Ontogeny of human B1 cells.

B1 cells, which are distinct from conventional B cells, are a rare B lymphocyte subpopulation that plays a pivotal role in innate immunity. Extensive previous studies have revealed the functions and ontogeny of murine B1 cells, but the properties of human B1 cells have just begun to be uncovered over the past decade. The phenotype of human B1 cells has recently been proposed, facilitating further studies. Here, we review the latest knowledge on human B1 cells, especially their ontogeny. A previous study using xenotransplantation models showed that human hematopoietic stem cells (HSCs) derived from cord blood or adult bone marrow can produce B1 cells in vivo. A recent study by our group reported that human B1 cells in peripheral blood are derived from adult HSCs and persist for approximately 3 years in situ. These findings suggest that adult human HSCs have the ability to produce B1 cells and contribute to maintenance of the adult B1 cell pool in peripheral blood. Further understanding of human B1 cell functions and ontogeny may elucidate the pathogenesis of B cell malignancies and autoimmune diseases.