The Plasmodium bottleneck: malaria parasite losses in the mosquito vector

Abstract

Nearly one million people are killed every year by the malaria parasite Plasmodium. Although the disease-causing forms of the parasite exist only in the human blood, mosquitoes of the genus Anopheles are the obligate vector for transmission. Here, we review the parasite life cycle in the vector and highlight the human and mosquito contributions that limit malaria parasite development in the mosquito host. We address parasite killing in its mosquito host and bottlenecks in parasite numbers that might guide intervention strategies to prevent transmission.

Fig. 1. : Plasmodium bottlenecks in the mosquito host. Shortly after ingesting an infectious blood meal, Plasmodium gametocytes are activated within the Anopheles midgut resulting in the emergence of male and female gametes. Following fertilisation, the resulting zygote differentiates into a motile ookinete. The ookinete must then penetrate the peritrophic matrix that completely surrounds the blood meal and invade the midgut epithelium. After traversal, ookinetes attach to the basal surface of the epithelium and differentiate into sessile oocysts that grow and produce thousands of sporozoites over an approximate two-week period. Upon maturation, sporozoites are released into the haemolymph from where they invade the salivary glands. The cycle is completed when the mosquito feeds on a new host and delivers sporozoites with the saliva. The illustration indicates development time, approximate parasite numbers during each stage of development (yellow bar) and the timing of anti-Plasmodium responses (bottom).

Fig. 3. : mechanisms of “early-phase” and “late-phase” immunity. Parasite development after traversal of the midgut epithelium is subjected to two “phases” of the mosquito innate immune response. An “early-phase” limits the ookinete survival before or at the transition to oocyst differentiation. As ookinetes traverse the midgut epithelium they undergo nitration (red dots) and in this way are “marked” for immune recognition by complement-like proteins circulating in the mosquito haemolymph [including thioester protein 1 (TEP1)]. Following recognition, TEP1 binds to the ookinete surface to initiate lysis or melanisation that result in parasite killing. A second, “late-phase” immune response limits oocyst survival and involves the production of nitric oxide (NO) by the signal transducer and activator of transcription (STAT) pathway leading to parasite killing. While increased levels of NO have been implicated in this process, it is unclear to what extent the midgut, fat body and possibly haemocytes may contribute to the “late-phase” response. This figure was adapted from Gupta et al. (2009) and Fraiture et al. (2009). APL1: Anopheles Plasmodium-responsive leucine-rich repeat protein 1; BL: basal lamina; LRIM1: leucine-rich immune molecule 1; ME: midgut epithelium; NOS: NO synthase; PM: peritrophic matrix.

Fig. 2. : factors that influence malaria parasite development in the mosquito midgut lumen. When a female Anopheles mosquito feeds on a malaria-infected person, it ingests sexual forms of the parasite: male and female gametocytes (1). Some of these gametocytes may be dead or non-infectious to the mosquito due to exposure to cytokines or nitric oxide (NO) produced in the infected human host (2). After the gametes egress from the red blood cell (RBC) (3) they become exposed to factors from the human blood that may negatively affect parasite development. These include damage caused by serum cytokines and NO (4), phagocytosis by lymphocytes (5), inhibition of fertilisation by transmission-blocking (TB) antibodies (6) and the attack of the vertebrate complement system (7, 8). The attack by the complement system can be initiated by two mechanisms: activation of the classical pathway (CPC) by opsonising antibodies (against Pfs230) that bind to gamete surface proteins (7) or activation of the alternative pathway (APC) by binding of C3 to the surface of the gamete (8). In both cases, lysis occurs after the formation of a membrane-attack complex on the parasite membrane. To evade the activation of the alternative complement pathway, the parasite uses the surface protein PfGAP50 to recruit factor H from the blood serum, thus inhibiting further activation of the system (9). Proteins from the complement system are degraded approximately 6 h after blood-feeding (10). Parasites that escape further develop into ookinetes which shares its niche with the midgut bacteria that multiplied exponentially after the ingestion of the blood (11). These bacteria may secrete antimalarial compounds, including reactive oxygen species (ROS), which impact ookinete viability (12). The ookinetes that survive invade and traverse the midgut epithelium after which they form oocysts on the basal side of the midgut epithelium (13). RNS: reactive nitrogen species.