Tissue macrophages suppress viral replication and prevent severe immunopathology in an interferon-I-dependent manner in mice

Characterization of the liver-draining lymph nodes in mice and their role in mounting regional immunity to HBV

The lymphatic system is important in mounting an immune response to foreign antigens and tumors in humans and animal models. The liver produces a large amount of lymph, and its lymphatic system is divided into three major components: the portal, sublobular and superficial lymphatic vessels. Despite the fact that mice are the most commonly used laboratory animals, detailed descriptions of the anatomical location and function of the lymph nodes (LNs) that drain the liver are surprisingly absent. In this study, we found that the portal and celiac LNs adjacent to mouse liver were stained with Evans blue within 5-8 min. Enhanced green fluorescence protein (EGFP)-positive cells from the liver also drained into the two aforementioned LNs. These data indicate that the portal and celiac LNs drain the mouse liver. Lymphadenectomy of the identified liver-draining LNs resulted in hepatitis B virus (HBV) persistence in immunocompetent mice compared with the sham group. In addition, the frequencies of CD8(+) T cells and dendritic cells (DCs) increased significantly in the liver-draining LNs after hydrodynamic injection of HBV plasmid. Liver-draining LN cells in HBV plasmid-injected mice also showed significant antigen-specific proliferation in response to stimulation with recombinant hepatitis B core antigen in vitro. Adoptive transfer of these cells into Rag1(-/-) mice induced a reduction in the serum concentration of hepatitis B surface antigen (HBsAg) compared to liver-draining LN cells in uninjected mice. Altogether our data characterize the liver-draining LNs and provide evidence that the liver-draining LNs induce an anti-HBV-specific immune response responsible for HBV clearance.

Adoptive transfer of macrophages from adult mice reduces mortality in mice infected with human enterovirus 71

Human enterovirus 71 (EV71) causes hand, foot and mouth disease in children under 6 years of age, and the neurological complications of this virus can lead to death. Until now, no vaccines or drugs have been available for the clinical control of this epidemic. Macrophages can engulf pathogens and mediate a series of host immune responses that play a role in the defence against infectious diseases. Using immunohistochemistry, we observed the localizations of virus in muscle tissues of EV71-infected mice. The macrophages isolated from the adult mice could kill the virus gradually in vitro, as shown using quantitative real-time PCR (qRT-PCR) and virus titration. Co-localisation of lysosomes and virus within macrophages suggested that the lysosomes were possibly responsible for the phagocytosis of EV71. Activation of the macrophages in the peritoneal cavity of mice four days pre-infection reduced the mortality of mice upon lethal EV71 infection. The adoptive transfer of macrophages from adult mice inhibited virus replication in the muscle tissues of infected mice, and this was followed by a relief of symptoms and a significant reduction of mortality, which suggested that the adoptive transfer of macrophages from adult humans represents a potential strategy to treat EV71-infected patients.

Antagonism of the interferon-induced OAS-RNase L pathway by murine coronavirus ns2 protein is required for virus replication and liver pathology

Many viruses induce hepatitis in humans, highlighting the need to understand the underlying mechanisms of virus-induced liver pathology. The murine coronavirus, mouse hepatitis virus (MHV), causes acute hepatitis in its natural host and provides a useful model for understanding virus interaction with liver cells. The MHV accessory protein, ns2, antagonizes the type I interferon response and promotes hepatitis. We show that ns2 has 2′,5′-phosphodiesterase activity, which blocks the interferon inducible 2′,5′-oligoadenylate synthetase (OAS)-RNase L pathway to facilitate hepatitis development. Ns2 cleaves 2′,5′-oligoadenylate, the product of OAS, to prevent activation of the cellular endoribonuclease RNase L and consequently block viral RNA degradation. An ns2 mutant virus was unable to replicate in the liver or induce hepatitis in wild-type mice, but was highly pathogenic in RNase L deficient mice. Thus, RNase L is a critical cellular factor for protection against viral infection of the liver and the resulting hepatitis.

Trafficking and replication patterns reveal splenic macrophages as major targets of dengue virus in mice

Human postmortem studies of natural dengue virus (DENV) infection have reported systemically distributed viral antigen. Although it is widely accepted that DENV infects mononuclear phagocytes, the sequence in which specific tissues and cell types are targeted remains uncharacterized. We previously reported that mice lacking alpha/beta and gamma interferon receptors permit high levels of DENV replication and show signs of systemic disease (T. R. Prestwood et al., J. Virol. 82:8411-8421, 2008). Here we demonstrate that within 6 h, DENV traffics to and replicates in both CD169(+) and SIGN-R1(+) macrophages of the splenic marginal zone or draining lymph node, respectively, following intravenous or intrafootpad inoculation. Subsequently, high levels of replication are detected in F4/80(+) splenic red pulp macrophages and in the bone marrow, lymph nodes, and Peyer's patches. Intravenously inoculated mice begin to succumb to dengue disease 72 h after infection, at which time viral replication occurs systemically, except in lymphoid tissues. In particular, high levels of replication occur in CD68(+) macrophages of the kidneys, heart, thymus, and gastrointestinal tract. Over the course of infection, proportionately large quantities of DENV traffic to the liver and spleen. However, late during infection, viral trafficking to the spleen decreases, while trafficking to the liver, thymus, and kidneys increases. The present study demonstrates that macrophage populations, initially in the spleen and other lymphoid tissues and later in nonlymphoid tissues, are major targets of DENV infection in vivo.

Mice deficient in STAT1 but not STAT2 or IRF9 develop a lethal CD4+ T-cell-mediated disease following infection with lymphocytic choriomeningitis virus

Interferon (IFN) signaling is crucial for antiviral immunity. While type I IFN signaling is mediated by STAT1, STAT2, and IRF9, type II IFN signaling requires only STAT1. Here, we studied the roles of these signaling factors in the host response to systemic infection with lymphocytic choriomeningitis virus (LCMV). In wild-type (WT) mice and mice lacking either STAT2 or IRF9, LCMV infection was nonlethal, and the virus either was cleared (WT) or established persistence (STAT2 knockout [KO] and IRF9 KO). However, in the case of STAT1 KO mice, LCMV infection was lethal and accompanied by severe multiorgan immune pathology, elevated expression of various cytokine genes in tissues, and cytokines in the serum. This lethal phenotype was unaltered by the coabsence of the gamma interferon (IFN-γ) receptor and hence was not dependent on IFN-γ. Equally, the disease was not due to a combined defect in type I and type II IFN signaling, as IRF9 KO mice lacking the IFN-γ receptor survived infection with LCMV. Clearance of LCMV is mediated normally by CD8(+) T cells. However, the depletion of these cells in LCMV-infected STAT1 KO mice was delayed, but did not prevent, lethality. In contrast, depletion of CD4(+) T cells prevented lethality in LCMV-infected STAT1 KO mice and was associated with a reduction in tissue immune pathology. These studies highlight a fundamental difference in the role of STAT1 versus STAT2 and IRF9. While all three factors are required to limit viral replication and spread, only STAT1 has the unique function of preventing the emergence of a lethal antiviral CD4(+) T-cell response.

Living in the liver: hepatic infections

The liver has vital metabolic and clearance functions that involve the uptake of nutrients, waste products and pathogens from the blood. In addition, its unique immunoregulatory functions mediated by local expression of co-inhibitory receptors and immunosuppressive mediators help to prevent inadvertent organ damage. However, these tolerogenic properties render the liver an attractive target site for pathogens. Although most pathogens that reach the liver via the blood are eliminated or controlled by local innate and adaptive immune responses, some pathogens (such as hepatitis viruses) can escape immune control and persist in hepatocytes, causing substantial morbidity and mortality worldwide. Here, we review our current knowledge of the mechanisms of liver targeting by pathogens and describe the interplay between pathogens and host factors that promote pathogen elimination and maintain organ integrity or that allow pathogen persistence.

iRhom2 regulation of TACE controls TNF-mediated protection against Listeria and responses to LPS

Innate immune responses are vital for pathogen defense but can result in septic shock when excessive. A key mediator of septic shock is tumor necrosis factor-α (TNFα), which is shed from the plasma membrane after cleavage by the TNFα convertase (TACE). We report that the rhomboid family member iRhom2 interacted with TACE and regulated TNFα shedding. iRhom2 was critical for TACE maturation and trafficking to the cell surface in hematopoietic cells. Gene-targeted iRhom2-deficient mice showed reduced serum TNFα in response to lipopolysaccharide (LPS) and could survive a lethal LPS dose. Furthermore, iRhom2-deficient mice failed to control the replication of Listeria monocytogenes. Our study has identified iRhom2 as a regulator of innate immunity that may be an important target for modulating sepsis and pathogen defense.

dsRNA sensors and plasmacytoid dendritic cells in host defense and autoimmunity

The innate immune system detects viruses through molecular sensors that trigger the production of type I interferons (IFN-I) and inflammatory cytokines. As viruses vary tremendously in size, structure, genomic composition, and tissue tropism, multiple sensors are required to detect their presence in various cell types and tissues. In this review, we summarize current knowledge of the diversity, specificity, and signaling pathways downstream of viral sensors and ask whether two distinct sensors that recognize the same viral component are complementary, compensatory, or simply redundant. We also discuss why viral sensors are differentially distributed in distinct cell types and whether a particular cell type dominates the IFN-I response during viral infection. Finally, we review evidence suggesting that inappropriate signaling through viral sensors may induce autoimmunity. The picture emerging from these studies is that disparate viral sensors in different cell types form a dynamic and integrated molecular network that can be exploited for improving vaccination and therapeutic strategies for infectious and autoimmune diseases.

Natural killer cell activation enhances immune pathology and promotes chronic infection by limiting CD8+ T-cell immunity

Infections with HIV, hepatitis B virus, and hepatitis C virus can turn into chronic infections, which currently affect more than 500 million patients worldwide. It is generally thought that virus-mediated T-cell exhaustion limits T-cell function, thus promoting chronic disease. Here we demonstrate that natural killer (NK) cells have a negative impact on the development of T-cell immunity by using the murine lymphocytic choriomeningitis virus. NK cell-deficient (Nfil3(-/-), E4BP4(-/-)) mice exhibited a higher virus-specific T-cell response. In addition, NK cell depletion caused enhanced T-cell immunity in WT mice, which led to rapid virus control and prevented chronic infection in lymphocytic choriomeningitis virus clone 13- and reduced viral load in DOCILE-infected animals. Further experiments showed that NKG2D triggered regulatory NK cell functions, which were mediated by perforin, and limited T-cell responses. Therefore, we identified an important role of regulatory NK cells in limiting T-cell immunity during virus infection.

Type I interferons: diversity of sources, production pathways and effects on immune responses

Type I interferons (IFN-I) were first described over 50 years ago as factors produced by cells that interfere with virus replication and promote an antiviral state. Innate and adaptive immune responses to viruses are also greatly influenced by IFN-I. In this article we discuss the diversity of cellular sources of IFN-I and the pathways leading to IFN-I production during viral infections. Finally, we discuss the effects of IFN-I on cells of the immune system with emphasis on dendritic cells.

Enforced viral replication activates adaptive immunity and is essential for the control of a cytopathic virus

The innate immune system limits viral replication via type I interferon and also induces the presentation of viral antigens to cells of the adaptive immune response. Using infection of mice with vesicular stomatitis virus, we analyzed how the innate immune system inhibits viral propagation but still allows the presentation of antigen to cells of the adaptive immune response. We found that expression of the gene encoding the inhibitory protein Usp18 in metallophilic macrophages led to lower type I interferon responsiveness, thereby allowing locally restricted replication of virus. This was essential for the induction of adaptive antiviral immune responses and, therefore, for preventing the fatal outcome of infection. In conclusion, we found that enforced viral replication in marginal zone macrophages was an immunological mechanism that ensured the production of sufficient antigen for effective activation of the adaptive immune response.

TLR7/9 versus TLR3/MDA5 signaling during virus infections and diabetes

IFN-I are pleiotropic cytokines that impact innate and adaptive immune responses. In this article, we discuss TLR7/9 versus TLR3/MDA5 signaling in antiviral responses and diabetes. pDCs are thought to have a critical role in antiviral defense because of their ability to rapidly secrete large amounts of IFN-I through TLR7/9 signaling. A recent study demonstrates that although pDCs are a source of IFN-I in vivo, their overall contribution to viral containment is limited and time-dependent, such that additional cellular sources of IFN-I are required to fully control viral infections. dsRNA sensors, such as TLR3 and MDA5, provide another important trigger for antiviral IFN-I responses, which can be exploited to enhance immune responses to vaccines. In the absence of infection, IFN-I production by pDCs or from signaling through dsRNA sensors has been implicated in the pathogenesis of autoimmune diseases such as diabetes. However, recent data demonstrate that IFN-I production via TLR3 and MDA5 is critical to counter diabetes caused by a virus with preferential tropism for pancreatic β-cells. This highlights the complexity of the host antiviral response and how multiple cellular and molecular components balance protective versus pathological responses.

Cell-type-specific type I interferon antagonism influences organ tropism of murine coronavirus

Previous studies have demonstrated that mouse hepatitis virus (MHV) hepatotropism is determined largely by postentry events rather than by availability of the viral receptor. In addition, mutation of MHV nonstructural protein 2 (ns2) abrogates the ability of the virus to replicate in the liver and induce hepatitis but does not affect replication in the central nervous system (CNS). Here we show that replication of ns2 mutant viruses is attenuated in bone marrow-derived macrophages (BMM) generated from wild-type (wt) mice but not in L2 fibroblasts, primary astrocytes, or BMM generated from type I interferon receptor-deficient (IFNAR(-/-)) mice. In addition, ns2 mutants are more sensitive than wt virus to pretreatment of BMM, but not L2 fibroblasts or primary astrocytes, with alpha/beta interferon (IFN-α/β). The ns2 mutants induced similar levels of IFN-α/β in wt and IFNAR(-/-) BMM, indicating that ns2 expression has no effect on the induction of IFN but rather that it antagonizes a later step in IFN signaling. Consistent with these in vitro data, the virulence of ns2 mutants increased to near that of wt virus after depletion of macrophages in vivo. These data imply that the ability of MHV to replicate in macrophages is a prerequisite for replication in the liver and induction of hepatitis but not for replication or disease in the CNS, underscoring the importance of IFN signaling in macrophages in vivo for protection of the host from hepatitis. Our results further support the notion that viral tissue tropism is determined in part by postentry events, including the early type I interferon response.

Kupffer cells hasten resolution of liver immunopathology in mouse models of viral hepatitis

Kupffer cells (KCs) are widely considered important contributors to liver injury during viral hepatitis due to their pro-inflammatory activity. Herein we utilized hepatitis B virus (HBV)-replication competent transgenic mice and wild-type mice infected with a hepatotropic adenovirus to demonstrate that KCs do not directly induce hepatocellular injury nor do they affect the pathogenic potential of virus-specific CD8 T cells. Instead, KCs limit the severity of liver immunopathology. Mechanistically, our results are most compatible with the hypothesis that KCs contain liver immunopathology by removing apoptotic hepatocytes in a manner largely dependent on scavenger receptors. Apoptotic hepatocytes not readily removed by KCs become secondarily necrotic and release high-mobility group box 1 (HMGB-1) protein, promoting organ infiltration by inflammatory cells, particularly neutrophils. Overall, these results indicate that KCs resolve rather than worsen liver immunopathology.

Kupffer cells (KCs), the resident macrophages of the liver, are considered important contributors to liver injury during viral hepatitis due to their pro-inflammatory activity. Herein we utilized two different mouse models of viral hepatitis (where liver damage is triggered, as during viral hepatitis in humans, by virus-specific CD8 T cells) to show that KCs do not directly induce liver injury nor do they affect the pathogenic potential of virus-specific CD8 T cells. Instead, KCs limit the severity of liver immunopathology. Mechanistically, our results are most compatible with the hypothesis that KCs contain liver immunopathology by removing dying hepatocytes. Dying hepatocytes not readily removed by KCs release high-mobility group box 1 (HMGB-1) protein, promoting organ infiltration by inflammatory cells, particularly neutrophils. These results indicate that KCs resolve rather than worsen liver disease.