A comparison of transmission-blocking activity with reactivity in a Plasmodium Falciparum 48/45-kD molecule-specific competition enzyme-linked immunosorbent assay

Monoclonal antibodies (MAbs) 32F1 and 32F3 react with two independent epitopes of a protein doublet with molecular weights of 48 and 45 kilodaltons (kD) expressed on the surface of Plasmodium falciparum (Pfs48/ 45) macrogametes and zygotes; only 32F3 blocks transmission. These MAbs were used to develop a Pfs48/45~specific competition enzyme-linked immunosorbent assay (ELISA) using 32F1 to capture antigen and labeled 32F3 for quan­ tification and analysis of the contribution of antibodies in human serum to transmission-blocking activity. A com­ parison analysis was used to determine agreement of competition ELISA titers and transmission-blocking activity as observed in the bioassay in three groups of serum samples: 37 from European travelers with previous exposure to malaria, 56 from gametocyte carriers, and 66 from schoolchildren from a malaria-endemic area in Cameroon. The index of agreement between outcomes of the ELISA and transmission-blocking assay in gametocyte carriers and in travelers was specifically defined as fair-to-moderate; in schoolchildren the agreement was not significant. The com­ bined analysis of all sera showed a significant and fai r-to-moderate agreement between the results of the competition ELISA and the transmission-blocking assay, with a relative specificity of 94% (of 105 cases negative in the trans­ mission-blocking assay, 99 were also negative in the competition ELISA) and a relative sensitivity of 44% (of 54 cases positive in the transmission-blocking assay, 24 were also positive in the competition ELISA). This study shows that a positive C48/45-ELISA is indicative for transmission-blocking activity in the mosquito assay, while a negative result does not exclude transmission-blocking activity. Gametocytes of Plasmodium falciparum synthesize 230- and 48/45-kilodalton (kD) molecules that remain exposed on the surface of macrogametes and zygotes.1" '3 Several mono­ clonal antibodies (MAbs) directed against these molecules block transmission of the parasite to the mosquito vector. One of the targets of transmission-blocking MAbs is the 48/ 45-kD glycoprotein doublet (Pfs48/45). Two independent, nonrepetitive epitopes of this protein react respectively with MAbs designated 32F1 and 32F3. The presence of 32F3 in a mosquito blood meal blocks transmission of the NF54 iso­ late of P. falciparum while 32F1 does not.4 The conventional assay measuring transmission reduction in mosquitoes using a feeder system, as described by Vermeulen and others,1 is costly, labor intensive, and thus limits the number of sera that can be tested. Development of a serologic test predicting transmission-blocking activity would greatly simplify epi­ demiologic studies. A positive correlation between antibody reactivity to Pfs230 and transmission-blocking activity has been found using immunoprecipitation to quantify the anti­ body response. Such a relation was not found for antibody reactivity to This paper describes the development of a two-site com­ petition enzyme-linked immunosorbent assay (ELISA) based on the 32F3 epitope. This competition ELISA was used to analyze several sets of field sera. In addition, the transmis­ sion-blocking activity of these field sera was determined us­ ing the feeder assay1 and the results obtained were compared with those of the competition ELISA using a statistical anal­ ysis for comparison of methods.6'7