Autoimmune Pituitary Disease: New Concepts With Clinical Implications.

Some endocrine disorders, including hypophysitis and isolated adrenocorticotropic hormone (ACTH) deficiency, are caused by an autoimmune response to endocrine organs. Although the pathogenesis of some autoimmune endocrine diseases has been elucidated, it remains obscure for most. Anti-PIT-1 hypophysitis (anti-PIT-1 antibody syndrome) is a newly described pituitary autoimmune disease characterized by acquired and specific growth hormone (GH), prolactin (PRL), and thyroid-stimulating hormone (TSH) deficiencies. This disorder is associated with a thymoma or neoplasm that ectopically expresses pituitary-specific transcription factor 1 (PIT-1) protein. Circulating anti-PIT-1 antibody is a disease marker, and PIT-1-reactive cytotoxic T cells (CTLs) play a pivotal role in disease development. In addition, isolated ACTH deficiency appears to be caused by autoimmunity to corticotrophs; however, the pathogenesis remains unclear. A recently described case of isolated ACTH deficiency with large cell neuroendocrine carcinoma (LCNEC) showed ectopically expressed proopiomelanocortin (POMC), and circulating anti-POMC antibody and POMC-reactive CTLs were also detected. As CTL infiltrations around corticotrophs were also observed, isolated ACTH deficiency may be associated at least in part with a paraneoplastic syndrome. Although several underlying mechanisms for pituitary autoimmunity have been proposed, these observations highlight the importance of paraneoplastic syndrome as a cause of pituitary autoimmune disease. In this review, we focus on the pathophysiology and connection of anti-PIT-1 hypophysitis and isolated ACTH deficiency and discuss the state-of-art knowledge for understanding pituitary autoimmunity.

Pathogenesis of Anti-PIT-1 Antibody Syndrome: PIT-1 Presentation by HLA Class I on Anterior Pituitary Cells.

CONTEXT: Anti-pituitary-specific transcriptional factor-1 (anti-PIT-1) antibody syndrome is characterized by acquired and specific deficiencies in growth hormone, prolactin, and thyroid-stimulating hormone. Although PIT-1-reactive cytotoxic T lymphocytes (CTLs) have been speculated to recognize anterior pituitary cells and to cause the injury in the pathogenesis of the syndrome, it remains unclear whether endogenous PIT-1 protein is processed through the proteolytic pathway and presented as an antigen on anterior pituitary cells. OBJECTIVE: To examine how PIT-1 protein is processed and whether its epitope is presented by major histocompatibility complex (MHC)/HLA class I on anterior pituitary cells. MATERIALS AND METHODS: Immunofluorescence staining and proximity ligation assay (PLA) were performed using anti-PIT-1 antibody and patients' sera on PIT-1-expressing cell line GH3 cells and human induced pluripotent stem cell (iPSC)-derived pituitary tissues. RESULTS: PIT-1 was colocalized with MHC class I molecules, calnexin, and GM130 in the cytosol. PLA results showed that PIT-1 epitope was presented by MHC/HLA class I molecules on the cell surface of GH3 cells and iPSC-derived pituitary cells. The number of PIT-1/HLA complexes on the cell surface of pituitary cells in the patient was comparable with that in the control subject. CONCLUSIONS: Our data indicate that PIT-1 protein is processed in the antigen presentation pathway and that its epitopes are presented by in MHC/HLA class I on anterior pituitary cells, supporting the hypothesis that PIT-1-reactive CTLs caused the cell-specific damage. It is also suggested that number of epitope presentation was not associated with the pathogenesis of anti-PIT-1 antibody syndrome.

Anti-PIT-1 antibody syndrome; a novel clinical entity leading to hypopituitarism.

Various hypothalamic-pituitary diseases cause hypopituitarism. Inflammation related to autoimmunity also causes hypopituitarism. Hypophysitis is a representative disease caused by autoimmunity. Generally, anterior pituitary hormones are non-specifically impaired in this condition, but specific hormone defects have been reported in some cases. Anti-PIT-1 (pituitary-specific transcription factor 1) antibody syndrome is a novel clinical entity that presents an acquired combined pituitary hormone deficiency characterized by a specific defect in growth hormone, prolactin, and thyroid-stimulating hormone. Circulating anti-PIT-1 antibody along with various autoantibodies are detected with multiple endocrine organopathy, meeting the definition of autoimmune polyglandular syndrome. Mechanistically, cytotoxic T lymphocytes that specifically react with PIT-1 protein play an important role in the development of this syndrome.

Involvement of PIT-1-reactive cytotoxic T lymphocytes in anti-PIT-1 antibody syndrome.

CONTEXT: Anti-pituitary-specific transcriptional factor 1 (PIT-1) antibody syndrome is characterized by acquired growth hormone (GH), prolactin (PRL), and thyroid-stimulating hormone (TSH) deficiencies associated with circulating anti-PIT-1 antibodies. Although autoimmunity to PIT-1 has been suggested as a pathogenesis, the precise mechanism of the syndrome remains unclarified. OBJECTIVE: To elucidate the involvement of antibody- or cell-mediated immunity in anti-PIT-1 antibody syndrome. MATERIALS AND METHODS: To investigate a direct effect of anti-PIT-1 antibody on pituitary cells, cell proliferation, and cytotoxicity detection assays were performed using patient serum. Enzyme-linked immunospot (ELISpot) assay was performed to evaluate the involvement of PIT-1-reactive cytotoxic T lymphocytes (CTLs). An immunohistochemical analysis using anti-CD4 or anti-CD8 antibody was performed to examine tissue infiltration by CTLs. RESULTS: Patient serum did not exhibit any inhibitory effect on cell proliferation and secretion of GH and PRL in GH3 cells. In addition, complement-dependent cytotoxicity was not detected in patient serum on GH3 cells or primary pituitary cells. The ELISpot assay revealed the presence of CTLs that specifically reacted to the recombinant PIT-1 protein in the patient's peripheral lymphocytes. CD8(+) cell infiltrations, which is the characteristic of CTLs, were observed in the pituitary gland, adrenal gland, stomach, thyroid gland, liver, and pancreas of the patient with anti-PIT-1 antibody syndrome. CONCLUSIONS: These results suggest that the anti-PIT-1 antibody is not a cause but a marker of anti-PIT-1 antibody syndrome, in which CTLs play a pivotal role in the pathogenesis.