Pediatric hospitalizations associated with 2009 pandemic influenza A (H1N1) in Argentina.

BACKGROUND: While the Northern Hemisphere experiences the effects of the 2009 pandemic influenza A (H1N1) virus, data from the recent influenza season in the Southern Hemisphere can provide important information on the burden of disease in children. METHODS: We conducted a retrospective case series involving children with acute infection of the lower respiratory tract or fever in whom 2009 H1N1 influenza was diagnosed on reverse-transcriptase polymerase-chain-reaction assay and who were admitted to one of six pediatric hospitals serving a catchment area of 1.2 million children. We compared rates of admission and death with those among age-matched children who had been infected with seasonal influenza strains in previous years. RESULTS: Between May and July 2009, a total of 251 children were hospitalized with 2009 H1N1 influenza. Rates of hospitalization were double those for seasonal influenza in 2008. Of the children who were hospitalized, 47 (19%) were admitted to an intensive care unit, 42 (17%) required mechanical ventilation, and 13 (5%) died. The overall rate of death was 1.1 per 100,000 children, as compared with 0.1 per 100,000 children for seasonal influenza in 2007. (No pediatric deaths associated with seasonal influenza were reported in 2008.) Most deaths were caused by refractory hypoxemia in infants under 1 year of age (death rate, 7.6 per 100,000). CONCLUSIONS: Pandemic 2009 H1N1 influenza was associated with pediatric death rates that were 10 times the rates for seasonal influenza in previous years.

Conserved cysteine residues within the attachment G glycoprotein of respiratory syncytial virus play a critical role in the enhancement of cytotoxic T-lymphocyte responses.

The cytotoxic T-lymphocyte (CTL) response plays an important role in the control of respiratory syncytial virus (RSV) replication and the establishment of a Th1-CD4+ T cell response against the virus. Despite lacking Major Histocompatibility Complex I (MHC I)-restricted epitopes, the attachment G glycoprotein of RSV enhances CTL activity toward other RSV antigens, and this effect depends on its conserved central region. Here, we report that RSV-G can also improve CTL activity toward antigens from unrelated pathogens such as influenza, and that a mutant form of RSV-G lacking four conserved cysteine residues at positions 173, 176, 182, and 186 fails to enhance CTL responses. Our results indicate that these conserved residues are essential for the wide-spectrum pro-CTL activity displayed by the protein.

Antiviral activity of Nrf2 in a murine model of respiratory syncytial virus disease.

RATIONALE: Respiratory syncytial virus (RSV) is the most frequent cause of significant lower respiratory illness in infants and young children, but its pathogenesis is not fully understood. The transcription factor Nrf2 protects lungs from oxidative injury and inflammation via antioxidant response element (ARE)-mediated gene induction. OBJECTIVES: The current study was designed to determine the role of Nrf2-mediated cytoprotective mechanisms in murine airway RSV disease. METHODS: Nrf2-deficient (Nrf2(-/-)) and wild-type (Nrf2(+/+)) mice were intranasally instilled with RSV or vehicle. In a separate study, Nrf2(+/+) and Nrf2(-/-) mice were treated orally with sulforaphane (an Nrf2-ARE inducer) or phosphate-buffered saline before RSV infection. MEASUREMENTS AND MAIN RESULTS: RSV-induced bronchopulmonary inflammation, epithelial injury, and mucus cell metaplasia as well as nasal epithelial injury were significantly greater in Nrf2(-/-) mice than in Nrf2(+/+) mice. Compared with Nrf2(+/+) mice, significantly attenuated viral clearance and IFN-gamma, body weight loss, heightened protein/lipid oxidation, and AP-1/NF-kappaB activity along with suppressed antioxidant induction was found in Nrf2(-/-) mice in response to RSV. Sulforaphane pretreatment significantly limited lung RSV replication and virus-induced inflammation in Nrf2(+/+) but not in Nrf2(-/-) mice. CONCLUSIONS: The results of this study support an association of oxidant stress with RSV pathogenesis and a key role for the Nrf2-ARE pathway in host defense against RSV.

Breastfeeding is associated with the production of type I interferon in infants infected with influenza virus.

BACKGROUND: Breast milk-mediated protection against respiratory viruses is well established. However, protective mechanisms are unclear. Type I interferons (IFN) mediate host defence against respiratory viruses, particularly influenza virus. The relationship among type I IFN, respiratory viral infections and breastfeeding has not been explored. METHODS: Type I IFN responses were studied by ELISA and real time PCR in nasal secretions of infants experiencing their first respiratory infection. Modulation of IFN by breastfeeding and other variables affecting severity during viral infection was explored. RESULTS: One hundred and twenty infants were positive by RT-PCR for influenza virus (n = 24), human metapneumovirus (hMPV) (n = 30) or respiratory syncytial virus (RSV) (n = 66). Type I IFNs were detected more frequently in infants infected with influenza virus than in those infected with RSV or hMPV. Breastfeeding promoted higher rates and levels of type I IFN only in infants infected with influenza virus. No effect on IFN production was observed for age, gender or smoking. CONCLUSION: Our study confirms that type I IFN production is detected more frequently in infants infected with influenza virus. Importantly, higher rates and levels of type I IFN in these infants are associated with breastfeeding. These observations suggest that breast milk can protect against respiratory viruses by activating innate antiviral mechanisms in the host.

The Role of Heterotypic DENV-specific CD8+T Lymphocytes in an Immunocompetent Mouse Model of Secondary Dengue Virus Infection.

Dengue is the most prevalent arthropod-borne viral disease worldwide and is caused by the four dengue virus serotypes (DENV-1-4). Sequential heterologous DENV infections can be associated with severe disease manifestations. Here, we present an immunocompetent mouse model of secondary DENV infection using non mouse-adapted DENV strains to investigate the pathogenesis of severe dengue disease. C57BL/6 mice infected sequentially with DENV-1 (strain Puerto Rico/94) and DENV-2 (strain Tonga/74) developed low platelet counts, internal hemorrhages, and increase of liver enzymes. Cross-reactive CD8+ T lymphocytes were found to be necessary and sufficient for signs of severe disease by adoptively transferring of DENV-1-immune CD8+T lymphocytes before DENV-2 challenge. Disease signs were associated with production of tumor necrosis factor (TNF)-α and elevated cytotoxicity displayed by heterotypic anti-DENV-1 CD8+ T lymphocytes. These findings highlight the critical role of heterotypic anti-DENV CD8+ T lymphocytes in manifestations of severe dengue disease.

T helper type 2 bias and type 17 suppression in primary dengue virus infection in infants and young children.

BACKGROUND: The immune response to dengue virus (DENV) primary infection in infants and young children is not well characterized. In Northern Argentina, >90% of the population was DENV-naïve before the 2009 outbreak, allowing evaluation of age-dependent primary responses to infection. METHODS: We conducted a comparative study of the immune response to DENV in 27 infected infants, young children and their mothers. Lymphocyte T helper (Th) 1, Th2, Th17 and inflammatory responses were assayed in blood during the 2009 DENV-1 epidemic. RESULTS: The immune response to DENV-1 was significantly biased to Th2 in infected infants and young children, compared to infants with other febrile illnesses (for IL-4 p < 0.001) and to their infected mothers (for IL-4 p < 0.01). In addition, IL-17 suppression was observed in the memory response to DENV-1 in infected infants (p < 0.01 vs placebo). CONCLUSION: Age-related differences in the primary response to DENV, characterized by an immature Th2 polarization and Th17 suppression in infants, should be studied further in order to expand our understanding of the mechanism of dengue pathogenesis.

Severe pandemic 2009 H1N1 influenza disease due to pathogenic immune complexes.

Pandemic influenza viruses often cause severe disease in middle-aged adults without preexisting comorbidities. The mechanism of illness associated with severe disease in this age group is not well understood. Here we find preexisting serum antibodies that cross-react with, but do not protect against, 2009 H1N1 influenza virus in middle-aged adults. Nonprotective antibody is associated with immune complex-mediated disease after infection. We detected high titers of serum antibody of low avidity for H1-2009 antigen, and low-avidity pulmonary immune complexes against the same protein, in severely ill individuals. Moreover, C4d deposition--a marker of complement activation mediated by immune complexes--was present in lung sections of fatal cases. Archived lung sections from middle-aged adults with confirmed fatal influenza 1957 H2N2 infection revealed a similar mechanism of illness. These observations provide a previously unknown biological mechanism for the unusual age distribution of severe cases during influenza pandemics.

Lack of antibody affinity maturation due to poor Toll-like receptor stimulation leads to enhanced respiratory syncytial virus disease.

Respiratory syncytial virus (RSV) is a leading cause of hospitalization in infants. A formalin-inactivated RSV vaccine was used to immunize children and elicited nonprotective, pathogenic antibody. Immunized infants experienced increased morbidity after subsequent RSV exposure. No vaccine has been licensed since that time. A widely accepted hypothesis attributed the vaccine failure to formalin disruption of protective antigens. Here we show that the lack of protection was not due to alterations caused by formalin but instead to low antibody avidity for protective epitopes. Lack of antibody affinity maturation followed poor Toll-like receptor (TLR) stimulation. This study explains why the inactivated RSV vaccine did not protect the children and consequently led to severe disease, hampering vaccine development for 42 years. It also suggests that inactivated RSV vaccines may be rendered safe and effective by inclusion of TLR agonists in their formulation, and it identifies affinity maturation as a key factor for the safe immunization of infants.

Differential gender response to respiratory infections and to the protective effect of breast milk in preterm infants.

OBJECTIVE: The protective role of breastfeeding against severe acute lung disease in infants is well established, but its mechanism is unclear. Most hypotheses assume that breastfeeding confers similar passive protection to every infant; however, a few observations have suggested that the benefits of breast milk against severe lung disease may differ according to gender. The objective of this study was to determine whether the effect of breastfeeding on susceptibility to severe acute lung disease among infants at high risk is different for girls and boys. METHODS: A cohort was analyzed prospectively by use of 2 different strategies: (1) predictors of first episode of rehospitalization by univariate and multivariate analyses using robust Poisson regression and (2) mean number of rehospitalizations between groups using multiple regression negative binomial models. RESULTS: A total of 119 high-risk, very low birth weight infants were enrolled. Breast milk protected girls but not boys against severe acute lung disease. The interaction between breastfeeding and gender was clinically and statistically significant, even after adjustment for variables that can affect severity of acute lung disease. Disease was most severe in formula-fed girls (versus formula-fed boys). CONCLUSIONS: Breastfeeding decreased the risk for severe acute lung disease in girls but not in boys. These findings suggest that breast milk protection is not universally conferred by passive transfer of humoral immunity (which should be gender indifferent), show that respiratory symptoms may be amenable to nonspecific modulation, and identify nonbreastfed preterm infant girls as an at-risk group for severe acute lung disease.

Mapping and characterization of the primary and anamnestic H-2(d)-restricted cytotoxic T-lymphocyte response in mice against human metapneumovirus.

Cytotoxic T lymphocytes (CTLs) are important for the control of virus replication during respiratory infections. For human metapneumovirus (hMPV), an H-2(d)-restricted CTL epitope in the M2-2 protein has been described. In this study, we screened the hMPV F, G, N, M, M2-1, and M2-2 proteins using three independent algorithms to predict H-2(d) CTL epitopes in BALB/c mice. A dominant epitope (GYIDDNQSI) in positions 81 to 89 of the antitermination factor M2-1 and a subdominant epitope (SPKAGLLSL) in N(307-315) were detected during the anti-hMPV CTL response. Passive transfer of CD8(+) T-cell lines against M2-1(81-89) and N(307-315) protected Rag1(-/-) mice against hMPV challenge. Interestingly, diversification of CTL targets to include multiple epitopes was observed after repetitive infections. A subdominant response against the previously described M2-2 epitope was detected after the third infection. An understanding of the CTL response against hMPV is important for developing preventive and therapeutic strategies against the virus.

Characterization of human metapneumovirus infection of myeloid dendritic cells.

Recent in vivo studies suggest that hMPV is a poor inducer of inflammatory cytokines and that clinical symptoms may not be related to immune-mediated pathogenesis as it has been proposed for respiratory syncytial virus (RSV) and human parainfluenza 3 (HPF3). Dendritic cells (DCs) are specialized antigen presenting cells, and very effective at inducing specific CTLs after encountering invading viruses. Interactions of hMPV with DCs have not been characterized. We hypothesized that the relatively mild inflammatory responses observed in vivo after hMPV infection might be at least in part due to hMPV's poor ability to stimulate and activate DCs. hMPV actively infected immature monocyte-derived CD11c+/HLA-DR+ DCs. However, in contrast to RSV or HPF3, hMPV caused no gross cytopathic effects such as syncytia, lytic infection, or massive apoptosis. DCs exposed to hMPV show no cytopathic effects under tissue culture conditions permissive for viral replication. The surface maturation markers CD83 and CD86 were not significantly up-regulated in infected DCs as compared to uninfected controls, while expression of CD80 appeared increased. Stimulation of hMPV-infected DCs with LPS resulted in the enhanced expression of all these surface markers indicating that hMPV is not generally suppressing DC maturation. Overall, cytokine expression remained low. These results indicate that hMPV does not induce effective DC maturation in vitro and suggest that the weak stimulation of DCs may account for the overall low immunogenicity of this virus observed in vivo.

Cytokine profiles in the respiratory tract during primary infection with human metapneumovirus, respiratory syncytial virus, or influenza virus in infants.

OBJECTIVES: We characterized the T helper cytokine profiles in the respiratory tract of infants infected with influenza virus, human metapneumovirus, and respiratory syncytial virus to examine whether these agents elicit similar cytokine responses and whether T helper type 2 polarization is associated with wheezing and severe disease. METHODS: A prospective study of infants who were seeking medical help for acute upper and/or lower respiratory tract infection symptoms for the first time and were found to be infected with influenza, human metapneumovirus, or respiratory syncytial virus was performed. Respiratory viruses were detected in nasal secretions with reverse transcriptase-polymerase chain reaction assays. The study was performed in emergency departments and outpatient clinics in Buenos Aires, Argentina. T cell cytokine responses were determined in nasal secretions with immunoassays and reverse transcriptase-polymerase chain reaction assays. RESULTS: Influenza elicited higher levels of interferon-gamma, interleukin-4, and interleukin-2 than did the other agents. Human metapneumovirus had the lowest interferon-gamma/interleukin-4 ratio (T helper type 2 bias). However, no association was found between T helper type 2 bias and overall wheezing or hospitalization rates. CONCLUSIONS: These findings show that viral respiratory infections in infants elicit different cytokine responses and that the pathogeneses of these agents should be studied individually.

C5 modulates airway hyperreactivity and pulmonary eosinophilia during enhanced respiratory syncytial virus disease by decreasing C3a receptor expression.

Enhanced respiratory syncytial virus disease, a serious pulmonary disorder that affected recipients of an inactivated vaccine against respiratory syncytial virus in the 1960s, has delayed the development of vaccines against the virus. The enhanced disease was characterized by immune complex-mediated airway hyperreactivity and a severe pneumonia associated with pulmonary eosinophilia. In this paper, we show that complement factors contribute to enhanced-disease phenotypes. Mice with a targeted disruption of complement component C5 affected by the enhanced disease displayed enhanced airway reactivity, lung eosinophilia, and mucus production compared to wild-type mice and C5-deficient mice reconstituted with C5. C3aR expression in bronchial epithelial and smooth muscle cells in the lungs of C5-deficient mice was enhanced compared to that in wild-type and reconstituted rodents. Treatment of C5-deficient mice with a C3aR antagonist significantly attenuated airway reactivity, eosinophilia, and mucus production. These results indicate that C5 plays a crucial role in modulating the enhanced-disease phenotype, by affecting expression of C3aR in the lungs. These findings reveal a novel autoregulatory mechanism for the complement cascade that affects the innate and adaptive immune responses.

The cysteine-rich region and secreted form of the attachment G glycoprotein of respiratory syncytial virus enhance the cytotoxic T-lymphocyte response despite lacking major histocompatibility complex class I-restricted epitopes.

The cytotoxic T-lymphocyte (CTL) response is important for the control of viral replication during respiratory syncytial virus (RSV) infection. The attachment glycoprotein (G) of RSV does not encode major histocompatibility complex class I-restricted epitopes in BALB/c mice (H-2(d)). Furthermore, studies to date have described an absence of significant CTL activity directed against this protein in humans. Therefore, G previously was not considered necessary for the generation of RSV-specific CTL responses. In this study, we demonstrate that, despite lacking H-2(d)-restricted epitopes, G enhances the generation of an effective CTL response against RSV. Furthermore, we show that this stimulatory effect is independent of virus titers and RSV-induced inflammation; that it is associated primarily with the secreted form of G; and that the effect depends on the cysteine-rich region of G (GCRR), a segment conserved in wild-type isolates worldwide. These findings reveal a novel function for the GCRR with potential implications for the generation of protective cellular responses and vaccine development.

The impact of infection with human metapneumovirus and other respiratory viruses in young infants and children at high risk for severe pulmonary disease.

We conducted a prospective, observational study to characterize the clinical manifestations of respiratory infections caused by human metapneumovirus (hMPV) and other viruses in 194 premature infants and young children with chronic lung disease or congenital heart disease in Buenos Aires. Children had 567 episodes of respiratory illness and were monitored until they were 2 years old or until the completion of the study. hMPV elicited 12 infections (2%) year-round; 30% were of moderate or greater severity. Human parainfluenza virus type 3 caused 24 infections (4%), and 5 (25%) of 20 lung infections led to hospitalization. Respiratory syncytial virus (RSV) caused 33 episodes--17% of infections and 32% of hospitalizations during the respiratory season. None of the 10 children infected with influenza virus had severe disease. The present study of at-risk children suggests that hMPV and influenza virus are infrequent agents of severe disease and highlights the need for preventive interventions against RSV in developing countries.

The cysteine-rich region of respiratory syncytial virus attachment protein inhibits innate immunity elicited by the virus and endotoxin.

The attachment protein (glycoprotein) of respiratory syncytial virus (RSV) has long been associated with disease potentiation and respiratory symptoms. The glycoprotein has a conserved cysteine-rich region (GCRR) whose function is unknown and which is not necessary for efficient viral replication. In this report, we show that the GCRR is a powerful inhibitor of the innate immune response against RSV, and that early secretion of glycoprotein is critical to modulate inflammation after RSV infection. Importantly, the GCRR is also a potent inhibitor of cytokine production mediated by several TLR agonists, indicating that this peptide sequence displays broad antiinflammatory properties. These findings have important implications for RSV pathogenesis and describe an inhibitor of TLR-mediated inflammatory responses that could have clinical applications.