An engineered viral protein activates STAT5 to prevent T cell suppression

T cell therapy efficacy can be compromised if cytokine-induced Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling is dysregulated or insufficient to sustain functionality. Here, we demonstrate that LCK kinase activity can be recruited to non-canonical protein substrates to directly activate targeted STAT proteins in T cells. STAT activation was accomplished by engineering the Herpesvirus saimiri (HVS) tyrosine kinase interacting protein (TIP) to provide a platform for the enforced recruitment of LCK to STAT proteins. We determined that a minimal region of TIP that binds to LCK could be combined with STAT binding sites derived from endogenous cytokine receptors. These constructs activated targeted STAT proteins in a cytokine-independent manner. We identified a STAT5 activator that sustained CD8+ T cell survival and cytotoxic function ex vivo in the absence of IL-2. Tumor outgrowth was reduced in vivo due to enhanced T cell persistence and functionality. Single-cell transcriptomics revealed the STAT5 activator prevented the expression of genes associated with an exhausted T cell fate. Our findings demonstrate that signaling pathways can be rewired in T cells to sustain their function in solid tumors.