Abstract
Immunotherapy utilizing checkpoint inhibitors has shown remarkable success in the treatment of cancers. In addition to immune checkpoint inhibitors, immune co-stimulation has the potential to enhance immune activation and destabilize the immunosuppressive tumor microenvironment. CD137, also known as 4-1BB, is one of the potent immune costimulatory receptors that could be targeted for effective immune co-stimulation. The interaction of the 4-1BB receptor with its natural ligand (4-1BBL) generates a strong costimulatory signal for T cell proliferation and survival. 4-1BBL lacks costimulatory activity in soluble form. To obtain co-stimulatory activity in soluble form, a recombinant 4-1BBL protein was generated by fusing the extracellular domains of murine 4-1BBL to a modified version of streptavidin (SA-4-1BBL). Treatment with SA-4-1BBL inhibited the development of lung tumors in A/J mice induced by weekly injections of the tobacco carcinogen NNK for eight weeks. The inhibition was dependent on the presence of T cells and NK cells; depletion of these cells diminished the SA-4-1BBL antitumor protective effect. The number of lung tumor nodules was significantly reduced by the administration of SA-4-1BBL to mice during ongoing exposure to NNK. The data presented in this paper suggest that utilizing an immune checkpoint stimulator as a single agent generate a protective immune response against lung cancer in the presence of a carcinogen. More broadly, this study suggests that immune checkpoint stimulation can be extended to a number of other cancer types, including breast and prostate cancers, for which improved diagnostics can detect disease at the preneoplastic stage.