Missing the target: DNAk is a dominant epitope in the humoral immune response of channel catfish (Ictalurus punctatus) to Flavobacterium columnare

• Exposure to Flavobacterium columnare elicited a humoral immune response in control and immunized catfish.
• The majority of antibodies generated in the catfish serum or the skin explants were to extracellular F. columnare antigens.
• Catfish antibodies generated against F. columnare are cross reactive to DNAk proteins in gram negative bacteria.
• DNAk is the dominant protein antigen in the channel catfish B-cell response to F. columnare.

Vaccination remains a viable alternative for bacterial disease protection in fish; however additional work is required to understand the mechanisms of adaptive immunity in the channel catfish. To assess the humoral immune response to Flavobacterium columnare; a group of channel catfish were first immunized with F. columnare LV-359-01 cultured in iron-depleted media, before being challenged with wild type F. columnare LV-359-01. The immunization protocol did not confer increased protection against F. columnare; however both control and immunized responders generated serum and skin IgM antibodies against F. columnare proteins. Western blot analyses of individuals from both groups showed that IgM antibodies were generated to the same 70 kDa extracellular protein, which was identified to be the bacterial chaperonin protein DNAk. Antibodies generated were cross reactive to DNAk proteins found in other gram negative bacteria. Our data suggests that DNAk is the dominant epitope in the channel catfish B-cell response to F. columnare.