Human monocyte spreading induced by activated factor B of the complement alternative pathway: Differential effects of Fab′ and F(ab′) 2 antibody fragments directed to C5, C6, and C7

Human peripheral blood mononuclear phagocytes are induced by activated Factor B (Bb) of the complement alternative pathway to undergo morphological shape changes in vitro which have been described as “spreading.” The spreading reaction induced by Bb has previously been shown to depend upon the enzymatic activity of Bb and to be inhibited by Fab′ antibody fragments directed to C5 (but not anti-C3 Fab′). The possibility that Bb may exert its effect on monocytes by initiating assembly of terminal complement complexes comprised of C5b, 6, 7, C5b-8, or C5b-9 was addressed in the present study. The effects were tested of Fab′ and F(ab′) 2 antibody fragments directed to C5, C6, C7, and C8 and to neoantigens expressed in the assembling terminal complement complexes on the monocyte spreading reaction induced by Bb. Differential effects of monovalent Fab′ and divalent F(ab′) 2 antibody fragments were observed. Anti-C5, C6, and C7 Fab′ were found to inhibit the spreading reaction induced by Bb in an immunologically specific manner. Divalent F(ab′) 2 fragments directed to these same proteins (but not to C3, C4, C8, or C9) induced monocyte spreading in the complete absence of Bb or other recognized inducing agents. Monocyte spreading induced by hybridoma immunoglobulin (Ig) directed to C5 and C7 was found to be correlated with the binding of 10 6 molecules Ig per cell. These findings support the notion that C5, C6, and C7 (or an analogous system of cellular proteins) are associated with the surface of human peripheral blood monocytes and that these proteins may play a role in certain reactions by which mononuclear phagocytes are induced to altered states of cellular physiology.