Tumor-associated T cells orchestrate cancer rejection after checkpoint blockade immunotherapy. T cell function depends on dynamic antigen recognition through the T cell receptor (TCR) resulting in T cell activation. Here, we present an approach to quantify the dynamics and magnitude of tumor-associated T cell activation at multiple time points in living mice using the genetically encoded calcium reporter Salsa6f and functional intravital microscopy (F-IVM). Our protocol allows researchers to measure the activation dynamics of various immune cells in vivo. For complete details on the use and execution of this protocol, please refer to Geels et al.1.

Foxp3⁺ T regulatory (Treg) cells promote immunological tumor tolerance, but how their immune-suppressive function is regulated in the tumor microenvironment (TME) remains unknown. Here, we used intravital microscopy to characterize the cellular interactions that provide tumor-infiltrating Treg cells with critical activation signals. We found that the polyclonal Treg cell repertoire is pre-enriched to recognize antigens presented by tumor-associated conventional dendritic cells (cDCs). Unstable cDC contacts sufficed to sustain Treg cell function, whereas T helper cells were activated during stable interactions. Contact instability resulted from CTLA-4-dependent downregulation of co-stimulatory B7-family proteins on cDCs, mediated by Treg cells themselves. CTLA-4-blockade triggered CD28-dependent Treg cell hyper-proliferation in the TME, and concomitant Treg cell inactivation was required to achieve tumor rejection. Therefore, Treg cells self-regulate through a CTLA-4- and CD28-dependent feedback loop that adjusts their population size to the amount of local co-stimulation. Its disruption through CTLA-4-blockade may off-set therapeutic benefits in cancer patients.