Germinal center B cells in Peyer's patches of aged mice exhibit a normal activation phenotype and highly mutated IgM genes.

Systemic and mucosal humoral immune responses have been reported to exhibit an age related decline. However, differences in primary and memory responses at mucosal sites in aged mice have been noted. In an effort to begin characterizing deficiencies in the mucosal system of aged mice, we examined the B cell compartment of gut associated Peyer's patch lymphoid tissue. To our surprise, we found that germinal center (GC) B cells from aged B6D2F1 mice (24-26 months) were present at similar frequencies and exhibited a normal activation phenotype such as upregulation of B7.1, B7.2 and CD44, and downregulation of CD23, CD62L and CD38 as that observed in younger mice (2.5-4 months). As expected, Peyer's patch GC B cells from aged mice expressing V(H)X24 genes displayed higher somatic mutation frequencies compared with younger mice. However, this was particularly striking in IgM sequences where high mutational loads suggested we were sampling memory cells. It is conceivable that B-cells expressing these genes reflect the presence of a mucosal memory compartment in aged mice that either retains flexibility in effector function or is committed to the secretion of IgM antibody.