An experimental therapy that harnesses the body's tumor-fighting immune cells may be effective for some women with advanced breast cancer, early research suggests.

The findings come from an ongoing trial at the U.S. National Cancer Institute (NCI). It is testing a new approach to treating women whose breast cancer has spread throughout the body and is not responding to standard therapies.

Researchers call it a "highly personalized" form of immunotherapy that uses patients' own immune system T cells to target their genetically unique cancer.

At this point, the team is reporting on only six women who've undergone the immunotherapy. Three responded and have been cancer-free for at least 3.5 years, according to findings published online Feb. 1 in the Journal of Clinical Oncology.

"This is highly experimental and is not yet approved by the Food and Drug Administration," stressed senior researcher Dr. Steven Rosenberg, chief of the surgery branch at the NCI's Center for Cancer Research.

But, he said, the initial findings offer "hope."

Rosenberg said the results also go against the traditional "dogma" that breast cancer typically does not illicit much of an immune response — and, therefore, is usually not susceptible to immunotherapy.

Immunotherapy refers to any treatment that enlists immune system defenses to battle a disease. There are various forms of cancer immunotherapy already approved, but not all types of cancer respond well to those drugs.

Melanoma is a prime example of a cancer that responds well, Rosenberg said. Melanoma tumors carry many mutations, which can draw a strong immune reaction. And immunotherapy (particularly drugs called checkpoint inhibitors) has transformed the treatment of advanced melanoma.

In contrast, breast tumors contain relatively few mutations, and there has been limited success so far in treating advanced breast cancer with available immunotherapies. Checkpoint inhibitors are approved for certain women with an aggressive subtype of breast cancer known as triple-negative.

The approach Rosenberg's team is developing is different. "We're using the patient's T cells as the drug," he said.

The researchers have so far used the approach to treat patients with advanced melanoma and certain other cancers, including a rare gastrointestinal cancer.

Specifically, the treatment relies on tumor-infiltrating lymphocytes — T cells found in and around a tumor. The idea is that, if those T cells are indeed reactive against the tumor, that can be enhanced and used as a weapon.

The current study involved 42 women with metastatic breast cancer that was not responding to conventional treatments. Metastatic means their cancer had spread. Each patient underwent surgery to remove a tumor sample, which the researchers genetically sequenced to identify its mutations.

They then isolated T cells from the tumor and, in the lab, tested the cells' reactivity to the tumor's specific mutations.

It turned out that in most patients — 67% — those T cells were reactive to at least one of their tumor mutations.