PROTECTIVE MONOCLONAL-ANTIBODY DEFINES A CIRCUMSPOROZOITE-LIKE GLYCOPROTEIN EXOANTIGEN OF CRYPTOSPORIDIUM-PARVUM SPOROZOITES AND MEROZOITES

The apicomplexan protozoan parasite Cryptosporidium parvum causes a di arrheal disease in humans and other mammals for which specific therapy and immunoprophylaxis are unavailable. Passive immunization with Abs against whole C. parvum organisms has Variable efficacy in immunocompr omised or neonatal hosts. Because apical and surface-exposed zoite Ags of the Apicomprexa are critical to infectivity and targets of protect ive immunity, we examined the ability of mAbs generated against such A gs in C. parvum sporozoites to passively protect against infection and identify biologically relevant parasite molecules. A panel of mAbs wa s produced against affinity-purified native Ags using sporozoite apica l- and surface-reactive mAb C4A1 as binding ligand. One resulting mAb, designated 3E2, elicited prominent morphologic changes in sporozoites and merozoites characterized by rapid and progressive formation, post erior movement, and release of membranous Ag-mAb precipitates. These c hanges had a striking resemblance-to the malarial circumsporozoite pre cipitate (CSP) reaction. Sporozoite infectivity was completely neutral ized after in vitro exposure to 3E2 and the CSP-like reaction. Further more, orally administered 3E2 completely prevented or markedly reduced infection in neonatal BALB/c mice. 3E2 bound to apical complex and su rface molecules of zoites and was demonstrated in membranous precipita tes by immunoelectron microscopy. In Western blots, 3E2 recognized mul tiple 46 to similar to 770 kDa sporozoite Ags and an similar to 1300-k Da Ag designated CSL, also expressed by merozoites. CSL was characteri zed as a soluble glycoprotein exoantigen released by infectious sporoz oites. Further, CSL was determined to be the molecular species mechani stically involved in the CSP-like reaction by its identification in SD S-PACE gels and Western blots of purified membranous precipitates. The se findings indicate that CSL has a functional role in sporozoite infe ctivity and is a candidate molecular target for passive or active immu nization against cryptosporidiosis.