IL-12 immunotherapy of BrafV600E-induced papillary thyroid cancer in a mouse model

Abstract

Papillary thyroid carcinoma (PTC) accounts for >80% thyroid malignancies, and BRAFV600E mutation is frequently found in >40% PTC. Interleukin-12 (IL-12) is a proinflammatory heterodimeric cytokine with strong antitumor activity. It is not known whether IL-12 immunotherapy is effective against BrafV600E-induced PTC. In the present study, we investigated the effectiveness of IL-12 immunotherapy against BrafV600E-induced PTC in LSL-BrafV600E/TPO-Cre mice. LSL-BrafV600E/TPO-Cre mice were created for thyroid-specific expression of BrafV600E under the endogenous Braf promoter, and spontaneous PTC developed at about 5 weeks of age. The mice were subjected to two treatment regimens: (1) weekly intramuscular injection of 50 μg plasmid DNA expressing a single-chain IL-12 fusion protein (scIL-12/CMVpDNA), (2) daily intraperitoneal injection of mouse recombinant IL-12 protein (mrIL-12, 100 ng per day). The role of T cells, natural killer (NK) cells, and transforming growth factor-β (TGF-β) in IL-12-mediated antitumor effects was determined by a 51Cr-release cytotoxicity assay. Tumor size and weight were significantly reduced by either weekly intramuscular injection of scIL-12/CMVpDNA or daily intraperitoneal injection of mrIL-12, and tumor became more localized. Survival was significantly increased when treatment started at 1 week of age as compared with that at the 6 weeks of age. Both NK and CD8+ T cells were involved in the cytotoxicity against tumor cells and their antitumor activity was significantly reduced in tumor-bearing mice. TGF-β also inhibited the antitumor activity of NK and CD8+ T cells. The immune suppression was completely reversed by IL-12 treatment and partially recovered by anti-TGF-β antibody. We conclude that both IL-12 gene therapy and recombinant protein therapy are effective against PTC. Given that the immune response is significantly suppressed in tumor-bearing mice and can be restored by IL-12, the current study raises a possibility of the application of IL-12 as an adjuvant therapy for thyroid cancer.