Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), one of the most important bacterial diseases of farmed channel catfish in the USA. Use of live attenuated vaccines (LAVs) is an effective strategy for combating mortalities in catfish farms. Our research group has developed three live attenuated E. ictaluri strains [EiΔevpB, EiΔgcvPΔsdhCΔfrdA (ESC-NDKL1), and EiΔhemRΔfrdAΔsdhC (triple-hemR)] that provide various levels of protection against ESC. However, the protective mechanisms of these vaccine candidates are mostly unknown. The overall objective of my study was to investigate protective mechanisms of these LAVs. To accomplish this, catfish fry were immersion challenged with EiΔevpB, ESC-NDKL1, and triple-hemR. Additional catfish fry were immersion challenged with Aquavac-ESC and E. ictaluri wild-type (EiWT) as controls. The internalization of antigens through the mucosal surfaces as well as the pathology and molecular immune responses were studied. The investigations showed that EiΔevpB and ESC-NDKL1 were highly safe and efficacious compared to Aquavac-ESC and EiWT. Pathologically, vaccination with EiΔevpB and ESC-NDKL1 decreased the pathological lesions, EiWT replication in catfish, and increased the ability of the immune system to resist and kill EiWT. On the other hand, triple-hemR was not safe causing severe tissue damage similar to EiWT. The gene expression data showed significantly high expression of innate and adaptive immune genes following vaccination and challenge with EiWT. Additionally, EiΔevpB and ESC-NDKL1 induced immune proliferation in pronephros after exposure to EiWT. Taken together, EiΔevpB and ESC-NDKL1 vaccine strains performed better to induce immune responses and reduce damage in the host tissues during EiWT challenge compared to Aquavac-ESC.

Vaccination against intracellular pathogens requires generation of pool of memory T cells, which can respond upon infection and mediate immune responses by either killing of infected host cells or induce killing mechanisms in infected cells. T cell-inducing vaccines aim to deliver the antigen to antigen presenting cells (APCs) by presenting on MHC molecules thus bridging innate and adaptive immunity. The intracellular pathogen Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), which is a devastating disease in catfish industry. E. ictaluri can survive in professional phagocytes and use them as an infection source. Two new live attenuated vaccine (LAV) strains, EiDELTAevpB and ESC-NDKL, were developed by our group. However, the role of LAVs in phagocytosis, bacterial killing, and antigen presentation is unexplored. Therefore, further research is necessary to determine immune responses in channel catfish against LAVs. The long-term goal of this project is to identify immunological APC-dependent mechanisms that underscore E. ictaluri pathogenesis to enable development of effective control strategies for ESC. The overall goal of this project is to assess the role of three professional APCs, dendritic cells (DCs), macrophages and B cells in the LAV-induced innate and adaptive immune responses in catfish. The central hypothesis is that efficacious LAV strains will enhance phagocytosis and microbial killing, and promote the generation of T cells that regulate and control protective B cell-mediated immunity. The rationale for this research is that more detailed knowledge about phenotype and function of catfish APCs will not only help gain insight into the evolution of vertebrate adaptive immune system but will provide valuable information for development and optimization of immunotherapies and vaccination protocols for aquaculture use. In this study, we first identified DC-like cells in immune-related organs of catfish and assessed their expression patterns in lymphoid organs of catfish in E. ictaluri infection. Although WT strain induces the functional inability of DC-like cells in migration and maturation, LAVs strains promote the migration and maturation of DC-like cells for antigen presentation. Two LAVs enhanced the phagocytosis and killing activity in catfish macrophages and B cells. Also, LAVs induce high expression of T cell-related genes without causing inflammation.

Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), a devastating disease in the channel catfish industry. Our research group has developed several E. ictaluri live attenuated vaccine (LAV) candidates (EiΔevpB, EiΔevpBΔfur, EiΔevpBΔhfq, EiΔevpBΔfurΔhfq), which were able to stimulate an immune response in vaccinated channel catfish and reduce ESC. However, innate, and adaptive immune responses in the lymphoid tissues of channel catfish to these LAVs are not known well. The overall goal of the project is to determine the role of adaptive and innate immune responses in catfish after vaccination with LAVs. Analysis of innate and adaptive immune-related gene expressions showed that the LAVs induced expression of adaptive immune-related genes in lymphoid tissues with less inflammation compared to wild type control. Also, the LAVs induced the expression of IgM in the sera of catfish.