Project Summary/Abstract Adoptive cell therapy has the potential to circumvent cancer immune evasion by employing patient-derived, tumor-specific cytotoxic T-cells to attack cancer. While promising responses of advanced cases to cancer immunotherapy bolster the optimism surrounding these therapeutic strategies, interpatient variations in treatment responses demand a new method to select highly effective tumor-specific cytotoxic T-cells for better and safe immunotherapy. We propose a high-throughput, single-cell level method for identifying and recovering highly functional T cells that secrete tumor-killing signals in the presence of tumors. Our proposed technology builds on transformational advances in the generation of droplet emulsions, secretomics, and new hydrogel-based sensing techniques we recently developed. In this assay, tumor-killing T cells and cancer cells will be encapsulated in pairs in a uniformly sized, secreted factor-sensitive hydrogel droplet using a microfluidic droplet generator. The resulting isolated reaction compartments for each cell pair will prevent the mixing of T cells' multiple secreted factors and will concentrate them to allow sensitive detection. The hydrogel will respond to a factor secreted by functional T cells via a dramatic size change, allowing hydrogels with active T cells to be reliably isolated from the heterogeneous mixture using a hydrodynamic size-based particle sorting mechanism. The sorted pairs will be individually recovered using an aspiration pipette and the hydrogel shell dissolved without cell damage, allowing for the recovery of functional individual T cells. We propose to use this high-throughput assay, capable of screening millions of T cells, to identify tumor-responsive T cells and to recover them for downstream analysis or proliferation. This assay is simple, sensitive, and versatile enough to be used by a wide research community or eventually within clinical settings for isolating and profiling rare cells with target secretory phenotypes.