New cytometry tools for immune monitoring during cancer immunotherapy.

The success of cancer immunotherapy (CIT) in the past decade has brought renewed excitement and the need to better understand how the human immune system functions during health and disease. Advances in single cell technologies have also inspired the creation of a Human Cell Atlas to identify and describe every cell in the human body with the intention of elucidating how to "fix" the ones that fail normal function. For example, treatment of cancer patients with immune checkpoint blockade (ICB) antibodies can reinvigorate their T cells and produce durable clinical benefit in a subset of patients, but a number of resistance mechanisms exist that prohibit full benefit to all treated patients. Early detection of biomarkers of response and mechanisms of resistance are needed to identify the patients who can benefit most from ICB. A noninvasive approach to predict treatment outcomes early after immunotherapies is a longitudinal analysis of peripheral blood immune cells using flow cytometry. Here we review some of the advances in our understanding of how ICB antibodies can re-invigorate tumor-specific T cells and also highlight the recent advances in high complexity flow cytometry, including spectral cytometers, that allow longitudinal sampling and deep immune phenotyping in clinical settings. We encourage the scientific community to utilize advanced cytometry platforms and analyses for immune monitoring in order to optimize CIT treatments for maximum clinical benefit.