Phagocytosis does not play a major role in naturally acquired transmission-blocking immunity to Plasmodium falciparum malaria

Phagocytosis of Plasmodium falciparum sexual stages in vitro and within the mosquito midgut was assayed in order to assess its role in transmission bl ocking immunity to malaria. Both monocytes/macrophages (MM) and polymorphon uclear neutrophils (PMN) phagocytosed malarial gametes in vitro, but levels of phagocytosis were low. Intraerythrocytic gametocytes were not susceptib le to phagocytosis, In vitro phagocytosis was positively correlated with le vels of antibodies against the gamete surface proteins Pfs230 and Pfs48/45. Immunoglobulin G (IgG) subclass analysis revealed that phagocytosis was co rrelated with levels of antigamete IgG1, In vivo membrane-feeding experimen ts were performed in the presence of both pooled and individual malaria imm une sera. The phagocytic process proceeded less efficiently in vivo than in vitro, which may be related to the lower ambient temperature (26 degrees C , compared with 37 degrees C), Finally, although we found a correlation bet ween the ability of a serum to promote phagocytosis in vitro and the presen ce of antibodies against transmission-blocking target antigens, we were una ble to demonstrate a role for MM- or PMN-mediated phagocytosis in reduction of infectivity of the malarial parasite to mosquitoes.