Thomas E. Morrison

NLRX1 is a regulator of mitochondrial antiviral immunity

The RIG-like helicase (RLH) family of intracellular receptors detect viral nucleic acid and signal through the mitochondrial antiviral signalling adaptor MAVS (also known as Cardif, VISA and IPS-1) during a viral infection. MAVS activation leads to the rapid production of antiviral cytokines, including type 1 interferons. Although MAVS is vital to antiviral immunity, its regulation from within the mitochondria remains unknown. Here we describe human NLRX1, a highly conserved nucleotide-binding domain (NBD)- and leucine-rich-repeat (LRR)-containing family member (known as NLR) that localizes to the mitochondrial outer membrane and interacts with MAVS. Expression of NLRX1 results in the potent inhibition of RLH- and MAVS-mediated interferon-β promoter activity and in the disruption of virus-induced RLH–MAVS interactions. Depletion of NLRX1 with small interference RNA promotes virus-induced type I interferon production and decreases viral replication. This work identifies NLRX1 as a check against mitochondrial antiviral responses and represents an intersection of three ancient cellular processes: NLR signalling, intracellular virus detection and the use of mitochondria as a platform for anti-pathogen signalling. This represents a conceptual advance, in that NLRX1 is a modulator of pathogen-associated molecular pattern receptors rather than a receptor, and identifies a key therapeutic target for enhancing antiviral responses.

Characterization of Ross River Virus Tropism and Virus-Induced Inflammation in a Mouse Model of Viral Arthritis and Myositis

ABSTRACT Mosquito-borne alphaviruses are a significant cause of both encephalitic and arthritic disease in humans worldwide. In contrast to the encephalitic alphaviruses, the pathogenesis of alphavirus-induced arthritic disease is not well understood. Utilizing a mouse model of Ross River virus (RRV) disease, we found that the primary targets of RRV infection are bone, joint, and skeletal muscle tissues of the hind limbs in both outbred CD-1 mice and adult C57BL/6J mice. Moreover, histological analyses demonstrated that RRV infection resulted in severe inflammation of these tissues. Characterization of the inflammatory infiltrate within the skeletal muscle tissue identified inflammatory macrophages, NK cells, and CD4+ and CD8+ T lymphocytes. To determine the contribution of the adaptive immune system, the outcome of RRV-induced disease was examined in C57BL/6J RAG-1−/− mice, which lack functional T and B lymphocytes. RAG-1−/− and wild-type mice developed similar disease signs, infiltration of inflammatory macrophages and NK cells, and muscle pathology, suggesting that the adaptive immune response does not play a critical role in the development of disease. These results establish the mouse model of RRV disease as a useful system for the identification of viral and host factors that contribute to alphavirus-induced arthritis and myositis.

A mouse model of chikungunya virus-induced musculoskeletal inflammatory disease: evidence of arthritis, tenosynovitis, myositis, and persistence.

Chikungunya virus (CHIKV), an emerging mosquito-borne Alphavirus, causes debilitating rheumatic disease in humans that can last for weeks to months. Starting in 2004, a CHIKV outbreak in the Indian Ocean region affected millions of people, and infected travelers introduced CHIKV to new regions. The pathogenesis of CHIKV is poorly understood, and no approved vaccines or specific therapies exist. A major challenge to the study of CHIKV disease is the lack of a small animal model that recapitulates the major outcomes of human infection. In this study, the pathogenesis of CHIKV in C57BL/6J mice was investigated using biological and molecular clones of CHIKV isolated from human serum (CHIKV SL15649). After 14-day-old mice were inoculated with CHIKV SL15649 in the footpad, they displayed reduced weight gain and swelling of the inoculated limb. Histologic analysis of hind limb sections revealed severe necrotizing myositis, mixed inflammatory cell arthritis, chronic active tenosynovitis, and multifocal vasculitis. Interestingly, these disease signs and viral RNA persisted in musculoskeletal tissues for at least 3 weeks after inoculation. This work demonstrates the development of a mouse model of CHIKV infection with clinical manifestations and histopathologic findings that are consistent with the disease signs of CHIKV-infected humans, providing a useful tool for studying viral and host factors that drive CHIKV pathogenesis and for evaluating potential therapeutics against this emerging viral disease.

Reemergence of Chikungunya Virus

ABSTRACT Chikungunya virus (CHIKV) is a mosquito-transmitted alphavirus that causes acute fever and acute and chronic musculoskeletal pain in humans. Since 2004, CHIKV has caused millions of cases of disease in the Indian Ocean region and has emerged in new areas, including Europe, the Middle East, and the Pacific region. The mosquito vectors for this virus are globally distributed in tropical and temperate zones, providing the opportunity for CHIKV to continue to expand into new geographic regions. In October 2013, locally acquired cases of CHIKV infection were identified on the Caribbean island of Saint Martin, signaling the arrival of the virus in the Western Hemisphere. In just 9 months, CHIKV has spread to 22 countries in the Caribbean and Central and South America, resulting in hundreds of thousands of cases. CHIKV disease can be highly debilitating, and large epidemics have severe economic consequences. Thus, there is an urgent need for continued research into the epidemiology, pathogenesis, prevention, and treatment of these infections.

Inhibition of IFN-γ Signaling by an Epstein-Barr Virus Immediate-Early Protein

Abstract Viruses have evolved elaborate mechanisms to target many aspects of the hosts immune response. The cytokine IFN-γ plays a central role in resistance of the host to infection via direct antiviral effects as well as modulation of the immune response. In this study, we demonstrate that the Epstein-Barr virus (EBV) immediate-early protein, BZLF1, inhibits the IFN-γ signaling pathway. BZLF1 decreases the ability of IFN-γ to activate a variety of important downstream target genes, such as IRF-1, p48, and CIITA, and prevents IFN-γ-induced class II MHC surface expression. Additionally, BZLF1 inhibits IFN-γ-induced STAT1 tyrosine phosphorylation and nuclear translocation. Finally, we demonstrate that BZLF1 decreases expression of the IFN-γ receptor, suggesting a mechanism by which EBV may escape antiviral immune responses during primary infection.

Chronic Joint Disease Caused by Persistent Chikungunya Virus Infection Is Controlled by the Adaptive Immune Response

ABSTRACT Chikungunya virus (CHIKV) is a reemerging mosquito-borne pathogen that causes incapacitating disease in humans characterized by intense joint pain that can persist for weeks, months, or even years. Although there is some evidence of persistent CHIKV infection in humans suffering from chronic rheumatologic disease symptoms, little is known about chronic disease pathogenesis, and no specific therapies exist for acute or chronic CHIKV disease. To investigate mechanisms of chronic CHIKV-induced disease, we utilized a mouse model and defined the duration of CHIKV infection in tissues and the associated histopathological changes. Although CHIKV RNA was readily detectable in a variety of tissues very early after infection, CHIKV RNA persisted specifically in joint-associated tissues for at least 16 weeks. Inoculation of Rag1 −/− mice, which lack T and B cells, resulted in higher viral levels in a variety of tissues, suggesting that adaptive immunity controls the tissue specificity and persistence of CHIKV infection. The presence of CHIKV RNA in tissues of wild-type and Rag1 −/− mice was associated with histopathological evidence of synovitis, arthritis, and tendonitis; thus, CHIKV-induced persistent arthritis is not mediated primarily by adaptive immune responses. Finally, we show that prophylactic administration of CHIKV-specific monoclonal antibodies prevented the establishment of CHIKV persistence, whereas therapeutic administration had tissue-specific efficacy. These findings suggest that chronic musculoskeletal tissue pathology is caused by persistent CHIKV infection and controlled by adaptive immune responses. Our results have significant implications for the development of strategies to mitigate the disease burden associated with CHIKV infection in humans.

Complement and viral pathogenesis.

Abstract The complement system functions as an immune surveillance system that rapidly responds to infection. Activation of the complement system by specific recognition pathways triggers a protease cascade, generating cleavage products that function to eliminate pathogens, regulate inflammatory responses, and shape adaptive immune responses. However, when dysregulated, these powerful functions can become destructive and the complement system has been implicated as a pathogenic effector in numerous diseases, including infectious diseases. This review highlights recent discoveries that have identified critical roles for the complement system in the pathogenesis of viral infection.

Differential Induction of Type I Interferon Responses in Myeloid Dendritic Cells by Mosquito and Mammalian-Cell-Derived Alphaviruses

ABSTRACT Dendritic cells (DCs) are an important early target cell for many mosquito-borne viruses, and in many cases mosquito-cell-derived arboviruses more efficiently infect DCs than viruses derived from mammalian cells. However, whether mosquito-cell-derived viruses differ from mammalian-cell-derived viruses in their ability to induce antiviral responses in the infected dendritic cell has not been evaluated. In this report, alphaviruses, which are mosquito-borne viruses that cause diseases ranging from encephalitis to arthritis, were used to determine whether viruses grown in mosquito cells differed from mammalian-cell-derived viruses in their ability to induce type I interferon (IFN) responses in infected primary dendritic cells. Consistent with previous results, mosquito-cell-derived Ross River virus (mos-RRV) and Venezuelan equine encephalitis virus (mos-VEE) exhibited enhanced infection of primary myeloid dendritic cells (mDCs) compared to mammalian-cell-derived virus preparations. However, unlike the mammalian-cell-derived viruses, which induced high levels of type I IFN in the infected mDC cultures, mos-RRV and mos-VEE were poor IFN inducers. Furthermore, the poor IFN induction by mos-RRV contributed to the enhanced infection of mDCs by mos-RRV. These results suggest that the viruses initially delivered by the mosquito vector differ from those generated in subsequent rounds of replication in the host, not just with respect to their ability to infect dendritic cells but also in their ability to induce or inhibit antiviral type I IFN responses. This difference may have an important impact on the mosquito-borne viruss ability to successfully make the transition from the arthropod vector to the vertebrate host.

Complement Contributes to Inflammatory Tissue Destruction in a Mouse Model of Ross River Virus-Induced Disease

ABSTRACT Arthritogenic alphaviruses, including Ross River virus (RRV) and chikungunya virus, are mosquito-borne viruses that cause significant human disease worldwide, including explosive epidemics that can result in thousands to millions of infected individuals. Similar to infection of humans, infection of C57BL/6 mice with RRV results in severe monocytic inflammation of bone, joint, and skeletal muscle tissues. We demonstrate here that the complement system, an important component of the innate immune response, enhances the severity of RRV-induced disease in mice. Complement activation products were detected in the inflamed tissues and in the serum of RRV-infected wild-type mice. Furthermore, mice deficient in C3 (C3−/−), the central component of the complement system, developed much less severe disease signs than did wild-type mice. Complement-mediated chemotaxis is essential for many inflammatory arthritides; however, RRV-infected wild-type and C3−/− mice had similar numbers and composition of inflammatory infiltrates within hind limb skeletal muscle tissue. Despite similar inflammatory infiltrates, RRV-infected C3−/− mice exhibited far less severe destruction of skeletal muscle tissue. In addition to these studies, complement activation was also detected in synovial fluid from RRV-infected patients. Taken together, these findings indicate that complement activation occurs in the tissues of humans and mice infected with RRV and suggest that complement plays an essential role in the effector phase, but not the inductive phase, of RRV-induced arthritis and myositis.

MyD88 is required for protection from lethal infection with a mouse-adapted SARS-CoV.

A novel human coronavirus, SARS-CoV, emerged suddenly in 2003, causing approximately 8000 human cases and more than 700 deaths worldwide. Since most animal models fail to faithfully recapitulate the clinical course of SARS-CoV in humans, the virus and host factors that mediate disease pathogenesis remain unclear. Recently, our laboratory and others developed a recombinant mouse-adapted SARS-CoV (rMA15) that was lethal in BALB/c mice. In contrast, intranasal infection of young 10-week-old C57BL/6 mice with rMA15 results in a nonlethal infection characterized by high titer replication within the lungs, lung inflammation, destruction of lung tissue, and loss of body weight, thus providing a useful model to identify host mediators of protection. Here, we report that mice deficient in MyD88 (MyD88−/−), an adapter protein that mediates Toll-like receptor (TLR), IL-1R, and IL-18R signaling, are far more susceptible to rMA15 infection. The genetic absence of MyD88 resulted in enhanced pulmonary pathology and greater than 90% mortality by day 6 post-infection. MyD88−/− mice had significantly higher viral loads in lung tissue throughout the course of infection. Despite increased viral loads, the expression of multiple proinflammatory cytokines and chemokines within lung tissue and recruitment of inflammatory monocytes/macrophages to the lung was severely impaired in MyD88−/− mice compared to wild-type mice. Furthermore, mice deficient in chemokine receptors that contribute to monocyte recruitment to the lung were more susceptible to rMA15-induced disease and exhibited severe lung pathology similar to that seen in MyD88−/−mice. These data suggest that MyD88-mediated innate immune signaling and inflammatory cell recruitment to the lung are required for protection from lethal rMA15 infection.