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Natural antibody and complement-mediated antigen processing and presentation by B lymphocytes.

Normal immune responses to primary protein Ags (those not seen previously by the immune system) have been shown to require C3 and the C3 receptor CR2 (CD21). This investigation tested the hypothesis that natural Abs to the primary protein Ag keyhole limpet hemocyanin (KLH) exist in normal serum and will form immune complexes (IC) that activate C and generate bound iC3b/C3dg, thereby promoting B cell CR2-dependent Ag processing. Both IgM and IgG anti-KLH were detectable in sera from 11 normal donors. IC generated with fresh serum bore iC3b and bound to CR1 (CD35), CR2, and CR3 (CD11b/CD18). Further treatment of IC with serum containing rCR1 formed IC-bearing C3dg that bound only to CR2. When ICs were mixed with B lymphoblastoid cell clones, CR2-dependent processing of the KLH occurred that was dependent on bound C3dg and CR2. Processing occurred regardless of whether the B cells bore KLH-specific surface Ig, although the efficiency of processing was greater with KLH-specific B cells. Both KLH-specific and nonspecific B cell clones presented KLH to KLH-specific T cells. The binding of KLH IC by normal B lymphocytes induced expression of the B7/BB1 Ag (CD80), the required co-stimulatory ligand for T cell CD28. Blocking experiments indicated that although bound C3 and CR2 were required to mediate IC binding to B cells, induction of CD80 expression required the secondary ligation of IC-associated IgG to B cell FcRII (CD32). These data support the hypothesis that responses to primary protein Ags involve IgG natural Abs and C3 that mediate Ag processing and presentation via B cell CR2 and FcRII.