Receptors for IgM on human lymphocytes. II. Mitogen-induced modulation of receptor expression

Human peripheral lymphocytes undergoing blastogenesis induced by phytohaemagglutinin (PHA) or concanavalin A did not express receptors for the Fc portion of IgM (RFcmu). Furthermore, treatment of lymphocytes with PHA produced a dose-dependent loss of RFcmu which began as early as 5 hr after exposure to the mitogen. Within 24 hr after the addition of PHA, the percentage of lymphocytes expressing RFcmu had decreased from 65% to 4%. Under the same conditions of treatment, the expression of the receptor for sheep erythrocytes (E) was unchanged. These findings seemed inconsistent with a direct blocking effect of PHA but suggested that PHA induced a time-dependent modulation (switch off) of expression of RFcmu. Pronase cleavage of surface proteins on cells incubated with PHA for 24 hr followed by overnight incubation showed an almost complete irreversibility of RFcmu modulation up to 72 hr later. Studies using T cells isolated by E-rosetting showed that RFcmu modulation predominantly occurred on T cells. The modulation of RFcmu expression is discussed in terms of its possible role in the immune response.