Immunogenicity of extracellular proteins of Flavobacterium covae in Ictalurus punctatus

Abstract

Abstract Background Columnaris disease, instigated by Gram-negative Flavobacterium covae (F. covae), is a dreadful disease of many different genera of freshwater fish with alarmingly high mortality rates. Ictalurus punctatus (I. punctatus) is extremely prone to this disease. Despite the terrible implications of this disease, very little is known about how the host and the bacterium's virulence factor interact. Secreted extracellular proteins (SEPs) are key bacterial virulence agents that frequently provide immunogenicity. In  a previous study, the major secreted extracellular proteins of F. covae were identified and their protective efficacy was verified in I. punctatus, and the results were published. In the current study, an endeavor has been made to examine the progressive shifts in I. punctatus immunity following immunization with F. covae SEPs in order to more thoroughly comprehend the host's intrinsic and acquired immune responses against F. covae infection. An enzyme-linked immunosorbent test (ELISA) is operated to analyze specific humoral immune responses against SEPs of F. covae. Moreover, the expressions of immune-associated genes that are related to inherent and acquired immunity were monitored in the anterior kidneys and spleens of the vaccinated I. punctatus. Results Marked rise in immunoglobulin M (IgM) levels were detected in the serum of I. punctatus vaccinated with different SEP formulations, including crude SEPs, heat-deactivated SEPs, and SEPs combined with adjuvant, at 14 and 21 days post-vaccination. At the same time points, a marked increase in the expression levels of clusters of differentiation (CD4-1 and CD8-β) and major histocompatibility complex (MHC-II) genes was detected in the vaccinated fish's anterior kidneys and spleens, while the highest levels of interleukin-8 (IL-8) and tumor necrosis factor alpha (TNF-α) up-regulations were detected in the immunized fish's anterior kidneys 14 days following vaccination. Conclusions This work revealed that SEPs are an effective basis for vaccination against F. covae infection, eliciting humoral and inherent immune responses of I. punctatus after SEPs immunization. Improvements are needed to further enhance the I. punctatus immunological responses to SEPs of F. covae since these antigens remain a prospective candidate for future optimization and clinical trials in aquaculture settings.