferon activated keratinocytes.

The CD6 glycoprotein is expressed by T lymphocytes and is hypothesized to interact with one or more ligands expressed on antigen presenting cells (APCs). We show that CD6 mediates binding of the transformed CD4+ T cell line Hut 78 to gamma-interferon activated keratinocytes (KCs). A recombinant CD6-Ig fusion protein has been reported to bind to a CD6 ligand ALCAM, but this is the first demonstration that cell-cell adhesion of human T lymphocytes can be CD6 dependent. The known CD6 ligand ALCAM (CD166) is expressed on cultured KCs but does not appear to mediate KC-Hut 78 binding, suggesting the existence of additional CD6 ligands expressed on KCs. In functional studies using autologous KCs as APCs for tetanus toxoid specific T cell clones, KCs +/- gamma-interferon are unable to stimulate autologous T cells with recall antigen. Therefore interaction of T cell CD6 with CD6 ligands on KCs does not provide sufficient co-stimulation of primed T cells to support responses to nominal antigen.

Cloning, mapping, and characterization of activated leukocyte-cell adhesion molecule (ALCAM), a CD6 ligand.

Antibody-blocking studies have demonstrated the role of CD6 in thymocyte-thymic epithelial (TE) cell adhesion. Here we report that CD6 expressed by COS cells mediates adhesion to TE cells and that this interaction is specifically blocked with an anti-CD6 monoclonal antibody (mAb) or with a mAb (J4-81) that recognized a TE cell antigen. We isolated and expressed a cDNA clone encoding this antigen and show that COS cells transfected with this cDNA bind a CD6 immunoglobulin fusion protein (CD6-Rg). This antigen, which we named ALCAM (activated leukocyte-cell adhesion molecule) because of its expression on activated leukocytes, appears to be the human homologue of the chicken neural adhesion molecule BEN/SC-1/DM-GRASP. The gene was mapped to human chromosome 3q13.1-q13.2 by fluorescence in situ hybridization of cDNA probes to metaphase chromosomes. We prepared an ALCAM-Rg fusion protein and showed that it binds to COS cell transfectants expressing CD6, demonstrating that ALCAM is a CD6 ligand. The observations that ALCAM is also expressed by activated leukocytes and that both ALCAM and CD6 are expressed in the brain suggest that ALCAM-CD6 interactions may play a role in the binding of T and B cells to activated leukocytes, as well as in interactions between cells of the nervous system.

Diversity of sites for measles virus binding and for inactivation of complement C3b and C4b on membrane cofactor protein CD46.

The complement system membrane cofactor protein (MCP) CD46 serves as a C3b/C4b inactivating factor for the protection of host cells from autologous complement attack and as a receptor for measles virus (MV). MCP consists of four short consensus repeats (SCR) which are the predominant extracellular structural motif. In the present study, we determined which of the four SCR of MCP contribute to its function using Chinese hamster ovary cell clones expressing each SCR deletion mutants. The results were as follows: 1) SCR1 and SCR2 are mainly involved in MV binding and infection; 2) SCR2, SCR3, and SCR4 contribute to protect Chinese hamster ovary cells from human alternative complement pathway-mediated cytolysis; and 3) SCR2 and SCR3 are essential for protection of host cells from the classical complement pathway. These results on cell protective activity of the mutants against the human classical and the alternative complement pathways were compatible with factor I-mediated inactivation profiles of C4b and C3b, respectively, in the fluid-phase assay using solubilized mutants and factor I; the results were mostly consistent with those reported by Adams et al. (Adams, E. M., Brown, M. C., Nunge, M., Krych, M., and Atkinson, J. P. (1991) J. Immunol. 147, 3005-3011). SCR2 and SCR3 were required for C3b and C4b inactivation, and SCR4-deleted MCP showed weak cofactor activity for C4b cleavage but virtually no cofactor activity for C3b cleavage. The functional domains of MCP for the three natural ligands C3b, C4b, and MV, therefore, map to different, although partly overlapping, SCR domains.

Identification and characterization of a 100-kD ligand for CD6 on human thymic epithelial cells.

CD6 is a 130-kD glycoprotein expressed on the surface of thymocytes and peripheral blood T cells that is involved in TCR-mediated T cell activation. In thymus, CD6 mediates interactions between thymocytes and thymic epithelial (TE) cells. In indirect immunofluorescence assays, a recombinant CD6-immunoglobulin fusion protein (CD6-Rg) bound to cultured human TE cells and to thymic fibroblasts. CD6-Rg binding to TF and TE cells was trypsin sensitive, and 54 +/- 4% of binding was divalent cation dependent. By screening the blind panel of 479 monoclonal antibodies (mAbs) from the 5th International Workshop on Human Leukocyte Differentiation Antigens for expression on human TE cells and for the ability to block CD6-Rg binding to TE cells, we found one mAb (J4-81) that significantly inhibited the binding of CD6-Rg to TE cells (60 +/- 7% inhibition). A second mAb to the surface antigen identified by mAb J4-81, J3-119, enhanced the binding of CD6-Rg to TE cells by 48 +/- 5%. Using covalent cross-linking and trypsin digestion, we found that mAb J4-81 and CD6-Rg both bound to the same 100-kD glycoprotein (CD6L-100) on the surface of TE cells. These data demonstrate that a 100-kD glycoprotein on TE cells detected by mAb J4-81 is a ligand for CD6.

Effect of liposomes containing sodium butyrate conjugated with anti-CD19 monoclonal antibody on in vitro and in vivo growth of malignant lymphoma.

A differentiation-inducer, sodium butyrate (SB), encapsulated in liposomes conjugated covalently to a monoclonal antibody directed to CD19 antigen, was successfully targeted to human lymphoma cell lines SKLY-18 and Ramos grown in vitro and in vivo in nude mice. Various control liposomes (lacking SB, lacking antibody, containing SB alone, or coupled to irrelevant antibody) were not effective targeters. Growth of tumor cells not expressing the relevant antigen was unaffected by the antibody-SB-liposome complex. Suppression of lymphoma growth in vivo was remarkable, and tumor regression was observed under certain conditions, although changes in morphology were not clearly observed. In view of the practical nonexistence of "tumor-specific" antigens, targeting of differentiation-inducers or growth modifiers, rather than cytotoxic drugs, to tumor cells, as exemplified in these experiments, may provide a useful approach for conversion of highly malignant to less malignant tumors, although the exact mechanism of the growth inhibitory effect on B-cell lymphoma of the anti-CD19 antibody-SB-liposome complex is unknown.

AFTR: a fifth human B-cell-specific surface antigen.

We report the identification and characterization of a fifth B-cell-specific surface antigen (AFTR) detected by three murine monoclonal antibodies. AFTR is present on all malignant human B-cell lines tested except those of plasma cells and on malignant B cells from 49/49 patients, including those with pre-B-cell acute lymphoblastic leukemia. Similarly, AFTR is found on all CD19+ B cells from normal bone marrow, peripheral blood, and tonsil, although it is only weakly expressed on Epstein-Barr virus-transformed normal B cells. AFTR is not expressed on T lymphocytes, thymocytes, myelocytes, monocytes, granulocytes, or erythrocytes, or on endothelial or epithelial cells. Isolation of AFTR from surface-iodinated cells by immune precipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals bands at 38 and 42 kd in the acute lymphoblastic leukemia cell lines Laz 221 and NALM-6. Molecular weights of these two bands vary slightly between different B-cell populations but change little under reducing and nonreducing conditions. In contrast, relative intensities of these bands vary considerably between different B-cell populations. Like the B-cell-specific surface immunoglobulin and CD19 antigens, but unlike CD20, expression of AFTR is specifically reduced or modulated when cells are incubated with monoclonal antibodies at 37 degrees C. The molecular weight, behavior, and cellular distribution of AFTR are distinct from those of known B-cell surface antigens. One of these MoAbs (J3-109) is a prototype reagent for the CD72 antigen cluster.

CD19 is functionally and physically associated with surface immunoglobulin.

The pan-B and B cell-specific sIg and CD19 antigens are functionally and physically associated in the presence of anti-Ig mAb. Incubation of B cells with anti-Ig antibodies causes rapid, specific, reversible, concentration-dependent, and unidirectional comodulation of CD19 on every mature B cell studied. Comodulation is produced by mAbs specific for the gamma, mu, kappa, and lambda chains of Ig, and by at least one idiotype-specific mAb. Comodulation is observed using 15 CD19-specific mAbs that detect at least three different CD19 epitopes. Of 18 surface antigens studied, only CD19 is comodulated. Loss of sIg and CD19 occurs concurrently during anti-Ig modulation and demonstrates a comparable dependence on anti-Ig concentration, suggesting that these are parallel rather than serial events. Incubation with anti-Ig specifically cocaps and suggests internalization of anti-CD19 mAb. Comodulation of sIg and CD19 by anti-Ig but not anti-CD19 mAbs suggests that ligand binding enables sIg to then interact with CD19. We propose that CD19 is a component of the B cell antigen receptor and suggest that it could facilitate signal transduction by sIg-antigen complexes.

A lymphocyte molecule implicated in lymph node homing is a member of the cartilage link protein family.

Monoclonal antibodies in the Hermes family recognize a lymphocyte structure that participates in lymphocyte adhesion to endothelium and has been suggested to be the human homolog of the murine Mel-14 lymph node homing receptor. Recently, antibodies against the Hermes antigen, the polymorphic glycoprotein Pgp-1 antigen, and the broadly expressed CDw44 antigen have been shown to recognize the same structure. In this work, cDNA clones encoding the CDw44 antigen were isolated and expressed in COS cells. Two forms were identified: a lymphoid form expressed in hematopoietic cells, and an epithelial form weakly expressed in normal epithelium but highly expressed in carcinomas. The extracellular domain of CDw44 bears homology to cartilage link proteins and a related segment of proteoglycan core protein. However, comparison with the recently identified sequence of the Mel-14 antigen shows that CDw44 and Mel-14 are unrelated.

HLA-DR molecules on all cells are electrophoretically unique.

Previous SDS-PAGE studies of autologous clonal ALL and normal B cell lines indicated that HLA-DR molecules on leukemic cells have an extra beta chain band. We now show that these results are not an artifact of cell lines, EBV transformation, or cell growth in culture. Leukemic cells from four ALL patients were surface labeled with 125I and their HLA-DR molecules compared with those on autologous normal peripheral blood B cells. The electrophoretic patterns of HLA-DR molecules on these in vivo cell populations are identical to those on the corresponding cell lines. Moreover, EBV-transformation does not alter the electrophoretic appearance of HLA-DR molecules on normal tonsil B cells. Cell lines can now be used to study the chemical basis for these electrophoretic differences.

Comparative reactivity of anti-Ia monoclonal antibodies in man and laboratory animals.

Sixteen murine monoclonal antibodies (MAb), reactive with HLA-DR, DR + DP or DR + DQ, were tested, using indirect immunocytofluorescence, for their reactivity with peripheral blood mononuclear cells (PBMC) from the dog, cat, guinea pig, sheep, rabbit and rat. In addition, the MAb were evaluated for inhibitory activity in the canine mixed lymphocyte culture (MLC). Fourteen of 16 MAb reacted with canine PBMC. There was a greater tendency for DR + DP reactive MAb to inhibit canine MLC and subsequently react with PBMC of the guinea pig, sheep, and, cat. MAb failing to block the canine MLC were generally nonreactive with guinea pig PBMC (7 of 9 nonreactive) suggesting the guinea pig may be a useful model to study the functional relevance of specific Ia molecules. One MAb, H81.98.21 (reactive with HLA-DR) blocked canine MLC and reacted with PBMC from all species tested suggesting the determinant it recognized to be very well conserved in nature.

Selective inhibition of the canine mixed lymphocyte response by HLA-DR and DP-reactive monoclonal antibodies.

Twenty-three of 37 anti-Ia McAb reactive with human B cells, as determined by indirect immunocytofluorescence, were shown to be reactive with canine peripheral blood mononuclear cells (PBMC). Using a panel of human B cell lines that differ in their expression of HLA-DR, -DP, and -DQ molecules, it was shown that 15 of these antibodies identify HLA-DR and DP molecules (i.e., broadly reactive), while 22 identify only HLA-DR molecules. Fourteen of the 15 broadly reactive McAb were reactive with canine PBMC while only 9 of the 22 HLA-DR-specific McAb reacted with canine PBMC, suggesting that broadly reactive anti-Ia McAb are much more likely to react with canine cells than narrowly reactive McAb. Ten of the canine reactive McAb that were shown to identify typical Ia bimolecular structures on canine cells using immune precipitation analysis were tested for blocking activity in the canine mixed lymphocyte culture (MLC). All four of the broadly reactive McAb (B1F6, J-70, 9-49, and HB10a) plus two of the six narrowly reactive McAbs (H81.98.21 and H40.164.3) blocked the canine MLC when added to culture wells on day 0, suggesting that inhibition may be related to the specificity of the anti-Ia McAb employed. Since the MLC may reflect cellular interactions occurring during graft-versus-host disease, this assay may be useful for screening functionally relevant broadly reactive McAb in experimental canine bone marrow transplantation studies. These data suggest that the dog may be a useful model to study anti-Ia immunotherapy.

Altered expression of surface antigens with appearance of HLA class II molecules on a malignant human B-cell line.

Class II molecules of the major histocompatibility complex play an important role in mediating cellular interactions and are differentiation antigens on lymphobohematopoietic cells. We have previously characterized a human B-cell line (BALM-3) whose cells fail to express HLA class II molecules unless treated with phorbol acetate (TPA). Recently, we identified a spontaneous variant of BALM-3 whose cells express HLA class II molecules in the absence of TPA. Since normal B cells lose HLA class II molecules on terminal differentiation, these two BALM-3 cell populations may provide a model for a discrete phase of B-cell maturation. Alternatively, they may reflect two B-cell activation states characterized by quantitative differences in their expression of class II molecules. Expression of six of 22 additional surface molecules (HLA class I, CD23, p60, p124, p129, p141) increases by a factor of three or more as BALM-3 cells spontaneously acquire class II molecules while that of one of the 22 (p45) decreases by a comparable amount. Expression of the plasma cell-associated T10/CD38 antigen decreases by a factor of two. These additional surface molecules might also reflect lymphoid differentiation/activation antigens and/or participate in HLA class II-mediated cellular interactions and require further study. Use of TPA to induce the expression of HLA class II molecules produces similar changes in several but not all of these surface antigens.

Evidence for chemical differences in HLA-DR molecules on autologous acute lymphoblastic leukemia and B-lymphoblastoid cell lines.

HLA-DR molecules on autologous acute lymphoblastic leukemia (ALL) and B-lymphoblastoid cell lines from two individuals were compared by immune precipitation and gel electrophoresis. Cells were surface labeled with 125I and proteins immunoprecipitated with specific monoclonal antibodies (MoAb). Two electrophoretically distinct bands were found in the HLA-DR beta chain region on both ALL cell lines in contrast to only one on each of the autologous B-lymphoblastoid cell lines. Differences in the electrophoretic mobility of the alpha chains were also observed with ALL and B-lymphoblastoid cell lines from one individual. Preclearing of radiolabeled cell lysates with MoAb specific for HLA-DQ and -DP molecules demonstrated that the complexity of the HLA-DR pattern is not the result of antibody cross-reactivity with alpha and beta chains from other class II products. Immunoprecipitation experiments indicated that two beta chain bands are observed with each of the parental HLA-DR molecules on the ALL but not the B-lymphoblastoid cell line from an HLA-DR3,7-positive individual. We conclude that the HLA-DR molecules expressed on ALL and B-lymphoblastoid cell lines from the same individual can differ chemically. Neuraminidase treatment reduced these electrophoretic differences, indicating that these molecules differ in their sialic acid content. Since small changes in class II molecules can profoundly alter cellular interactions, the functional significance of these differences requires further investigation.

Antibody-induced antigenic modulation is antigen dependent: characterization of 22 proteins on a malignant human B cell line.

Expression of several of the surface antigens on normal and malignant hematopoietic cells is reduced or is modulated by incubation with specific antibodies. Although antigenic modulation provides a means by which cells can escape antibody-mediated immune destruction, the physiologic significance and frequency of this phenomenon are both poorly understood. To begin to address these issues, we identified and characterized surface antigens on the malignant B cell line Laz 221 established from a patient with acute lymphoblastic leukemia (ALL). Indirect immunofluorescence analysis with the use of 26 hematopoietic cell populations and immune precipitation studies with the use of iodinated ALL cells indicate that 163 monoclonal antibodies (MoAb) identify 22 different proteins on this cell line, including at least six previously described surface molecules. Seven of these antigens are expressed by all nucleated cells examined, whereas only the mu chain of immunoglobulin is B cell specific. Incubation of specific MoAb with cultures of Laz 221 cells at 37 degrees C reduces or modulates surface expression of five of these 22 antigens (p45, immunoglobulin mu chain, transferrin receptor, common ALL antigen (CD10), and p105). Studies that made use of multiple MoAb specific for the same antigen suggest that the capacity for antigenic modulation is an intrinsic property of individual antigens. These studies also suggest that the murine immune response to shared human antigens varies from one immunizing cell population to another. For example, three of the antigens present on Laz 221 cells were only identified by MoAb raised to the Burkitt's cell line Ramos and vice-versa. Only one of these six shared antigens is present in greater amounts on the immunogenic cell population. Immunogenicity of individual human antigens in the mouse may be a function of their cell surface environment.

Monoclonal antibodies to CALLA do not alter polymorphonuclear functions.

The common acute lymphoblastic leukemia antigen (CALLA) is present on the malignant cells of most patients with acute lymphoblastic leukemia (ALL). Monoclonal antibodies (MoAbs) to CALLA have been useful for differentiating lymphoblastic from nonlymphoblastic leukemias as well as for serotherapy in ALL. Since this MoAb also reacts with polymorphonuclear (PMN) leukocytes, it was of interest to determine whether or not MoAbs to CALLA affected PMN function. We therefore evaluated four MoAbs to CALLA for their effect on PMN function. Two of the MoAbs were IgG and two were IgM. Chemotaxis was studied using Micro-Boyden chambers. Intraleukocyte bacterial killing was studied using Staph:PMN of 2:1 and 10:1, and metabolic activation was evaluated by hexose monophosphate shunt activity. The results showed that these MoAbs to CALLA did not impair the parameters of PMN function studied. These findings have relevance to the usefulness of MoAbs to CALLA for serotherapy and also as a probe for understanding the surface molecule properties that have a bearing on PMN host defense functions.

HLA-DP can be expressed with or without -DR molecules on a malignant B cell line.

HLA class II molecules (HLA-DR, -DQ, -DP) appear to differ in their ability to serve as corecognition elements in antigen presentation to T lymphocytes. Expression of these molecules on the autologous malignant B cell lines BALM-3, -4, and -5 was studied by binding of alloantisera and by indirect immunofluorescence and immune precipitation performed with well-characterized monoclonal antibodies. The following phenotypes were identified: BALM-5, HLA-DR+, -DP+; BALM-4, HLA-DR-, -DP+; BALM-3, HLA-DR-, -DP-. When treated with phorbol acetate (TPA), all three cell lines synthesize and express both HLA-DR and -DP molecules, indicating that the structural genes for these molecules remain intact. Thus HLA-DP can be expressed with or without HLA-DR molecules. Immune precipitation studies of metabolically labeled BALM-4 cells detect HLA-DR molecules in cells from TPA-treated but not control cultures, suggesting that TPA acts by inducing the transcription and/or translation of the genes for these class II molecules. HLA-DP molecules are detected in cells from both BALM-4 cultures. Recent studies suggest that BALM-5 but not BALM-3 or BALM-4 cells are HLA-DQ positive. TPA also appears to induce expression of HLA-DQ molecules on the latter two cell lines. The unique class II phenotypes of these autologous B cell lines in the resting state therefore appear to reflect differences in their ability to execute discrete steps leading to the surface expression of individual class II molecules. Variable expression of individual class II molecules by different cell populations may affect their ability to present cell-associated antigens to T lymphocytes.

Differential expression of HLA-DR, -DQ, and -DP antigens on malignant B cells.

HLA class II antigens mediate interactions among cells involved in the immune response and play an important role in the process of self recognition. We made use of conventional alloantisera and six well-characterized monoclonal antibodies (MoAb) to study the HLA class II antigens on CALLA-positive malignant B cell populations and autologous normal B cell lines. Forty additional HLA class II-specific MoAb were also tested for their ability to bind to these cells. By using indirect immunofluorescence and immune precipitation assays, we find that malignant B cells often fail to express one or more of the three known types of HLA class II antigens. Cell lines with the following five phenotypes have been identified: HLA-DR+, -DQ+, -DP+; HLA-DR+, -DQ-, -DP+; HLA-DR-, -DQ+, -DP+; HLA-DR-, -DQ-, -DP+; and HLA-DR-, -DQ-, -DP-. These cell lines have been used to characterize the subregion specificity of MoAb that react with HLA class II antigens. This work confirms the existence of complicated patterns of serologic cross-reactivity between the three different types of HLA class II molecules. It also increases our understanding of the specificity of individual MoAb, thereby facilitating future investigation of the distribution and function of individual antigens. Our studies are consistent with the proposal that altered expression of HLA antigens on tumors might impair recognition of these cells by the immune system of the host, thereby contributing to the proliferation of a malignant clone.

Malignant human B cells express two populations of p24 surface antigens.

Monoclonal antibodies (MoAb) to a leukemia-associated p24 cell surface antigen are currently being used to purge bone marrow of malignant cells before autologous transplantation for acute lymphoblastic leukemia (ALL). Their use as potential diagnostic reagents for hematologic disorders is also under investigation. It has been assumed throughout these investigations that the p24-specific MoAb produced by different laboratories all identify the same antigen. Our present studies indicate that at least two populations of p24 antigens, having different chemical properties and cellular distributions, exist on malignant B cells. For example, eight MoAb raised to ALL cells (ALL-MoAb) identify a p24 antigen on these cells but do not react with the Burkitt's lymphoma cell line Ramos. In contrast, six MoAb raised to Ramos (Ramos-MoAb) identify a p24 antigen on both Ramos and ALL cells. Quantitative binding of both sets of MoAb to ALL cells is comparable. The ALL-MoAb react with platelets, granulocytes, and activated but not resting T lymphocytes, whereas the Ramos-MoAb react with both resting and activated T lymphocytes but not with platelets or granulocytes. The ALL-MoAb react with 11 of 34 human hematopoietic cell lines tested; the Ramos-MoAb react with all 34. Both sets of MoAb react with most of the nonhematopoietic human cell lines tested. Reciprocal exhaustive radioimmune precipitation experiments performed with an ALL cell line indicate that the antigenic determinants recognized by these two sets of MoAb are present on different molecules. Similarly, proteolytic digests of iodinated antigens identified by these two sets of MoAb on ALL cells confirm the unique chemical identities of these molecules and suggest that they reflect the products of different genetic loci. The presence of the antigen identified by the Ramos-MoAb on every cell population tested except granulocytes suggests that it may serve an important cellular function. The existence of two populations of p24 antigens on at least some hematopoietic cells indicates the need for caution when comparing the results of studies of these antigens by groups employing different MoAb.

Anti-CALLA antibodies identify unique antigens on lymphoid cells and granulocytes.

The common acute lymphoblastic leukemia antigen (CALLA) is a 100-kd surface glycoprotein that is present on normal and malignant lymphoid cells. It is a useful marker for distinguishing between clinically important types of acute leukemia. Anti-CALLA monoclonal antibodies (MoAb) also react with mature myeloid cells (granulocytes), where they identify an antigen having a similar molecular weight (mol wt). We now report that the antigens detected by anti-CALLA MoAb on human lymphoid and myeloid cells differ in their behavior and chemistry. Surface-labeling studies indicate that the antigen on lymphoid cells has a mol wt of approximately 100 kd v 110 kd for that on granulocytes. When cells are metabolically labeled with 35S-methionine, differences in the mol wt of these antigens are again observed. Unlike the lymphoid antigen, expression of that on purified granulocytes is not modulated by incubation with specific antibody. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of proteolytic digests of the two antigens fails to clarify their chemical relationship. Thus the antigens detected on these two cell types may share an epitope(s) but be chemically distinct, or CALLA may exist in distinct forms and behave differently on lymphoid cells and granulocytes.

Nonhuman primates express human leukemia-associated antigens.

Serologic studies using four murine monoclonal antibodies specific for the common acute lymphoblastic leukemia antigen (CALLA) and five monoclonal antibodies specific for the gp24 surface antigen indicate that these leukemia-associated antigens are present on cells of comparable tissues in man and in four nonhuman primates. As in man, adherent cell populations obtained from skin, lung, and bone marrow of Macaca fascicularis, M mulatta, M nemestrina, and Papio cynocephalus react with these antibodies. Similarly, granulocytes from both man and these nonhuman primates bind CALLA- and gp24-specific antibodies. Radioimmune precipitation experiments confirm the identity of these antigens. Our studies suggest that nonhuman primates can be used to screen serologic reagents to leukemia-associated antigens for potential toxic effects on normal tissues prior to their use in man. Similarly, nonhuman primates could be employed to assess the possible role of antigen-positive stromal cells in the reconstitution of bone marrow following transplantation.