Transcytosis of IgG from the basolateral to the apical membrane of human thyrocytes in autoimmune thyroid disease

Unidirectional transport of IgG by neonatal Fc receptor in human thyrocytes varies across different IgG subclasses

Neonatal Fc receptor (FcRn) is down-regulated in Hashimoto's thyroiditis (HT) thyrocytes and mediates IgG endocytosis in thyrocytes. The serum distribution of IgG subclasses (of TgAb and TPOAb) differs between HT patients and normal individuals. We aimed to explore the direction and regulation of FcRn-mediated IgG transport in thyrocyte monolayers and the difference between various IgG subclass transport. IgG was transported by FcRn from the basolateral to apical side in the thyrocyte monolayers grown on Transwell filters and the transport was inhibited by IFN-γ and TNF-α. Stimulation by T3 and TSH down-regulated FcRn expression in thyrocytes. IgG1 was transported preferentially over IgG2 and IgG4, which might be related to the differences in FcRn-binding affinities as shown by SPR. FcRn mediates unidirectional IgG transport in thyrocytes in a tissue-specific manner. Down-regulation of FcRn is speculated to play a protective role in HT pathogenesis by mainly reducing IgG1 transport in thyrocytes.

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

OBJECTIVE

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.

METHODS

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).

RESULTS

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.

CONCLUSIONS

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.

Structural and functional aspects of thyroid peroxidase

Thyroperoxidase (TPO) is the enzyme involved in thyroid hormone synthesis. Although many studies have been carried out on TPO since it was first identified as being the thyroid microsomal antigen involved in autoimmune thyroid disease, previous authors have focused more on the immunological than on the biochemical aspects of TPO during the last few years. Here, we review the latest contributions in the field of TPO research and provide a large reference list of original publications. Given this promising background, scientists and clinicians will certainly continue in the future to investigate the mechanisms whereby TPO contributes to hormone synthesis and constitutes an important autoantigen involved in autoimmune thyroid disease, and the circumstances under which the normal physiological function of this enzyme takes on a pathological role.

Androgen-dependent expression of FcgammaRIIB2 by thyrocytes from patients with autoimmune Graves' disease: a possible molecular clue for sex dependence of autoimmune disease

Thyrocyte expression of HLA class I and class II antigens and related accessory molecules would convert these epithelial cells into functional antigen-presenting cells. Here we show that whereas normal thyrocytes express FcRn, Graves' disease thyrocytes also express FcgammaRIIB2. We further find that expression of FcgammaRIIB2, but not FcRn, is repressed by dihydrotestosterone. By mediating the uptake and transport of autoantibodies, we suggest that these IgG Fc receptors contribute in various ways to the onset and/or progression of autoimmune thyroid diseases. The androgen-mediated decrease of FcgammaRIIB2 expression in Graves' disease thyrocytes also provides a rationale for the predominant susceptibility of women to develop an autoimmune thyroid disease. Our findings open up a new prospect to autoimmunity, linking the role of the target organ to the sex dependence in autoimmune disease.