Expression of HIS50 Ag: a rat homologue of mouse heat-stable antigen and human CD24 on B lymphoid cells in the rat.

Recently, a cDNA encoding a newly identified rat antigen (HIS50 Ag) that binds to monoclonal antibody (mAb) HIS50 was cloned and shown to be homologous to cDNA encoding murine heat-stable antigen (HSA) and human CD24. Here we show that, like CD24 and HSA, at least part of HIS50 Ag is inserted into the plasma membrane by a glycosylphosphatidylinosito: (GPI)-lipid linkage and we describe its expression in rat haemolymphopoietic tissues. HIS50 Ag expression was almost exclusively confined to B lymphoid cells, the vast majority of T lymphoid cells, erythroid and myeloid cells were HIS50+. Cell suspension analysis indicated that in bone marrow (BM) almost all Thy-1+ cells, HIS24+ cells [HIS24 recognizes the B-cell form of leucocyte common antigen (LCA)], terminal deoxynucleotidyl transferase-positive (TdT+) cells and (c + s)kappa cells expressed HIS50 Ag, and all (c + s)mu 1 cells. A presumably early population of B lymphoid cells, expressing HIS24 Ag without HIS50 Ag, TdT or immunoglobulin HIS24+HIS50+ TdT Ig+), constituted 1.6% of BM nucleated cells. In blood, one-fifth of mononuclear cells were HIS50+, and about 85% of these expressed mu and/or kappa chains. In spleen, flow cytometry analysis and immunohistology demonstrated heterogeneous expression of HIS50 Ag: immunoglobulin M (IgM)bright cells (as found largely in red pulp and marginal zone) were HIS50bright, while IgMdull cells expressed low or undetectable levels of HIS50 Ag. Germinal centre B cells expressed high levels of HIS50 Ag. Germinal centres of lymph nodes and tonsil of man also bound HIS50. We conclude that HIS50 Ag expression in the haemolymphopoietic system of rat is virtually restricted to the B lineage.