Recruiting memory B cells with changed antigenic specificity.

During T cell-dependent antibody responses, the V region genes of responding B lymphocytes are physiologically mutated at a high rate. An intense selection process expands subclones of B cells producing mutant antibodies that bind Ag optimally. This implies that most mutant B cells and their antibody products are unselected and not often observed by conventional hybridoma or serum sampling procedures. Herein we show that the pool of mutant B cells includes unselected members that have acquired new antigenic specificities. Mice were immunized with a haptenated carrier protein to recruit and somatically diversity hapten-specific B cells producing antibodies with a defined V region bearing a major idiotype. During the primary immune response, the mice were given booster injections with a second related hapten conjugated to the same carrier. Hybridomas were isolated that produced idiotypic antibodies binding the second hapten but not the first. V gene sequencing analyses conclusively demonstrated that one of these was derived from a precursor B cell that expressed the defined unmutated V region with specificity for the first hapten. Sequence of the V genes expressed by the remaining six hybridomas supported this interpretation. In essence, single B cell clones were mutationally diversified to include members that had lost an original antigenic specificity while acquiring a new one, and the mutants were recruited into the memory compartment by antigenic selection. These results support the view that selection processes in vivo normally reveals only a small fraction of a mutationally diversified B cell clone. They also suggest a potential route by which antibodies of differing antigenic specificities can be generated from a single B cell clone of predefined origin and antigenic specificity.