Armwood

Comparative immune responses to Edwardsiella ictaluri challenge in vaccinated and non-vaccinated channel and channel ♀ x blue ♂ hybrid catfish

Abigail R. Armwood,1 Wenlong Cai,2 John Leary,1 Adrián López-Porras,3 Bradley M. Richardson,4 Cynthia Ware,5 Sara L. Purcell,6 David Wise,5 Mark D. Fast,6 Matt J. Griffin,5 and Alvin C. Camus1

1Department of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA, USA; 2Department of Infectious Disease and Public Health, City University of Hong Kong, Hong Kong, China; 3Department of Biosciences, University of Oslo, Oslo, Norway; 4Warmwater Aquaculture Research Unit, Agricultural Research Service, United States Department of Agriculture, Stoneville, MS, USA; 5Thad Cochran National Warmwater Aquaculture Center, Delta Research and Extension Center, Mississippi State University, Stoneville, MS, USA; 6Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, Prince Edward Island, Canada

ABSTRACT

Edwardsiella ictaluri is a commercially significant bacterial pathogen of US farm-raised catfish, mostly affecting channel catfish (Ictalurus punctatus) fingerlings. Virulence varies with catfish type, causing more severe disease in channel catfish compared to channel ♀ x blue ♂ (Ictalurus furcatus) catfish hybrids. Recently, a live-attenuated E. ictaluri vaccine has been widely adopted by the US catfish industry. Variation between catfish types in terms of susceptibility to infection and the protective immune response induced by vaccination are poorly understood. This study compared susceptibility and immune responses to E. ictaluri challenge in vaccinated and non-vaccinated channel and hybrid catfish fingerlings. Catfish were stocked into 24 aquaria (12 channels, 12 hybrids; 55 fish/aquaria) for sample collection and 24 non-sampling aquaria (12 channels, 12 hybrids; 20 fish/aquaria) to calculate challenge mortality. Fish were immunized via immersion with the live-attenuated E. ictaluri vaccine strain. Thirty-three days post vaccination (dpv), fish were challenged by intraperitoneal injection with E. ictaluri (~3.59 x 103 CFU/fish) or sterile brain-heart infusion broth as a control. Posterior kidneys were collected from 2 fish from each sampling aquarium (6 fish/treatment group) at 5 days post challenge (dpc). RNA was isolated, and expression of select immune genes (e.g., TNFα, IFNγ, IL1β) was quantified by qPCR. A subset of 5 fish/treatment group were also subjected to RNA-Seq analysis. ELISAs were performed against E. ictaluri and E. piscicida antigen with serum collected from select treatment groups at 28 dpv and 28 dpc. Vaccinated hybrid and channel catfish were significantly (p < 0.001) more likely to survive than their unvaccinated counterparts when challenged with E. ictaluri. In vaccinated fish, the relative percent survival (RPS) in response to E. ictaluri challenge was 93.2% in hybrids and 87.7% in channels. At 28 dpv, serum IgM levels against E. ictaluri antigen were significantly elevated in vaccinated channels compared to non-vaccinated fish (p < 0.001). Comparably, serum IgM levels at 28 dpv in vaccinated hybrids were not significantly different than non-vaccinated cohorts (p = .13). At 28 dpc, serum IgM against E. ictaluri antigen was significantly increased in vaccinated, challenged channel (p < 0.001) and hybrid fish (p < 0.05) against E. ictaluri antigen compared to non-challenged, vaccinated and non-vaccinated controls. Interestingly, despite reports of a cross-protective effect of the vaccine against E. piscicida, only vaccinated and subsequently challenged channels produced a significant antibody response to E. piscicida antigen (p <0.05), regardless of sampling time (28 dpv or 28 dpc). TNFα, IFNγ, and IL1β were differentially expressed (p < 0.05), with increased expression predominantly in challenged fish (both vaccinated and non-vaccinated). Differentially expressed genes (DEGs) were identified between fish types and across treatments, although no DEGs were attributed to vaccination alone. Notable variably regulated genes were largely related to innate immune responses, complement, and coagulation. These data illustrate how bacterial infection, vaccination, and fish type influence the host response to E. ictaluri infection in channel and hybrid catfish and lay the foundation for future work optimizing Edwardsiella vaccine efficacy in channel and hybrid catfish.