Antibodies in malarial sera to parasite antigens in the membrane of erythrocytes infected with early asexual stages of Plasmodium falciparum.

Abstract

Monolayers of human erythrocytes (E) infected with P. falciparum were fixed, exposed to sera from patients with P. falciparum malaria or from donors immune to this parasite and then tested in an indirect immunofluorescence assay (IFA). Immunofluorescence (IF) was restricted to the surface of infected E. No antibody binding was detected unless the E were air dried, suggesting that the relevant antigens were not available on the outer layers of the E surface. Antibodies in sera from different parts of Africa, Colombia or Sweden reacted similarly with E infected with a Tanzanian P. falciparum strain kept in culture for many years and with parasitized E freshly drawn from African, Swedish or Colombian patients. All sera from residents of a holoendemic area (Liberia) were IFA positive. In contrast, some sera from Colombian or Swedish patients with primary infection gave negative results. The results of the IFA and of an enzyme-linked immunosorbent assay in which fixed and dried E were the targets were well-correlated, suggesting that the same antibodies were detected by these assays. The antigens involved in the IFA were susceptible to pronase but not to trypsin or neuraminidase. E surface IF was inhibited by lysates of infected E, merozoite extracts or soluble antigens present in P. falciparum culture supernatants but not by lysates of normal E or ghosts extracts. The inhibitory antigens were heat stable (100.degree. C, 5 min). Antibodies eluted from monolayers of infected E reacted consistently with a predominant polypeptide of Mr 155,000 and 2-4 minor polypeptides of lower MW. Metabolic labeling of the parasites with 75Se-methionine indicated that these antigens were parasite derived. It is concluded that the antigens involved in these reactions are released from bursting schizonts or merozoites and are deposited in the E membrane in the course of invasion. A possible protective significance of the immune response to these antigens is suggested by preliminary experiments which showed that the antibodies eluted from infected E very efficiently inhibit E reinvasion by merozoites in vitro.