Plasmodium gallinaceum: Differential Lysis of Two Developmental Stages of Malaria Sporozoites by the Alternative Pathway of Complement

Abstract

During sporogonic development of Plasmodium gallinaceum in the mosquito vector, two developmentally distinct sporozoite stages can be isolated. Sporozoites obtained from oocysts in abdomens of mosquitoes 10 days after an infective blood meal are poorly infectious to the vertebrate host (chicken); days later, sporozoites isolated from mosquito salivary glands are highly infectious. In a first step toward understanding the physiologic basis of this developmentally regulated infectivity to the vertebrate host, we determined the relative resistance of the two sporozoite stages to lysis by the complement system of the vertebrate 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 oocyst sporozoites in a homogenate of female Aedes aegypti salivary glands did not diminish their susceptibility to lysis by serum, indicating that lysis was mediated by the interaction of serum factors with parasite-specific molecules. The lytic activity of fresh chicken serum was abrogated by incubating for 45 min at 56°C or chelating with EDTA. Fresh chicken serum specifically depleted of Ca2+, by chelating with EGTA in the presence of Mg2+, retained its ability to lyse oocyst sporozoites. The lysis of salivary gland sporozoites mediated by chicken antisporozoite serum is abrogated by EGTA, indicating that the antibody-dependent complement pathway in chicken serum is blocked by EGTA. Because oocyst sporozoite lysis by serum was heat sensitive and Mg2+ dependent, but Ca2+ independent, we conclude that lysis was mediated by the alternative pathway of complement. One factor in the development from noninfectious oocyst sporozoites to infectious salivary gland sporozoites is the ability to resist lysis by the alternative pathway of complement.