Innate immunity to recombinant QseC, a bacterial adrenergic receptor, may regulate expression of virulence genes of avian pathogenic Escherichia coli

Certain bacterial pathogens rely on a membrane bound sensor kinase, QseC, to coordinate their virulence gene expression in a process called quorum sensing. The present study evaluated the effect of host immunity to a recombinant QseC protein, on the virulence gene expression of avian pathogenic Escherichia coli (APEC) of O78 serogroup (APECO78). For this purpose, we constructed a plasmid expressing QseC protein which is 50 kDa in size and stimulated avian macrophage-like cells (AMCs) with the native form of QseC protein at different concentrations. The cell culture medium of QseC-stimulated AMCs was then used to investigate its effect on APECO78 growth rate and virulence gene expression. Growth curve analysis of APECO78 indicated that growth rate of APECO78 in Luria Bertani (LB) broth containing the culture medium of stimulated AMCs was significantly lower and was impeded at entering the exponential phase. The expression of virulence genes of APECO78 such as aufA, fliC, fimH, fyuA, iucC, iutA, msbB and vat were also significantly down-regulated. On the other hand, APECO78 grown in LB containing the cell culture medium of non-stimulated AMCs did not exhibit these changes. Additionally, stimulation with QseC effectively induced interferon gamma (IFN-γ), Toll-like receptor 4 (TLR-4) and Toll like receptor 15 (TLR-15) expression in AMCs. To summarize, our results demonstrated that recombinant QseC protein could be immunogenic and induces host immunity that regulates selective, yet major, virulence gene expression of APECO78 bacteria. Thus, present data provide evidence that QseC, a bacterial functional analog of adrenergic receptor, holds a promise as one of the vaccine candidates against APEC infections.