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.