Plasmodium falciparum antigens on the surface of the gametocyte-infected erythrocyte

Abstract

Background: The asexual blood stages of the human malaria parasite Plasmodium falciparum produce highly immunogenic polymorphic antigens that are expressed on the surface of the host cell. In contrast, few studies have examined the surface of the gametocyte-infected erythrocyte.

Methodology/principal findings: We used flow cytometry to detect antibodies recognising the surface of live cultured erythrocytes infected with gametocytes of P. falciparum strain 3D7 in the plasma of 200 Gambian children. The majority of children had been identified as carrying gametocytes after treatment for malaria, and each donated blood for mosquito-feeding experiments. None of the plasma recognised the surface of erythrocytes infected with developmental stages of gametocytes (I-IV), but 66 of 194 (34.0%) plasma contained IgG that recognised the surface of erythrocytes infected with mature (stage V) gametocytes. Thirty-four (17.0%) of 200 plasma tested recognised erythrocytes infected with trophozoites and schizonts, but there was no association with recognition of the surface of gametocyte-infected erythrocytes (odds ratio 1.08, 95% C.I. 0.434-2.57; P = 0.851). Plasma antibodies with the ability to recognise gametocyte surface antigens (GSA) were associated with the presence of antibodies that recognise the gamete antigen Pfs 230, but not Pfs48/45. Antibodies recognising GSA were associated with donors having lower gametocyte densities 4 weeks after antimalarial treatment.

Conclusions/significance: We provide evidence that GSA are distinct from antigens detected on the surface of asexual 3D7 parasites. Our findings suggest a novel strategy for the development of transmission-blocking vaccines.

Figure 1. Purification of early gametocyte stages from cultures of 3D7a parasites.

The top panel shows stage I gametocytes stained with ethidium bromide (blue) and the sexual parasite-specific anti-Pfs16 antibody (green). The majority of nucleated erythrocytes are also Pfs16 positive and therefore gametocytes. The lower panel shows a Giemsa–stained thin film of the same culture 2 days later, when all parasites have reached Stage IIb of gametocyte development. Asexual parasite contamination was estimated at less than 1%.

Figure 2. Serum antibodies recognise the surface of mature gametocyte-infected RBCs.

Serum from a Dutch individual with previously demonstrated transmission-blocking antibodies was incubated with mature asexual parasites or stage V gametocytes and analysed by flow cytometry. Parasites were dual labeled with FITC conjugate, indirectly recognising human IgG, and EB staining nuclear DNA. Axes denote the number of number of cells counted (events) in each dimension.

Figure 3. Recognition profiles of plasma IgG from 202 Gambian children.

Erythrocytes harbouring P. falciparum clone 3D7a stage V gametocytes (left column) and asexual parasite stages (right) were tested with each of 194 plasma (rows). Plasma are arranged in increasing order of the proportion of gametocyte recognition events in the right upper quadrant of the flow cytometry dot-blot. Positive antibody recognition is scored as dark grey. Pale fill indicates that antibodies could not be detected above the level of controls (see text).