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The defense system of teleost fish organized on innate and adaptive immunity protects them against a wide variety of pathogenic microorganisms in the aquatic environment. Phagocytosis is one of the most effective defense strategies against microbial challenge mainly performed by classical 'professional' phagocytes (including monocytes, macrophages and granulocytes). They contain, kill and process the internalized pathogens for antigen presentation by providing antigenic ligands to initiate activation and clonal expansion of T and B cells, which bridge the innate and adaptive immunity. The discovery of phagocytic B cells in teleost fish has broken the paradigm that primary vertebrate B cells are lack of phagocytosis of particulates, as well as led to the investigation of phagocytic activity of mammalian B-1 B cells. The active phagocytic, microbicidal capabilities and antigen presentation in teleost phagocytic B cell have demonstrated to be similar as professional phagocytes, providing a potential impact on development of new vaccination strategies to prevent and control infectious diseases. In this review, we aim to address current progress on the antimicrobial role of phagocytic B cells in teleost fish by comparing it with other professional phagocytes and mammalian B-1 B cells, and provide the application prospect of phagocytic B cells in developing vaccines as well as the prevention of fish diseases. [ABSTRACT FROM AUTHOR]

Plasma cells are terminally differentiated antibody-secreting B lymphocytes that contribute to humoral immunity by producing large numbers of antibodies. Increasing evidence suggests that teleost fish B cells share certain characteristics with mammalian B1 B cells, including antibody-secreting, phagocytic, and antigen-presenting capacities. However, the difference between mature B cells and plasma cells remains unclear. In this study, we found that, based on their light-scattering characteristics, tilapia anterior kidney (AK) leukocytes can be categorized into two IgM+ B-cell subsets: the lymphoid (L) gate and granulocyte–monocyte/macrophage (G-M) subsets. G-M gate cells are more numerous than L-gate cells and have higher mean fluorescence, but lower forward scatter and side scatter. We analyzed the morphological and ultrastructural features of sorted IgM+ cells and found that L-gate IgM+ cells have a high nucleus–cytoplasm ratio and lymphocyte-like morphology, whereas G-M gate IgM+ cells have a small nucleus, more abundant endoplasmic reticulum, and a larger number of mitochondria, and have a plasma cell-like or macrophage-like morphology. To further characterize the cell types, we examined the specific patterns of expression of B-cell- and T-cell-related genes. We found that B-cell-specific genes were expressed by both L-gate and G-M gate IgM+ cells, and that G-M gate IgM+ cells secreted extremely high levels of IgM. However, T-cell-related genes were highly expressed only in L-gate IgM– cells. These results suggest that G-M gate IgM+ cells are similar to plasma-like cells, with high antibody-secreting capacity. Given that G-M gate cells include the granulocyte, monocyte, and macrophage cell types, but not B cells, monocyte/macrophage markers were used to investigate the cell types further. A macrophage receptor with a collagenous structure was frequently observed, and macrophage-expressed gene-1 was highly expressed, in the G-M gate IgM+ cells. Phagocytic capacity, as determined by ingestion of beads or bacteria, was significantly higher in G-M gate IgM+ cells than in L-gate IgM+ cells, as was antigen-processing capacity. Our findings show that tilapia AK leukocytes can be divided into two IgM+ B-cell subsets and that G-M gate IgM+ cells resemble plasma-like cells, having high antibody-secreting, phagocytic, and antigen-presenting capacities. Thus, this study increases our understanding of the functions of teleost fish plasmalike cells. [ABSTRACT FROM AUTHOR]