Type I interferons (IFN-I) are critical for antiviral immunity; however, chronic IFN-I signaling is associated with hyperimmune activation and disease progression in persistent infections. We demonstrated in mice that blockade of IFN-I signaling diminished chronic immune activation and immune suppression, restored lymphoid tissue architecture, and increased immune parameters associated with control of virus replication, ultimately facilitating clearance of the persistent infection. The accelerated control of persistent infection induced by blocking IFN-I signaling required CD4 T cells and was associated with enhanced IFN-γ production. Thus, we demonstrated that interfering with chronic IFN-I signaling during persistent infection redirects the immune environment to enable control of infection.

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-α/β 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.

Influenza and other respiratory viral infections are the most common type of acute respiratory infection. Viral infections predispose patients to secondary bacterial infections, which often have a more severe clinical course. The mechanisms underlying post-viral bacterial infections are complex, and include multifactorial processes mediated by interactions between viruses, bacteria, and the host immune system. Studies over the past 15 years have demonstrated that unique microbial communities reside on the mucosal surfaces of the gastrointestinal tract and the respiratory tract, which have both direct and indirect effects on host defense against viral infections. In addition, antiviral immune responses induced by acute respiratory infections such as influenza are associated with changes in microbial composition and function (“dysbiosis”) in the respiratory and gastrointestinal tract, which in turn may alter subsequent immune function against secondary bacterial infection or alter the dynamics of inter-microbial interactions, thereby enhancing the proliferation of potentially pathogenic bacterial species. In this review, we summarize the literature on the interactions between host microbial communities and host defense, and how influenza, and other acute respiratory viral infections disrupt these interactions, thereby contributing to the pathogenesis of secondary bacterial infections.

Sepsis results in a state of relative immunosuppression, rendering critically ill patients susceptible to secondary infections and increased mortality. Monocytes isolated from septic patients and experimental animals display a "deactivated" phenotype, characterized by impaired inflammatory and antimicrobial responses, including hyporesponsiveness to LPS. We investigated the role of the LPS/TLR4 axis and its inhibitor, IL-1 receptor-associated kinase-M (IRAK-M), in modulating the immunosuppression of sepsis using a murine model of peritonitis-induced sepsis followed by secondary challenge by intratracheal Pseudomonas aeruginosa. Septic mice demonstrated impaired alveolar macrophage function and increased mortality when challenged with intratracheal Pseudomonas as compared with nonseptic controls. TLR2 and TLR4 expression was unchanged in the lung following sepsis, whereas levels of IRAK-M were upregulated. Macrophages from IRAK-M-deficient septic mice produced higher levels of proinflammatory cytokines ex vivo and greater costimulatory molecule expression in vivo as compared with those of their WT counterparts. Following sepsis and secondary intrapulmonary bacterial challenge, IRAK-M -/-animals had higher survival rates and improved bacterial clearance from lung and blood compared with WT mice. In addition, increased pulmonary chemokine and inflammatory cytokine production was observed in IRAK-M -/-animals, leading to enhanced neutrophil recruitment to airspaces. Collectively, these findings indicate that IRAK-M mediates critical aspects of innate immunity that result in an immunocompromised state during sepsis.

The contribution of neutrophils to lethal sensitivity and cytokine balance governing T1 and T2 host responses was assessed in a murine model of Legionella pneumophila pneumonia. Neutrophil depletion by administration of granulocyte-specific mAb RB6-8C5 at 1 day before infection rendered mice ∼100-fold more susceptible to lethal pneumonia induced by L. pneumophila. However, this treatment did not alter early bacterial clearance, despite a substantial decrease in neutrophil influx at this time point. Cytokine profiles in the lungs of control mice demonstrated strong T1 responses, characterized by an increase of IFN-γ and IL-12. In contrast, neutrophil-depleted mice exhibited significantly lower levels of IFN-γ and IL-12, and elevation of T2 cytokines, IL-4 and IL-10. Immunohistochemistry of bronchoalveolar lavage cells demonstrated the presence of IL-12 in neutrophils, but not alveolar macrophages. Moreover, IL-12 was detected in lavage cell lysates by ELISA, which was paralleled to neutrophil number. However, intratracheal administration of recombinant murine IL-12 did not restore resistance, whereas reconstitution of IFN-γ drastically improved bacterial clearance and survival in neutrophil-depleted mice. Taken together, these data demonstrated that neutrophils play crucial roles in primary L. pneumophila infection, not via direct killing but more immunomodulatory effects. Our results suggest that the early recruitment of neutrophils may contribute to T1 polarization in a murine model of L. pneumophila pneumonia.

Neutrophils clear viruses, but excessive neutrophil responses induce tissue injury and worsen disease. Aging increases mortality to influenza infection; however, whether this is due to impaired viral clearance or a pathological host immune response is unknown. Here, we show that aged mice have higher levels of lung neutrophils than younger mice after influenza viral infection. Depleting neutrophils after, but not before, infection substantially improves the survival of aged mice without altering viral clearance. Aged alveolar epithelial cells (AECs) have a higher frequency of senescence and secrete higher levels of the neutrophil-attracting chemokines CXCL1 and CXCL2 during influenza infection. These chemokines are required for age-enhanced neutrophil chemotaxis in vitro . Our work suggests that aging increases mortality from influenza in part because senescent AECs secrete more chemokines, leading to excessive neutrophil recruitment. Therapies that mitigate this pathological immune response in the elderly might improve outcomes of influenza and other respiratory infections.

The roles of CXC chemokine-mediated host responses were examined with an A/J mouse model of Legionella pneumophila pneumonia. After intratracheal inoculation of 10 6 CFU of L. pneumophila, the bacterial numbers in the lungs increased 10-fold by day 2; this increase was accompanied by the massive accumulation of neutrophils. Reverse transcription-PCR data demonstrated the up-regulation of CXC chemokines, such as keratinocyte-derived chemokine, macrophage inflammatory protein 2 (MIP-2), and lipopolysaccharide-induced CXC chemokine (LIX). Consistent with these data, increased levels of KC, MIP-2, and LIX proteins were observed in the lungs and peaked at days 1, 2, and 2, respectively. Although the administration of anti-KC or anti-MIP-2 antibody resulted in an approximately 20% decrease in neutrophil recruitment on day 2, no increase in mortality was observed. In contrast, the blockade of CXC chemokine receptor 2 (CXCR2), a receptor for CXC chemokines, including KC and MIP-2, strikingly enhanced mortality; this effect coincided with a 67% decrease in neutrophil recruitment. Interestingly, anti-CXCR2 antibody did not affect bacterial burden by day 2, even in the presence of a lethal challenge of bacteria. Moreover, a significant decrease in interleukin-12 (IL-12) levels, in contrast to the increases in KC, MIP-2, and LIX levels, was demonstrated for CXCR2-blocked mice. These data indicated that CXCR2-mediated neutrophil accumulation may play a crucial role in host defense against L. pneumophila pneumonia in mice. The increase in lethality without a change in early bacterial clearance suggested that neutrophils may exert their protective effect not through direct killing but through more immunomodulatory actions in L. pneumophila pneumonia. We speculate that a decrease in the levels of the protective cytokine IL-12 may explain, at least in part, the high mortality in the setting of reduced neutrophil recruitment.

Macrolides exert their effects on the host by modulation of immune responses. In this study, we assessed the therapeutic efficacy of azithromycin in a murine model of mucoid Pseudomonas aeruginosa endobronchial infection. The clearance of Pseudomonas from the airway of mice treated with the macrolide azithromycin was not different than untreated mice challenged with Pseudomonas beads. However, the azithromycin-treated mice showed a remarkable reduction in lung cellular infiltrate in response to Pseudomonas beads, as compared with untreated mice. This effect was associated with significant decreases in lung levels of tumor necrosis factor-alpha and keratinocyte-derived chemokine in azithromycin-treated mice compared with untreated mice. Furthermore, there was a significant reduction in the response of both mouse and human neutrophils to chemokine-dependent and -independent chemoattractants when studied in vitro. Inhibition of chemotaxis correlated with azithromycin-mediated inhibition of extracellular signal-regulated kinase-1 and -2 activation. This study indicates that the azithromycin treatment in vivo results in significant reduction in airway-specific inflammation, which occurs in part by inhibition of neutrophil recruitment to the lung through reduction in proinflammatory cytokine expression and inhibition of neutrophil migration via the extracellular signal-regulated kinase-1 and -2 signal transduction pathway.