Enteropathogenic Escherichia coli (EPEC) is a diarrhoeagenic gut bacteria that utilises a type III secretion system (T3SS) to translocate effector proteins into host cells to modify cell signalling for persistent infection and dissemination. One such effector is EspL, which has been characterised to inhibit necroptosis and associated inflammatory pathways by directly cleaving RHIM domain-containing proteins, RIPK1 and RIPK3. This suggests a novel role for RHIM proteins in bacterial clearance.
Preliminary experiments on Ripk1‑/‑Ripk3‑/‑deficient mice infected with the EPEC-like mouse pathogen, Citrobacter rodentium, demonstrated heightened susceptibility to infection and a significant decrease in serum amyloid A2 (SAA2) within the colon, typically associated with T helper 17 responses important for bacterial clearance. As the physiological importance of RHIM proteins remain unclear, this study sought to clarify the role of these proteins and the underlying pathway mediating exacerbated pathology in Ripk1‑/‑Ripk3‑/‑deficient mice during gut infection.
Examination of disease in various single and compound knockout mice found that RIPK3 plays a larger role in mediating local gut pathology, whereas RIPK1 restricts bacterial systemic dissemination. Additionally, flow cytometry analysis of colonic lamina propria from infected Ripk1‑/‑Ripk3‑/‑deficient mice showed a significant reduction in T helper 17 and T regulatory cells relative to wildtype controls. This was consistent with qPCR evaluation of inflammatory cytokine levels where a larger fold reduction in Saa2, Il22 and Il17a gene expression was exhibited in mice infected with wildtype C. rodentium compared to those with an espL deletion mutant. Overall, these results show for the first time, a link between RHIM-proteins (innate immunity) and T cell responses (adaptive immunity), which will be further investigated to better inform the significance of RHIM proteins in bacterial pathogenesis and maintenance of gut homeostasis.