In-vivo imaging of tumor-infiltrating immune cells. Implications for cancer immunotherapy

Dynamic interactions between tumor cells and immune cells promote the initiation, progression, metastasis and therapy-resistance of cancer. With respect to immunotherapy, immune cell populations such as cytotoxic CD8+ T-cells, CD56+ NK cells and myeloid phagocytic cells play decisive roles. From an imaging perspective, the immune system displays unique challenges, which have implications for the design and performance of studies. The immune system comprises highly mobile cells that undergo distinct phases of development and activation. These cells circulate through several compartments during their active life span and accumulate in rather limited numbers in cancer lesion, where their effector phenotype further diversifies. Given these features, accurate evaluation of the tumor microenvironment and its cellular components during anti-cancer immunotherapy is challenging. In-vivo imaging currently offers quantitative and sensitive modalities that exploit long-lived tracers to interrogate, e.g. distinct immune cell populations, metabolic phenotypes, specific targets relevant for therapy or critical for their effector function. This review provides a comprehensive overview of current status for in-vivo imaging tumor-infiltrating immune cell populations, focusing on lymphocytes, NK cells and myeloid phagocytic cells, with emphasis on clinical translation.