CD300a is differentially expressed among B cell subsets, although its expression in immunoglobulin (Ig)M^{+} B cells is not well known. We identified a B cell subset expressing CD300a and high levels of IgM (IgM^{hi}CD300a^{+}). The results showed that IgM^{hi}CD300a^{+} B cells were CD10^{-}CD27^{+}CD25^{+}IgD^{lo}CD21^{hi}CD23^{-}CD38^{lo}CD1c^{hi}, suggesting that they are circulating marginal zone (MZ) IgM memory B cells. Regarding the immunoglobulin repertoire, IgM^{hi}CD300a^{+} B cells exhibited a higher mutation rate and usage of the IgH-VDJ genes than the IgM^{+}CD300a^{-} counterpart. Moreover, the shorter complementarity-determining region 3 (CDR3) amino acid (AA) length from IgM^{hi}CD300a^{+} B cells together with the predicted antigen experience repertoire indicates that this B cell subset has a memory phenotype. IgM memory B cells are important in T cell-independent responses. Accordingly, we demonstrate that this particular subset secretes higher amounts of IgM after stimulation with pneumococcal polysaccharides or a toll-like receptor 9 (TLR9) agonist than IgM^{+}CD300a^{-} cells. Finally, the frequency of IgM^{hi}CD300a^{+} B cells was lower in people living with HIV-1 (PLWH) and it was inversely correlated with the years with HIV infection. Altogether, these data help to identify a memory B cell subset that contributes to T cell-independent responses to pneumococcal infections and may explain the increase in severe pneumococcal infections and the impaired responses to pneumococcal vaccination in PLWH.
【저자키워드】 IgM, HIV, memory, B cell, CD300a, IGD, marginal zone, pneumococcus, CD300, polysaccharides.,