Toll-Like Receptors in CNS Viral InfectionsToll-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

Herpes Simplex Life CycleHerpes Simplex Life Cycle

Frampton et al. Gene Therapy (2005) 12,

891–901. doi:10.1038/sj.gt.3302545

a) 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.

Herpes Simplex Encephalitis (HSE)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-, -, - production

http://pathology.mc.duke.edu/neuropath/CNSlecture2/hsv.jpg

Whitley. Herpes simplex encephalitis: Adolescents and adults. Antiviral Res. 2006 Sep;71(2-3):141-8.

Impaired TLR3-dependent induction of IFN-, -, - is involved in HSE

A Heterozygous TLR3 Mutation in Two A Heterozygous TLR3 Mutation in Two Children with HSEChildren with HSE

The P554S TLR3 mutation conferred an autosomal dominant predisposition to HSE with incomplete clinical penetrance.

Impaired Responsiveness of Fibroblasts to Impaired Responsiveness of Fibroblasts to Poly(I:C) StimulationPoly(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.

Dominant-negative Effect of the P554S Dominant-negative Effect of the P554S TLR3TLR3 Allele in Fibroblasts 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.

Impaired IFN-dependent Control of Viruses Impaired IFN-dependent Control of Viruses in TLR3-deficient Fibroblastsin TLR3-deficient Fibroblasts

...

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

Impaired Response to Poly(I:C) Stimulation Impaired Response to Poly(I:C) Stimulation in MDDCs, NK and CD8 T Cellsin 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.

Response of Blood DCs and Keratinocytes Response of Blood DCs and Keratinocytes to Poly(I:C) Stimulationto 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.

HSV-1 does not spread to epithelia during or following HSE.

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.

Most Viruses Trigger IFNs in TLR3 Most Viruses Trigger IFNs in TLR3 Heterozygous CellsHeterozygous Cells

Induction of IFN-, - and –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.

ConclusionsConclusions• After autosomal recessive UNC93B-deficiency, autosomal dominant TLR3

deficiency was identified as the second genetic etiology of isolated HSE.• Molecular pathogenesis of HSE primarily involves impaired TLR3-dependent IFN-, - and - 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.

• The findings provide further support for the treatment of HSE patients with IFN- 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).