Human IgA1 initiates complement-mediated killing of Neisseria meningitidis

We studied the effect of human IgA1, the predominant IgA subclass in serum, on C-mediated killing of Neisseria meningitidis. We purified monomeric IgA1 from normal human serum and tetravalent meningococcal polysaccharide vaccinate serum by using the following successive chromatographic steps: jacalin lectin affinity, Superose 12 FPLC gel filtration, Mono Q FPLC anion exchange, and anti-IgG affinity. SDS-PAGE, ELISA, and Western immunoblot analyses of the IgA1 detected no trace of contaminating IgG or IgM. IgA1 initiated partial or complete lysis (62 to 100%) of nine group C strains by using either normal, hypogammaglobulinemic, factor B-depleted, or properdin-deficient human serum as a C source, but IgA1 was unable to effect killing in serum chelated with 10 mM MgCl2 and 10 mM EGTA. Lytic activity was dependent on the group C strain and the source of the IgA1; neither IgA1 preparation was bactericidal for all nine strains. Removal of the Fc portion of IgA1 with pepsin completely abolished bactericidal activity. We purified and radiolabeled C component C3, and found that IgA1 did not increase C3 deposition. With the use of a group C polysaccharide ELISA, we found that the vaccinate IgA1 had a high titer of group C polysaccharide antibody, whereas the IgA1 purified from normal human serum had no detectable group C polysaccharide specificity. Absorption of the vaccinate IgA1 with alum-bound group C polysaccharide did not affect the killing of a sensitive strain, but it did potentiate the killing of a previously resistant strain. Western immunoblots of whole cell lysates, outer membrane complex, and purified lipooligosaccharide showed that the bactericidal IgA1 was specific for several outer membrane proteins. Four of the proteins recognized by both IgA1 preparations had apparent Mr of 29, 42, 66, and 74 kDa. We conclude that IgA1, when bound to specific outer membrane proteins, can initiate lysis of group C meningococci via the classical C pathway, and that initiation of lysis is an Fc-dependent event which occurs without an increase in C3 deposition.