Toll-Like Receptors in CNS Viral Infections

2. Toll-Like Receptors in CNS Viral Infections • Unclear role of TLRs in • natural infections • TLR3, -7, -8, -9 recognize • viral nucleic acids. • Localized intracellularly in • endosomal compartments • Ligands for TLR3: • - dsRNA (Reovirus) • - dsRNA intermediates (RNA, DNA viruses) • - cellular RNA, released • upon tissue damage (ssRNA, mRNA) • TLR3 is expressed abundantly in astrocytes, as well as in microglial cells and neurons • upon viral infections and brain disorders • Menager et al. PLoS Pathog 5(2): e1000315. doi:10.1371/journal.ppat.1000315; • Tammy Kielian. Toll-like Receptors: Roles in Infection and Neuropathology. • Current Topics in Microbiology and Immunology ISSN 0070-217x

3. Herpes Simplex Life Cycle • Primary lytic infection of epithelial or mucosal cells; • b) Progeny viral particles are transported to nerve cell body. Virus can be spread to CNS, or • c) Viral genome can persist as a latent episome within the neuron; • d) Upon stimulation virus can reactivate and travel to neuron termini; • e) Virus initiates productive recurrent infection upon encountering an epithelial or mucosal cell. Frampton et al. Gene Therapy (2005) 12, 891–901. doi:10.1038/sj.gt.3302545

4. Herpes Simplex Encephalitis (HSE) • severe viral infection of the human central nervous system • affects at least 1 in 500,000 individuals per year • both primary and recurrent HSV infections can cause disease of the CNS • the route of access of virus to the CNS in primary infection, especially in humans, is a subject of debate. Classic studies defined pathways for access of HSV to the brain in animals and include both the olfactory and trigeminal nerves among others • IRAK4-deficient patients (fail to signal through TLR7, -8 and -9) are not succeptible to HSE • UNC93B-deficient patients (fail to signal through TLR3, -7, 8- and -9) display impaired TLR3-dependent IFN-a, -b, -l production http://pathology.mc.duke.edu/ neuropath/CNSlecture2/hsv.jpg Impaired TLR3-dependent induction of IFN-a, -b, -l is involved in HSE Whitley. Herpes simplex encephalitis: Adolescents and adults. Antiviral Res. 2006 Sep;71(2-3):141-8.

5. A Heterozygous TLR3 Mutation in Two Children with HSE The P554S TLR3 mutation conferred an autosomal dominant predisposition to HSE with incomplete clinical penetrance.

6. Impaired Responsiveness of Fibroblasts to Poly(I:C) Stimulation The cosegregation of genotype and fibroblastic phenotype suggests that heterozygosity for the P554S TLR3 allele confers autosomal dominant hyporesponsiveness to poly(I:C) in fibroblasts.

7. Dominant-negative Effect of the P554S TLR3 Allele in Fibroblasts The P554S TLR3 protein is C-terminally truncated. P554S mutation leads to loss-of-function for poly(I:C) responsiveness, and is dominant negative in fibroblasts, at least for IFN induction.

8. Impaired IFN-dependent Control of Viruses in TLR3-deficient Fibroblasts

9. ... The P554S TLR3 mutation leads to TLR3 impaired signaling, abnormally weak IFN-a/b and –l production, enhanced viral replication, and higher levels of fibroblast cell death upon viral infection.

10. Impaired Response to Poly(I:C) Stimulation in MDDCs, NK and CD8 T Cells TLR3 mutation affects heterozygous children, but contribution of NK cells and CD8 T cells to the pathogenesis of HSE is probably modest.

11. Response of Blood DCs and Keratinocytes to Poly(I:C) Stimulation The lack of clinical HSV-1 dissemination by the blood in patients with HSE may be due to the induction of IFNs by MDCs and PDCs stimulated with dsRNA and other viral intermediates. The poly(I:C) responsiveness of DCs and keratinocytes probably operated through TLR3-independent pathways, although residual TLR3 signaling or lack of dominance of P554S TLR3 mutant cannot be excluded in these cells. HSV-1 does not spread to epithelia during or following HSE.

12. Most Viruses Trigger IFNs in TLR3 Heterozygous Cells Induction of IFN-a, -b and –l in blood cells and fibroblasts from TLR3-heterozygous patients after stimulation with most of the viruses tested, was consistent with the natural resistance of these patients to most viruses other than HSV-1.

13. Conclusions • Molecular pathogenesis of HSE primarily involves impaired TLR3-dependent IFN-a, -b and -l responses. Pathogenic cellular mechanism involves intrinsic defect affecting CNS-resident cells: - neurotropic infection of the CNS by HSV-1, - CNS-restricted clinical course of HSE, - preferential expression of TLR3 in CNS, - poly(I:C) inducible production of antiviral IFNs by blood DCs in TLR3 heterozygotes, - the absence of HSE in patients with conventional primary immunodeficiencies. • After autosomal recessive UNC93B-deficiency, autosomal dominant TLR3 deficiency was identified as the second genetic etiology of isolated HSE. • The findings provide further support for the treatment of HSE patients with IFN-a in addition to acyclovir. • TLR3-deficiency appears to be redundant in antiviral immunity, in humans as well as in mice. Nevertheless, human TLR3 is essential for primary immunity to HSV-1 in the CNS (although multiple factors may affect clinical penetrance: age at infection with HSV-1, viral inoculum, and human modifier genes).