The complement system—particularly component C3—has been demonstrated to be a key link between innate and adaptive immunity. The trimolecular complex of complement receptor type 2 (CR2), CD19, and CD81 is known to promote B cell activation when coligated with the B cell Ag receptor. In the present study, we aimed to elucidate the role of human complement receptor type 1 (CR1), the other C3-receptor on B cells. As ligand, aggregated C3 and aggregated C3(H₂O), i.e., multimeric “C3b-like C3”, are used, which bind to CR1, but not to CR2. In experiments studying the functional consequences of CR1-clustering, the multimeric ligand is shown to inhibit the proliferation of tonsil B cells activated with a suboptimal dose of anti-IgM F(ab′)₂. Importantly, this inhibitory activity also occurs in the presence of the costimulatory cytokines IL-2 and IL-15. The anti-IgM-induced transient increase in the concentration of intracellular free Ca²⁺ and phosphorylation of several cytoplasmic proteins are strongly reduced in the presence of the CR1 ligand. Data presented indicate that CR1 has a negative regulatory role in the B cell Ag receptor mediated activation of human B lymphocytes.

Defence against the reactive oxidants produced during aerobic metabolism is a complex process and is provided by a system of enzymes and antioxidant compounds capable of preventing excess radical production, neutralising free radicals and repairing the damage caused by them. Regulation of the antioxidant system must provide sufficient, properly located, antioxidant compounds and enzymes. Damage to this system has been proved to play a role in various disorders. Long-term complications of diabetes mellitus are supposed to be partially mediated by oxidative stress. The authors summarise experimental and clinical investigations in this field and analyse the possible importance of the changes in the antioxidant system in the development of diabetic vascular complications.

Murine cells of the B lymphoblastoid line A20 and concanavalin A-elicited peritoneal macrophages are shown to activate and fix C3 fragments covalently when incubated in fresh, autologous serum under conditions allowing the initiation of the alternative complement pathway. For the detection of cell-bound C3, cytofluorimetry was performed using FITC-labeled F(ab')2 fragments of anti-mouse C3. Cell-bound C3 fragments are not internalized or shed by the cells under culture conditions for at least two hours. When the antigen-presenting capacity of serum-treated cells was tested using various antigens and experimental systems, augmentation of the proliferation of antigen-specific T cells was found. This enhancing effect was particularly pronounced at suboptimal antigen doses. The elevation of T cell proliferation induced by C3-opsonized antigen-presenting cells (APC) could be abrogated by F(ab')2 fragments of goat anti-mouse C3, suggesting the involvement of C3 receptors expressed by T cells in the process. Using the 7G6 mAb recognizing murine CR1/CR2, the presence of these complement receptors on activated T cells is demonstrated by cytofluorimetry and immunoprecipitation, as well. These results point to the role of C3 bound to acceptor sites on APC in the facilitation of antigen presentation, providing a further link between innate and adaptive immunity.