Necroptosis blockade prevents lung injury in severe influenza

Severe influenza A virus (IAV) infections can result in hyper-inflammation, lung injury and acute respiratory distress syndrome1-5 (ARDS), for which there are no effective pharmacological therapies. Necroptosis is an attractive entry point for therapeutic intervention in ARDS and related inflammatory conditions because it drives pathogenic lung inflammation and lethality during severe IAV infection6-8 and can potentially be targeted by receptor interacting protein kinase 3 (RIPK3) inhibitors. Here we show that a newly developed RIPK3 inhibitor, UH15-38, potently and selectively blocked IAV-triggered necroptosis in alveolar epithelial cells in vivo. UH15-38 ameliorated lung inflammation and prevented mortality following infection with laboratory-adapted and pandemic strains of IAV, without compromising antiviral adaptive immune responses or impeding viral clearance. UH15-38 displayed robust therapeutic efficacy even when administered late in the course of infection, suggesting that RIPK3 blockade may provide clinical benefit in patients with IAV-driven ARDS and other hyper-inflammatory pathologies.

Pulmonary inflammation and fibroblast immunoregulation: from bench to bedside

In recent years, there has been an explosion of interest in how fibroblasts initiate, sustain, and resolve inflammation across disease states. Fibroblasts contain heterogeneous subsets with diverse functionality. The phenotypes of these populations vary depending on their spatial distribution within the tissue and the immunopathologic cues contributing to disease progression. In addition to their roles in structurally supporting organs and remodeling tissue, fibroblasts mediate critical interactions with diverse immune cells. These interactions have important implications for defining mechanisms of disease and identifying potential therapeutic targets. Fibroblasts in the respiratory tract, in particular, determine the severity and outcome of numerous acute and chronic lung diseases, including asthma, chronic obstructive pulmonary disease, acute respiratory distress syndrome, and idiopathic pulmonary fibrosis. Here, we review recent studies defining the spatiotemporal identity of the lung-derived fibroblasts and the mechanisms by which these subsets regulate immune responses to insult exposures and highlight past, current, and future therapeutic targets with relevance to fibroblast biology in the context of acute and chronic human respiratory diseases. This perspective highlights the importance of tissue context in defining fibroblast-immune crosstalk and paves the way for identifying therapeutic approaches to benefit patients with acute and chronic pulmonary disorders.

Indoleamine 2,3-dioxygenase 1 regulates cell permissivity to astrovirus infection

Astroviruses cause a spectrum of diseases spanning asymptomatic infections to severe diarrhea, but little is understood about their pathogenesis. We previously determined that small intestinal goblet cells were the main cell type infected by murine astrovirus-1. Here, we focused on the host immune response to infection and inadvertently discovered a role for indoleamine 2,3-dioxygenase 1 (Ido1), a host tryptophan catabolizing enzyme, in the cellular tropism of murine and human astroviruses. We identified that Ido1 expression was highly enriched among infected goblet cells, and spatially corresponded to the zonation of infection. Because Ido1 can act as a negative regulator of inflammation, we hypothesized it could dampen host antiviral responses. Despite robust interferon signaling in goblet cells, as well as tuft cell and enterocyte bystanders, we observed delayed cytokine induction and suppressed levels of fecal lipocalin-2. Although we found Ido-/- animals were more resistant to infection, this was not associated with fewer goblet cells nor could it be rescued by knocking out interferon responses, suggesting that IDO1 instead regulates cell permissivity. We characterized IDO1-/- Caco-2 cells and observed significantly reduced human astrovirus-1 infection. Together this study highlights a role for Ido1 in astrovirus infection and epithelial cell maturation.

SARS-CoV-2 infection results in immune responses in the respiratory tract and peripheral blood that suggest mechanisms of disease severity

Respiratory tract infection with SARS-CoV-2 results in varying immunopathology underlying COVID-19. We examine cellular, humoral and cytokine responses covering 382 immune components in longitudinal blood and respiratory samples from hospitalized COVID-19 patients. SARS-CoV-2-specific IgM, IgG, IgA are detected in respiratory tract and blood, however, receptor-binding domain (RBD)-specific IgM and IgG seroconversion is enhanced in respiratory specimens. SARS-CoV-2 neutralization activity in respiratory samples correlates with RBD-specific IgM and IgG levels. Cytokines/chemokines vary between respiratory samples and plasma, indicating that inflammation should be assessed in respiratory specimens to understand immunopathology. IFN-α2 and IL-12p70 in endotracheal aspirate and neutralization in sputum negatively correlate with duration of hospital stay. Diverse immune subsets are detected in respiratory samples, dominated by neutrophils. Importantly, dexamethasone treatment does not affect humoral responses in blood of COVID-19 patients. Our study unveils differential immune responses between respiratory samples and blood, and shows how drug therapy affects immune responses during COVID-19.

Identifying distinct T cell subsets in the context of pediatric ARDS

Acute respiratory distress syndrome (ARDS) is a condition defined by sudden respiratory failure caused by severe lung injury, with disease etiology that can differ across pediatric and adult patients. While T lymphocytes have been implicated in ARDS, the role of T cell subsets in pediatric ARDS (pARDS) is not fully understood. In this study, we sought to characterize the potential role of T cells in pARDS caused by viral lower respiratory tract infection (LRTI). To do this, we performed single cell RNA-Seq on tracheal aspirate samples from three categories of mechanically ventilated pediatric patients: 1) LRTI patients with pARDS, 2) LRTI patients without pARDS, and 3) ventilated patients without underlying LRTI or pARDS. All LRTI patients were diagnosed with respiratory viral infections, 70 percent of which were respiratory syncytial virus (RSV). Within the T lymphocytes, we identified populations of CD8+ T cells, CD4+ regulatory T cells, and γδ T cells that each expressed unique gene signatures with distinct distribution across disease states. We compared these data to T cell subsets within healthy lung tissue at multiple ages, and within the periphery of healthy adults. These analyses identified age, location, and disease-specific differences within T cell subsets, highlighting the unique role of T cells in pARDS.

Supported by grants from NIH (R01 AI54470, R01 AI121832)

Influenza virus and SARS-CoV-2: pathogenesis and host responses in the respiratory tract

Influenza viruses cause annual epidemics and occasional pandemics of respiratory tract infections that produce a wide spectrum of clinical disease severity in humans. The novel betacoronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in December 2019 and has since caused a pandemic. Both viral and host factors determine the extent and severity of virus-induced lung damage. The host's response to viral infection is necessary for viral clearance but may be deleterious and contribute to severe disease phenotypes. Similarly, tissue repair mechanisms are required for recovery from infection across the spectrum of disease severity; however, dysregulated repair responses may lead to chronic lung dysfunction. Understanding of the mechanisms of immunopathology and tissue repair following viral lower respiratory tract infection may broaden treatment options. In this Review, we discuss the pathogenesis, the contribution of the host response to severe clinical phenotypes and highlight early and late epithelial repair mechanisms following influenza virus infection, each of which has been well characterized. Although we are still learning about SARS-CoV-2 and its disease manifestations in humans, throughout the Review we discuss what is known about SARS-CoV-2 in the context of this broad knowledge of influenza virus, highlighting the similarities and differences between the respiratory viruses.

ADAMTS4 promotes chronic airway inflammation and hyperresponsiveness in HDM-based mouse model of asthma: A potential therapeutic target for asthma traits

Asthma is a chronic airway inflammation characterized by combination of immune responses and structural remodeling that involves both large and small airways. The global prevalence of asthma is on the rise with a sizable portion of asthmatics not responding to existing medication, thus posing a substantial health and economic burden. Matrix proteases including A Disintegrin and Metalloproteinase with Thrombospondin motifs (ADAMTS) contribute to lung damage and repair responses. Our lab has recently reported a critical role for ADAMTS4 in the immunopathology of IAV infection-induced acute lung inflammation through exuberant activity of lung fibroblasts and remodeling of the extracellular matrix. Given the divergent nature of immune responses in chronic airway inflammation compared to infection, we sought to investigate the sustaining role of ADAMTS4 upon persistent exposure to aeroallergen. Using HDM-based asthma model, ADAMTS4 knockout mice exhibited a robust reduction in lung eosinophils, mucus production, and airway resistance compared to their wildtype counterparts. This was associated with a marked decrease in IL-4, IL-5, and IL-13 cytokine production and IgE secretion. Interestingly, ADAMTS4 gene deletion differentially blocked the myeloid cell recruitment to the lungs without affecting their development. Such effects were also associated with significant reduction in alveolar macrophages and peribranchial inflammation. These results suggest that the modulating immune responses by ADAMTS-4 is sustainable beyond the acute phase of inflammation and may be extensively contributing to the immunopathology of chronic airway inflammation, thus harboring a therapeutic potential for the treatment of asthma traits.

Necroptosis restricts influenza A virus as a stand-alone cell death mechanism

Influenza A virus (IAV) activates ZBP1-initiated RIPK3-dependent parallel pathways of necroptosis and apoptosis in infected cells. Although mice deficient in both pathways fail to control IAV and succumb to lethal respiratory infection, RIPK3-mediated apoptosis by itself can limit IAV, without need for necroptosis. However, whether necroptosis, conventionally considered a fail-safe cell death mechanism to apoptosis, can restrict IAV—or indeed any virus—in the absence of apoptosis is not known. Here, we use mice selectively deficient in IAV-activated apoptosis to show that necroptosis drives robust antiviral immune responses and promotes effective virus clearance from infected lungs when apoptosis is absent. We also demonstrate that apoptosis and necroptosis are mutually exclusive fates in IAV-infected cells. Thus, necroptosis is an independent, “stand-alone” cell death mechanism that fully compensates for the absence of apoptosis in antiviral host defense.

Publisher Correction: Exuberant fibroblast activity compromises lung function via ADAMTS4

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Influenza Virus Z-RNAs Induce ZBP1-Mediated Necroptosis

Influenza A virus (IAV) is a lytic RNA virus that triggers receptor-interacting serine/threonine-protein kinase 3 (RIPK3)-mediated pathways of apoptosis and mixed lineage kinase domain-like pseudokinase (MLKL)-dependent necroptosis in infected cells. ZBP1 initiates RIPK3-driven cell death by sensing IAV RNA and activating RIPK3. Here, we show that replicating IAV generates Z-RNAs, which activate ZBP1 in the nucleus of infected cells. ZBP1 then initiates RIPK3-mediated MLKL activation in the nucleus, resulting in nuclear envelope disruption, leakage of DNA into the cytosol, and eventual necroptosis. Cell death induced by nuclear MLKL was a potent activator of neutrophils, a cell type known to drive inflammatory pathology in virulent IAV disease. Consequently, MLKL-deficient mice manifest reduced nuclear disruption of lung epithelia, decreased neutrophil recruitment into infected lungs, and increased survival following a lethal dose of IAV. These results implicate Z-RNA as a new pathogen-associated molecular pattern and describe a ZBP1-initiated nucleus-to-plasma membrane "inside-out" death pathway with potentially pathogenic consequences in severe cases of influenza.

One hundred years of (influenza) immunopathology

It has been over 100 years since the 1918 influenza pandemic, one of the most infamous examples of viral immunopathology. Since that time, there has been an inevitable repetition of influenza pandemics every few decades and yearly influenza seasons, which have a significant impact on human health. Recently, noteworthy progress has been made in defining the cellular and molecular mechanisms underlying pathology induced by an exuberant host response to influenza virus infection. Infection with influenza viruses is associated with a wide spectrum of disease, from mild symptoms to severe complications including respiratory failure, and the severity of influenza disease is driven by a complex interplay of viral and host factors. This chapter will discuss mechanisms of infection severity using concepts of disease resistance and tolerance as a framework for understanding the balance between viral clearance and immunopathology. We review mechanistic studies in animal models of infection and correlational studies in humans that have begun to define these factors and discuss promising host therapeutic targets to improve outcomes from severe influenza disease.

Astrovirus infects actively secreting goblet cells and alters the gut mucus barrier

Astroviruses are a global cause of pediatric diarrhea, but they are largely understudied, and it is unclear how and where they replicate in the gut. Using an in vivo model, here we report that murine astrovirus preferentially infects actively secreting small intestinal goblet cells, specialized epithelial cells that maintain the mucus barrier. Consequently, virus infection alters mucus production, leading to an increase in mucus-associated bacteria and resistance to enteropathogenic E. coli colonization. These studies establish the main target cell type and region of the gut for productive murine astrovirus infection. They further define a mechanism by which an enteric virus can regulate the mucus barrier, induce functional changes to commensal microbial communities, and alter host susceptibility to pathogenic bacteria.

Human CD8+ T cell cross-reactivity across influenza A, B and C viruses

Influenza A, B and C viruses (IAV, IBV and ICV, respectively) circulate globally and infect humans, with IAV and IBV causing the most severe disease. CD8+ T cells confer cross-protection against IAV strains, however the responses of CD8+ T cells to IBV and ICV are understudied. We investigated the breadth of CD8+ T cell cross-recognition and provide evidence of CD8+ T cell cross-reactivity across IAV, IBV and ICV. We identified immunodominant CD8+ T cell epitopes from IBVs that were protective in mice and found memory CD8+ T cells directed against universal and influenza-virus-type-specific epitopes in the blood and lungs of healthy humans. Lung-derived CD8+ T cells displayed tissue-resident memory phenotypes. Notably, CD38+Ki67+CD8+ effector T cells directed against novel epitopes were readily detected in IAV- or IBV-infected pediatric and adult subjects. Our study introduces a new paradigm whereby CD8+ T cells confer unprecedented cross-reactivity across all influenza viruses, a key finding for the design of universal vaccines.

Lung γδ T Cells Mediate Protective Responses during Neonatal Influenza Infection that Are Associated with Type 2 Immunity

Compared to adults, infants suffer higher rates of hospitalization, severe clinical complications, and mortality due to influenza infection. We found that γδ T cells protected neonatal mice against mortality during influenza infection. γδ T cell deficiency did not alter viral clearance or interferon-γ production. Instead, neonatal influenza infection induced the accumulation of interleukin-17A (IL-17A)-producing γδ T cells, which was associated with IL-33 production by lung epithelial cells. Neonates lacking IL-17A-expressing γδ T cells or Il33 had higher mortality upon influenza infection. γδ T cells and IL-33 promoted lung infiltration of group 2 innate lymphoid cells and regulatory T cells, resulting in increased amphiregulin secretion and tissue repair. In influenza-infected children, IL-17A, IL-33, and amphiregulin expression were correlated, and increased IL-17A levels in nasal aspirates were associated with better clinical outcomes. Our results indicate that γδ T cells are required in influenza-infected neonates to initiate protective immunity and mediate lung homeostasis.

IL-1 cytokines promote ADAMTS-mediated remodeling of the lung extracellular matrix during severe influenza A infection in mice

The lung extracellular matrix (ECM) is extensively remodeled following respiratory viral infection. This remodeling occurs during inflammation when immune cells migrate to the site of infection and during repair as the ECM is modified to regenerate damaged tissue. Matrix proteases, including the A Disintegrin and Metalloproteinase with Thrombospondin motifs (ADAMTS) gene family, play a critical role in these remodeling processes. The immune mechanisms that regulate matrix protease activity and their effect on host responses to lung infection have not been well characterized. Previously, we found that ADAMTS-4 knockout mice exhibited improved survival following lethal influenza A (IAV) challenge and signs of reduced immunopathology, including fewer numbers of inflammatory T cells. Here, by flow cytometric sorting of immune and stromal cell populations, we identified fibroblasts as the main producers of ADAMTS-4 in the lung during severe IAV infection, with ~50-fold higher levels of mRNA compared to other lung cell populations. Stimulation of primary lung fibroblasts in vitro with a wide range of inflammatory cytokines demonstrated that only IL-1β and IL-1α induced expression of ADAMTS-4. Co-stimulation of lung fibroblasts with IL-1 cytokines and TNF-α resulted in a synergistic effect and an increase of ADAMTS-4 expression to greater than 6-fold above baseline levels. IL-1 treatment of lung fibroblasts also induced expression of the lung ECM proteoglycan versican (VCAN), a major enzymatic target of ADAMTS-4. These results suggest a role for IL-1 cytokines in regulating lung ECM remodeling, and highlight the potential of modulating the IL-1/ADAMTS-4 pathway to reduce immunopathology following severe respiratory infection.

Lung extracellular matrix changes mediated by ADAMTS-4 modulate immune responses to influenza A infection

The lung extracellular matrix (ECM) undergoes dramatic changes during and after respiratory viral infection. Matrix proteases, including members of matrix metalloproteinases (MMPs) and A Disintegrin and Metalloproteinase with Thrombospondin motifs (ADAMTS) gene families, play a critical role in remodeling the ECM following tissue damage. Alterations in the ECM feed into immunological pathways, which subsequently regulate repair and regeneration. The mechanisms that regulate matrix proteases and the effect of lung ECM changes on immune responses to viral infection have not been well characterized. We previously found that ADAMTS-4 was upregulated in the lung within 3 days after influenza infection and that genetic knockout of ADAMTS-4 resulted in improved survival following infection in mice. Using quantitative PCR, we determined the transcriptional levels of 96 ECM-related genes in lung homogenates from ADAMTS-4 −/− and +/+ mice during a 15-day infection time course. At early time points, lungs from ADAMTS-4 −/− mice exhibited lower expression of inflammatory cytokines, including IL-6 and IL-1β, but higher expression of several ECM remodeling enzymes, including neutrophil elastase and MMP-13. Flow cytometric analysis of immune cell populations from lungs revealed altered neutrophil and T cell kinetics between ADAMTS-4 −/− and +/+ mice over the course of influenza infection. Lungs from ADAMTS-4 −/− mice exhibited higher numbers of neutrophils at 3 dpi and a higher percentage of IFNγ-producing CD4+ and CD8+ T cells at 6 dpi, but lower numbers of these cells by 12 dpi. These results suggest that ADAMTS-4-mediated alterations of the lung ECM modulate immune responses to influenza infection and may contribute to immunopathology.

Towards integrating extracellular matrix and immunological pathways

The extracellular matrix (ECM) is a complex and dynamic structure made up of an estimated 300 different proteins. The ECM is also a rich source of cytokines and growth factors in addition to numerous bioactive ECM degradation products that influence cell migration, proliferation, and differentiation. The ECM is constantly being remodeled during homeostasis and in a wide range of pathological contexts. Changes in the ECM modulate immune responses, which in turn regulate repair and regeneration of tissues. Here, we review the many components of the ECM, enzymes involved in ECM remodeling, and the signals that feed into immunological pathways in the context of a dynamic ECM. We highlight studies that have taken an integrative approach to studying immune responses in the context of the ECM and studies that use novel proteomic strategies. Finally, we discuss research challenges relevant to the integration of immune and ECM networks and propose experimental and translational approaches to resolve these issues.

DAI Senses Influenza A Virus Genomic RNA and Activates RIPK3-Dependent Cell Death

Influenza A virus (IAV) is an RNA virus that is cytotoxic to most cell types in which it replicates. IAV activates the host kinase RIPK3, which induces cell death via parallel pathways of necroptosis, driven by the pseudokinase MLKL, and apoptosis, dependent on the adaptor proteins RIPK1 and FADD. How IAV activates RIPK3 remains unknown. We report that DAI (ZBP1/DLM-1), previously implicated as a cytoplasmic DNA sensor, is essential for RIPK3 activation by IAV. Upon infection, DAI recognizes IAV genomic RNA, associates with RIPK3, and is required for recruitment of MLKL and RIPK1 to RIPK3. Cells lacking DAI or containing DAI mutants deficient in nucleic acid binding are resistant to IAV-triggered necroptosis and apoptosis. DAI-deficient mice fail to control IAV replication and succumb to lethal respiratory infection. These results identify DAI as a link between IAV replication and RIPK3 activation and implicate DAI as a sensor of RNA viruses.

Adapting SHIVs In Vivo Selects for Envelope-Mediated Interferon-α Resistance

Lentiviruses are able to establish persistent infection in their respective hosts despite a potent type-I interferon (IFN-I) response following transmission. A number of IFN-I-induced host factors that are able to inhibit lentiviral replication in vitro have been identified, and these studies suggest a role for IFN-induced factors as barriers to cross-species transmission. However, the ability of these factors to inhibit viral replication in vivo has not been well characterized, nor have the viral determinants that contribute to evasion or antagonism of the host IFN-I response. In this study, we hypothesized that the host IFN-I response serves as a strong selective pressure in the context of SIV/HIV chimeric virus (SHIV) infection of macaques and sought to identify the viral determinants that contribute to IFN-I resistance. We assessed the ability of SHIVs encoding HIV-1 sequences adapted by serial passage in macaques versus SHIVs encoding HIV sequences isolated directly from infected individuals to replicate in the presence of IFNα in macaque lymphocytes. We demonstrate that passage in macaques selects for IFNα resistant viruses that have higher replication kinetics and increased envelope content. SHIVs that encode HIV-1 sequences derived directly from infected humans were sensitive to IFNα –induced inhibition whereas SHIVs obtained after passage in macaques were not. This evolutionary process was directly observed in viruses that were serially passaged during the first few months of infection–a time when the IFNα response is high. Differences in IFNα sensitivity mapped to HIV-1 envelope and were associated with increased envelope levels despite similar mRNA expression, suggesting a post-transcriptional mechanism. These studies highlight critical differences in IFNα sensitivity between HIV-1 sequences in infected people and those used in SHIV models.

The innate immune system is an important host defense against viral infection. Recently, there has been significant interest in characterizing the innate immune response to HIV-1 infection, in particular the role of type-I interferon (IFN-I). Understanding the interaction of HIV-1 with the innate immune system is particularly important for the development of animal models of infection as innate host factors present potential species-specific barriers to the establishment of persistent infection. One of the most commonly used animal models of HIV-1 infection is chimeric SIV/HIV (SHIV) infection of macaques. Here, we demonstrate that the process of adapting SHIVs for replication in macaques selects for viruses that are resistant to the IFNα response, and we identity important viral determinants that contribute to this resistance. This improved understanding of virus interactions with the innate immune response may facilitate the development of improved animal models of HIV-1 infection.

ADAMTS4 modulates lung tissue repair following lethal influenza A infection in mice

Lung tissue damage is a major contributor to morbidity and mortality following infection with influenza A virus. Tissue repair, including remodeling of the extracellular matrix (ECM), is a critical process in recovery from infection. A Disintegrin and Metalloproteinase with Thrombospondin motifs (ADAMTS) family proteins are secreted zinc metalloendopeptidases and important mediators of ECM remodeling. The role of ADAMTS proteins in lung tissue repair following influenza A infection remains unclear. To investigate the regulation of ADAMTS genes in the context of influenza A infection, we performed a microarray analysis of whole lung homogenates harvested at 24 hours post-infection from B6 mice that were uninfected or infected with mouse-adapted influenza A PR8. In the context of infection, several ADAMTS genes, including ADAMTS4, −6, and −9 exhibited a ≥ 3-fold increase in gene expression compared to uninfected lungs. ADAMTS-4 exhibited the most dramatic increase in gene expression (>100-fold) at this early timepoint. We further investigated the role of ADAMTS4 in survival and recovery from influenza A infection by infecting wild-type (WT) and ADAMTS4 −/− knockout B6 mice with a lethal dose of influenza A PR8. Interestingly, ADAMTS4 −/− mice exhibited significantly improved survival compared to WT littermates (log-rank test, p= 0.04) with 75% vs. 41% survival at 15 days post-infection, despite no observable difference in weight loss. Consistent with a role in tissue repair and recovery, differences in survival were observed after 9 days post-infection. These results raise the possibility that increased expression of ADAMTS4 following influenza A infection modulates the tissue repair process and contributes to lung pathology.

Development of SHIVs with circulating, transmitted HIV-1 variants

SHIV/macaque model is critical for pre-clinical HIV-1 research. The ability of this model to predict efficacious intervention(s) in humans depends on how faithfully the model recapitulates key features of HIV-1 transmission and pathogenesis. Here, we provide insights for rationally designing SHIVs with transmitted HIV-1 variants for vaccine and prevention research.

HIV-1 superinfection occurs less frequently than initial infection in a cohort of high-risk Kenyan women

HIV superinfection (reinfection) has been reported in several settings, but no study has been designed and powered to rigorously compare its incidence to that of initial infection. Determining whether HIV infection reduces the risk of superinfection is critical to understanding whether an immune response to natural HIV infection is protective. This study compares the incidence of initial infection and superinfection in a prospective seroincident cohort of high-risk women in Mombasa, Kenya. A next-generation sequencing-based pipeline was developed to screen 129 women for superinfection. Longitudinal plasma samples at <6 months, >2 years and one intervening time after initial HIV infection were analyzed. Amplicons in three genome regions were sequenced and a median of 901 sequences obtained per gene per timepoint. Phylogenetic evidence of polyphyly, confirmed by pairwise distance analysis, defined superinfection. Superinfection timing was determined by sequencing virus from intervening timepoints. These data were combined with published data from 17 additional women in the same cohort, totaling 146 women screened. Twenty-one cases of superinfection were identified for an estimated incidence rate of 2.61 per 100 person-years (pys). The incidence rate of initial infection among 1910 women in the same cohort was 5.75 per 100 pys. Andersen-Gill proportional hazards models were used to compare incidences, adjusting for covariates known to influence HIV susceptibility in this cohort. Superinfection incidence was significantly lower than initial infection incidence, with a hazard ratio of 0.47 (CI 0.29-0.75, p = 0.0019). This lower incidence of superinfection was only observed >6 months after initial infection. This is the first adequately powered study to report that HIV infection reduces the risk of reinfection, raising the possibility that immune responses to natural infection are partially protective. The observation that superinfection risk changes with time implies a window of protection that coincides with the maturation of HIV-specific immunity.

Inhibition of the potassium current IK(SO), in cerebellar granule cells, by the inhibitors of MEK1 activation, PD 98059 and U 0126

IK(SO) is a standing-outward potassium current found in cerebellar granule neurons which is inhibited by the activation of muscarinic M(3) receptors. However the pathway between muscarinic receptor activation and current inhibition is unknown. Using two structurally distinct inhibitors of the activation of MEK1 (mitogen activated protein (MAP) kinase kinase 1), PD 98059 and U 0126, we have shown that the MAP kinase signalling cascade does not appear to underlie muscarinic inhibition of IK(SO), recorded using whole-cell patch-clamp methods. Nevertheless, both PD 98059 and U 0126 caused an inhibition of IK(SO) when applied acutely with 30 microM of each compound producing around 50% inhibition of the current. In addition, U 0125, which is structurally related to U 0126 but has a much lower potency for inhibiting MEK1 activation, was also able to inhibit IK(SO) to a similar degree. Neither the inhibition by PD 98059 nor that by U 0126 was found to be voltage dependent. This was true whether the IK(SO) current was outward or inward. Block of IK(SO) by these two compounds may compromise interpretation of studies in intact neuronal preparations when they are used as MEK1 inhibitors.

The role of Ca2+ stores in the muscarinic inhibition of the K+ current IK(SO) in neonatal rat cerebellar granule cells

Cerebellar granule neurons (CGNs) possess a standing outward potassium current (IK(SO)) which shares many similarities with current through the two-pore domain potassium channel TASK-1 and which is inhibited following activation of muscarinic acetylcholine receptors. The action of muscarine on IK(SO) was unaffected by the M2 receptor antagonist methoctramine (100 nM) but was blocked by the M3 antagonist zamifenacin, which, at a concentration of 100 nM, shifted the muscarine concentration-response curve to the right by around 50-fold. Surprisingly, M3 receptor activation rarely produced a detectable increase in [Ca2+]i unless preceded by depolarization of the cells with 25 mM K+. Experiments with thapsigargin and ionomycin suggested that the endoplasmic reticulum Ca2+ stores in CGNs were depleted at rest. In contrast, cerebellar glial cells in the same fields of cells possessed substantial endoplasmic reticulum Ca2+ stores at rest. Pretreatment of the cells with BAPTA AM, thapsigargin or the phospholipase C (PLC) inhibitor U-73122 all blocked the muscarine-induced Ca2+ signal but had little or no effect on muscarinic inhibition of IK(SO). Raising [Ca2+]i directly with ionomycin caused a small but significant inhibition of IK(SO). It is concluded that muscarine acts on M3 muscarinic acetylcholine receptors both to inhibit IK(SO) and to mobilize Ca2+ from intracellular stores in CGNs. While the mobilization of Ca2+ occurs through activation of PLC, this does not seem to be the primary mechanism underlying muscarinic inhibition of IK(SO).

Inhibition of delayed rectifier K+ conductance in cultured rat cerebellar granule neurons by activation of calcium-permeable AMPA receptors

Activation of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors in cerebellar granule cells during perforated-patch whole-cell recordings activated an inward current at negative voltages which was followed, after a delay, by the inhibition of an outward potassium current at voltages positive to -20 mV. The activated inward current was inwardly rectifying suggesting that the AMPA receptors were Ca2+-permeable. This was confirmed by direct measurements of intracellular calcium where Ca2+ rises were seen following AMPA receptor activation in Na+-free external solution. Ca2+ rises were equally large in the presence of 100 microM Cd2+ to block voltage-gated Ca2+ channels. Specific voltage-protocols, allowing selective activation of the delayed rectifier potassium current (KV) and the transient A current (KA), showed that kainate inhibited KV, but not to any great extent KA. The inhibition of KV was blocked by the AMPA receptor antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) and was no longer observed when the KV current was abolished with high concentrations of Ba2+. The responses to kainate were not altered by pre-treating the cells with pertussis toxin, suggesting that the AMPA receptor stimulation of the G-protein Gi cannot account for the effects observed. Replacing extracellular Na+ with choline did not alter the inhibition of KV by kainate, however, removing extracellular Ca2+ reduced the kainate response. The inhibition of KV by kainate was unaffected by the presence of 100 microM Cd2+. The guanylyl cyclase inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), did not alter kainate inhibition of KV. It is concluded that ion influx (particularly Ca2+ ions) through AMPA receptor channels following receptor activation leads to an inhibition of KV currents in cerebellar granule neurons.

Enhancement of 5-HT1B and 5-HT1D receptor antagonist effects on extracellular 5-HT levels in the guinea-pig brain following concurrent 5-HT1A or 5-HT re-uptake site blockade

The effects of selective serotonin re-uptake inhibitor (SSRI), paroxetine, and 5-HT1A, 5-HT1B and 5-HT1B/1D receptor antagonists on in vivo extracellular 5-HT levels in the guinea-pig frontal cortex and dorsal hippocampus were investigated using the technique of microdialysis. The aim of the study was to further investigate the autoreceptor roles of the 5-HT1A, 5-HT1B and 5-HT1D receptors in the median vs dorsal raphe nuclei. In the frontal cortex, 5-HT1A (WAY 100635, 1 mg/kg i.p.) or 5-HT1B (SB-224289, 4 mg/kg i.p.) receptor antagonists had no effect on extracellular levels of 5-HT, whilst the mixed 5-HT1B/1D receptor antagonist (GR 127935, 0.3 mg/kg i.p) produced a significant decrease in extracellular 5-HT levels. Paroxetine (10 microM) significantly increased extracellular 5-HT levels when perfused locally into the cortex. Administration of SB-224289, followed 120 min later by WAY 100635, had no effect on extracellular 5-HT levels. In contrast, sequential administration of either WAY 100635 and GR 127935, or SB-224289 and paroxetine significantly increased extracellular 5-HT levels. In the dorsal hippocampus, whilst 5-HT1A receptor antagonism elicited by administration of WAY 100635 had no effect, both 5-HT1B and mixed 5-HT1B/1D receptor blockade significantly increased extracellular 5-HT levels. Administration of SB-224289 followed 120 min later with WAY 100635, or WAY 100635 followed 30 min later with GR 127935, potentiated the effect of the three compounds alone, significantly increasing extracellular 5-HT levels. These data demonstrate that to simultaneously increase extracellular 5-HT in both frontal cortex and dorsal hippocampus of the guinea-pig brain concurrent 5-HTA1A, 5-HT1B and 5-HT1D receptor blockade is required. Whereas in the dorsal hippocampus, 5-HT1B receptor blockade is sufficient to elicit an increase in extracellular 5-HT levels.