SPECIES-SPECIFIC AND STAGE-SPECIFIC ANTIGENS IN EXOERYTHROCYTIC STAGES OF PLASMODIUM-FALCIPARUM

Numerous exoerythrocyte forms of P. 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-.mu.m sections of liver fragments collected on day 5 following sporozoite inoculation and fixed in Carnoy''s solution or kept in liquid N2 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., 1:200-1:1600). 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 on blood stages. No age-dependent pattern of fluorescence titers was found in 30 sera from individuals ranging in age from 2-78 yr living in a malaria-endemic area. Monoclonal antibodies (26) 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 negative when using a monoclonal antibody directed to P. falciparum sporozoite surfaces and one to P. falciparum gametes. Apparently, 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.