THE ALTERNATIVE COMPLEMENT PATHWAY PROMOTES IGM ANTIBODY-DEPENDENT AND ANTIBODY-INDEPENDENT ADHERENCE OF BACTEROIDES TO POLYMORPHONUCLEAR LEUKOCYTES THROUGH CR3 AND CR-1

Natural immunoglobulin M (IgM) antibodies act synergistically with the alternative complement pathway to promote opsonization and adherence of Bacteroides thetaiotaomicron and Bacteroides fragilis to polymorpho nuclear leukocytes (PMNs). This study characterizes the PMN receptors involved in adherence of Bacteroides opsonized by the alternative path way and determines the effect of natural IgM antibodies on receptor in volvement and on the molecular form of C3 deposited on the bacteria. A model system consisting of the six isolated proteins of the alternati ve pathway with or without supplemental isolated normal IgM was used f or opsonization, and results were compared to those obtained with seru m. The results demonstrate that the alternative pathway promotes adher ence of Bacteroides to PMNs through complement receptors 1 and 3 (CR3) , and epitopes in CR3 besides those mediating iC3b binding do not part icipate. The results also demonstrate that natural IgM antibodies do n ot influence the character of the ligands or receptors involved in thi s interaction.