TLR8 Couples SOCS-1 and Restrains TLR7-Mediated Antiviral Immunity, Exacerbating West Nile Virus Infection in Mice

Abstract

West Nile virus (WNV) is a neurotropic, single-stranded RNA (ssRNA) flavivirus that can cause encephalitis, meningitis, and death in humans and mice. Human Toll-like receptor (TLR) 7, 8, and mouse TLR7 recognize viral ssRNA motifs and induce antiviral immunity. However, the role of mouse TLR8 in antiviral immunity is poorly understood. Here, we report that TLR8 deficient (Tlr8−/−) mice were resistant to WNV infection compared to wild-type (WT) controls. Efficient WNV clearance and moderate susceptibility to WNV-mediated neuronal death in Tlr8−/− mice was attributed to overexpression of Tlr7 and an interferon-stimulated gene Isg-56 expression, while reduced expression of the pro-apoptotic gene coding Bcl2-associated X protein (Bax) was observed. Interestingly, suppressor of cytokine signaling -1 (SOCS-1) directly associated with TLR8, but not with TLR7, indicating a novel role for TLR8 regulation of SOCS-1 function, while selective siRNA knockdown of Socs-1 resulted in induced Isg-56 and Tlr7 expression following WNV infection. Collectively, we report that TLR8 coupling with SOCS-1 inhibits TLR7-mediated antiviral immunity during WNV infection in mice.

Current Understanding of West Nile Virus Clinical Manifestations, Immune Responses, Neuroinvasion, and Immunotherapeutic Implications

“…Human TLR8 can recognize viral ssRNA, but mouse TLR8 was described as nonfunctional, which may be due to a deletion of 5 amino acids in the leucine-rich repeat ectodomain of TLR8 in mice [42]. Although TLR8 may not directly sense WNV RNA, it interacts with suppressor of cytokine signaling (SOCS)-1 and restrains TLR7 mediated antiviral immunity in mice [43].…”

Tumor Necrosis Factor-Alpha Signaling May Contribute to Chronic West Nile Virus Post-infectious Proinflammatory State

“…First, the clinical data provide evidence that WNV infection in humans induces a significant upregulation of TNF-α. This is not surprising since previous studies in mice and cell cultures have demonstrated essential protective roles for TNF-α and other proinflammatory cytokines against acute WNV infection (10,(12)(13)(14) Moreover, high TNF-α levels have been reported in other flavivirus human infections, including dengue (21,22), Zika virus (23,24), and Japanese encephalitis virus (JEV) (25). Children infected with dengue virus showed significantly higher serum levels of TNF-α, with the highest levels in those with severe dengue disease (21), formerly "dengue shock syndrome" and "dengue hemorrhagic fever."…”

“…On a molecular level, acute WNV infection induces a significant upregulation of various proinflammatory proteins, including astroglial protein S100B and the receptor for advanced glycation end products (RAGE) (7), high-mobility group box-1 (HMGB1) (8), and osteopontin (OPN) (9), which initiate and maintain an inflammatory cascade that increases production of pro-inflammatory cytokines, interleukins (ILs), and chemokines (10)(11)(12) as part of the initial antiviral immune response. Previous studies in mice and cell cultures have demonstrated essential protective roles for these proinflammatory proteins against acute WNV infection (13,14). However, these antiviral molecules have also been reported to persist for months following the acute illness, leading to a post-infectious proinflammatory state that may contribute to long-term neuroinflammation and cytotoxicity in some WNV survivors (15,16).…”

Inhibition of murine herpesvirus-68 replication by IFN-gamma in macrophages is counteracted by the induction of SOCS1 expression

“…Our study indicated that the virus-induced increase in SOCS1 expression in primary macrophages was mediated by TLR3 and NF-κB activation. Both TLRs [ 46 ] and NF-κB [ 47 , 48 ] have been reported to be regulators of SOCS1 expression. In addition to MHV-68 infection, we demonstrated that when TLR3 on BMMs was activated by poly(I:C), the SOCS1 expression was also induced ( Fig 6B ) in a manner dependent on NF-κB activation ( Fig 7D ).…”