Marine L. B. Hillaire

Cross-protective immunity against influenza pH1N1 2009 viruses induced by seasonal influenza A (H3N2) virus is mediated by virus-specific T-cells.

Influenza A (H1N1) viruses of swine origin were introduced into the human population in 2009 and caused a pandemic. The disease burden in the elderly was relatively low, which was attributed to the presence of cross-reacting serum antibodies in this age group, which were raised against seasonal influenza A (H1N1) viruses that circulated before 1957. It has also been described how infection with heterosubtypic influenza viruses can induce some degree of protection against infection by a novel strain of influenza virus. Here, we assess the extent of protective immunity against infection with the 2009 influenza A (H1N1) pandemic influenza virus that is afforded by infection with a seasonal influenza A (H3N2) virus in mice. Mice that experienced a primary A (H3N2) influenza virus infection displayed reduced weight loss after challenge infection and cleared the 2009 influenza A (H1N1) virus infection more rapidly. To elucidate the correlates of protection of this heterosubtypic immunity to pandemic H1N1 virus infection, adoptive transfer experiments were carried out by using selected post-infection lymphocyte populations. Virus-specific CD8(+) T-cells in concert with CD4(+) T-cells were responsible for the observed protection. These findings may not only provide an explanation for epidemiological differences in the incidence of severe pandemic H1N1 infections, they also indicate that the induction of cross-reactive virus-specific CD8(+) and CD4(+) T-cell responses may be a suitable approach for the development of universal influenza vaccines.

Induction of virus-specific cytotoxic T lymphocytes as a basis for the development of broadly protective influenza vaccines

There is considerable interest in the development of broadly protective influenza vaccines because of the continuous emergence of antigenic drift variants of seasonal influenza viruses and the threat posed by the emergence of antigenically distinct pandemic influenza viruses. It has been recognized more than three decades ago that influenza A virus-specific cytotoxic T lymphocytes recognize epitopes located in the relatively conserved proteins like the nucleoprotein and that they cross-react with various subtypes of influenza A viruses. This implies that these CD8+ T lymphocytes may contribute to protective heterosubtypic immunity induced by antecedent influenza A virus infections. In the present paper, we review the evidence for the role of virus-specific CD8+ T lymphocytes in protective immunity against influenza virus infections and discuss vaccination strategies that aim at the induction of cross-reactive virus-specific T-cell responses.

Human Cytotoxic T Lymphocytes Directed to Seasonal Influenza A Viruses Cross-React with the Newly Emerging H7N9 Virus

ABSTRACT In February 2013, zoonotic transmission of a novel influenza A virus of the H7N9 subtype was reported in China. Although at present no sustained human-to-human transmission has been reported, a pandemic outbreak of this H7N9 virus is feared. Since neutralizing antibodies to the hemagglutinin (HA) globular head domain of the virus are virtually absent in the human population, there is interest in identifying other correlates of protection, such as cross-reactive CD8+ T cells (cytotoxic T lymphocytes [CTLs]) elicited during seasonal influenza A virus infections. These virus-specific CD8+ T cells are known to recognize conserved internal proteins of influenza A viruses predominantly, but it is unknown to what extent they cross-react with the newly emerging H7N9 virus. Here, we assessed the cross-reactivity of seasonal H3N2 and H1N1 and pandemic H1N1 influenza A virus-specific polyclonal CD8+ T cells, obtained from HLA-typed study subjects, with the novel H7N9 virus. The cross-reactivity of CD8+ T cells to H7N9 variants of known influenza A virus epitopes and H7N9 virus-infected cells was determined by their gamma interferon (IFN-γ) response and lytic activity. It was concluded that, apart from recognition of individual H7N9 variant epitopes, CD8+ T cells to seasonal influenza viruses display considerable cross-reactivity with the novel H7N9 virus. The presence of these cross-reactive CD8+ T cells may afford some protection against infection with the new virus.

Vaccination with whole inactivated virus vaccine affects the induction of heterosubtypic immunity against influenza virus A/H5N1 and immunodominance of virus-specific CD8 + T-cell responses in mice

It was recently shown that the use of an experimental subunit vaccine protected mice against infection with a human A/H3N2 influenza virus, but consequently affected the induction of heterosubtypic immunity to a highly pathogenic A/H5N1 influenza virus, which was otherwise induced by the A/H3N2 infection. As whole inactivated virus (WIV) vaccines are widely used to protect against seasonal influenza and also contain inner viral proteins such as the nucleoprotein (NP), the potential of a WIV vaccine to induce protective immunity against infection was tested with a homologous A/H3N2 (A/Hong Kong/2/68) and a heterosubtypic A/H5N1 influenza virus (A/Indonesia/5/05). As expected, the vaccine afforded protection against infection with the A/H3N2 virus only. In addition, it was demonstrated that the use of WIV vaccine for protection against A/H3N2 infection affected the induction of heterosubtypic immunity that was otherwise afforded by A/H3N2 influenza virus infection. The reduction in protective immunity correlated with changes in the immunodominance patterns of the CD8(+) T-cell responses directed to the epitopes located in the acid polymerase subunit of the viral RNA polymerase (PA(224-233)) and the NP (NP(366-374)). In unvaccinated mice that experienced infection with the A/H3N2 influenza virus, the magnitude of the CD8(+) T-cell response to both peptides was similar on secondary infection with A/H5N1 influenza virus. In contrast, prior vaccination with WIV affected the immunodominance pattern and skewed the response after infection with influenza virus A/Indonesia/5/05 towards a dominant NP(366-374)-specific response. These findings may have implications for vaccination strategies aimed at the induction of protective immunity to seasonal and/or pandemic influenza.

Human T-cells directed to seasonal influenza A virus cross-react with 2009 pandemic influenza A (H1N1) and swine-origin triple-reassortant H3N2 influenza viruses

Virus-specific CD8(+) T-cells contribute to protective immunity against influenza A virus (IAV) infections. As the majority of these cells are directed to conserved viral proteins, they may afford protection against IAVs of various subtypes. The present study assessed the cross-reactivity of human CD8(+) T-lymphocytes, induced by infection with seasonal A (H1N1) or A (H3N2) influenza virus, with 2009 pandemic influenza A (H1N1) virus [A(H1N1)pdm09] and swine-origin triple-reassortant A (H3N2) [A(H3N2)v] viruses that are currently causing an increasing number of human cases in the USA. It was demonstrated that CD8(+) T-cells induced after seasonal IAV infections exerted lytic activity and produced gamma interferon upon in vitro restimulation with A(H1N1)pdm09 and A(H3N2)v influenza A viruses. Furthermore, CD8(+) T-cells directed to A(H1N1)pdm09 virus displayed a high degree of cross-reactivity with A(H3N2)v viruses. It was concluded that cross-reacting T-cells had the potential to afford protective immunity against A(H1N1)pdm09 viruses during the pandemic and offer some degree of protection against infection with A(H3N2)v viruses.

Characterization of the human CD8+ T cell response following infection with 2009 pandemic influenza H1N1 virus

ABSTRACT The 2009 H1N1 influenza pandemic provided an opportunity to study human virus-specific T cell responses after infection with a novel influenza virus against which limited humoral immunity existed in the population. Here we describe the magnitude, kinetics, and nature of the virus-specific T cell response using intracellular gamma interferon (IFN-γ) staining and fluorochrome-labeled major histocompatibility complex (MHC) class I-peptide complexes. We demonstrate that influenza virus-infected patients develop recall T cell responses that peak within 1 week postinfection and that contract rapidly. In particular, effector cell frequencies declined rapidly postinfection in favor of relatively larger proportions of central memory cells.

Pulmonary Surfactant Protein D in First-Line Innate Defence against Influenza A Virus Infections

Influenza A viruses (IAV) cause respiratory tract infections annually associated with excess mortality and morbidity. Nonspecific, innate immune mechanisms play a key role in protection against viral invasion at early stages of infection. A soluble protein present in mucosal secretions of the lung, surfactant protein D (SP-D), is an important component of this initial barrier that helps to prevent and limit IAV infections of the respiratory epithelium. This collagenous C-type lectin binds IAVs and thereby inhibits attachment and entry of the virus but also contributes to enhanced clearance of SP-D-opsonized virus via interactions with phagocytic cells. In addition, SP-D modulates the inflammatory response and helps to maintain a balance between effective neutralization/killing of IAV, and protection against alveolar damage resulting from IAV-induced excessive inflammatory responses. The mechanisms of interaction between SP-D and IAV not only depend on the structure and binding properties of SP-D but also on strain-specific features of IAV, and both issues will be discussed. SP-D from pigs exhibits distinct anti-IAV properties and is discussed in more detail. Finally, the potential of SP-D as a prophylactic and/or therapeutic antiviral agent to protect humans against infections by IAV is discussed.

Assessment of the antiviral properties of recombinant porcine SP-D against various influenza A viruses in vitro.

The emergence of influenza viruses resistant to existing classes of antiviral drugs raises concern and there is a need for novel antiviral agents that could be used therapeutically or prophylacticaly. Surfactant protein D (SP-D) belongs to the family of C-type lectins which are important effector molecules of the innate immune system with activity against bacteria and viruses, including influenza viruses. In the present study we evaluated the potential of recombinant porcine SP-D as an antiviral agent against influenza A viruses (IAVs) in vitro. To determine the range of antiviral activity, thirty IAVs of the subtypes H1N1, H3N2 and H5N1 that originated from birds, pigs and humans were selected and tested for their sensitivity to recombinant SP-D. Using these viruses it was shown by hemagglutination inhibition assay, that recombinant porcine SP-D was more potent than recombinant human SP-D and that especially higher order oligomeric forms of SP-D had the strongest antiviral activity. Porcine SP-D was active against a broad range of IAV strains and neutralized a variety of H1N1 and H3N2 IAVs, including 2009 pandemic H1N1 viruses. Using tissue sections of ferret and human trachea, we demonstrated that recombinant porcine SP-D prevented attachment of human seasonal H1N1 and H3N2 virus to receptors on epithelial cells of the upper respiratory tract. It was concluded that recombinant porcine SP-D holds promise as a novel antiviral agent against influenza and further development and evaluation in vivo seems warranted.

Human influenza A virus-specific CD8+ T cell response is long-lived

Animal and human studies have demonstrated the importance of influenza A virus (IAV)-specific CD8(+) cytotoxic T lymphocytes (CTLs) in heterosubtypic cross-protective immunity. Using peripheral blood mononuclear cells obtained intermittently from healthy HLA-typed blood donors between 1999 and 2012, we were able to demonstrate that IAV-specific CTLs are long-lived. Intercurrent IAV infections transiently increase the frequency of functionally distinct subsets of IAV-specific CTLs, in particular effector and effector memory T cells.

Clearance of influenza virus infections by T cells: risk of collateral damage?

Influenza A viruses are a major cause of respiratory infections in humans. To protect against influenza, vaccines mainly aim at the induction of antibodies against the two surface proteins and do not protect against influenza A viruses from other subtypes. There is an increasing interest in heterosubtypic immunity that does protect against different subtypes. CD8 and CD4 T cells have a beneficial effect on the course of influenza A virus infection and can recognize conserved IAV epitopes. The T cell responses are tightly regulated to avoid collateral damage due to overreaction. Different studies have shown that an aberrant T cell response to an influenza virus infection could be harmful and could contribute to immunopathology. Here we discuss the recent findings on the balance between the beneficial and detrimental effects of T cell responses in influenza virus infections.