Mature IgM+ plasma cells sense antigen and develop competence for cytokine production upon antigenic challenge

Dogma holds that plasma cells (PC), as opposed to B cells, cannot bind antigen (Ag) because they have switched from expression of membrane-bound immunoglobulins that constitute the B cell Ag receptor (BCR) to production of the secreted form of immunoglobulins. We have compared the phenotypical and functional attributes of PC generated by the T-cell-dependent and T-cell-independent forms of the hapten NP. We unexpectedly found that the nature of the secreted Ig isotype rather than the chemical structure of the immunizing Ag defines two functionally distinct populations of PC. Fully mature IgM-expressing bone marrow PC retain expression of a functional BCR while their IgG+ counterparts do not. We show that Ag boost modifies the gene expression profile of IgM+ PC and initiates a cytokine production program, characterized by upregulation of CCL5 and IL-10. Our results demonstrate that IgM-expressing PC can sense Ag and acquire competence for cytokine production upon antigenic challenge. Two experimental groups (control and boost) composed of 5 distinct samples each