Gametocyte-specific immunity provides a rationale for novel transmission blocking interventions in Plasmodium falciparum

Abstract

The recent decline in global malaria burden has stimulated a renewed interest in malaria elimination and eradication. Understanding the biology of malaria transmission and identifying opportunities to interfere with the development and infectivity of Plasmodium falciparum transmission stages are essential to elimination efforts. Transmission-blocking vaccine efforts to date have focused on mosquito-stage antigens while a vaccine targeting immature gametocytes—the transmission stages developing in the human host—would have distinct advantages. Immature (stage I-IV) gametocytes sequester in human bone marrow before being released into the circulation as mature stage V gametocytes. As in asexual parasite sequestration, this process is likely mediated by interactions between specific host receptors and immunogenic adhesins on the surface of infected red blood cells (iRBCs). Antibodies recognizing immature gametocytes could confer protection by 1) inhibiting binding necessary for entry and/or development in the bone marrow, 2) increasing killing by effector cells, and/or 3) inducing uptake by other immune cells. Hypothesizing that early-stage gametocytes are targets of host antibody responses, we performed the first systematic characterization of immune responses targeting developing gametocytes. Using a newly developed flow cytometry assay, we identified a subset of Malawian sera that strongly recognize the surface of immature, but not mature, gametocyte-iRBCs (giRBCs). Stage I/IIA giRBCs were most highly recognized, suggesting that giRBC surface antigen expression is highest in early stages and progressively decreases throughout gametocyte development. The strength and prevalence of both asexual- and gametocyte-iRBC surface recognition increased with age. In order to determine the identity of parasite antigens bound by human sera, we took 3 approaches: 1) mass spectrometry, 2) immunization of mice with giRBC membranes and evaluation of the resulting sera by protein array, and 3) correlation between serum recognition of specific proteins by array and recognition of giRBCs by flow cytometry. We identified 47 novel candidate surface antigens, some shared between asexual- and gametocyte-iRBCs and others enriched in giRBCs. The surface expression of 5 of these antigens has been validated by multiple methods. Our findings demonstrate for the first time that immune responses to giRBCs are acquired following malaria exposure, and present several possible targets of these responses. This work may form a starting point for designing vaccines that prevent malaria transmission by interfering with gametocyte maturation.