Synergy between the classical and alternative pathways of complement is essential for conferring effective protection against the pandemic influenza A(H1N1) 2009 virus infection

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.

Complement Evasion Strategies of Viruses: An Overview

“…Thus, it can be envisaged that the acquired CD59 would protect the virus from the MAC-mediated lysis. However, it should be mentioned here that multiple reports have now established that coating of the INFLV surface with C3b is enough for its neutralization (Jayasekera et al, 2007;Rattan et al, 2017). Therefore, though recruited CD59 would protect it from lysis, it would not shield it from neutralization.…”

The protective potential of Fc‐mediated antibody functions against influenza virus and other viral pathogens

“…Influenza virus subverts complement by blocking the interaction of C1q with antibodies (via M1 protein) and acquiring a membrane attack complex‐inhibitory protein (CD59) in its envelope during budding . Further, human C3b is not efficiently deposited onto the surface of H1N1 2009 pandemic A/California/07/2009 [A(H1N1)pdm09] influenza virions in vitro . Thus, the A(H1N1)pdm09 virus is only susceptible to classical and alternative complement pathways when bound by influenza‐specific antibodies .…”

“…Further, human C3b is not efficiently deposited onto the surface of H1N1 2009 pandemic A/California/07/2009 [A(H1N1)pdm09] influenza virions in vitro . Thus, the A(H1N1)pdm09 virus is only susceptible to classical and alternative complement pathways when bound by influenza‐specific antibodies . Pre‐existing antibodies with complement‐dependent lysis activity correlate with protection against clinical A(H1N1)pdm09 influenza in children, independently of ADCC …”

Non-neutralizing Antibodies Directed at Conservative Influenza Antigens

“…In 1983, it was established that human serum contains antibodies capable of neutralizing the influenza virus by activating the classical complement pathway [45]. To date it is known that influenza virions can activate both the classical and alternative complement pathways, and that antibody opsonization is required for efficient lysis of virions [46].…”