Horst von Bernuth

TLR3 Deficiency in Patients with Herpes Simplex Encephalitis

Some Toll and Toll-like receptors (TLRs) provide immunity to experimental infections in animal models, but their contribution to host defense in natural ecosystems is unknown. We report a dominant-negative TLR3 allele in otherwise healthy children with herpes simplex virus 1 (HSV-1) encephalitis. TLR3 is expressed in the central nervous system (CNS), where it is required to control HSV-1, which spreads from the epithelium to the CNS via cranial nerves. TLR3 is also expressed in epithelial and dendritic cells, which apparently use TLR3-independent pathways to prevent further dissemination of HSV-1 and to provide resistance to other pathogens in TLR3-deficient patients. Human TLR3 appears to be redundant in host defense to most microbes but is vital for natural immunity to HSV-1 in the CNS, which suggests that neurotropic viruses have contributed to the evolutionary maintenance of TLR3.

Mutations in STAT3 and IL12RB1 impair the development of human IL-17–producing T cells

The cytokines controlling the development of human interleukin (IL) 17–producing T helper cells in vitro have been difficult to identify. We addressed the question of the development of human IL-17–producing T helper cells in vivo by quantifying the production and secretion of IL-17 by fresh T cells ex vivo, and by T cell blasts expanded in vitro from patients with particular genetic traits affecting transforming growth factor (TGF) β, IL-1, IL-6, or IL-23 responses. Activating mutations in TGFB1, TGFBR1, and TGFBR2 (Camurati-Engelmann disease and Marfan-like syndromes) and loss-of-function mutations in IRAK4 and MYD88 (Mendelian predisposition to pyogenic bacterial infections) had no detectable impact. In contrast, dominant-negative mutations in STAT3 (autosomal-dominant hyperimmunoglobulin E syndrome) and, to a lesser extent, null mutations in IL12B and IL12RB1 (Mendelian susceptibility to mycobacterial diseases) impaired the development of IL-17–producing T cells. These data suggest that IL-12Rβ1– and STAT-3–dependent signals play a key role in the differentiation and/or expansion of human IL-17–producing T cell populations in vivo.

Clinical features and outcome of patients with IRAK-4 and MyD88 deficiency

Autosomal recessive interleukin-1 receptor-associated kinase (IRAK)-4 and myeloid differentiation factor (MyD)88 deficiencies impair Toll-like receptor (TLR)- and interleukin-1 receptor-mediated immunity. We documented the clinical features and outcome of 48 patients with IRAK-4 deficiency and 12 patients with MyD88 deficiency, from 37 kindreds in 15 countries. The clinical features of IRAK-4 and MyD88 deficiency were indistinguishable. There were no severe viral, parasitic, and fungal diseases, and the range of bacterial infections was narrow. Noninvasive bacterial infections occurred in 52 patients, with a high incidence of infections of the upper respiratory tract and the skin, mostly caused by Pseudomonas aeruginosa and Staphylococcus aureus, respectively. The leading threat was invasive pneumococcal disease, documented in 41 patients (68%) and causing 72 documented invasive infections (52.2%). P. aeruginosa and Staph. aureus documented invasive infections also occurred (16.7% and 16%, respectively, in 13 and 13 patients, respectively). Systemic signs of inflammation were usually weak or delayed. The first invasive infection occurred before the age of 2 years in 53 (88.3%) and in the neonatal period in 19 (32.7%) patients. Multiple or recurrent invasive infections were observed in most survivors (n = 36/50, 72%). Clinical outcome was poor, with 24 deaths, in 10 cases during the first invasive episode and in 16 cases of invasive pneumococcal disease. However, no death and invasive infectious disease were reported in patients after the age of 8 years and 14 years, respectively. Antibiotic prophylaxis (n = 34), antipneumococcal vaccination (n = 31), and/or IgG infusion (n = 19), when instituted, had a beneficial impact on patients until the teenage years, with no seemingly detectable impact thereafter. IRAK-4 and MyD88 deficiencies predispose patients to recurrent life-threatening bacterial diseases, such as invasive pneumococcal disease in particular, in infancy and early childhood, with weak signs of inflammation. Patients and families should be informed of the risk of developing life-threatening infections; empiric antibacterial treatment and immediate medical consultation are strongly recommended in cases of suspected infection or moderate fever. Prophylactic measures in childhood are beneficial, until spontaneous improvement occurs in adolescence. Abbreviations: CRP = C-reactive protein, ELISA = enzyme-linked immunosorbent assay, IFN = interferon, IKBA = I&kgr;B&agr;, IL = interleukin, IL-1R = interleukin-1 receptor, InvBD = invasive bacterial disease, IRAK = interleukin-1 receptor-associated kinase, MyD = myeloid differentiation factor, NEMO = nuclear factor-kappaB essential modulator, NInvBD = noninvasive bacterial disease, TIR = Toll/IL-1R, TLR = Toll-like receptor, TNF = tumor necrosis factor.

Human TLR-7-, -8-, and -9-Mediated Induction of IFN-α/β and -λ Is IRAK-4 Dependent and Redundant for Protective Immunity to Viruses

Summary Five TLRs are thought to play an important role in antiviral immunity, sensing viral products and inducing IFN-α/β and -λ. Surprisingly, patients with a defect of IRAK-4, a critical kinase downstream from TLRs, are resistant to common viruses. We show here that IFN-α/β and -λ induction via TLR-7, TLR-8, and TLR-9 was abolished in IRAK-4-deficient blood cells. In contrast, IFN-α/β and -λ were induced normally by TLR-3 and TLR-4 agonists. Moreover, IFN-β and -λ were normally induced by TLR-3 agonists and viruses in IRAK-4-deficient fibroblasts. We further show that IFN-α/β and -λ production in response to 9 of 11 viruses tested was normal or weakly affected in IRAK-4-deficient blood cells. Thus, IRAK-4-deficient patients may control viral infections by TLR-3- and TLR-4-dependent and/or TLR-independent production of IFNs. The TLR-7-, TLR-8-, and TLR-9-dependent induction of IFN-α/β and -λ is strictly IRAK-4 dependent and paradoxically redundant for protective immunity to most viruses in humans.

Neutrophils: Between host defence, immune modulation, and tissue injury.

Neutrophils, the most abundant human immune cells, are rapidly recruited to sites of infection, where they fulfill their life-saving antimicrobial functions. While traditionally regarded as short-lived phagocytes, recent findings on long-term survival, neutrophil extracellular trap (NET) formation, heterogeneity and plasticity, suppressive functions, and tissue injury have expanded our understanding of their diverse role in infection and inflammation. This review summarises our current understanding of neutrophils in host-pathogen interactions and disease involvement, illustrating the versatility and plasticity of the neutrophil, moving between host defence, immune modulation, and tissue damage.

IRAK-4- and MyD88-Dependent Pathways Are Essential for the Removal of Developing Autoreactive B Cells in Humans

Most autoreactive B cells are normally counterselected during early B cell development. To determine whether Toll-like receptors (TLRs) regulate the removal of autoreactive B lymphocytes, we tested the reactivity of recombinant antibodies from single B cells isolated from patients deficient for interleukin-1 receptor-associated kinase 4 (IRAK-4), myeloid differentiation factor 88 (MyD88), and UNC-93B. Indeed, all TLRs except TLR3 require IRAK-4 and MyD88 to signal, and UNC-93B-deficient cells are unresponsive to TLR3, TLR7, TLR8, and TLR9. All patients suffered from defective central and peripheral B cell tolerance checkpoints, resulting in the accumulation of large numbers of autoreactive mature naive B cells in their blood. Hence, TLR7, TLR8, and TLR9 may prevent the recruitment of developing autoreactive B cells in healthy donors. Paradoxically, IRAK-4-, MyD88-, and UNC-93B-deficient patients did not display autoreactive antibodies in their serum or develop autoimmune diseases, suggesting that IRAK-4, MyD88, and UNC-93B pathway blockade may thwart autoimmunity in humans.

Inherited disorders of human Toll-like receptor signaling: immunological implications

Summary:  In vitro nine of 10 known human Toll‐like receptors (TLRs) are engaged by well‐defined chemical agonists that mimic microbial compounds, raising the possibility that human TLRs play a critical role in protective immunity in vivo. We thus review here the recently described human primary immunodeficiencies caused by germline mutations in genes encoding molecules involved in cell signaling downstream from TLRs. Subjects with anhidrotic ectodermal dysplasia with immunodeficiency (EDA‐ID) carry either X‐linked recessive hypomorphic mutations in NEMO or autosomal dominant hypermorphic mutations in IKBA. Their cells show a broad defect in nuclear factor‐κB (NF‐κB) activation, with an impaired, but not abolished response to a large variety of stimuli including TLR agonists. EDA‐ID patients show developmental anomalies of skin appendages and a broad spectrum of infectious diseases. Patients with autosomal recessive amorphic mutations in IRAK4 present a purely immunological syndrome and more restricted defects, with specific impairment of the Toll and interleukin‐1 receptor (TIR)–interleukin‐1 receptor‐associated kinase (IRAK) signaling pathway. In these subjects, the NF‐κB‐ and mitogen‐activated protein kinase‐mediated induction of inflammatory cytokines in response to TIR agonists is impaired. The patients present a narrow range of pyogenic bacterial infections that become increasingly rare with age. Altogether, these data suggest that human TLRs play a critical role in host defense. However, they do not provide compelling evidence, as even the infectious phenotype of patients with mutations in IRAK4 may result from impaired signaling via receptors other than TLRs. Paradoxically, these experiments of nature raise the possibility that the entire set of human TLRs is largely redundant in protective immunity in vivo.

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

Summary: 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)‐α, IFN‐β, and IFN‐λ, 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‐α, IFN‐β, and IFN‐λ is largely redundant in human antiviral immunity, whereas the TLR3‐dependent induction of IFN‐α, IFN‐β, and IFN‐λ is critical for primary immunity to HSV‐1 in the central nervous system in children but redundant for immunity to most other viral infections.

IRAK4 and NEMO mutations in otherwise healthy children with recurrent invasive pneumococcal disease

Background: About 2% of childhood episodes of invasive pneumococcal disease (IPD) are recurrent, and most remain unexplained. Objective: To report two cases of otherwise healthy, unrelated children with recurrent IPD as the only clinical infectious manifestation of an inherited disorder in nuclear factor-κB(NF-κB)-dependent immunity. Results: One child carried two germline mutations in IRAK4, and had impaired cellular responses to interleukin (IL)1 receptor and toll-like receptor (TLR) stimulation. The other child carried a hemizygous mutation in NEMO, associated with a broader impairment of NF-κB activation, with an impaired cellular response to IL-1R, TLR and tumour necrosis factor receptor stimulation. The two patients shared a narrow clinical phenotype, associated with two related but different genotypes. Conclusions: Otherwise healthy children with recurrent IPD should be explored for underlying primary immunodeficiencies affecting the IRAK4-dependent and NEMO-dependent signalling pathways.

Experimental and natural infections in MyD88‐ and IRAK‐4‐deficient mice and humans

Most Toll‐like‐receptors (TLRs) and interleukin‐1 receptors (IL‐1Rs) signal via myeloid differentiation primary response 88 (MyD88) and interleukin‐1 receptor‐associated kinase 4 (IRAK‐4). The combined roles of these two receptor families in the course of experimental infections have been assessed in MyD88‐ and IRAK‐4‐deficient mice for almost fifteen years. These animals have been shown to be susceptible to 46 pathogens: 27 bacteria, eight viruses, seven parasites, and four fungi. Humans with inborn MyD88 or IRAK‐4 deficiency were first identified in 2003. They suffer from naturally occurring life‐threatening infections caused by a small number of bacterial species, although the incidence and severity of these infections decrease with age. Mouse TLR‐ and IL‐1R‐dependent immunity mediated by MyD88 and IRAK‐4 seems to be vital to combat a wide array of experimentally administered pathogens at most ages. By contrast, human TLR‐ and IL‐1R‐dependent immunity mediated by MyD88 and IRAK‐4 seems to be effective in the natural setting against only a few bacteria and is most important in infancy and early childhood. The roles of TLRs and IL‐1Rs in protective immunity deduced from studies in mutant mice subjected to experimental infections should therefore be reconsidered in the light of findings for natural infections in humans carrying mutations as discussed in this review.