Pf332-C231-reactive antibodies affect growth and development of intra-erythrocytic Plasmodium falciparum parasites.

The Plasmodium falciparum antigen 332 (Pf332), is a megadalton parasite protein expressed at the surface of infected red cells during later stages of the parasite's developmental cycle. Antibodies to different parts of this antigen have been shown to inhibit parasite growth and adherence to host cells with or without ancillary cells. However, the mechanisms involved in these inhibitions remain largely unknown. We further analysed the activities of specific antibodies with regard to their specific mechanisms of action. For these analyses, affinity purified human antibodies against epitopes in the C-terminal fragment of Pf332 (Pf332-C231) were employed. All purified antibodies recognized Pf332-C231 both by immunofluorescence and ELISA. IgG was the main antibody isotype detected, although all sera investigated had varying proportions of IgG and IgM content. All the antibodies showed a capacity to inhibit parasite growth in P. falciparum cultures to different extents, mainly by acting on the more mature parasite stages. Morphological analysis revealed the antibody effects to be characterized by the presence of a high proportion of abnormal schizonts (15-30%) and pyknotic parasites. There was also an apparent antibody effect on the red cell integrity, as many developing parasites (up to 10% of trophozoites and schizonts) were extracellular. In some cases, the infected red cells appeared to be disintegrating/fading, staining paler than surrounding infected and uninfected cells. Antigen reversal of inhibition confirmed that these inhibitions were antigen specific. Furthermore, the growth of parasites after 22-42h exposure to antibodies was investigated. Following the removal of antibody pressure, a decreased growth rate of these parasites was seen compared to that of control parasites. The present study confirms the potential of Pf332 as a target antigen for parasite neutralizing antibodies, and further indicates that epitopes within the C231 region of Pf332 should constitute important tools in the dissection of the role of Pf332 in the biology of the malaria parasite, as well as in the design of a malaria vaccine.

Mechanisms of malarial anaemia: potential involvement of the Plasmodium falciparum low molecular weight rhoptry-associated proteins.

Plasmodium falciparum malaria is a major cause of morbidity and mortality throughout the tropics. Anaemia is a constant feature of the disease. Pregnant women mostly primigravidae and children below the age of 5 years are the most afflicted. Its pathogenesis is multifactorial and incompletely understood. Among several factors, the destruction of erythrocytes (RBCs) is the most frequently observed cause of severe malarial anaemia and the removal of non-parasitized RBCs (nEs) is thought to be the most important, accounting for approximately 90% of the reduction in haematocrit in acute malaria. Previous studies demonstrated that the tagging of nEs with the parasite antigen RAP-2 (rhoptry-associated protein-2; also designated RSP-2) due to either failed or aborted invasion by merozoites resulted in the destruction of these cells. In this study we further investigated the mechanisms mediating the destruction of nEs in the development of severe malarial anaemia and the possible involvement of RAP-2/RSP-2 and other members of the low molecular weight rhoptry complex (RAP-1: rhoptry-associated protein-1 and RAP-3: rhoptry-associated protein-3). Antibodies to the rhoptry-associated proteins were found to recognise the surface of nEs in a parasitaemia-dependent manner after merozoite release in P. falciparumin vitro cultures. These cells, as well as erythroblasts co-cultured with infected RBCs (IEs), could then be destroyed by either phagocytosis or lysis after complement activation. The ability of anti-rhoptry antibodies to mediate the destruction of RAP-2/RSP-2-tagged erythroblasts in the presence of effector cells was also investigated. Data obtained suggest that mouse monoclonal antibodies to the low molecular weight RAP proteins mediate the death of RAP-2/RSP-2-tagged erythroblasts on interaction with adherent monocytes. The mechanism of cell death is not yet fully known, but seems to involve primarily apoptosis. The above observations suggest that the antibody response against RAP-2/RSP-2 and other members of the complex could trigger the destruction of RAP-2/RSP-2-tagged host cells. Taken together it appears that during severe anaemia a defective bone marrow or dyserythropoiesis possibly due to erythroblast cell death, may overlap with the accelerated destruction of normal erythroid cells, either by opsonisation or complement activation further aggravating the anaemia which may become fatal. These observations could therefore have implications in the design, development and deployment of future therapeutic interventions against malaria.