Concurrent intratumoural Treg cell depletion and CD8+ T cell expansion via a cleavable anti-4-1BB–interleukin-15 fusion protein

Potent agonists of the inducible co-stimulatory receptor 4-1BB are too toxic for patients with advanced cancer. Here, on the basis of observations of a weak agonist of 4-1BB depleting regulatory T (T_reg) cells within the tumour microenvironment without leading to substantial restoration of dysfunctional cytotoxic T cells (CTLs), we show that effective tumour control can be achieved via concurrent T_reg cell depletion and CTL expansion through an anti-4-1BB antibody fused to interleukin-15 (IL-15) via a peptide sensitive to tumour proteases. In mouse models of advanced cancers, intraperitoneal injection of the bifunctional protein attenuated the activity of the interleukin mostly in the periphery of the primary tumour while allowing for the expansion of CTLs within the tumour microenvironment, led to more effective tumour inhibition and to lower systemic toxicity than treating the cancers with combinatorial treatment with unlinked anti-4-1BB antibody and IL-15, and reduced the resistance of tumours to checkpoint blockade. Concurrent eradication of T_reg cells and activation of tumour-infiltrating lymphocytes may represent a general strategy for the effective control of advanced metastatic tumours.