They are positive for B-cell markers and CD38, negative for CD138 (a marker for plasma cells; [45]) and exhibit low PNA-binding activity. Histology revealed the average number of IgG1+ memory B cells in splenic follicles to be comparable between wild-type and mutant mice with conditional deletion of Bcl6 in B cells, consistent with the results of the flow cytometric analysis. These results suggest that in the spleen both unmutated and mutated memory B cells mostly localize to B-cell follicles. In wild-type mice, large numbers of GC B cells accumulate mutations in their Ig

VH genes by day 7 after immunization Palbociclib [2]. In parallel with GC development, studies of the frequency of mutated VH genes among IgG1 memory B cells showed an increase from ≤5% at day 7 to 50–60% at day 40 after immunization. There was no significant increase in the frequency of mutated cells in the memory B-cell population from day 40 to day 100 after immunization [2]. This observation supports the notion that memory B cells that develop independently of GCs within the first week of the response are maintained for a long period, and

then are joined by mutated GC B-cell progeny as the immune response progresses. Despite the recruitment of substantial numbers of GC B-cell progeny into the memory compartment over time, the absolute numbers of memory cells were similar between normal wild type mice and mice in which the GC response was ablated by Bcl6 U0126 deletion. These results indicate that the splenic environment has a limited capacity to sustain memory B cells. We speculate that GC-dependent and -independent memory B cells compete for hypothetical “niches” in the spleen for survival. It seems unlikely that competition for mafosfamide antigen is the determining factor in this process, since memory B cells persist over a long period in the apparent absence of immunizing antigen,

under competitive conditions [44]. The postulated niches for memory B cell maintenance in peripheral lymphoid organs may thus serve some trophic function. For over two decades, GCs have been considered to be the sole site for memory B-cell generation. Recent studies challenge this dogma and demonstrate that memory B cells can also develop before the onset and independently of the GC reaction (Fig. 2). How important are such low-affinity memory B cells and their immune response for protective immunity? We have recently shown that IgG1 memory B cells proliferate in response to antigen re-exposure and accumulate somatic mutations in their rearranged VH genes [10], regardless of whether they express unmutated or mutated VH genes. In this process, unmutated memory B cells generate large numbers of progeny expressing somatically mutated antibodies with high affinity for the antigen to which they responded.