Metabolic Fitness as a Determinant of CAR-T Cell Persistence

Introduction

Solid tumors create metabolically hostile environments that challenge the function and persistence of immune cells. One of the defining characteristics of the tumor microenvironment is hypoxia, a condition in which oxygen levels fall significantly below those found in normal tissues. Oxygen concentrations within tumors typically range from approximately 0.2% to 4%, considerably lower than normal tissue oxygen levels (physoxia) of 3–7.4%, and far below the ~21% found in standard cell culture conditions and atmospheric air (Bigos et al., 2024; Vaupel & Mayer, 2007).

For T cells, oxygen availability is closely tied to metabolic activity, differentiation, and effector function. As immunotherapies such as CAR-T and tumor-infiltrating lymphocyte (TIL) therapies continue to advance, understanding how hypoxic conditions influence T cell biology has become increasingly important. The metabolic state of therapeutic immune cells can strongly influence their ability to persist, proliferate, and function within the tumor microenvironment (Arner et al., 2023; Longo et al., 2025).

This article explores how hypoxia shapes T cell metabolism and function, and why oxygen levels represent an important parameter in the development and manufacturing of cell-based immunotherapies.

Oxygen Gradients in Solid Tumors

Solid tumors frequently outgrow their vascular supply, creating regions with limited oxygen delivery. Tumor-associated blood vessels are often irregular and inefficient, resulting in heterogeneous oxygen distribution throughout the tumor mass (Bigos et al., 2024). Median tumor oxygen levels across cancer types range from approximately 0.3% to 4.2%, with most tumors exhibiting median oxygenation below 2%.

These oxygen gradients create microenvironments where immune cells encounter conditions dramatically different from those experienced during ex vivo cell expansion. It is also worth noting that even normal tissue oxygen levels (physoxia, ~3–7%) are substantially lower than the 21% used in standard culture conditions, meaning manufacturing environments are metabolically mismatched with the in vivo context from the outset. In addition to hypoxia, immune cells in tumors must contend with nutrient depletion, acidic extracellular pH, and metabolic competition with rapidly proliferating cancer cells (Arner et al., 2023).

Collectively, these conditions impose significant metabolic constraints on infiltrating T cells. Cells that cannot adapt to these stresses may lose functionality or undergo exhaustion, limiting the effectiveness of immune-mediated tumor clearance.

Hypoxia-Inducible Signaling in T Cells

A central mediator of cellular responses to low oxygen is hypoxia-inducible factor-1α (HIF-1α). Under hypoxic conditions, HIF-1α becomes stabilized and translocates to the nucleus, where it regulates transcriptional programs involved in metabolism, survival, and stress adaptation.

In T cells, HIF-1α influences several key processes including metabolic reprogramming, cytokine production, and differentiation pathways (Shi et al., 2025). Importantly, these effects are context-dependent. Under hypoxic conditions specifically, the HIF-1α–glycolysis axis has been shown to regulate IFN-γ production in activated T cells, a relationship that does not operate through the same mechanism under normoxic conditions (Shen et al., 2024). 

Additionally, HIF-1α is indispensable for CD8+ T cells to sustain glycolytic metabolism and execute effective tumor cell killing, underscoring its dual role as both a stress-response mediator and a positive effector of anti-tumor function (Shi et al., 2025).

HIF-1α also drives upregulation of immune checkpoint ligands, particularly PD-L1, on tumor cells and stromal cells. This PD-L1 expression engages PD-1 on cytotoxic T cells, contributing to T cell exhaustion and reduced effector activity, a mechanism of direct relevance for immunotherapy design (Shi et al., 2025). Prolonged hypoxic signaling can further contribute to metabolic stress and immune dysfunction depending on the broader metabolic context (Shen et al., 2024).

Understanding how hypoxia influences these signaling pathways is therefore essential for predicting immune cell behavior within tumors.

Metabolic Adaptation of T Cells to Hypoxia

Activated T cells rely heavily on aerobic glycolysis (the Warburg effect) to support rapid growth and effector function under oxygen-replete conditions. Under hypoxic conditions, cells shift toward anaerobic glycolysis, producing lactate without requiring oxygen, as oxidative phosphorylation becomes less efficient due to limited oxygen availability (Longo et al., 2025).

Key metabolic adaptations observed in hypoxic T cells include:

• increased glucose uptake and glycolytic flux

• upregulation of glucose transporters such as GLUT1

• shift from oxidative phosphorylation to anaerobic glycolysis

• accumulation of biosynthetic metabolic intermediates

These metabolic changes allow T cells to maintain effector functions such as cytokine production and proliferation under stress conditions. However, prolonged metabolic stress can impair mitochondrial function and contribute to T cell exhaustion (Liu et al., 2020; Li et al., 2025).

Tumor cells themselves often consume large quantities of glucose and other nutrients, further limiting metabolic resources available to infiltrating immune cells (Arner et al., 2023). As a result, the ability of therapeutic T cells to maintain metabolic flexibility may be a critical determinant of their effectiveness in solid tumors.

Implications for Cell Therapy

Many cell therapy manufacturing workflows expand T cells in standard culture conditions at ~21% oxygen. While these environments support robust proliferation, they differ substantially from the hypoxic conditions cells encounter following infusion into patients, and even from normal tissue physoxia (~3–7% O₂). This mismatch between manufacturing conditions and the in vivo tumor microenvironment may influence the metabolic state of therapeutic cells at the time of infusion.

Cells conditioned exclusively under high-oxygen conditions may require significant metabolic reprogramming after entering hypoxic tumor environments. Hypoxia-driven PD-L1 upregulation within tumors may also limit the activity of infused T cells regardless of their intrinsic functional state, highlighting the importance of combining metabolic conditioning strategies with checkpoint blockade approaches.

Recent research suggests that environmental conditioning during cell expansion may influence the metabolic phenotype, persistence, and functionality of therapeutic T cells (Corrado et al., 2022; Cunha et al., 2023). Controlled oxygen environments during culture may therefore represent an important variable for optimizing cell therapy products.

Looking Forward

The metabolic landscape of the tumor microenvironment presents both challenges and opportunities for the development of effective immunotherapies. As researchers continue to investigate the interplay between oxygen availability, metabolic programming, and immune cell function, new strategies are emerging to improve the performance of cell-based therapies.

By integrating insights from immunometabolism with advances in cell manufacturing technology, it may become possible to better prepare therapeutic immune cells for the conditions they will encounter in patients. Continued exploration of hypoxia-driven metabolic pathways, including HIF-1α signaling, anaerobic glycolytic reprogramming, and checkpoint ligand regulation, will play a central role in shaping the next generation of cancer immunotherapies.

Frequently Asked Questions

What is metabolic fitness in CAR-T cells?

Metabolic fitness refers to the ability of T cells to efficiently generate energy and maintain metabolic flexibility under stress conditions. CAR-T cells with strong mitochondrial function and balanced metabolic pathways are more likely to persist and remain functional after infusion.

Why is mitochondrial metabolism important for CAR-T therapy?

Mitochondria support energy production and metabolic resilience. T cells with higher mitochondrial capacity often exhibit improved survival, proliferation, and long-term persistence, which are key factors for effective CAR-T therapies.

How does the tumor microenvironment affect CAR-T metabolism?

Tumors often create metabolically stressful environments characterized by hypoxia, nutrient depletion, and high levels of metabolic by-products such as lactate. These conditions can impair T cell metabolism and promote exhaustion.

Can manufacturing conditions influence CAR-T metabolic fitness?

Yes. Factors such as activation signals, cytokine exposure, oxygen levels, and nutrient availability during cell expansion can influence T cell differentiation and metabolic programming. Optimizing these parameters may help produce CAR-T cells with improved persistence.

References

Arner, E.N. et al. (2023). Metabolic programming and immune suppression in the tumor microenvironment. Cancer Cell.

Bigos, K.J.A. et al. (2024). Tumour response to hypoxia: understanding the hypoxic tumour microenvironment. Frontiers in Oncology.

Cunha, P.P. et al. (2023). Oxygen levels at the time of activation determine T cell persistence and immunotherapeutic efficacy.

Corrado, M. et al. (2022). Targeting memory T cell metabolism to improve immunity. Journal of Clinical Investigation.

Li, F. et al. (2025). Mitochondrial metabolism in T-cell exhaustion. Cell Death & Disease.

Liu, Y.N. et al. (2020). Hypoxia induces mitochondrial defects that promote T cell exhaustion. Nature Immunology.

Longo, J. et al. (2025). Nutrient allocation fuels T-cell-mediated immunity. Cell Metabolism.

Shen, H. et al. (2024). The HIF-1α–glycolysis axis regulates IFN-γ production in T cells under hypoxia. Nature Communications.

Shi, S. et al. (2025). Research progress of HIF-1α in immunotherapy outcomes. Frontiers in Immunology.

Vaupel, P., & Mayer, A. (2007). Hypoxia in cancer: significance and impact on clinical outcome. Cancer Metastasis Reviews.

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