TREX1 Deficiency Triggers Cell-Autonomous Immunity in a cGAS-Dependent Manner

Cytosolic detection of DNA is crucial for the initiation of antiviral immunity but can also cause autoimmunity in the context of endogenous nucleic acids being sensed. Mutations in the human 3' repair exonuclease 1 (TREX1) have been linked to the type I IFN-associated autoimmune disease Aicardi-Goutières syndrome. The exact mechanisms driving unabated type I IFN responses in the absence of TREX1 are only partly understood, but it appears likely that accumulation of endogenous DNA species triggers a cell-autonomous immune response by activating a cytosolic DNA receptor. In this article, we demonstrate that knocking out the DNA sensor cyclic GMP-AMP synthase completely abrogates spontaneous induction of IFN-stimulated genes in TREX1-deficient cells. These findings indicate a key role of cyclic GMP-AMP synthase for the initiation of self-DNA-induced autoimmune disorders, thus providing important implications for novel therapeutic approaches.

Induction of type I IFNs by intracellular DNA-sensing pathways

Andrea Ablasser

A successful antimicrobial immune response involves the coordinate action of cells and soluble factors, with the cytokine family of type I interferons (IFNs) having a central role. Type I IFNs are not only crucial in conferring immediate antimicrobial, most importantly antiviral effects, but they also have an essential role in bridging the innate with the adaptive immune response. Therefore, production of these key cytokines must be tightly controlled. To this effect the host has evolved a set of pattern recognition receptors (PRRs) that reliably and specifically detect the presence of microbial pathogens before mounting an IFN response. Most PRR pathways that are known to induce type I IFNs are triggered upon recognition of nucleic acids. This mode of sensing is not straightforward, as large amounts of RNA and DNA are also present within the host. Nevertheless, in some cases distinct molecular features that are present within foreign nucleic acids but absent in endogenous nucleic acids, allow the host to reliably discriminate between 'self' and 'non-self'. At the same time, compartmentalization of PRRs within subcellular organelles that are usually devoid of host nucleic acids, but are sites of pathogen localization, is another principle that enables the host to distinguish self from non-self. The latter mode of sensing applies to the detection of microbial DNA within the cytoplasm, a compartment in which host DNAs are usually not present. Despite the past years' tremendous progress in the field of innate immunity, our understanding of cytoplasmic DNA sensing mechanisms is only beginning to form/take form. In this review, we outline the recent advancements in the elucidation of intracellular DNA-sensing pathways and discuss the future directions of this emerging field.

2012

Polynucleotide cytidine deaminases, innate immunity and hepatitis B infection

Stéphanie Jost

More than 350 millions individuals worldwide suffer from chronic hepatitis B virus (HBV) infection, a condition that evolves towards liver insufficiency and hepatocellular carcinoma in approximately 15 to 40 percent of cases. Yet the majority of de novo HBV infections, except for those occurring during the perinatal period, are cleared during the acute phase. This largely appears to result from non-cytopathic host defense mechanisms involving cytokines and notably interferons (IFNs). A key role for IFNs in the biology of HBV infection has been further illustrated by studies in animal models of the disease. Besides, IFN-α is the standard treatment for chronic hepatitis B. Although type I and II IFNs are the most important immune-regulators of HBV replication, the downstream effectors mediating the inhibitory action of IFNs on HBV replication have not yet been identified. In a first place we showed that HBV replication is blocked by human APOBEC3G and APOBEC3F, two closely related editing enzymes belonging to the family of polynucleotide cytidine deaminases, whose catalytic activity results in the conversion of cytidine residues to uridine residues. Both proteins were initially recognized for their ability to inhibit retroviruses, a property which mainly depends on their editing activity, and as the targets of the virion infectivity factor (Vif) protein of human immunodeficiency virus (HIV), which induces their degradation. The family of polynucleotide cytidine deaminases also comprises Activation-Induced cytidine Deaminase (AID), APOBEC1, APOBEC2, and the products of the APOBEC3A through APOBEC3H gene cluster. AID is essential for class switch recombination and somatic hypermutation of the immunoglobulin locus in pre-B lymphocytes, thus for the generation of antibody diversity, and APOBEC1 edits the apolipoprotein B mRNA in the guts, thereby regulating cholesterol metabolism. By comparison, little is known about the physiological functions of the other family members. However, apart from APOBEC2 and APOBEC3A, sequence analysis of the genes encoding these enzymes suggested that they possibly evolved as a host defense against pathogens. This hypothesis was confirmed for most of APOBEC3 proteins, which were independently found to restrict the replication of various exogenous and endogenous retroelements, as well as the adeno-associated virus. For this reason, we next explored the range of cytidine deaminases active on HBV and found that besides APOBEC3G and APOBEC3F, APOBEC3B, AID and APOBEC1 are also potent inhibitors of HBV. In parallel, while examining in details the mechanisms underlying the antiviral activity of APOBEC3G against HBV, we found that this cytidine deaminase blocks HBV infection not by lethal editing of nascent reverse transcripts, as for retroviruses, but rather by impairing the accumulation of pregenomic RNA (pgRNA)-containing viral capsids, the subviral structures in which HBV reverse transcription takes place before virions containing a partially double-stranded DNA genome are released. Remarkably, this is similar to the decreased steady-state levels of pgRNA-containing capsids which are observed upon exposure of HBV infected cells to IFNs. It suggested that IFNs and cytidine deaminases might affect HBV replication through the same pathway. The first step to unveil the in vivo relevance of antiviral cytidine deaminases for the control of HBV replication consisted in our demonstration that type I and II IFN treatment of human hepatocytes induces the production of APOBEC3G, and to a smaller extent, that of APOBEC3F and APOBEC3B, but not that of AID and APOBEC1. These observations led us to ask whether the induction of antiviral cytidine deaminases by cytokines contributes to the IFN-induced clearance of HBV, as IFN-α and IFN-γ are both found in high amounts in the liver of acutely infected individuals. Importantly, we revealed that blocking APOBEC3G, APOBEC3F and APOBEC3B in hepatic cell lines does not abrogate the inhibitory effect of IFNs on HBV, indicating that antiviral cytidine deaminases are not the main mediators of the action of IFNs on this pathogen. Nevertheless, while HBV replication was not impaired in hepatic cells when these enzymes were expressed at their baseline levels, they could edit the HBV genome in these conditions, suggesting that cytidine deaminases might participate in the emergence of new HBV strains, notably those that have been recognized as being of clinical importance. Finally, the results of this work indicate that a therapeutic strategy based on the expression of antiviral cytidine deaminases in the liver could be envisioned to cure HBV chronic infections.

EPFL2007

Adeno-Associated Virus Activates an Innate Immune Response in Normal Human Cells but Not in Osteosarcoma Cells

Peter Martin Beard

Adeno-associated virus (AAV) is a small, DNA-containing dependovirus with promising potential as a gene delivery vehicle. Given the variety of applications of AAV-based vectors in the treatment of genetic disorders, numerous studies have focused on the immunogenicity of recombinant AAV. In general, AAV vectors appear not to induce strong inflammatory responses. We have found that AAV2, when it infects the osteosarcoma cells U2OS, can initiate part of its replicative cycle in the absence of helper virus. This does not occur in untransformed cells. We set out to test whether the cellular innate antiviral defenses control this susceptibility and found that, in nonimmune normal human fibroblasts, AAV2 induces type I interferon production and release and the accumulation of nuclear promyelocytic leukemia bodies. AAV fails to mobilize this defense pathway in the U2OS cells. This permissiveness is in large part due to impairment of the viral sensing machinery in these cells. Our investigations point to Toll-like receptor 9 as a potential intracellular sensor that detects AAV2 and triggers the antiviral state in AAV-infected untransformed cells. Efficient sensing of the AAV genome and the ensuing activation of an innate antiviral response are thus crucial cellular events dictating the parvovirus infectivity in host cells.

2011

Polynucleotide cytidine deaminases, innate immunity and hepatitis B infection

Stéphanie Jost

EPFL2007