Abstract A major challenge in the development of immune checkpoint blockades (ICB) is the variability in outcome of such therapeutics. This leads to a large number of non-responder patients receiving costly therapies that are also associated with some toxicities. Surrogates of efficacy are needed both to select the right patient population for the clinical trials and in later stages, to identify the right patients for the right ICB, ideally with assays that are rapid, high throughput and noninvasive. T cell clonal expansions caused by ICB are mainly local. As a result, the expanded T cells are mostly found in the tumor deposit and hard to detect in the peripheral blood. We optimized an antigen recall assay that assesses the ability of PD-1 blocking antibody to further enhance antigen specific CD8 T cell expansion and thereby discriminates responders from non-responders. A simple version of this assay has been used for functional screening of ICBs. By improving each component of the assay and its readout, we have increased the “signal to noise” window allowing us to rapidly detect PD-1 blockades’ effects on the host. We use a cocktail of optimized recall peptides, a characterized conditioned media and 96-well plates to support rapid (7 days) expansion of memory T cells. Finally, as a readout system, cocktail of MHC class I tetramers generated by peptide exchange was used to detect the overall expanded clonotypes. Currently, we have optimized the assay with commercially available anti-PD-1 and anti-PD-L1 antibodies. This assay system has the potential to assist in predicting those who are more likely to respond to ICB therapies. We have initiated blinded studies using clinical samples for clinical validation.