By engineering a special molecule to track certain immune cells in the body, scientists at the Stanford University School of Medicine have invented a litmus test for the effectiveness of a newly devised cancer therapy.

The molecule is a radioactive tracer that latches onto immune cells when they’re activated—the status that immune cells, in particular T cells, assume when they’re poised to kill tumor cells.

“It’s not good enough to just image all T cells; you need to image activated T cells because those are the ones that are going to kill the tumor,” said Sanjiv “Sam” Gambhir, MD, PhD, professor and chair of radiology at Stanford. “The problem that occurs in other approaches, including ones we’ve previously developed, is that they’re sometimes not specific enough. I could image tumor patients who’ve yet to receive an immunotherapy; they’ll sometimes show T cells in their tumors, but those T cells aren’t always activated and killing tumor cells—so we need a way to track activated T cells more specifically, and I think we’ve done that here.”

With the tracer, doctors can theoretically see if a cancer vaccine has successfully galvanized T cells into a protective state, though the research conducted in this study was exclusively in mice. The PET tracer’s capabilities aren’t limited to cancer therapies, Gambhir added. Because the tracer latches onto a molecule that flags any activated T cell, it also makes for a powerful tool to detect autoimmune diseases, which occur when the immune system erroneously activates T cells to attack healthy tissue.

A study describing the tracer was published online May 14 in the Journal of Clinical Investigation. Postdoctoral scholar Israt Alam, PhD, and graduate student Aaron Mayer share lead authorship of the study. Gambhir, the Virginia and D.K. Ludwig Professor for Clinical Investigation in Cancer Research, is the senior author.