Amino acid auxotrophy as a system of immunological control nodes

Abstract

Cells of the immune system are auxotrophs for most amino acids, including several nonessential ones. Arginine and tryptophan are used within the regulatory immune networks to control proliferation and function through pathways that actively deplete the amino acid from the microenvironment or that create regulatory molecules such as nitric oxide or kynurenines. How immune cells integrate information about essential amino acid supplies and then transfer these signals to growth and activation pathways remains unclear but has potential for pathway discovery about amino sensing. In applied research, strategies to harness amino acid auxotrophy so as to block cancerous lymphocyte growth have been attempted for decades with limited success. Emerging insights about amino acid metabolism may lead to new strategies in clinical medicine whereby both amino acid auxotrophy and the immunoregulatory pathways controlled by amino acids can be manipulated.

Figure 1. Regulated amino acid metabolizing enzymes in immunity

Shown are structures and reaction mechanisms of iNOS, IDO1, Arg1 derived from deposited coordinates in the PDB (pdb.org). The structure of IDO2 has not been reported but is anticipated to be similar to IDO1. The structure of Arg2 is similar to Arg1 (Ref.). The primary sequence structure of mammalian IL4i1 is conserved compared to snake venom L-amino acid oxidases.

Figure 2. Arg1 in immune responses

(a) Signaling pathways that increase Arg1 gene expression in the mouse. Two main pathways control Arg1 expression (STAT6 and STAT3). In addition, other pathways modulate the STAT3 and STAT6 pathways such as IL-10-mediated increases in IL-4Rα expression, or act independently of the main pathways, such as hypoxia, lactate and adenosine pathways^-. (b) Schematic of the how macrophage Arg1 regulates T cell proliferation in liver granulomas. In normal animals, Arg1⁺ macrophage encase schistosome eggs. The granulomas are embedded within the liver paranchyma, of which all hepatocytes constitutively express Arg1. When Arg1 is deleted only from macrophages, T cell proliferation is enhanced, leading to a feed-forward lethal non-resolving granulomatous reaction^, . (c) Possible mechanism of macrophage Arg1 in controlling worm motility. Macrophages are recruited to deposited antibodies on the surface of worms, which in turn activates Arg1 expression and ornithine production. Ornithine is proposed to contribute to stunting worm movement. (d) Simplified schematic of the competition mechanism between Arg1 and iNOS when both enzymes are co-expressed^, .

Figure 3. Arginine regeneration and the anaplerotic Krebs cycle in M1 macrophages

Key regulatory events in the links between the pathways include the supply of citrulline as a substrate, the role of NO in blocking mitochondrial respiration, the expression of the key enzymes and the flux through both pathways at a given time^{, ,} . Many features of this reaction cycle remain to be uncovered.

Figure 4. Clinical exploitation of amino acid auxotrophy

Shown are three examples of proteins that degrade amino acids (ASNase, ADI and Arg1) that can be used to starve target cells of amino acids. Two examples of inhibition strategies for amino avid degrading enzymes are shown: arginases and IDO proteins. In the case of inhibition, the goal is to block the immunoregulatory effects of arginases or IDOs and enhance T cell responses. Possible complications are listed below.