Characterization of B cell responses in rainbow trout (Oncorhynchus mykiss) experimentally exposed to red mark syndrome (RMS)

Introduction: Red mark syndrome (RMS) is a disease that affects farmed rainbow trout throughout Europe. Although the etiological agent of RMS has not yet been isolated, the disease is thought to be caused by a Midichloria-like organism (MLO). RMS causes no or low mortalities in the affected stocks but is characterized by red hyperaemic skin lesions, which lead to economic losses due to downgrading. Previous immunohistochemistry and transcriptomic studies pointed to a B cell dominated local immune response, which potentially could be involved in the pathology.

Methodology: In the current work, we further investigated the B cell response to RMS. For this, skin leukocytes were isolated from experimentally infected fish and non-infected controls and the presence of B cells studied by flow cytometry. The number of B cells secreting IgM in the skin was also determined by ELISPOT. Finally, a transcriptomic analysis of a panel of genes related with B cell function was also carried out in the skin as well as in systemic immune organs (spleen and head kidney).

Results: Flow cytometry confirmed that the presence of IgM+ B cells greatly increased in the skin lesions of rainbow trout affected by RMS. Additionally, we established that the number of cells secreting IgM locally also significantly increased, suggesting a local differentiation of B cells to plasmablasts / plasma cells. The gene expression pattern obtained in the skin further confirmed this local differentiation of B cells in RMS lesions. Finally, some changes in B cell associated genes in spleen and head kidney suggested that B cells were also affected to some degree in systemic immune organs.

Conclusions: Our results confirm the prevalence of B cell responses in RMS-related skin lesions and further demonstrate an associated local differentiation of B cells to plasmablasts / plasma cells in the affected fish.