Manipulating Tregs is a promising anti-cancer strategy

Manipulating Tregs is a promising anti-cancer strategy.  There are several Treg targeted therapies currently under study. In addition, targeted Tregs combined with other cancer treatments will be another good choice.

Preface

Regulatory T cells (Tregs), as an important mechanism for regulating the homeostasis of the immune system and immune tolerance of the body, play a vital role in tumor immune regulation and are currently a research hotspot in this field.

In the tumor microenvironment, traditional T cells can induce and differentiate Tregs, have a strong immunosuppressive function, inhibit anti-tumor immunity, and promote the occurrence and development of tumors. Tregs can also inhibit the function of immune effector cells through a variety of mechanisms, and are a key factor in tumor immune escape.

CD4+CD25+Foxp3+ is currently considered as a classic combinatorial marker of Tregs. In fact, in addition to being a marker of Treg, Foxp3 also controls the function of Treg. Only its continuous expression can ensure the maintenance of complete Treg inhibition. In the process of tumor immune escape, Tregs can secrete TGF-β, IL-10 and IL-35, down-regulate anti-tumor immunity, inhibit DC antigen presentation, CD4+ T helper cell (Th) function, and produce tumor-specific CD8+ cytotoxicity T lymphocytes (CTL).

Tregs have shown their potential as clinical targets, and the newly discovered new mechanisms of Tregs help us further understand the role of Tregs in potential treatment strategies and develop Treg targeted therapies.

01 Classification and functions of Tregs

Tregs can be divided into two categories: natural regulatory T cells (nTregs) and induced regulatory T cells (iTregs). Both types of Tregs ubiquitously express Foxp3. nTregs develop naturally in the thymus, and their inhibitory effect is achieved through cell-to-cell contact. Their main function is to maintain normal immune tolerance and control inflammation.

Source Reference 1

iTregs are derived from peripheral primitive T cells induced by tumor microenvironmental signals, including tumor antigens, cytokines (such as TGF-β) and other soluble molecules. iTregs inhibit the anti-tumor immunity of effector T cells (Teff), NK cells and DC through a variety of mechanisms that promote tumor progression.

Source Reference 1

Tregs mainly have the following five functional mechanisms:

① Tregs secrete inhibitory cytokines, including IL-10, TGF-β and IL-35.

② Tregs kill effector cells through granzyme and perforin.

③ Tregs affect the function of effector cells: Treg competes with effector T cells to consume IL-2, thereby inhibiting the growth of effector T cells; Tregs promote the production of adenosine in TME by producing extracellular enzymes CD39 and CD73, and induce the inhibition of effector cells. And anti-proliferative effect; Tregs transfer a large amount of cAMP to effector T cells through gap junctions and interfere with their metabolism.

④Tregs induce DC tolerance through stimulatory and inhibitory receptors (CTLA-4 or LAG3), which further inhibit the ability of T cells through IDO.

⑤The factors produced by MDSCs and Tregs form a positive feedback loop to promote proliferation and enhance the inhibition of the environment.

Source Reference 1

In addition, a study used a new strategy to define Tregs, Th1-like Tregs (T-bet+IFNγ+Foxp3+), Th2-like Tregs (Gata3+IRF4+IL4+Foxp3+) and Th17-like Tregs (IL-17+RORγt) +Foxp3+), which provides new ideas for targeted Tregs therapy.

02 New mechanism of Tregs action

New metabolic mechanism

The core of Tregs activation is the changes in lipid metabolism that support their survival and function, which can be promoted by fatty acid binding proteins (FABPs). In addition, glycolysis and oxidative metabolism both contribute to the expansion of Tregs, because the relative advantage of Tregs in tumors in glucose uptake may promote FA synthesis.

Source: Reference 1

Tregs show another unique metabolic feature is the increase of mitochondrial metabolism. Mitochondrial transcription factor A (Tfam) and mitochondrial complex III are necessary to maintain the immunosuppressive function of Tregs to regulate gene expression. In addition, the lack of Foxp3 can lead to dysregulation of mTORC2 signal transduction, and enhance aerobic glycolysis and oxidative phosphorylation.

It is worth noting that CD36 is selectively upregulated in Tregs in tumors and acts as a central metabolic regulator. CD36 finely regulates mitochondrial fitness through PPARβ signaling and reprograms Tregs to adapt to the lactate-rich TME. In the course of anti-PD-1 therapy, CD36 targeting induced additional anti-tumor responses.

Liver kinase B1 (LKB1) regulates the metabolism and functional fitness of Tregs, and acts as a key inhibitor of DCs immunogenicity. In Tregs, the specific deletion of LKB1 can lead to a fatal inflammatory disease characterized by excessive Th2-type dominant response. It not only destroys the survival, mitochondrial fitness and metabolism of Tregs, but also induces abnormalities in immune regulatory molecules. Express, including PD-1 and TNF receptor superfamily proteins GITR and OX40.

New genetic mechanism

Helios can enhance the preferential differentiation of human fetal CD4+ naive T cells into Tregs, as a key transcription factor that stabilizes Tregs during the inflammatory response. Helios-deficient Tregs in tumors acquire the function of effector T cells, and promote the immune response to cancer by up-regulating effector cytokines with high affinity to self-antigens, such as increased expression of GITR/PD-1 and increased responsiveness to self-antigens.

Foxp1 is a forkhead transcription factor. The loss of Foxp1 will cause the function of Tregs to be impaired. TSDR, CNS2, and CNS3 are all Foxp3 related elements, which regulate the expression and function of Foxp3.

YAP is a co-activator of the Hippo pathway. It is highly expressed in Tregs and promotes the expression and function of Foxp3. TAZ is a co-activator of TEAD transcription factor that induces Hippo signal transduction. It attenuates the development of Tregs by reducing the acetylation of Foxp3 mediated by histone acetyltransferase Tip60.

The nuclear receptor Nr4a is a key transcription factor that maintains the Treg gene program and participates in Treg-mediated TME anti-tumor immunosuppression. The autophagy-promoting protein AMBRA1 is also a key regulator of T cells. In addition, SUMO-specific protease 3 (SENP3)-mediated deSUMOylation of BACH2 prevents nuclear export of BACH2, thereby inhibiting genes related to the differentiation of CD4+T effector cells and stabilizing Treg-specific gene characteristics.

New molecular mechanism

Some new progress has also been made in the molecular mechanism of Tregs. Tregs have abundant IL-2 receptor (IL-2R) expression and depend on IL-2 produced by activated T cells, suggesting that the consumption of IL-2 by Tregs is related to its inhibitory function. And the activation of IL-2R-dependent transcription factor STAT5 plays an important role in the inhibitory function of Tregs.

Serine-threonine kinase Mst1 was identified as a signal-dependent amplifier of IL-2-STAT5 activity in Tregs. The high activity of Mst1 and Mst2 in Tregs is essential to prevent tumor resistance and autoimmunity.

In the process of T cell activation, the phosphorylation of Foxp3 in Tregs can be regulated by the TAK1 Nemo-like kinase (NLK) signaling pathway. In addition, the specific deletion of Tregs kinase TAK1 can reduce the number of Tregs in peripheral lymphoid organs, while Tregs lacking the E3 ubiquitinated ligase TRAF6 are dysfunctional in vivo.

ST2 is considered to be the only receptor of IL-33. Tregs expressing ST2 respond to IL-33, and the percentage of Tregs increases under IL-33 stimulation, especially Foxp3+GATA3+Tregs. In mouse models of colorectal cancer (CRC), tumor-infiltrating Tregs preferentially up-regulate ST2, and IL-33/ST2 signaling is positively correlated with tumor burden.

Activated Tregs express glycoprotein-A-based repeats (GARP) on the surface, which bind to and activate latent TGF-β. In addition, the activation of integrin α4β1 can also increase the inhibitory ability of Tregs.

Tregs homeostasis is closely related to the function of mucosa-associated lymphoid tissue 1 (Malt1) through Tregs internal and external mechanisms. In contrast, CARD11-BCL10-MALT1 (CBM) signaling is essential for mediating the inhibitory function of Tregs in a Malt1 protease-dependent manner.

03 Tregs-targeted tumor immunotherapy strategy

Tregs exhaustion

The deletion of Tregs or the weakened inhibitory activity can enhance tumor immunity. Tregs in TME showed several cell surface markers, including CD25, CTLA-4, GITR, OX40, ICOS, PD-1, LAG3, TIM3, TIGIT, CCR4, folate receptor (FR) 4 and CD15s, these cells Surface marker-specific monoclonal antibodies can be used to deplete Tregs or hinder their function.

Treg and immune checkpoint suppression

Immune checkpoint molecules, including CTLA-4 and PD-1, are highly expressed in activated Tregs and Teff cells. Anti-CTLA-4 antibody can eliminate the inhibitory function of Tregs and release the cytotoxic effect of effector cells. However, compared with anti-PD-1 therapy, CTLA-4 targeted therapy faces two challenges: poor efficacy and increased toxicity.

Two monoclonal antibodies, ipilimumab and tremelimumab, block the function of CTLA-4, and by enhancing the immune response mediated by effector T cells, they have shown lasting clinical activity in some patients with advanced solid malignant tumors. Both ipilimumab and tremelimumab can increase the infiltration of CD4+ and CD8+ cells in the tumor without significantly changing or depleting the Foxp3+ cells in the TME. This indicates that the Fc part of the monoclonal antibody can be modified to enhance the Fc-mediated Treg in the tumor Depletion to enhance its efficacy.

The inhibitory receptor PD-1’s effect on Teff cells has been confirmed, but its role in Tregs is unclear. Tregs in TME showed that the expression level of PD-1 was comparable to that of Teff cells. Various studies reported that the anti-PD-1 monoclonal antibody nivolumab reduced the immunosuppressive activity of Tregs. However, another study suggests that in some cancer patients, PD-1 inhibition induces immunosuppressive activity mediated by Tregs. Therefore, it is necessary to further study the role of PD-1 in Tregs.

Treg modulation factor in TME

The TGF-β and IL-2 signaling pathways are essential to maintain the differentiation and survival of Tregs in the thymus and peripheral tissues. In an animal model of melanoma treated with anti-CTLA-4 monoclonal antibody, type I TGF-β receptor inhibitor (galunisertib) increased the ratio of CD8+ T cells to Treg. In addition, the combination therapy of galusertib and anti-PD-1 or anti-PD-L1 monoclonal antibody is currently undergoing clinical trials.

The PI3K signaling pathway is critical to the maintenance and function of Treg, and it is a promising target for Treg targeted therapy. At present, research on the combination of pembrolizumab and PI3Kδ inhibitors in the treatment of advanced solid tumors is underway. In addition, in the clinical trial of tyrosine kinase inhibitor, Dasatinib, it was observed that the Treg of patients with chronic myelogenous leukemia decreased and showed good clinical results.

Tregs produce extracellular adenosine through the activity of CD39 and CD73 on the cell surface. Adenosine increases tolerance to APC and enhances the immunosuppressive activity of Tregs. Therefore, CD39 and CD73 are important targets for targeting adenosine metabolism and regulating Tregs.

VEGFR-2 plays an important role in tumor angiogenesis. In animal models, this signaling pathway has been shown to increase the infiltration of Tregs into tumors. Blocking VEGF-VEGFR2 signaling inhibits tumor growth by reducing the accumulation of immunosuppressive cells in TME. Therefore, targeting the VEGFR2 molecules expressed by activated Tregs or blocking VEGF-VEGFR2 signaling may promote tumor immunotherapy by inhibiting Tregs.

Outlook

There are several Treg targeted therapies currently under study. In addition, targeted Tregs combined with other cancer treatments will be another good choice.

However, the mode of these combinations during treatment requires more in-depth research on the dynamics of the tumor microenvironment to determine how to obtain the best combination. Therefore, it is necessary to conduct in-depth research on the role and function of Tregs in order to give full play to the potential of Tregs as immunotherapy targets and provide new strategies for tumor immunotherapy.

(source:internet, reference only)

Disclaimer of medicaltrend.org