Immunoglobulin M-dependent classical complement pathway activation in killing of Pentatrichomonas hominis.

Complement pathway activity in the killing of Pentatrichomonas hominis was investigated in this study. At 10(5) organisms per ml, P. hominis was completely killed by the presence of 1% normal human serum. In contrast, no killing effect on P. hominis was observed when specific antibodies were absorbed or when the complement was destroyed. Moreover, Mg2+-ethylene glycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-treated serum had no killing effect on P. hominis, while serum heated at 50 degrees C or treated with zymosan killed P. hominis as well as did normal human serum. Further study using gel filtration (Sephacryl S-300) and affinity chromatography (protein A) revealed that immunoglobulin M (IgM; 20 micrograms/ml) alone was responsible for the complement activation in the killing of P. hominis, but both IgA (24 micrograms/ml) and IgG (180 micrograms/ml) had no effect on complement-mediated lysis. On the other hand, IgG at 1,260 micrograms/ml completely inhibited complement-mediated killing by IgM, suggesting that a blocking factor is present in IgG. The results of this study indicate that a mechanism of IgM-dependent classical complement pathway activation contributes to the killing effect of normal human serum on P. hominis.