Characterization of B Cell Responses in Rainbow Trout (Oncorhynchus Mykiss) Affected by Red Mark Syndrome

Abstract Red mark syndrome (RMS) is a disease affecting rainbow trout. Although the precise etiology of this disease is still under debate, a Midichloria‐like organism (MLO) is suspected as the triggering agent. RMS provokes characteristic skin lesions. Previous investigations pointed to a local immune response characterized by a B cell influx. To elaborate on these findings, here, this study extensively characterizes systemic and local B cell responses in RMS‐affected fish. The local influx of IgM+ B cells to the skin lesions is confirmed, and a differentiation of these B cells to plasma‐like cells is demonstrated. IgM repertoires suggested a polyclonal activation of local B cells and some trafficking between skin and head kidney. Finally, the fact that transcripts of the gene coding for MLO 16S rRNA are found in sorted skin IgM⁺ B cells and non‐IgM⁺ skin leukocytes reveals the capacity of this intracellular bacterium to be internalized by B cells, where it may possibly directly interfere with intracellular pathways. The data significantly advances the knowledge on RMS and provides another example of a fish pathology in which fish B cells seem pivotal, possibly because of the specific ways in which teleost B cells intrinsically sense and respond to pathogens.