Induction of natural killer cell responses by ectromelia virus controls infection

Differential Activation of Splenic cDC1 and cDC2 Cell Subsets following Poxvirus Infection of BALB/c and C57BL/6 Mice

Conventional dendritic cells (cDCs) are innate immune cells that play a pivotal role in inducing antiviral adaptive immune responses due to their extraordinary ability to prime and polarize naïve T cells into different effector T helper (Th) subsets. The two major subpopulations of cDCs, cDC1 (CD8α+ in mice and CD141+ in human) and cDC2 (CD11b+ in mice and CD1c+ in human), can preferentially polarize T cells toward a Th1 and Th2 phenotype, respectively. During infection with ectromelia virus (ECTV), an orthopoxvirus from the Poxviridae family, the timing and activation of an appropriate Th immune response contributes to the resistance (Th1) or susceptibility (Th2) of inbred mouse strains to the lethal form of mousepox. Due to the high plasticity and diverse properties of cDC subpopulations in regulating the quality of a specific immune response, in the present study we compared the ability of splenic cDC1 and cDC2 originating from different ECTV-infected mouse strains to mature, activate, and polarize the Th immune response during mousepox. Our results demonstrated that during early stages of mousepox, both cDC subsets from resistant C57BL/6 and susceptible BALB/c mice were activated upon in vivo ECTV infection. These cells exhibited elevated levels of surface MHC class I and II, and co-stimulatory molecules and showed enhanced potential to produce cytokines. However, both cDC subsets from BALB/c mice displayed a higher maturation status than that of their counterparts from C57BL/6 mice. Despite their higher activation status, cDC1 and cDC2 from susceptible mice produced low amounts of Th1-polarizing cytokines, including IL-12 and IFN-γ, and the ability of these cells to stimulate the proliferation and Th1 polarization of allogeneic CD4+ T cells was severely compromised. In contrast, both cDC subsets from resistant mice produced significant amounts of Th1-polarizing cytokines and demonstrated greater capability in differentiating allogeneic T cells into Th1 cells compared to cDCs from BALB/c mice. Collectively, our results indicate that in the early stages of mousepox, splenic cDC subpopulations from the resistant mouse strain can better elicit a Th1 cell-mediated response than the susceptible strain can, probably contributing to the induction of the protective immune responses necessary for the control of virus dissemination and for survival from ECTV challenge.

Transcriptional reprogramming of natural killer cells by vaccinia virus shows both distinct and conserved features with mCMV

Natural killer (NK) cells have an established role in controlling poxvirus infection and there is a growing interest to exploit their capabilities in the context of poxvirus-based oncolytic therapy and vaccination. How NK cells respond to poxvirus-infected cells to become activated is not well established. To address this knowledge gap, we studied the NK cell response to vaccinia virus (VACV) in vivo, using a systemic infection murine model. We found broad alterations in NK cells transcriptional activity in VACV-infected mice, consistent with both direct target cell recognition and cytokine exposure. There were also alterations in the expression levels of specific NK surface receptors (NKRs), including the Ly49 family and SLAM receptors, as well as upregulation of memory-associated NK markers. Despite the latter observation, adoptive transfer of VACV-expercienced NK populations did not confer protection from infection. Comparison with the NK cell response to murine cytomegalovirus (MCMV) infection highlighted common features, but also distinct NK transcriptional programmes initiated by VACV. Finally, there was a clear overlap between the NK transcriptional response in humans vaccinated with an attenuated VACV, modified vaccinia Ankara (MVA), demonstrating conservation between the NK response in these different host species. Overall, this study provides new data about NK cell activation, function, and homeostasis during VACV infection, and may have implication for the design of VACV-based therapeutics.

Resistance to lethal ectromelia virus infection requires Type I interferon receptor in natural killer cells and monocytes but not in adaptive immune or parenchymal cells

Type I interferons (IFN-I) are antiviral cytokines that signal through the ubiquitous IFN-I receptor (IFNAR). Following footpad infection with ectromelia virus (ECTV), a mouse-specific pathogen, C57BL/6 (B6) mice survive without disease, while B6 mice broadly deficient in IFNAR succumb rapidly. We now show that for survival to ECTV, only hematopoietic cells require IFNAR expression. Survival to ECTV specifically requires IFNAR in both natural killer (NK) cells and monocytes. However, intrinsic IFNAR signaling is not essential for adaptive immune cell responses or to directly protect non-hematopoietic cells such as hepatocytes, which are principal ECTV targets. Mechanistically, IFNAR-deficient NK cells have reduced cytolytic function, while lack of IFNAR in monocytes dampens IFN-I production and hastens virus dissemination. Thus, during a pathogenic viral infection, IFN-I coordinates innate immunity by stimulating monocytes in a positive feedback loop and by inducing NK cell cytolytic function.

Viral infection modulates Qa-1b in infected and bystander cells to properly direct NK cell killing

Natural killer (NK) cell activation depends on the signaling balance of activating and inhibitory receptors. CD94 forms inhibitory receptors with NKG2A and activating receptors with NKG2E or NKG2C. We previously demonstrated that CD94-NKG2 on NK cells and its ligand Qa-1b are important for the resistance of C57BL/6 mice to lethal ectromelia virus (ECTV) infection. We now show that NKG2C or NKG2E deficiency does not increase susceptibility to lethal ECTV infection, but overexpression of Qa-1b in infected cells does. We also demonstrate that Qa-1b is down-regulated in infected and up-regulated in bystander inflammatory monocytes and B cells. Moreover, NK cells activated by ECTV infection kill Qa-1b-deficient cells in vitro and in vivo. Thus, during viral infection, recognition of Qa-1b by activating CD94/NKG2 receptors is not critical. Instead, the levels of Qa-1b expression are down-regulated in infected cells but increased in some bystander immune cells to respectively promote or inhibit their killing by activated NK cells.

Natural Killer Cells: From Innate to Adaptive Features

Natural killer (NK) cells are innate lymphocytes that provide critical host defense against pathogens and cancer. Originally heralded for their early and rapid effector activity, NK cells have been recognized over the last decade for their ability to undergo adaptive immune processes, including antigen-driven clonal expansion and generation of long-lived memory. This review presents an overview of how NK cells lithely partake in both innate and adaptive responses and how this versatility is manifest in human NK cell-mediated immunity.

Poxvirus-encoded TNF receptor homolog dampens inflammation and protects from uncontrolled lung pathology during respiratory infection

Ectromelia virus (ECTV) causes mousepox, a surrogate mouse model for smallpox caused by variola virus in humans. Both orthopoxviruses encode tumor necrosis factor receptor (TNFR) homologs or viral TNFR (vTNFR). These homologs are termed cytokine response modifier (Crm) proteins, containing a TNF-binding domain and a chemokine-binding domain called smallpox virus-encoded chemokine receptor (SECRET) domain. ECTV encodes one vTNFR known as CrmD. Infection of ECTV-resistant C57BL/6 mice with a CrmD deletion mutant virus resulted in uniform mortality due to excessive TNF secretion and dysregulated inflammatory cytokine production. CrmD dampened pathology, leukocyte recruitment, and inflammatory cytokine production in lungs including TNF, IL-6, IL-10, and IFN-γ. Blockade of TNF, IL-6, or IL-10R function with monoclonal antibodies reduced lung pathology and provided 60 to 100% protection from otherwise lethal infection. IFN-γ caused lung pathology only when both the TNF-binding and SECRET domains were absent. Presence of the SECRET domain alone induced significantly higher levels of IL-1β, IL-6, and IL-10, likely overcoming any protective effects that might have been afforded by anti-IFN-γ treatment. The use of TNF-deficient mice and those that express only membrane-associated but not secreted TNF revealed that CrmD is critically dependent on host TNF for its function. In vitro, recombinant Crm proteins from different orthopoxviruses bound to membrane-associated TNF and dampened inflammatory gene expression through reverse signaling. CrmD does not affect virus replication; however, it provides the host advantage by enabling survival. Host survival would facilitate virus spread, which would also provide an advantage to the virus.

Langerhans Cells Orchestrate the Protective Antiviral Innate Immune Response in the Lymph Node

During disseminating viral infections, a swift innate immune response (IIR) in the draining lymph node (dLN) that restricts systemic viral spread is critical for optimal resistance to disease. However, it is unclear how this IIR is orchestrated. We show that after footpad infection of mice with ectromelia virus, dendritic cells (DCs) highly expressing major histocompatibility complex class II (MHC class II^hi DCs), including CD207⁺ epidermal Langerhans cells (LCs), CD103⁺CD207⁺ double-positive dermal DCs (DP-DCs), and CD103⁻CD207⁻ double-negative dermal DCs (DN-DCs) migrate to the dLN from the skin carrying virus. MHC class II^hi DCs, predominantly LCs and DP-DCs, are the first cells upregulating IIR cytokines in the dLN. Preventing MHC class II^hi DC migration or depletion of LCs, but not DP-DC deficiency, suppresses the IIR in the dLN and results in high viral lethality. Therefore, LCs are the architects of an early IIR in the dLN that is critical for optimal resistance to a disseminating viral infection.

Wong et al. show that by producing chemokines that recruit monocytes and by upregulating NKG2D ligands that activate ILCs, Langerhans cells are responsible for the innate immune cascade in the lymph node that is critical for survival of infection with a disseminating virus.

Viral cGAMP nuclease reveals the essential role of DNA sensing in protection against acute lethal virus infection

Cells contain numerous immune sensors to detect virus infection. The cyclic GMP-AMP (cGAMP) synthase (cGAS) recognizes cytosolic DNA and activates innate immune responses via stimulator of interferon genes (STING), but the impact of DNA sensing pathways on host protective responses has not been fully defined. We demonstrate that cGAS/STING activation is required to resist lethal poxvirus infection. We identified viral Schlafen (vSlfn) as the main STING inhibitor, and ectromelia virus was severely attenuated in the absence of vSlfn. Both vSlfn-mediated virulence and STING inhibitory activity were mapped to the recently discovered poxin cGAMP nuclease domain. Animals were protected from subcutaneous, respiratory, and intravenous infection in the absence of vSlfn, and interferon was the main antiviral protective mechanism controlled by the DNA sensing pathway. Our findings support the idea that manipulation of DNA sensing is an efficient therapeutic strategy in diseases triggered by viral infection or tissue damage-mediated release of self-DNA.

TNF deficiency dysregulates inflammatory cytokine production, leading to lung pathology and death during respiratory poxvirus infection

Excessive tumor necrosis factor (TNF) is known to cause significant pathology. Paradoxically, deficiency in TNF (TNF^-/-) also caused substantial pathology during respiratory ectromelia virus (ECTV) infection, a surrogate model for smallpox. TNF^-/- mice succumbed to fulminant disease whereas wild-type mice, and those engineered to express only transmembrane TNF (mTNF), fully recovered. TNF deficiency did not affect viral load or leukocyte recruitment but caused severe lung pathology and excessive production of the cytokines interleukin (IL)-6, IL-10, transforming growth factor beta (TGF-β), and interferon gamma (IFN-γ). Short-term blockade of these cytokines significantly reduced lung pathology in TNF^-/- mice concomitant with induction of protein inhibitor of activated STAT3 (PIAS3) and/or suppressor of cytokine signaling 3 (SOCS3), factors that inhibit STAT3 activation. Consequently, inhibition of STAT3 activation with an inhibitor reduced lung pathology. Long-term neutralization of IL-6 or TGF-β protected TNF^-/- mice from an otherwise lethal infection. Thus, mTNF alone is necessary and sufficient to regulate lung inflammation but it has no direct antiviral activity against ECTV. The data indicate that targeting specific cytokines or cytokine-signaling pathways to reduce or ameliorate lung inflammation during respiratory viral infections is possible but that the timing and duration of the interventive measure are critical.

Mechanisms of Natural Killer Cell Evasion Through Viral Adaptation

The continuous interactions between host and pathogens during their coevolution have shaped both the immune system and the countermeasures used by pathogens. Natural killer (NK) cells are innate lymphocytes that are considered central players in the antiviral response. Not only do they express a variety of inhibitory and activating receptors to discriminate and eliminate target cells but they can also produce immunoregulatory cytokines to alert the immune system. Reciprocally, several unrelated viruses including cytomegalovirus, human immunodeficiency virus, influenza virus, and dengue virus have evolved a multitude of mechanisms to evade NK cell function, such as the targeting of pathways for NK cell receptors and their ligands, apoptosis, and cytokine-mediated signaling. The studies discussed in this article provide further insights into the antiviral function of NK cells and the pathways involved, their constituent proteins, and ways in which they could be manipulated for host benefit.

Chronic Lymphocytic Choriomeningitis Infection Causes Susceptibility to Mousepox and Impairs Natural Killer Cell Maturation and Function

Chronic viral infections. like those of humans with cytomegalovirus, human immunodeficiency virus (even when under antiretroviral therapy), and hepatitis C virus or those of mice with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13), result in immune dysfunction that predisposes the host to severe infections with unrelated pathogens. It is known that C57BL/6 (B6) mice are resistant to mousepox, a lethal disease caused by the orthopoxvirus ectromelia virus (ECTV), and that this resistance requires natural killer (NK) cells and other immune cells. We show that most B6 mice chronically infected with CL13 succumb to mousepox but that most of those that recovered from acute infection with the LCMV Armstrong (Arm) strain survive. We also show that B6 mice chronically infected with CL13 and those that recovered from Arm infection have a reduced frequency and a reduced number of NK cells. However, at steady state, NK cells in mice that have recovered from Arm infection mature normally and, in response to ECTV, get activated, become more mature, proliferate, and increase their cytotoxicity in vivo Conversely, in mice chronically infected with CL13, NK cells are immature and residually activated, and following ECTV infection, they do not mature, proliferate, or increase their cytotoxicity. Given the well-established importance of NK cells in resistance to mousepox, these data suggest that the NK cell dysfunction caused by CL13 persistence may contribute to the susceptibility of CL13-infected mice to mousepox. Whether chronic infections similarly affect NK cells in humans should be explored.IMPORTANCE Infection of adult mice with the clone 13 (CL13) strain of lymphocytic choriomeningitis virus (LCMV) is extensively used as a model of chronic infection. In this paper, we show that mice chronically infected with CL13 succumb to challenge with ectromelia virus (ECTV; the agent of mousepox) and that natural killer (NK) cells in CL13-infected mice are reduced in numbers and have an immature and partially activated phenotype but do respond to ECTV. These data may provide additional clues why humans chronically infected with certain pathogens are less resistant to viral diseases.

α2β1 Integrin Is Required for Optimal NK Cell Proliferation during Viral Infection but Not for Acquisition of Effector Functions or NK Cell-Mediated Virus Control

NK cells play an important role in antiviral resistance. The integrin α2, which dimerizes with integrin β1, distinguishes NK cells from innate lymphoid cells 1 and other leukocytes. Despite its use as an NK cell marker, little is known about the role of α2β1 in NK cell biology. In this study, we show that in mice α2β1 deficiency does not alter the balance of NK cell/ innate lymphoid cell 1 generation and slightly decreases the number of NK cells in the bone marrow and spleen without affecting NK cell maturation. NK cells deficient in α2β1 had no impairment at entering or distributing within the draining lymph node of ectromelia virus (ECTV)-infected mice or at becoming effectors but proliferated poorly in response to ECTV and did not increase in numbers following infection with mouse CMV (MCMV). Still, α2β1-deficient NK cells efficiently protected from lethal mousepox and controlled MCMV titers in the spleen. Thus, α2β1 is required for optimal NK cell proliferation but is dispensable for protection against ECTV and MCMV, two well-established models of viral infection in which NK cells are known to be important.

ECTV Abolishes the Ability of GM-BM Cells to Stimulate Allogeneic CD4 T Cells in a Mouse Strain-Independent Manner

Ectromelia virus (ECTV) is the etiological agent of mousepox, an acute and systemic disease with high mortality rates in susceptible strains of mice. Resistance and susceptibility to mousepox are triggered by the dichotomous T-helper (Th) immune response generated in infected animals, with strong protective Th1 or nonprotective Th2 profile, respectively. Th1/Th2 balance is influenced by dendritic cells (DCs), which were shown to differ in their ability to polarize naïve CD4⁺ T cells in different mouse strains. Therefore, we have studied the inner-strain differences in the ability of conventional DCs (cDCs), generated from resistant (C57BL/6) and susceptible (BALB/c) mice, to stimulate proliferation and activation of Th cells upon ECTV infection. We found that ECTV infection of GM-CSF-derived bone marrow (GM-BM) cells, composed of cDCs and macrophages, affected initiation of allogeneic CD4⁺ T cells proliferation in a mouse strain-independent manner. Moreover, infected GM-BM cells from both mouse strains failed to induce and even inhibited the production of Th1 (IFN-γ and IL-2), Th2 (IL-4 and IL-10) and Th17 (IL-17A) cytokines by allogeneic CD4⁺ T cells. These results indicate that in in vitro conditions ECTV compromises the ability of cDCs to initiate/polarize adaptive antiviral immune response independently of the host strain resistance/susceptibility to lethal infection.

Chemokines cooperate with TNF to provide protective anti-viral immunity and to enhance inflammation

The role of cytokines and chemokines in anti-viral defense has been demonstrated, but their relative contribution to protective anti-viral responses in vivo is not fully understood. Cytokine response modifier D (CrmD) is a secreted receptor for TNF and lymphotoxin containing the smallpox virus-encoded chemokine receptor (SECRET) domain and is expressed by ectromelia virus, the causative agent of the smallpox-like disease mousepox. Here we show that CrmD is an essential virulence factor that controls natural killer cell activation and allows progression of fatal mousepox, and demonstrate that both SECRET and TNF binding domains are required for full CrmD activity. Vaccination with recombinant CrmD protects animals from lethal mousepox. These results indicate that a specific set of chemokines enhance the inflammatory and protective anti-viral responses mediated by TNF and lymphotoxin, and illustrate how viruses optimize anti-TNF strategies with the addition of a chemokine binding domain as soluble decoy receptors.

Comparison of Host Gene Expression Profiles in Spleen Tissues of Genetically Susceptible and Resistant Mice during ECTV Infection

Ectromelia virus (ECTV), the causative agent of mousepox, has emerged as a valuable model for investigating the host-Orthopoxvirus relationship as it relates to pathogenesis and the immune response. ECTV is a mouse-specific virus and causes high mortality in susceptible mice strains, including BALB/c and C3H, whereas C57BL/6 and 129 strains are resistant to the disease. To understand the host genetic factors in different mouse strains during the ECTV infection, we carried out a microarray analysis of spleen tissues derived from BALB/c and C57BL/6 mice, respectively, at 3 and 10 days after ECTV infection. Differential Expression of Genes (DEGs) analyses revealed distinct differences in the gene profiles of susceptible and resistant mice. The susceptible BALB/c mice generated more DEGs than the resistant C57BL/6 mice. Additionally, gene ontology and KEGG pathway analysis showed the DEGs of susceptible mice were involved in innate immunity, apoptosis, metabolism, and cancer-related pathways, while the DEGs of resistant mice were largely involved in MAPK signaling and leukocyte transendothelial migration. Furthermore, the BALB/c mice showed a strong induction of interferon-induced genes, which, however, were weaker in the C57BL/6 mice. Collectively, the differential transcriptome profiles of susceptible and resistant mouse strains with ECTV infection will be crucial for further uncovering the molecular mechanisms of the host-Orthopoxvirus interaction.

A systemic macrophage response is required to contain a peripheral poxvirus infection

The goal of the innate immune system is to reduce pathogen spread prior to the initiation of an effective adaptive immune response. Following an infection at a peripheral site, virus typically drains through the lymph to the lymph node prior to entering the blood stream and being systemically disseminated. Therefore, there are three distinct spatial checkpoints at which intervention to prevent systemic spread of virus can occur, namely: 1) the site of infection, 2) the draining lymph node via filtration of lymph or 3) the systemic level via organs that filter the blood. We have previously shown that systemic depletion of phagocytic cells allows viral spread after dermal infection with Vaccinia virus (VACV), which infects naturally through the skin. Here we use multiple depletion methodologies to define both the spatial checkpoint and the identity of the cells that prevent systemic spread of VACV. Subcapsular sinus macrophages of the draining lymph node have been implicated as critical effectors in clearance of lymph borne viruses following peripheral infection. We find that monocyte populations recruited to the site of VACV infection play a critical role in control of local pathogenesis and tissue damage, but do not prevent dissemination of virus. Following infection with virulent VACV, the subcapsular sinus macrophages within the draining lymph node become infected, but are not exclusively required to prevent systemic spread. Rather, small doses of VACV enter the bloodstream and the function of systemic macrophages, but not dendritic cells, is required to prevent further spread. The results illustrate that a systemic innate response to a peripheral virus infection may be required to prevent widespread infection and pathology following infection with virulent viruses, such as poxviruses.

Dendritic cells during mousepox: The role of delayed apoptosis in the pathogenesis of infection

Dendritic cells (DCs) are effector cells linking the innate immune system with the adaptive immune response. Many viruses eliminate DCs to prevent host response, induce immunosuppression and to maintain chronic infection. In this study, we examined apoptotic response of dendritic cells during in vitro and in vivo infection with ectromelia virus (ECTV), the causative agent of mousepox. ECTV-infected bone marrow dendritic cells (BMDCs) from BALB/c mice underwent apoptosis through mitochondrial pathway at 48 h post infection, up-regulated FasL and decreased expression of anti-apoptotic Bcl-2 and pro-apoptotic Fas. Similar pattern of Bcl-2, Fas and FasL expression was observed for DCs early during in vivo infection of BALB/c mice. Both BMDCs and DCs from BALB/c mice showed no maturation upon ECTV infection. We conclude that ECTV-infected DCs from BALB/c mouse strain help the virus to spread and to maintain infection.

Monocytic myeloid-derived suppressor cells regulate T-cell responses against vaccinia virus

Vaccinia virus (VV) can potently activate NK- and T-cell responses, leading to efficient viral control and generation of long-lasting protective immunity. However, immune responses against viral infections are often tightly controlled to avoid collateral damage and systemic inflammation. We have previously shown that granulocytic myeloid-derived suppressor cells (g-MDSCs) can suppress the NK-cell response to VV infection. It remains unknown what regulates T-cell responses to VV infection in vivo. In this study, we first showed that monocytic MDSCs (m-MDSCs), but not g-MDSCs, from VV-infected mice could directly suppress CD4⁺ and CD8⁺ T-cell activation in vitro. We then demonstrated that defective recruitment of m-MDSCs to the site of VV infection in CCR2^-/- mice enhanced VV-specific CD8⁺ T-cell response and that adoptive transfer of m-MDSCs into VV-infected mice suppressed VV-specific CD8⁺ T-cell activation, leading to a delay in viral clearance. Mechanistically, we further showed that T-cell suppression by m-MDSCs is mediated by indication of iNOS and production of NO upon VV infection, and that IFN-γ is required for activation of m-MDSCs. Collectively, our results highlight a critical role for m-MDSCs in regulating T-cell responses against VV infection and may suggest potential strategies using m-MDSCs to modulate T-cell responses during viral infections.

Ectromelia virus N1L is essential for virulence but not dissemination in a classical model of mousepox

Mousepox is caused by the orthopoxvirus ectromelia virus (ECTV), and is thought to be transmitted via skin abrasions. We studied the ECTV virulence factor N1 following subcutaneous infection of mousepox-susceptible BALB/c mice. In this model, ECTV lacking N1L gene was attenuated more than 1000-fold compared with wild-type virus and replication was profoundly reduced as early as four days after infection. However, in contrast to data from an intranasal model, N1 protein was not required for virus dissemination. Further, neither T cell nor cytokine responses were enhanced in the absence of N1. Together with the early timing of reduced virus titres, this suggests that in a cutaneous model, N1 exerts its function at the level of infected cells or in the inhibition of the very earliest effectors of innate immunity.

Exposure of rhesus monkeys to cowpox virus Brighton Red by large-particle aerosol droplets results in an upper respiratory tract disease

We previously demonstrated that small-particle (0.5-3.0 µm) aerosol infection of rhesus monkeys (Macaca mulatta) with cowpox virus (CPXV)-Brighton Red (BR) results in fulminant respiratory tract disease characterized by severe lung parenchymal pathology but only limited systemic virus dissemination and limited classic epidermal pox-like lesion development (Johnson et al., 2015). Based on these results, and to further develop CPXV as an improved model of human smallpox, we evaluated a novel large-particle aerosol (7.0-9.0 µm) exposure of rhesus monkeys to CPXV-BR and monitored for respiratory tract disease by serial computed tomography (CT). As expected, the upper respiratory tract and large airways were the major sites of virus-induced pathology following large-particle aerosol exposure. Large-particle aerosol CPXV exposure of rhesus macaques resulted in severe upper airway and large airway pathology with limited systemic dissemination.

A lack of Fas/FasL signalling leads to disturbances in the antiviral response during ectromelia virus infection

Ectromelia virus (ECTV) is an orthopoxvirus (OPV) that causes mousepox, the murine equivalent of human smallpox. Fas receptor-Fas ligand (FasL) signaling is involved in apoptosis of immune cells and virus-specific cytotoxicity. The Fas/FasL pathway also plays an important role in controlling the local inflammatory response during ECTV infection. Here, the immune response to the ECTV Moscow strain was examined in Fas (-) (lpr), FasL (-) (gld) and C57BL6 wild-type mice. During ECTV-MOS infection, Fas- and FasL mice showed increased viral titers, decreased total numbers of NK cells, CD4(+) and CD8(+) T cells followed by decreased percentages of IFN-γ expressing NK cells, CD4(+) and CD8(+) T cells in spleens and lymph nodes. At day 7 of ECTV-MOS infection, Fas- and FasL-deficient mice had the highest regulatory T cell (Treg) counts in spleen and lymph nodes in contrast to wild-type mice. Furthermore, at days 7 and 10 of the infection, we observed significantly higher numbers of PD-L1-expressing dendritic cells in Fas (-) and FasL (-) mice in comparison to wild-type mice. Experiments in co-cultures of CD4(+) T cells and bone-marrow-derived dendritic cells showed that the lack of bilateral Fas-FasL signalling led to expansion of Tregs. In conclusion, our results demonstrate that during ECTV infection, Fas/FasL can regulate development of tolerogenic DCs and Tregs, leading to an ineffective immune response.

Evidence for Persistence of Ectromelia Virus in Inbred Mice, Recrudescence Following Immunosuppression and Transmission to Naïve Mice

Orthopoxviruses (OPV), including variola, vaccinia, monkeypox, cowpox and ectromelia viruses cause acute infections in their hosts. With the exception of variola virus (VARV), the etiological agent of smallpox, other OPV have been reported to persist in a variety of animal species following natural or experimental infection. Despite the implications and significance for the ecology and epidemiology of diseases these viruses cause, those reports have never been thoroughly investigated. We used the mouse pathogen ectromelia virus (ECTV), the agent of mousepox and a close relative of VARV to investigate virus persistence in inbred mice. We provide evidence that ECTV causes a persistent infection in some susceptible strains of mice in which low levels of virus genomes were detected in various tissues late in infection. The bone marrow (BM) and blood appeared to be key sites of persistence. Contemporaneous with virus persistence, antiviral CD8 T cell responses were demonstrable over the entire 25-week study period, with a change in the immunodominance hierarchy evident during the first 3 weeks. Some virus-encoded host response modifiers were found to modulate virus persistence whereas host genes encoded by the NKC and MHC class I reduced the potential for persistence. When susceptible strains of mice that had apparently recovered from infection were subjected to sustained immunosuppression with cyclophosphamide (CTX), animals succumbed to mousepox with high titers of infectious virus in various organs. CTX treated index mice transmitted virus to, and caused disease in, co-housed naïve mice. The most surprising but significant finding was that immunosuppression of disease-resistant C57BL/6 mice several weeks after recovery from primary infection generated high titers of virus in multiple tissues. Resistant mice showed no evidence of a persistent infection. This is the strongest evidence that ECTV can persist in inbred mice, regardless of their resistance status.

The Inhibitory Receptor NKG2A Sustains Virus-Specific CD8⁺ T Cells in Response to a Lethal Poxvirus Infection

CD8(+) T cells and NK cells protect from viral infections by killing virally infected cells and secreting interferon-γ. Several inhibitory receptors limit the magnitude and duration of these anti-viral responses. NKG2A, which is encoded by Klrc1, is a lectin-like inhibitory receptor that is expressed as a heterodimer with CD94 on NK cells and activated CD8(+) T cells. Previous studies on the impact of CD94/NKG2A heterodimers on anti-viral responses have yielded contrasting results and the in vivo function of NKG2A remains unclear. Here, we generated Klrc1(-/-) mice and found that NKG2A is selectively required for resistance to ectromelia virus (ECTV). NKG2A functions intrinsically within ECTV-specific CD8(+) T cells to limit excessive activation, prevent apoptosis, and preserve the specific CD8(+) T cell response. Thus, although inhibitory receptors often cause T cell exhaustion and viral spreading during chronic viral infections, NKG2A optimizes CD8(+) T cell responses during an acute poxvirus infection.

The Pathogenesis and Immunobiology of Mousepox

Ectromelia virus is a mouse-specific orthopoxvirus that, following footpad infection or natural transmission, causes mousepox in most strains of mice, while a few strains, such as C57BL/6, are resistant to the disease but not to the infection. Mousepox is an acute, systemic, highly lethal disease of remarkable semblance to smallpox, caused by the human-specific variola virus. Starting in 1929 with its discovery by Marchal, work with ECTV has provided essential information for our current understanding on how viruses spread lympho-hematogenously, the genetic control of antiviral resistance, the role of different components of the innate and adaptive immune system in the control of primary and secondary infections with acute viruses, and how the mechanisms of immune evasion deployed by the virus affect virulence in vivo. Here, I review the literature on the pathogenesis and immunobiology of ECTV infection in vivo.

Roles of natural killer cells in antiviral immunity

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NK cells can kill virus-infected cells and protect against severe infections.

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Long-lived memory NK cells may develop after vaccination or infection.

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NK cells are potent regulatory of antiviral T and B cell responses.

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The role of NK cells in human infection is complex and context-dependent.

Natural killer (NK) cells are important in immune defense against virus infections. This is predominantly considered a function of rapid, innate NK-cell killing of virus-infected cells. However, NK cells also prime other immune cells through the release of interferon gamma (IFN-γ) and other cytokines. Additionally, NK cells share features with long-lived adaptive immune cells and can impact disease pathogenesis through the inhibition of adaptive immune responses by virus-specific T and B cells. The relative contributions of these diverse and conflicting functions of NK cells in humans are poorly defined and likely context-dependent, thereby complicating the development of therapeutic interventions. Here we focus on the contributions of NK cells to disease in diverse virus infections germane to human health.

Natural Killer Cells and Innate Interferon Gamma Participate in the Host Defense against Respiratory Vaccinia Virus Infection

In establishing a respiratory infection, vaccinia virus (VACV) initially replicates in airway epithelial cells before spreading to secondary sites of infection, mainly the draining lymph nodes, spleen, gastrointestinal tract, and reproductive organs. We recently reported that interferon gamma (IFN-γ) produced by CD8 T cells ultimately controls this disseminated infection, but the relative contribution of IFN-γ early in infection is unknown. Investigating the role of innate immune cells, we found that the frequency of natural killer (NK) cells in the lung increased dramatically between days 1 and 4 postinfection with VACV. Lung NK cells displayed an activated cell surface phenotype and were the primary source of IFN-γ prior to the arrival of CD8 T cells. In the presence of an intact CD8 T cell compartment, depletion of NK cells resulted in increased lung viral load at the time of peak disease severity but had no effect on eventual viral clearance, disease symptoms, or survival. In sharp contrast, RAG(-/-) mice devoid of T cells failed to control VACV and succumbed to infection despite a marked increase in NK cells in the lung. Supporting an innate immune role for NK cell-derived IFN-γ, we found that NK cell-depleted or IFN-γ-depleted RAG(-/-) mice displayed increased lung VACV titers and dissemination to ovaries and a significantly shorter mean time to death compared to untreated NK cell-competent RAG(-/-) controls. Together, these findings demonstrate a role for IFN-γ in aspects of both the innate and adaptive immune response to VACV and highlight the importance of NK cells in T cell-independent control of VACV in the respiratory tract.

IMPORTANCE

Herein, we provide the first systematic evaluation of natural killer (NK) cell function in the lung after infection with vaccinia virus, a member of the Poxviridae family. The respiratory tract is an important mucosal site for entry of many human pathogens, including poxviruses, but precisely how our immune system defends the lung against these invaders remains unclear. Natural killer cells are a type of cytotoxic lymphocyte and part of our innate immune system. In recent years, NK cells have received increasing levels of attention following the discovery that different tissues contain specific subsets of NK cells with distinctive phenotypes and function. They are abundant in the lung, but their role in defense against respiratory viruses is poorly understood. What this study demonstrates is that NK cells are recruited, activated, and contribute to protection of the lung during a severe respiratory infection with vaccinia virus.

Vaccinia virus strain LC16m8 defective in the B5R gene keeps strong protection comparable to its parental strain Lister in immunodeficient mice

BACKGROUND

Attenuated vaccinia virus strain, LC16m8, defective in the B5R envelope protein gene, is used as a stockpile smallpox vaccine strain in Japan against bioterrorism: the defect in the B5R gene mainly contributes to its highly attenuated properties.

METHODS

The protective activity of LC16m8 vaccine against challenge with a lethal dose of vaccinia Western Reserve strain was assessed in wild-type and immunodeficient mice lacking CD4, MHC class I, MHC class II or MHC class I and II antigens.

RESULTS

The immunization with LC16m8 induced strong protective activity comparable to that of its parent strain, Lister (Elstree) strain, in wild-type mice from 2 days to 1 year after vaccination, as well as in immunodeficient mice at 2 or 3 weeks after vaccination. These results implicated that the defect in the B5R gene hardly affected the potential activity of LC16m8 to induce innate, cell-mediated and humoral immunity, and that LC16m8 could be effective in immunodeficient patients.

CONCLUSION

LC16m8 with truncated B5 protein has an activity to induce immunity, such as innate immunity and subsequent cell-mediated and humoral immunity almost completely comparable to the activity of its parental strain Lister.

Redundant Function of Plasmacytoid and Conventional Dendritic Cells Is Required To Survive a Natural Virus Infection

Viruses that spread systemically from a peripheral site of infection cause morbidity and mortality in the human population. Innate myeloid cells, including monocytes, macrophages, monocyte-derived dendritic cells (mo-DC), and dendritic cells (DC), respond early during viral infection to control viral replication, reducing virus spread from the peripheral site. Ectromelia virus (ECTV), an orthopoxvirus that naturally infects the mouse, spreads systemically from the peripheral site of infection and results in death of susceptible mice. While phagocytic cells have a requisite role in the response to ECTV, the requirement for individual myeloid cell populations during acute immune responses to peripheral viral infection is unclear. In this study, a variety of myeloid-specific depletion methods were used to dissect the roles of individual myeloid cell subsets in the survival of ECTV infection. We showed that DC are the primary producers of type I interferons (T1-IFN), requisite cytokines for survival, following ECTV infection. DC, but not macrophages, monocytes, or granulocytes, were required for control of the virus and survival of mice following ECTV infection. Depletion of either plasmacytoid DC (pDC) alone or the lymphoid-resident DC subset (CD8α(+) DC) alone did not confer lethal susceptibility to ECTV. However, the function of at least one of the pDC or CD8α(+) DC subsets is required for survival of ECTV infection, as mice depleted of both populations were susceptible to ECTV challenge. The presence of at least one of these DC subsets is sufficient for cytokine production that reduces ECTV replication and virus spread, facilitating survival following infection.

IMPORTANCE

Prior to the eradication of variola virus, the orthopoxvirus that causes smallpox, one-third of infected people succumbed to the disease. Following successful eradication of smallpox, vaccination rates with the smallpox vaccine have significantly dropped. There is now an increasing incidence of zoonotic orthopoxvirus infections for which there are no effective treatments. Moreover, the safety of the smallpox vaccine is of great concern, as complications may arise, resulting in morbidity. Like many viruses that cause significant human diseases, orthopoxviruses spread from a peripheral site of infection to become systemic. This study elucidates the early requirement for innate immune cells in controlling a peripheral infection with ECTV, the causative agent of mousepox. We report that there is redundancy in the function of two innate immune cell subsets in controlling virus spread early during infection. The viral control mediated by these cell subsets presents a potential target for therapies and rational vaccine design.

Small particle aerosol inoculation of cowpox Brighton Red in rhesus monkeys results in a severe respiratory disease

Cowpox virus (CPXV) inoculation of nonhuman primates (NHPs) has been suggested as an alternate model for smallpox (Kramski et al., 2010, PLoS One, 5, e10412). Previously, we have demonstrated that intrabronchial inoculation of CPXV-Brighton Red (CPXV-BR) into cynomolgus monkeys resulted in a disease that shared many similarities to smallpox; however, severe respiratory tract disease was observed (Smith et al., 2011, J. Gen. Virol.). Here we describe the course of disease after small particle aerosol exposure of rhesus monkeys using computed tomography (CT) to monitor respiratory disease progression. Subjects developed a severe respiratory disease that was uniformly lethal at 5.7 log10 PFU of CPXV-BR. CT indicated changes in lung architecture that correlated with changes in peripheral blood monocytes and peripheral oxygen saturation. While the small particle aerosol inoculation route does not accurately mimic human smallpox, the data suggest that CT can be used as a tool to monitor real-time disease progression for evaluation of animal models for human diseases.

Peptide-MHC-I from Endogenous Antigen Outnumber Those from Exogenous Antigen, Irrespective of APC Phenotype or Activation

Naïve anti-viral CD8⁺ T cells (T_CD8+) are activated by the presence of peptide-MHC Class I complexes (pMHC-I) on the surface of professional antigen presenting cells (pAPC). Increasing the number of pMHC-I in vivo can increase the number of responding T_CD8+. Antigen can be presented directly or indirectly (cross presentation) from virus-infected and uninfected cells, respectively. Here we determined the relative importance of these two antigen presenting pathways in mousepox, a natural disease of the mouse caused by the poxvirus, ectromelia (ECTV). We demonstrated that ECTV infected several pAPC types (macrophages, B cells, and dendritic cells (DC), including DC subsets), which directly presented pMHC-I to naïve T_CD8+ with similar efficiencies in vitro. We also provided evidence that these same cell-types presented antigen in vivo, as they form contacts with antigen-specific T_CD8+. Importantly, the number of pMHC-I on infected pAPC (direct presentation) vastly outnumbered those on uninfected cells (cross presentation), where presentation only occurred in a specialized subset of DC. In addition, prior maturation of DC failed to enhance antigen presentation, but markedly inhibited ECTV infection of DC. These results suggest that direct antigen presentation is the dominant pathway in mice during mousepox. In a broader context, these findings indicate that if a virus infects a pAPC then the presentation by that cell is likely to dominate over cross presentation as the most effective mode of generating large quantities of pMHC-I is on the surface of pAPC that endogenously express antigens. Recent trends in vaccine design have focused upon the introduction of exogenous antigens into the MHC Class I processing pathway (cross presentation) in specific pAPC populations. However, use of a pantropic viral vector that targets pAPC to express antigen endogenously likely represents a more effective vaccine strategy than the targeting of exogenous antigen to a limiting pAPC subpopulation.

To induce a protective cell type (CD8+ T cells) following virus infection, it is necessary to present degraded fragments of viral protein in complex with self molecules on the surface of so-called antigen presenting cells (APC). This process can occur in infected or uninfected APC and has been studied and quantified extensively in experimental setups in the lab. However, the extent to which presentation by infected or uninfected cells contribute to the induction of a protective CD8+ T cell response has not been studied extensively during a natural infection in a mouse model. Here we use a natural mouse virus to examine importantly, quantify, the contribution of presentation of the fragments of viral protein by infected or uninfected cells. We find that the presentation by infected cells dwarfs that seen by uninfected cells. The importance of this work lies in the fact that, if infected cells present way more antigen than uninfected cells, successful vaccine design should utilize this observation to make a vaccine where infected cells expressing virus proteins are the prevalent mode of induction of CD8+ T cells.

Safety and Immunogenicity of Modified Vaccinia Ankara-Bavarian Nordic Smallpox Vaccine in Vaccinia-Naive and Experienced Human Immunodeficiency Virus-Infected Individuals: An Open-Label, Controlled Clinical Phase II Trial

Background.  First- and second-generation smallpox vaccines are contraindicated in individuals infected with human immunodeficiency virus (HIV). A new smallpox vaccine is needed to protect this population in the context of biodefense preparedness. The focus of this study was to compare the safety and immunogenicity of a replication-deficient, highly attenuated smallpox vaccine modified vaccinia Ankara (MVA) in HIV-infected and healthy subjects. Methods.  An open-label, controlled Phase II trial was conducted at 36 centers in the United States and Puerto Rico for HIV-infected and healthy subjects. Subjects received 2 doses of MVA administered 4 weeks apart. Safety was evaluated by assessment of adverse events, focused physical exams, electrocardiogram recordings, and safety laboratories. Immune responses were assessed using enzyme-linked immunosorbent assay (ELISA) and a plaque reduction neutralization test (PRNT). Results.  Five hundred seventy-nine subjects were vaccinated at least once and had data available for analysis. Rates of ELISA seropositivity were comparably high in vaccinia-naive healthy and HIV-infected subjects, whereas PRNT seropositivity rates were higher in healthy compared with HIV-infected subjects. Modified vaccinia Ankara was safe and well tolerated with no adverse impact on viral load or CD4 counts. There were no cases of myo-/pericarditis reported. Conclusions.  Modified vaccinia Ankara was safe and immunogenic in subjects infected with HIV and represents a promising smallpox vaccine candidate for use in immunocompromised populations.

Fas/FasL pathway participates in regulation of antiviral and inflammatory response during mousepox infection of lungs

Fas receptor-Fas ligand (FasL) signalling is involved in apoptosis of immune cells as well as of the virus infected target cells but increasing evidence accumulates on Fas as a mediator of apoptosis-independent processes such as induction of activating and proinflammatory signals. In this study, we examined the role of Fas/FasL pathway in inflammatory and antiviral response in lungs using a mousepox model applied to C57BL6/J, B6. MRL-Faslpr/J, and B6Smn.C3-Faslgld/J mice. Ectromelia virus (ECTV) infection of Fas- and FasL-deficient mice led to increased virus titers in lungs and decreased migration of IFN-γ expressing NK cells, CD4+ T cells, CD8+ T cells, and decreased IL-15 expression. The lungs of ECTV-infected Fas- and FasL-deficient mice showed significant inflammation during later phases of infection accompanied by decreased expression of anti-inflammatory IL-10 and TGF-β1 cytokines and disturbances in CXCL1 and CXCL9 expression. Experiments in vitro demonstrated that ECTV-infected cultures of epithelial cells, but not macrophages, upregulate Fas and FasL and are susceptible to Fas-induced apoptosis. Our study demonstrates that Fas/FasL pathway during ECTV infection of the lungs plays an important role in controlling local inflammatory response and mounting of antiviral response.

Deficiency in Th2 cytokine responses exacerbate orthopoxvirus infection

Ectromelia virus (ECTV) causes mousepox in mice, a disease very similar to smallpox in humans. ECTV and variola virus (VARV), the agent of smallpox, are closely related orthopoxviruses. Mousepox is an excellent small animal model to study the genetic and immunologic basis for resistance and susceptibility of humans to smallpox. Resistance to mousepox is dependent on a strong polarized type 1 immune response, associated with robust natural killer (NK) cell, cytotoxic T lymphocyte (CTL) and gamma interferon (IFN-γ) responses. In contrast, ECTV-susceptible mice generate a type 2 response, associated with weak NK cell, CTL and IFN-γ responses but robust IL-4 responses. Nonetheless, susceptible strains infected with mutant ECTV lacking virus-encoded IFN-γ binding protein (vIFN-γbp) (ECTV-IFN-γbp^Δ) control virus replication through generation of type 1 response. Since the IL-4/IL-13/STAT-6 signaling pathways polarize type 2/T helper 2 (Th2) responses with a corresponding suppression of IFN-γ production, we investigated whether the combined absence of vIFN-γbp, and one or more host genes involved in Th2 response development, influence generation of protective immunity. Most mutant mouse strains infected with wild-type (WT) virus succumbed to disease more rapidly than WT animals. Conversely, the disease outcome was significantly improved in WT mice infected with ECTV-IFN-γbp^Δ but absence of IL-4/IL-13/STAT-6 signaling pathways did not provide any added advantage. Deficiency in IL-13 or STAT-6 resulted in defective CTL responses, higher mortality rates and accelerated deaths. Deficiencies in IL-4/IL-13/STAT-6 signaling pathways significantly reduced the numbers of IFN-γ producing CD4 and CD8 T cells, indicating an absence of a switch to a Th1-like response. Factors contributing to susceptibility or resistance to mousepox are far more complex than a balance between Th1 and Th2 responses.

EVM005: an ectromelia-encoded protein with dual roles in NF-κB inhibition and virulence

Poxviruses contain large dsDNA genomes encoding numerous open reading frames that manipulate cellular signalling pathways and interfere with the host immune response. The NF-κB signalling cascade is an important mediator of innate immunity and inflammation, and is tightly regulated by ubiquitination at several key points. A critical step in NF-κB activation is the ubiquitination and degradation of the inhibitor of kappaB (IκBα), by the cellular SCF^β-TRCP ubiquitin ligase complex. We show here that upon stimulation with TNFα or IL-1β, Orthopoxvirus-infected cells displayed an accumulation of phosphorylated IκBα, indicating that NF-κB activation was inhibited during poxvirus infection. Ectromelia virus is the causative agent of lethal mousepox, a natural disease that is fatal in mice. Previously, we identified a family of four ectromelia virus genes (EVM002, EVM005, EVM154 and EVM165) that contain N-terminal ankyrin repeats and C-terminal F-box domains that interact with the cellular SCF ubiquitin ligase complex. Since degradation of IκBα is catalyzed by the SCF^β-TRCP ubiquitin ligase, we investigated the role of the ectromelia virus ankyrin/F-box protein, EVM005, in the regulation of NF-κB. Expression of Flag-EVM005 inhibited both TNFα- and IL-1β-stimulated IκBα degradation and p65 nuclear translocation. Inhibition of the NF-κB pathway by EVM005 was dependent on the F-box domain, and interaction with the SCF complex. Additionally, ectromelia virus devoid of EVM005 was shown to inhibit NF-κB activation, despite lacking the EVM005 open reading frame. Finally, ectromelia virus devoid of EVM005 was attenuated in both A/NCR and C57BL/6 mouse models, indicating that EVM005 is required for virulence and immune regulation in vivo.

Poxviruses are large dsDNA viruses that are renowned for regulating cellular pathways and manipulating the host immune response, including the NF-κB pathway. NF-κB inhibition by poxviruses is a growing area of interest and this family of viruses has developed multiple mechanisms to manipulate the pathway. Here, we focus on regulation of the NF-κB pathway by ectromelia virus, the causative agent of mousepox. We demonstrate that ectromelia virus is a potent inhibitor of the NF-κB pathway. Previously, we identified a family of four ectromelia virus genes that contain N-terminal ankyrin repeats and a C-terminal F-box domain that interacts with the cellular SCF ubiquitin ligase. Significantly, expression of the ankyrin/F-box protein, EVM005, inhibited NF-κB, and the F-box domain was critical for NF-κB inhibition and interaction with the SCF complex. Ectromelia virus devoid of EVM005 still inhibited NF-κB, indicating that multiple gene products contribute to NF-κB inhibition. Importantly, mice infected with ectromelia virus lacking EVM005 had a robust immune response, leading to viral clearance during infection. The data present two mechanisms, one in which EVM005 inhibits NF-κB activation through manipulation of the host SCF ubiquitin ligase complex, and an additional, NF-κB-independent mechanism that drives virulence.

Safety of attenuated smallpox vaccine LC16m8 in immunodeficient mice

Freeze-dried live attenuated smallpox vaccine LC16m8 prepared in cell culture has been the sole smallpox vaccine licensed in Japan since 1975 and was recently recommended as a WHO stockpile vaccine. We evaluated the safety of recently remanufactured lots of LC16m8 using a series of immunodeficient mouse models. These models included suckling mice, severe combined immunodeficiency disease (SCID) mice, and wild-type mice treated with cyclosporine. LC16m8 showed extremely low virulence in each of the three mouse models compared with that of its parental strains, Lister and LC16mO. These results provide further evidence that LC16m8 is one of the safest replication-competent smallpox vaccines in the world and may be considered for use in immunodeficient patients.

The granzyme B-Serpinb9 axis controls the fate of lymphocytes after lysosomal stress

Cytotoxic lymphocytes (CLs) contain lysosome-related organelles (LROs) that perform the normal degradative functions of the lysosome, in addition to storage and release of powerful cytotoxins employed to kill virally infected or abnormal cells. Among these cytotoxins is granzyme B (GrB), a protease that has also been implicated in activation (restimulation)-induced cell death of natural killer (NK) and T cells, but the underlying mechanism and its regulation are unclear. Here we show that restimulation of previously activated human or mouse lymphocytes induces lysosomal membrane permeabilisation (LMP), followed by GrB release from LROs into the CL cytosol. The model lysosomal stressors sphingosine and Leu-Leu-methyl-ester, and CLs from gene-targeted mice were used to show that LMP releases GrB in both a time- and concentration-dependent manner, and that the liberated GrB is responsible for cell death. The endogenous GrB inhibitor Serpinb9 (Sb9) protects CLs against LMP-induced death but is decreasingly effective as the extent of LMP increases. We also used these model stressors to show that GrB is the major effector of LMP-mediated death in T cells, but that in NK cells additional effectors are released, making GrB redundant. We found that limited LMP and GrB release occurs constitutively in proliferating lymphocytes and in NK cells engaged with targets in vitro. In Ectromelia virus-infected lymph nodes, working NK cells lacking Sb9 are more susceptible to GrB-mediated death. Taken together, these data show that a basal level of LMP occurs in proliferating and activated lymphocytes, and is increased on restimulation. LMP releases GrB from LROs into the lymphocyte cytoplasm and its ensuing interaction with Sb9 dictates whether or not the cell survives. The GrB-Sb9 nexus may therefore represent an additional mechanism of limiting lymphocyte lifespan and populations.

Memory CD8⁺ T cells can outsource IFN-γ production but not cytolytic killing for antiviral protection

Immunization with vaccinia virus (VACV), the virus comprising the smallpox vaccine, induces memory CD8(+) T cells that protect from subsequent infections with smallpox in humans or the related ectromelia virus (ECTV) in mice. Memory CD8(+) T cells largely mediate these effects by expanding into secondary effectors that secrete the antiviral cytokine interferon-γ (IFN-γ) and induce cytolysis via releasing factors such as perforin, which permeabilizes target cells. We show that protection from ECTV infection after VACV immunization depends on the initial memory cell frequency and ability of expanded secondary effectors to kill infected targets in a perforin-dependent manner. Although IFN-γ is essential for antiviral protection, it can be produced by either secondary effectors or concomitant primary effector CD8(+) T cells recruited to the response. Thus, during lethal virus challenge, memory CD8(+) T cells are required for cytolytic killing of infected cells, but primary effectors can play important roles by producing IFN-γ.

Monkeypox virus infection of rhesus macaques induces massive expansion of natural killer cells but suppresses natural killer cell functions

Natural killer (NK) cells play critical roles in innate immunity and in bridging innate and adaptive immune responses against viral infection. However, the response of NK cells to monkeypox virus (MPXV) infection is not well characterized. In this intravenous challenge study of MPXV infection in rhesus macaques (Macaca mulatta), we analyzed blood and lymph node NK cell changes in absolute cell numbers, cell proliferation, chemokine receptor expression, and cellular functions. Our results showed that the absolute number of total NK cells in the blood increased in response to MPXV infection at a magnitude of 23-fold, manifested by increases in CD56+, CD16+, CD16-CD56- double negative, and CD16+CD56+ double positive NK cell subsets. Similarly, the frequency and NK cell numbers in the lymph nodes also largely increased with the total NK cell number increasing 46.1-fold. NK cells both in the blood and lymph nodes massively proliferated in response to MPXV infection as measured by Ki67 expression. Chemokine receptor analysis revealed reduced expression of CXCR3, CCR7, and CCR6 on NK cells at early time points (days 2 and 4 after virus inoculation), followed by an increased expression of CXCR3 and CCR5 at later time points (days 7-8) of infection. In addition, MPXV infection impaired NK cell degranulation and ablated secretion of interferon-γ and tumor necrosis factor-α. Our data suggest a dynamic model by which NK cells respond to MPXV infection of rhesus macaques. Upon virus infection, NK cells proliferated robustly, resulting in massive increases in NK cell numbers. However, the migrating capacity of NK cells to tissues at early time points might be reduced, and the functions of cytotoxicity and cytokine secretion were largely compromised. Collectively, the data may explain, at least partially, the pathogenesis of MPXV infection in rhesus macaques.

Quantitative immunophenotypic analysis of antigen-presenting cells involved in ectromelia virus antigen presentation in BALB/c and C57BL/6 mice

During mousepox in resistant (C57BL/6) or susceptible (BALB/c) strains of mice, stimulation of Th1 or Th2 cytokine immune response, respectively, is observed. Because mechanisms of different polarization of T cells remain elusive, in this study, we quantitatively assessed the phenotype of antigen-presenting cells (APCs) involved in ectromelia virus (ECTV) antigen presentation and cluster formation with effector cells in secondary lymphoid organs of BALB/c and C57BL/6 mice. We showed that both strains of mice display similar dynamics and kinetics of viral antigen presentation by CD11c(+) , CD11b(+) , and CD19(+) cells. CD11c(+) and CD11b(+) cells highly participated in viral antigen presentation during all stages of mousepox, whereas CD19(+) cells presented viral peptides later in infection. The main population of dendritic cells (DCs) engaged in ECTV antigen presentation and cell junction formation with effector cells was a population of myeloid CD11b(+) DCs (mDCs). We suggest that, on the one hand, ECTV may differentially affect the functions of APCs depending on the strain of mice. On the other hand, we suggest that some types of APCs, such as mDCs or other DCs subsets, have different abilities to direct the shape of immune response depending on the host resistance to mousepox.

Immunosenescence and age-related viral diseases

Immunosenescence is described as a decline in the normal functioning of the immune system associated with physiologic ageing. Immunosenescence contributes to reduced efficacy to vaccination and increased susceptibility to infectious diseases in the elderly. Extensive studies of laboratory animal models of ageing or donor lymphocyte analysis have identified changes in immunity caused by the ageing process. Most of these studies have identified phenotypic and functional changes in innate and adaptive immunity. However, it is unclear which of these defects are critical for impaired immune defense against infection. This review describes the changes that occur in innate and adaptive immunity with ageing and some age-related viral diseases where defects in a key component of immunity contribute to the high mortality rate in mouse models of ageing.

Vaccinia virus protein K7 is a virulence factor that alters the acute immune response to infection

Vaccinia virus (VACV) encodes many proteins that antagonize the innate immune system including a family of intracellular proteins with a B-cell lymphoma (Bcl)-2-like structure. One of these Bcl-2 proteins called K7 binds Toll-like receptor-adaptor proteins and the DEAD-box RNA helicase DDX3 and thereby inhibits the activation of NF-κB and interferon regulatory factor 3. However, the contribution of K7 to virus virulence is not known. Here a VACV lacking the K7R gene (vΔK7) was constructed and compared with control viruses that included a plaque purified wt (vK7), a revertant with the K7R gene reinserted (vK7-rev) and a frame-shifted virus in which the translational initiation codon was mutated to prevent K7 protein expression (vK7-fs). Data presented show that loss of K7 does not affect virus replication in cell culture or in vivo; however, viruses lacking the K7 protein were less virulent than controls in murine intradermal (i.d.) and intranasal (i.n.) infection models and there was an altered acute immune response to infection. In the i.d. model, vΔK7 induced smaller lesions than controls, and after i.n. infection vΔK7 induced a reduced weight loss and signs of illness, and more rapid clearance of virus from infected tissue. Concomitantly, the intrapulmonary innate immune response to infection with vΔK7 showed increased infiltration of NK cells and CD8⁺ T-cells, enhanced MHC class II expression by macrophages, and enhanced cytolysis of target cells by NK cells and VACV-specific CD8⁺ T-cells. Thus protein K7 is a virulence factor that affects the acute immune response to infection.

Suppression of NK cell-mediated cytotoxicity against PRRSV-infected porcine alveolar macrophages in vitro

The adaptive immunity against PRRSV has already been studied in depth, but only limited data are available on the innate immune responses against this pathogen. In the present study, we analyzed the interaction between porcine natural killer (NK) cells and PRRSV-infected primary porcine alveolar macrophages (PAMs), since NK cells are one of the most important components of innate immunity and PAMs are primary target cells of PRRSV infection. NK cytotoxicity assays were performed using enriched NK cells as effector cells and virus-infected or mock-inoculated PAMs as target cells. The NK cytotoxicity against PRRSV-infected PAMs was decreased starting from 6h post inoculation (hpi) till the end of the experiment (12 hpi) and was significantly lower than that against pseudorabies virus (PrV)-infected PAMs. UV-inactivated PRRSV also suppressed NK activity, but much less than infectious PRRSV. Furthermore, co-incubation with PRRSV-infected PAMs inhibited degranulation of NK cells. Finally, using the supernatant of PRRSV-infected PAMs collected at 12 hpi showed that the suppressive effect of PRRSV on NK cytotoxicity was not mediated by soluble factors. In conclusion, PRRSV-infected PAMs showed a reduced susceptibility toward NK cytotoxicity, which may represent one of the multiple evasion strategies of PRRSV.

NK cells and poxvirus infection

In recent years, our understanding of the role of natural killer (NK) cells in the response to viral infection has grown rapidly. Not only do we realize viruses have many immune-evasion strategies to escape NK cell responses, but that stimulation of NK cell subsets during an antiviral response occurs through receptors seemingly geared directly at viral products and that NK cells can provide a memory response to viral pathogens. Tremendous knowledge has been gained in this area through the study of herpes viruses, but appreciation for the significance of NK cells in the response to other types of viral infections is growing. The function of NK cells in defense against poxviruses has emerged over several decades beginning with the early seminal studies showing the role of NK cells and the NK gene complex in susceptibility of mouse strains to ectromelia, a poxvirus pathogen of mice. More recently, greater understanding has emerged of the molecular details of the response. Given that human diseases caused by poxviruses can be as lethal as smallpox or as benign as Molluscum contagiosum, and that vaccinia virus, the prototypic member of the pox family, persists as a mainstay of vaccine design and has potential as an oncolytic virus for tumor therapy, further research in this area remains important. This review focuses on recent advances in understanding the role of NK cells in the immune response to poxviruses, the receptors involved in activation of NK cells during poxvirus infection, and the viral evasion strategies poxviruses employ to avoid the NK response.

The orchestrated functions of innate leukocytes and T cell subsets contribute to humoral immunity, virus control, and recovery from secondary poxvirus challenge

A pivotal role for antigen-specific recall responses to secondary virus infection is well established, but the contribution of innate immune cells to this process is unknown. Recovery of mice from a primary orthopoxvirus (ectromelia virus [ECTV]) infection requires the function of natural killer (NK) cells, granulocytes, plasmacytoid dendritic cells (pDC), T cells, and B cells. However, during a secondary challenge, resolution of infection is thought to be dependent on antibody but not T cell function. We investigated the contribution of NK cells, granulocytes, and pDC to virus control during a secondary virus challenge in mice that had been primed with an avirulent, mutant strain of ECTV. Mice depleted of NK cells, granulocytes, or pDC effectively controlled virus, as did mice depleted of both CD4 and CD8 T cell subsets. However, mice concurrently depleted of all three innate cell subsets had elevated virus load, but this was significantly exacerbated in mice also depleted of CD4 and/or CD8 T cells. Increased viral replication in mice lacking innate cells plus CD4 T cells was associated with a significant reduction in neutralizing antibody. Importantly, in addition to T-dependent neutralizing antibody responses, the function of CD8 T cells was also clearly important for virus control. The data indicate that in the absence of innate cell subsets, a critical role for both CD4 and CD8 T cells becomes apparent and, conversely, in the absence of T cell subsets, innate immune cells help contain infection.

Genetic dissection of NK cell responses

The association of Natural Killer (NK) cell deficiencies with disease susceptibility has established a central role for NK cells in host defence. In this context, genetic approaches have been pivotal in elucidating and characterizing the molecular mechanisms underlying NK cell function. To this end, homozygosity mapping and linkage analysis in humans have identified mutations that impact NK cell function and cause life-threatening diseases. However, several critical restrictions accompany genetic studies in humans. Studying NK cell pathophysiology in a mouse model has therefore proven a useful tool. The relevance of the mouse model is underscored by the similarities that exist between cell-structure-sensing receptors and the downstream signaling that leads to NK cell activation. In this review, we provide an overview of how human and mouse quantitative trait locis (QTLs) have facilitated the identification of genes that modulate NK cell development, recognition, and killing of target cells.

Interferon-γ mediates chemokine-dependent recruitment of natural killer cells during viral infection

Natural killer (NK) cells provide in vivo control of orthopoxvirus infections in association with their expansion in the draining lymph node (LN), where they are normally very rare. The mechanism of this expansion is unclear. Herein, we determined that NK-cell depletion results in enhanced infection following footpad inoculation of cowpox virus, a natural pathogen of rodents. Following cowpox virus infection in normal mice, NK cells were greatly expanded in the draining LN, were not replicating, and displayed markers similar to splenic NK cells, suggesting specific recruitment of splenic NK cells rather than in situ proliferation. Moreover, NK-cell expansion was abrogated by prior injection of clodronate-loaded liposomes, indicating a role for subcapsular sinus macrophages. Furthermore, recruitment of transferred splenic NK cells to the draining LN was pertussis toxin-sensitive, suggesting involvement of chemokine receptors. Comprehensive analysis of chemokine mRNA expression in the draining LN following infection suggested the selective involvement of CCR2, CCR5, and/or CXCR3. Mice deficient for CCR2 or CCR5 had normal NK-cell recruitment, whereas CXCR3-deficient mice displayed a major defect, which was NK cell-intrinsic. Interestingly, both induction of transcripts for CXCR3 ligands (Cxcl9 and Cxcl10) and NK-cell recruitment required IFN-γ. These data indicate that NK-cell recruitment is mediated by subcapsular sinus macrophages, IFN-γ, and CXCR3 during orthopoxvirus infection.

Safety, immunogenicity, and surrogate markers of clinical efficacy for modified vaccinia Ankara as a smallpox vaccine in HIV-infected subjects

BACKGROUND

Human immunodeficiency virus (HIV)-infected persons are at higher risk for serious complications associated with traditional smallpox vaccines. Alternative smallpox vaccines with an improved safety profile would address this unmet medical need.

METHODS

The safety and immunogenicity of modified vaccinia Ankara (MVA) was assessed in 91 HIV-infected adult subjects (CD4(+) T-cell counts, ≥350 cells/mm(3)) and 60 uninfected volunteers. The primary objectives were to evaluate the safety of MVA and immunogenicity in HIV-infected and uninfected subjects. As a measure of the potential efficacy of MVA, the ability to boost the memory response in people previously vaccinated against smallpox was evaluated by the inclusion of vaccinia-experienced HIV-infected and HIV-uninfected subjects.

RESULTS

MVA was well tolerated and immunogenic in all subjects. Antibody responses were comparable between uninfected and HIV-infected populations, with only 1 significantly lower total antibody titer at 2 weeks after the second vaccination, while no significant differences were observed for neutralizing antibodies. MVA rapidly boosted the antibody responses in vaccinia-experienced subjects, supporting the efficacy of MVA against variola.

CONCLUSIONS

MVA is a promising candidate as a safer smallpox vaccine, even for immunocompromised individuals, a group for whom current smallpox vaccines have an unacceptable safety profile.

Cowpox virus but not Vaccinia virus induces secretion of CXCL1, IL-8 and IL-6 and chemotaxis of monocytes in vitro

Orthopoxviruses are large DNA viruses which can cause disease in numerous host species. Today, after eradication of Variola virus and the end of vaccination against smallpox, zoonotic Orthopoxvirus infections are emerging as potential threat to human health. The most common causes of zoonotic Orthopoxvirus infections are Cowpox virus in Europe, Monkeypox virus in Africa and Vaccinia virus in South America. Although all three viruses are genetically and antigenically closely related, the human diseases caused by each virus differ considerably. This observation may reflect different capabilities of these viruses to modulate the hosts' immune response. Therefore, we aimed at characterizing the specific cytokine response induced by Orthopoxvirus infection in vitro. We analysed the gene expression of nine human pro-inflammatory cytokines and chemokines in response to infection of HeLa cells and could identify an upregulation of cytokine gene expression following Cowpox virus and Monkeypox virus infection but not following Vaccinia virus infection. This was verified by a strong induction of especially IL-6, IL-8 and CXCL1 secretion into the cell culture supernatant following Cowpox virus infection. We could further show that supernatants derived from Cowpox virus-infected cells exhibit an increased chemotactic activity towards monocytic and macrophage-like cells. On the one hand, increased cytokine secretion by Cowpox virus-infected cells and subsequent monocyte/macrophage recruitment may contribute to host defence and facilitate clearance of the infection. On the other hand, given the assumed important role of circulating macrophages in viral spread, this may also point towards a mechanism facilitating delivery of the virus to further tissues in vivo.

NK cell response to vaccinia virus is regulated by myeloid-derived suppressor cells

NK cells are critical for the innate immune control of poxviral infections. Previous studies have shown that NK cells are efficiently activated in response to infection with vaccinia virus (VV), the most studied member of the poxvirus family. However, it remains unknown whether the activation of NK cells in response to VV infection is tightly regulated. In this study, we showed that myeloid-derived suppressor cells (MDSCs) rapidly accumulated at the site of VV infection. In vivo depletion of MDSCs led to enhanced NK cell proliferation, activation, and function in response to VV infection. This was accompanied by an increase in mortality and systemic IFN-γ production. We further demonstrated that the granulocytic-MDSC (G-MDSC) subset was responsible for the suppression on NK cells and that this suppression was mediated by reactive oxygen species. These results indicate that G-MDSCs can negatively regulate NK cell activation and function in response to VV infection and suggest that manipulation of G-MDSCs could represent an attractive strategy for regulating NK cell activities for potential therapeutic benefits.