B lymphocytes from early vertebrates have potent phagocytic and microbicidal abilities

Nature Immunology 7, 1116-1124 (2006)

Speaker: 王壹                                 Time: 14:00~15:00, Nov. 1, 2006

Commentator: 謝奇璋 醫師                      Place: Room 601

Abstract:

Phagocytosis is responsible for uptake and destruction of microbes and accomplished mainly by professional phagocytes, including monocytes, macrophages, and polymorphonuclear cells, but not B cells (1). According to previous studies, bipotent B macrophage progenitors have been identified in adult bone marrow (2). Therefore, the authors want to identify a cell type with common B cells and macrophage features in primitive vertebrates. In this article, the authors demonstrated that B cells from a kind of primitive vertebrate, rainbow trout, have potent phagocytic and microbicidal activity. The authors analyzed peripheral blood leukocytes (PBLs) from rainbow trouts by flow cytometry and observed cellular morphology by Giemsa staining. They found IgM⁺ phagocytic cells with small size, low intracellular complexity, and high nucleus/cytoplasm ratio. These phagocytic IgM⁺ cells express B cell-specific genes. The results from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed that phagocytic IgM⁺ cells can ingest fluorescent beads in the process like professional phagocytes. As professional phagocytes, size, concentration, and opsonization of target particles would influence phagocytic activity of IgM⁺ cells. By using fluorescent red dextran localized at acidic lysosomes, fusion of lysosomes and phagosome vesicles were observed and this indicated that phagocytic IgM⁺ cells have microbicidal activity. Finally, phagocytic IgM⁺ cells are present in Xenopus laevis. Thus, phagocytic capacity of B cells is present in a common ancestor at the time of the phylogenetic split of teleosts from amphibians. This paper supports the idea that there is an evolutionary relationship between mammalian B cells and macrophages.

References:

1.          Aderem, A. et al. Mechanisms of phagocytosis in macrophages. Annu. Rev. Immunol. 17, 593–623 (1999).

2.          Montecino-Rodriguez. et al. Bipotential B-macrophage progenitors are present in adult bone marrow. Nat. Immunol. 2, 83–88 (2001).