MDA5

Abstract: As cutaneous pigment-producing cells, melanocytes can become targets of primary and secondary immune response as can be seen in diseases like vitiligo and Vogt-Koyanagi-Harada (VKH) syndrome. Viral infections have previously been implicated as a possible precipitating factor in the destruction of melanocytes in these disorders. During viral replication, double-stranded RNA (dsRNA) is produced as an intermediate metabolite, which induces antiviral and inflammatory responses through Toll-like receptor 3 (TLR3) in cells of innate immune system. The functional responses of melanocytes to dsRNA, however, remain unclear. Herein, we demonstrated that human melanocytes expressed TLR3 at a constitutive and inducible level. Stimulation with poly(I:C), a synthetic dsRNA analogue, triggered apoptosis of melanocytes. The apoptosis-inducing effect was shown by RNA interference to be largely dependent on TLR3, but occurred independently of NF-κB activation since treatment with specific NF-κB inhibitor Bay 11-7082 failed to prevent the process. In contrast, IFN-β neutralizing Ab blocked the apoptosis-inducing effect of dsRNA, indicating the involvement of IFN-β autocrine signalling. Furthermore, studies on the intracellular signal transduction pathways revealed that dsRNA induces the activation of p38, ERK1/2 and JNK1/2 in melanocytes. Using specific inhibitors, we demonstrated that activation of p38 and ERK1/2 controlled both IFN-β secretion and IFN-β mediated cell death. Taken together, these data suggest that viral dsRNA stimulates TLR3 in human melanocytes and triggers the cellular apoptosis through autocrine of IFN-β.

Abstract: dsRNA, as genomic fragment, replicative intermediate, or stem and loop structure in cells infected by viruses, can act to signal the immune system of the presence of viral infections. Although most viral infections are associated with strong Th1 immune responses, Th2-type responses have also been observed. In this study, we characterize the effects of dsRNA on the induction of Th2 responses in human lymphocytes. We report that in addition to the well-known Th1-inducing capabilities of dsRNA, treatment of human lymphocytes with low concentrations of dsRNA (0.1–1 μg/ml) leads to the expression of the prototypic Th2 cytokine IL-4. This induction was accompanied with the concentration-dependent activation of NF-κB and NF-AT2 but not NF-AT1. In addition, dsRNA can directly activate an IL-4 promoter-driven chloramphenicol acetyltransferase reporter gene in transiently transfected Jurkat cells. These results are the first demonstration of a non-TCR-associated activator of NF-AT in human cells and suggest that dsRNA directly influences IL-4 gene expression through its effect on NF-AT activation. Our data provide support for the idea that dsRNA at low concentrations in vivo may induce a Th2-dominant response that is not optimal for protective immunity to the virus.

Abstract: Chikungunya virus (CHIKV) is a single-stranded positive-sense RNA virus, belonging to the genus Alphavirus of the Togaviridae family. It causes multiple symptoms, including headache, fever, severe joint and muscle pain, and arthralgia. Since CHIKV was first isolated in Tanzania in 1952, there have been multiple outbreaks of chikungunya fever. However, its pathogenesis and mechanisms of viral immune evasion have been poorly understood. In addition, the exact roles of individual CHIKV genes on the host innate immune response remain largely unknown. To investigate if CHIKV-encoded genes modulate the type I interferon (IFN) response, each and every CHIKV gene was screened for its effects on the induction of the IFN-β promoter. Here we report that CHIKV nsP2, E2 and E1 strongly suppressed activation of the IFN-β promoter induced by the MDA5/RIG-I receptor signaling pathway, suggesting that nsP2, E2, and E1 are the major antagonists against induction of IFN-β. Delineation of underlying mechanisms of CHIKV-mediated inhibition of the IFN-β pathway may help develop virus-specific therapeutics and vaccines.