Cellular stress signal found to drive immune exhaustion and weaken cancer therapy

Cancer-fighting T cells do not simply “run out of energy.” They are molecularly reprogrammed. For years, mitochondrial dysfunction has been recognized as a hallmark of exhausted T cells in tumors. Yet how metabolic stress translates into stable transcriptional reprogramming remained unclear.

The new study, published in the journal Nature, uncovers a decisive molecular bridge.

When mitochondria become depolarized, CD8⁺ T cells increase proteasome activity. This heightened protein degradation selectively dismantles mitochondrial hemoproteins, releasing excess regulatory heme.

Rather than remaining a byproduct, heme becomes a signal.

It translocates to the nucleus, where it binds and destabilizes the transcription factor Bach2, relieving repression of Blimp1, a master regulator of terminal exhaustion. The collapse of the Bach2–Blimp1 axis locks T cells into a dysfunctional state and erodes their stem-like potential.

Mechanistically, the researchers identify CBLB as a driver of mitochondrial protein ubiquitination and PGRMC2 as a chaperon enabling nuclear heme transport.

A molecular switch and a therapeutic opportunity

“We uncovered a metabolic signaling switch that converts mitochondrial stress into a permanent transcriptional decision,” says Professor Ping-Chih Ho, senior author of the study. “This pathway explains how energy failure becomes immune failure.”

Crucially, the axis is actionable.

The team shows that transient low-dose bortezomib treatment during CAR-T cell manufacturing dampens proteasome-driven heme signaling, reduces exhaustion-associated programs, and promotes durable epigenetic reprogramming toward a memory-like state.

Clinical relevance is reinforced by data from B-ALL patients: CAR-T cells exhibiting high proteasome activity correlate with poorer therapeutic outcomes.

“Our last paper identified mitochondrial damage as the cause of T cell failure and this one reveals the molecular switch behind it, and how to turn exhaustion off. For a long time, mitochondrial dysfunction was an observation without a clear mechanistic explanation,” says Y. Xu, first author of the study.

“Discovering that regulatory heme acts as the signaling mediator was unexpected and it gives us a tangible way to intervene.”

Reframing T cell exhaustion

The findings redefine T cell exhaustion not merely as a consequence of chronic antigen stimulation, but as the outcome of a dysregulated metabolic signaling circuit.