TLR3 signaling is either protective or pathogenic for the development of Theiler's virus-induced demyelinating disease depending on the time of viral infection

Abstract

BackgroundWe have previously shown that toll-like receptor 3 (TLR3)-mediated signaling plays an important role in the induction of innate cytokine responses to Theiler's murine encephalomyelitis virus (TMEV) infection. In addition, cytokine levels produced after TMEV infection are significantly higher in the glial cells of susceptible SJL mice compared to those of resistant C57BL/6 mice. However, it is not known whether TLR3-mediated signaling plays a protective or pathogenic role in the development of demyelinating disease.MethodsSJL/J and B6;129S-Tlr3tm1Flv/J (TLR3KO-B6) mice, and TLR3KO-SJL mice that TLR3KO-B6 mice were backcrossed to SJL/J mice for 6 generations were infected with Theiler's murine encephalomyelitis virus (2 × 105 PFU) with or without treatment with 50 μg of poly IC. Cytokine production and immune responses in the CNS and periphery of infected mice were analyzed.ResultsWe investigated the role of TLR3-mediated signaling in the protection and pathogenesis of TMEV-induced demyelinating disease. TLR3KO-B6 mice did not develop demyelinating disease although they displayed elevated viral loads in the CNS. However, TLR3KO-SJL mice displayed increased viral loads and cellular infiltration in the CNS, accompanied by exacerbated development of demyelinating disease, compared to the normal littermate mice. Late, but not early, anti-viral CD4+ and CD8+ T cell responses in the CNS were compromised in TLR3KO-SJL mice. However, activation of TLR3 with poly IC prior to viral infection also exacerbated disease development, whereas such activation after viral infection restrained disease development. Activation of TLR3 signaling prior to viral infection hindered the induction of protective IFN-γ-producing CD4+ and CD8+ T cell populations. In contrast, activation of these signals after viral infection improved the induction of IFN-γ-producing CD4+ and CD8+ T cells. In addition, poly IC-pretreated mice displayed elevated PDL-1 and regulatory FoxP3+ CD4+ T cells in the CNS, while poly IC-post-treated mice expressed reduced levels of PDL-1 and FoxP3+ CD4+ T cells.ConclusionsThese results suggest that TLR3-mediated signaling during viral infection protects against demyelinating disease by reducing the viral load and modulating immune responses. In contrast, premature activation of TLR3 signal transduction prior to viral infection leads to pathogenesis via over-activation of the pathogenic immune response.

The Level of Viral Infection of Antigen-Presenting Cells Correlates with the Level of Development of Theiler's Murine Encephalomyelitis Virus-Induced Demyelinating Disease

“…The DCs treated with the ligands for TLR2, TLR3, TLR4, and TLR7 induced Th17 development significantly and further increased Th1 development, suggesting that signaling functions for the TLRs are similar in these DCs. The upregulated Th17 development in the presence of TLR3 and TLR4 ligands was consistent with the previous reports (18,31). In addition, the effects of a given TLR were consistent with the different TMEV-specific stimulants, such as VP2 peptide, UV-TMEV, and TMEV.…”

“…Our present study (Fig. 7) showed that the Th17 polarization can be achieved equally well in APCs from susceptible and resistant mice with agonists of multiple TLR receptors (TLR2, -3, -4, and -7), which are either known or suspected to be associated with the innate immune responses following TMEV infection (18,26,(29)(30)(31). Therefore, the potential deficiency of the signal induction in any of these TLRs is unlikely to be associated with the differential Th17 polarization.…”

The Role of Interleukin-6 in the Expression of PD-1 and PDL-1 on Central Nervous System Cells following Infection with Theiler's Murine Encephalomyelitis Virus

“…However, T cells expressing elevated PD-1 levels in HIV-1 infection do not appear to be functionally exhausted (42). Furthermore, we have repeatedly demonstrated that T cells in the CNS of TMEV-infected resistant B6 mice vigorously function for cytolysis and IFN-␥ production (34), yet the majority (ϳ70%) of the T cells express PD-1, similar to functionally compromised T cells in the CNS of infected susceptible SJL mice (Fig. 2).…”

“…Moreover, there was no impairment in the expression of PD-1 and PDL-1 on macrophages from TLR3 KO mice after in vitro TMEV infection (not shown). Interestingly, type I IFN (IFN-␣ and IFN-␤) and IL-6 levels are higher in TLR3 KO mice than in control mice due to the elevated viral load in the CNS (34). Thus, the elevated levels of type I IFNs caused by high viral loads in TLR3 KO mice may circumvent TLR3-dependent signaling for the induction of type I IFNs and IL-6 (46,47).…”

“…It was previously reported that innate immune responses to viral infection, such as the TLR response, upregulate the expression of PD-1 and PDL-1 molecules on various cell types (32,33). In addition, TMEV infection induces strong TLR3 and melanoma differentiation-associated gene 5 (MDA5) responses, and these innate immune responses play important protective roles in controlling viral loads in the CNS and the development of demyelinating disease (34,35). If a high viral load in the CNS affects the expression levels of PD-1 and PDL-1, we predicted that mice lacking these receptors for innate immunity to TMEV would display an elevated expression of PD-1 and PDL-1 molecules.…”