Tumour-infiltrating lymphocytes and the emerging role of immunotherapy in breast cancer.

Breast cancer has not previously been considered a highly immunogenic cancer. Observations of tumour-infiltrating lymphocytes (TILs) in and around neoplastic cells in tumour samples, and associations with improved pathological complete response and clinical survival end points have changed our perspective on this. Lymphocytic infiltrates have long been observed in breast cancer; however, more recently, retrospective analysis of prospectively collected tissue samples from clinical trials has demonstrated the potential role of host immunosurveillance in influencing the biology of breast cancer. This association appears to be strongest in triple negative and HER2 positive breast cancer subtypes. Contrastingly, the association in luminal tumours is less clear, and is potentially limited by substantial tumoural heterogeneity. Several methodologies have been employed to quantify, and describe the composition of TILs, each with its own advantages and disadvantages. The results of these analyses have been generally consistent, and valuable efforts are currently underway to standardise the evaluation of TILs toward a universal approach. More technical methods of TIL characterisation remain important in the research setting. The evaluation of TILs becomes increasingly relevant with the emerging role of immunotherapy in breast cancer. Early phase trials of checkpoint blockade show promising results; however, it is likely that some patients will require combination treatments to maximise therapeutic benefits. Equally, some patients may not derive any benefit from immunotherapies. This underscores the importance of the development of relevant predictive biomarkers. As a key representative of the immune interaction between host and tumour, lymphocytic infiltrates are ideally placed for continued research into the determinants of immunogenicity, and response to immunotherapeutic approaches. In this review, we will discuss the current methodologies of evaluation, and the clinical relevance of TILs. Additionally, we discuss the emerging role of immunotherapy in breast cancer, and the future of TIL characterisation in this context.