Cis-delivering releasable IL-15 superagonist enhances antitumor immunity in cold tumors by invigorating preexisting CD25 ⁺ CD8 ⁺ T cells

Antibody-mediated cis-delivery and trans-delivery both direct cytokines to tumors and have been extensively investigated in clinical trials. However, a comparative analysis of their differential effects on cytokine activity is still lacking. In this study, we initially verify that cis-delivery of cytokine demonstrates a markedly stronger antitumor effect than trans-delivery, but it also exhibits certain drawbacks, including severe toxicity, insufficient activation of CD25 ⁺ CD8 ⁺ T cells, and enhanced stimulation of intratumoral regulatory T cells (Tregs). To further address these issues, we developed a conditionally releasable and cis-delivering IL-15 immunocytokine (termed PMIS), which can selectively release a free IL-15 superagonist within tumors rather than immobilizing IL-15 to PD-1 ⁺ Tregs, thereby potently stimulating CD25 ⁺ CD8 ⁺ T cells. Mice treated with PMIS showed significantly reduced systemic toxicities while achieving notably stronger antitumor effects. Administered either alone or in combination with other therapies, PMIS exhibits great potential for inhibiting orthotopic cold tumor and its metastases. Mechanistically, the significant activation of the pre-existing intratumoral CD25 ⁺ CD8 ⁺ T cells and the improved CD25 ⁺ CD8/Treg ratio contribute to the enhanced antitumor response of PMIS. These findings underscore the indispensable role of CD25 ⁺ CD8 ⁺ T cells in cis-delivering IL-15 and provide a promising strategy for overcoming resistance to therapies and effectively controlling advanced cold tumors.