Molecular Characterization of Diverse Mechanisms of Mucosal Immunity in Teleost Fishes

Abstract

The mucosal barriers (gill, skin and intestine) of fish constitute the first line of defense against aquatic pathogens invasion. By exploring and targeting fish mucosal immune system, we can rationally design better vaccines and immunostimulants to maximize fish health and prevent infectious diseases. Flavobacterium columnare, the causative agent of columnaris disease, causes ulcerative, necrotic, ectopic infection resulting in tremendous losses in farmed fish globally. A recently developed mucosal vaccine has been demonstrated to provide superior protection for channel catfish (Ictalurus punctatus) against columnaris. I examined the mechanisms of this protection by comparing transcriptional responses to F. columnare challenge in vaccinated and unvaccinated control catfish. Transcriptome profiling at early time points post F. columnare infection revealed a basal polarization in vaccinated fish gills and a central role for eosinophilic granular cells. Vaccinated fish were armed with an array of preformed mediators by vaccination, but had lower expression of pro-inflammatory genes after secondary infection. The rhamnose-binding lectin (RBL1a) was previously identified by our group as a potential mediator of F. columnare adhesion and exposure of fish to its carbohydrate ligand, L-rhamnose, prior to challenge decreased pathogen adherence and protected catfish from columnaris mortality. However, rhamnose is prohibitively expensive. Therefore, in my second study, I examined whether rhamnolipids (RLs), an affordable and commercially available alternative, would offer host protection against F. columnare similar to that provided by L-rhamnose through feeding trials. Dietary RLs studied here increased susceptibility of channel catfish to columnaris disease, associated with a robust upregulation of RBL1a expression immediately following challenge and a suppression of mucin and lysozyme production. In my final project, considering the importance of RBL in fish immunity, I identified four putative RBL genes from Nile tilapia (Oreochromis niloticus) and characterized their expression profiles. They were significantly expressed by exposure to Streptococcus agalactiae, another important fish pathogen and one of the major causes of streptococcosis in farmed tilapia, in at least one tissue (gill or intestine) or time point. Taken together, my research will expand our knowledge of fish mucosal immunity and contribute to the development of effective immunotherapies for disease prevention.