Characterization of a Cytokine-Independent STAT5 Activator

Background: Cytokine-induced JAK–STAT signaling becomes dysregulated in chronic human diseases, including cancer and autoimmunity, and contributes to immune cell dysfunction. A cytokine-independent approach to activating STAT proteins could “hardwire” pro-survival and effector programs in immune cells to sustain function within diseased tissues. Engineered variants of the herpesvirus saimiri tyrosine kinase interacting protein (TIP) can recruit the SRC family kinase (SFK) LCK to drive STAT phosphorylation and activation. Here, we evaluated the interactome of a TIP-derived, cytokine-independent STAT5 activator and determined whether it could induce STAT5 activation in immune cell lines and primary human CD8+ T cells. Methods: A STAT5 activator (aSTAT5) was characterized by proteomics using affinity purification mass spectrometry (AP-MS) to define its interactome and STAT5 binding specificity. STAT5 phosphorylation was assessed in hematopoietic cell lines and primary human CD8+ T cells. Results: Proteomic analysis confirmed preferential association of aSTAT5 with STAT5 relative to other proteins. In cell-based assays, aSTAT5 induced robust STAT5 phosphorylation in LCK-expressing NK-92 and Jurkat T cells, whereas phosphorylation was not observed in Raji B cells or RAW 264.7 macrophages despite expression of closely related SFKs and STAT5. Cytokine-independent STAT5 phosphorylation supported the viability of NK-92 cells and primary human CD8+ T cells during cytokine withdrawal and preserved the cytotoxic function of CAR T cells. Conclusions: We defined the interactome of a cytokine-independent STAT5 activator and demonstrated its capacity to maintain survival and function in human CD8+ T cells and NK-92 cells. These findings underscore the translational potential of engineered, cytokine-independent STAT5 activation for immune cell therapies.