Emerging Role of TILs in Breast Cancer Leads to Expanding Pathology Practices

K.P. Siziopikou, MD, PhD

Just as immunotherapies are transforming clinical practice for oncologists, these new therapies are also changing the practice of pathology and offering new ways for oncologists and pathologists to collaborate and improve patient care, said K.P. Siziopikou, MD, PhD, at the 2018 Lynn Sage Breast Cancer Symposium.

“As our understanding increases of the role the tumor immune microenvironment plays in the evolution of cancer and the clinical utility of tumor infiltrating lymphocytes (TILs) as a prognostic and predictive biomarker becomes more clear, the pathologist’s role in evaluating and quantifying immune infiltrates will become even more important,” Siziopikou told colleagues at the symposium.

Siziopikou began by noting that pathologists have a variety of methods to evaluate the patient’s immune response to tumors. These include hematoxylin and eosin (H-E) sections using qualitative or semi-quantitative scoring, immunohistochemistry (IHC) to identify immune cell subsets, digital image analysis, multiplex fluorescent IHC with multispectral imaging, flow cytometry, and messenger RNA tumor profiling to detect immune gene signatures.

According to Siziopikou, the tumor microenvironment in breast carcinomas, which includes tumor cells, stroma, immune infiltrates, blood vessels and nerves, is largely defined by the presence and composition of TILs. This interplay drives tumor behavior and the patient’s response to treatment and prognosis, she said, adding that recent evidence suggests that both ductal carcinoma in situ (DCIS) and certain breast carcinomas feature active tumor immune microenvironments.

TILs have assumed a greater importance in the diagnosis and treatment of breast cancer, said Siziopikou, because breast carcinomas are often composed of more than 50% to 60% lymphocytes, especially in triple negative breast cancer and HER2-positive breast cancer. “Assessing TILs in breast cancer provides great clinical utility, particularly given the growing potential of immunotherapy in this disease,” she said. “These assessments can assist with risk prediction models, as well as adjuvant and neoadjuvant chemotherapy decisions.”

Siziopikou, Director of Breast Pathology at Northwestern University’s Robert H. Lurie Comprehensive Cancer Center, the symposium’s lead sponsor, also outlined the work of the International Immuno-Oncology Biomarker Working Group. This group had the goal of issuing guidelines for pathologists that met criteria like being applicable in a variety of settings and circumstances, from large randomized clinical trials to everyday pathology practice. Additionally, they wanted to focus on measures that are affordable and accessible in countries of all income levels.

The working group met their objective, publishing Guidelines for Assessment of TILs in Breast Tumors in 2015.¹ Under these guidelines, pathologists should:

• Evaluate TILs within the tumor’s borders

• Exclude TILs outside tumor borders, DCIS, or normal lobules

• Count all the mononuclear cells but no neutrophils

• Use full sections when possible instead of biopsies

• Assess the entire sample, not just focus on “hot spots”

• Assess TILs as a continuous variable (from 0% to 100%) • Use a semiquantitative approach for evaluation and reporting (ie, reporting a result as 20% rather than the more precise 19.58%)

According to the guidelines, pathologists should take the following steps when assessing TILs in solid tumors:

1. Select tumor area

2. Define stromal area

3. Scan at low magnification

4. Determine the type of inflammatory infiltrate

5. Assess the percentage of stromal TILs

Siziopikou also discussed TILs in DCIS, noting that different subtypes of DCIS are associated with varying degrees of lymphocytic infiltrate. Additionally, TILs are associated with DCIS grade, young patient age and are more common in HER2-positive subtypes. Compared to low- or intermediate-grade DCIS, high-grade DCIS shows increased TILs, CD4 T cells, CD20 B-cells, FOXP3 + regulatory T cells (Tregs) and CD68-positive macrophages. She noted that pathologists often find it challenging to define the tumor area in DCIS and to differentiate reactive lymphocytes from bystander lymphocytes.

When evaluating TILs in metastatic lesions, pathologists compare the immune response with metastatic tumor deposits, Siziopikou said. They also consider the immune response in primary versus metastatic lesions, focusing on issues of clinical relevance like whether changes have occurred over time, differences in specific locations or metastatic sites, and the effect of tumor phenotype. “Evaluating TILs in the most recent tumor sample may be more relevant for the current status instead of basing therapies on old information from the primary tumor,” she said.

Siziopikou also touched on future directions in pathologic evaluation. Pathologists would like guidance on how to assess TILs in multifocal tumors and lymph nodes or other metastatic sites, as well as clarifying the role of tumor heterogeneity. They also hope to define the minimum amount of tumor needed for a reliable representation of the tumor immune microenvironment, she said.

Siziopikou believes that surgical pathologists have a critical role in assessing specimens for elements of the tumor immune microenvironment, performing assays with standardized methodologies, reporting the immune findings according to the new guidelines, and addressing questions from clinicians and patients regarding predictive and prognostic immune biomarkers. “We have reached a crossroads in needing constant discussion and collaboration between the pathologist and the oncologist,” she said. “That’s the only way checkpoint inhibitors will reach their full potential in the clinic.”

Salgado R, Denkert C, Demaria S, et al; International TILs Working Group 2014. The evaluation of tumor-infiltrating lymphocytes (TILs) in breast cancer: recommendations by an International TILs Working Group 2014. Ann Oncol. 2015;26(2):259-271. doi: 10.1093/annonc/mdu450.