Acute myeloid leukemia (AML) is an aggressive cancer with high rates of relapse and mortality. Currently, the only treatment with curative potential for AML is allogeneic hematopoietic stem cell transplantation (allo‐HSCT). Allo-HSCT consists of lymphodepletion, followed by reconstitution of the patient's hematopoietic system with healthy donor stem cells. In addition, the donor hematopoietic cells produce cytotoxic CD8⁺ T cells and NK cells that have the potential to recognize and kill residual AML cells.

Nevertheless, relapse of leukemia remains a major cause of death after allo-HSCT, potentially supported by protective mechanisms that AML cells employ against cytotoxic T cells as evidenced by the low response rate to donor lymphocyte infusions after allo-HSCT relapse. In addition, a variety of immunomodulatory agents, including hypomethylating agents, cytokines, and immune checkpoint inhibitors have shown little effect in AML.¹

To address the need for more durable responses after allo-HSCT, Robert Zeiser and his group set out to investigate how AML cells escape killing by donor CD8⁺ T cells and how cytotoxic activity of T cells can be restored. In their study, published in October in Science Translational Medicine, Franziska Uhl, Zeiser, and co-workers report a mechanism for T-cell exhaustion in AML and suggest an intriguingly simple therapeutic strategy for overcoming it.²

A metabolic escape route from immune attack

It was shown previously that T cells with reduced metabolic fitness are impaired in their ability for cytotoxic killing. Furthermore, T cells displaying an “exhausted” phenotype have been found in patients with AML relapse and patients with measurable residual disease, compared with patients maintaining long-term complete responses.³

However, when the authors compared T cells from healthy volunteers and patients at AML diagnosis, there was no difference in metabolic fitness as measured by glycolytic activity or oxygen consumption rate. Also, T cells isolated from patients displayed increased interferon (IFN)-g, tumor necrosis factor (TNF)-a, and perforin expression indicating intact metabolic activity at diagnosis.

By contrast, metabolic activity of donor T cells at time of relapse after allo-HSCT was significantly reduced as measured by oxidative phosphorylation activity and extracellular acidification rate compared with T cells harvested before relapse. Also, patients who remained in remission had metabolically healthy donor T cells. Concomitant with this metabolic inhibition in patients with relapse were reduced levels of intracellular IFN-g in T cells, indicating impaired cytotoxic killing ability.

Experiments in graft-versus-leukemia (GvL) mouse models confirmed that the presence of AML cells inhibited metabolic activity of allogeneic T cells. Further in vitro experiments led to the identification of a soluble factor as the main inhibitor of CD8⁺ T-cell energy metabolism, proliferation, and cytotoxic function. Infusion with allogeneic T cells that had been exposed to supernatant from AML cells lost its ability to rescue mice bearing AML tumors in three different mouse models (WEHI-3B luc, MLL^PTD/+;Flt3^ITD/+, and MOLM-13 luc).

Mass spectrometric analysis of cell culture supernatants revealed that lactic acid was highly abundant in AML supernatant compared to control medium or T-cell culture supernatant, which led to characteristic acidification of the culture medium. The abundance of lactic acid was also significantly increased in the serum of AML patients who relapsed after allo-HSCT compared to levels before the intervention. For patients who stayed in remission, no changes in lactic acid were detected.

Sodium bicarbonate, a clinically approved agent, re-energizes metabolically impaired T cells

Adding lactic acid to murine T-cell cultures in vitro reduced cell cycle progression and T-cell proliferation in a dose-dependent manner. Since sodium bicarbonate (NaBi) is used as a clinical treatment for acidosis, the authors investigated whether it might also help to overcome lactate-induced inhibition in T cells. Indeed, addition of NaBi in vitro completely rescued T-cell metabolism, proliferation, and cell cycle progression, and even enabled them to use extracellular lactic acid as an additional fuel source for energy production.

Was lactic acid-mediated acidification of the medium, as seen in AML culture, the cause of T-cell inhibition? Interestingly, manipulation of the extracellular pH with hydrochloric acid, HEPES buffer, or sodium hydroxide had little effect on T-cell function. However, CD8⁺ T cells strongly express transmembrane transporters for lactate and bicarbonate, which could explain a selective ability of these two agents to affect the intracellular pH. Further analysis revealed lactic acid uptake inhibited several core metabolic pathways involved in energy generation, growth, and cell division. Addition of NaBi reversed this inhibition and enabled CD8⁺ T cells to remove intracellular lactate by using it as an energy source. Similar effects were observed in murine CD4⁺T cells as well. Therefore, NaBi appears to act by normalizing the intracellular environment of T cells exposed to high extracellular lactate concentrations.

To assess the efficacy of NaBi to restore GvL activity in two allo-transplanted AML bearing mouse models (WEHI-3B luc, MLL^PTD/+;Flt3^ITD/+)in vivo, mice received NaBi in the drinking water. In both models, NaBi restored the GvL activity of allogeneic donor T cells and significantly improved survival of these mice. Importantly, this treatment did not aggravate the development of graft-versus-host disease (GvHD), suggesting that it might be safe for use in the clinic.

Translation into the clinic

Like murine T cells, human CD8⁺ T cells from healthy donors reduced metabolic activity when challenged with AML culture supernatant or lactic acid but retained their metabolic function in the presence of NaBi. Encouraged by these results, the authors performed an exploratory trial (NCT04321161) in 10 patients who had relapsed after allo-HSCT and had received additional donor lymphocyte infusions. All individuals received NaBi treatment for 1 week. Blood samples taken before and after the treatment revealed an increase in pH and bicarbonate concentrations. Encouragingly, NaBi rescued oxidative phosphorylation and increased inflammatory cytokine production in donor T cells but did not alter glycolytic activity or the expression of other T-cell activation markers.

Outlook

To date, allo-HSCT is the only treatment with curative potential for AML and related blood cancers. Nevertheless, prognosis is poor for a significant number of patients who relapse after cell transfer. The results published by Uhl et al., if confirmed in a larger controlled trial, could lead to a simple and cost-effective therapeutic strategy to overcome impaired T-cell cytotoxicity and GvL activity after allo-HSCT potentially without increasing the risk for GvHD.

A number of interesting questions remain:

Firstly, will all subtypes of AML respond to NaBi treatment after allo-HSCT in the same way?

Secondly, could NaBi be useful to increase the efficacy of CAR-T cells, which are being tested in 26 ongoing clinical trials in patients with AML with currently limited success?

Thirdly, could NaBi be a useful for other immune therapy approaches such as checkpoint inhibitors, or adoptive immune cell therapies, in cancers apart from AML?

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