Hypothyroidism‐associated immunosuppression involves induction of galectin‐1‐producing regulatory T cells

Hypothyroidism exerts deleterious effects on immunity, but the precise role of the hypothalamic–pituitary–thyroid (HPT) axis in immunoregulatory and tolerogenic programs is barely understood. Here, we investigated the mechanisms underlying hypothyroid‐related immunosuppression by examining the regulatory role of components of the HPT axis. We first analyzed lymphocyte activity in mice overexpressing the TRH gene (Tg‐Trh). T cells from Tg‐Trh showed increased proliferation than wild‐type (WT) euthyroid mice in response to polyclonal activation. The release of Th1 pro‐inflammatory cytokines was also increased in Tg‐Trh and TSH levels correlated with T‐cell proliferation. To gain further mechanistic insights into hypothyroidism‐related immunosuppression, we evaluated T‐cell subpopulations in lymphoid tissues of hypothyroid and control mice. No differences were observed in CD3/CD19 or CD4/CD8 ratios between these strains. However, the frequency of regulatory T cells (Tregs) was significantly increased in hypothyroid mice, and not in Tg‐Trh mice. Accordingly, in vitro Tregs differentiation was more pronounced in naïve T cells isolated from hypothyroid mice. Since Tregs overexpress galectin‐1 (Gal‐1) and mice lacking this lectin (Lgals1−/−) show reduced Treg function, we investigated the involvement of this immunoregulatory lectin in the control of Tregs in settings of hypothyroidism. Increased T lymphocyte reactivity and reduced frequency of Tregs were found in hypothyroid Lgals1−/− mice when compared to hypothyroid WT animals. This effect was rescued by the addition of recombinant Gal‐1. Finally, increased expression of Gal‐1 was found in Tregs purified from hypothyroid WT mice compared with their euthyroid counterpart. Thus, a substantial increase in the frequency and activity of Gal‐1‐expressing Tregs underlies immunosuppression associated with hypothyroid conditions, with critical implications in immunopathology, metabolic disorders, and cancer. The induction of galectin‐1 (Gal‐1)‐producing regulatory T cells (Tregs) contributes to hypothyroidism‐related immunosuppression. Hypothyroid wild‐type (WT) mice display increased frequency and activity of Tregs expressing high levels of Gal‐1, which contributed to lower T‐cell proliferation and immunosuppression compared with euthyroid mice. These effects were abrogated in hypothyroid mice lacking Gal‐1 (Lgals1−/−).