Human LFA-1 is a widely expressed leukocyte antigen present on cells of myeloid and lymphoid lineage. Monoclonal antibodies to LFA-1 have been shown to inhibit in vitro T cell immune functions. However, a role for LFA-1 in B cell activation has not been documented. To investigate this possibility, we examined the distribution of LFA-1 on normal, neoplastic, and EBV-transformed B cells as well as the ability of a monoclonal anti-LFA-1 antibody (NB-107) to inhibit B cell mitogenesis. NB-107 immunoprecipitates a noncovalently linked heterodimer of approximately 170,000 and 95,000 daltons. Sequential immunoprecipitation and cross-blocking studies showed that NB-107 identified a distinct epitope on the LFA-1 molecule. NB-107-defined LFA-1 was present on peripheral blood mononuclear cells (PBMC) from all normal individuals (N = 27) and on EBV-transformed cell lines (N = 9), but was absent from four of seven neoplastic B lymphoma lines. NB-107 was observed to profoundly inhibit the response of PBMC to the B cell mitogens anti-IgM (mean 71% inhibition) and lipopolysaccharide (mean 80% inhibition). In order to investigate the mechanism of inhibition, B cells were sequentially purified from PBMC by using a combination of E rosette depletion of T cells, monocyte removal by glass adherence, and finally cell sorting. These extensively enriched populations of B cells, although still responding to anti-mu, showed no evidence of inhibition by NB-107. Growth of EBV-transformed cell lines, cultured in the presence of NB-107, also was not inhibited by this antibody. When tested in assays for T cell function, NB-107 was shown to inhibit the mixed lymphocyte response, but had no effect on phytohemagglutinin stimulation of PBMC nor on the clonal growth and differentiation of granulopoietic, erythropoietic, and pluripotent progenitor cells. We conclude that anti-LFA-1 monoclonal antibody inhibits B cell mitogens via indirect effects on monocytes and/or T cells, rather than by a direct antiproliferative effect on B cells.

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