Ia ANTIGENS ON MURINE LYMPHOID CELLS: DISTRIBUTION, SURFACE MOVEMENT AND PARTIAL CHARACTERIZATION *

SUMMARY Antisera directed against the I (immune response gene) segment of the murine major histocompatibility complex were prepared, and the distribution of lymphoid cells binding the antisera studied by radioautography. A clear bimodal distribution of grain counts was observed in lymph node and spleen. Bone marrow lymphocytes showed heavy but heterogeneous labelling, while thymocytes were lightly but definitely labelled. Approximately 30% of lymph node cells and 60% of spleen cells were heavily labelled. The heavily labelled cells were shown to be B cells. Ia antigens are present on plaque forming cells, but probably in smaller amounts than on resting B cells. One plasma cell tumour was studied, and was found to possess little or no Ia antigen. There was no demonstrable labelling of thoracic duct lymphocytes from preparations of T cells activated to histocompatibility antigens. However, several T cell lymphomas probably possess Ia antigens, since they were rejected when transplanted into mice differing at the I region. Overall, the data indicate that Ia antigens are expressed on at least some T cells. The Ia antigens were shown to move independently of surface immunoglobulin on B cells, demonstrating the non-identity of these molecules. Pretreatment of B cells with anti-immunoglobulin did not inhibit binding of anti-Ia sera. The Ia antigens could be capped by a sandwich technique in 75% of labelled cells. The Ia antigens were specifically precipitated from lysates of surface radioiodinated spleen cells, and the products analysed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. A single specific peak of radioactivity corresponding to a molecular weight of approximately 30,000 was found. The significance of these results in relation to immune response genes, receptors for antigen on T cells, and receptors for the Fc portion of IgG on B cells, is discussed.