Functional characterization of IgM+ B cells and adaptive immunity in lumpfish (Cyclopterus lumpus L.)

• IgM+ B cells had phagocytic ability and were the predominant phagocytes in blood.
• Small agranular IgM− cells and monocytes had highest phagocytic capacity among PBL.
• Immunoglobulin M isolated from serum is tetrameric.
• Adaptive immunity was verified by detecting specific antibodies upon immunization.

The innate immune responses in lumpfish (Cyclopterus lumpus L.) have been shown to be functional, but little is currently known about the B cells, immunoglobulins or adaptive immune responses in this species. We have used anti-IgM antiserum to isolate B cells and compared them morphologically and functionally with other cell types. The fraction of IgM+ cells among isolated peripheral blood leukocytes (PBL), head kidney leukocytes (HKL) and spleen leukocytes (SL) was in the range of 40%, 12% and 34%, respectively. The IgM+ B cells had high phagocytic ability and were the predominant phagocytes in blood with higher capacity than IgM+ B cells in HKL. Interestingly, among PBL, the most potent phagocytes were, in addition to monocytes, some small agranular uncharacterized IgM− cells. The IgM+ B cells were positive for acid phosphatases (AcP), but negative for myeloperoxidase (MPO). Neutrophils were positive for MPO, while monocytes/macrophages and dendritic-like cells stained negatively. Monocytes/macrophages and the small, agranular IgM− cells stained most strongly positive for AcP corresponding to their high phagocytic capacity. Further, the ability to produce specific antibodies upon immunization verified adaptive immunity in the species.The high proportion of phagocytic IgM+ B cells and their phagocytic ability indicate a significant role of phagocytic B cells in lumpfish innate immunity. The present analyses also give strong indications that vaccination and immunostimulation of farmed lumpfish can be used to prevent disease and mortality caused by pathogenic organisms.

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