Complement pathway activity in the opsonization of mutant strains of Salmonella typhimurium

Two separate complement pathway activities and their interaction in the opsonization of mutant strains of Salmonella typhimurium by human neutrophils were investigated in 1% and 10% sera. In the absence of antibody, all mutant strains of S. typhimurium could activate complement by either the classical or the alternative pathway. The presence of antibody did not alter alternative pathway activity but enhanced the classical pathway activity, especially for those strains possessing intact core structure and in the presence of 10% serum. This effect of antibody on classical pathway activation decreased as the lipopolysaccharide chain length decreased. The finding that the presence of antibody reduced the classical pathway activity in the opsonization of SL 1032 (Rd1) strain in 10% serum was unexpected. This suggests that if complement activity in the absence of antibody is already marked, the presence of antibody cannot increase it further. Inhibition was also found in the interaction between classical and alternative pathways. For SL 1032 (Rd1) strain, in the absence of antibody, either classical or alternative pathway activity alone could be maximally activated in 10% serum and when present together were in competition with each other. The interaction, either enhancement or inhibition, was not only dependent on the individual strain but also on the concentration of serum. In general, enhancement occurred in the presence of 1% serum while inhibition was observed in the presence of 10% serum. Such antagonistic interaction has not previously been reported.