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Enhancement of lectin pathway haemolysis by immunoglobulins

We recently reported that indicator sheep erythrocytes (E) coated with mannan and sensitized with mannan‐binding lectin (MBL) (E‐M‐MBL) are lysed by human serum in the absence of calcium via the lectin pathway of complement activation by a process which requires alternative pathway amplification and is associated with increased binding of and control by complement regulatory proteins C4 bp and factor H. In the present study, we investigated the effect of immunoglobulin (Ig) on this haemolysis. Co‐sensitization of indicator E with anti‐E haemolysin led to threefold enhancement of lectin pathway haemolysis in the absence of calcium, associated with increased binding of C3 and C5. Lysis was enhanced approximately twofold when E‐M‐MBL were chemically or immunologically coated with IgM or IgA, and fourfold when coated with IgG, prior to lysis in human serum‐Mg‐ethyleneglycol tetraacetic acid. The presence of haemolysin did not reduce the binding or inhibitory activity of C4 bp, and the enhancing activity of haemolysin was retained in serum depleted of C4 bp. By contrast, binding of factor H was greatly reduced in the presence of haemolysin, which had no enhancing effect in serum depleted of factor H. These experiments demonstrate the ability of IgG, IgM and IgA to enhance lectin pathway cytolysis, and that this enhancement occurs by neutralization of the inhibitory activity of factor H. Immunoglobulin enhancement of lectin pathway cytolysis represents another interaction between the innate and adaptive systems of immunity.