A New Mechanism of Lactate Regulation of Immune Checkpoint Expression

Recently, it has been reported that tumor-infiltrating regulatory T (Treg) cells mediate pre-mRNA splicing of cytotoxic T-cell-associated protein-4 (CTLA-4) through massive uptake of lactic acid, which promotes the expression of CTLA-4, and thus improves the efficacy of CTLA-4 monoclonal antibody. Revealing this delicate network could have significant ramifications for the development of innovative immune checkpoint therapeutic approaches, particularly in cancer medicine.

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Lactate Regulation and CTLA-4 in Treg Cells

In solid tumors, not only the presence of tumor cells alone, but also blood vessels, various immune cells, fibroblasts, bone marrow-derived inflammatory cells, various signaling molecules, and extracellular matrix, etc., which together form the tumor microenvironment (TME). Tumor-infiltrating Treg cells are a member of this group, which is a unique subpopulation of immune T cells with immunosuppressive effects that inhibit or down-regulate the proliferation and differentiation of effector T cells. Treg cells constitutively express a very important molecule, CTLA-4, which is on par with PD-1 and PD-L1 as immune checkpoints. When CTLA-4 binds to CD80/CD86 on the surface of antigen-presenting cells, it acts as an "off" switch, which contributes to the suppressive function of Treg cells.

According to the Warburg effect, tumor cells rely on aerobic glycolysis to support their survival, which often leads to excessive lactate accumulation in the TME. It has been reported that Treg cells utilize lactate to maintain their immunosuppressive function. However, the regulation of the expression of CTLA-4, an important molecule in Treg cells, by lactate is not known.

Lactate Modulates RNA Splicing to Promote CTLA-4 Expression in Tumor-infiltrating Regulatory T Cells

The researchers found that tumor-infiltrating Treg cells must promote CTLA-4 expression through lactate uptake to maintain their high suppressive properties.

Treg cells utilize the monocarboxylate transporter protein 1 (MCT1) on their cell membrane to uptake lactate from the TME. Thus, when deletion of MCT1 is induced in Treg cells, lactate uptake is not possible, which markedly slows tumor growth and enhances anti-tumor immune effects.
However, when drug-triggered deletion of MCT1 in Treg cells was combined with anti-CTLA-4 to treat mice, no synergistic effect was demonstrated compared to control, suggesting that lactate-induced CTLA-4 expression is required for the therapeutic response to CTLA-4 blockade, presumably because CTLA-4 expression enhances the efficiency of contact with the blocker.

How Does Lactate Regulate CTLA-4 Expression?

Lactate uptake by Treg cells promoted not only the expression of CTLA-4 but also the expression of the important transcription factor Foxp3. In turn, Foxp3 induced high expression of ubiquitin-specific peptidase 39 (USP39). Although USP39 is an inactive deubiquitinating enzyme that lacks key catalytic cysteine residues, it is involved in RNA spliceosome assembly and will directly affect RNA splicing after transcription. Therefore, when researchers deleted USP39 in Treg cells, it not only disrupted the effective splicing of CTLA-4 pre-mRNA, but also impaired Treg cell-mediated immunosuppression.

Therefore, lactate increased Foxp3-dependent USP39 expression and triggered USP39-dependent RNA splicing-mediated CTLA-4 expression.
In addition, the researchers further analyzed Treg cells in tumor samples and blood samples from colorectal cancer patients and also observed a significant increase in CTLA-4 pre-mRNA splicing efficiency.

In conclusion, lactate modulates RNA splicing to increase CTLA-4 expression and enhance immunosuppressive function of Treg cells, which contributes to the efficacy of CTLA-4 blockade therapy.

Lactate as a Regulator of Immune Checkpoint Expression

Lactate acts as a signaling molecule that affects the expression of immune checkpoint molecules on tumor cells and immune cells.

Lactate promotes the upregulation of PD-L1 on tumor cells.
Lactate enhances CTLA-4 expression on Tregs.
The increased acidity of lactate impairs the migration, function, and survival of cytotoxic T-cells and natural killer (NK) cells, thereby attenuating the immune response.

New evidence that lactate regulates immune checkpoint expression certainly provides a potential therapeutic target.

One approach currently being investigated is to inhibit lactate production within cancer cells, which has the potential to reverse the immunosuppressive tumor microenvironment that has developed, allowing immune cells to function effectively.
Innovative strategies targeting lactate transporters (MCTs) are also being developed to prevent lactate from being excreted from tumor cells and affecting the microenvironment. Obstacles to MCT-coupled lactate shuttling would disrupt the lactate-dependent immune regulatory cascade.

By unraveling the intricate crosstalk between metabolism and immunity, Creative Biolabs researchers are pioneering new approaches in cancer immunotherapy and beyond.

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