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 noncanonical protein substrates to directly activate targeted STAT proteins in T cells. STAT activation was accomplished by engineering the herpesvirus saimiri 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 interleukin-2. Tumor outgrowth was reduced in vivo because of enhanced T cell persistence and functionality. Single-cell transcriptomics revealed that 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.