Human neutrophils stimulated by
T. cruzi generate neutrophil extracellular traps (NETs), which are fibrous traps of DNA, histones, and granules that are involved in pathogen killing. A study reported that NET release was reduced as a result of treatment with antibodies against TLR2 and TLR4
[62][51].
T. cruzi-infected macrophages are able to produce higher amounts of extracellular vesicles (EVs) with respect to non-infected cells. These EVs interact with TLR2 and induce translocation of NF-κB to the nucleus, thereby activating the production of pro-inflammatory cytokines (TNF-α, IL-6 and IL-1β) which are able to maintain the inflammatory response
[63][52].
TLR2 inhibition increases the histopathological damage induced by parasites, reduces IL-6 and IL-10 secretion and the expression of proliferation and differentiation markers, while increasing the expression of cell death markers
[68][53].
Galectin-3, a β-galactoside-binding lectin, acting in several biological processes
[69][54], allow macrophages and epithelial cells to bind galactosides of membrane debris obtained from the vacuoles that are used to evade the phagolysosomal pathway of the host by some intra-vacuolar pathogens
[45][29]. Galectin-3 is able to favor cellular infiltration in the hearts of mice infected by the pathogen, collagen deposition and cardiac fibrosis.
5. Virulence Factors
Trypanosoma cruzi, during its different stages, can infect different host cells using several virulence mechanisms: resistance to oxidative damage, humoral immune response evasion and cell invasion
[71][55]. Different virulence factors act in a sequential manner during the different phases of the
T. cruzi life cycle. Upon infection, metacyclic trypomastigotes (MT) mainly invade local macrophages, fibroblasts and tissues at the site of infection
[72][56]. The antioxidant mechanisms used by
T. cruzi are crucial for the inactivation of reactive oxygen and nitrogen species released by the host cells at the early stage of the infection
[73][57]. The parasite produces several enzymes, such as peroxidases, that act on different molecules from the cellular oxidative pathway. Glutathione peroxidase TcGPXI (present in the cytosol) deactivates exogenous hydroper-oxides and TcGPXII (present in the endoplasmatic reticulum) inactivates lipid-hydroperoxides
[74][58]. Ascorbate-dependent heme peroxidase TcAPX disables the binding of hydroxyl ions with oxygen in conjunction with the cytosolic tryparedoxin peroxidase TcCPX and mitochondrial TcMPX.
T. cruzi also has iron superoxide dismutases (FeSOD) that detoxify reactive oxygen species generated in the cytosol, glycosomes and mitochondria
[75][59]. The expression of enzymes of the
T. cruzi antioxidative network is related with its life cycle. After transforming into bloodstream trypomastigotes (BT),
T. cruzi is able to resist the humoral immune response and the lytic effects of the complement system
[76][60]. The evasion mechanism is mediated by the surface glycoproteins of
T. cruzi trypomastigotes, which restrict the activation of the classical and alternative complement pathways
[77][61]. The trypomastigote decay-accelerating factor (T-DAF) is a surface glycoprotein that interferes with the C3 convertase-mediated assembly of the classical and alternative pathways
[78][62]. The complement regulatory protein (CRP) is a surface-anchored glycoprotein expressed only by trypomastigotes, which inhibits the activation pathway of the complement system
[77][61]. T-DAF and CRP are trans-sialidase-like glycoproteins belonging to the
T. cruzi trans-sialidase superfamily
[79][63]. Both proteins impair C3b formation by interacting with C4b and C3b
[35][22]. Calreticulin (TcCRT) is a surface molecule that interacts with C1q to inhibit the activation of the classical complement pathway
[80][64]. The complement C2 receptor inhibitor trispanning (TcCRIT) factor impairs the activation of complement cascades via both the classical and lectin pathways through the cleavage of the shared C2 factor and impairs the formation of C3 convertase via its interaction with C4
[35][22]. The proline racemases (PRs) TcPRACA and TcPRACB are secreted and intracellular enzymes, respectively
[81][65]. TcPRACA is a B cell mitogen which initiates the activation of nonspecific polyclonal lymphocytes and is important for
T. cruzi evasion and persistence
[82][66]. The overexpression of TcPRAC isoforms results in increased parasite differentiation and cell invasion
[83][67]. Tc52 is a secreted protein responsible for suppressing T cell proliferation
[84][68]. It is able to modulate the expression of macrophage cytokines and iNOS and the production of NO
[85][69]. Once they parasites have differentiated into extracellular amastigotes (EA), they start a new cycle of infection and invade new host cells, therefore they require molecules that allow them an efficient cellular invasion that favors adhesion and the activation of signaling cascades
[86][70]. P21 and TcMVK proteins released by EAs favour host cell invasion. P21 rearranges actin filaments of the host cells and induces actin polymerization and phagocytosis
[87][71]. TcMVK is bound to the membrane of the host cells and induces parasite uptake into HeLa cells
[88][72].
T. cruzi has developed surface proteins (transialidases, mucins, mucin-associated surface glycoproteins and phospholipases) that allow the adhesion of metacyclic trypomastigotes and extracellular amastigotes to host cells through interactions with carbohydrates
[86][70]. Gp82 is a surface protein of the metacyclic phase of
T. cruzi that is responsible for adhesion to the host cell and activation of the Ca2+ signaling cascade, leading to internalization of the parasite
[89][73]. Transialidase enzymes (TS) are important for
T. cruzi virulence
[90][74] as they allow the pathogen to acquire sialic acid from host cells and modify trypomastigote surface proteins, making them capable of inducing cell paralysis and cell lysis.