Construction of Aeromonas salmonicida subsp. achromogenes AsaP1-toxoid strains and study of their ability to induce immunity in Arctic char, Salvelinus alpinus L

The metalloendopeptidase AsaP1 is one of the major extracellular virulence factors of A. salmonicida subsp. achromogenes, expressed as a 37-kDa pre-pro-peptide and processed to a 19-kDa active peptide. The aim of this study was to construct mutant strains secreting an AsaP1-toxoid instead of AsaP1-wt, to study virulence of these strains and to test the potency of the AsaP1-toxoid bacterin and the recombinant AsaP1-toxoids to induce protective immunity in Arctic char. Two A. salmonicida mutants were constructed that secrete either AsaP1_E294A or AsaP1_Y309F . The secreted AsaP1_Y309F -toxoid had weak caseinolytic activity and was processed to the 19-kDa peptide, whereas the AsaP1_E294A -toxoid was found as a 37-kDa pre-pro-peptide suggesting that AsaP1 is auto-catalytically processed. The LD₅₀ of the AsaP1_Y309F -toxoid mutant in Arctic char was significantly higher than that of the corresponding wt strain, and LD₅₀ of the AsaP1_E294A -toxoid mutant was comparable with that of an AsaP1-deficient strain. Bacterin based on AsaP1_Y309F -toxoid mutant provided significant protection, comparable with that induced by a commercial polyvalent furunculosis vaccine. Detoxification of AsaP1 is very hard, expensive and time consuming. Therefore, an AsaP1-toxoid-secreting mutant is more suitable than the respective wt strain for production of fish bacterins aimed to protect against atypical furunculosis.

Innate and adaptive immune responses of Arctic charr (Salvelinus alpinus, L.) during infection with Aeromonas salmonicida subsp. achromogenes and the effect of the AsaP1 toxin

Aeromonas salmonicida subsp. achromogenes, the causative agent of atypical furunculosis in many fish species, secretes the toxic metalloendopeptidase AsaP1. This study aimed to analyze innate and adaptive immune parameters induced in Arctic charr (Salvelinus alpinus, L.) infected with wild type (wt) A. salmonicida subsp. achromogenes and its isogenic asaP1 deletion mutant (AsaP1-deficient). Head-kidney, liver and spleen were obtained from i.p. infected charr (wt, AsaP1-deficient), during a time schedule of 7 d post infection. Reverse transcription quantitative real-time PCR (RT-qPCR) was applied to study the expression of immune parameters: pro-inflammatory cytokines IL-1β and TNF-α; anti-inflammatory cytokine IL-10; chemokines CXCL-8 (IL-8) and CC-chemokine; the cytokines IFN-γ and IL-4/13A as tracers for Th1 and Th2 immune responses, respectively; and the cell markers CD8α and CD83. In addition, lymphoid organs were histopathologically examined at days 3 and 7 post infection, including B (IgM) and T (CD3ε) cell staining. The detected immune responses were initially driven by innate mechanisms represented by the up-regulation of pro-inflammatory cytokines and chemokines and later on by adaptive Th2 related responses cumulating in B-cell recruitment as shown by regulation of immune parameters in spleen and head-kidney, with significant differences between mutant and wt infected fish. Histological sections revealed IgM-positive cells around ellipsoid arterioles in spleen, while CD3ε positive cells were found in clusters scattered all over the section. However, histopathological differences were only detected between infected and non-infected fish, but not between AsaP1-deficient mutant and wt infected fish. This work represents the first study on innate and adaptive immune responses of Arctic charr induced by a bacterial infection.