Mechanisms of regulatory T cell infiltration in tumors: implications for innovative immune precision therapies

Abstract

With the broad application of cancer immunotherapies such as immune checkpoint inhibitors in multiple cancer types, the immunological landscape in the tumor microenvironment (TME) has become enormously important for determining the optimal cancer treatment. Tumors can be immunologically divided into two categories: inflamed and non-inflamed based on the extent of immune cell infiltration and their activation status. In general, immunotherapies are preferable for the inflamed tumors than for non-inflamed tumors. Regulatory T cells (Tregs), an immunosuppressive subset of CD4⁺ T cells, play an essential role in maintaining self-tolerance and immunological homeostasis. In tumor immunity, Tregs compromise immune surveillance against cancer in healthy individuals and impair the antitumor immune response in tumor-bearing hosts. Tregs, therefore, accelerate immune evasion by tumor cells, leading to tumor development and progression in various types of cancer. Therefore, Tregs are considered to be a crucial therapeutic target for cancer immunotherapy. Abundant Tregs are observed in the TME in many types of cancer, both in inflamed and non-inflamed tumors. Diverse mechanisms of Treg accumulation, activation, and survival in the TME have been uncovered for different tumor types, indicating the importance of understanding the mechanism of Treg infiltration in each patient when selecting the optimal Treg-targeted therapy. Here, we review recent advances in the understanding of mechanisms leading to Treg abundance in the TME to optimize Treg-targeted therapy. Furthermore, in addition to the conventional strategies targeting cell surface molecules predominantly expressed by Tregs, reagents targeting molecules and signaling pathways specifically employed by Tregs for infiltration, activation, and survival in each tumor type are illustrated as novel Treg-targeted therapies. The effectiveness of immune precision therapy depends on conditions in the TME of each cancer patient.

Keywords: CD4-positive T-lymphocytes; costimulatory and inhibitory molecules; immunotherapy; tumor microenvironment.

Figure 1

Mechanisms of immunosuppression by eTregs. (A) FOXP3⁺CD4⁺ T cells can be classified into three fractions based on FOXP3 (and/or CD25) and CD45RA expression levels: fraction 1 (Fr. 1), naive Tregs defined as FOXP3^low(CD25^low)CD45RA⁺ cells; fraction 2 (Fr. 2), eTregs, defined as FOXP3^high(CD25^high)CD45RA⁻ cells; and fraction 3 (Fr. 3), non-Tregs, defined as FOXP3^low(CD25^low)CD45RA⁻ cells. Naive Tregs (Fr. 1) that recently left the thymus have weak immunosuppressive activity. Once naive Tregs (Fr. 1) receive TCR stimulation, they differentiate into eTregs (Fr. 2), which have strong immunosuppressive activity. Non-Tregs (Fr. 3) do not have immunosuppressive activity. (B) Coinhibitory receptor cytotoxic T lymphocyte antigen 4 (CTLA-4) in Tregs inhibits costimulatory signaling via CD80/B7-1 and CD86/B7-2 in antigen presenting cells (APCs) due to its high affinity binding to CD80/B7-1 and CD86/B7-2. when these costimulatory molecules interact with CTLA-4, they are captured from APCs by transendocytosis. Compared with effector T cells, Tregs harbor receptors with higher affinity for IL-2: much higher CD25 expression in Tregs than in effector T cells. This provides Tregs with a competitive advantage in utilizing the limited amount of IL-2 in the TME. Tregs produce TGF-β, IL-10, and IL-35 for immunosuppression. TGF-β reduces the cytotoxic function of effector T cells. Fgl2 secreted by Tregs binds to FcγRIIB in CD8⁺ T cells and leads to their apoptosis. CD39 and CD73 expressed on the cell surface of Tregs act as ectonucleotidases that hydrolyze ATP or ADP to AMP and AMP to adenosine, respectively. Adenosine suppresses effector T cells. eTregs, effector Tregs; FcγRIIB, Fc fragment of IgG receptor IIb; FOXP3, Forkhead box P3; IL-10, interleukin 10; TCR, T cell receptor; TGF-β, tumor growth factor beta; TME, tumor microenvironment; Tregs, regulatory T cells.

Figure 2

Mechanisms leading to Treg infiltration and adaptation in the inflamed TME. Inflammation-associated infiltration: chemokine and cytokine dependent recruitment. Tregs possess multiple chemokine receptors. Chemokine gradients such as CCR4-CCL17/22, CCR8-CCL1, CCR5-CCL5, and CCR10-CCL28 are involved in recruiting Tregs into the TME. Hyperactivation of focal adhesion kinase (FAK) is correlated with Treg infiltration and CD8⁺ T cell exclusion via regulation of the production of chemokines such as CCL5 by tumor cells. Tumor hypoxia induces the expression of CCL28 and promotes the recruitment of Tregs via CCR10. activated CD8⁺ T cells also produce CCL17/22 that recruit Tregs. On the other hand, chemokine gradients such as CXCR3-CXCL9/10/11 are involved in CD8⁺ T cell recruitment. Metabolic adaptation. Effector T cells and Tregs employ different metabolic system in normal versus inflammatory conditions. TCR stimulation provokes a specific metabolic program through the PI3K-mTOR signaling pathway, leading to increased uptake of glucose through glucose transporter (GLUT) to enhance aerobic glycolysis. Activated effector T cells shift their metabolic program from oxidative phosphorylation (OXPHOS) to aerobic glycolysis. Metabolic reprogramming in tumor cells changes the TME into a nutrient-restricted, lactate-rich, and hypoxic environment, which is unfavorable for the survival and function of effector T cells. FOXP3 plays an essential role in this distinct metabolic program of Tregs by suppressing glycolysis, promoting OXPHOS, and enabling the use of lactate through monocarboxylate transporter 1 (MCT1) as an energy source. While tumor-infiltrating non-Tregs convert pyruvate to lactate to maintain glycolysis, Tregs in the TME convert pyruvate to acetyl-CoA in the mitochondria to trigger the tricarboxylic acid (TCA) cycle, which provides a survival benefit to Tregs over effector T cells in the low-glucose, high-extracellular lactate TME. mTOR, mammalian target of rapamycin; PI3K phosphoinositide 3-kinase; TME, tumor microenvironment; Tregs, regulatory T cells.

Figure 3

Mechanisms leading to Treg infiltration and adaptation in the non-inflamed TME. Tumor cell intrinsic signal-dependent infiltration. Certain gene alterations can modify the chemokine profile in tumor cells by modulating downstream signaling pathways. EGFR mutations in lung adenocarcinoma reduce CXCL10 production through interferon regulatory factor 1 (IRF1) inhibition. CCL22 production is increased via JUN induction, leading to CCR4-dependent Treg infiltration in the TME. In gastric cancers with RHOA mutations, CXCL10/11 levels are reduced through IRF1 suppression. RHOA mutations in gastric cancer produce large amounts of fatty acids through upregulation of fatty acid synthase (FASN) compared with tumors without RHOA mutations, leading to more Tregs and fewer effector T cells in the TME. Fatty acids produced in the TME can be used by Tregs as an energy source. Tregs use mechanisms in the fatty acid metabolism, such as upregulating the fatty acid transporter CD36, to adapt to the fatty acid-rich TME. FAK, focal adhesion kinase; GLUT, glucose through glucose transporter; OXPHOS, oxidative phosphorylation; PI3K, phosphoinositide 3-kinase; TME, tumor microenvironment; Tregs, regulatory T cells.