Phenotypic and transcriptional changes in peripheral blood mononuclear cells during alphavirus encephalitis in mice.

Sindbis virus (SINV) infection of mice provides a model system for studying the pathogenesis of alphaviruses that infect the central nervous system (CNS) to cause encephalomyelitis. While studies of human viral infections typically focus on accessible cells from the blood, this compartment is rarely evaluated in mice. To bridge this gap, single-cell RNA sequencing (scRNAseq) was combined with flow cytometry to characterize the transcriptional and phenotypic changes of peripheral blood mononuclear cells (PBMCs) from SINV-infected mice. Twenty-one clusters were identified by scRNAseq at 7 days after infection, with a unique cluster and overall increase in naive B cells for infected mice. Uninfected mice had fewer immature T cells and CCR9+ CD4 T cells and a unique immature T cell cluster. Gene expression was most altered in the Ki67+ CD8 T cell cluster, with chemotaxis and proliferation-related genes upregulated. Global analysis indicated metabolic changes in myeloid cells and increased expression of Ccl5 by NK cells. Phenotypes of PBMCs and cells infiltrating the CNS were analyzed by flow cytometry over 14 days after infection. In PBMCs, CD8 and Th1 CD4 T cells increased in representation, while B cells showed a transient decrease at day 5 in total, Ly6a+, and naive cells, and an increase in activated B cells. In the brain, CD8 T cells increased for the first 7 days, while Th1 CD4 T cells and naive and Ly6a+ B cells continued to accumulate for 14 days. Therefore, dynamic immune cell changes can be identified in the blood as well as the CNS during viral encephalomyelitis. IMPORTANCE: The outcome of viral encephalomyelitis is dependent on the host immune response, with clearance and resolution of infection mediated by the adaptive immune response. These processes are frequently studied in mouse models of infection, where infected tissues are examined to understand the mechanisms of clearance and recovery. However, studies of human infection typically focus on the analysis of cells from the blood, a compartment rarely examined in mice, rather than inaccessible tissue. To close this gap, we used single-cell RNA sequencing and flow cytometry to profile the transcriptomic and phenotypic changes of peripheral blood mononuclear cells (PBMCs) before and after central nervous system (CNS) infection in mice. Changes to T and B cell gene expression and cell composition occurred in PBMC and during entry into the CNS, with CCL5 being a differentially expressed chemokine. Therefore, dynamic changes occur in the blood as well as the CNS during the response of mice to virus infection, which will inform the analysis of human studies.

SARS-CoV-2 reservoir in post-acute sequelae of COVID-19 (PASC).

Millions of people are suffering from Long COVID or post-acute sequelae of COVID-19 (PASC). Several biological factors have emerged as potential drivers of PASC pathology. Some individuals with PASC may not fully clear the coronavirus SARS-CoV-2 after acute infection. Instead, replicating virus and/or viral RNA-potentially capable of being translated to produce viral proteins-persist in tissue as a 'reservoir'. This reservoir could modulate host immune responses or release viral proteins into the circulation. Here we review studies that have identified SARS-CoV-2 RNA/protein or immune responses indicative of a SARS-CoV-2 reservoir in PASC samples. Mechanisms by which a SARS-CoV-2 reservoir may contribute to PASC pathology, including coagulation, microbiome and neuroimmune abnormalities, are delineated. We identify research priorities to guide the further study of a SARS-CoV-2 reservoir in PASC, with the goal that clinical trials of antivirals or other therapeutics with potential to clear a SARS-CoV-2 reservoir are accelerated.

Interferon regulatory factor 7 modulates virus clearance and immune responses to alphavirus encephalomyelitis.

IMPORTANCE: Viral encephalomyelitis outcome is dependent on host responses to neuronal infection. Interferon (IFN) is an important component of the innate response, and IFN regulatory factor (IRF) 7 is an inducible transcription factor for the synthesis of IFN-α. IRF7-deficient mice develop fatal paralysis after CNS infection with Sindbis virus, while wild-type mice recover. Irf7 -/- mice produce low levels of IFN-α but high levels of IFN-ß with induction of IFN-stimulated genes, so the reason for this difference is not understood. The current study shows that Irf7 -/- mice developed inflammation earlier but failed to clear virus from motor neuron-rich regions of the brainstem and spinal cord. Levels of IFN-γ and virus-specific antibody were comparable, indicating that IRF7 deficiency does not impair expression of these known viral clearance factors. Therefore, IRF7 is either necessary for the neuronal response to currently identified mediators of clearance or enables the production of additional antiviral factor(s) needed for clearance.

Effect of remdesivir post-exposure prophylaxis and treatment on pathogenesis of measles in rhesus macaques.

Measles is a systemic disease initiated in the respiratory tract with widespread measles virus (MeV) infection of lymphoid tissue. Mortality can be substantial, but no licensed antiviral therapy is available. We evaluated both post-exposure prophylaxis and treatment with remdesivir, a broad-spectrum antiviral, using a well-characterized rhesus macaque model of measles. Animals were treated with intravenous remdesivir for 12 days beginning either 3 days after intratracheal infection (post-exposure prophylaxis, PEP) or 11 days after infection at the onset of disease (late treatment, LT). As PEP, remdesivir lowered levels of viral RNA in peripheral blood mononuclear cells, but RNA rebounded at the end of the treatment period and infectious virus was continuously recoverable. MeV RNA was cleared more rapidly from lymphoid tissue, was variably detected in the respiratory tract, and not detected in urine. PEP did not improve clinical disease nor lymphopenia and reduced the antibody response to infection. In contrast, LT had little effect on levels of viral RNA or the antibody response but also did not decrease clinical disease. Therefore, remdesivir transiently suppressed expression of viral RNA and limited dissemination when provided as PEP, but virus was not cleared and resumed replication without improvement in the clinical disease parameters evaluated.

Infection of Pro- and Anti-Inflammatory Macrophages by Wild Type and Vaccine Strains of Measles Virus: NLRP3 Inflammasome Activation Independent of Virus Production.

In humans and non-human primates, wild type (WT) measles virus (MeV) replicates extensively in lymphoid tissue and induces an innate response characteristic of NF-κB and inflammasome activation without type I interferon. In contrast, the live attenuated MeV vaccine (LAMV) replicates poorly in lymphoid tissue with little detectable in vivo cytokine production. To characterize the innate responses of macrophages to WT MeV and LAMV infection, we analyzed primary human monocyte-derived macrophages and phorbol myristic acid-matured monocytic THP-1 cells (M0) polarized to inflammatory (M1) and anti-inflammatory (M2) phenotypes 24 h after MeV infection. LAMV infected macrophages more efficiently than WT MeV but produced less virus than WT MeV-infected macrophages. Both strains induced production of NF-κB-responsive cytokines IL-6 and TNFα and inflammasome products IL-1ß and IL-18 without evidence of pyroptosis. Analysis of THP-1 cells deficient in inflammasome sensors NOD-like receptor pyrin (NLRP)3, IFN-γ-inducible protein 16 (IFI16) or absent in melanoma (AIM)2; adaptor apoptosis-associated speck-like protein containing a CARD (ASC) or effector caspase 1 showed that IL-18 production was dependent on NLRP3, ASC, and caspase 1. However, M1 cells produced IL-1ß in the absence of ASC or caspase 1 indicating alternate pathways for MeV-induced pro-IL-1ß processing. Therefore, the innate response to in vitro infection of macrophages with both LAMV and WT MeV includes production of IL-6 and TNFα and activation of the NLRP3 inflammasome to release IL-1ß and IL-18. LAMV attenuation impairs production of infectious virus but does not reduce ability to infect macrophages or innate responses to infection.

Pro-Viral and Anti-Viral Roles of the RNA-Binding Protein G3BP1.

Viruses depend on host cellular resources to replicate. Interaction between viral and host proteins is essential for the pathogens to ward off immune responses as well as for virus propagation within the infected cells. While different viruses employ unique strategies to interact with diverse sets of host proteins, the multifunctional RNA-binding protein G3BP1 is one of the common targets for many viruses. G3BP1 controls several key cellular processes, including mRNA stability, translation, and immune responses. G3BP1 also serves as the central hub for the protein-protein and protein-RNA interactions within a class of biomolecular condensates called stress granules (SGs) during stress conditions, including viral infection. Increasing evidence suggests that viruses utilize distinct strategies to modulate G3BP1 function-either by degradation, sequestration, or redistribution-and control the viral life cycle positively and negatively. In this review, we summarize the pro-viral and anti-viral roles of G3BP1 during infection among different viral families.

Cell-Type-Dependent Role for nsP3 Macrodomain ADP-Ribose Binding and Hydrolase Activity during Chikungunya Virus Infection.

Chikungunya virus (CHIKV) causes outbreaks of rash, arthritis, and fever associated with neurologic complications, where astrocytes are preferentially infected. A determinant of virulence is the macrodomain (MD) of nonstructural protein 3 (nsP3), which binds and removes ADP-ribose (ADPr) from ADP-ribosylated substrates and regulates stress-granule disruption. We compared the replication of CHIKV 181/25 (WT) and MD mutants with decreased ADPr binding and hydrolase (G32S) or increased ADPr binding and decreased hydrolase (Y114A) activities in C8-D1A astrocytic cells and NSC-34 neuronal cells. WT CHIKV replication was initiated more rapidly with earlier nsP synthesis in C8-D1A than in NSC-34 cells. G32S established infection, amplified replication complexes, and induced host-protein synthesis shut-off less efficiently than WT and produced less infectious virus, while Y114A replication was close to WT. However, G32S mutation effects on structural protein synthesis were cell-type-dependent. In NSC-34 cells, E2 synthesis was decreased compared to WT, while in C8-D1A cells synthesis was increased. Excess E2 produced by G32S-infected C8-D1A cells was assembled into virus particles that were less infectious than those from WT or Y114A-infected cells. Because nsP3 recruits ADP-ribosylated RNA-binding proteins in stress granules away from translation-initiation factors into nsP3 granules where the MD hydrolase can remove ADPr, we postulate that suboptimal translation-factor release decreased structural protein synthesis in NSC-34 cells while failure to de-ADP-ribosylate regulatory RNA-binding proteins increased synthesis in C8-D1A cells.

The Integral Membrane Protein ZMPSTE24 Protects Cells from SARS-CoV-2 Spike-Mediated Pseudovirus Infection and Syncytia Formation.

COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has had a devastating impact on global public health, emphasizing the importance of understanding innate immune mechanisms and cellular restriction factors that cells can harness to fight viral infections. The multimembrane-spanning zinc metalloprotease ZMPSTE24 is one such restriction factor. ZMPSTE24 has a well-characterized proteolytic role in the maturation of prelamin A, precursor of the nuclear scaffold protein lamin A. An apparently unrelated role for ZMPSTE24 in viral defense involves its interaction with the interferon-inducible membrane proteins (IFITMs), which block virus-host cell fusion by rigidifying cellular membranes and thereby prevent viral infection. ZMPSTE24, like the IFITMs, defends cells against a broad spectrum of enveloped viruses. However, its ability to protect against coronaviruses has never been examined. Here, we show that overexpression of ZMPSTE24 reduces the efficiency of cellular infection by SARS-CoV-2 Spike-pseudotyped lentivirus and that genetic knockout or small interfering RNA-mediated knockdown of endogenous ZMPSTE24 enhances infectivity. We further demonstrate a protective role for ZMPSTE24 in a Spike-ACE2-dependent cell-cell fusion assay. In both assays, a catalytic dead version of ZMPSTE24 is equally as protective as the wild-type protein, indicating that ZMPSTE24's proteolytic activity is not required for defense against SARS-CoV-2. Finally, we demonstrate by plaque assays that Zmpste24-/- mouse cells show enhanced infection by a genuine coronavirus, mouse hepatitis virus (MHV). This study extends the range of viral protection afforded by ZMPSTE24 to include coronaviruses and suggests that targeting ZMPSTE24's mechanism of viral defense could have therapeutic benefit. IMPORTANCE The COVID-19 pandemic caused by the coronavirus SARS-CoV-2 has underscored the importance of understanding intrinsic cellular components that can be harnessed as the cell's first line of defense to fight against viral infection. Our paper focuses on one such protein, the integral membrane protease ZMPSTE24, which interacts with interferon-inducible transmembrane proteins (IFITMs). IFITMs interfere with virus entry by inhibiting fusion between viral and host cell membranes, and ZMPSTE24 appears to contribute to this inhibitory activity. ZMPSTE24 has been shown to defend cells against several, but not all, enveloped viruses. In this study, we extend ZMPSTE24's reach to include coronaviruses, by showing that ZMPSTE24 protects cells from SARS-CoV-2 pseudovirus infection, Spike protein-mediated cell-cell fusion, and infection by the mouse coronavirus MHV. This work lays the groundwork for further studies to decipher the mechanistic role of ZMPSTE24 in blocking the entry of SARS-CoV-2 and other viruses into cells.

Treatment of Sindbis Virus-Infected Neurons with Antibody to E2 Alters Synthesis of Complete and nsP1-Expressing Defective Viral RNAs.

Alphaviruses are positive-sense RNA viruses that are important causes of viral encephalomyelitis. Sindbis virus (SINV), the prototype alphavirus, preferentially infects neurons in mice and is a model system for studying mechanisms of viral clearance from the nervous system. Antibody specific to the SINV E2 glycoprotein plays an important role in SINV clearance, and this effect is reproduced in cultures of infected mature neurons. To determine how anti-E2 antibody affects SINV RNA synthesis, Oxford Nanopore Technologies direct long-read RNA sequencing was used to sequence viral RNAs following antibody treatment of infected neurons. Differentiated AP-7 rat olfactory neuronal cells, an in vitro model for mature neurons, were infected with SINV and treated with anti-E2 antibody. Whole-cell RNA lysates were collected for sequencing of poly(A)-selected RNA 24, 48, and 72 h after infection. Three primary species of viral RNA were produced: genomic, subgenomic, and defective viral genomes (DVGs) encoding the RNA capping protein nsP1. Antibody treatment resulted in overall lower production of SINV RNA, decreased synthesis of subgenomic RNA relative to genomic RNA, and suppressed production of the nsP1 DVG. The nsP1 DVG was packaged into virus particles and could be translated. Because antibody-treated cells released a higher proportion of virions with noncapped genomes and transient transfection to express the nsP1 DVG improved viral RNA capping in antibody-treated cells, we postulate that one mechanism by which antibody inhibits SINV replication in neurons is to suppress DVG synthesis and thus decrease production of infectious virions containing capped genomes. IMPORTANCE Alphaviruses are important causes of viral encephalomyelitis without approved treatments or vaccines. Antibody to the Sindbis virus (SINV) E2 glycoprotein is required for immune-mediated noncytolytic virus clearance from neurons. We used direct RNA nanopore sequencing to evaluate how anti-E2 antibody affects SINV replication at the RNA level. Antibody altered the viral RNAs produced by decreasing the proportion of subgenomic relative to genomic RNA and suppressing production of a previously unrecognized defective viral genome (DVG) encoding nsP1, the viral RNA capping enzyme. Antibody-treated neurons released a lower proportion of SINV particles with capped genomes necessary for translation and infection. Decreased nsP1 DVG production in antibody-treated neurons led to lower expression of nsP1 protein, decreased genome capping efficiency, and release of fewer infectious virus particles. Capping was increased with exogenous expression of the nsP1 DVG. These studies identify a novel alphavirus DVG function and new mechanism for antibody-mediated control of virus replication.

Effect of IL-10 Deficiency on TGFß Expression during Fatal Alphavirus Encephalomyelitis in C57Bl/6 Mice.

Sindbis virus (SINV) causes viral encephalitis in mice with strain-dependent virulence. Fatal encephalomyelitis in C57Bl/6 mice infected with a neuroadapted strain of SINV (NSV) is an immunopathogenic process that involves Th17 cells modulated by the regulatory cytokine IL-10. To further characterize the pathogenic immune response to NSV, we analyzed the regulation of transforming growth factor (TGF)-b in both wild-type (WT) and IL-10-deficient mice. NSV infection upregulated the expression of TGFb1 and TGFb3 in the central nervous system (CNS). In the absence of IL-10, levels of brain Tgfb1 mRNA and brain and spinal cord mature active TGFß1 and TGFß3 proteins were higher than in WT mice. Compared to WT mice, IL-10-deficient mice had more TGFß1-expressing type 3 innate lymphoid cells (ILC3s) and CD4+ T cells infiltrating the CNS, but similar numbers in the cervical lymph nodes. Expression of glycoprotein A repetitions predominant protein (GARP) that binds pro-TGFb on the surface of regulatory T cells was decreased on CNS cells from IL-10-deficient mice. Higher CNS TGFb was accompanied by more expression of TGFbRII receptor, activation of SMAD transcription factors, increased PCKα mRNA, and more RORγt-positive and IL-17A-expressing cells. These results suggest a compensatory role for TGFß in the absence of IL-10 that fosters Th17-related immunopathology and more rapid death after NSV infection.

Why does viral RNA sometimes persist after recovery from acute infections?

DNA viruses often persist in the body of their host, becoming latent and recurring many months or years later. By contrast, most RNA viruses cause acute infections that are cleared from the host as they lack the mechanisms to persist. However, it is becoming clear that viral RNA can persist after clinical recovery and elimination of detectable infectious virus. This persistence can either be asymptomatic or associated with late progressive disease or nonspecific lingering symptoms, such as may be the case following infection with Ebola or Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Why does viral RNA sometimes persist after recovery from an acute infection? Where does the RNA come from? And what are the consequences?

B-Cell Responses in Hospitalized Severe Acute Respiratory Syndrome Coronavirus 2-Infected Children With and Without Multisystem Inflammatory Syndrome.

Multisystem inflammatory syndrome in children (MIS-C) can complicate infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but differences in the immune responses during MIS-C compared to coronavirus disease 2019 (COVID-19) are poorly understood. We longitudinally compared the amounts and avidity of plasma anti-nucleocapsid (N) and spike (S) antibodies, phenotypes of B cells, and numbers of virus-specific antibody-secreting cells in circulation of children hospitalized with COVID-19 (n = 10) and with MIS-C (n = 12). N-specific immunoglobulin G (IgG) was higher early after presentation for MIS-C than COVID-19 patients and avidity of N- and S-specific IgG at presentation did not mature further during follow-up as it did for COVID-19. Both groups had waning proportions of B cells in circulation and decreasing but sustained production of virus-specific antibody-secreting cells for months. Overall, B-cell responses were similar, but those with MIS-C demonstrated a more mature antibody response at presentation compared to COVID-19, suggesting a postinfectious entity.

Continued Virus-Specific Antibody-Secreting Cell Production, Avidity Maturation and B Cell Evolution in Patients Hospitalized with COVID-19.

Understanding the development and sustainability of the virus-specific protective immune response to infection with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) remains incomplete with respect to the appearance and disappearance of virus-specific antibody-secreting cells (ASCs) in circulation. Therefore, we performed cross-sectional and longitudinal analyses of peripheral blood mononuclear cells and plasma collected from 55 hospitalized patients up to 4 months after onset of COVID-19 symptoms. Spike (S)- and nucleocapsid (N)-specific IgM and IgG ASCs appeared within 2 weeks accompanied by flow cytometry increases in double negative plasmablasts consistent with a rapid extrafollicular B cell response. Total and virus-specific IgM and IgG ASCs peaked at 3-4 weeks and were still being produced at 3-4 months accompanied by increasing antibody avidity consistent with a slower germinal center B cell response. N-specific ASCs were produced for longer than S-specific ASCs and avidity maturation was greater for antibody to N than S. Patients with more severe disease produced more S-specific IgM and IgG ASCs than those with mild disease and had higher levels of N- and S-specific antibody. Women had more B cells in circulation than men and produced more S-specific IgA and IgG and N-specific IgG ASCs. Flow cytometry analysis of B cell phenotypes showed an increase in circulating B cells at 4-6 weeks with decreased percentages of switched and unswitched memory B cells. These data indicate ongoing antigen-specific stimulation, maturation, and production of ASCs for several months after onset of symptoms in patients hospitalized with COVID-19.

The Conserved Macrodomain Is a Potential Therapeutic Target for Coronaviruses and Alphaviruses.

Emerging and re-emerging viral diseases pose continuous public health threats, and effective control requires a combination of non-pharmacologic interventions, treatment with antivirals, and prevention with vaccines. The COVID-19 pandemic has demonstrated that the world was least prepared to provide effective treatments. This lack of preparedness has been due, in large part, to a lack of investment in developing a diverse portfolio of antiviral agents, particularly those ready to combat viruses of pandemic potential. Here, we focus on a drug target called macrodomain that is critical for the replication and pathogenesis of alphaviruses and coronaviruses. Some mutations in alphavirus and coronaviral macrodomains are not tolerated for virus replication. In addition, the coronavirus macrodomain suppresses host interferon responses. Therefore, macrodomain inhibitors have the potential to block virus replication and restore the host's protective interferon response. Viral macrodomains offer an attractive antiviral target for developing direct acting antivirals because they are highly conserved and have a structurally well-defined (druggable) binding pocket. Given that this target is distinct from the existing RNA polymerase and protease targets, a macrodomain inhibitor may complement current approaches, pre-empt the threat of resistance and offer opportunities to develop combination therapies for combating COVID-19 and future viral threats.

Primary differentiated respiratory epithelial cells respond to apical measles virus infection by shedding multinucleated giant cells.

Measles virus (MeV) is highly infectious by the respiratory route and remains an important cause of childhood mortality. However, the process by which MeV infection is efficiently established in the respiratory tract is controversial with suggestions that respiratory epithelial cells are not susceptible to infection from the apical mucosal surface. Therefore, it has been hypothesized that infection is initiated in lung macrophages or dendritic cells and that epithelial infection is subsequently established through the basolateral surface by infected lymphocytes. To better understand the process of respiratory tract initiation of MeV infection, primary differentiated respiratory epithelial cell cultures were established from rhesus macaque tracheal and nasal tissues. Infection of these cultures with MeV from the apical surface was more efficient than from the basolateral surface with shedding of viable MeV-producing multinucleated giant cell (MGC) syncytia from the surface. Despite presence of MGCs and infectious virus in supernatant fluids after apical infection, infected cells were not detected in the adherent epithelial sheet and transepithelial electrical resistance was maintained. After infection from the basolateral surface, epithelial damage and large clusters of MeV-positive cells were observed. Treatment with fusion inhibitory peptides showed that MeV production after apical infection was not dependent on infection of the basolateral surface. These results are consistent with the hypothesis that MeV infection is initiated by apical infection of respiratory epithelial cells with subsequent infection of lymphoid tissue and systemic spread.

Stress granule formation, disassembly, and composition are regulated by alphavirus ADP-ribosylhydrolase activity.

While biomolecular condensates have emerged as an important biological phenomenon, mechanisms regulating their composition and the ways that viruses hijack these mechanisms remain unclear. The mosquito-borne alphaviruses cause a range of diseases from rashes and arthritis to encephalitis, and no licensed drugs are available for treatment or vaccines for prevention. The alphavirus virulence factor nonstructural protein 3 (nsP3) suppresses the formation of stress granules (SGs)-a class of cytoplasmic condensates enriched with translation initiation factors and formed during the early stage of infection. nsP3 has a conserved N-terminal macrodomain that hydrolyzes ADP-ribose from ADP-ribosylated proteins and a C-terminal hypervariable domain that binds the essential SG component G3BP1. Here, we show that macrodomain hydrolase activity reduces the ADP-ribosylation of G3BP1, disassembles virus-induced SGs, and suppresses SG formation. Expression of nsP3 results in the formation of a distinct class of condensates that lack translation initiation factors but contain G3BP1 and other SG-associated RNA-binding proteins. Expression of ADP-ribosylhydrolase-deficient nsP3 results in condensates that retain translation initiation factors as well as RNA-binding proteins, similar to SGs. Therefore, our data reveal that ADP-ribosylation controls the composition of biomolecular condensates, specifically the localization of translation initiation factors, during alphavirus infection.

U.S.-Japan cooperative medical sciences program: 22nd International Conference on Emerging Infectious Diseases in the Pacific Rim.

This review summarizes the presentations given at the 22nd International conference on Emerging Infectious Diseases in the Pacific Rim. The purpose of this annual meeting is to foster international collaborations and address important public health issues in the Asia-Pacific region. This meeting was held in Bangkok in February 2020 and focused on emerging virus infections. Unexpectedly, the SARS-CoV-2 pandemic was in the initial stages leading to a special session on COVID-19 in addition to talks on dengue, influenza, hepatitis, AIDS, Zika, chikungunya, rabies, cervical cancer and nasopharyngeal carcinoma.

Measles immunity and immunosuppression.

Effects of measles on the immune system are only partially understood. Lymphoid tissue is a primary site of measles virus (MeV) replication where CD150 is the receptor for infection of both B and T cells. Lymphocyte depletion occurs during the acute phase of infection, but initiation of the adaptive immune response leads to extensive lymphocyte proliferation, production of MeV-specific antibody and T cells, the rash and clearance of infectious virus. Viral RNA persists in lymphoid tissue accompanied by ongoing germinal center proliferation, production of antibody-secreting cells, functionally distinct populations of T cells and antibody avidity maturation to establish life-long immunity. However, at the same time diversity of pre-existing antibodies and numbers of memory and naive B cells are reduced and susceptibility to other infections is increased.

The NF-κB/leukemia inhibitory factor/STAT3 signaling pathway in antibody-mediated suppression of Sindbis virus replication in neurons.

Alphaviruses are positive-sense, enveloped RNA viruses that are important causes of viral encephalomyelitis. Sindbis virus (SINV) is the prototype alphavirus and preferentially infects neurons in rodents to induce an encephalomyelitis similar to the human disease. Using a mouse model of SINV infection of the nervous system, many of the immune processes involved in recovery from viral encephalomyelitis have been identified. Antibody specific to the SINV E2 glycoprotein plays an important role in recovery and is sufficient for noncytolytic suppression of virus replication in vivo and in vitro. To investigate the mechanism of anti-E2 antibody-mediated viral suppression, a reverse-phase protein array was used to broadly survey cellular signaling pathway activation following antibody treatment of SINV-infected differentiated AP-7 neuronal cells. Anti-E2 antibody induced rapid transient NF-κB and later sustained Y705 STAT3 phosphorylation, outlining an intracellular signaling cascade activated by antiviral antibody. Because NF-κB target genes include the STAT3-activating IL-6 family cytokines, expression of these messenger RNAS (mRNAs) was assessed. Expression of leukemia inhibitory factor (LIF) cytokine mRNA, but not other IL-6 family member mRNAs, was up-regulated by anti-E2 antibody. LIF induced STAT3 Y705 phosphorylation in infected differentiated AP-7 cells but did not inhibit virus replication. However, anti-E2 antibody localized the LIF receptor to areas of E2 expression on the infected cell surface, and LIF enhanced the antiviral effects of antibody. These findings identify activation of the NF-κB/LIF/STAT3 signaling cascade as involved in inducing antibody-mediated viral suppression and highlight the importance of nonneutralizing antibody functions in viral clearance from neurons.