Parathyroid Diseases and T Cells.

PURPOSE OF REVIEW: This review summarizes studies into the permissive role of T cells in the bone catabolic effects of hyperparathyroidism and parathyroid hormone (PTH).

RECENT FINDINGS: Work in animals combined with recent translational studies in humans now highlight the potent amplificatory action of T cells on PTH-induced bone resorption. Mechanistic animal studies reveal a complex pathway by which PTH exploits natural self-renewal functions of CD4+ T cells, to drive TNFα production that promotes formation of IL-17A secreting Th17 T cells. TNFα and IL-17 further amplify osteoblastic receptor activator of NF-κB ligand (RANKL) production and down-modulate osteoprotegerin (OPG), establishing conditions propitious for osteoclastic bone resorption. These findings are consistent with, and add to, the traditional view of PTH-induced bone loss involving only osteoblast-lineage cells. T cells potently amplify traditional pathways and provide permissive costimulatory signals to bone marrow stromal cells, facilitating the development of an increased RANKL/OPG ratio favourable to bone resorption and bone loss.