Abstract
Trypanosoma cruzi is the causal agent of Chagas disease that affects 6–7 million people around the world, principally in Latin America. This disease is characterized for the presence of an acute phase in which the host immune response plays a central role in the elimination of the parasite. If the parasite is not efficiently eliminated, patients can remain asymptomatic or develop a chronic infection. One of the cells that are primarily infected with this intracellular parasite is macrophages (Mϕ). Mϕ present a wide array of activation states with classically activated macrophages in one pole (CAMϕ) and alternatively activated macrophages (AAMϕ) in the other. One of the most important differences between these two activation states is the presence of the inducible nitric oxide synthase (iNOS or NOS2) in CAMϕ and arginase 1 (Arg-1) in AAMϕ; both enzymes share the same substrate, l-arginine, and are reciprocally regulated by the action of Th1 cytokines in the case of NOS2 and Th2 cytokines in the case of Arg-1. The activation of CAMϕ permits the production of nitric oxide (NO), highly trypanotoxic, while the activation of AAMϕ allows the synthesis of polyamines, necessary for parasite duplication. l-arginine is a very important metabolite situated in the center between the elimination and perpetuation of T. cruzi.
Keywords
- arginase-1
- l-arginine
- inducible nitric oxide synthase
- macrophages
- trypanosoma cruzi
1. Introduction
This intracellular obligate parasite enters the human host in the form of metacyclic promastigotes that are released from the triatomine feces during the blood meal, through damaged skin or mucosae. Alternatively, infection can occur through other routes such as oral, congenital, blood transfusions, or organ transplants. After entering the host, trypomastigotes are phagocytized mainly by macrophages, where they transform to amastigotes, the intracellular form that has the ability to replicate. In order to evade the host immune response and ensure its persistence inside macrophages,
2. Phases of the infection with Trypanosoma cruzi
The infection with
3. Generalities of l-arginine
ʟ-arginine is one of the most versatile amino acids at the metabolic level. Besides serving as a precursor for protein synthesis, it is also a precursor of multiple compounds of great biologic importance such as urea, nitric oxide, polyamines, l-proline, glutamate, creatinine, and agmatine (Figure 1) [6, 7].
In adult mammals, ʟ-arginine is a nonessential amino acid; nevertheless, during childhood and certain physiologic or pathologic conditions (e.g., pregnancy, sepsis, trauma, catabolic stress, intestinal or renal damage), it is considered as a semi-essential amino acid or conditioned nonessential, due to the fact that its consumption exceeds the capacity of being synthesized by the organism and has to be supplied exogenously [8, 9, 10]. In mammals, the provision of l-arginine depends on its procurement through the protein diet, endogenous synthesis (de novo synthesis), and its release during the process of protein replacement (Figure 1) [6]. Approximately 40% of the l-arginine that is obtained from the protein diet is catabolized in the intestine before entering the circulation [11]. In the absence of the contribution by the protein diet, approximately 80% of the ʟ-arginine that enters the circulation derives from the protein replacement, and the remaining percentage is obtained through the novo synthesis [11]. l-arginine metabolism occurs basically in the liver and kidney; nevertheless, other tissues and cells also possess the required enzymes to metabolize it, including some cells of the immune response [12]. Regarding last point, it is interesting to note that a complete urea cycle has been described in macrophages [13]. Although only two enzymes directly involved in l-arginine synthesis have been identified (arginine succinate synthetase and arginine succinate lyase that are the third and fourth enzymes of the urea cycle), four enzymes utilize this amino acid as substrate: arginine decarboxylase, arginine/glycine aminotransferase, different isoforms of arginase (Arg), and the different isoforms of the nitric oxide synthases (NOS), the last two being the most studied and characterized [12]. In mammals two arginase isoforms exist, Arg-1 and Arg-2, that catalyze the same reaction but differ in cellular expression and subcellular localization. Arg-1 is cytosolic and is highly expressed in the liver and some cells of the immune response. Compared to Arg-1, Arg-2 is mitochondrial and is expressed in a great variety of peripheral tissues, mainly in the kidney, prostate, small intestine, and mammary glands during lactation [14]. Regarding NOS, this enzyme is present in three isoforms: neuronal NOS (nNOS or NOS1), inducible nitric oxide synthase (iNOS or NOS2), and endothelial NOS (eNOS or NOS3). NOS 1 is expressed in specific neurons of the central nervous system (CNS), and NOS3 is mostly expressed in endothelial cells [15]. NOS 2 is not usually expressed in cells, but its expression can be induced by bacterial lipopolysaccharide, cytokines, and other agents. Although primarily identified in macrophages, the expression of this enzyme can be stimulated in almost any cell or tissue, provided that the appropriate inducing agents are present [16].
4. l-arginine metabolism in the immune response: special emphasis in macrophages
In the immune response, ʟ-arginine metabolism through NOS2 and Arg-1 has a pivotal role in the regulation of the effector capabilities of macrophages, dendritic cells, and neutrophils [17, 18, 19, 20] during infectious processes caused by a great variety of microorganisms: different species of
l-arginine metabolism in the immune response acquired great relevance with the discovery that murine macrophages express both NOS2 and Arg-1 and that their expression is reciprocally regulated by the action of Th1/proinflammatory cytokines (e.g., IFN-γ and TNF-α) and Th2/anti-inflammatory (e.g., IL-4, IL-10, and IL-13) that determine the activation state of macrophages [19, 23, 24, 25, 26, 27]. Th1 cytokines activate macrophages in a classical way (CAMФ) and induce the expression and function of NOS2, while Th2 cytokines activate macrophages in an alternative way (AAMФ) and induce the expression and function of Arg-1.
NOS2 or iNOS is an oxide-reductase responsible for the synthesis of l-citrulline and nitric oxide (NO•) from ʟ-arginine in the presence of NADPH and oxygen. This reaction occurs through two successive reactions: the monooxygenation of ʟ-arginine that drives to the production of the intermediary Nω-OH- l-arginine (NOHA) and the subsequent hydrolysis of this last compound, thus producing ʟ-citrulline and NO• (Figure 2). NOS2 generates both NO• and superoxide (O2−) that together react to form the radical peroxynitrite (ONOO−) [28]. This last compound has been identified as a reactive species derived both from oxygen and nitrogen (RONS) that constitutes the principal cytostatic or cytotoxic mechanism of CAMФ to fight the infections generated by virus, bacteria, fungi, and protozoan parasites [20, 23, 26, 29].
5. Immune response to Trypanosoma cruzi
Inside the mammalian host, macrophages represent an important site for the duplication of
In response to the defense mechanisms of the host, parasites have developed several strategies in order to escape host immune response and take advantage of some host’s molecules. In this way, parasites must reduce the production of toxic molecules, including nitric oxide and its derivatives, that are synthesized by the immune system, in particular by macrophages [32, 33, 34]. In addition, internalized parasites of different
6. Role of Arg-1 in the infection with Trypanosoma
The induction of Arg-1 in macrophages promotes the infection of parasites of the genus
7. Conclusion
Acknowledgments
This work was funded by project number IN218119 from Papiit, DGAPA, UNAM, to LGK.
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