IMMOBILIZATION OF SCHISTOSOMA-MANSONI MIRACIDIA BY ACTIVATION OF THE ALTERNATE COMPLEMENT PATHWAY AT UNUSUALLY HIGH SERUM DILUTION

Free-swimming Schistosoma mansoni miracidia were immobilized by adding normal mammalian serum to the water. Miracidial immobilizing activity (MIA) was shown to result from activating the alternate pathway of co mplement (APC). MIA in normal sera was heat-sensitive and anibody inde pendent; it was greatly reduced in factor B-depleted or C6-depleted, b ut not in Cl-depleted, human serum. Addition of purified factor B to B -depleted serum totally restored MIA. Half-maximal MIA in normal human , rabbit, and guinea pig sera was detectable at final dilutions exceed ing 1/100, 1/200, and 1/500, respectively, and normal rat serum was pa rticularly potent, with MIA at dilutions exceeding 1/2000. Detection o f APC activity at such high dilutions is quite extraordinary and attri buted to the hypotonic conditions. We confirmed and extended previous findings that heat-inactivated infection sera also display MIA. Immobi lizing activity in irradiated-cercarial vaccine rat serum cofractionat ed with rat IgG and anti-SWAP antibody activity. Antibody-dependent MI A titres were much lower than for APC-dependent MIA. Based upon light microscope and transmission EM studies, immobilization of miracidia by APC activation was attributed to severe tegumental damage. Miracidia within egg shells were insensitive to MIA.