Antibody-dependent alternate pathway of complement activation in opsonophagocytosis of Porphyromonas gingivalis

It has been suggested that the ability of Porphyromonas gingivalis to proteolyse complement, as well as its production of a capsule, contributes to resistance to phagocytosis by polymorphonuclear leukocytes. In this report, the opsonic role of serum complement and its activation pathways were investigated, using individual sera heat treated or depleted of factors B, C2, and C1q and the divalent cations Mg2+ and Ca2+. A fluorochrome microassay was used to quantitate phagocytosis of P. gingivalis A7436 by human polymorphonuclear leukocytes. Heat treatment of rabbit antiserum to P. gingivalis (RaPg) (56 degrees C, 30 min) resulted in a reduction in phagocytosis from 100% to 55% +/- 5%, while heat treatment of chronic adult periodontal disease serum abrogated phagocytosis. The heat-labile activity of RaPg was fully restored with MgEGTA-chelated rabbit serum but not EDTA- or EGTA-chelated rabbit serum. The addition of serum depleted of factor B but not C2 or C1q restored most of the heat-labile activity; however, the factor B-depleted serum was suspect, due to low-level opsonization of zymosan (inhibitable by EDTA but not MgEGTA). Adding C1q at 80 micrograms/ml to serum depleted of C1q restored much but not all of the activity lost through heat treatment or through depletion of C1q. A large part of opsonic activity with C2- and C1q-depleted sera was enhanced by the addition of 4 x 10(-3) M Mg2+. The data indicate that although opsonophagocytosis of P. gingivalis A7436 is dependent on the classical complement pathway, a significant contribution is made by an antibody-dependent alternate pathway.