The Toll-like receptor (TLR) 7 response represents a vital host-defence mechanism in a murine model of systemic West Nile virus (WNV) infection. Here, we investigated the role of the TLR7-induced immune response following cutaneous WNV infection. We found that there was no difference in susceptibility to WNV encephalitis between wild-type and TLR7"/" mice upon intradermal injection or infected mosquito feeding. Viral load analysis revealed similar levels of WNV RNA in the peripheral tissues and brains of these two groups of mice following intradermal infection. There was a higher level of cytokines in the blood of wild-type mice at early stages of infection; however, this difference was diminished in the blood and brains at later stages. Langerhans cells (LCs) are permissive to WNV infection and migrate from the skin to draining lymph nodes upon intradermal challenge. Our data showed that WNV infection of TLR7"/"keratinocytes was significantly higher than that of wild-type keratinocytes. Infection of wild-type keratinocytes induced higher levels of alpha interferon and interleukin-1b (IL-1b), IL-6 and IL-12, which might promote LC migration from the skin. Co-culture of naïve LCs of wild-type mice with WNV-infected wild-type keratinocytes resulted in the production of more IL-6 and IL-12 than with TLR7 "/" keratinocytes or by cultured LCs alone. Moreover, LCs in the epidermis were reduced in wild-type mice, but not in TLR7 "/" mice, following intradermal WNV infection. Overall, our results suggest that the TLR7 response following cutaneous infection promotes LC migration from the skin, which might compromise its protective effect in systemic infection.
INTRODUCTIONWest Nile virus (WNV), a single-stranded (ss)RNA flavivirus, has been endemic in Africa, Asia, Europe and, more recently, North America (Granwehr et al., 2004; Pletnev et al., 2006). The virus is maintained in an enzootic cycle that involves mosquitoes and birds. Human infection results primarily from mosquito bites, and symptoms include fever, headache, myalgia, meningitis and encephalitis. Neurological disease (including encephalitis) has been observed in .30 % of confirmed WNV cases, with a higher frequency in the elderly and immunocompromised (Campbell et al., 2002; Pletnev et al., 2006). Human vaccines are not yet available. Treatment is mostly nonspecific and supportive.Murine infection via subcutaneous or intraperitoneal injection has been an effective in vivo experimental model for human infections to investigate WNV pathogenesis and host immune response (Beasley et al., 2002;. Work from the murine model has demonstrated that type I interferons (IFNs), cd T cells and humoral immunity are critical in controlling the dissemination of WNV (Anderson & Rahal, 2002;Diamond et al., 2003;Fredericksen et al., 2008;Klein et al., 2005;Lucas et al., 2003;Roehrig et al., 2001;Wang et al., 2003a Intradermal WNV infection, which presumably mimics natural infection in humans, has also been studied in mice (Johnston et al., 1996(Johnston et al., , 2000. Follow...