Quantitative subcellular proteome and secretome profiling of influenza A virus-infected human primary macrophages

Influenza A viruses are important pathogens that cause acute respiratory diseases and annual epidemics in humans. Macrophages recognize influenza A virus infection with their pattern recognition receptors, and are involved in the activation of proper innate immune response. Here, we have used high-throughput subcellular proteomics combined with bioinformatics to provide a global view of host cellular events that are activated in response to influenza A virus infection in human primary macrophages. We show that viral infection regulates the expression and/or subcellular localization of more than one thousand host proteins at early phases of infection. Our data reveals that there are dramatic changes in mitochondrial and nuclear proteomes in response to infection. We show that a rapid cytoplasmic leakage of lysosomal proteins, including cathepsins, followed by their secretion, contributes to inflammasome activation and apoptosis seen in the infected macrophages. Also, our results demonstrate that P2X₇ receptor and src tyrosine kinase activity are essential for inflammasome activation during influenza A virus infection. Finally, we show that influenza A virus infection is associated with robust secretion of different danger-associated molecular patterns (DAMPs) suggesting an important role for DAMPs in host response to influenza A virus infection. In conclusion, our high-throughput quantitative proteomics study provides important new insight into host-response against influenza A virus infection in human primary macrophages.     

Bordetella pertussis infection exacerbates influenza virus infection through pertussis toxin-mediated suppression of innate immunity

Pertussis (whooping cough) is frequently complicated by concomitant infections with respiratory viruses. Here we report the effect of Bordetella pertussis infection on subsequent influenza virus (PR8) infection in mouse models and the role of pertussis toxin (PT) in this effect. BALB/c mice infected with a wild-type strain of B. pertussis (WT) and subsequently (up to 14 days later) infected with PR8 had significantly increased pulmonary viral titers, lung pathology and mortality compared to mice similarly infected with a PT-deficient mutant strain (ΔPT) and PR8. Substitution of WT infection by intranasal treatment with purified active PT was sufficient to replicate the exacerbating effects on PR8 infection in BALB/c and C57/BL6 mice, but the effects of PT were lost when toxin was administered 24 h after virus inoculation. PT had no effect on virus titers in primary cultures of murine tracheal epithelial cells (mTECs) in vitro, suggesting the toxin targets an early immune response to increase viral titers in the mouse model. However, type I interferon responses were not affected by PT. Whole genome microarray analysis of gene expression in lung tissue from PT-treated and control PR8-infected mice at 12 and 36 h post-virus inoculation revealed that PT treatment suppressed numerous genes associated with communication between innate and adaptive immune responses. In mice depleted of alveolar macrophages, increase of pulmonary viral titers by PT treatment was lost. PT also suppressed levels of IL-1β, IL-12, IFN-γ, IL-6, KC, MCP-1 and TNF-α in the airways after PR8 infection. Furthermore PT treatment inhibited early recruitment of neutrophils and NK cells to the airways. Together these findings demonstrate that infection with B. pertussis through PT activity predisposes the host to exacerbated influenza infection by countering protective innate immune responses that control virus titers.     

Host cellular signaling induced by influenza virus

A wide range of host cellular signal transduction pathways can be stimulated by influenza virus infection. Some of these signal transduction pathways induce the host cell’s innate immune response against influenza virus, while others are essential for efficient influenza virus replication. This review examines the cellular signaling induced by influenza virus infection in host cells, including host pattern recognition receptor (PRR)-related signaling, protein kinase C (PKC), Raf/MEK/ERK and phosphatidylinositol- 3-kinase (PI3K)/Akt signaling, and the corresponding effects on the host cell and/or virus, such as recognition of virus by the host cell, viral absorption and entry, viral ribonucleoprotein (vRNP) export, translation control of cellular and viral proteins, and virus-induced cell apoptosis. Research into influenza virus-induced cell signaling promotes a clearer understanding of influenza virus-host interactions and assists in the identification of novel antiviral targets and antiviral strategies.     

Distinct RIG-I and MDA5 signaling by RNA viruses in innate immunity

Alpha/beta interferon immune defenses are essential for resistance to viruses and can be triggered through the actions of the cytoplasmic helicases retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). Signaling by each is initiated by the recognition of viral products such as RNA and occurs through downstream interaction with the IPS-1 adaptor protein. We directly compared the innate immune signaling requirements of representative viruses of the Flaviviridae, Orthomyxoviridae, Paramyxoviridae, and Reoviridae for RIG-I, MDA5, and interferon promoter-stimulating factor 1 (IPS-1). In cultured fibroblasts, IPS-1 was essential for innate immune signaling of downstream interferon regulatory factor 3 activation and interferon-stimulated gene expression, but the requirements for RIG-I and MDA5 were variable. Each was individually dispensable for signaling triggered by reovirus and dengue virus, whereas RIG-I was essential for signaling by influenza A virus, influenza B virus, and human respiratory syncytial virus. Functional genomics analyses identified cellular genes triggered during influenza A virus infection whose expression was strictly dependent on RIG-I and which are involved in processes of innate or adaptive immunity, apoptosis, cytokine signaling, and inflammation associated with the host response to contemporary and pandemic strains of influenza virus. These results define IPS-1-dependent signaling as an essential feature of host immunity to RNA virus infection. Our observations further demonstrate differential and redundant roles for RIG-I and MDA5 in pathogen recognition and innate immune signaling that may reflect unique and shared biologic properties of RNA viruses whose differential triggering and control of gene expression may impact pathogenesis and infection.     

甲型流感病毒感染过程中干扰素介导的天然免疫应答机制

甲型流感病毒作为引起人类和动物急性呼吸道传染病的一个主要病原体,在世界范围内广泛流行。研究表明,甲型流感病毒感染宿主后会诱导宿主的天然免疫应答。甲型流感病毒感染可引起Toll样受体(Toll like receptors,TLRs)和RIG-Ⅰ样受体(RIG-Ⅰ like receptors,RLRs)等宿主模式识别受体介导的抗病毒信号通路的活化,并在多种机制调控下诱导干扰素和其他细胞因子的表达,如Ⅰ型干扰素、Ⅲ型干扰素等,从而启动干扰素刺激基因(Interferon stimulated genes,ISGs)的转录及其抗病毒蛋白的表达,进而实现抗病毒作用。本文就甲型流感病毒感染与干扰素介导的天然免疫应答相关的信号通路和调控机制进行综述。     

Baculovirus induces an innate immune response and confers protection from lethal influenza virus infection in mice

A recombinant baculovirus expressing the hemagglutinin gene of the influenza virus, A/PR/8/34 (H1N1), under the control of the chicken beta-actin promoter, was constructed. To determine the induction of protective immunity in vivo, mice were inoculated with the recombinant baculovirus by intramuscular, intradermal, i.p., and intranasal routes and then were challenged with a lethal dose of the influenza virus. Intramuscular or i.p. immunization with the recombinant baculovirus elicited higher titers of antihemagglutinin Ab than intradermal or intranasal administration. However, protection from a lethal challenge of the influenza virus was only achieved by intranasal immunization of the recombinant baculovirus. Surprisingly, sufficient protection from the lethal influenza challenge was also observed in mice inoculated intranasally with a wild-type baculovirus, as evaluated by reductions in the virus titer, inflammatory cytokine production, and pulmonary consolidations. These results indicate that intranasal inoculation with a wild-type baculovirus induces a strong innate immune response, which protects mice from a lethal challenge of influenza virus.     

Role of MyD88 in route-dependent susceptibility to vesicular stomatitis virus infection

TLRs are important components of the innate immune response. The role of the TLR signaling pathway in host defense against a natural viral infection has been largely unexplored. We found that mice lacking MyD88, an essential adaptor protein in TLR signaling pathway, were extremely sensitive to intranasal infection with vesicular stomatitis virus, and this susceptibility was dose dependent. We demonstrated that this increased susceptibility correlates with the impaired production of IFN-alpha and defective induction and maintenance of neutralizing Ab. These studies outline the important role of the TLR signaling pathway in nasal mucosae-respiratory tracts-neuroepithelium environment in the protection against microbial pathogen infections. We believe that these results explain how the route of infection, probably by virtue of activating different cell populations, can lead to entirely different outcomes of infection based on the underlying genetics of the host.     

TLR7 Recognition Is Dispensable for Influenza Virus A Infection but Important for the Induction of Hemagglutinin-Specific Antibodies in Response to the 2009 Pandemic Split Vaccine in Mice

Recognition of pathogen-associated molecular patterns by pattern recognition receptors of the innate immune system is crucial for the initiation of innate and adaptive responses and for immunological memory. We investigated the role of TLR7 in the induction of adaptive immunity and long-term memory following influenza virus infection and vaccination in C57BL/6 mice. During infection with influenza A/PR8/34 virus, the absence of either TLR7 or MyD88 leads to reduced virus-specific antibodies in the serum and antibody-secreting cells in their secondary lymphoid organs, particularly in bone marrow. In spite of this, the absence of TLR7/MyD88 signaling did not impair the production of protective antibodies. Following immunization with the 2009 pandemic inactivated split vaccine, TLR7(-/-) mice had significantly lower levels of germinal center formation, antibody-secreting cells, and circulating influenza virus-specific antibodies than control animals. Consequently, TLR7(-/-) mice failed to develop protective immunological memory upon challenge. Furthermore, the immunogenicity of the split vaccine was likely due to TLR7 recognition of virion RNA, as its removal from the split vaccine significantly reduced the levels of influenza virus-specific antibodies and compromised the vaccine protective efficacy in mice. Taken together, our data demonstrate that TLR7 plays an important role in vaccine-induced humoral immune responses to influenza virus through the interaction with viral RNA present in the split vaccine.     

Interleukin-21 Receptor Signalling Is Important for Innate Immune Protection against HSV-2 Infections

Interleukin (IL) -21 is produced by Natural Killer T (NKT) cells and CD4⁺ T cells and is produced in response to virus infections, where IL-21 has been shown to be essential in adaptive immune responses. Cells from the innate immune system such as Natural Killer (NK) cells and macrophages are also important in immune protection against virus. These cells express the IL-21 receptor (IL-21R) and respond to IL-21 with increased cytotoxicity and cytokine production. Currently, however it is not known whether IL-21 plays a significant role in innate immune responses to virus infections. The purpose of this study was to investigate the role of IL-21 and IL-21R in the innate immune response to a virus infection. We used C57BL/6 wild type (WT) and IL-21R knock out (KO) mice in a murine vaginal Herpes Simplex Virus type 2 (HSV-2) infection model to show that IL-21 – IL-21R signalling is indeed important in innate immune responses against HSV-2. We found that the IL-21R was expressed in the vaginal epithelium in uninfected (u.i) WT mice, and expression increased early after HSV-2 infection. IL-21R KO mice exhibited increased vaginal viral titers on day 2 and 3 post infection (p.i.) and subsequently developed significantly higher disease scores and a lower survival rate compared to WT mice. In addition, WT mice infected with HSV-2 receiving intra-vaginal pre-treatment with murine recombinant IL-21 (mIL-21) had decreased vaginal viral titers on day 2 p.i., significantly lower disease scores, and a higher survival rate compared to infected untreated WT controls. Collectively our data demonstrate the novel finding that the IL-21R plays a critical role in regulating innate immune responses against HSV-2 infection.     

The role of alpha/beta and gamma interferons in development of immunity to influenza A virus in mice

During influenza virus infection innate and adaptive immune defenses are activated to eliminate the virus and thereby bring about recovery from illness. Both arms of the adaptive immune system, antibody neutralization of free virus and termination of intracellular virus replication by antiviral cytotoxic T cells (CTLs), play pivotal roles in virus elimination and protection from disease. Innate cytokine responses, such as alpha/beta interferon (IFN-alpha/beta) or IFN-gamma, can have roles in determining the rate of virus replication in the initial stages of infection and in shaping the initial inflammatory and downstream adaptive immune responses. The effect of these cytokines on the replication of pneumotropic influenza A virus in the respiratory tract and in the regulation of adaptive antiviral immunity was examined after intranasal infection of mice with null mutations in receptors for IFN-alpha/beta, IFN-gamma, and both IFNs. Virus titers in the lungs of mice unable to respond to IFNs were not significantly different from congenic controls for both primary and secondary infection. Likewise the mice were comparably susceptible to X31 (H3N2) influenza virus infection. No significant disruption to the development of normal antiviral CTL or antibody responses was observed. In contrast, mice bearing the disrupted IFN-alpha/beta receptor exhibited accelerated kinetics and significantly higher levels of neutralizing antibody activity during primary or secondary heterosubtypic influenza virus infection. Thus, these observations reveal no significant contribution for IFN-controlled pathways in shaping acute or memory T-cell responses to pneumotropic influenza virus infection but do indicate some role for IFN-alpha/beta in the regulation of antibody responses. Recognizing the pivotal role of CTLs and antibody in virus clearance, it is reasonable to assume a redundancy in IFN-mediated antiviral effects in pulmonary influenza. However, IFN-alpha/beta seems to be a valid factor in determining tissue tropism and replicative rates of highly virulent influenza virus strains as reported previously by others, and this aspect is discussed here.     

The Essential,Nonredundant Roles of RIG-I and MDA5 in Detecting and Controlling West Nile Virus Infection

Virus recognition and response by the innate immune system are critical components of host defense against infection. Activation of cell-intrinsic immunity and optimal priming of adaptive immunity against West Nile virus (WNV), an emerging vector-borne virus, depend on recognition by RIG-I and MDA5, two cytosolic pattern recognition receptors (PRRs) of the RIG-I-like receptor (RLR) protein family that recognize viral RNA and activate defense programs that suppress infection. We evaluated the individual functions of RIG-I and MDA5 both in vitro and in vivo in pathogen recognition and control of WNV. Lack of RIG-I or MDA5 alone results in decreased innate immune signaling and virus control in primary cells in vitro and increased mortality in mice. We also generated RIG-I^−/− × MDA5^−/− double-knockout mice and found that a lack of both RLRs results in a complete absence of innate immune gene induction in target cells of WNV infection and a severe pathogenesis during infection in vivo, similar to findings for animals lacking MAVS, the central adaptor molecule for RLR signaling. We also found that RNA products from WNV-infected cells but not incoming virion RNA display at least two distinct pathogen-associated molecular patterns (PAMPs) containing 5′ triphosphate and double-stranded RNA that are temporally distributed and sensed by RIG-I and MDA5 during infection. Thus, RIG-I and MDA5 are essential PRRs that recognize distinct PAMPs that accumulate during WNV replication. Collectively, these experiments highlight the necessity and function of multiple related, cytoplasmic host sensors in orchestrating an effective immune response against an acute viral infection.     

Protective role of beta interferon in host defense against influenza A virus

Type I interferon (IFN), which includes the IFN-alpha and -beta subtypes, plays an essential role in host defense against influenza A virus. However, the relative contribution of IFN-beta remains unresolved. In mice, type I IFN is effective against influenza viruses only if the IFN-induced resistance factor Mx1 is present, though most inbred mouse strains, including the recently developed IFN-beta-deficient mice, bear only defective Mx1 alleles. We therefore generated IFN-beta-deficient mice carrying functional Mx1 alleles (designated Mx-BKO) and compared them to either wild-type mice bearing functional copies of both IFN-beta and Mx1 (designated Mx-wt) or mice carrying functional Mx1 alleles but lacking functional type I IFN receptors (designated Mx-IFNAR). Influenza A virus strain SC35M (H7N7) grew to high titers and readily formed plaques in monolayers of Mx-BKO and Mx-IFNAR embryo fibroblasts which showed no spontaneous expression of Mx1. In contrast, Mx-wt embryo fibroblasts were found to constitutively express Mx1, most likely explaining why SC35M did not grow to high titers and formed no visible plaques in such cells. In vivo challenge experiments in which SC35M was applied via the intranasal route showed that the 50% lethal dose was about 20-fold lower in Mx-BKO mice than in Mx-wt mice and that virus titers in the lungs were increased in Mx-BKO mice. The resistance of Mx-BKO mice to influenza A virus strain PR/8/34 (H1N1) was also substantially reduced, demonstrating that IFN-beta plays an important role in the defense against influenza A virus that cannot be compensated for by IFN-alpha.     

Human immune responses elicited by an intranasal inactivated H5 influenza vaccine

Intranasally administered influenza vaccines could be more effective than injected vaccines, because intranasal vaccination can induce virus-specific immunoglobulin A (IgA) antibodies in the upper respiratory tract, which is the initial site of infection. In this study, immune responses elicited by an intranasal inactivated vaccine of influenza A(H5N1) virus were evaluated in healthy individuals naive for influenza A(H5N1) virus. Three doses of intranasal inactivated whole-virion H5 influenza vaccine induced strong neutralizing nasal IgA and serum IgG antibodies. In addition, a mucoadhesive excipient, carboxy vinyl polymer, had a notable impact on the induction of nasal IgA antibody responses but not on serum IgG antibody responses. The nasal hemagglutinin (HA)-specific IgA antibody responses clearly correlated with mucosal neutralizing antibody responses, indicating that measurement of nasal HA-specific IgA titers could be used as a surrogate for the mucosal antibody response. Furthermore, increased numbers of plasma cells and vaccine antigen-specific Th cells in the peripheral blood were observed after vaccination, suggesting that peripheral blood biomarkers may also be used to evaluate the intranasal vaccine-induced immune response. However, peripheral blood immune cell responses correlated with neutralizing antibody titers in serum samples but not in nasal wash samples. Thus, analysis of the peripheral blood immune response could be a surrogate for the systemic immune response to intranasal vaccination but not for the mucosal immune response. The current study suggests the clinical potential of intranasal inactivated vaccines against influenza A(H5N1) viruses and highlights the need to develop novel means to evaluate intranasal vaccine-induced mucosal immune responses.     

Galectin-1 binds to influenza virus and ameliorates influenza virus pathogenesis

Innate immune response is important for viral clearance during influenza virus infection. Galectin-1, which belongs to S-type lectins, contains a conserved carbohydrate recognition domain that recognizes galactose-containing oligosaccharides. Since the envelope proteins of influenza virus are highly glycosylated, we studied the role of galectin-1 in influenza virus infection in vitro and in mice. We found that galectin-1 was upregulated in the lungs of mice during influenza virus infection. There was a positive correlation between galectin-1 levels and viral loads during the acute phase of viral infection. Cells treated with recombinant human galectin-1 generated lower viral yields after influenza virus infection. Galectin-1 could directly bind to the envelope glycoproteins of influenza A/WSN/33 virus and inhibit its hemagglutination activity and infectivity. It also bound to different subtypes of influenza A virus with micromolar dissociation constant (K(d)) values and protected cells against influenza virus-induced cell death. We used nanoparticle, surface plasmon resonance analysis and transmission electron microscopy to further demonstrate the direct binding of galectin-1 to influenza virus. More importantly, we show for the first time that intranasal treatment of galectin-1 could enhance survival of mice against lethal challenge with influenza virus by reducing viral load, inflammation, and apoptosis in the lung. Furthermore, galectin-1 knockout mice were more susceptible to influenza virus infection than wild-type mice. Collectively, our results indicate that galectin-1 has anti-influenza virus activity by binding to viral surface and inhibiting its infectivity. Thus, galectin-1 may be further explored as a novel therapeutic agent for influenza.     

Antiviral activity of the long chain pentraxin PTX3 against influenza viruses

Proteins of the innate immune system can act as natural inhibitors of influenza virus, limiting growth and spread of the virus in the early stages of infection before the induction of adaptive immune responses. In this study, we identify the long pentraxin PTX3 as a potent innate inhibitor of influenza viruses both in vitro and in vivo. Human and murine PTX3 bound to influenza virus and mediated a range of antiviral activities, including inhibition of hemagglutination, neutralization of virus infectivity and inhibition of viral neuraminidase. Antiviral activity was associated with binding of the viral hemagglutinin glycoprotein to sialylated ligands present on PTX3. Using a mouse model we found PTX3 to be rapidly induced following influenza infection and that PTX3-/- mice were more susceptible than wild-type mice to infection by PTX3-sensitive virus strains. Therapeutic treatment of mice with human PTX3 promoted survival and reduced viral load in the lungs following infection with PTX3-sensitive, but not PTX3-resistant, influenza viruses. Together, these studies describe a novel antiviral role for PTX3 in early host defense against influenza infections both in vitro and in vivo and describe the therapeutic potential of PTX3 in ameliorating disease during influenza infection.     

Interferon-lambda contributes to innate immunity of mice against influenza A virus but not against hepatotropic viruses

Virus-infected cells secrete a broad range of interferon (IFN) subtypes which in turn trigger the synthesis of antiviral factors that confer host resistance. IFN-alpha, IFN-beta and other type I IFNs signal through a common universally expressed cell surface receptor, whereas IFN-lambda uses a distinct receptor complex for signaling that is not present on all cell types. Since type I IFN receptor-deficient mice (IFNAR1(0/0)) exhibit greatly increased susceptibility to various viral diseases, it remained unclear to which degree IFN-lambda might contribute to innate immunity. To address this issue we performed influenza A virus infections of mice which carry functional alleles of the influenza virus resistance gene Mx1 and which, therefore, develop a more complete innate immune response to influenza viruses than standard laboratory mice. We demonstrate that intranasal administration of IFN-lambda readily induced the antiviral factor Mx1 in mouse lungs and efficiently protected IFNAR1(0/0) mice from lethal influenza virus infection. By contrast, intraperitoneal application of IFN-lambda failed to induce Mx1 in the liver of IFNAR1(0/0) mice and did not protect against hepatotropic virus infections. Mice lacking functional IFN-lambda receptors were only slightly more susceptible to influenza virus than wild-type mice. However, mice lacking functional receptors for both IFN-alpha/beta and IFN-lambda were hypersensitive and even failed to restrict usually non-pathogenic influenza virus mutants lacking the IFN-antagonistic factor NS1. Interestingly, the double-knockout mice were not more susceptible against hepatotropic viruses than IFNAR1(0/0) mice. From these results we conclude that IFN-lambda contributes to inborn resistance against viral pathogens infecting the lung but not the liver.     

A20 Deficiency in Lung Epithelial Cells Protects against Influenza A Virus Infection

A20 negatively regulates multiple inflammatory signalling pathways. We here addressed the role of A20 in club cells (also known as Clara cells) of the bronchial epithelium in their response to influenza A virus infection. Club cells provide a niche for influenza virus replication, but little is known about the functions of these cells in antiviral immunity. Using airway epithelial cell-specific A20 knockout (A20^AEC-KO) mice, we show that A20 in club cells critically controls innate immune responses upon TNF or double stranded RNA stimulation. Surprisingly, A20^AEC-KO mice are better protected against influenza A virus challenge than their wild type littermates. This phenotype is not due to decreased viral replication. Instead host innate and adaptive immune responses and lung damage are reduced in A20^AEC-KO mice. These attenuated responses correlate with a dampened cytotoxic T cell (CTL) response at later stages during infection, indicating that A20^AEC-KO mice are better equipped to tolerate Influenza A virus infection. Expression of the chemokine CCL2 (also named MCP-1) is particularly suppressed in the lungs of A20^AEC-KO mice during later stages of infection. When A20^AEC-KO mice were treated with recombinant CCL2 the protective effect was abrogated demonstrating the crucial contribution of this chemokine to the protection of A20^AEC-KO mice to Influenza A virus infection. Taken together, we propose a mechanism of action by which A20 expression in club cells controls inflammation and antiviral CTL responses in response to influenza virus infection.     

Avian influenza virus H5N1 induces rapid interferon-beta production but shows more potent inhibition to retinoic acid-inducible gene I expression than H1N1 in vitro

ABSTRACT: BACKGROUND: The mechanisms through which the avian influenza virus H5N1 modulate the host's innate immune defense during invasion, remains incompletely understood. RIG-I as a pattern recognition receptor plays an important role in mediating innate immune response induced by influenza virus. So, modulating RIG-I might be adopted as a strategy by influenza virus to antagonize the host's innate immune defense. Methods: Here we chose an avian influenza virus A/tree sparrow/Henan/1/04 (H5N1) directly isolated from a free-living tree sparrow in Mainland China which is amplified in egg allantoic cavity, and researched its interferon induction and manipulation of RIG-I expression compared with influenza virus A/WSN/1933(H1N1), a well charaterized mouse adapted strain, in human lung epithelial A549 cells and human embryonic kidney 293T cells. Results: Although the avian influenza virus H5N1 infection initiated a rapid IFN-beta production early on, it eventually presented a more potent inhibition to IFN-beta production than H1N1. Correspondingly, the H5N1 infection induced low level expression of endogenous RIG-I, an Interferon Stimulating Gene (ISG), and showed more potent inhibition to the expression of endogenous RIG-I triggered by exogenous interferon than H1N1. Conclusions: Manipulating endogenous RIG-I expression might constitute one of the mechanisms through which avian influenza virus H5N1 control the host's innate immune response during infection.     

Natural antibody and complement mediate neutralization of influenza virus in the absence of prior immunity

Early control of virus replication by the innate immune response is essential to allow time for the generation of a more effective adaptive immune response. As an important component of innate immunity, complement has been shown to be necessary for protection against numerous microbial infections. This study was undertaken to investigate the role of complement in neutralizing influenza virus. Results demonstrated that the classical pathway of complement mediated serum neutralization of influenza virus. Although nonimmune serum neutralized influenza virus, the mechanism of virus neutralization (VN) required antibody, as sera from RAG1-deficient mice lacked VN activity; moreover, purified natural immunoglobulin M (IgM) restored VN activity to antibody-deficient sera. The mechanism of VN by natural IgM and complement was associated with virion aggregation and coating of the viral hemagglutinin receptor; however, viral lysis did not significantly contribute to VN. Additionally, reconstitution of RAG1-deficient mice with natural IgM resulted in delayed morbidity during influenza virus infection. Collectively, these results provide evidence that natural IgM and the early components of the classical pathway of complement work in concert to neutralize influenza virus and that this interaction may have a significant impact on the course of influenza viral pneumonia.     

Toll-like receptor 2 plays a role in the early inflammatory response to murine pneumococcal pneumonia but does not contribute to antibacterial defense

Toll-like receptors (TLR) are crucial pattern recognition receptors in innate immunity. The importance of TLR2 in host defense against Gram-positive bacteria has been suggested by the fact that this receptor recognizes major Gram-positive cell wall components, such as peptidoglycan and lipoteichoic acid. To determine the role of TLR2 in pulmonary Gram-positive infection, we first established that TLR2 is indispensable for alveolar macrophage responsiveness toward Streptococcus pneumoniae. Nonetheless, TLR2 gene-deficient mice intranasally inoculated with S. pneumoniae at doses varying from nonlethal (with complete clearance of the infection) to lethal displayed only a modestly reduced inflammatory response in their lungs and an unaltered antibacterial defense when compared with normal wild-type mice. These data suggest that TLR2 plays a limited role in the innate immune response to pneumococcal pneumonia, and that additional pattern recognition receptors likely are involved in host defense against this common respiratory pathogen.