Abstract
Edwardsiella ictaluri is a Gram-negative intracellular pathogen that causes enteric septicemia of catfish (ESC). Successful vaccination against intracellular pathogens requires T cell priming by antigen presenting cells (APCs) that bridge innate and adaptive immunity. However, the evidence on immunological mechanisms that underscore E. ictaluri pathogenesis and the protective role of live attenuated vaccines (LAVs) is scarce. We assessed the expression of immune genes related to antigen presentation by real-time PCR and the distribution patterns of Langerhans-like (L/CD207+) cells by immunohistochemistry in the immune-related tissues of channel catfish challenged with two novel E. ictaluri LAVs, EiΔevpB, and ESC-NDKL1 and wild type (WT) strain. Our results indicated significantly elevated expression of IFN-γ gene in the anterior kidney (AK) and spleen of vaccinated catfish at the early stages of exposure, which correlated with increased numbers of L/CD207+ cells. In general, the ESC-NDKL1-induced IFN-γ gene expression patterns in the AK resembled that of the patterns induced by EiΔevpB. However the MHCII gene expression patterns differed between the strains with significant increases at 6 h post-challenge (pc) with the EiΔevpB and at 7 d pc with the ESC-NDKL1 strains, respectively. Significant increases in activity of T helper type polarization genes such as IFN-γ and T cell co-receptors after exposure to ESC-NDKL1, in combination with elevated numbers of L/CD207+ cells at 7 d pc with both LAVs compared to uninfected and the WT-exposed counterparts, were documented in the spleen. The dominant pro-inflammatory environment with dramatically overexpressed inflammatory genes in the AK and 7 d pc in the spleen in response to E. ictaluri was found in exposed catfish. In general, the pro-inflammatory gene expression profiles in the ESC-NDKL1 pc showed more similarities to the WT strain-induced gene profiles compared to the EiΔevpB counterpart. In addition, E. ictaluri WT significantly decreased the numbers of Langerhans-like L/CD207+ cells in the AK and spleen at 3 and 7 days pc. In conclusion, we report the differential framework of initiation of innate and adaptive immune responses between E. ictaluri strains with both LAVs having a potential of satisfying the stringent requirements for successful vaccines.
Highlights
Edwardsiella ictaluri (E. ictaluri) is a Gram-negative facultative intracellular pathogen that causes enteric septicemia of channel catfish (ESC), one of the most devastating diseases in the US catfish industry [1,2,3,4]
Changes in the cytokine/chemokine and lymphocyte-specific gene expression induced by WT E. ictaluri were evident at 6 h post-exposure, resulting in significant upregulation of most of the genes evaluated in the anterior kidney (AK) of catfish that survived infection (Figures 2A–I)
Due to the lack of information available on the morphological and functional markers of catfish Dendritic cells (DCs), we applied the IHC approach described previously [40] to evaluate the Langerhans cells (LCs)-like expression patterns in the immune-related tissues of catfish challenged with two live attenuated vaccines (LAVs) and WT E. ictaluri strains to underscore their immune effector mechanisms
Summary
Edwardsiella ictaluri (E. ictaluri) is a Gram-negative facultative intracellular pathogen that causes enteric septicemia of channel catfish (ESC), one of the most devastating diseases in the US catfish industry [1,2,3,4]. A live E. ictaluri vaccine (Aquavac-ESC) against ESC was developed by Klesius and Shoemaker, and this vaccine can provide efficient protection to juvenile catfish [5]. Immersion studies demonstrated that Aquavac-ESC stimulated the protective immunity in catfish fry, fingerlings, and eyed catfish eggs [6,7,8,9]. Avirulent E. ictaluri isolate (S97-773) was developed by Wise et al and oral vaccination with this live attenuated isolate protected fingerlings from E. ictaluri infection [10]. Edwardsiella ictaluri can survive and replicate in channel catfish macrophages, and E. ictaluri live attenuated vaccines (LAVs) induced cell-mediated immunity to protect catfish against ESC [11,12,13]. Catfish vaccinated with LAVs triggered humoral immune responses which augmented the bacterial killing activity of macrophages [12, 14]
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