Endocrine Abstracts

The immune system is a major target of thyroid hormones (TH), as they have been shown to act on innate as well as adaptive immune cells affecting their proliferation, activation and survival. However, the local TH action within the immune system is still poorly understood. To improve our understanding of this interaction, we performed an immunophenotyping of mice at baseline using flow cytometry and ELISA. For this, we focused on mice with TRα or TRβ mutations leading to TR deficiency (KO strains) or impaired DNA binding (GS strains). Flow cytometric analysis of the main leukocyte populations in TRβKO and GS mice revealed only minor differences in the composition of circulating and lymphoid immune cells. Similarly, TRαKO and GS mice displayed only minor differences in the innate immune compartment. However, significantly elevated frequencies of circulating T cells were found in TRαGS mice, which was related to an increase in the CD4⁺ but not the CD8⁺ subpopulation. Nevertheless, both CD4⁺ and CD8⁺ T cells exhibited a distinct phenotype in TRαGS mice which could be linked to either T cell activation or T cell exhaustion. Overall our data suggest a minor effect of TRβ on immunity, as well as a minor role of both TR in the innate immune system. It should be noted that these data were obtained in a naïve state and therefore do not necessarily correspond to the situation after stimulation. Nevertheless, our results imply an important function of TRα in the adaptive immune response already at steady state. The impact of TRα action on the adaptive immunity upon activation requires further investigation.