Role of Antibodies against Antigens of PLASMODIUM MEROZOITE and Infected RBC Surface in Malaria Immunity

Abstract

Background: Malaria infection is caused by parasites belonging to the genus Plasmodium. There are five (5) species of Plasmodium that infect humans, namely: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium knowlesi and Plasmodium malariae. Malaria ensuing from Plasmodium falciparum infection is a principal source of disease and death annually, especially among African children in the Sub-Saharan Regions. The erythrocytic stages of the parasites of Plasmodium falciparum are linked to malaria symptoms and its complications. Several Plasmodium antigens, expressed on the surface of the parasite during the blood stage have been shown to be relevant for RBC invasion. These antigens include merozoite surface protein (MSP1)175 -1, apical membrane antigen (AMA)-1, serine-rich antigen (SERA), erythrocyte binding antigen (EBA)-175, among others. Although humoral and cellular immunity are believed to be involved in malaria immunity, their relative importance in offering protection against the disease is not well established. Antibodies produced against merozoite surface antigens and those against variant surface antigens, VSAs (especially PfEMP-1) on the infected red blood cell (iRBC) membrane have been studied individually but their relative contributions that could lead to protection against clinical malaria using the same samples have not been documented. General Aim: The aim of this study was to assess the plasma levels and functional effects of antibodies to merozoite surface and RBC surface antigens between children with asymptomatic infection and clinical malaria. Methodology: This was a case-control study which utilized a total of 153 archived plasma samples collected (from 81 children with asymptomatic infection and 72 children with clinical malaria) by a larger study (cross sectional) that was conducted at Bongo in the Northern Region and Navrongo in the Upper East Region of Ghana. The following procedures were then carried out: Culture of Schizont Stage P. falciparum, Isolation of late stage P. falciparum trophozoites and quantification of antigen (specific antibodies to iRBC membrane antigens and preparation of free merozoites). Bradford Assay was used to measure the concentration of Plasmodium merozoite antigen protein. Measurement of antibodies to merozoite surface antigens was also done by enzyme-linked immunosorbent assay (ELISA) and that of antibodies to variant surface antigens (VSAs) was done by Flow cytometry. Spin purification protocol was employed in purification and concentration of IgG. Growth inhibition assay (GIA) as well as ligand binding inhibition assay (BIA) were subsequently done. Results: In determining the antibody levels to merozoite surface antigens by Bradford Assay, a mean sample OD of 0.4798 measured at 590 – 595nm was observed. Sample concentration determined was 0.0839 mg/ml = 83.9 ug/ml. Asymptomatic individuals had statistically (p<0.0001) higher antibody levels against merozoite surface antigens compared to the symptomatic individuals. Antibody levels to variant surface antigens on infected RBC surface between asymptomatic and symptomatic individuals was not statistically significant (p=0.6469). Meanwhile, there was no statistically significant difference (p=0.2487) between asymptomatic and symptomatic patients in terms of growth inhibition ability of antibodies against antigens of Plasmodium merozoite and infected RBC surface. Similarly, there seem to be no difference in the ligand binding inhibitory ability of antibodies against antigens on Plasmodium infected RBC surface. Conclusion: Antibodies against antigens of Plasmodium merozoite and infected RBC surface seem to give similar contribution in malaria immunity.