Immune-related adverse events induced by anti–programmed cell death–1 antibodies (PD-1-Ab), including destructive thyroiditis (thyroid-irAE), are thought to be caused by activated T cells. However, the T cell subsets that are directly responsible for damaging self-organs remain unclear. To clarify which T cell subsets are involved in the development of thyroid-irAE, a mouse model of thyroid-irAE was analyzed. PD-1-Ab administration 2.5 months after immunization with thyroglobulin caused destructive thyroiditis. Thyroiditis was completely prevented by previous depletion of CD4+ T cells and partially prevented by depleting CD8+ T cells. The frequencies of central and effector memory CD4+ T cell subsets and the secretion of interferon-y after stimulation with thyroglobulin were increased in the cervical lymph nodes of mice with thyroid-irAE compared with controls. Histopathological analysis revealed infiltration of CD4+ T cells expressing granzyme B in thyroid glands and major histocompatibility complex class II expression on thyrocytes in mice with thyroid-irAE. Adoptive transfer of CD4+ T cells from cervical lymph nodes in mice with thyroid-irAE caused destruction of thyroid follicular architecture in the irradiated recipient mice. Flow cytometric analyses showed that the frequencies of central and effector memory CD4+ T cells expressing the cytotoxic marker CD27 were higher in peripheral blood mononuclear cells collected from patients with thyroid-irAE induced by PD-1-Ab versus those without. These data suggest a critical role for cytotoxic memory CD4+ T cells activated by PD-1-Ab in the pathogenesis of thyroid-irAE.
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