Phosphoprotein dynamics of interacting tumor and T cells by HySic

Functional interactions between cytotoxic T cells and tumor cells are central to anti-cancer immunity. Some of the proteins involved, particularly immune checkpoints expressed by T cells, serve as promising clinical targets in immunotherapy. However, our understanding of the complexity and dynamics of the interactions between tumor cells and T cells is only rudimentary. Here we present HySic (for Hybrid quantification of SILAC (Stable Isotope Labelling by Amino acids in Cell culture)-labeled interacting cells) as an innovative method to quantify protein and phosphorylation dynamics between and within physically interacting (heterotypic) cells. We show that co-cultured HLA/antigen-matched tumor and T cells engage in physical and stable interactions, allowing for in-depth HySic analysis. This method does not require physical separation of the two cell types for subsequent MS proteome and phosphoproteome measurement using label free quantification (LFQ). We demonstrate that HySic can be used to unravel proteins contributing to functional T cell:tumor cell interactions. We validated HySic with established interactions, including those mediating IFN{gamma} signaling. Using HySic we identified the RHO/RAC/PAK1 signaling pathway to be activated upon interaction of T cells and tumor cells. Pharmacologic inhibition of PAK1 sensitized tumor cells to T cell killing. Thus, HySic is an innovative and simple method to study short-term protein signaling dynamics in physically interacting cells, which can be easily extended to other biological systems.