Fine structure of the interaction of T. cruzi with host cells

The first steps of the T. cruzi—mammalian host cell interaction process can be divided into three stages: (1) adhesion and recognition; (2) signaling; and (3) invasion. The adhesion step involves the recognition of molecules present on the surface of both the parasite and the host cell. Initially, the interaction process with the host cell was described as split into two mechanisms: active penetration and phagocytosis. The term active penetration was first used by Romana and Meyer¹² as a process in which mechanical activity of the protozoan is prevalent, and the parasites penetrate into the cell through the plasma membrane. In this classic paper, it was stated that “in general, the active penetration was more visible with metacyclic forms that, with their great motility, easily crossed the cell surface, penetrating into the protoplasm of fibroblasts and myocytes.” Posteriorly, Meyer and Xavier de Oliveira⁵⁹ demonstrated that the parasite “can touch the surface of the host cells without penetration. Occasionally, they adhere to the cell surface and suddenly penetrate into the cell.” The same idea was presented in 1973 by Dvorak and Hyde.^60,61 In the 1970s, electron microscopy data demonstrated the parasite is always present in an intracellular vacuole surrounded by a membrane originated from the host cell plasma membrane, without direct contact with the host cell cytoplasm. The early step to internalization known as adhesion is a process that depends on receptors restricted to membrane domains.

Different strains of T. cruzi as well as different forms of the parasite (tissue culture-derived trypomastigotes, metacyclic trypomastigotes, and amastigotes) express different molecules on their surface. In the case of trypomastigotes, several surface-exposed glycoproteins have been described that play roles in the interaction process, including the following: (1) gp90, which seems to have glycosidase activity, downregulates host cell invasion probably due to its lack of Ca²¹ signal- inducing activity.^62,63 (2) Mucins, which are the major T. cruzi surface glycoproteins, have sugar residues that interact with mammalian cells.^64,65 Many mucins have been implicated in the host cell infection process.^{65 -67} (3) The Tc85 molecule, which is abundant in trypomastigotes, is part of the gp85/trans-sialidase family, which includes other proteins such as gp85, gp82, TSA-1, and trans- sialidases. Tc85 forms a population of heterogeneous GPI-glycoproteins with similar molecular masses but different electric charges.^68-71 Because the Tc85 family is composed of multiadhesive glycoproteins, its members are capable of binding to different receptor molecules either located on the cell surface, like host cell cyto- keratin 18,⁷² or belonging to components of the extracellular matrix, like fibronec- tin⁷³ and laminin.⁷² (4) Two groups of glycoproteins, gp82 and gp35/50, are also involved in parasite invasion. Both proteins are expressed on the surface of metacyclic trypomastigotes.⁷⁴ In 1998, Favoretto and colleagues⁷⁵ demonstrated that gp82 is the signaling receptor that mediates protein tyrosine phosphorylation, which is necessary for host cell invasion. Phospholipase C and IP3 are also involved in this signaling cascade, which is initiated at the parasite cell surface by gp82 and leads to the Ca²¹ mobilization required to target cell invasion.^75,76 Besides these glycoproteins, a new class of surface proteins of T. cruzi was described: the TcSMP⁷⁷ expressed in all developmental stages of T. cruzi and also present in other members of the Trypanosoma genus. (5) Present in all strains of T. cruzi, Gp83 is a ligand employed by the parasite to attach and enter both professional and nonprofessional phagocytic cells.^78,79 It is expressed only in infective trypomastigotes.⁸⁰ (6) Some T. cruzi proteases, including cruzipain, oligopeptidase B and Tc80, have been implicated in the process of host-cell infection. Oligopeptidase B, a serine endopeptidase, is secreted by T. cruzi trypomastigotes.^81,82 This soluble factor is generated by the action of a 120-kDa alkaline peptidase on precursors present only in infective trypomastigotes. This 80-kDa cytosolic serine peptidase indirectly induces [Ca²¹]_rtransients during T. cruzi invasion.^82,83 Tc80 is a prolyl oligopepti- dase and a member of the serine protease family that hydrolyzes human collagens type I and IV at neutral pH. Fibronectin is important for the parasite’s transit through the extracellular matrix^84,85 showed that the parasite’s entry into the host cell was blocked by treatment with selective inhibitors of Tc80.

Several surface-exposed molecules on the host cell have been shown to be involved in the process of T. cruzi—host cell interaction. These include proteins/gly- coproteins released by trypsin and/or neuraminidase, lectin-binding sites, and lectin-like molecules. ^, Galectin-3, a (3-galactosyl-binding lectin, is a type of lectin involved in T. cruzi attachment, and its binding has been suggested to mediate parasite attachment and entry into both dendritic cells and smooth muscle cells.⁸⁹ This molecule is recruited during parasite invasion of host cells and influences the intracellular trafficking of amastigotes.⁸⁴ Besides, Reignault and coworkers⁹⁰ demonstrated that galectin 3 is recruited to parasitophorous vacuole of T. cruzi and the in presence of galectin 3 this vacuole remains for 3 h, and may be involved in the lysis process. Another galectin that is involved in T. cruzi infection is galectin-1.⁹¹ Galectin-1 not only reduced infection by T. cruzi but also diminished parasite phosphatidylserine exposure. Tc85, which is present in the T. cruzi membrane and has been associated with parasite invasion, contains fibronectin-like binding sequences⁹² and a laminin-binding domain.⁷² Besides functioning as a cellular link to laminin or fibronectin, integrins also function as receptors that can activate PI-3 kinase signaling pathways. Another host cell receptor utilized by trypomastigotes that trigger PI3K is LDL receptor.⁹³ Tc85 can bind to cytokeratin 18, a cytoskeletal protein that was suggested to function as a T. cruzi receptor.⁸⁶ However, when cytokeratin 18 expression was silenced by RNAi, the binding of trypomastigotes to host cells was not affected.⁹² Another molecule present on the host cell surface and involved in trypomastigote entry is the TGF(3 receptor. Signal transduction through TGF-(3 receptors facilitates T. cruzi entry into epithelial cells.^84,94 The trypomastigote molecule that is capable of binding to the TGF(3 receptor has not yet been identified. Ming and colleagues⁹⁵ proposed that infective stages of T. cruzi secrete a TGF(3-like molecule or a factor capable of activating latent host TGF(3. The exposure of phosphatidylserine on the surface of T. cruzi trypomastigotes⁹⁶ and its deactivating effect on macrophages by the induction of TGF(3 suggests that phosphatidylserine is a possible activator of host-cell TGF(3 receptor.