Arginine metabolism in immune cells

Arginine metabolism in immune cells is the network of enzymatic pathways that channels the semi-essential amino acid L-arginine into nitric oxide (NO), ornithine, urea, polyamines, proline, and creatine to regulate immune-cell activation, differentiation, and effector function. It sits at the intersection of nutrient sensing, inflammation, and tissue adaptation.

Amino acid availability is now recognized as a decisive input into immune-cell fate. Arginine is particularly influential because it is not only consumed for protein synthesis but also acts as a substrate for enzymes whose products shape antimicrobial defense, immune suppression, tissue repair, and anti-tumor immunity. Shifts in arginine flux therefore represent an immunometabolic checkpoint across steady-state physiology, infection, chronic inflammation, and cancer.¹,²

Arginine as an immunometabolic hub

Arginine sits at the branch point of two competing enzymatic routes. Nitric oxide synthases (NOS1, NOS2/iNOS, NOS3) oxidize arginine to NO and citrulline, driving antimicrobial and inflammatory functions. Arginases (ARG1, ARG2) hydrolyze arginine to ornithine and urea, feeding polyamine and proline synthesis that support proliferation, extracellular matrix deposition, and wound repair.¹,³

The intracellular pool of arginine is further shaped by transport and biosynthesis. Cationic amino acid transporters (CAT family) mediate arginine uptake, while argininosuccinate synthase 1 (ASS1) and argininosuccinate lyase (ASL) regenerate arginine from citrulline and aspartate. In parallel, neutral amino acid transporters such as ASCT2/SLC1A5 and the heavy-chain adaptor CD98/SLC3A2 coordinate glutamine and essential amino acid uptake, linking arginine flux to mTORC1 signaling and mitochondrial activity.²,⁴ Arginine metabolism therefore never acts in isolation — it is embedded within a broader nitrogen and nutrient economy that immune cells remodel upon activation.

Arginine metabolism and T cell function

T cells are among the most arginine-sensitive immune cells. Sufficient extracellular arginine sustains T cell receptor (TCR) signaling, expression of activation markers such as CD3ζ, CD25, and CD69, and clonal expansion. When local arginine is depleted, for example, by arginase-expressing myeloid cells in tumors or chronic inflammation, TCR signaling is blunted and proliferation stalls, generating an immunosuppressive microenvironment.⁵

Arginine also shapes T cell differentiation through NO- and polyamine-dependent mechanisms. Low NO concentrations can favor Th1 polarization and IFN-γ production, whereas high NO suppresses proliferation and alters cytokine output; effects on Th17 and regulatory T cells are strongly context-dependent.³ Polyamines such as spermidine, derived from ornithine, influence autophagy, mitochondrial fitness, and immune aging, and have been linked to restoration of CD8+ T cell function in aged systems. Creatine, another arginine-derived metabolite, contributes to energy buffering and effector persistence.⁶

Macrophage polarisation: NOS versus arginase activity

Macrophages provide the clearest illustration of how arginine metabolism dictates immune-cell phenotype. Classically activated (M1-like) macrophages upregulate iNOS/NOS2, generating NO for antimicrobial and cytotoxic activity. Alternatively activated (M2-like) macrophages, in many experimental systems, express ARG1 and divert arginine toward ornithine, polyamines, and proline to support tissue repair, matrix remodeling, and fibrosis.¹,⁷

ARG2, unlike cytosolic ARG1, resides in mitochondria and can influence redox balance, oxidative phosphorylation, and inflammatory signalling. In myeloid cells, ARG2 activity contributes to local arginine depletion and can drive suppressive or tissue-remodeling phenotypes depending on microenvironmental cues.⁷

This NOS–arginase balance is not a rigid pro- versus anti-inflammatory dichotomy but a flexible metabolic program responsive to hypoxia, cytokines, tissue damage, and tumor-derived signals. In tumors, macrophages leveraging arginine-derived polyamines can reinforce immunosuppression and dampen CD8+ T cell activity. Metabolic crosstalk involving ASS1-mediated arginine synthesis between tumor cells and macrophages further shapes the microenvironment.⁸

Dendritic cells and antigen presentation

Dendritic cells (DCs) link innate sensing to adaptive priming, and arginine metabolism modulates their maturation and stimulatory capacity. iNOS-derived NO in DCs can influence mitochondrial function and cytokine output; at high concentrations, NO suppresses T cell proliferation and reduces DC stimulatory capacity, while reactive nitrogen species may contribute to tumor-cell killing and antigen release in other settings.³

Arginase activity in DCs further limits T cell responses by depleting local arginine, an effect particularly relevant in tumor and chronic inflammatory niches where CD8+ T cell priming is compromised. The functional outcome depends on metabolite concentration, inflammatory context, and tissue location.⁵

Neutrophils, NK cells, B cells, and MDSCs

Arginine metabolism extends well beyond T cells and macrophages. Neutrophils store high levels of arginase and release it during degranulation, lowering extracellular arginine and dampening T cell responses. The same depletion can, however, restrict pathogen growth — illustrating how a single metabolic axis can support either host defense or immune suppression.⁹

Natural killer (NK) cells also require adequate arginine for cytotoxicity and cytokine production; arginine limitation reduces NK effector function, weakening innate anti-tumor responses. B cells and plasma cells are less well characterized in this axis, but iNOS has been implicated in plasma-cell homeostasis, suggesting arginine-derived NO contributes to antibody-producing cell biology.²

Myeloid-derived suppressor cells (MDSCs) are perhaps the most potent arginine-metabolizing suppressive population. Co-expression of arginase and iNOS enables MDSCs to deplete arginine and generate reactive nitrogen species that impair TCR signaling, block T cell proliferation, and reduce immune-cell trafficking into tumors. Arginase inhibition in MDSCs is therefore actively investigated as a strategy to relieve immune suppression.⁵,⁹

Integration with amino acid transport and biosynthesis

Activated immune cells increase amino acid uptake to fuel proliferation and effector function. ASCT2/SLC1A5 supports glutamine influx, while CD98/SLC3A2 partners with light chains to import essential amino acids, driving mTORC1 activation and metabolic reprogramming.⁴ ASS1 enables de novo arginine synthesis from citrulline and aspartate, sustaining intracellular arginine when extracellular supply is limited. In tumors, differential ASS1 expression between malignant and immune compartments can determine whether arginine flux favors immune activation or immune escape.⁸

Relevance in inflammatory disease and cancer

In pulmonary inflammation, asthma, and chronic obstructive pulmonary disease, arginine metabolism shapes airway inflammation, macrophage phenotype, NO output, and fibrotic remodeling. In infection, NO supports microbial killing, while dysregulated NO contributes to collateral tissue damage. Arginase-driven ornithine and proline synthesis fuels collagen deposition in fibrotic disease.¹,⁷

In cancer, arginine metabolism is a major axis of immune escape. Tumors and suppressive myeloid populations deplete arginine, expand polyamine pools, and impair CD8+ T cell function. Conversely, tumors with reduced ASS1 expression become auxotrophic for extracellular arginine, opening therapeutic opportunities for arginine-deprivation strategies. This dual character, metabolic vulnerability in tumor cells alongside metabolic suppression of immune cells, makes arginine metabolism a compelling therapeutic axis.⁸,¹⁰

How does arginine depletion suppress T cell function?

Arginine depletion suppresses T cells primarily by destabilizing the CD3ζ chain, impairing TCR signal transduction, and arresting cell-cycle progression. Reduced arginine also limits polyamine and creatine synthesis needed for proliferation and effector metabolism, and it dampens mTORC1 activity, collectively blunting activation, expansion, and cytokine production.⁵,⁶

Why do macrophages express both NOS and arginase pathways?

Macrophages retain both pathways because they must switch between antimicrobial and reparative programs. NOS2-driven NO supports pathogen killing during acute infection, while arginase-driven ornithine, polyamine, and proline synthesis supports resolution, matrix deposition, and wound healing. The balance is dynamically tuned by cytokines, hypoxia, and tissue signals rather than fixed to a single phenotype.¹,⁷

How does arginine metabolism contribute to tumor immune escape?

Tumor-associated myeloid cells, including MDSCs and tumor-associated macrophages, deplete arginine through arginase and iNOS activity, generating reactive nitrogen species and polyamines that suppress CD8+ T cell and NK-cell effector function. Concurrent ASS1-dependent metabolic crosstalk between tumor and stromal cells can further reinforce an immunosuppressive microenvironment.⁵,⁸

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References

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