Abstract
The pandemic influenza A(H1N1) 2009 virus caused significant morbidity and mortality worldwide thus necessitating the need to understand the host factors that influence its control. Previously, the complement system has been shown to provide protection during the seasonal influenza virus infection, however, the role of individual complement pathways is not yet clear. Here, we have dissected the role of intact complement as well as of its individual activation pathways during the pandemic influenza virus infection using mouse strains deficient in various complement components. We show that the virus infection in C3-/- mice results in increased viral load and 100% mortality, which can be reversed by adoptive transfer of naïve wild-type (WT) splenocytes, purified splenic B cells, or passive transfer of immune sera from WT, but not C3-/- mice. Blocking of C3a and/or C5a receptor signaling in WT mice using receptor antagonists and use of C3aR-/- and C5aR-/- mice showed significant mortality after blocking/ablation of C3aR, with little or no effect after blocking/ablation of C5aR. Intriguingly, deficiency of C4 and FB in mice resulted in only partial mortality (24%-32%) suggesting a necessary cross-talk between the classical/lectin and alternative pathways for providing effective protection. In vitro virus neutralization experiments performed to probe the cross-talk between the various pathways indicated that activation of the classical and alternative pathways in concert, owing to coating of viral surface by antibodies, is needed for its efficient neutralization. Examination of the virus-specific complement-binding antibodies in virus positive subjects showed that their levels vary among individuals. Together these results indicate that cooperation between the classical and alternative pathways not only result in efficient direct neutralization of the pandemic influenza virus, but also lead to the optimum generation of C3a, which when sensed by the immune cells along with the antigen culminates in generation of effective protective immune responses.
Highlights
Influenza viruses, the members of the family Orthomyxoviridae, are globally important acute respiratory pathogens known to cause significant morbidity and mortality [1,2,3]
We demonstrate that presence of intact complement is essential for clearing the pandemic influenza virus infection, wherein complement synthesized by B cells plays a major role
We show that activation of the classical as well as alternative pathways is a requisite for efficient neutralization of the virus as well as the optimum generation of C3a, which is necessary for boosting the protective immune responses
Summary
The members of the family Orthomyxoviridae, are globally important acute respiratory pathogens known to cause significant morbidity and mortality [1,2,3] These negative sense RNA viruses possess a segmented genome encompassing a constellation of genes, which facilitate genetic reassortment upon infection with more than one influenza A virus strains [4,5,6]. This may result in generation of a novel virus with the ability to cause pandemics in humans [3]. Identification of factors influencing the pathogenesis and control of the pandemic influenza virus is essential
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