The airway epithelium acts as a frontline defense against respiratory viruses, not only as a physical barrier and through the mucociliary apparatus but also through its immunological functions. It initiates multiple innate and adaptive immune mechanisms which are crucial for efficient antiviral responses. The interaction between respiratory viruses and airway epithelial cells results in production of antiviral substances, including type I and III interferons, lactoferrin, β-defensins, and nitric oxide, and also in production of cytokines and chemokines, which recruit inflammatory cells and influence adaptive immunity. These defense mechanisms usually result in rapid virus clearance. However, respiratory viruses elaborate strategies to evade antiviral mechanisms and immune responses. They may disrupt epithelial integrity through cytotoxic effects, increasing paracellular permeability and damaging epithelial repair mechanisms. In addition, they can interfere with immune responses by blocking interferon pathways and by subverting protective inflammatory responses toward detrimental ones. Finally, by inducing overt mucus secretion and mucostasis and by paving the way for bacterial infections, they favor lung damage and further impair host antiviral mechanisms.

https://cmr.asm.org/content/cmr/24/3/631.full.pdf

The Airway Epithelium: Soldier in the Fight against Respiratory Viruses

Influenza-related complications continue to be a major cause of mortality worldwide. Due to unclear mechanisms, a substantial number of influenza-related deaths result from bacterial superinfections, particularly secondary pneumococcal pneumonia. Here, we report what we believe to be a novel mechanism by which influenza-induced type I IFNs sensitize hosts to secondary bacterial infections. Influenza-infected mice deficient for type I IFN-alpha/beta receptor signaling (Ifnar-/- mice) had improved survival and clearance of secondary Streptococcus pneumoniae infection from the lungs and blood, as compared with similarly infected wild-type animals. The less effective response in wild-type mice seemed to be attributable to impaired production of neutrophil chemoattractants KC (also known as Cxcl1) and Mip2 (also known as Cxcl2) following secondary challenge with S. pneumoniae. This resulted in inadequate neutrophil responses during the early phase of host defense against secondary bacterial infection. Indeed, influenza-infected wild-type mice cleared secondary pneumococcal pneumonia after pulmonary administration of exogenous KC and Mip2, whereas neutralization of Cxcr2, the common receptor for KC and Mip2, reversed the protective phenotype observed in Ifnar-/- mice. These data may underscore the importance of the type I IFN inhibitory pathway on CXC chemokine production. Collectively, these findings highlight what we believe to be a novel mechanism by which the antiviral response to influenza sensitizes hosts to secondary bacterial pneumonia.

/pdf/type-i-ifns-mediate-development-of-postinfluenza-bacterial-1vtkpwcsio.pdf

Type I IFNs mediate development of postinfluenza bacterial pneumonia in mice

Infection with pathogenic influenza virus induces severe pulmonary immune pathology, but the specific cell types that cause this have not been determined. We characterized inflammatory cell types in mice that overexpress MCP-1 (CCL2) in the lungs, then examined those cells during influenza infection of wild-type (WT) mice. Lungs of both naive surfactant protein C-MCP mice and influenza-infected WT mice contain increased numbers of CCR2+ monocytes, monocyte-derived DC (moDC), and exudate macrophages (exMACs). Adoptively transferred Gr-1+ monocytes give rise to both moDC and exMACs in influenza-infected lungs. MoDC, the most common inflammatory cell type in infected lungs, induce robust naive T cell proliferation and produce NO synthase 2 (NOS2), whereas exMACs produce high levels of TNF-α and NOS2 and stimulate the proliferation of memory T cells. Relative to WT mice, influenza-infected CCR2-deficient mice display marked reductions in the accumulation of monocyte-derived inflammatory cells, cells producing NOS2, the expression of costimulatory molecules, markers of lung injury, weight loss, and mortality. We conclude that CCR2+ monocyte-derived cells are the predominant cause of immune pathology during influenza infection and that such pathology is markedly abrogated in the absence of CCR2.

CCR2+ monocyte-derived dendritic cells and exudate macrophages produce influenza-induced pulmonary immune pathology and mortality.

Background: Vitamin D deficiency has been shown to be independently associated with increased risk of viral acute respiratory infection (ARI) in a number of observational studies, and meta-analysis of clinical trials of vitamin D supplementation for prevention of ARI has demonstrated protective effects. Several cellular studies have investigated the effects of vitamin D metabolites on immune responses to respiratory viruses, but syntheses of these reports are lacking. Scope: In this article, we review the literature reporting results of in vitro experiments investigating immunomodulatory actions of vitamin D metabolites in human respiratory epithelial cells infected with respiratory viruses. Key findings: Vitamin D metabolites do not consistently influence replication or clearance of rhinovirus, respiratory syncytial virus (RSV) or influenza A virus in human respiratory epithelial cell culture, although they do modulate expression and secretion of type 1 interferon, chemokines including CXCL8 and CXCL10 and pro-inflammatory cytokines, such as TNF and IL-6. Future research: More studies are needed to clarify the effects of vitamin D metabolites on respiratory virus-induced expression of cell surface markers mediating viral entry and bacterial adhesion to respiratory epithelial cells.

/pdf/modulation-of-the-immune-response-to-respiratory-viruses-by-1uob7dn5md.pdf

Modulation of the immune response to respiratory viruses by vitamin D.

Acute lung injury due to influenza infection is associated with high mortality, an increase in neutrophils in the airspace, and increases in tissue myeloperoxidase (MPO). Because IL-17A and IL-17F, ligands for IL-17 receptor antagonist (IL-17RA), have been shown to mediate neutrophil migration into the lung in response to LPS or Gram-negative bacterial pneumonia, we hypothesized that IL-17RA signaling was critical for acute lung injury in response to pulmonary influenza infection. IL-17RA was critical for weight loss and both neutrophil migration and increases in tissue myeloperoxidase (MPO) after influenza infection. However, IL-17RA was dispensable for the recruitment of CD8+ T cells specific for influenza hemagglutinin or nucleocapsid protein. Consistent with this, IL-17RA was not required for viral clearance. However, in the setting of influenza infection, IL-17RA−/− mice showed significantly reduced levels of oxidized phospholipids, which have previously been shown to be an important mediator in several models of acute lung injury, including influenza infection and gastric acid aspiration. Taken together, these data support targeting IL-17 or IL-17RA in acute lung injury due to acute viral infection.

Critical Role of IL-17RA in Immunopathology of Influenza Infection

Neutrophils traffic to the lungs in large numbers during influenza virus infection. Although the ability of these cells to respond to numerous chemotactic stimuli has been described in other systems, the chemokine receptors mediating recruitment of neutrophils to the lungs during influenza virus infection and the role of this cell type in viral clearance are currently undefined. In the present study, we used CXCR2(/) mice to investigate the role of the chemokine receptor CXCR2 in neutrophil recruitment to the lungs during influenza virus infection and to determine the role of neutrophils in viral clearance. We infected CXCR2(/) or wild-type mice with influenza and assessed the level of inflammation, the cellular composition of the inflammatory infiltrate, and viral titers in the lungs. Absence of CXCR2 ablated neutrophil recruitment to the lungs, but had no effect on peak viral titers or on the kinetics of viral clearance. Thus, it appears that CXCR2 is the major receptor mediating neutrophil trafficking to the lung during influenza virus infection, but that neutrophils do not play an essential role in viral clearance.

CXCR2 Is Required for Neutrophil Recruitment to the Lung during Influenza Virus Infection, But Is Not Essential for Viral Clearance

Influenza virus infections induce chemokines and cytokines, which regulate the immune response. The chemokine receptor CCR2 plays an important role in macrophage recruitment and in the development of T1 immunity. In the present study, we addressed the role of CCR2 in influenza A virus infection. CCR2 knockout (-/-) mice are protected against influenza A virus infection, despite delayed recruitment of macrophages. We show that low-dose influenza infection of CCR2-/- mice leads to increased neutrophilia between Days 5 and 10 after infection and decreased monocyte/macrophage and CD4(+) T cell recruitment to the lungs between Days 5 and 7 after infection. These changes in leukocyte recruitment did not result from or cause increased viral titers or delayed viral clearance. Neutrophilia in the lungs correlated with increased keratinocyte-derived chemokine (KC) and/or MIP-2 expression in CCR2-/- mice between Days 5 to 10 after infection, although the kinetics of neutrophil recruitment was not altered. MIP-2 mRNA and protein expression was increased three- to fivefold, and KC protein levels were increased two- to threefold in CCR2-/- compared with CCR2 wild-type mice at Day 5 after infection. This preceded the peak neutrophil influx, which occurred 7 days after infection. In vitro studies confirmed that MIP-2 and KC accounted for neutrophil chemotactic activity in the bronchoalveolar lavage. CCR2 deficiency also resulted in increased MIP-1alpha, MIP-1beta, MCP-1, and IFN-inducible protein 10 and decreased RANTES mRNA expression. Furthermore, IL-6 and TNF-alpha cytokine production were elevated after infection. These studies suggest that CCR2 plays a multifactorial role in the development of the immune response to influenza.

https://jlb.onlinelibrary.wiley.com/doi/pdf/10.1189/jlb.0506299

Chemokine regulation of the inflammatory response to a low-dose influenza infection in CCR2–/– mice

CD4 T cells are not essential for primary clearance of replicating murine gammaherpesvirus 68 (MHV-68) but are required for effective long-term control. The virus reactivates in the lungs of major histocompatibility complex class II-deficient (CII−/−) mice that lack functional CD4 T cells. CD40 ligand (CD40L) is upregulated on activated CD4 T cells, and it is thought that CD40-CD40L interactions are an important component of CD4 T-cell help. Our previous studies have shown that agonistic antibodies to CD40 can substitute for CD4 T-cell function in the long-term control of MHV-68. In the present study, we sought to identify the CD40-positive cell type mediating this effect. To address this question, we adoptively transferred MHV-68 peptide-pulsed CII −/− dendritic cells (DC) that had been treated with an agonistic antibody to CD40 into MHV-68-infected CII −/− recipients. Viral reactivation was significantly lower in mice injected with anti-CD40-treated DC than in those injected with control DC or in mice that did not receive any DC. However, in similar experiments with B cells, anti-CD40 treatment had no effect. We also investigated the requirement for CD40 expression on T cells by adoptive transfer of T cells from CD40 +/+ or CD40 −/− mice into T-cell-deficient recipients that were subsequently infected with MHV-68. The results showed that CD40 expression on T cells is not necessary for preventing viral reactivation. Taken together, our data suggest that CD40 engagement on DC, but not on T or B cells, is essential for effective long-term control of MHV-68.

CD40 Engagement on Dendritic Cells, but Not on B or T Cells, Is Required for Long-Term Control of Murine Gammaherpesvirus 68

CD4 T cells are dispensable for acute control of murine gammaherpesvirus 68 (MHV-68) but are necessary for effective long-term control of the virus by CD8 T cells. In contrast, protein kinase C theta (PKCtheta) is not essential for either acute or long-term viral control. However, we found that while either CD4 or CD8 T cells could mediate the clearance of MHV-68 from the lungs of PKCtheta(+/+) mice, PKCtheta(-/-) mice depleted of either subset failed to clear the virus. These data suggest that there are two alternative pathways for MHV-68 clearance, one dependent on CD4 T cells and the other on PKCtheta. Protection mediated by the latter appears to be short-lived. These observations may help to explain the differential requirement for PKCtheta in various models of CD8 T-cell activation and differences in the costimulatory requirements for acute and long-term viral control.

Chandra Inglis

Papers

CXCR2 Is Required for Neutrophil Recruitment to the Lung during Influenza Virus Infection, But Is Not Essential for Viral Clearance

Chemokine regulation of the inflammatory response to a low-dose influenza infection in CCR2–/– mice

CD40 Engagement on Dendritic Cells, but Not on B or T Cells, Is Required for Long-Term Control of Murine Gammaherpesvirus 68

Primary Clearance of Murine Gammaherpesvirus 68 by PKCθ−/− CD8 T Cells Is Compromised in the Absence of Help from CD4 T Cells