The Parasitic Intracellular Lifestyle of Trypanosomatids: Parasitophorous Vacuole Development and Survival

Abstract

The trypanosomatid (protozoan) parasites Trypanosoma cruzi and Leishmania spp. are causative agents of Chagas disease and Leishmaniasis, respectively. They display high morphological plasticity, are capable of developing in both invertebrate and vertebrate hosts, and are the only trypanosomatids that can survive and multiply inside mammalian host cells. During internalization by host cells, these parasites are lodged in "parasitophorous vacuoles" (PVs) comprised of host cell endolysosomal system components. PVs effectively shelter parasites within the host cell. PV development and maturation (acidification, acquisition of membrane markers, and/or volumetric expansion) precede parasite escape from the vacuole and ultimately from the host cell, which are key determinants of infective burden and persistence. PV biogenesis varies, depending on trypanosomatid species, in terms of morphology (e.g., size), biochemical composition, and parasite-mediated processes that coopt host cell machinery. PVs play essential roles in the intracellular development (i.e., morphological differentiation and/or multiplication) of T. cruzi and Leishmania spp. They are of great research interest as potential gateways for drug delivery systems and other therapeutic strategies for suppression of parasite multiplication and control of the large spectrum of diseases caused by these trypanosomatids. This mini-review focuses on mechanisms of PV biogenesis, and processes whereby PVs of T. cruzi and Leishmania spp. promote parasite persistence within and dissemination among mammalian host cells.

Keywords: Leishmania; Trypanosoma cruzi; intracellular pathogen; parasitophorous vacuole; vacuole.

FIGURE 1

Entry of Trypanosoma cruzi metacyclic trypomastigote (MT) forms into non-professional phagocytic cell (A). 1: MT entry requires parasite surface protein gp82 and recruitment of lysosomes to the infection site in a Ca²⁺-dependent manner. 2: MT invasion activates PI3K/PKC-mTOR signaling and F-actin disruption. 3: Acidic parasitophorous vacuole is formed as T. cruzi begins internalization. Mitochondria are localized near flagellar pocket (blue arrow). T. cruzi vacuolar closure and maturation/acidification involve continuous communication with ER/Golgi vesicles. Glutamine synthetase (GS) controls pH by regulating NH4⁺ vacuolar content (small pink squares). 4: T. cruzi vacuolar degradation. 5: T. cruzi vacuolar escape into cytoplasm. MT form remains intact, or an intermediate form between MT and intracellular amastigote (IA) is present. 6: T. cruzi extravacuolar differentiation into IA forms. 7: Free T. cruzi IA in cytoplasm. 8: T. cruzi IA cytoplasmic replication. T. cruzi culture-derived trypomastigote (CDT) invasion by lysosome-dependent pathway (early lysosome fusion; B). 1: CDT induces membrane damage. 2: CDT requires parasite oligopeptidase B surface protein and recruitment of lysosomes to the infection site in a Ca²⁺-dependent manner. F-actin disruption also plays a role in CDT invasion. 3-8: Similar to (A). T. cruzi culture-derived trypomastigote (CDT) invasion by lysosome-independent pathway (late lysosome fusion; C). 1: CDT invades by membrane invagination resulting in a PI3K-dependent PIP3 accumulation. In deprivation of nutrients host cell increases CDT internalization through ATG5/Beclin1 pathway. 2: lysosome markers are acquired only during PV maturation. ER membrane is donated to PV during its membrane construction. 3-8: Similar to (A). Model of L. amazonensis non-phagocytic cell internalization as proposed by Cavalcante-Costa et al. (2019); D). 1: L. amazonensis induces membrane damage and Ca²⁺ recruitment during invasion process. 2: L. amazonensis vacuolar formation and maturation. 3: L. amazonensis multiplication into large vacuoles.

FIGURE 2

Entry of trypanosomatids in a professional phagocytic cell. (A) 1: Uptake of Trypanosoma cruzi trypomastigote by phagocytosis. This process can be facilitated through expression of phosphatidylserine (PS) in parasite surface. VAMP3 (SNARE) recruitment to entry site in early-phase infection. 2: T. cruzi vacuolar closure and maturation. VAMP7 (SNARE) is recruited and maintained during PV maturation. Parasite uses Fe-SOD to reduce the amount of O₂^– produced by host (NOX-2) inside phagosome. PS exposition creates a permissive state to parasite survival. (B) 1: Uptake of L. amazonensis amastigote by phagocytosis. 2: L. amazonensis vacuolar formation and maturation. 3: L. amazonensis and T. cruzi vacuolar fusion. 4: Chimeric vacuole formation with L. amazonensis amastigotes and T. cruzi trypomastigotes. (C) 1: Uptake of L. amazonensis promastigote by phagocytosis. 2: L. amazonensis vacuolar formation and maturation. Rab5 and EEA1 endosome markers are acquired after internalization. 3: L. amazonensis multiplication into large vacuoles. ATP6V0d2 subunit of V-ATPase participates in cholesterol influx, an essential process in PV maintenance. Syntaxin 15 and Sec22b are SNAREs that play important roles in PV development. (D) 1: Uptake of L. major by phagocytosis. 2: L. major vacuolar formation and maturation. The parasite is developed in a single compact PV. 3: L. major multiplication into tight individual vacuoles. Parasite virulence factors GP63 and LPG are transferred to host cell (ER/ERGIC; “ER-Golgi intermediate compartment”) by SNAREs. Both promastigotes and amastigotes of L. amazonesis and L. major can present PS in its surface with the same functions as in T. cruzi facilitating parasite phagocytosis and, once the parasite is inside the cell, aiding in its survival.