αTIGIT-guided IL-15 mimetics drive potent and safe antitumor immunity by restoring tumor-infiltrating T cells

Cytokines are powerful modulators of antitumor immunity, but their clinical use is limited by structural instability, short half-life, poor drug-like properties, and severe systemic toxicity. Antibody-based cytokine mimetics have recently emerged to activate immune cells in vitro, but whether these mimetics can overcome the shortcomings of cytokines and exert therapeutic efficacy in vivo remains unclear. Here, we engineered bispecific antibody-based IL-15 mimetics using immunized Alpaca-derived phage display coupled with AlphaFold3-assisted structural screening and comparative screening of multiple formats to identify tandem IL-15 mimetics with strong in vitro bioactivity. Importantly, tandem IL-15R agonistic bispecifics, which simultaneously engage IL-15Rβ and γc, clearly demonstrated antitumor activity in vivo. To better target tumor-infiltrating lymphocytes (TILs), we incorporated a high-affinity anti-TIGIT antibody to guide tandem IL-15Rβγ agonists. This tri-antibody design, αTIGIT-αIL-15Rβγ, resulted in a striking improvement in antitumor activity without detectable systemic toxicity even at high doses. Mechanistically, αTIGIT-αIL-15Rβγ enhanced CD8⁺ T effector function and expanded the number of intratumoral stem-like T cells. Our study highlights a strategy to use TIL-targeted cytokine mimetics to overcome the limitations of native cytokines and enable dose pairing with immune checkpoint blockade (ICB), offering a path toward safer and more effective cytokine immunotherapy.