Immunization with immune complex alters the repertoire of antigen‐reactive B cells in the germinal centers

The differentiation of memory B cells in germinal centers (GC) is selectively enhanced upon administration of antigen‐antibody complexes. To characterize the repertoire of this response, we examined the rearranged immunoglobulin heavy chain variable (VH) genes from mouse splenic GC after a single immunization with either antigen, nitrophenyl (NP) hapten coupled to keyhole limpet hemocyanin, or with a preformed complex of antigen with a monoclonal anti‐NP antibody of γ1 isotype. Among antigen‐immunized mice, NP‐reactive GC B cell populations in the antigen‐induced GC consisted mostly of cells expressing the canonical V186.2 gene which contained, on average, 0.8 point mutations/VH gene by day 8 after immunization. These results are indicative of the beginning of somatic hypermutation and consistent with previously published analyses of NP antigen‐driven GC. In contrast, the NP‐specific B cells in GC that were elicited by administration of immune complex represented a heterogeneous cell population expressing nine different germ‐line segments of the V186.2/V3 (J558) gene family, i.e. V23, V24.8, C1H4, V3, CH10, V165.1, V102, V671.5 and V186.2. Moreover, the average frequency of mutations in these genes was 1.7, reaching up to 4 mutations/VH in some GC. Administration of the antigen NP in complex with specific antibody apparently alters the process of interclonal competition in the GC and results in loss of dominance by V186.2+ cells and nearly stochastic representation of diverse clono‐types. These results suggest an important feedback regulation of the B cell repertoire by antibody and indicate a role for immune complexes in the activation of somatic hypermutation.