Essential role for Cmtm7 in cell-surface phenotype, BCR signaling, survival and Igμ repertoire of splenic B-1a cells.

B-1a cells represent a distinct B cell population with unique phenotype, self-renewing capacity and restricted Igμ repertoire. They primarily locate in body cavity and also exist in spleen. The different subpopulations of B-1a cells are heavily affected by local environment. Our previous studies revealed that MARVEL-domain-containing membrane protein, CMTM7, was involved in B-1a cell development. Here, we focused its influence on peritoneal and splenic B-1a cells. Unlike peritoneal B-1a cells, we found that splenic Cmtm7^-/- B-1a cells expressed higher level of CD5, CD80 and CD86 compared with WT counterparts. They also exhibited an enhanced tonic BCR signals in steady state. Though the cell viability was unaffected in vitro, Cmtm7 knockout markedly promoted splenic B-1a cell apoptosis in situ, which was likely associated with down-regulation of Il-5rα. With regard to Igμ repertoire, peritoneal and splenic Cmtm7^-/- B-1a cells exhibit similar changes exemplified by the loss of V_H11 and gain of V_H12, whereas an increase in V_H1 usage and skewed J segments from J_H1 to J_H2 and J_H4 families could only be detected within splenic Cmtm7^-/- B-1a cells. Overall, these data indicate that Cmtm7 functions differently in peritoneal and splenic B-1a cells and plays a more important role in splenic cells.

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