Keer Sun

Inhibition of pulmonary antibacterial defense by interferon-γ during recovery from influenza infection

Secondary bacterial infection often occurs after pulmonary virus infection and is a common cause of severe disease in humans, yet the mechanisms responsible for this viral-bacterial synergy in the lung are only poorly understood. We now report that pulmonary interferon-γ (IFN-γ) produced during T cell responses to influenza infection in mice inhibits initial bacterial clearance from the lung by alveolar macrophages. This suppression of phagocytosis correlates with lung IFN-γ abundance, but not viral burden, and leads to enhanced susceptibility to secondary pneumococcal infection, which can be prevented by IFN-γ neutralization after influenza infection. Direct inoculation of IFN-γ can mimic influenza infection and downregulate the expression of the class A scavenger receptor MARCO on alveolar macrophages. Thus, IFN-γ, although probably facilitating induction of specific anti-influenza adaptive immunity, suppresses innate protection against extracellular bacterial pathogens in the lung.

Interleukin-12 Promotes Gamma Interferon-Dependent Neutrophil Recruitment in the Lung and Improves Protection against Respiratory Streptococcus pneumoniae Infection

ABSTRACT The ability of exogenous interleukin-12 (IL-12) to elicit protective innate immune responses against the extracellular pathogen Streptococcus pneumoniae was tested by infecting BALB/c mice intranasally (i.n.) with S. pneumoniae after i.n. administration of IL-12. It was found that administration of IL-12 resulted in lower bacterial burdens in the infected mice and significantly improved survival rates. All IL-12-treated mice contained higher levels of pulmonary gamma interferon (IFN-γ) after infection and significantly more neutrophils than infected mice not treated with IL-12. IFN-γ was found to be essential for IL-12-induced resistance and for neutrophil influx into the lungs, and the observed changes correlated with increased levels of the IL-8 homologue keratinocyte-derived chemokine (KC). In addition, in vitro tumor necrosis factor alpha (TNF-α) production by alveolar macrophages stimulated with heat-killed pneumococci was enhanced by IFN-γ, and TNF-α in turn could enhance production of KC by lung cells. Finally, IL-12-induced protection was dependent upon the presence of neutrophils and the KC receptor CXCR2. Taken together, the results indicate that exogenous IL-12 can improve innate defense in the lung against S. pneumoniae by inducing IFN-γ production, which in turn enhances chemokine expression, and promotes pulmonary neutrophil recruitment into the infected lung. The findings show that IL-12 and IFN-γ can mediate a protective effect against respiratory infection caused by extracellular bacterial pathogens.

Immune Dysfunction and Bacterial Coinfections following Influenza

Secondary pulmonary infections by encapsulated bacteria including Streptococcus pneumoniae and Staphylococcus aureus following influenza represent a common and challenging clinical problem. The reasons for this polymicrobial synergy are still not completely understood, hampering development of effective prophylactic and therapeutic interventions. Although it has been commonly thought that viral-induced epithelial cell damage allows bacterial invasiveness, recent studies by several groups have now implicated dysfunctional innate immune defenses following influenza as the primary culprit for enhanced susceptibility to secondary bacterial infections. Understanding the immunological imbalances that are responsible for virus/bacteria synergy will ultimately allow the design of effective, broad-spectrum therapeutic approaches for prevention of enhanced susceptibility to these pathogens.

An Important Role for Polymeric Ig Receptor-Mediated Transport of IgA in Protection against Streptococcus pneumoniae Nasopharyngeal Carriage

The importance of IgA for protection at mucosal surfaces remains unclear, and in fact, it has been reported that IgA-deficient mice have fully functional vaccine-induced immunity against several bacterial and viral pathogens. The role of respiratory Ab in preventing colonization by Streptococcus pneumoniae has now been examined using polymeric IgR knockout (pIgR−/−) mice, which lack the ability to actively secrete IgA into the mucosal lumen. Intranasal vaccination with a protein conjugate vaccine elicited serotype-specific anti-capsular polysaccharide Ab locally and systemically, and pIgR−/− mice produced levels of total serum Ab after vaccination that were similar to wild-type mice. However, pIgR−/− mice had ∼5-fold more systemic IgA and 6-fold less nasal IgA Ab than wild-type mice due to defective transport into mucosal tissues. Wild-type, but not pIgR−/− mice were protected against infection with serotype 14 S. pneumoniae, which causes mucosal colonization but does not induce systemic inflammatory responses in mice. The relative importance of secretory IgA in host defense was further shown by the finding that intranasally vaccinated IgA gene-deficient mice were not protected from colonization. Although secretory IgA was found to be important for protection against nasal carriage, it does not appear to have a crucial role in immunity to systemic pneumococcus infection, because both vaccinated wild-type and pIgR−/− mice were fully protected from lethal systemic infection by serotype 3 pneumococci. The results demonstrate the critical role of secretory IgA in protection against pneumococcal nasal colonization and suggest that directed targeting to mucosal tissues will be needed for effective vaccination in humans.

Seasonal FluMist Vaccination Induces Cross-Reactive T Cell Immunity against H1N1 (2009) Influenza and Secondary Bacterial Infections

T cell epitopes have been found to be shared by circulating, seasonal influenza virus strains and the novel pandemic H1N1 influenza infection, but the ability of these common epitopes to provide cross-protection is unknown. We have now directly tested this by examining the ability of live seasonal influenza vaccine (FluMist) to mediate protection against swine-origin H1N1 influenza virus infection. Naive mice demonstrated considerable susceptibility to H1N1 Cal/04/09 infection, whereas FluMist-vaccinated mice had markedly decreased morbidity and mortality. In vivo depletion of CD4+ or CD8+ immune cells after vaccination indicated that protective immunity was primarily dependent upon FluMist-induced CD4+ cells but not CD8+ T cells. Passive protection studies revealed little role for serum or mucosal Abs in cross-protection. Although H1N1 influenza infection of naive mice induced intensive phagocyte recruitment, pulmonary innate defense against secondary pneumococcal infection was severely suppressed. This increased susceptibility to bacterial infection was correlated with augmented IFN-γ production produced during the recovery stage of H1N1 influenza infection, which was completely suppressed in mice previously immunized with FluMist. Furthermore, susceptibility to secondary bacterial infection was decreased in the absence of type II, but not type I, IFN signaling. Thus, seasonal FluMist treatment not only promoted resistance to pandemic H1N1 influenza infection but also restored innate immunity against complicating secondary bacterial infections.

A Detrimental Effect of Interleukin-10 on Protective Pulmonary Humoral Immunity during Primary Influenza A Virus Infection

ABSTRACT Interleukin-10 (IL-10) is an important anti-inflammatory molecule that can cause immunosuppression and long-term pathogen persistence during chronic infection of mice with viruses such as lymphocytic choriomeningitis virus. However, its specific role in immunity to acute viral infections is not fully understood. We found that IL-10 plays a detrimental role in host responses to acute influenza A virus since IL-10−/− mice had improved viral clearance and survival after infection compared to wild-type mice. Enhanced viral clearance in IL-10−/− mice was not correlated with increased CD4+ or CD8+ T-cell recruitment into the lung but was correlated with increased pulmonary anti-influenza virus antibody titers, and this was dependent upon the presence of T cells, primarily CD4+ T cells. In addition, virus-specific antibody produced during the early stages of infection in the respiratory tract of IL-10−/− but not wild-type mice was sufficient to mediate passive protection against viral challenge of naïve mice. Complement was necessary for this antibody-mediated passive protection, but FcγR or neutrophil deficiency did not significantly influence viral clearance. Our results show that an absence of IL-10 at the time of primary infection leads to enhanced local virus-specific antibody production and, thus, increased protection against influenza A virus infection.

Influenza Infection Suppresses NADPH Oxidase–Dependent Phagocytic Bacterial Clearance and Enhances Susceptibility to Secondary Methicillin-Resistant Staphylococcus aureus Infection

Methicillin-resistant Staphylococcus aureus (MRSA) has emerged as a leading contributor to mortality during recent influenza pandemics. The mechanism for this influenza-induced susceptibility to secondary S. aureus infection is poorly understood. In this study, we show that innate antibacterial immunity was significantly suppressed during the recovery stage of influenza infection, even though MRSA superinfection had no significant effect on viral burdens. Compared with mice infected with bacteria alone, postinfluenza MRSA–infected mice exhibited impaired bacterial clearance, which was not due to defective phagocyte recruitment, but rather coincided with reduced intracellular reactive oxygen species levels in alveolar macrophages and neutrophils. NADPH oxidase is responsible for reactive oxygen species production during phagocytic bacterial killing, a process also known as oxidative burst. We found that gp91phox-containing NADPH oxidase activity in macrophages and neutrophils was essential for optimal bacterial clearance during respiratory MRSA infections. In contrast to wild-type animals, gp91phox−/− mice exhibited similar defects in MRSA clearance before and after influenza infection. Using gp91phox+/− mosaic mice, we further demonstrate that influenza infection inhibits a cell-intrinsic contribution of NADPH oxidase to phagocyte bactericidal activity. Taken together, our results establish that influenza infection suppresses NADPH oxidase–dependent bacterial clearance and leads to susceptibility to secondary MRSA infection.

Analysis of Murine Genetic Predisposition to Pneumococcal Infection Reveals a Critical Role of Alveolar Macrophages in Maintaining the Sterility of the Lower Respiratory Tract

ABSTRACT The study of pathogenic mechanisms of disease can be greatly facilitated by studying genetic differences in susceptibility to infection. In the present study, we compared the severity of pneumococcal infection in C57BL/6 (B6) and 129Sv mice. The results showed that 129Sv mice were remarkably more susceptible to pneumococcal infection than B6 mice. Bacterial clearance, proinflammatory mediators, leukocyte recruitment, and phagocyte activities were measured to examine potential immune factors associated with differences in susceptibility to pneumococcal infection. The greater susceptibility of 129Sv mice was associated only with inadequate alveolar macrophage bacterial killing, as indicated by significantly decreased initial bacterial clearance from the respiratory tract. Effective pneumococcal clearance was not dependent upon Toll-like receptor 2 (TLR2) expression, oxidative stress, or matrix metallopeptidase 12 (MMP-12) expression. Furthermore, phagocytosis analysis suggested that the deficiency found in 129Sv alveolar macrophages was not due to a lack of bacterial recognition but, rather, to reduced bacterial uptake. In conclusion, our findings indicate a crucial role of alveolar macrophage phagocytosis during innate defense against pneumococcal infection, which may explain the association of host genetic risk factors with predisposition to pneumococcal infection.

Limited Efficacy of Antibacterial Vaccination Against Secondary Serotype 3 Pneumococcal Pneumonia Following Influenza Infection

BACKGROUND Secondary bacterial infections following influenza represent a major cause of mortality in the human population, which, in turn, has led to a call for stockpiling of bacterial vaccines for pandemic preparedness. METHODS To investigate the efficacy of bacterial vaccination for protection against secondary pneumococcal infection, mice were immunized with pneumococcal capsular polysaccharide conjugate vaccine, and then sequentially coinfected 5 weeks later with PR8 influenza virus and A66.1 Streptococcus pneumoniae. RESULTS In the absence of influenza virus exposure, vaccination with polysaccharide conjugate vaccine was highly effective, as indicated by 100% survival from lethal pneumococcal pneumonia and 10 000-fold greater efficiency in clearance of bacteria from the lung compared to unvaccinated mice. Enhanced clearance after vaccination was dependent upon Fc receptor (FcR) expression. However, following influenza, <40% of vaccinated mice survived bacterial coinfection and FcR-dependent clearance of antibody-opsonized bacteria reduced bacterial levels in the lungs only 5-10 fold. No differences in lung myeloid cell numbers or in FcR cell surface expression were observed following influenza. CONCLUSIONS The results show that induction of antibacterial humoral immunity is only partially effective in protection against secondary bacterial infections that occur following influenza, and suggest that additional therapeutic strategies to overcome defective antibacterial immunity should be explored.

Expression of Suppressor of Cytokine Signaling 1 (SOCS1) Impairs Viral Clearance and Exacerbates Lung Injury during Influenza Infection

Suppressor of cytokine signaling (SOCS) proteins are inducible feedback inhibitors of cytokine signaling. SOCS1−/− mice die within three weeks postnatally due to IFN-γ-induced hyperinflammation. Since it is well established that IFN-γ is dispensable for protection against influenza infection, we generated SOCS1−/−IFN-γ−/− mice to determine whether SOCS1 regulates antiviral immunity in vivo. Here we show that SOCS1−/−IFN-γ−/− mice exhibited significantly enhanced resistance to influenza infection, as evidenced by improved viral clearance, attenuated acute lung damage, and consequently increased survival rates compared to either IFN-γ−/− or WT animals. Enhanced viral clearance in SOCS1−/−IFN-γ−/− mice coincided with a rapid onset of adaptive immune responses during acute infection, while their reduced lung injury was associated with decreased inflammatory cell infiltration at the resolution phase of infection. We further determined the contribution of SOCS1-deficient T cells to antiviral immunity. Anti-CD4 antibody treatment of SOCS1−/−IFN-γ−/− mice had no significant effect on their enhanced resistance to influenza infection, while CD8+ splenocytes from SOCS1−/−IFN-γ−/− mice were sufficient to rescue RAG1−/− animals from an otherwise lethal infection. Surprisingly, despite their markedly reduced viral burdens, RAG1−/− mice reconstituted with SOCS1−/−IFN-γ−/− adaptive immune cells failed to ameliorate influenza-induced lung injury. In conclusion, in the absence of IFN-γ, the cytoplasmic protein SOCS1 not only inhibits adaptive antiviral immune responses but also exacerbates inflammatory lung damage. Importantly, these detrimental effects of SOCS1 are conveyed through discrete cell populations. Specifically, while SOCS1 expression in adaptive immune cells is sufficient to inhibit antiviral immunity, SOCS1 in innate/stromal cells is responsible for aggravated lung injury.