Resistance of highly pathogenic Naegleria fowleri amoebae to complement-mediated lysis.

Weakly pathogenic and nonpathogenic Naegleria spp. are readily lysed by human and guinea pig complement. Highly pathogenic Naegleria fowleri are resistant to complement-mediated lysis. Electrophoretic analysis of normal human serum (NHS) incubated with pathogenic or nonpathogenic Naegleria spp. demonstrates that amoebae activate the complement cascade, resulting in the production of C3 and C5 complement cleavage products. To determine whether surface constituents play a role in resistance to complement lysis, trophozoites of Naegleria spp. were subjected to enzymatic treatments prior to incubation in NHS. Treatment of trophozoites with papain or trypsin for 1 h, but not with neuraminidase, increased susceptibility of highly pathogenic Naegleria fowleri to complement lysis. Treatment of trophozoites with actinomycin D or cycloheximide during incubation with NHS or pretreatment with various protease inhibitors for 4 h did not increase the susceptibility of N. fowleri amoebae to lysis. Neither a repair process involving de novo protein synthesis nor a complement-inactivating protease appears to account for the increased resistance of N. fowleri amoebae to complement-mediated lysis.