Recombinant Costimulatory Fusion Proteins as Functional Immunomodulators Enhance Antitumor Activity in Murine B16F10 Melanoma.

Blocking inhibitory signaling and engaging stimulatory signaling have emerged as important therapeutic modalities for cancer immunotherapy. This study aimed to investigate immunomodulatory features of three recombinant costimulatory ligand proteins in a mouse model, which are extracellular domains of OX40-ligand (OX40L), 4-1BB-ligand (4-1BBL), or two domains in tandem, fused with the transmembrane domain of diphtheria toxin (DTT), named DTT-COS1, DTT-COS2, and DTT-COS12, respectively. In vitro study showed that DTT-COS1 and DTT-COS12 had immunological activity increasing the ratio of CD8/CD4 T cells. Treatments with DTT-COS1 and DTT-COS12 dramatically generated immune protection against the B16F10 tumor challenge in both prophylactic and therapeutic efficacy. Furthermore, regarding tumor microenvironment (TME) immunomodulation, DTT-COS1 treatment increased the proportion of CD4+ effector T cells (Teff) and decreased the expression of a suppressive cytokine. Meanwhile, DTT-COS12 reduced regulatory T cells (Treg) and improved the level of stimulatory cytokines. In addition, endogenous antibodies against OX40L/4-1BBL were generated, which may help with antitumor responses. Unexpectedly, DTT-COS2 lacked antitumor effects in vitro and in vivo. Importantly, serum analysis of liver-function associated factors and pro-inflammatory cytokines demonstrated that treatments were safe formulations in mice without signs of systemic toxicity. Remarkably, DTT-COS1 and DTT-COS12 are functional immunomodulators for mouse B16F10 melanoma, creating practical preclinical value in cancer immunotherapy.