Factor B Binding to Lymphoid Cells

A receptor for fluid phase human complement (C) components C3 and C3b (1, 2) was found on the surface membranes of human lymphoblastoid cells. No other C proteins or serum factors were needed to mediate binding of C3 or C3b to this receptor. In studying its biological importance, we demonstrated that Raji cells cultured in medium containing fresh normal human serum (NHS) 1 and cobra venom factor (CoF) were lysed (1). Results were similar when C3b-bearing Raji cells were cultured in medium with fresh NHS. In other experiments, lysis was eliminated by inactivating serum factor B. 2 Therefore, we concluded that C3b bound to the Raji cells activated the C system through C3b-dependent C3 convertase (3, 4) which resulted in fixation of the later C components directly on the cell membranes, thereby leading to their damage and lysis. Joseph et al. (5) observed that immune human serum, containing antimeasles virus antibody and C, lysed HeLa cells infected with measles virus via activation of the alternative C pathway (6). By using immunofluorescence, properdin (P) was detected on the surfaces of the measles virus-infected HeLa cells previously incubated with the immune serum (Perrin, L., and M. B. A. Oldstone, unpublished observations). Therefore we questioned, do components of the P system bind to cells, and if so, what events lead to their fixation on the cell surface? We now show that P, as well as factor B, can bind to surfaces of certain lymphoblastoid cells and of human B type peripheral lymphocytes. The fixation