Interaction of human lymphocytes with fluid phase human C3b detected by immunofluorescence

Human lymphocytes obtained from tonsils and peripheral blood were found to bind human fluid phase C3b, obtained by trypsin treatment. This binding was detected by indirect immunofluorescence (IIF) using specific anti-C3 antisera. Lymphocytes isolated from thymus tissue scored low percentages in IIF, indicating that the main population of thymus-derived lymphocytes are T cells. The distribution pattern of C3b-binding cells was compared with that of cells forming rosettes with sheep erythrocytes coated with antibody and complement (EAC) and with sheep erythrocytes (E) only, as well as with that of Ig-bearing lymphocytes, as detected by direct immunofluorescence. It appeared that the distribution pattern of lymphocytes which can bind fluid phase C3b is similar to that of EAC rosette-forming and of Ig-bearing lymphocytes. Pre-incubation of the lymphocytes with C3b and pretreatment of the cells with trypsin decreased the capacity to form rosettes and to bind C3b to their surface. Human monocytes granulocytes and erythrocytes did not bind fluid phase C3b, as judged by IIF. Therefore, the selective binding of fluid phase C3b to lymphocytes provides a specific method for the detection of complement-reactive lymphocytes in lymphoid cell preparations.