Regulation of cGAS and STING signaling during inflammation and infection.

Stimulator of interferon genes (STING) is a sensor of cyclic dinucleotides including cyclic GMP-AMP, which is produced by cyclic GMP-AMP synthase (cGAS) in response to cytosolic DNA. The cGAS-STING signaling pathway regulates both innate and adaptive immune responses, as well as fundamental cellular functions such as autophagy, senescence, and apoptosis. Mutations leading to constitutive activation of STING cause devastating human diseases. Thus, the cGAS-STING pathway is of great interest because of its role in diverse cellular processes and because of the potential therapeutic implications of targeting cGAS and STING. Here, we review molecular and cellular mechanisms of STING signaling, and we propose a framework for understanding the immunological and other cellular functions of STING in the context of disease.

Transferrable protection by gut microbes against STING-associated lung disease.

STING modulates immunity by responding to bacterial and endogenous cyclic dinucleotides (CDNs). Humans and mice with STING gain-of-function mutations develop a syndrome known as STING-associated vasculopathy with onset in infancy (SAVI), which is characterized by inflammatory or fibrosing lung disease. We hypothesized that hyperresponsiveness of gain-of-function STING to bacterial CDNs might explain autoinflammatory lung disease in SAVI mice. We report that depletion of gut microbes with oral antibiotics (vancomycin, neomycin, and ampicillin [VNA]) nearly eliminates lung disease in SAVI mice, implying that gut microbes might promote STING-associated autoinflammation. However, we show that germ-free SAVI mice still develop severe autoinflammatory disease and that transferring gut microbiota from antibiotics-treated mice to germ-free animals eliminates lung inflammation. Depletion of anaerobes with metronidazole abolishes the protective effect of the VNA antibiotics cocktail, and recolonization with the metronidazole-sensitive anaerobe Bacteroides thetaiotaomicron prevents disease, confirming a protective role of a metronidazole-sensitive microbe in a model of SAVI.

HSV-1 and Zika Virus but Not SARS-CoV-2 Replicate in the Human Cornea and Are Restricted by Corneal Type III Interferon.

Here, we report our studies of immune-mediated regulation of Zika virus (ZIKV), herpes simplex virus 1 (HSV-1), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in the human cornea. We find that ZIKV can be transmitted via corneal transplantation in mice. However, in human corneal explants, we report that ZIKV does not replicate efficiently and that SARS-CoV-2 does not replicate at all. Additionally, we demonstrate that type III interferon (IFN-λ) and its receptor (IFNλR1) are expressed in the corneal epithelium. Treatment of human corneal explants with IFN-λ, and treatment of mice with IFN-λ eye drops, upregulates antiviral interferon-stimulated genes. In human corneal explants, blockade of IFNλR1 enhances replication of ZIKV and HSV-1 but not SARS-CoV-2. In addition to an antiviral role for IFNλR1 in the cornea, our results suggest that the human cornea does not support SARS-CoV-2 infection despite expression of ACE2, a SARS-CoV-2 receptor, in the human corneal epithelium.

Immunopathology of Zika virus infection.

Zika virus (ZIKV) is a mosquito-borne virus of the flavivirus genus in the Flaviviridae family. Flaviviruses are single-stranded, positive-sense RNA viruses that have been responsible for numerous human epidemics. Notable flaviviruses include mosquito-borne viruses such as yellow fever virus (YFV), Dengue virus (DENV), West Nile virus (WNV), Japanese encephalitis virus (JEV), as well as tick-borne viruses including Powassan virus (POWV) and tick-borne encephalitis virus (TBEV). Despite having been relatively obscure until the past decade, ZIKV has become a major global health concern, and is a topic of active research following multiple outbreaks across the globe. Here, we discuss ZIKV pathogenesis and the associated immunopathology, as well as advances in research, therapies, and vaccines developed using models of ZIKV pathogenesis.

STING Gain-of-Function Disrupts Lymph Node Organogenesis and Innate Lymphoid Cell Development in Mice.

STING gain-of-function causes autoimmunity and immunodeficiency in mice and STING-associated vasculopathy with onset in infancy (SAVI) in humans. Here, we report that STING gain-of-function in mice prevents development of lymph nodes and Peyer's patches. We show that the absence of secondary lymphoid organs is associated with diminished numbers of innate lymphoid cells (ILCs), including lymphoid tissue inducer (LTi) cells. Although wild-type (WT) α4ß7+ progenitors differentiate efficiently into LTi cells, STING gain-of-function progenitors do not. Furthermore, STING gain-of-function impairs development of all types of ILCs. Patients with STING gain-of-function mutations have fewer ILCs, although they still have lymph nodes. In mice, expression of the STING mutant in RORγT-positive lineages prevents development of lymph nodes and reduces numbers of LTi cells. RORγT lineage-specific expression of STING gain-of-function also causes lung disease. Since RORγT is expressed exclusively in LTi cells during fetal development, our findings suggest that STING gain-of-function prevents lymph node organogenesis by reducing LTi cell numbers in mice.

A Human STAT1 Gain-of-Function Mutation Impairs CD8+ T Cell Responses against Gammaherpesvirus 68.

Autosomal dominant STAT1 mutations in humans have been associated with chronic mucocutaneous candidiasis (CMC), as well as with increased susceptibility to herpesvirus infections. Prior studies have focused on mucosal and Th17-mediated immunity against Candida, but mechanisms of impaired antiviral immunity have not previously been examined. To begin to explore the mechanisms of STAT1-associated immunodeficiency against herpesviruses, we generated heterozygous STAT1 R274W knock-in mice that have a frequently reported STAT1 mutation associated in humans with susceptibility to herpesvirus infections. In primary macrophages and fibroblasts, we found that STAT1 R274W had no appreciable effect on cell-intrinsic immunity against herpes simplex virus 1 (HSV-1) or gammaherpesvirus 68 (γHV68) infection. However, intraperitoneal inoculation of mice with γHV68 was associated with impaired control of infection at day 14 in STAT1 R274W mice compared with that in wild-type (WT) littermate control animals. Infection of STAT1 R274W mice was associated with paradoxically decreased expression of IFN-stimulated genes (ISGs) and gamma interferon (IFN-γ), likely secondary to defective CD4+ and CD8+ T cell responses, including diminished numbers of antigen-specific CD8+ T cells. Viral pathogenesis studies in WT and STAT1 R274W mixed bone marrow chimeric mice revealed that the presence of WT leukocytes was sufficient to limit infection and that antigen-specific STAT1 R274W CD8+ T cell responses were impaired even in the presence of WT leukocytes. Thus, in addition to regulating Th17 responses against Candida, a STAT1 gain-of-function mutant impedes antigen-specific T cell responses against a common gammaherpesvirus in mice.IMPORTANCE Mechanisms of immunodeficiency related to STAT1 gain of function have not been previously studied in an animal model of viral pathogenesis. Using virological and immunological techniques, we examined the immune response to γHV68 in heterozygous mice that have an autosomal dominant mutation in the STAT1 coiled-coil domain (STAT1 R274W). We observed impaired control of infection, which was associated with diminished production of gamma interferon (IFN-γ), fewer effector CD4+ and CD8+ T cells, and a reduction in the number of antigen-specific CD8+ T cells. These findings indicate that a STAT1 gain-of-function mutation limits production of antiviral T cells, likely contributing to immunodeficiency against herpesviruses.

STING-associated lung disease in mice relies on T cells but not type I interferon.

BACKGROUND: Monogenic interferonopathies are thought to be mediated by type I interferon. For example, a gain-of-function mutation in stimulator of interferon genes (STING; N153S) upregulates type I interferon-stimulated genes and causes perivascular inflammatory lung disease in mice. The equivalent mutation in human subjects also causes lung disease, which is thought to require signaling through the cyclic GMP-AMP synthase (cGAS)-STING pathway and subsequent activation of interferon regulatory factors (IRFs) 3 and 7, type I interferon, and interferon-stimulated genes. OBJECTIVE: We set out to define the roles of cGAS, IRF3, IRF7, the type I interferon receptor (IFN-α and IFN-ß receptor subunit 1 [IFNAR1]), T cells, and B cells in spontaneous lung disease in STING N153S mice. METHODS: STING N153S mice were crossed to animals lacking cGAS, IRF3/IRF7, IFNAR1, adaptive immunity, αß T cells, and mature B cells. Mice were evaluated for spontaneous lung disease. Additionally, bone marrow chimeric mice were assessed for lung disease severity and survival. RESULTS: Lung disease in STING N153S mice developed independently of cGAS, IRF3/IRF7, and IFNAR1. Bone marrow transplantation revealed that certain features of STING N153S-associated disease are intrinsic to the hematopoietic compartment. Recombination-activating gene 1 (Rag1)-/- STING N153S mice that lack adaptive immunity had no lung disease, and T-cell receptor ß chain (Tcrb)-/- STING N153S animals only had mild disease. STING N153S led to a reduction in percentages and numbers of naive and regulatory T cells, as well as an increased frequency of cytokine-producing effector T cells. CONCLUSION: Spontaneous lung disease in STING N153S mice develops independently of type I interferon signaling and cGAS. STING N153S relies primarily on T cells to promote lung disease in mice.

A Human Gain-of-Function STING Mutation Causes Immunodeficiency and Gammaherpesvirus-Induced Pulmonary Fibrosis in Mice.

We previously generated STING N153S knock-in mice that have a human disease-associated gain-of-function mutation in STING. Patients with this mutation (STING N154S in humans) develop STING-associated vasculopathy with onset in infancy (SAVI), a severe pediatric autoinflammatory disease characterized by pulmonary fibrosis. Since this mutation promotes the upregulation of antiviral type I interferon-stimulated genes (ISGs), we hypothesized that STING N153S knock-in mice may develop more severe autoinflammatory disease in response to a virus challenge. To test this hypothesis, we infected heterozygous STING N153S mice with murine gammaherpesvirus 68 (γHV68). STING N153S mice were highly vulnerable to infection and developed pulmonary fibrosis after infection. In addition to impairing CD8+ T cell responses and humoral immunity, STING N153S also promoted the replication of γHV68 in cultured macrophages. In further support of a combined innate and adaptive immunodeficiency, γHV68 infection was more severe in Rag1-/- STING N153S mice than in Rag1-/- littermate mice, which completely lack adaptive immunity. Thus, a gain-of-function STING mutation creates a combined innate and adaptive immunodeficiency that leads to virus-induced pulmonary fibrosis.IMPORTANCE A variety of human rheumatologic disease-causing mutations have recently been identified. Some of these mutations are found in viral nucleic acid-sensing proteins, but whether viruses can influence the onset or progression of these human diseases is less well understood. One such autoinflammatory disease, called STING-associated vasculopathy with onset in infancy (SAVI), affects children and leads to severe lung disease. We generated mice with a SAVI-associated STING mutation and infected them with γHV68, a common DNA virus that is related to human Epstein-Barr virus. Mice with the human disease-causing STING mutation were more vulnerable to infection than wild-type littermate control animals. Furthermore, the STING mutant mice developed lung fibrosis similar to that of patients with SAVI. These findings reveal that a human STING mutation creates severe immunodeficiency, leading to virus-induced lung disease in mice.

Zika virus-related neurotropic flaviviruses infect human placental explants and cause fetal demise in mice.

Although Zika virus (ZIKV) infection in pregnant women can cause placental damage, intrauterine growth restriction, microcephaly, and fetal demise, these disease manifestations only became apparent in the context of a large epidemic in the Americas. We hypothesized that ZIKV is not unique among arboviruses in its ability to cause congenital infection. To evaluate this, we tested the capacity of four emerging arboviruses [West Nile virus (WNV), Powassan virus (POWV), chikungunya virus (CHIKV), and Mayaro virus (MAYV)] from related (flavivirus) and unrelated (alphavirus) genera to infect the placenta and fetus in immunocompetent, wild-type mice. Although all four viruses caused placental infection, only infection with the neurotropic flaviviruses (WNV and POWV) resulted in fetal demise. WNV and POWV also replicated efficiently in second-trimester human maternal (decidua) and fetal (chorionic villi and fetal membrane) explants, whereas CHIKV and MAYV replicated less efficiently. In mice, RNA in situ hybridization and histopathological analysis revealed that WNV infected the placenta and fetal central nervous system, causing injury to the developing brain. In comparison, CHIKV and MAYV did not cause substantive placental or fetal damage despite evidence of vertical transmission. On the basis of the susceptibility of human maternal and fetal tissue explants and pathogenesis experiments in immunocompetent mice, other emerging neurotropic flaviviruses may share with ZIKV the capacity for transplacental transmission, as well as subsequent infection and injury to the developing fetus.

Consequences of congenital Zika virus infection.

The 2015 Zika virus (ZIKV) epidemic in the Americas led to the discovery that ZIKV causes congenital abnormalities including microcephaly, intrauterine growth restriction, and eye disease that can result in blindness. Studies in animal models and human organoid cultures, together with human epidemiological studies, have shown that ZIKV crosses the placenta and subsequently replicates within fetal tissues including the developing brain. Preferential infection of neural cell precursors causes damage to the developing fetal brain. However, a majority of congenitally infected humans do not develop microcephaly or other overt congenital abnormalities, so longitudinal epidemiological studies are necessary to more completely define the long-term consequences of in utero ZIKV infection.

STING-associated vasculopathy develops independently of IRF3 in mice.

Patients with stimulator of interferon genes (STING)-associated vasculopathy with onset in infancy (SAVI) develop systemic inflammation characterized by vasculopathy, interstitial lung disease, ulcerative skin lesions, and premature death. Autosomal dominant mutations in STING are thought to trigger activation of IRF3 and subsequent up-regulation of interferon (IFN)-stimulated genes (ISGs) in patients with SAVI. We generated heterozygous STING N153S knock-in mice as a model of SAVI. These mice spontaneously developed inflammation within the lung, hypercytokinemia, T cell cytopenia, skin ulcerations, and premature death. Cytometry by time-of-flight (CyTOF) analysis revealed that the STING N153S mutation caused myeloid cell expansion, T cell cytopenia, and dysregulation of immune cell signaling. Unexpectedly, we observed only mild up-regulation of ISGs in STING N153S fibroblasts and splenocytes and STING N154S SAVI patient fibroblasts. STING N153S mice lacking IRF3 also developed lung disease, myeloid cell expansion, and T cell cytopenia. Thus, the SAVI-associated STING N153S mutation triggers IRF3-independent immune cell dysregulation and lung disease in mice.

Zika virus infection damages the testes in mice.

Infection of pregnant women with Zika virus (ZIKV) can cause congenital malformations including microcephaly, which has focused global attention on this emerging pathogen. In addition to transmission by mosquitoes, ZIKV can be detected in the seminal fluid of affected males for extended periods of time and transmitted sexually. Here, using a mouse-adapted African ZIKV strain (Dakar 41519), we evaluated the consequences of infection in the male reproductive tract of mice. We observed persistence of ZIKV, but not the closely related dengue virus (DENV), in the testis and epididymis of male mice, and this was associated with tissue injury that caused diminished testosterone and inhibin B levels and oligospermia. ZIKV preferentially infected spermatogonia, primary spermatocytes and Sertoli cells in the testis, resulting in cell death and destruction of the seminiferous tubules. Less damage was caused by a contemporary Asian ZIKV strain (H/PF/2013), in part because this virus replicates less efficiently in mice. The extent to which these observations in mice translate to humans remains unclear, but longitudinal studies of sperm function and viability in ZIKV-infected humans seem warranted.

Structural Basis of Zika Virus-Specific Antibody Protection.

Zika virus (ZIKV) infection during pregnancy has emerged as a global public health problem because of its ability to cause severe congenital disease. Here, we developed six mouse monoclonal antibodies (mAbs) against ZIKV including four (ZV-48, ZV-54, ZV-64, and ZV-67) that were ZIKV specific and neutralized infection of African, Asian, and American strains to varying degrees. X-ray crystallographic and competition binding analyses of Fab fragments and scFvs defined three spatially distinct epitopes in DIII of the envelope protein corresponding to the lateral ridge (ZV-54 and ZV-67), C-C' loop (ZV-48 and ZV-64), and ABDE sheet (ZV-2) regions. In vivo passive transfer studies revealed protective activity of DIII-lateral ridge specific neutralizing mAbs in a mouse model of ZIKV infection. Our results suggest that DIII is targeted by multiple type-specific antibodies with distinct neutralizing activity, which provides a path for developing prophylactic antibodies for use in pregnancy or designing epitope-specific vaccines against ZIKV.

Broadly Neutralizing Activity of Zika Virus-Immune Sera Identifies a Single Viral Serotype.

Recent epidemics of Zika virus (ZIKV) have been associated with congenital malformation during pregnancy and Guillain-Barré syndrome. There are two ZIKV lineages (African and Asian) that share >95% amino acid identity. Little is known regarding the ability of neutralizing antibodies elicited against one lineage to protect against the other. We investigated the breadth of the neutralizing antibody response following ZIKV infection by measuring the sensitivity of six ZIKV strains to neutralization by ZIKV-confirmed convalescent human serum or plasma samples. Contemporary Asian and early African ZIKV strains were similarly sensitive to neutralization regardless of the cellular source of virus. Furthermore, mouse immune serum generated after infection with African or Asian ZIKV strains was capable of neutralizing homologous and heterologous ZIKV strains equivalently. Because our study only defines a single ZIKV serotype, vaccine candidates eliciting robust neutralizing antibody responses should inhibit infection of both ZIKV lineages, including strains circulating in the Americas.

A Mouse Model of Zika Virus Pathogenesis.

The ongoing Zika virus (ZIKV) epidemic and unexpected clinical outcomes, including Guillain-Barré syndrome and birth defects, has brought an urgent need for animal models. We evaluated infection and pathogenesis with contemporary and historical ZIKV strains in immunocompetent mice and mice lacking components of the antiviral response. Four- to six-week-old Irf3(-/-)Irf5(-/-)Irf7(-/-) triple knockout mice, which produce little interferon α/ß, and mice lacking the interferon receptor (Ifnar1(-/-)) developed neurological disease and succumbed to ZIKV infection, whereas single Irf3(-/-), Irf5(-/-), and Mavs(-/-) knockout mice exhibited no overt illness. Ifnar1(-/-) mice sustained high viral loads in the brain and spinal cord, consistent with evidence that ZIKV causes neurodevelopmental defects in human fetuses. The testes of Ifnar1(-/-) mice had the highest viral loads, which is relevant to sexual transmission of ZIKV. This model of ZIKV pathogenesis will be valuable for evaluating vaccines and therapeutics as well as understanding disease pathogenesis.