Teleost IgM+ plasma-like cells: beyond antibody secretion.

Upon antigen encounter, B cells start a differentiation process toward antibody-secreting cells (ASCs), initially plasmablasts, and eventually long-lived plasma cells. All these ASCs specialize in secreting important amounts of antibodies and usually lose other functionalities of naïve B cells. This differentiation process is scarcely characterized in teleost fish, in which B cells have been shown to share many functional and phenotypic characteristics of mammalian B1 innate subsets. In this context, we were prompted to investigate further the functionalities of ASCs in teleosts, using rainbow trout (Oncorhynchus mykiss) as a model. Our results demonstrate that IgM+ plasma-like cells in the rainbow trout head kidney exhibit a strong IgM secreting capacity along with phagocytic and antigen-presenting capacities, even higher than those of naïve B cells. These IgM+ plasma-like cells were capable of surviving in vitro for 2 wk secreting IgM. Interestingly, they retained a functional B cell receptor that responded to TNP conjugated to lipopolysaccharide, a thymus-independent model antigen, which also rendered these cells more reactive to B cell receptor crosslinking. These findings shed light on the differentiation process of teleost B cells, demonstrating that teleost plasma-like cells conserve other phenotypical attributes beyond immunoglobulin secretion, being capable of directly responding to antigens. These findings point to an exclusive differentiation process of teleost B cells, which might provide mechanistic insights on how mammalian innate subsets such as B1 cells or IgM-expressing plasma cells differentiate.