In Vivo Imaging of Pro- and Antitumoral Cellular Components of the Tumor Microenvironment.

Tumor development and growth, as well as metastatic spread, are strongly influenced by various, mostly innate, immune cells, which are recruited to the tumor site and driven to establish a specific tumor-supportive microenvironment. The contents of this microenvironment, such as myeloid cells, are a major factor in the overall prognosis of malignant disease, addressed by a constantly growing armament of therapeutic interventions targeting tumor-supportive immune cells. Current clinical imaging has long ignored the growing need for diagnostic approaches addressing these microenvironmental contents-approaches enabling a sensitive and specific classification of tumor immune crosstalk and the resulting tumor-associated immune cell activity. In this focus article we review the present status of, and promising developments in, the in vivo molecular imaging of tumor immune components designed to allow for inferences to be made on the cross-talk between tumor cells and the immune system. Current imaging modalities based on the infiltrating cell types are briefly discussed.