Abstract Background: Patients with advanced thyroid cancer have few treatment options and poor survival e.g. 10% at 10 years for radioactive iodine-refractory differentiated thyroid cancer (DTC) and immune checkpoint inhibitors (ICI) have not shown benefit in these patients. The goal of this project is to identify novel strategies to improve the efficacy of ICI therapy in thyroid cancer. Prior work showed that suppressive myeloid cells are important contributors to immune escape and ICI resistance in thyroid cancer. In addition, clinical studies have shown that the presence of MDSCs in the peripheral blood of thyroid cancer patients correlates with more advanced disease and recurrence. In melanoma, colon, and lung tumor models, the TLR9 agonist CpG can act on myeloid cells to reduce suppressive function, increase antigen presentation and augment T cell proliferation, activation, and ingress into the tumor. We hypothesized that combined ICI therapy with TLR9 agonist CpG reverse of MDSC suppression to improve anti-tumor efficacy of ICI in DTC. Methods: Mice bearing poorly differentiated BRAFV600E+ papillary thyroid cancer tumors were randomized to treatment with isotype control or ICI (anti-PD1 or anti-PDL1; 250mg/dose i.p., 2x weekly) antibodies, with or without CpG-1826 peritumorally. In addition, we tested ICI with directly conjugated CpG given intraperitoneally compared to ICI alone as a potential systemic therapy. Immune populations in tumor and spleen were evaluated using immunofluorescence, flow cytometry, and in vitro functional assays. Tumor volume was measured thrice weekly. Results: ICI treatment alone (anti-PD1 or anti-PDL1) had no effect on tumor growth compared to isotype control. There were no differences in CD44+ IFNγ+CD8+ effector T cells and CD11b+Gr1hi MDSCs in tumor-bearing mice. Interestingly, peritumoral injection of CpG with concurrent ICI treatment resulted in significantly slower tumor growth (p0.05) and increased tumor-infiltrating cytotoxic IFNγ+CD8+ T cells. Ex vivo assessment of tumor-associated immune cells showed that CpG treatment abrogated MDSC-mediated suppression of CD8+ T cell proliferation and IFNg production. CpG showed modest but not significant direct effects on CD8+ T cells. Furthermore, PD1+CD8+ T cells and PDL1+ MDSCs were enriched in the TME and wondered whether this checkpoint protein enrichment could be leveraged for targeting of CpG therapy to the tumor. Systemic delivery of CpG is limited by systemic toxicity, whereas intratumoral CpG delivery has limited use for metastatic disease and short intratumoral half-life. allowing for it to be specific targets for Cpg conjugates to deliver directly to the TME. ICI-conjugated CpG (anti-PD1 or anti-PDL1) administered effectively reduced tumor growth and increased population of cytotoxic effector CD44+ IFNγ+ CD8+ T cells in the TME (p0.05 for anti-PDL1-CpG, trend for anti-PD1-CpG). Conclusion: Our study underscores the role of MDSCs in ICI-resistance seen in advanced thyroid cancer and suggests that TLR9 agonists may improve clinical outcomes for patients. Citation Format: Joah E. Lee, Jaden Nguyen, Jarod Olay, Kyleigh Kimbrell, Amber Lu, Aime T. Franco, Alan L. Epstein, Trevor E. Angell, Melissa G. Lechner. Reversing immune checkpoint inhibitor resistance rherapy in advanced thyroid cancer abstract. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr LB-A008.
Lee et al. (Thu,) studied this question.