Interaction of fluid phase C1/C1q and macrophage membrane-associated C1q with gram-negative bacteria

Many gram-negative bacteria are killed after treatment with normal non-immune sera and directly bind and activate C1 in the absence of antibodies. For the immediate killing of such serum-sensitive bacteria, like R-forms of Salmonella strains, all serum complement components are essential. When purified serum C1 to C9 are used, further activation of the cascade requires an additional serum factor. This glycoprotein differs from antibody and mediates the attachment of C4b to the bacterial cell surface. The antibody-independent interaction with C1 occurs via C1q, which binds to LPS. In addition outer membrane proteins bind C1q and C1. The association of these porins with LPS may potentiate the antibody-independent C1q and C1 binding to serum-sensitive bacteria. Porins can contribute to complement activation mainly through the classical pathway. LPS and porins from bacterial cell walls are also involved in the binding of gram-negative bacteria to macrophages. This antibody-independent attachment and ingestion of gram-negative bacteria is mediated by endogenous macrophage-membrane associated C1q.