CHARACTERIZATION OF AN IGM FC-BINDING RECEPTOR ON HUMAN T-CELLS

The existence of an IgM receptor on human T cells has been suggested b y T cell rosetting with IgM-coated erythrocytes. In this study we used immunofluorocytometry to demonstrate that 1) after short-term culture , a majority of the T cells can bind IgM at easily detectable levels, 2) human and mouse IgM preparations bind to human T cells but other Ig isotypes do not, 3) the IgM binding is saturable and inhibitable only by Ig of IgM isotype and 4) the Fc portion of IgM is involved in the binding to a protease-sensitive cell surface protein. Biochemical anal ysis of the T cell receptor for IgM reveals a cell surface protein of 60 kDa in comparison with the 58 kDa Fcmu receptors (FcmuR) on B-linea ge cells. Although FcmuR expression is up-regulated after B cell activ ation, the reverse is true after T cell activation. In addition, the T cell FcmuR is relatively resistant to phospholipase C treatment. Thes e results indicate that T- and B-lineage cells express FcmuR with diff erent biologic characteristics.