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Glutaminyl cyclase (QC) modifies N-terminal glutamine or glutamic acid residues of target proteins into cyclic pyroglutamic acid (pGlu). Here, we report the biochemical and functional analysis of Plasmodium QC. We show that Plasmodium sporozoites of QC-null mutants are recognized by the mosquito immune system and melanized when they reach the hemocoel. Sporozoite numbers in salivary glands are also reduced in mosquitoes infected with QC-null or QC catalytically-dead mutants. This phenotype can be rescued by genetic complementation or by disrupting mosquito hemocytes or melanization immune responses. Mutation of a single QC-target glutamine of the major sporozoite surface protein (CSP) also results in immune recognition of sporozoites. These findings reveal QC-mediated post-translational modification of surface proteins as a major mechanism of mosquito immune evasion by Plasmodium sporozoites.

Immune priming in Anopheles gambiae mosquitoes following infection with Plasmodium parasites is mediated by the systemic release of a hemocyte differentiation factor (HDF), a complex of lipoxin A4 bound to Evokin, a lipid carrier. HDF increases the proportion of circulating granulocytes and enhances mosquito cellular immunity. We found that Evokin is constitutively produced by hemocytes and fat-body cells, but expression increases in response to infection. Insects synthesize lipoxins, but lack lipoxygenases. Here, we show that the Double Peroxidase (DBLOX) enzyme, present in insects but not in vertebrates, is essential for HDF synthesis. DBLOX is highly expressed in oenocytes in the fat body tissue, and these cells proliferate in response to Plasmodium challenge. We provide direct evidence that modifications mediated by the histone acetyltransferase AgTip60 (AGAP01539) are essential for sustained oenocyte proliferation, HDF synthesis and immune priming. We propose that oenocytes function as a population of “memory” cells that continuously release lipoxin to orchestrate and maintain a broad, systemic and long-lasting state of enhanced immune surveillance.