Co-ordinated expression of lymphoid and myeloid specific transcription factors during B-1b cell differentiation into mononuclear phagocytes in vitro.

We previously demonstrated that B-1b cells can undergo differentiation to acquire a mononuclear phagocyte phenotype upon attachment to substrate in vitro. Here we followed the expression of surface markers and transcription factors during this differentiation. B-1b cells spontaneously express both myeloid and lymphoid restricted transcription factors. When induced to differentiate into a phagocyte, the lymphoid genes E box protein (E2A), early B-cell factor (EBF), paired box 5 (Pax5) are down-modulated, while expression of genes related to myeloid commitment is sustained. Furthermore, B-1b cell-derived phagocytes (B-1CDPs) decrease immunoglobulin M (IgM) expression but retain the expression of the heavy chain variable gene VH11 or VH12, an immunoglobulin gene rearrangement predominantly expressed by B-1 cells. The maintenance of lymphoid characteristics in B-1CDPs characterizes a unique type of phagocyte, not related to monocyte-derived macrophages.