BINDING OF SOLUBLE IMMUNE-COMPLEXES TO RAJI LYMPHOCYTES - ROLE OF RECEPTORS FOR COMPLEMENT COMPONENTS, C1Q AND C3-C3B

Abstract

Although the binding of particulate antigen-antibody complement [c] complexes such as EAC [erythrocyte antibody complement complex] to [human Burkitt''s] lymphoblastoid Raji cells is mediated largely through receptors for C3b, the binding of C-containing soluble complexes such as those prepared with aggregated human Ig[immunoglobulin]G (AHG) occurs also via receptors for C1q. Evidence supporting this conclusion included the following points: binding of AHG to Raji cells occurred after incubation in EDTA serum; binding of AHG does not occur in C1q-deficient EDTA serum but did occur after addition of C1q; the extent of binding of AHG in EDTA serum was a function of the amount of C1q present; Raji cells bound up to 5.4 .times. 105 molecules of 125I C1q per cell which could be blocked by unlabeled C1q; AGH preincubated with C can bind to a [human] T[thymus-derived]-cell line MOLT, which lacks receptors for C3b but possesses receptors for C1q to the same extent as Raji cells; immunoassays for immune complexes in human sera yield similar results whether Raji cells, MOLT cells or C1q precipitation is used for assay; and EAC-Raji cell rosettes can be inhibited with inulin-treated, C1q-deficient serum containing C3b or C3d, whereas binding of AHG or immune complexes in patient samples to Raji or MOLT cells is not inhibited by this reagent. Receptors for C1q on certain B [bone marrow-derived] and T lymphocytes may play an important role in physiologic functions of lymphocytes depending on binding of soluble immune complexes to their surfaces.