Contrasting effects of cyclooxygenase-1 (COX-1) and COX-2 deficiency on the host response to influenza A viral infection

Influenza is a significant cause of morbidity and mortality worldwide despite extensive research and vaccine availability. The cyclooxygenase (COX) pathway is important in modulating immune responses and is also a major target of nonsteroidal anti-inflammatory drugs (NSAIDs) and the newer COX-2 inhibitors. The purpose of the present study was to examine the effect of deficiency of COX-1 or COX-2 on the host response to influenza. We used an influenza A viral infection model in wild type (WT), COX-1-/-, and COX-2-/- mice. Infection induced less severe illness in COX-2-/- mice in comparison to WT and COX-1-/- mice as evidenced by body weight and body temperature changes. Mortality was significantly reduced in COX-2-/- mice. COX-1-/- mice had enhanced inflammation and earlier appearance of proinflammatory cytokines in the BAL fluid, whereas the inflammatory and cytokine responses were blunted in COX-2-/- mice. However, lung viral titers were markedly elevated in COX-2-/- mice relative to WT and COX-1-/- mice on day 4 of infection. Levels of PGE2 were reduced in COX-1-/- airways whereas cysteinyl leukotrienes were elevated in COX-2-/- airways following infection. Thus, deficiency of COX-1 and COX-2 leads to contrasting effects in the host response to influenza infection, and these differences are associated with altered production of prostaglandins and leukotrienes following infection. COX-1 deficiency is detrimental whereas COX-2 deficiency is beneficial to the host during influenza viral infection.     

Inhibition of Nox2 oxidase activity ameliorates influenza A virus-induced lung inflammation

Influenza A virus pandemics and emerging anti-viral resistance highlight the urgent need for novel generic pharmacological strategies that reduce both viral replication and lung inflammation. We investigated whether the primary enzymatic source of inflammatory cell ROS (reactive oxygen species), Nox2-containing NADPH oxidase, is a novel pharmacological target against the lung inflammation caused by influenza A viruses. Male WT (C57BL/6) and Nox2(-/y) mice were infected intranasally with low pathogenicity (X-31, H3N2) or higher pathogenicity (PR8, H1N1) influenza A virus. Viral titer, airways inflammation, superoxide and peroxynitrite production, lung histopathology, pro-inflammatory (MCP-1) and antiviral (IL-1β) cytokines/chemokines, CD8(+) T cell effector function and alveolar epithelial cell apoptosis were assessed. Infection of Nox2(-/y) mice with X-31 virus resulted in a significant reduction in viral titers, BALF macrophages, peri-bronchial inflammation, BALF inflammatory cell superoxide and lung tissue peroxynitrite production, MCP-1 levels and alveolar epithelial cell apoptosis when compared to WT control mice. Lung levels of IL-1β were ～3-fold higher in Nox2(-/y) mice. The numbers of influenza-specific CD8+D(b)NP(366)+ and D(b)PA(224)+ T cells in the BALF and spleen were comparable in WT and Nox2(-/y) mice. In vivo administration of the Nox2 inhibitor apocynin significantly suppressed viral titer, airways inflammation and inflammatory cell superoxide production following infection with X-31 or PR8. In conclusion, these findings indicate that Nox2 inhibitors have therapeutic potential for control of lung inflammation and damage in an influenza strain-independent manner.     

Multiple T and B cell epitopes in the S1 subunit ("A"-monomer) of the pertussis toxin molecule

The immunogenicity and reactogenicity of Bordetella pertussis vaccine are mediated in part by the S1 subunit of pertussis toxin (PT). To identify the immune epitopes in the S1 subunit of PT, synthetic peptides were prepared and tested for their capacity to induce antibodies in mice with different MHC genotypes. In BALB/c mice, peptides corresponding to sequences 1-17, 70-82 and 189-199 generate T cell proliferative responses, induce the production of antibodies capable of neutralization of the toxin in the Chinese hamster ovary-cell assay, and protect mice from a shock-like syndrome caused by alternate injections of BSA and PT. Protection and neutralization correlated with the ability of these peptides to elicit high anti-PT titers. Different B cell epitopes were detected in other inbred mouse strains. The antibody reactivity against synthetic peptides from two infants vaccinated with pertussis vaccine was tested. These infants had antibodies reactive to a variety of epitopes in the S1 subunit, including peptides 1-17, 70-82, 99-112, 135-145, and 189-199. Thus, it appears that there are multiple T and B cell epitopes in the S1 subunit of PT.     

Multifunctional CD4 cells expressing gamma interferon and perforin mediate protection against lethal influenza virus infection

CD4 effectors generated in vitro can promote survival against a highly pathogenic influenza virus via an antibody-independent mechanism involving class II-restricted, perforin-mediated cytotoxicity. However, it is not known whether CD4 cells activated during influenza virus infection can acquire cytolytic activity that contributes to protection against lethal challenge. CD4 cells isolated from the lungs of infected mice were able to confer protection against a lethal dose of H1N1 influenza virus A/Puerto Rico 8/34 (PR8). Infection of BALB/c mice with PR8 induced a multifunctional CD4 population with proliferative capacity and ability to secrete interleukin-2 (IL-2) and tumor necrosis factor alpha (TNF-α) in the draining lymph node (DLN) and gamma interferon (IFN-γ) and IL-10 in the lung. IFN-γ-deficient CD4 cells produced larger amounts of IL-17 and similar levels of TNF-α, IL-10, and IL-2 compared to wild-type (WT) CD4 cells. Both WT and IFN-γ(-/-) CD4 cells exhibit influenza virus-specific cytotoxicity; however, IFN-γ-deficient CD4 cells did not promote recovery after lethal infection as effectively as WT CD4 cells. PR8 infection induced a population of cytolytic CD4 effectors that resided in the lung but not the DLN. These cells expressed granzyme B (GrB) and required perforin to lyse peptide-pulsed targets. Lethally infected mice given influenza virus-specific CD4 cells deficient in perforin showed greater weight loss and a slower time to recovery than mice given WT influenza virus-specific CD4 cells. Taken together, these data strengthen the concept that CD4 T cell effectors are broadly multifunctional with direct roles in promoting protection against lethal influenza virus infection.     

Heat-killed Lactobacillus gasseri TMC0356 protects mice against influenza virus infection by stimulating gut and respiratory immune responses

This study investigated whether heat-killed Lactobacillus protects host animal against influenza virus infection and stimulates their immunity. Heat-killed Lactobacillus gasseri TMC0356 was orally administered to BALB/c mice for 19 days; the mice were intranasally infected with Flu A/PR/8/34 (H1N1) on day 14, and clinical symptoms were monitored. After 6 days, the mice were sacrificed, and pulmonary virus titres were determined. Splenic activation of natural killer (NK) cells and the mRNA expression of cytokines and other immune molecules in the lung and Peyer's patch (PP) were analysed. Clinical symptom scores of mice orally fed TMC0356 ameliorated significantly (P < 0.01); their pulmonary virus titres decreased significantly compared with those of control mice (P < 0.05); their mRNA expression of interleukin (IL)-12, IL-15 and IL-21 in PP and the pulmonary mRNA expression of IFN-γ, TNF, IL-12a, IL-12rbl, IL-2rb and perforin 1 increased significantly (P < 0.05). Oral administration of heat-killed lactobacilli may protect against influenza virus infection by stimulating local and systemic immune responses. Cellular components of lactobacilli may be pivotal in protecting against viral infection by enhancing gut and respiratory immune responses.     

alpha-Galactosylceramide can act as a nasal vaccine adjuvant inducing protective immune responses against viral infection and tumor

alpha-Galactosylceramide (alpha-GalCer) is a ligand of invariant Valpha14+ NKT cells and is presented by CD1d molecule on APC. NKT cells produce a large amount of Th1 and Th2 cytokines in response to alpha-GalCer-presented APC. In this study, we assessed whether alpha-GalCer could act as an effective nasal vaccine adjuvant for mucosal vaccine that would be capable of inducing systemic as well as mucosal immune responses. When alpha-GalCer was administered with OVA via the intranasal route to C57BL/6 and BALB/c mice, significant OVA-specific mucosal secretory IgA, systemic IgG, and CTL responses were induced with mixed Th1 and Th2 cytokine profiles seen in both strains of mice. Interestingly, as BALB/c mice were intranasally immunized with PR8 hemagglutinin Ag isolated from influenza virus A/PR/8/34 together with alpha-GalCer, significant protection was afforded against influenza viral infection. When alpha-GalCer was coimmunized with a replication-deficient live adenovirus to BALB/c mice, it significantly induced both humoral and cellular immune responses. In addition, intranasal administration of OVA with alpha-GalCer showed complete protection against EG7 tumor challenge in C57BL/6. The adjuvant effects induced by intranasal coadministration with alpha-GalCer were blocked in CD1d-/- mice, indicating that the immune responses were exclusively mediated by CD1d molecule on APC. Most interestingly, intranasally coadministered alpha-GalCer activated naive T cells and triggered them to differentiate into functional effector T cells when CFSE-labeled OT-1 cells were adoptively transferred into syngeneic mice. Overall, our results are the first to show that alpha-GalCer can act as a nasal vaccine adjuvant inducing protective immune responses against viral infections and tumors.     

Protective effect of ginseng polysaccharides on influenza viral infection

Ginseng polysaccharide has been known to have multiple immunomodulatory effects. In this study, we investigated whether Panax ginseng polysaccharide (GP) would have a preventive effect on influenza infection. Administration of mice with GP prior to infection was found to confer a survival benefit against infection with H1N1 (A/PR/8/34) and H3N2 (A/Philippines/82) influenza viruses. Mice infected with the 2009 H1N1 virus suspended in GP solution showed moderately enhanced survival rates and lower levels of lung viral titers and the inflammatory cytokine (IL-6). Daily treatment of vaccinated mice with GP improved their survival against heterosubtypic lethal challenge. This study demonstrates the first evidence that GP can be used as a remedy against influenza viral infection.     

IL-15 participates in the respiratory innate immune response to influenza virus infection

Following influenza infection, natural killer (NK) cells function as interim effectors by suppressing viral replication until CD8 T cells are activated, proliferate, and are mobilized within the respiratory tract. Thus, NK cells are an important first line of defense against influenza virus. Here, in a murine model of influenza, we show that virally-induced IL-15 facilitates the trafficking of NK cells into the lung airways. Blocking IL-15 delays NK cell entry to the site of infection and results in a disregulated control of early viral replication. By the same principle, viral control by NK cells can be therapeutically enhanced via intranasal administration of exogenous IL-15 in the early days post influenza infection. In addition to controlling early viral replication, this IL-15-induced mobilization of NK cells to the lung airways has important downstream consequences on adaptive responses. Primarily, depletion of responding NK1.1+ NK cells is associated with reduced immigration of influenza-specific CD8 T cells to the site of infection. Together this work suggests that local deposits of IL-15 in the lung airways regulate the coordinated innate and adaptive immune responses to influenza infection and may represent an important point of immune intervention.     

Treatment with the reactive oxygen species scavenger EUK-207 reduces lung damage and increases survival during 1918 influenza virus infection in mice

The 1918 influenza pandemic caused over 40 million deaths worldwide, with 675,000 deaths in the United States alone. Studies in several experimental animal models showed that 1918 influenza virus infection resulted in severe lung pathology associated with dysregulated immune and cell death responses. To determine if reactive oxygen species produced by host inflammatory responses play a central role in promoting severity of lung pathology, we treated 1918 influenza virus-infected mice with the catalytic catalase/superoxide dismutase mimetic, salen–manganese complex EUK-207 beginning 3 days postinfection. Postexposure treatment of mice infected with a lethal dose of the 1918 influenza virus with EUK-207 resulted in significantly increased survival and reduced lung pathology without a reduction in viral titers. In vitro studies also showed that EUK-207 treatment did not affect 1918 influenza viral replication. Immunohistochemical analysis showed a reduction in the detection of the apoptosis marker cleaved caspase-3 and the oxidative stress marker 8-oxo-2′-deoxyguanosine in lungs of EUK-207-treated animals compared to vehicle controls. High-throughput sequencing and RNA expression microarray analysis revealed that treatment resulted in decreased expression of inflammatory response genes and increased lung metabolic and repair responses. These results directly demonstrate that 1918 influenza virus infection leads to an immunopathogenic immune response with excessive inflammatory and cell death responses that can be limited by treatment with the catalytic antioxidant EUK-207.     

Sex differences in the response to influenza virus infection: modulation by stress

Influenza virus infection is a significant public health problem; however factors affecting the incidence and severity of disease have not been fully elucidated. The present study sought to examine the role of sex and stress in mediating susceptibility to an influenza viral infection in mice. Male and female mice underwent repeated cycles of restraint (RST) stress, followed by an influenza A/PR8 virus infection. Following these manipulations, levels of circulating corticosterone, lung proinflammatory cytokine gene expression and sickness behavior were examined. The data indicate sex differences in several aspects of the response to the A/PR8 virus infection. The kinetics of lung interleukin-1β mRNA expression were faster in infected males compared to females, while circulating corticosterone levels were elevated in infected females, but not in males. Anorexia and reduced saccharin consumption began earlier and symptoms were more pronounced in infected males than in females. In addition, RST modulated the response to the A/PR8 virus infection. Proinflammatory cytokine gene expression in response to infection was enhanced and sickness behavior was modulated by RST in both males and females. These data suggest that males mount more vigorous immune and behavioral responses to influenza viral infection compared to females, and stress exacerbates the response in both males and females. In conclusion, complex interactions between biological and behavioral factors are involved in mediating individual differences in health and disease. Additional studies may help uncover some of the factors contributing to the individual differences in susceptibility to influenza infection.     

Dose dependence of CTL precursor frequency induced by a DNA vaccine and correlation with protective immunity against influenza virus challenge

Intramuscular injection of BALB/c mice with a DNA plasmid encoding nucleoprotein (NP) from influenza virus A/PR/8/34 (H1N1) provides cross-strain protection against lethal challenge with influenza virus A/HK/68 (H3N2). CTL specific for the H-2Kd-restricted epitope NP147-155 are present in these mice and are thought to play a role in the protection. To assess the effectiveness of NP DNA immunization in comparison with influenza virus infection in the induction of CTL responses, we monitored the frequency of CTL precursors (CTLp) in mice following i.m. injection with NP DNA or intranasal infection with influenza virus and showed that the CTLp frequency in NP DNA-immunized mice can reach levels found in mice that had been infected with influenza virus. We also measured the CTLp frequency, anti-NP Ab titers, and T cell proliferative responses in mice that were injected with titrated dosages of NP DNA and documented a correlation of the CTLp frequency and the Ab titers, but not proliferative responses, with the injection dose. Furthermore, we observed a positive correlation between the frequency of NP147-155 epitope-specific CTLp and the extent of protective immunity against cross-strain influenza challenge induced by NP DNA injection. Collectively, these results and our early observations from adoptive transfer experiments of in vitro activated lymphocytes from NP DNA-immunized mice suggest a protective function of NP-specific CTLp in mice against cross-strain influenza virus challenge.     

Clara cell secretory protein modulates lung inflammatory and immune responses to respiratory syncytial virus infection

Clara cell secretory protein (CCSP) has been shown to have anti-inflammatory and immunomodulatory functions in the lung. Respiratory syncytial virus (RSV) is the most common cause of respiratory infection in infants and young children. RSV usually infects small airways and likely interacts with the Clara cells of bronchioles. To determine a possible role for CCSP during acute RSV infection, CCSP-deficient (CCSP(-/-)) and wild-type (WT) mice were intratracheally infected with RSV and the lung inflammatory and immune responses to RSV infection were assessed. RSV-F gene expression was increased in the lungs of CCSP(-/-) mice as compared with WT mice following RSV infection, consistent with increased viral persistence. Lung inflammation was significantly increased in CCSP(-/-) mice as compared with WT mice after infection. Moreover, although the levels of Th1 cytokines were similar, the levels of Th2 cytokines and neutrophil chemokines were increased in the lungs of CCSP(-/-) mice following infection. Physiologic endpoints of exacerbated lung disease, specifically airway reactivity and mucus production, were increased in CCSP(-/-) mice after RSV infection. Importantly, restoration of CCSP in the airways of CCSP(-/-) mice abrogated the increased viral persistence, lung inflammation, and airway reactivity. These findings suggest a role for CCSP and Clara cells in regulating lung inflammatory and immune responses to RSV infection.     

Parasite-mediated upregulation of NK cell-derived gamma interferon protects against severe highly pathogenic H5N1 influenza virus infection

Outbreaks of influenza A viruses are associated with significant human morbidity worldwide. Given the increasing resistance to the available influenza drugs, new therapies for the treatment of influenza virus infection are needed. An alternative approach is to identify products that enhance a protective immune response. In these studies, we demonstrate that infecting mice with the Th1-inducing parasite Toxoplasma gondii prior to highly pathogenic avian H5N1 influenza virus infection led to decreased lung viral titers and enhanced survival. A noninfectious fraction of T. gondii soluble antigens (STAg) elicited an immune response similar to that elicited by live parasites, and administration of STAg 2 days after H5N1 influenza virus infection enhanced survival, lowered viral titers, and reduced clinical disease. STAg administration protected H5N1 virus-infected mice lacking lymphocytes, suggesting that while the adaptive immune response was not required for enhanced survival, it was necessary for STAg-mediated viral clearance. Mechanistically, we found that administration of STAg led to increased production of gamma interferon (IFN-γ) from natural killer (NK) cells, which were both necessary and sufficient for survival. Further, administration of exogenous IFN-γ alone enhanced survival from H5N1 influenza virus infection, although not to the same level as STAg treatment. These studies demonstrate that a noninfectious T. gondii extract enhances the protective immune response against severe H5N1 influenza virus infections even when a single dose is administered 2 days postinfection.     

MyD88 signaling is indispensable for primary influenza A virus infection but dispensable for secondary infection

Recent studies have revealed that innate immunity is involved in the development of adaptive immune responses; however, its role in protection is not clear. In order to elucidate the exact role of Toll-like receptor (TLR) or RIG-I-like receptor (RLR) signaling on immunogenicity and protective efficacy against influenza A virus infection (A/PR/8/34 [PR8]; H1N1), we adapted several innate signal-deficient mice (e.g., TRIF(-/-), MyD88(-/-), MyD88(-/-) TRIF(-/-), TLR3(-/-) TLR7(-/-), and IPS-1(-/-)). In this study, we found that MyD88 signaling was required for recruitment of CD11b(+) granulocytes, production of early inflammatory cytokines, optimal proliferation of CD4 T cells, and production of Th1 cytokines by T cells. However, PR8 virus-specific IgG and IgA antibody levels in both systemic and mucosal compartments were normal in TLR- and RLR-deficient mice. To further assess the susceptibility of these mice to influenza virus infection, protective efficacy was determined after primary or secondary lethal challenge. We found that MyD88(-/-) and MyD88(-/-) TRIF(-/-) mice were more susceptible to primary influenza virus infection than the B6 mice but were fully protected against homologous (H1N1) and heterosubtypic (H5N2) secondary infection when primed with a nonlethal dose of PR8 virus. Taken together, these results show that MyD88 signaling plays an important role for resisting primary influenza virus infection but is dispensable for protection against a secondary lethal challenge.     

Inhibition of lectin-mediated innate host defences in vivo modulates disease severity during influenza virus infection

Host-mediated recognition of mannose-rich glycans on the surface of pathogens represents an ancient mechanism of innate immune defence. In this study, we demonstrate that the virus strains that differ in the degree of N-linked glycosylation on the globular head of their hemagglutinin glycoprotein also differed in their (i) sensitivity to neutralization by a mannose-specific lectin in mouse lung fluids and (ii) ability to infect (and, therefore, to be destroyed) by airway macrophages. Virus strain BJx109 (H3N2), but not PR8 (H1N1), was sensitive to neutralization by mouse lung fluids and infected airway macrophages efficiently in vitro and these antiviral activities were blocked by mannan, a complex polymer of mannose residues. Although intranasal (i.n.) infection of mice with PR8 led to severe disease and mortality, mice infected with an equivalent dose of BJx109 displayed no signs of disease. However, i.n. treatment of BJx109-infected mice with mannan led to viral pneumonia, severe disease and death characterized by excessive virus replication, pulmonary inflammation, vascular leak and lung edema. Thus, when mannose-specific innate defences were inhibited in vivo, virus strain BJx109 induced severe viral pneumonia similar to that of PR8. Together, these findings highlight the importance of N-linked glycans as a target for recognition and destruction of influenza viruses by the innate immune system. Moreover, soluble and cell-associated lectins coordinate to modulate disease severity following influenza virus infection of mice.     

Acellular and whole cell pertussis vaccines protect against the lethal effects of intracerebral challenge by two different T-cell dependent humoral routes

Athymic (nu/nu) and euthymic (+/nu) BALB/c mice were immunized with a whole cell pertussis vaccine or with an acellular vaccine which contained detoxified pertussis toxin (PT) and filamentous hemagglutinin (FHA). Only the euthymic mice were protected against intracerebral challenge with virulent Bordetella pertussis which implies involvement of T-cells. As a cell transfer from mice immunized with whole cell or acellular vaccine prior to the challenge did not protect naive euthymic recipients, cellular immunity seems to be non-protective as an effector mechanism. Mice could be protected passively against a challenge by administration of immune sera. Therefore, T-cell dependent humoral immune responses to B. pertussis appear to be crucial for protection. The humoral response was further studied with athymic and euthymic mice. In euthymic mice the whole cell vaccine induced antibodies to FHA, pililipopolysaccharides (LPS) and an outer membrane protein (OMP) preparation, whereas the acellular vaccine induced antibodies to PT, FHA and OMP. Both IgM and IgG could be detected. From the nude mice only those immunized with the whole cell vaccine showed an antibody response which consisted of low titres of IgM directed to LPS. Sera from both +/nu and nu/nu mice immunized with the whole cell vaccine were bactericidal in vitro. These data demonstrate that in the mouse model protection to intracerebral challenge with B. pertussis is T-cell dependent as is the humoral response to PT, FHA, OMP and pili. The T-independent B-cell activation by the whole cell preparation is due to the presence of LPS.(ABSTRACT TRUNCATED AT 250 WORDS)     

Virus-like particle vaccine induces protective immunity against homologous and heterologous strains of influenza virus

Recurrent outbreaks of highly pathogenic avian influenza virus pose the threat of pandemic spread of lethal disease and make it a priority to develop safe and effective vaccines. Influenza virus-like particles (VLPs) have been suggested to be a promising vaccine approach. However, VLP-induced immune responses, and their roles in inducing memory immune responses and cross-protective immunity have not been investigated. In this study, we developed VLPs containing influenza virus A/PR8/34 (H1N1) hemagglutinin (HA) and matrix (M1) proteins and investigated their immunogenicity, long-term cross-protective efficacy, and effects on lung proinflammatory cytokines in mice. Intranasal immunization with VLPs containing HA induced high serum and mucosal antibody titers and neutralizing activity against PR8 and A/WSN/33 (H1N1) viruses. Mice immunized with VLPs containing HA showed little or no proinflammatory lung cytokines and were protected from a lethal challenge with mouse-adapted PR8 or WSN viruses even 5 months postimmunization. Influenza VLPs induced mucosal immunoglobulin G and cellular immune responses, which were reactivated rapidly upon virus challenge. Long-lived antibody-secreting cells were detected in the bone marrow of immunized mice. Immune sera administered intranasally were able to confer 100% protection from a lethal challenge with PR8 or WSN, which provides further evidence that anti-HA antibodies are primarily responsible for preventing infection. Taken together, these results indicate that nonreplicating influenza VLPs represent a promising strategy for the development of a safe and effective vaccine to control the spread of lethal influenza viruses.     

Pneumocystis infection enhances antibody-mediated resistance to a subsequent influenza infection

In contrast to the detrimental outcomes most often associated with the resolution of coinfections, the model presented here involving a localized Pneumocystis infection of the lung, followed 2 wk later by an influenza virus infection, results in a significant beneficial outcome for the host. In the week following the influenza infection, immunocompetent coinfected animals exhibited an accelerated rate of virus clearance, an accelerated appearance of higher influenza-specific neutralizing Ab titers in their serum and bronchoalveolar lavage fluid (BALF), significantly reduced inflammatory cytokine levels in their BALF, and reduced levels of morbidity relative to animals infected only with influenza virus. The beneficial outcome observed in coinfected immunocompetent animals was dependent on the ongoing resolution of a viable Pneumocystis infection. No differences in viral clearance were detected between coinfected and influenza-only-infected muMT mice or likewise for SCID mice. The accelerated anti-influenza response did not appear to be associated with influenza-specific CD8 T cell-mediated responses or NK cell responses in the lung. Rather, the increased rate of viral clearance was due to the enhancement of the influenza-specific Ab response, which in turn was transiently dependent upon the resolution of the ongoing Pneumocystis infection.     

Neutrophils sustain effective CD8(+) T-cell responses in the respiratory tract following influenza infection

Neutrophils have an important role in early host protection during influenza A virus infection. Their ability to modulate the virus-specific adaptive immune response is less clear. Here, we have used a mouse model to examine the impact of neutrophils on CD8(+) T-cell responses during influenza virus infection. CD8(+) T-cell priming, expansion, migration, cytokine secretion and cytotoxic capacity were investigated in the virus-infected airways and secondary lymphoid organs. To do this, we utilised a Ly6G-specific monoclonal antibody (mAb; 1A8) that specifically depletes neutrophils in vivo. Neutrophil depletion early after infection with influenza virus strain HKx31 (H3N2) did not alter influenza virus-derived antigen presentation or na?ve CD8(+) T-cell expansion in the secondary lymphoid organs. Trafficking of virus-specific CD8(+) T cells into the infected pulmonary airways was also unaltered. Instead, early neutropenia reduced both the overall magnitude of influenza virus-specific CD8(+) T cells, together with impaired cytokine production and cytotoxic effector function. Therefore, neutrophils are important participants in anti-viral mechanisms that sustain effective CD8(+) T-cell responses in the respiratory tract of influenza virus-infected mice.