A major membrane glycoprotein with mol wt of approximately 54,000 has been isolated from membrane preparations of B-type lymphoid cell lines. Antiserum prepared against the isolated material specifically precipitated this glycoprotein from membranes labeled by surface radioiodination or by metabolic labeling. This antiserum was shown by complement-mediated cytotoxicity assay, membrane immunofluorescent staining, and by quantitative absorption analysis to react preferentially with certain B-lymphoblastoid cell lines, with a minor population of peripheral blood B lymphocytes, and a major population of tonsillar B lymphocytes. Certain B-cell leukemias also expressed the antigen, whereas others did not. Considerable variability was observed among positive B cells in the intensity of fluorescent staining even among the leukemic cells from a single individual. Although T cells, including T cells, were negative by direct immunofluorescent and cytotoxicity assay, evidence for low levels of the antigen on the cells of T cell lines was obtained. The whole specific antiserum and its F(ab')2 fragments stimulated B lymphocytes to proliferate. This proliferation did not produce differentiation to plasma cells and was T-cell independent. The monovalent Fab fragments had no effect. None of these preparations timulated T cells. The possibility that this antigen, termed gp54, may play some role in B-cell activation is discussed.

We report in this paper the generation and characterization of three monoclonal antibodies, designated alpha BL1, alpha BL2, and alpha BL3, that recognize distinctive antigens unrelated to complement, Fc, and mouse erythrocyte rosette receptors, which are preferentially expressed on B lymphocytes. alpha BL1 recognizes a heat stable nonimmunoprecipitable antigen, possibly glycolipid in nature. Alpha BL2 recognizes a nonreducible single polypeptide with a m.w. of 68,000 that occasionally co-precipitates with a p29,34 complex of HLA-DR antigens. Alpha BL3 recognizes a nonreducible single polypeptide with a m.w. of 105,000 with an acidic pI point. We demonstrated that BL1 is expressed on fetal liver hematopoietic cells, a small subset (5 to 15%) of Ficoll-Hypaque-separated normal bone marrow cells, and on a subpopulation of nonadherent, non-E rosette-forming cells and granulocytes. BL2 is expressed on fetal liver hematopoietic cells, on 3 to 7% of normal bone marrow cells, and on a majority (40 to 70%) of nonadherent, non-E rosette-forming cells with a distinctive pattern similar to that of HLA-DR. BL3 is expressed on a subpopulation of nonadherent, non-E rosette-forming cells, and on occasional cells in the monocyte-enriched adherent cell population. The peak fluorescence for BL2 is substantially higher than that of BL1 and BL3, indicating higher BL2 antigen density. All three antigens are absent from thymocytes and E rosette-positive T cell fractions obtained from various lymphoid tissues. Cellular distribution of the BL antigens on various well-characterized established hematopoietic cell lines, leukemias, and malignant lymphomas, in conjunction with the results of the in vitro activation and TPA-induction experiments, suggest that BL1 is expressed during early developmental stages of B cell differentiation, whereas BL3 is expressed at the later stages. BL2 expression spans immature and mature stages of B cell differentiation, with the exception of mature plasma cells. The alpha BL antibodies described here should prove to be useful in the investigation of B cell differentiation and in the clinical diagnosis of lymphoid neoplasms.