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Host responses to Renibacterium salmoninarum and specific components of the pathogen reveal the mechanisms of immune suppression and activation

During infection, Renibacterium salmoninarum survives within the pronephric macrophages of salmonid fish. Therefore, to study the initial phases of the interaction we infected macrophages with live bacteria and analysed the responses of host and pathogen. It was found that the expression of msa encoding the p57 antigen of R. salmoninarum, was constitutive, while the expression of hly and rsh, encoding haemolysins, and lysB and grp was reduced after infection. Macrophages showed a rapid inflammatory response in which the expression of interleukin‐1β (IL‐1β), major histocompatibility complex class II (MHC II), inducible cyclo‐oxygenase (Cox‐2), and inducible nitric oxide synthase (iNOS) was enhanced, but tumour necrosis factor‐α (TNF‐α) expression was greatly reduced initially and then increased. After 5 days, except for TNF‐α and MHC II, expression returned to levels approaching those of uninfected macrophages. We propose that R. salmoninarum survives initial contact with macrophages by avoiding and/or interfering with TNF‐α‐dependent killing pathways. The effects of specific R. salmoninarum components were studied in vivo by injecting fish with DNA vaccine constructs expressing msa, hly, rsh, lysB, or grp. We found that msa reduced the expression of IL‐1β, Cox‐2, and MHC II but stimulated TNF‐α while hly, rsh and grp stimulated MHC II but down‐regulated TNF‐α. Constructs expressing hly or lysB stimulated iNOS expression and additionally, lysB stimulated TNF‐α. The results show how p57 suppresses the host immune system and suggest that the immune mechanisms for the containment of R. salmoninarum infections rely on MHC II‐ and TNF‐α‐dependent pathways. Moreover, prolonged stimulation of TNF‐α may contribute to the chronic inflammatory pathology of bacterial kidney disease.