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.