Studies on the mechanism of the complement-mediated inhibition of the Fc and C3 receptors of B lymphocytes

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.

The selective localization of B lymphocytes in the spleen and the role of complement receptors

The role of complement receptors on the localization of T and B cells in the spleen of mice was studied using short-term homing experiments in cobra venom factor (CoF)-treated animals. The localization ratio of B and T cells in the spleen of CoF-treated mice decreased significantly compared to control recipients. No changes could be found in the relative distribution of resident T and B cells in the spleen or other lymphoid organs of CoF-treated animals and when their spleen or lymph node cells were transferred, the localization pattern was normal. When cells were incubated in serum prior to transfer a disturbed localization ratio in the spleen of untreated recipients was observed. This was due to a blockade of complement receptors as determined by the inability of the incubated cells to form EAC rosettes. No blockade of EAC rosettes and no changes in localization ratios upon transfer could be observed when the cells were incubated in functionally C3-depleted serum. The results suggest a role for the complement-receptor on B cells in the initial localization in the spleen, whereas no influence upon the selective localization in high endothelial venules-bearing organs was found.

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.

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

The binding of nascent human C3b (i.e. the fragment of C3 just after trypsin cleavage) to mouse peritoneal macrophages was demonstrated by immune adherence. Acceptor-bound C3b could be detected longer than 24 h on the cell membrane. The rosette formation and phagocytosis of SRBC coated with anti-SRBC rat IgG was inhibited by preincubation of the cells with C3 and trypsin (15 min, 37 degrees C). However, the phagocytosis of opsonized yeast particles was not influenced by acceptor-bound C3b, proving that C3b-C3b acceptor interaction did not alter the function of C3b-receptors. Acceptor-bound C3b on the macrophages failed to mediate phagocytosis of human 0,Rh+ red cells having C3b-receptors.

Pathopysiological aspects of immune complex diseases. Part II. Phagocytosis, exocytosis, and pathogenic depositions

Elimination of IC by the phagocytic system occurs mainly by macrophages and contrarotates to the pathogenic effect. Decisive to prevent systemic IC disease is the capacity of the phagocytic system. In the case of its saturation, the danger of the occurrence of IC disease is greatly enhanced. Conclusive evidence seems to exist that IC of extremely small or extremely high lattice structure (precipitates) are less pathogenic than soluble IC of medium network. Small IC in extreme antigen and antibody excess or precipitates exhibit a reduced complement activating potency. Small IC in extreme antigen or antibody excess hardly interact in vitro with membrane receptors and do not induce IC disease when injected or formed in vivo. Highly lattices IC, especially precipitates, are eliminated extremely quickly from the circulation, mainly by macrophages and there deposition in the kidney is significantly reduced. Thus, lack of quality of the antibody to precipitate the antigen and a reduced capacity and effectivity of the phagocytic system to eliminate the IC may be extremely important in the generation of IC diseases. Facing the overwhelming and partly even inconsistant data of this topic, one may doubt whether IC diseases may be regarded to be a defined and coherent disease. Too many variables and questions exist concerning the nature of the antigen, especially in tumor and autoimmune diseases, concerning the quality of the antibody and the characteristics of the pathogenic IC and concerning localization and the elimination process. Nevertheless, common pathophysiological pathways of IC diseases may be recognized.