Basal polarization of the mucosal compartment in Flavobacterium columnare susceptible and resistant channel catfish (Ictalurus punctatus).

The freshwater bacterial pathogen, Flavobacterium columnare, infects a variety of ornamental and farmed fish species worldwide through mucosal attachment points on the gill and skin. While previous studies have demonstrated a chemotactic response of F. columnare to fish mucus, little is known about how host gill mucosal molecular and cellular constituents may impact rates of adhesion, tissue invasion, and ultimately, mortality. Here, we describe the use of RNA-seq to profile gill expression differences between channel catfish (Ictalurus punctatus) differing in their susceptibility to F. columnare both basally (before infection) and at three early timepoints post-infection (1 h, 2 h, and 8 h). After sequencing and de novo assembly of over 350 million 100 base-pair transcript reads, between group comparisons revealed 1714 unique genes differentially expressed greater than 1.5-fold at one or more timepoints. In the large dataset, we focused our analysis on basal differential expression between resistant and susceptible catfish as these genes could potentially reveal genetic and/or environmental factors linked with differential rates of infection. A number of critical innate immune components including iNOS2b, lysozyme C, IL-8, and TNF-alpha were constitutively higher in resistant catfish gill, while susceptible fish showed high expression levels of secreted mucin forms, a rhamnose-binding lectin previously linked to susceptibility, and mucosal immune factors such as CD103 and IL-17. Taken together, the immune and mucin profiles obtained by RNA-seq suggest a basal polarization in the gill mucosa, with susceptible fish possessing a putative mucosecretory, toleragenic phenotype which may predispose them to F. columnare infection.