PLASMODIUM-GALLINACEUM - DIFFERENTIAL LYSIS OF 2 DEVELOPMENTAL STAGESOF MALARIA SPOROZOITES BY THE ALTERNATIVE PATHWAY OF COMPLEMENT

During sporogonic development of Plasmodium gallinaceum in the mosquit o vector, two developmentally distinct sporozoite stages can be isolat ed. Sporozoites obtained from oocysts in abdomens of mosquitoes 10 day s after an infective blood meal are poorly infectious to the vertebrat e host (chicken); days later, sporozoites isolated from mosquito saliv ary glands are highly infectious. In a first step toward understanding the physiologic basis of this developmentally regulated infectivity t o the vertebrate host, we determined the relative resistance of the tw o sporozoite stages to lysis by the complement system of the vertebrat e host. Whereas 86% of oocyst sporozoites were lysed when incubated in fresh chicken serum in vitro, only 24% of salivary gland sporozoites were lysed under identical incubation conditions. Preincubation of ooc yst sporozoites in a homogenate of female Aedes aegypti salivary gland s did not diminish their susceptibility to lysis by serum, indicating that lysis was mediated by the interaction of serum factors with paras ite-specific molecules. The lytic activity of fresh chicken serum was abrogated by incubating for 45 min at 56 degrees C or chelating with E DTA. Fresh chicken serum specifically depleted of Ca2+, by chelating w ith EGTA in the presence of Mg2+, retained its ability to lyse oocyst sporozoites. The lysis of salivary gland sporozoites mediated by chick en antisporozoite serum is abrogated by EGTA, indicating that the anti body-dependent complement pathway in chicken serum is blocked by EGTA. Because oocyst sporozoite lysis by serum was heat sensitive and Mg2dependent, but Ca2+ independent, we conclude that lysis was mediated b y the alternative pathway of complement. One factor in the development from noninfectious oocyst sporozoites to infectious salivary gland sp orozoites is the ability to resist lysis by the alternative pathway of complement. (C) 1994 Academic Press,Inc.