Disruption of PTH Receptor 1 in T Cells Protects against PTH-Induced Bone Loss

Abstract

BackgroundHyperparathyroidism in humans and continuous parathyroid hormone (cPTH) treatment in mice cause bone loss by regulating the production of RANKL and OPG by stromal cells (SCs) and osteoblasts (OBs). Recently, it has been reported that T cells are required for cPTH to induce bone loss as the binding of the T cell costimulatory molecule CD40L to SC receptor CD40 augments SC sensitivity to cPTH. However it is unknown whether direct PTH stimulation of T cells is required for cPTH to induce bone loss, and whether T cells contribute to the bone catabolic activity of PTH with mechanisms other than induction of CD40 signaling in SCs.Methodology/Principal FindingsHere we show that silencing of PTH receptor 1 (PPR) in T cells blocks the bone loss and the osteoclastic expansion induced by cPTH, thus demonstrating that PPR signaling in T cells is central for PTH-induced reduction of bone mass. Mechanistic studies revealed that PTH activation of the T cell PPR stimulates T cell production of the osteoclastogenic cytokine tumor necrosis factor α (TNF). Attesting to the relevance of this effect, disruption of T cell TNF production prevents PTH-induced bone loss. We also show that a novel mechanism by which TNF mediates PTH induced osteoclast formation is upregulation of CD40 expression in SCs, which increases their RANKL/OPG production ratio.Conclusions/SignificanceThese findings demonstrate that PPR signaling in T cells plays an essential role in PTH induced bone loss by promoting T cell production of TNF. A previously unknown effect of TNF is to increase SC expression of CD40, which in turn increases SC osteoclastogenic activity by upregulating their RANKL/OPG production ratio. PPR-dependent stimulation of TNF production by T cells and the resulting TNF regulation of CD40 signaling in SCs are potential new therapeutic targets for the bone loss of hyperparathyroidism.

PTH expands short-term murine hemopoietic stem cells through T cells

“…To control for strain-dependent confounders, the study included TCR␤ Ϫ/Ϫ mice subjected to adoptive transfer of WT T cells 3 weeks before initiation of iPTH treatment, a procedure that is followed by the engraftment and homeostatic expansion of the donor T cells. 23,24 Flow cytometric analysis of splenocytes harvested at death from mice subjected to adoptive transfer of T cells confirmed the engraftment and the expansion of adoptively transferred T cells (supplemental Figure 1). Analysis of BM samples revealed that vehicle-treated WT mice, TCR␤ Ϫ/Ϫ mice, and TCR␤ Ϫ/Ϫ mice reconstituted with T cells had a similar number of BM mononucleated cells and LSK cells (supplemental Figure 2) and a similar relative frequency of LSK cells ( Figure 1A-C).…”

“…18 Whereas SCs, OBs, and osteocytes represent the major targets of PTH in bone, [19][20][21] reports from our laboratory have disclosed that T lymphocytes play an unexpected role in the mechanism of action of PTH in bone. [22][23][24][25] Importantly, treatment with daily PTH increases the T-cell production of Wnt10b, 23,25 a Wnt ligand that stimulates osteoblastogenesis by activating Wnt signaling in SCs and OBs, 23 through direct targeting of the T-cell PTH/PTH-related protein receptor (PPR). 25 As a result, the bone anabolic activity of daily PTH treatment is markedly reduced in T cell-deficient mice and in mice with a specific disruption of Wnt10b production by T cells.…”

Osteoimmunology and Its Implications for Transplantation

“…Therefore, SCs from T cell deficient mice or mice lacking T cell expression of CD40L have a lower capacity to support osteoclast formation in vivo and in vitro (71). Moreover, cPTH stimulates T cell production of TNF (75). TNF further increases the SC production of RANKL, and upregulates the expression of CD40 in SCs, thus increasing their response to T cell expressed CD40L (Figure 2).…”

“…TNF further increases the SC production of RANKL, and upregulates the expression of CD40 in SCs, thus increasing their response to T cell expressed CD40L (Figure 2). Attesting to the relevance of T cell produced TNF, cPTH fails to induce bone loss and stimulate bone resorption in mice specifically lacking T cell TNF production (75). Deletion of the PTH receptor PPR in T cells blunts the stimulation of bone resorption induced by cPTH without affecting bone formation, thus blocking cortical bone loss and converting the effects of cPTH in trabecular bone from catabolic to anabolic (75).…”

Osteoimmunology at the nexus of arthritis, osteoporosis, cancer, and infection

“…A recent analysis of several strains of genetically modified mice revealed that T cells play a role in promoting increased bone resorption observed with continuous PTH administration (76,77). These studies showed that PTH signals through PTH receptor 1 on bone marrow T cells to augment production of TNF-α (77).…”