Polymeric IgA-secreting and mucosal homing pre-plasma cells in normal human peripheral blood

Clear identification of recently activated mucosal B cells in human blood would greatly facilitate study of mucosal vaccines, immune response to infection and the ongoing mucosal IgA response. We examined blood lymphocytes from normal, healthy individuals to identify IgA-secreting pre-plasma cells' (PPC) functional and phenotypic relevance to mucosal antibody production, in the absence of infection, disease or recent vaccination. PPC are the most recently activated B lymphocytes in blood and are considered in transit between lymphoid tissue and effector tissues, where they terminally differentiate into plasma cells. We observed that all IgA-secreting PPC expressed surface IgA (sIgA) and intracellular IgA (iclgA) and secreted primarily polymeric IgA (plgA), as determined by flow cytometry, ELISPOT and size exclusion chromatography. A large sub-population of PPC in blood expresses the mucosal chemokine receptor CCR10 and contains the largest fraction of sIgA and iclgA PPC that secrete plgA. The majority of CCR10(+) PPC expresses high levels of Ki67, indicative of recently activated blasts. In contrast, most CCR10(-) PPC secrete IgG, but a small population secretes plgA and stains for iclgA. The mucosal integrin alpha(4)beta(7) was detected on a subset of PPC, but this subset did not account for all CCR10(+) PPC or all PPC with sIgA expression. Our data clearly demonstrate that PPC defined by surface expression of CD19, CD27(hi), IgA and CCR10 secrete only plgA and are the dominant mucosal PPC subset in human blood. These mucosal PPC can now be investigated routinely as indicators of recent human mucosal IgA responses.