Isoleucine and Valine, Not Leucine, Drive T Cell Proliferation Through c-Myc

A new study published in EMBO Reports on 28 October 2026 shows that, at least for mouse CD8 T cells, this lumping conceals a striking division of labor. Crucially, these effects do not depend on how the amino acids are broken down inside the cell, but on how they are sensed.

The team isolated CD8 T cells from the spleens of mice and activated them with antibodies against CD3 and CD28, the standard mimic of antigen recognition. They then cultured the cells in medium stripped of branched-chain amino acids, adding back leucine, isoleucine or valine in different combinations at 0.8 millimolar each. Across experiments repeated two or more independent times with technical replicates, BCAA restriction had minimal impact on cell viability during the first day of activation. Yet the amino acid requirements for function were sharply asymmetric. Valine alone was sufficient to drive upregulation of the activation markers CD25 and CD69, and the combination of isoleucine and valine was sufficient for entry into the G1 phase of the cell cycle. Leucine, the amino acid most famous for its anabolic signaling, contributed nothing essential.

The pattern held when the researchers flipped the timing. When T cells were activated for two days in complete medium and then switched into BCAA-depleted conditions for days two through four, isoleucine and valine again maintained proliferative capacity in a dose-dependent manner, with triple-concentration addbacks performing better than the standard dose. Extending restriction across days zero to three produced the same dose-dependent recovery of cell division with isoleucine and valine, although over this longer period the loss of any single BCAA eventually harmed viability, consistent with their status as essential amino acids. Extracellular valine alone was even sufficient to support cytokine production, including interferon-gamma and TNF-alpha, in cultures that were not proliferating. The researchers also ruled out a scavenging explanation: when they blocked macropinocytosis, a process cells can use to gulp down extracellular protein, with the inhibitor EIPA, leucine-restricted T cells still proliferated while isoleucine-restricted cells retained their defect.

To understand the mechanism, the authors turned to the two master regulators of anabolic growth in T cells: mTORC1, the nutrient-sensitive kinase complex, and c-Myc, the transcription factor that drives much of the metabolic reprogramming after antigen stimulation. Downstream of the T cell receptor, both factors promote expression of the System L transporter Slc7a5, which imports large neutral amino acids including the BCAAs, creating a feedforward loop that reinforces T cell expansion. Prior work had established that leucine uptake is required for sustained mTORC1 activation, but which imported amino acids sustain c-Myc and clonal expansion had remained unresolved, with studies in natural killer cells hinting that c-Myc regulation might be independent of leucine.

The new experiments confirmed and extended that hint. When cells activated for one day in replete medium were switched into BCAA-restricted conditions for two to four hours, leucine or isoleucine availability promoted mTORC1 signaling, measured by phosphorylation of S6K and S6, whereas valine alone could not. But the c-Myc story was the reverse. Isoleucine and valine together were sufficient to maintain c-Myc expression, and in a rescue experiment where amino acids were withdrawn for two hours and then reintroduced, provision of isoleucine and valine restored c-Myc to levels comparable with complete medium, with valine alone maintaining near-control levels. Leucine, by contrast, showed signs of antagonism toward c-Myc during nutrient stress. The integrated stress response, which triggers phosphorylation of eIF2-alpha and translation of ATF4 when amino acids run low, was induced differentially by loss of each BCAA, and the conditions with the strongest stress response showed elevated CD98, the heavy chain partner of SLC7A5 and a known stress target, further evidence that each amino acid is individually sensed.

How does the cell regulate c-Myc this way? The researchers checked Myc transcript levels by quantitative PCR and found minimal change under any BCAA condition, so transcription was not the answer. Leucine restriction did not alter c-Myc protein stability or the rate of protein accumulation when the proteasome was blocked with MG132. Isoleucine and valine restriction, however, each impaired the rate at which c-Myc accumulated in the presence of MG132 while minimally affecting its half-life, pointing to a defect in translation. A broader readout agreed: puromycin incorporation, a surrogate for global protein synthesis, was unaffected by leucine absence but reduced when isoleucine or valine were missing. In short, isoleucine and valine support c-Myc at the level of protein synthesis, not gene expression or protein degradation.

One obvious alternative explanation was catabolism. Once inside a cell, BCAAs are transaminated by BCAT1 or BCAT2 and committed to breakdown by the branched-chain alpha-ketoacid dehydrogenase complex, after which their pathways diverge, particularly for leucine versus the other two. To test whether these divergent catabolic fates explained the different phenotypes, the team generated T cell-specific conditional knockout mice lacking Dbt, one of four essential components of that enzyme complex, by crossing CD4-cre mice with Dbt-floxed animals. The knockout worked: tracing of carbon-13-labeled isoleucine showed impaired conversion to succinyl-CoA in the knockout T cells, and the animals showed no alterations in T cell development or homeostasis. Yet BCAA catabolism proved dispensable for cell cycle entry, c-Myc expression and proliferation in both restricted and replete media, with no changes in activation markers or one-day viability. The distinct roles of the amino acids must therefore arise from sensing, not burning.

The most surprising finding concerns leucine’s dark side during scarcity. When the researchers applied the mTOR inhibitor Torin during acute BCAA restriction, mTOR inhibition fully or partially restored c-Myc expression in isoleucine- and valine-deprived cells, while ISRIB, a drug that relieves the translational effects of eIF2-alpha phosphorylation, suppressed ATF4 as expected but barely moved c-Myc. Rapamycin, a more selective mTORC1 inhibitor, enhanced T cell proliferation during isoleucine withdrawal and promoted S-phase entry, measured by EdU incorporation, while having minimal to slightly detrimental effects in leucine-restricted or replete conditions. At the molecular level, mTORC1 inhibition reduced accumulation of the cell cycle brake p21 and restored phosphorylation of Rb, indicating relief of cell cycle arrest. The authors propose that leucine sensing by mTORC1 imposes metabolic demands that worsen nutrient stress in actively cycling cells, uncoupling mTORC1 from c-Myc.

This antagonism, however, is stage-specific. When the same rapamycin experiment was repeated during the first day of activation, mTORC1 signaling was still required for quiescence exit even in leucine-restricted cells, and inhibition failed to restore cell cycle entry in isoleucine- or valine-deprived cells, which also remained small in size. So during the earliest metabolic reprogramming, mTORC1 behaves as the canonical pro-growth factor alongside c-Myc; once cells are cycling, the two axes part ways. The authors frame this as two discrete modules, a leucine-mTORC1 axis and an isoleucine/valine-c-Myc axis, that cooperate during activation but become uncoupled, and even opposed, under amino acid stress.

Because the work was done entirely in mouse CD8 T cells cultured in vitro, with some experiments using OT-I mice to obtain larger cell numbers, its implications for human immunity remain to be tested, and the authors note that the sensors for isoleucine and valine are still unknown, as is the precise mechanism by which mTORC1 antagonizes c-Myc translation. Even so, the study offers a provocative reframing. The branched-chain amino acids that dominate discussions of muscle and metabolism are not interchangeable signals, and for a proliferating immune cell, the leucine that activates mTORC1 is not the nutrient that matters most. The authors suggest that rapamycin sensitivity in tumor settings could inform strategies for T cell-based cancer therapies, where blunting the leucine-mTORC1 axis might help engineered T cells withstand the nutrient-poor tumor microenvironment, though such applications remain speculative. What is established is a mechanistic demonstration that differential nutrient sensing, rather than catabolism, assigns each branched-chain amino acid its own job in cell growth.

Subject of Research: Differential sensing of leucine, isoleucine and valine by mTORC1 and c-Myc in mouse CD8 T cell proliferation

Article Title: Distinct sensing of BCAAs by mTOR and c-Myc governs T cell proliferation, independent of catabolism

Article References: Rome, K., Hall, E., Le, T. N. V., Jaccard, A., Wu, A., Snyder, N. W., Wellen, K. E., & Bailis, W. (2026). Distinct sensing of BCAAs by mTOR and c-Myc governs T cell proliferation, independent of catabolism. EMBO Reports. https://doi.org/10.1038/s44319-026-00919-w

Image Credits: AI Generated

DOI: 10.1038/s44319-026-00919-w

Keywords: branched-chain amino acids, leucine, isoleucine, valine, mTORC1, c-Myc, CD8 T cells, T cell proliferation, BCAA catabolism, Slc7a5, integrated stress response, rapamycin