TLR3 deficiency in patients with herpes simplex encephalitis

Human Toll-like receptor-dependent induction of interferons in protective immunity to viruses

Five of the 10 human Toll-like receptors (TLRs) (TLR3, TLR4, TLR7, TLR8, and TLR9), and four of the 12 mouse TLRs (TLR3, TLR4, TLR7, TLR9) can trigger interferon (IFN)-alpha, IFN-beta, and IFN-lambda, which are critical for antiviral immunity. Moreover, TLR3, TLR7, TLR8, and TLR9 differ from TLR4 in two particularly important ways for antiviral immunity: they can be activated by nucleic acid agonists mimicking compounds produced during the viral cycle, and they are typically present within the cell, along the endocytic pathway, where they sense viral products in the intraluminal space. Investigations in mice have demonstrated that the TLR7/9-IFN and TLR3-IFN pathways are different and critical for protective immunity to various experimental viral infections. Investigations in humans with interleukin-1 receptor-associated kinase-4 (IRAK-4) deficiency (unresponsive to TLR7, TLR8, and TLR9), UNC-93B deficiency (unresponsive to TLR3, TLR7, TLR8, and TLR9), and TLR3 deficiency have recently shed light on the role of these two pathways in antiviral immunity in natural conditions. UNC-93B- and TLR3-deficient patients appear to be specifically prone to herpes simplex virus 1 (HSV-1) encephalitis, although clinical penetrance is incomplete, whereas IRAK-4-deficient patients appear to be normally resistant to most viruses, including HSV-1. These experiments of nature suggest that the TLR7-, TLR8-, and TLR9-dependent induction of IFN-alpha, IFN-beta, and IFN-lambda is largely redundant in human antiviral immunity, whereas the TLR3-dependent induction of IFN-alpha, IFN-beta, and IFN-lambda is critical for primary immunity to HSV-1 in the central nervous system in children but redundant for immunity to most other viral infections.

Maturation-dependent responses of human neuronal cells to western equine encephalitis virus infection and type I interferons

Innate cell-autonomous antiviral responses are essential first lines of defense against central nervous system infections but may also contribute to neuropathogenesis. We investigated the relationships between innate immunity and neuronal differentiation using an in vitro culture system with human cell lines to analyze cellular responses to the neurotropic alphavirus western equine encephalitis virus. Human neuronal cells displayed a maturation-dependent reduction in virus-induced cytopathology that was independent of autocrine interferon alpha or beta activity. In addition, maturation was associated with enhanced responsiveness to exogenous stimuli, such that differentiated neurons required five- to ten-fold less type I interferon to suppress viral replication or virus-induced cytopathology compared to immature cells, although this enhanced responsiveness extended to only a subset of unique type I interferons. These results demonstrate that maturation-dependent changes in human neuronal cells may be key determinants in the innate immune response to infections with neurotropic alphaviruses.

Immunological intervention in human diseases

A recent Keystone Symposium Meeting on "Immunological Intervention in Human Disease" was held in Big Sky, Montana on January, 6–11, 2007, organized by Jacques Banchereau, Federica Sallusto and Robert Coffman. It brought together basic scientists and clinicians from both academia and the pharmaceutical industry to discuss how the immune system is involved in the development of human diseases, including cancer, allergy, autoimmunity, and infectious diseases. We highlight advances in our understanding of the pathogenesis of immune-mediated diseases and future approaches in the immune therapeutic interventions. Considerable progress in the development of model systems and methodologies to monitor human immune responses will help to develop and to evaluate new immune-based therapies at pre-clinical and clinical studies.

Selective predisposition to bacterial infections in IRAK-4-deficient children: IRAK-4-dependent TLRs are otherwise redundant in protective immunity

Human interleukin (IL) 1 receptor–associated kinase 4 (IRAK-4) deficiency is a recently discovered primary immunodeficiency that impairs Toll/IL-1R immunity, except for the Toll-like receptor (TLR) 3– and TLR4–interferon (IFN)-a/b pathways. The clinical and immunological phenotype remains largely unknown. We diagnosed up to 28 patients with IRAK-4 deficiency, tested blood TLR responses for individual leukocyte subsets, and TLR responses for multiple cytokines. The patients' peripheral blood mononuclear cells (PBMCs) did not induce the 11 non-IFN cytokines tested upon activation with TLR agonists other than the nonspecific TLR3 agonist poly(I:C). The patients' individual cell subsets from both myeloid (granulocytes, monocytes, monocyte-derived dendritic cells [MDDCs], myeloid DCs [MDCs], and plasmacytoid DCs) and lymphoid (B, T, and NK cells) lineages did not respond to the TLR agonists that stimulated control cells, with the exception of residual responses to poly(I:C) and lipopolysaccharide in MDCs and MDDCs. Most patients (22 out of 28; 79%) suffered from invasive pneumococcal disease, which was often recurrent (13 out of 22; 59%). Other infections were rare, with the exception of severe staphylococcal disease (9 out of 28; 32%). Almost half of the patients died (12 out of 28; 43%). No death and no invasive infection occurred in patients older than 8 and 14 yr, respectively. The IRAK-4–dependent TLRs and IL-1Rs are therefore vital for childhood immunity to pyogenic bacteria, particularly Streptococcus pneumoniae. Conversely, IRAK-4–dependent human TLRs appear to play a redundant role in protective immunity to most infections, at most limited to childhood immunity to some pyogenic bacteria.