If MEK becomes excessively active, it can drive T cells into terminal exhaustion, a severely depleted state in which immunotherapy can no longer reactivate them.

What did the new study find?

Surprisingly, the team discovered that exhausted T cells were not metabolically sluggish. They were highly active. When researchers treated the cells with MEK inhibitors, the T cells multiplied more while consuming less energy.

"That paradox made us ask where all that energy was going, and we discovered that these cells were investing enormous resources into making proteins," says Tanmana Mitra, PhD, first author of the study and a student in the Vardhana lab. "It changed how we think about T cell exhaustion — from a problem of too little energy to one of excessive energy demand."

Mitra adds: "We realized T cell exhaustion isn't simply a loss of function — it reflects an imbalance between what these cells are being asked to do and the energy they have available."

When should doctors preserve T cells versus push them harder?

Whether a short, powerful immune response or a slower, longer-lasting one is preferable may depend on each patient's cancer. Dr. Vardhana notes that two factors can indicate that a patient is likely to respond well to traditional immunotherapy:

• The tumors are small.
• The patient has a high number of immune cells attacking the tumor, often because the tumor carries many mutations that make it recognizable.

"In these patients, conservation of T cells is not that important," he says. "It's like being in a car with 1/8 of a tank left, but you can see the finish line. In these patients, you would just let the car keep burning the gas — in other words, take the traditional immunotherapy approach. These are the patients in whom MEK inhibition is probably not needed."

The situation may differ for patients with large tumors or relatively few tumor-fighting immune cells. In those cases, creating a slower, sustained response with a MEK inhibitor could help T cells remain present for longer, even if they are partly exhausted.

Could MEK inhibition work across different immunotherapies?

Dr. Vardhana says carefully applied MEK inhibition could potentially improve several treatment types:

• Checkpoint inhibitors: MEK inhibition has already shown effectiveness in melanoma when combined with checkpoint inhibitors and a BRAF inhibitor.
• CAR T cell therapy: "We think this approach could dramatically boost T cell persistence, which has been a big problem with CAR T cell therapy," Dr. Vardhana says.
• Tumor-infiltrating lymphocyte (TIL) therapy: Using MEK inhibition before or after TIL therapy could help the most effective tumor-fighting TILs survive longer.
• Bispecific antibodies: These laboratory-made molecules attach to two targets at once, strongly activating T cells but possibly accelerating exhaustion.

What are the limits of this approach?

The researchers caution that suppressing MEK is unlikely to suit every cancer patient. The strategy involves a tradeoff: the immune attack may become less intense, but the cancer-fighting cells can survive longer.

MSK immunologist Andrea Schietinger, PhD, previously found that T cell exhaustion can act as a survival strategy, helping cells avoid becoming overstimulated and dying.

"As we've learned more about T cell exhaustion, we've increasingly understood that it's not the case that exhausted T cells are bad, so let's try to reverse the process with a drug," Dr. Vardhana says. "Instead, exhaustion is more of an equilibrium state that lets the cells survive and keep going — almost like a 'safe mode' for T cells."

The findings come from laboratory and animal models and have not yet been tested in human patients for this specific use. Clinical trials would be required to confirm whether MEK inhibition can extend the effectiveness of cancer immunotherapy.