Immune Checkpoint Inhibitor-Induced Thyroiditis Is Associated with Increased Intrathyroidal T Lymphocyte Subpopulations

Background: Immune checkpoint inhibitors (ICIs) frequently cause thyroid dysfunction but their underlying mechanism remains unclear. We have previously demonstrated increased circulating natural killer (NK) cells and human leukocyte antigen (HLA)-DR surface expression on inflammatory intermediate CD14⁺CD16⁺ monocytes in programmed cell death protein-1 (PD-1) inhibitor-treated patients. This study characterizes intrathyroidal and circulating immune cells and class II HLA in ICI-induced thyroiditis. Methods: This is a single-center prospective cohort study of 10 patients with ICI-induced thyroiditis by flow cytometry of thyroid fine needle aspirates (n = 9) and peripheral blood (n = 7) as compared with healthy thyroid samples (n = 5) and healthy volunteer blood samples (n = 44); HLA class II was tested in n = 9. Results: ICI-induced thyroiditis samples demonstrated overall increased T lymphocytes (61.3% vs. 20.1%, p = 0.00006), CD4^-CD8^- T lymphocytes (1.9% vs. 0.7%, p = 0.006), and, as a percent of T lymphocytes, increased CD8⁺T lymphocytes (38.6% vs. 25.7%; p = 0.0259) as compared with healthy thyroid samples. PD-1 inhibitor-induced thyroiditis had increased CD4⁺PD1⁺ T lymphocytes (40.4% vs. 0.8%; p = 0.021) and CD8⁺PD1⁺ T lymphocytes (28.8% vs. 1.5%; p = 0.038) in the thyroid compared with the blood. Circulating NK cells, certain T lymphocytes (CD4⁺CD8⁺, CD4^-CD8^- T, gamma-delta), and intermediate monocytes were increased in ICI-induced thyroiditis. Six patients typed as HLA-DR4-DR53 and three as HLA-DR15. Conclusions: ICI-induced thyroiditis is a T lymphocyte-mediated process with intra-thyroidal predominance of CD8⁺ and CD4^-CD8^- T lymphocytes. The HLA haplotypes may be involved but need further evaluation. These findings expand the limited understanding of ICI-induced thyroiditis, which could be further translated to guide immunomodulatory therapies for advanced thyroid cancer.

PD-L1 Inhibitor-Induced Thyroiditis Is Associated with Better Overall Survival in Cancer Patients

Background: Thyroid immune-related adverse events (IRAEs) have been reported more frequently with programmed cell death protein-1 (PD-1) inhibitors than cytotoxic T lymphocyte antigen 4 (CTLA-4) inhibitors, but there is limited data describing endocrinopathies from programmed cell death protein-ligand 1 (PD-L1) inhibitors. This study characterizes thyroid IRAEs in cancer patients treated with PD-L1 inhibitors and examines the impact on overall survival. Methods: This is a retrospective cohort study of cancer patients treated with atezolizumab and avelumab at the Mayo Clinic, Rochester, Minnesota, from June 1, 2016 to January 30, 2018, and followed for a median of 10.1 months. Thyroid IRAEs were characterized as new onset hypothyroidism, thyrotoxicosis, and worsening of pre-existing hypothyroidism. Results: Of 91 patients treated with a PD-L1 inhibitor, 19 (21%) developed new onset thyroid dysfunction, of whom 14 presented with hypothyroidism and 5 with thyrotoxicosis (3 progressed to hypothyroidism and 2 returned to euthyroidism), and 4 (4%) had worsening of pre-existing hypothyroidism. Thyroid IRAEs occurred after a median of two doses (6 weeks), 48% required thyroid hormone replacement, and none required steroids or discontinuation of immunotherapy. Two out of four patients with thyroid peroxidase (TPO) antibody >9 IU/mL at baseline developed thyroid IRAEs. Median TPO antibody titer was not different between those with and without thyroid IRAEs but was higher in those with overt than those with subclinical hypothyroidism (5 vs. 0.3 IU/mL, p = 0.003) and those prescribed thyroid hormone replacement as compared with observation (5.5 vs. 0.3, p = 0.008). Diffusely increased thyroid 18-fluorodeoxyglucose (¹⁸FDG) uptake on positron emission tomography (PET) scan occurred in 71% with thyroid IRAE as compared with 6% without thyroid IRAEs (p = 0.001). Patients who developed thyroid IRAEs had longer overall survival (p = 0.027) and lower mortality (hazard ratio 0.49 [95% CI 0.25-0.99], p = 0.034) after adjusting for potential confounders. Conclusions: PD-L1 inhibitors lead to immune-mediated thyroiditis, the most frequent endocrine IRAE. In most cases, management is supportive without requiring steroids or discontinuation of immunotherapy. Diffusely increased thyroid ¹⁸FDG uptake on PET scan may predict the occurrence of thyroiditis, whereas TPO antibodies may help identify its severity. Thyroiditis may be a biomarker for antitumor immune response, highlighting the need to further characterize its underlying mechanism.