Tumor-infiltrating lymphocytes in breast cancer according to tumor subtype: Current state of the art

The recent success of the immune checkpoint blockade in cancer immunotherapy has modified the treatment algorithms in a variety of aggressive neoplastic diseases. Nevertheless, optimal selection of ideal candidates to these drugs remains a challenge. The presence, location and composition of a pre-existing tumor immune infiltrate seem to impact on the benefit from these treatments. The association between the presence of baseline tumor-infiltrating lymphocytes (TIL) and patients' outcomes has been widely investigated in breast cancer, although immunotherapeutic strategies have historically been less successful with respect to other neoplastic diseases such as melanoma and kidney cancer. TIL extent varies and has different associations with outcomes in the various breast cancer subtypes. Furthermore, the presence of baseline high TIL has been associated with an increased benefit from some chemotherapeutic and targeted agents even though some conflicting results have been observed on this regard. This review aims to summarize the state of the art of TIL in breast cancer with a focus on their assessment, prevalence and clinical implications in the different subtypes.