CD21(int) CD23(+) follicular B cells express antigen‐specific secretory IgM mRNA as primary and memory responses

CD21(int) CD23(+) IgM(+) mouse follicular B cells comprise the bulk of the mature B‐cell compartment, but it is not known whether these cells contribute to the humoral antibody response. We show using a direct RT‐PCR method for antigen‐specific VH, that FACS‐sorted mouse CD21(int) CD23(+) B cells express specific secretory IgM VH transcripts in response to immunization and also exhibit a memory response. The secretory IgM expressed is distinct from the IgG expressed by cells of this phenotype, which we also analyse here, having a distinct broader distribution of CDR‐H3 sequences and zero or low levels of somatic mutation in the region analysed. These results imply that cells of the CD21(int) CD23(+) phenotype have distinct IgM(+) and IgG(+) populations that contribute directly to the humoral antibody and memory responses by expressing antigen‐specific secretory immunoglobulin. We also argue that the more diverse CDR‐H3 sequences expressed by antigen‐experienced IgM(+) CD21(int) CD23(+) follicular B cells would place them at the bottom of a recently hypothesized memory B‐cell hierarchy.