Immune Regulation of Is Species Specific and Infection Intensity Dependent.

Malaria parasite ookinetes must traverse the vector mosquito midgut epithelium to transform into sporozoite-producing oocysts. The innate immune system is a key regulator of this process, thereby determining vector competence and disease transmission. The role of innate immunity factors as agonists or antagonists of malaria parasite infection has been previously determined using specific single - species combinations. Here we show that the two C-type lectins CTL4 and CTLMA2 exert differential agonistic and antagonistic regulation of parasite killing in African and South American species. The C-type lectins regulate both parasite melanization and lysis through independent mechanisms, and their implication in parasite melanization is dependent on infection intensity rather than mosquito-parasite species combination. We show that the leucine-rich repeat protein LRIM1 acts as an antagonist on the development of ookinetes and as a regulator of oocyst size and sporozoite production in the South American mosquito Our findings explain the rare observation of human melanization and define a key factor mediating the poor vector competence of for and Malaria, one of the world's deadliest diseases, is caused by parasites that are vectored to humans by the bite of mosquitoes. The mosquito's innate immune system is actively engaged in suppressing infection. Studies on mosquito immunity revealed multiple factors that act as either facilitators or inhibitors of infection, but these findings were mostly based on single - species combinations, not taking into account the diversity of mosquito and parasite species. We show that the functions of CTL4 and CTLMA2 have diverged in different vector species and can be both agonistic and antagonistic for infection. Their protection against parasite melanization in is dependent on infection intensity, rather than the mosquito-parasite combination. Importantly, we describe for the first time how LRIM1 plays an essential role in infection of , suggesting it is a key regulator of the poor vector competence of this species.