The Prognostic Impact of Tumor-Infiltrated Lymphocytes (TILs) in Breast Cancer

The immune compartment of tumor microenvironment is formed by cells of innate immune system, such as neutrophils and tumor-associated macrophages and cells of the adaptive immune system, namely, tumor-infiltrating lymphocytes (TILs). The role of TILs in invasive breast cancer has been studied since several decades. Scientists first noted the presence of a brisk lymphocytic infiltrate in the peritumoral stroma of the so-called “medullary carcinoma” and suggested that its better prognosis should be related to this type of infiltrate as a sort of “host antitumoral response.” It is been a long time since then, and many subsequent studies examined the association between tumor lymphocytic infiltrates and prognosis in other histological types of breast cancer, not without contradicting results. A systematic review was conducted using studies sourced from the PubMed, Embase, and Cochrane literature databases. The relevant publications discussing TILs in breast cancer are analyzed. The guidelines for histopathological assessment of TILs are discussed and summarized. TILs play a prognostic role in breast cancer, especially in triple-negative and HER-2-positive tumors. TIL assessment is mandatory in every breast cancer subtype and also in the neoadjuvant chemotherapy setting. It seems to have also therapeutic implications in conventional chemotherapy and chemotherapy/immunotherapy regimens.