Humoral immune responses to Plasmodium falciparum antigens in children and adults living in a hypoendemic area of Punjab (Pakistan).

We have investigated the prevalence and the levels of antibodies reactive with repeat sequences in Plasmodium falciparum asexual blood-stage Pf155/ring-infected erythrocyte surface antigen (RESA), Pf332 antigen, and the circumsporozoite (CS) protein in 532 children and adults residing in a hypoendemic area in the Punjab (Pakistan). We show here that the levels of antibodies reactive with synthetic peptides corresponding to repeat sequences in these antigens increased gradually with age. However, the prevalence and levels of antibodies reactive with the peptides were quite low as compared with the high prevalence of such antibodies in donors from areas holoendemic and hyperendemic for malaria. The levels of Ag332 (2-12)2-reactive antibodies in individual sera as well as in the study population as a whole correlated well with the levels of antibodies to the Pf155/RESA peptide (EENV)6. However, there was no correlation between anti-(EENV)6 and anti-Ag332 (2-12)2 antibody levels and malaria parasitemia. There was a significant negative correlation between anti-(NANP)6-reactive antibodies and the parasite rate, suggesting that a heavy load of blood-stage parasites may be one factor that exerts an immunosuppressive effect on the antibody response to the sporozoites.