Immune Activity Shift a Key Early Step in Breast Cancer Formation

Kornelia Polyak, MD, PhD, has been trying to understand how breast cancer forms ever since starting her lab at Dana-Farber in 1998. Specifically, she has wanted to know how ductal carcinoma in situ (DCIS), a pre-cancerous state that appears only in the milk ducts in the breast, changes into invasive breast cancer, which is breast cancer that has spread outside of the ducts and into the breast.

In new research, Polyak and her team found that as DCIS transitions into early-stage invasive breast cancer, a shift occurs in levels of regulatory T cells (Tregs), immune cells that keep the immune system in check to prevent damage to healthy tissue. A specialized set of regulatory T cells called cycling Tregs expands rapidly and suppresses the immune system around the tumor, giving cancer cells a chance to grow and spread into the breast. In the study, decreasing the number of cycling Tregs within tumors turned the tumors more immune active and reduced the development of breast tumors in animal models.

Polyak’s study validated these cellular dynamics in human samples, suggesting that the findings support a clinical strategy for reducing the likelihood of a transition from DCIS to invasive breast cancer. Targeting cycling Tregs directly, however, isn’t clinically viable yet, so Polyak’s lab will focus next on understanding what triggers the rapid expansion of these cells.

“If we know what triggers this expansion, then we could have a very specific way of targeting these cells only in the tumor,” says Polyak, a Dana-Farber clinician-scientist and senior author on a new publication in Cancer Cell. “A systemic blockade of cycling Tregs and all other Tregs would have undesirable side effects in patients.”

About a quarter of breast cancer diagnoses among those who receive mammograms are DCIS. According to the American Cancer Society, an estimated 50,000 to 60,000 people are diagnosed with DCIS in the U.S. each year. When DCIS transitions into invasive breast cancer, which is typically diagnosed as early-stage breast cancer, it then has the potential to spread outside of the breast and become advanced or metastatic breast cancer.

Pinpointing and blocking specialized T cells

T cells are complex; they can be active fighters against diseased cells, or they can be suppressors of active fighters, helping protect the body from autoimmunity. Identifying a highly specialized subset of T cells that act together to make big biological shifts, like a transition from precancer to cancer, required advanced scientific methods.

Polyak developed animal models of DCIS that enabled her to reliably study the cellular dynamics that occur when DCIS transitions to breast cancer. She and her team then used single cell sequencing to learn as much as possible about all the cells involved in the transition.

“What is amazing about single cell technology is you can basically see every cell, and we were able to see this population of Tregs that had specific markers showing they were actively proliferating,” says Polyak. “This is how we discovered these cycling Tregs.”

Examination of these cells from both animal and human samples helped the team, including first author Triet Minh Bui, PhD, MSCI, cancer immunologist and instructor in the Polyak lab, recognize that the cells helped shift the immune system from active against the tumor to suppressive.

The team also found ways to shut down the immune-suppressive effects using existing drugs. They targeted a marker called OX40 that was highly expressed in cycling Tregs. They also targeted an immune checkpoint called PD-1/PD-L1 that suppresses immune activity more generally. In rats, a blockade of both reduced cycling Tregs, increased the activity of T cells that fight cancer, and reduced the development of tumors.

The team also tested a blockade of IL-33, a cytokine that directs T cell specialization. The blockade of IL-33 had similar effects, restoring an environment where the immune system could fight against the DCIS cells.

Patient samples the key to discovery

To validate these findings in patients, Polyak and team collaborated widely to work with various cohorts of patients, including one repository with patients that had transitioned from DCIS to invasive breast cancer, one comprised of patients with DCIS only, and two very large databases, the Cancer Genome Atlas and METABRIC, including over a thousand patients with early and more advanced breast cancer.

Using these databases, Polyak’s team found that cycling Tregs are present in patients with DCIS and invasive breast cancer. Levels of cycling Tregs predict higher grade disease in DCIS, increased tumor growth after the transition to breast cancer, an increase in the likelihood of recurrence of DCIS, and poor long-term outcomes for patients who developed hormone receptor-positive breast cancer.

“This research involved a lot of teamwork and collaboration to be both immunologically detailed and clinically relevant,” says Polyak, who with her team collaborated closely with Dana-Farber investigator Glenn Dranoff, MD, and was funded by the National Institutes of Health, The Breast Cancer Research Foundation, and the Human Tumor Atlas Consortium.

Medically Reviewed By: Kornelia Polyak, MD, PhD