Species- and Stage-Specific Antigens in Exoerythrocytic Stages of Plasmodium falciparum

Abstract

Numerous exoerythrocytic forms of Plasmodium falciparum (PFEEF) were obtained from the liver of the South American monkey, Cebus apella, for analysis of the antigens on this stage. As antigen for the fluorescent assay, 5-µm sections of liver fragments collected on day 5 following sporozoite inoculation and fixed in Carnoy's solution or kept in liquid nitrogen were used. Two types of fluorescent labeling of the PFEEF were identified: diffuse and peripheral. Each of 23 sera from individuals with P. falciparum infection acquired naturally by mosquito bite showed the diffuse and peripheral patterns of fluorescence at low serum dilutions (i.e., 1:10–1:100), but only peripheral staining at higher serum dilutions (i.e., 1:200–1:1,600). All other polyclonal sera tested showed only the diffuse pattern of fluorescence whatever the serum dilution used; this was true for P. falciparum infections acquired accidentally by blood transfusion, heterologous human infections with P. vivax, P. ovale, P. malariae or P. cynomolgi, and experimental animal infections with P. berghei, P. gallinaceum, or P. cynomolgi. Fluorescent antibody titers on PFEEF were generally 1–4 dilutions lower than on blood stages. No age-dependent pattern of fluorescence titers was found in 30 sera from individuals ranging in age from 2–78 years living in a malaria-endemic area. Twenty-six monoclonal antibodies directed to P. falciparum blood stages which reacted at high titers with rings, schizonts, merozoites, and gametocytes did not react with PFEEF antigen even when using the undiluted ascitic fluid. Results were also negative when using a monoclonal antibody directed to P. falciparum sporozoite surfaces and one to P. flaciparum gametes. The results indicate that, in addition to antigens of broad specificity which are shared with blood stages and other malaria species, the hepatic stage of P. falciparum exhibits stage- and species-specific antigens which are located at the periphery of the schizont.