More than just antibodies: protective mechanisms of a muscosal vaccine against fish pathogen Flavobacterium columnare

Authors: ZHANG, DONGDONG - Auburn University; THONGDA, WILAWAN - Auburn University; LI, CHAO - Qingdao Agricultural University; ZHAO, HONGGANG - Auburn University; Beck, Benjamin; MOHAMMED, HAITHAM - Auburn University; ARIAS, COVADONGA - Auburn University; PEATMAN, ERIC - Auburn University

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2017
Publication Date: 10/5/2017
Citation: Zhang, D., Thongda, W., Li, C., Zhao, H., Beck, B.H., Mohammed, H., Arias, C., Peatman, E. 2017. More than just antibodies: protective mechanisms of a muscosal vaccine against fish pathogen Flavobacterium columnare. Fish and Shellfish Immunology. 71:160-170.

Interpretive Summary: Columnaris disease caused by Flavobacterium columnare continues to be a global problem in farmed fish, particularly in the U.S. catfish industry. A recently developed vaccine (17-23) for columnaris has been shown to provide superior protection for channel catfish against this costly disease. We were interested in determining the mechanisms of this protection by comparing the molecular immune responses to an experimental columnaris infection (a “challenge”) in vaccinated and unvaccinated juvenile catfish. In this study, two month old fingerling catfish (28 days post-vaccination or unvaccinated controls) were challenged with a highly virulent type of columnaris and gill tissues were collected just before challenge (0 hours), and at 1 hour and 2 hours post-infection. Using RNA-sequencing, a powerful platform for studying changes in the utilization of genes in animals, we found several patterns and pathways potentially underlying the improved survival of vaccinated fish. Most striking was a pattern of dramatically higher expression of an array of neuropeptides, hormones, immune factors, and proteases at 0 h in vaccinated fish. Following challenge, these elements fell to almost undetectable levels (>100-fold decrease) by 1 h in vaccinated fish, suggesting a rapid release of their contents or reduced production of their contents following stimulation. In addition, the levels of pro-inflammatory chemical messengers were induced in unvaccinated fish, while in vaccinated catfish, we observed widespread induction of genes needed for wound repair and tissue remodeling. Taken together, our results reveal the immune characteristics triggered by vaccination that are protective against columnaris disease.

Technical Abstract: A recently developed attenuated vaccine (17-23) for Flavobacterium columnare has been demonstrated to provide superior protection for channel catfish, Ictalurus punctatus, against genetically diverse columnaris isolates (Mohammed et al. 2013). We were interested in examining the mechanisms of this protection by comparing transcriptional responses to F. columnare challenge in vaccinated and unvaccinated juvenile catfish. Accordingly, 58 day old fingerling catfish (28 days post-vaccination or unvaccinated control) were challenged with a highly virulent F. columnare isolate (BGSF-27) and gill tissues collected pre-challenge (0 h), and 1 h and 2 h post infection, time points previously demonstrated to be critical in early host-pathogen interactions. Following RNA-sequencing and transcriptome assembly, differential expression (DE) analysis within and between treatments revealed several patterns and pathways potentially underlying improved survival of vaccinated fish. Most striking was a pattern of dramatically higher basal expression of an array of neuropeptides (e.g. somatostatin), hormones, complement factors, and proteases at 0 h in vaccinated fish. Previous studies indicate these are likely the preformed mediators of neuroendocrine cells and/or eosinophilic granular (mast-like) cells within the fish gill. Following challenge, these elements fell to almost undetectable levels (>100-fold downregulated) by 1 h in vaccinated fish, suggesting their rapid release and/or cessation of synthesis following degranulation. Concomitantly, levels of pro-inflammatory cytokines (IL-1b, IL-8, IL-17) were induced in unvaccinated fish. In contrast, in vaccinated catfish, we observed widespread induction of genes needed for collagen deposition and tissue remodeling. Taken together, our results indicate an important component of vaccine protection in fish mucosal tissues may be the sensitization, proliferation and arming of resident secretory cells in the period between primary and secondary challenge.