Receptor interacting serine/threonine protein kinase (RIPK) 1 and 3 are key signaling factors in inflammation and programmed cell death. In particular, the interaction of RIPK1 and RIPK3 through their conserved RHIM domains results in necroptosis, a cell death modality where membrane pore formation releases danger-associated molecular patterns to drive an inflammatory response. Consequently, the diarrhoeagenic bacteria enteropathogenic Escherichia coli (EPEC) has evolved to express an effector termed EspL, which cleaves the RHIM domains of RIPK1 and RIPK3, thus inhibiting necroptosis in vitro. The physiological significance of this event to host immunity however, remains to be examined.
Using Citrobacter rodentium – the model organism for EPEC, evaluation of disease in a panel of RIPK knockout mice revealed that compound deletion of RIPK1, RIPK3 and caspase-8 greatly heightened the diarrhoeal severity and bacterial burden experienced at peak of infection. Notably, only RIPK3 played a significant role in moderating colonic pathology and bacterial burden in a manner independent from necroptosis. While RIPK1 kinase activity was shown to be dispensable for mediating protection, RIPK1 does contribute to immunity through a separate yet undefined mechanism. More interestingly, flow cytometry analysis of RIPK3-deficient mice found a marked reduction in the T-helper 1 (Th1), Th17 and T-regulatory cell populations in the colonic lamina propria. This loss of RIPK3 also correlated with an altered colonic expression of Cd59a and Clca1 that are involved in the regulation of leukocyte function. Dysregulation of these key immune responses is proposed to contribute to host decline upon EPEC infection.
Together, our results describe an association between RIPK1/3 (innate immunity) and T cell responses (adaptive immunity) that is involved in host mucosal protection against enteric bacteria. Further characterisation of the underlying signaling pathways will be important to inform future management and treatment of serious gastrointestinal diseases.