Identification of a major rif transcript common to gametocytes and sporozoites of Plasmodium falciparum

In this study, the transcript profiles of rif and var genes, known to encode surface antigens in asexual blood stage parasites, were investigated at different stages of 3D7/NF54 gametocytogenesis and in sporozoites.Gametocytes exhibited a rif transcript profile unlinked to the rif and var transcript profile of the asexual progenitors. At stage V, mature gametocytes produced high levels of a single rif gene, PF13_0006, which also dominated the rif transcript profile of sporozoites. All var genes appeared to be silenced in sporozoites.The most prominent variant surface antigen transcribed in both gametocytes and sporozoites of 3D7/NF54 is a single variant of the RIFIN protein family. This discovery may lead to the identification of the parasites binding ligands responsible for the adhesion during sexual stages and potentially to novel vaccine candidates.Plasmodium falciparum is transmitted from infected humans to mosquitoes via sexual stages called gametocytes. Immature gametocytes arise from erythrocytic asexual stages and sequester preferentially in the bone marrow [1] for about 9-12 days before reaching maturity and emerging into peripheral blood [2]. Unlike other species of malaria parasites, five different stages of P. falciparum gametocyte development have been described in vitro by morphological criteria [2], known as stages I to V. Only at stage V are gametocytes released to the bloodstream and become infectious to mosquitoes of the Anopheles sp. A mean circulation time of 6.4 days with a range of 1.3-22.2 days has been reported [3], but aggregation of the mature stage V gametocyte in the sub-dermal capillaries has been proposed [4,5]. In the mosquito midgut, the male gametocyte transforms into eight motile microgametes and emerges from the erythrocyte within minutes (exflagellation), whereas the female emerges as a single round shaped macrogamete. Fertilization is required to initiate the mosquito stages of parasite development, resulting 10-18 days later in