Activation of the alternative complement pathway by Trichomonas vaginalis.

The mechanism underlying the lysis of Trichomonas vaginalis by normal human or guinea pig serum was investigated. The involvement of the complement system was demonstrated by the failure of human serum deficient in C3 or C8 to mediate parasite killing and by the ablation of lytic activity observed when fresh sera were heated at 56 degrees C or treated with ethylenediaminetetraacetate. Fixation of human C3 on the parasite surface was demonstrated by indirect immunofluorescence. The involvement of the alternative complement pathway was demonstrated (i) by the inability of properdin-depleted human serum to lyse T. vaginalis and (ii) by the normal killing observed with guinea pig serum lacking C4 and with normal human or guinea pig serum treated with ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid and Mg2+ to selectively inhibit the classical pathway.