The expression and function of the neonatal Fc receptor in thyrocytes of Hashimoto's thyroiditis

Thyroglobulin (Tg) and thyroid peroxidase (TPO) antibodies (TgAb and TPOAb), which are primarily of the immunoglobulin G (IgG) class, can mediate antibody-dependent cell-mediated cytotoxicity in vitro. However, it is unclear whether any thyrocyte molecules can facilitate the transport and elimination of TgAb and TPOAb. The IgG transport receptor neonatal Fc receptor (FcRn) is a candidate mediator of these processes. In this study, we aimed to evaluate FcRn expression and function in normal and Hashimoto's thyroiditis (HT) thyrocytes.

FcRn expression in primary thyrocyte cultures (four normal and four HT groups) was examined by polymerase chain reaction (PCR) and Western blotting. Localization of FcRn was demonstrated by immunoelectron microscopy. A double immunofluorescence staining method was adopted to detect FcRn and internalized human TgAb IgG. Stimulation experiments were performed to assess the regulation of FcRn expression by T helper cell 1 (Th1) (IFN-γ and TNF-α) and Th2 cytokines (IL-10 and IL-4).

FcRn expression was lower in HT thyrocytes than in normal thyrocytes. FcRn was located in the cytoplasm, membranes, mitochondria and transport vesicles of thyrocytes. Both human IgG and TgAb IgG were internalized by thyrocytes in a pH-dependent manner and co-localized with FcRn in thyrocytes. FcRn expression was downregulated by Th1 and Th2 cytokines in both normal and HT thyrocytes in a dose-dependent manner.

Our results suggest that FcRn may be associated with the transport and metabolism of IgG in thyrocytes and that transport is independent of IgG type. FcRn may be involved in HT pathogenesis.