The Intestinal Immune Responses of Channel Catfish Against Live-Attenuated Edwardsiella Ictaluri Vaccines

Edwardsiella ictaluri is a facultative intracellular pathogen causing gastrointestinal septicemia in catfish. E. ictaluri invades catfish intestine within 15 minutes and establishes a systemic infection. Despite the importance of intestinal route in E. ictaluri infections, intestinal immune responses against E. ictaluri are largely unknown. Recently, we developed two live attenuated vaccines protecting catfish against E. ictaluri infection. In this work, we report catfish intestinal immune responses against E. ictaluri wild-type and attenuated vaccine strains. To achieve our goal, intestinal ligated loops were constructed surgically in live catfish and loops were injected with E. ictaluri strains. Histopathological examination showed no significant tissue damage or inflammation in the intestinal tissue exposed to the vaccine strains. In contrast, massive expansion of lamina propria with congested and dilated blood vessels and intestinal inflammation were observed in the intestinal tissue exposed to wild-type E. ictaluri. Also, immunohistochemical staining revealed a moderate level of neutrophil and B cell infiltration in the vaccine injected intestinal tissue compared to that of wild-type injected intestinal tissue. We further assessed the expression levels of IL-1β, IL-8, TNF-α, CD4-1, and CD8-α in catfish intestine injected with E. ictaluri strains using quantitative real-time PCR. Vaccine exposed tissue showed significantly lower expression of TNF-α, CD4-1, and CD8-α compared to that of the wild-type exposed tissue. All these data suggest that our live attenuated vaccines are capable of priming immune responses in catfish without causing damage to the host.