Innate immune responses to rotavirus infection in macrophages depend on MAVS but involve neither the NLRP3 inflammasome nor JNK and p38 signaling pathways.

Rotavirus infection is a major cause of life-threatening infantile gastroenteritis. The innate immune system provides an immediate mechanism of suppressing viral replication and is necessary for an effective adaptive immune response. Innate immunity involves host recognition of viral infection and establishment of a powerful antiviral state through the expression of pro-inflammatory cytokines such as type-1 interferon (IFN). Macrophages, the front-line cells of innate immunity, produce IFN and other cytokines in response to viral infection. However, the role of macrophages during rotavirus infection is not well defined. We demonstrate here that RRV rotavirus triggers the production of proinflammatory cytokines from mouse bone marrow-derived macrophages. IFN and antiviral cytokine production was abolished in rotavirus-infected MAVS (-/-) macrophages. This indicates that rotavirus triggers innate immunity in macrophages through RIG-I and/or MDA5 viral recognition, and MAVS signaling is essential for cytokine responses in macrophages. Rotavirus induced IFN expression in both wild type and MDA5 (-/-) macrophages, showing that MDA5 is not essential for IFN secretion following infection, and RIG-I and MDA5 may act redundantly in promoting rotavirus recognition. Interestingly, rotavirus neither stimulated mitogen-activated protein kinases p38 and JNK nor activated the NLRP3 inflammasome, demonstrating that these components might not be involved in innate responses to rotavirus infection in macrophages. Our results indicate that rotavirus elicits intracellular signaling in macrophages, resulting in the induction of IFN and antiviral cytokines, and advance our understanding of the involvement of these cells in innate responses against rotavirus.

NSP1 of human rotaviruses commonly inhibits NF-κB signalling by inducing ß-TrCP degradation.

Rotavirus is a leading cause of severe gastroenteritis in infants worldwide. Rotavirus nonstructural protein 1 (NSP1) is a virulence factor that inhibits innate host immune responses. NSP1 from some rotaviruses targets host interferon response factors (IRFs), leading to inhibition of type I interferon expression. A few rotaviruses encode an NSP1 that inhibits the NF-κB pathway by targeting ß-TrCP, a protein required for IκB degradation and NF-κB activation. Available evidence suggests that these NSP1 properties involve proteosomal degradation of target proteins. We show here that NSP1 from several human rotaviruses and porcine rotavirus CRW-8 inhibits the NF-κB pathway, but cannot degrade IRF3. Furthermore, ß-TrCP levels were much reduced in cells infected with these rotaviruses. This provides strong evidence that ß-TrCP degradation is required for NF-κB pathway inhibition by NSP1 and demonstrates the relevance of ß-TrCP degradation to rotavirus infection. C-terminal regions of NSP1, including a serine-containing motif resembling the ß-TrCP recognition motif of IκB, were required for NF-κB inhibition. CRW-8 infection of HT-29 intestinal epithelial cells induced significant levels of IFN-ß and CCL5 but not IL-8. This contrasts with monkey rotavirus SA11-4F, whose NSP1 inhibits IRF3 but not NF-κB. Substantial amounts of IL-8 but not IFN-ß or CCL5 were secreted from HT-29 cells infected with SA11-4F. Our results show that human rotaviruses commonly inhibit the NF-κB pathway by degrading ß-TrCP and thus stabilizing IκB. They suggest that NSP1 plays an important role during human rotavirus infection by inhibiting the expression of NF-κB-dependent cytokines, such as IL-8.