Immunity to sexual stages of human malaria parasites: immune modulation during natural infections, antigenic determinants, and the induction of transmission-blocking immunity.

Four antigens of Plasmodium falciparum have so far been identified as targets of transmission-blocking antibodies; three of them (Pfs 230, 48/45) are detectable in gametocytes and expressed on gametes, the fourth (Pfs 25) appears only after fertilization. Epitope analyses of each antigen were made with competitive immunoassays, and the extent of antigenic diversity determined amongst numerous isolates of P. falciparum. There was minimal variation within one of the two epitopes on Pfs 230 both of which induce transmission-blocking antibodies. The epitopes on Pfs 25 to which blocking monoclonal antibodies respond showed a variability amongst different isolates by immunofluorescence which was unexpected in view of sequence data on the molecule. Five epitope regions have been identified on Pfs 48/45 and antibodies to them interact in a complex manner. Antigenic diversity affecting these epitopes was minimal. In P. vivax malaria much greater polymorphism was seen amongst gamete surface antigens. Natural P. falciparum infections induce antibody responses to gametocyte/gamete surface antigens that will suppress infectivity to mosquitoes but these responses may involve reactivity with any of a series of different epitopes, interactions between antibodies, and may be sequential. In P. vivax infections antibody to the sexual stage antigens may suppress or enhance transmission depending on the antibody level. Cytokine production induced by sexual stage antigens may also modulate transmission, by rendering gametocytes non-infective. Experimental studies showed marked MHC-restriction of immune responses to gamete antigens (but not to the Pfs 25 zygote antigen); the evidence from studies in humans is less convincing. Antibody responses to the sexual stage antigens seem to be more frequent in persons who have experienced only one or a few attacks of malaria as opposed to those who have been exposed frequently. Some form of down-regulation may therefore be occurring.