Plasmodium falciparum Antigens on the Surface of the Gametocyte-Infected Erythrocyte Maha Saeed 1 , Will Roeffen 2 , Neal Alexander1, Christopher J. Drakeley1, Geoffrey A. T. Targett1, Colin J. 2 Sutherland * 1 Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, United Kingdom, 2 Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands 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. Citation: Saeed M, Roeffen W, Alexander N, Drakeley CJ, Targett GAT, et al. (2008) Plasmodium falciparum Antigens on the Surface of the Gametocyte-Infected Erythrocyte. PLoS ONE 3(5): e2280. doi:10.1371/journal.pone.0002280 Editor: Mauricio Martins Rodrigues, Federal University of Sa¨o Paulo, Brazil Received February 28, 2008; Accepted April 7, 2008; Published May 28, 2008 Copyright: 2008 Saeed et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by the Wellcome Trust (Project # 061910; ISRCTN11805747), the UK Medical Research Council, and the Gates Malaria Partnership. MS is supported by the Council for Assisting Refugee Academics (CARA). CJS is supported by the UK Health Protection Agency. None of these bodies played any role in the design and conduct of the study, in the collection, analysis, and interpretation of the data, or in the preparation, review, or approval of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: colin.sutherland@lshtm.ac.uk Introduction Available evidence suggests that there are specific immune responses to different stages of the malaria parasite life cycle. Natural human immune responses to malaria recognise extracel- lular sporozoites and merozoites, which both have surface-exposed antigens, and are the targets of various vaccines currently under development [1]. Blood-stage immunity also involves the acqui- sition of a repertoire of antibodies (IgG) directed against parasite- encoded variant surface antigens (VSA) on the surface of the infected erythrocyte [2,3]. Carriage of IgG which recognise VSA, including P. falciparum erythrocyte membrane protein-1 (PfEMP-1) [4,5], is associated with protection from clinical malaria [6–11]. Transmissible sexual stages of the malaria parasite, gametocytes, frequently die in the host without being passed on to a mosquito, and in doing so release intracellular antigens into the host circulation. Among these antigens are a number that elicit humoral responses which mediate transmission blocking immuni- ty. This occurs when human antibodies, taken up by a mosquito in a potentially infective blood-meal containing male and female Plasmodium gametocytes, are able to block further parasite development and prevent infection of the mosquito. This immunity is known to be antibody-mediated [12] and is directed against the parasites in the mid-gut of the mosquitoes immediately after ingestion of a blood meal by the mosquito [13–17]. Targets of this immunity include the gamete surface proteins Pfs230 and Pfs48/45, but other antigens may be involved. These gamete proteins are not present on the surface of intact gametocytes and thus antibodies against these antigens are unlikley to have any effect on the parasite in the human host. By comparison, little is known about any specific immune responses that may recognise the surface of erythrocytes infected with sexual stages of malaria parasites during their development in the human body. We know that erythrocytes infected with early forms of P. falciparum gametocytes sequester away from the peripheral circulation until they reach maturity [18], which suggests the presence of adhesins on the gametocyte-infected erythrocyte surface. Analysis of the adhesion phenotype of stage I– V gametocytes that mediates binding to C32 melanoma cells [19] and transformed human bone marrow endothelial cells trHBMEC PLoS ONE | www.plosone.org 1 May 2008 | Volume 3 | Issue 5 | e2280