Natural Antibodies against Three Distinct and Defined Antigens of Plasmodium falciparum in Residents of a Mesoendemic Area in Gabon

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Abstract
The magnitude of the antibody response to three distinct and defined antigens of Plasmodium falciparum was assessed in 144 inhabitants of the Kassa district (Haut Ogooué Province, Gabon), a region where malaria is mesoendemic. Antibodies against a polypeptide consisting of 40 (Asn-Ala-Asn-Pro) repeats of P. falciparum circumsporozoite protein [(NANP)40] were detected by ELISA. Antibodies against the fusion peptide 31.1, which consists of the N-terminal portion of the 190–200 kDa glycoprotein, were also detected by ELISA. Antibodies against ring-infected erythrocyte surface antigens (RESA), mainly the P. falciparum 155 kDa antigen (Pf155), were detected by IFA on glutaraldehyde-fixed P. falciparum infected red blood cells (IRBC). In addition, a standard IFA employing air-dried P. falciparum IRBC was used to detect antibodies against intraerythrocytic asexual forms. Parasitemia and spleen enlargement were also recorded. The frequency of sera positive for specific antibodies increased with age for all the antigens tested. Plateau antibody levels were reached at different ages for the different antigens. Individual antibody responses varied in titer to the different antigens. Subjects with parasite-negative thick smears showed higher titers of anti-31.1 as well as an increased frequency of anti-RESA antibodies compared to subjects having positive smears. No differences in the titer and in the prevalence of anti-(NANP)40 antibodies were found between these groups. The results suggest that the antibody response against asexual blood stage antigens, especially anti-RESA and anti-31.1, may play a role in controlling parasitemia.

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