Haemopoietic cells carry a variety of cell-surface molecules, some of which are known to have allotypic variation. In rats, the RT7 alloantigenic system has been well documented using alloantisera. We have produced the first mouse hybridoma cell line secreting an antibody, HIS41, which binds to leucocytes of rat strains carrying the RT7.2 but not the RT7.1 determinant. An IgG2b isotype switch variant (HIS41.2b) of the original HIS41 (IgG1 isotype) was also made. HIS41 showed a clear and discrete binding in immunofluorescent and histological experiments and has already been used in several studies on haemopoietic cell turnover and differentiation employing PVG rats congenic for RT7. The present study addresses the question of whether the RT7 gene products are members of the L-CA family, which has been a matter of controversy over the last decade. When using HIS41 for the analysis of tissue distribution and molecular weight of RT7 gene products, a strong similarity was evident with the data reported for the L-CA detected by MRC OX-1 and MRC OX-30. These two MoAb have been reported to bind to all members of the L-CA family. All haemopoietic cells, excluding erythrocytes and the more mature stages of erythropoiesis, stained with HIS41. The molecular weights of HIS41 binding molecules on thymocytes and peripheral T cells were comparable to the L-CA precipitated by MRC OX-1. Capping and sequential immunoprecipitation studies indicated that HIS41 and MRC OX-30-binding molecules were identical. MRC OX-1, however, appeared to bind only a subset of these molecules. Thus, our study confirms the identity of RT7.2 gene products and L-CA. It also revealed a difference between MRC OX-1 and MRC OX-30 not noticed previously.

A hybridoma producing the monoclonal antibody HIS42 was isolated from a fusion between spleen cells from a BALB/c mouse immunized with rat thymocytes and the fusion partner SP2/0. This antibody recognizes a minor subset of T cells in every haplotype of rat tested so far. The subpopulation of HIS42 positive T cells contains both CD4+ and CD8+ cells, in the same ratio as found in the peripheral T cell population. When bound to Sepharose beads, HIS42 induces T cell proliferation in the presence of interleukin-2. In contrast to lymph node T cells, a number of thymocytes were found to express HIS42 only in the cytoplasm or together with membrane expression. Most bright HIS42 surface labelled thymocytes were also positive for MRC OX-44, a marker predominantly identifying mature thymocytes. SDS-PAGE analyses of the membrane molecules immunoprecipitated by HIS42 show two bands on unreduced gels. One of these bands (85 kd) runs as two separate bands at 35 and 48 kd on reduction. The other much weaker broad band (approximately 100 kd) is hardly affected by reduced conditions. Taken together these data suggest that HIS42 is directed against a determinant on the rat T cell receptor for antigen, which is common to a small number of T cells.

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