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 IgMB cells and non-IgMskin 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.