Fig 1.
The Plasmodium life cycle in the mosquito vector.
Following an infected blood meal, the ingested Plasmodium gametocytes develop into male and female gametes that can fuse together to form a zygote. The zygote then differentiates into a motile ookinete and invades the midgut epithelium after traversing the peritrophic matrix separating the ingested blood meal from the epithelial cells. Right after it reaches the extracellular space between the epithelium and the basal lamina, the ookinete develops into an early oocyst, which in turn undergoes sporogony to give rise to a mature oocyst holding thousands of sporozoites. Following rupture of the oocyst, the released sporozoites invade the salivary glands and are ready for transmission to a new host. Proteins mediating the various phases of the life cycle are mentioned in the panels below the corresponding developmental stages. Created in BioRender. Saab, S. (2025) https://BioRender.com/p65s417.
Fig 2.
Mosquito immune responses against the early ookinete stage.
Plasmodium ookinete midgut invasion induces nitration, modifying the parasite surface for TEP1 recruitment. TEP1 is secreted into the hemolymph in its full form (TEP1-F). Upon activation, TEP1-F is cleaved by an unknown factor into the active cut form (TEP1cut), which is stabilized in the hemolymph by binding to the APL1C/LRIM1 heterodimer. Recognition of the ookinete results in the deposition and accumulation of TEP1 cut on the parasite surface which is promoted by SPCLIP1 due to the cleavage of more TEP1-F by an unknown TEP1 convertase. TEP1-marked ookinetes are then targeted for lysis or, in rare cases, melanization, which mainly occurs in certain refractory backgrounds (left panel). Melanization requires a cascade of catalytic and noncatalytic CLIP serine proteases that regulate the cleavage of PPO zymogen into active PO, the key protein in melanin synthesis. Altogether, thus far, TEP1 seems to be the factor farthest upstream, followed by the core cSPH module (SPCLIP1-CLIPA8-CLIPA28) that acts upstream of the catalytic CLIPC9. Downstream of the CLIPC9 comes CLIPBs, with CLIPB4 and CLIPB17 seeming to be the farthest upstream; the cascade then bifurcates into two different branches: one converging on CLIPB8 and the other on CLIPB10. Dashed arrows indicate that the steps require further characterization (right panel). Created in BioRender. Saab, S. (2025) https://BioRender.com/r16n889.
Fig 3.
Mosquito late-phase immune responses against the oocyst stage.
The main cellular immune response known to be triggered against the oocyst stage involves hemocyte differentiation driven by the action of the LITAF-Like 3 and STAT pathway. Hemocytes are the unique suppliers of PPOs, and PPO2, PPO3, and PPO9 limit oocyst survival without triggering melanin deposition (left panel). However, the oocyst stage can evade most of the hemolymph-mediated immune responses by exploiting its capsule components, such as laminin, collagen, LYSC1, MMP1, and CSP (right panel). Created in BioRender. Saab, S. (2025) https://BioRender.com/q60q481.
Fig 4.
Mosquito immune responses against the sporozoite stage.
Following the rupture of the mature oocysts, the released sporozoites become directly exposed to all the cellular and humoral immune responses in the hemolymph. Apart from hemocyte-mediated phagocytosis, sporozoites can also be subjected to fat body-secreted AMPs that can play an essential role in sporozoite lysis (right panel). After infection, the high hemolymph flow drives the hemocyte accumulation around the periostial region, promoting direct contact between circulating pathogens and hemocytes, and therefore phagocytosis. Also, AMPs are produced in pericardial cells and may be involved in the lysis of sporozoites that accumulate in the heart (right panel). AMPs, lysozyme-related genes, C-type lectins, serine proteases, leucine-rich immune proteins, and TEP1 are all expressed in the mosquito salivary glands; however, their effect on the sporozoite stage needs to be further investigated (left panel). Created in BioRender. Saab, S. (2025) https://BioRender.com/c22y701.