EXPRESSION OF HIS50 AG - A RAT HOMOLOG 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) th at binds to monoclonal antibody (mAb) HIS50 was cloned and shown to be homologous to cDNA encoding murine heat-stable antigen (HSA) and huma n CD24. Here we show that, like CD24 and HSA, at least part of HIS50 A g is inserted into the plasma membrane by a glycosylphosphatidylinosit ol (GPI)-lipid linkage and we describe its expression in rat haemolymp hopoietic 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 t hat in bone marrow (BM) almost all Thy-1(+) cells, HIS24(+) cells [HIS 24 recognizes the B-cell form of leucocyte common antigen (LCA)], term inal deoxynucleotidyl transferase-positive (TdT(+)) cells and (c + s)k appa(+) cells expressed HIS50 Ag, and all (c + s)mu(+) cells. A presum ably early population of B lymphoid cells, expressing HIS24 Ag without HIS50 Ag, TdT or immunoglobulin (HIS24(+)HIS50(-)TdT(-)lg(-)), consti tuted 1.6% of BM nucleated cells. In blood, one-fifth of mononuclear c ells were HIS50(+), and about 85% of these expressed mu and/or kappa c hains. In spleen, flow cytometry analysis and immunohistology demonstr ated heterogeneous expression of HIS50 Ag: immunoglobulin M (IgM)(brig ht) cells (as found largely in red pulp and marginal zone) were HIS50( bright), while IgM(dull) 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.