Mechanisms of naturally acquired transmission-blocking immunity to Plasmodium falciparum malaria

Monoclonal antibodies to the gamete surface antigens of Plasmodiurn falciparum, Pfs230 and Pfs48/45, can abolish the infectivity of gametes to mosquitoes, hence these antigens have been proposed as candidates for inclusion in a malaria transmission-blocking vaccine. One possible mechanism of antibody-mediated transmission-blocking is complementmediated gamete lysis. A cohort of human sera, from geographically distinct malaria-endemic regions, was used to investigate whether this may be a mechanism of naturally acquired transmission-blocking immunity to P.falciparum. By immunoprecipitation, it was shown that antibody recognition of Pfs230 and Pfs48/45 is limited, despite universal exposure to P.falciparum gametocytes. In vitro complement-mediated lysis of P.falciparum gametes was positively associated with the presence of antibodies to Pfs230, but not with antibodies to the N-terminal region of the precursor molecule (Pfs260) which is shed from the gametocyte surface at the time of gametogenesis. Similarly, antibodies to two other gametocyte-specific proteins, Pfs48/45 and Pfg27/25 were not associated with gamete lysis. All sera which mediated gamete lysis contained IgGi and / or IgG3 antibodies to gamete surface proteins as determined by ELISA. These data suggest that Pfs230 is a major target of complement-fixing antibodies which may be important for antibody-mediated transmissionblocking immunity. A selection of these malaria-immune human sera were tested for their ability to affect the infectivity of P. falciparum gametocytes to Anopheles mosquitoes. It was found that transmission-reducing effects of the sera were associated with the presence of IgGi antibodies to the gamete surface, specifically against the protein, Pfs230. Enhancement of transmission, manifest as elevated numbers of oocysts relative to controls, was observed for a number of sera, but was not found to be significantly associated with antibodies against Pfs230 or Pfs48/ 45. Consistent with the data above, the ability to mediate in vitro complement-dependent gamete lysis was also significantly associated with reduction of infectivity to mosquitoes. The role of complement in transmission blocking immunity was further investigated by testing the effects, on gametocyte infectivity, of serially diluted malaria-immune sera in the presence of active and inactive human complement. These studies revealed a complex interaction of mosquito bloodmeal components, which, in the absence of complement, led to enhancement of transmission at low serum concentrations and reduction of transmission at high serum concentrations. In the presence of active complement, levels of enhancement were less than in inactive complement and infectivity was lowest at the highest serum concentration. Phagocytosis of sexual stages in the presence of malaria-immune sera was assayed to assess its role in transmission-blocking immunity. In vitro phagocytosis was positively correlated with the level of specific antibody to both Pfs230 and Pfs48/45, and with IgGi to a crude extract of gamete surface proteins. Despite this, transmission-blocking could not be associated with phagocytosis in vivo. Taken together, these data support the role of antibodies to the gamete surface protein Pfs230, in naturally acquired transmission blocking immunity against P. falciparum. P. falciparurn oocysts on An. stephensi midguts oocyst containing sporozoites