Susceptibility of Giardia lamblia trophozoites to the lethal effect of human serum.

To define the potential role of complement and antibody in host defense against Giardia lamblia, the effect of human serum on axenically cultured G. lamblia trophozoites was studied. Sera from patients without a history of giardiasis and with no detectable antibody by an indirect immunofluorescence antibody (IFA) assay (IFA titers less than 1:2) killed from 8 to 76% of trophozoites (n = 23 sera). Whereas less than 10% of parasites incubated in phosphate-buffered saline alone were killed, 16 sera killed from 10 to 25% and six sera killed from 25 to 75%. One serum with an anti-G lamblia antibody titer of 1:128 killed greater than 98% of the parasites. The complement dependency of killing was demonstrated by abrogation of the lethal effect when serum was chelated with EDTA (7 mM) or was heat-inactivated (56 degrees C, 30 min), and, in an immunofluorescence assay, by detection of C3 on parasites killed by normal serum. When the classical complement pathway was selectively blocked by using serum chelated with Mg+2 (2 mM)-EGTA (8 mM), or serum congenitally deficient for the second component of complement, there was no killing; thus, killing was dependent on the presence of an intact classical pathway. In each of three sera from donors with negative IFA titers, an absorbable factor specific for G. lamblia, possibly antibody not detected by IFA, was required for classical pathway activation. To determine if alteration of the surface of G. lamblia would render it an activator of the alternative pathway of complement, trophozoites were studied after cell death or after treatment with neuraminidase or trypsin. In MgEGTA-chelated serum, dead trophozoites activated the alternative pathway as determined by consumption of Factor B and deposition of C3 on their surface. In contrast, untreated or enzyme-treated living parasites did not activate the alternative pathway.