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.