C3b acceptors on macrophages: inhibition of Fc gamma-receptor-mediated phagocytosis by acceptor-bound C3b

Appearance of acceptor-bound C3b on HLA-DR positive macrophages and on stimulated U937 cells; inhibition of Fc gamma-receptors by the covalently fixed C3 fragments

The appearance and the functional role of acceptor-bound C3b during differentiation of human monocytes into macrophages were studied. Acceptor-bound C3b could be detected by the immune adherence (IA) test parallel to the expression of antigenic determinants specific to mature cells--i.e. on days 4-5 of culture. Consequently, the capacity of these phagocytes to fix C3b covalently via C3b-acceptors (C3bAs) can be considered as one of the signs of their activation/differentiation. All the mature macrophages positive in the IA test were also found to express HLA-DR antigens on their membrane. Using solubilized extracts of stimulated, 35S-cysteine-labelled cells of the human monocytic cell line, U937, we demonstrate that C3 synthesized by these cells can bind to C3bAs of the same cells. Covalently fixed C3 fragments were found to inhibit Fc gamma-receptor-mediated ingestion of immune complexes and also antibody-dependent cellular cytotoxicity of monocyte-derived macrophages.

Effector or target cell selection mediated by C3 bridges

Potential effector cells (including stimulated lymphocytes and cultured monocytes) and potential target cells of NK and AK type cytotoxic reactions (including several lymphoblastoid cell lines) cleave the third complement component (C3). As a result of expression of C3bA sites such cells are able to bind covalently the activated C3b through its metastable binding site and thereby become "armed" by the C3b. This permits C3b-bridge formation between these cells and CR1-bearing cells. The "effector selection" (i.e. when C3b is bound covalently to potential target cells) or "target selection" (when C3b is covalently bound to C3bA sites on potential effector cells) mediated by C3b bridges results in enhanced killing capacity. Macrophages activate and bind C3b as well; but the covalent binding of C3b by these cells inhibits Fc receptor mediated ADCC type killing.

A possible self-regulating mechanism mediated by C3b-acceptor-bound C3b generated by stimulated macrophages

Macrophages have been shown to produce C3 and to bear Fc receptors (FcR), and besides the various C3 receptors, they possess C3b acceptors (C3bA) as well as surface proteases capable of cleaving C3. Using the immune adherence method, we demonstrated that the amount of covalently fixed (i.e., C3bA-bound) C3b is markedly increased upon cell stimulation by phorbol myristate acetate or aggregated IgG, even in the absence of C3. The enhancement of nascent C3b (C3bx) binding to C3bA on these cells could be reversed by inhibiting the process at different stages, using either cycloheximide, phenyl-methyl-sulphonyl-fluoride, salycil hydroxamic acid, or methylamine. On the basis of our present results and earlier results, we propose a self-regulatory mechanism by which activated, C3-producing macrophages cleave C3 by their surface proteases. C3bx generated in this way fixes covalently to C3bA of the producer cells, resulting in the inhibition of FcR on these cells.

Functional cooperation of C3b-acceptors, Fc gamma-receptors and cell-surface proteases on macrophages

Macrophages are FcR-positive cells, synthetize complement components and express proteolytic enzymes on their surface. In this paper a functional cooperation of C3b acceptor (C3bA) sites, which bind covalently nascent C3b molecules via their metastable binding site, IgG FcRs and cell surface proteases are described and the possible importance of this cooperation in regulation of immune response is discussed. It was found that isolated monocytes did not express C3bA in contrast to cultured macrophages which showed immune adherence positivity. Stimulation of macrophages resulted in enhanced expression of C3bA. C3 synthetized by macrophages was shown to be cleaved by cellular proteases which resulted in the binding of nascent C3b to C3bA. C3bA-nascent C3b interaction inhibited FcR-dependent effector functions, such as immune complex phagocytosis and antibody-dependent cellular cytotoxicity.