Osteoclasts are multinucleated giant cells that resorb bone, ensuring development and continuous remodelling of the skeleton and the bone marrow haematopoietic niche. Defective osteoclast activity leads to osteopetrosis and bone marrow failure1–9, whereas excess activity can contribute to bone loss and osteoporosis10. Osteopetrosis can be partially treated by bone marrow transplantation in humans and mice11–18, consistent with a haematopoietic origin of osteoclasts13,16,19 and studies suggesting that they develop by fusion of monocytic precursors derived from haematopoietic stem cells in the presence of CSF1 and RANK ligand1,20. However, the developmental origin and lifespan of osteoclasts, and the mechanisms that ensure maintenance of osteoclast function throughout life in vivo remain largely unexplored. Here we report that osteoclasts that colonize fetal ossification centres originate from embryonic erythro-myeloid progenitors21,22. These erythro-myeloid progenitor-derived osteoclasts are required for normal bone development and tooth eruption. Yet, timely transfusion of haematopoietic stem cells derived monocytic cells in newborn mice is sufficient to rescue bone development in early-onset autosomal recessive osteopetrosis. We also found that the postnatal maintenance of osteoclasts, bone mass and the bone marrow cavity involve iterative fusion of circulating blood monocytic cells with long-lived osteoclast syncytia. As a consequence, parabiosis or transfusion of monocytic cells results in long-term gene transfer in osteoclasts in the absence of haematopoietic stem cell chimerism, and can rescue an adult-onset osteopetrotic phenotype caused by cathepsin K deficiency23,24. In sum, our results identify the developmental origin of osteoclasts and a mechanism that controls their maintenance in bones after birth. These data suggest new strategies to rescue osteoclast deficiency in osteopetrosis and to modulate osteoclast activity in vivo.

Elucidating the rules for receptor triggering in cell-cell and cell-matrix contacts requires precise control of ligand positioning in three dimensions. Here, we use the T cell receptor (TCR) as a model and subject T cells to different geometric arrangements of ligands, using a nanofabricated single-molecule array platform. This is comprised of monovalent TCR ligands anchored to lithographically patterned nanoparticle clusters surrounded by mobile adhesion molecules on a supported lipid bilayer (SLB). The TCR ligand could be co-planar with the SLB (2D), excluding the CD45 transmembrane tyrosine phosphatase, or elevated by 10 nm on solid nanopedestals (3D), allowing closer access of CD45 to engaged TCR. The two configurations resulted in different T cell responses, depending on the lateral spacing between the ligands. These results identify the important contributions of lateral and axial components of ligand positioning and create a more complete foundation for receptor engineering for immunotherapy.

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BackgroundPatients with heterozygous germline mutations in phosphatase and tensin homolog deleted on chromosome 10 (PTEN) experience autoimmunity and lymphoid hyperplasia.ObjectivesBecause regulation of the phosphoinositide 3-kinase (PI3K) pathway is critical for maintaining regulatory T (Treg) cell functions, we investigate Treg cells in patients with heterozygous germline PTEN mutations (PTEN hamartoma tumor syndrome [PHTS]).MethodsPatients with PHTS were assessed for immunologic conditions, lymphocyte subsets, forkhead box P3 (FOXP3)+ Treg cell levels, and phenotype. To determine the functional importance of phosphatases that control the PI3K pathway, we assessed Treg cell induction in vitro, mitochondrial depolarization, and recruitment of PTEN to the immunologic synapse.ResultsAutoimmunity and peripheral lymphoid hyperplasia were found in 43% of 79 patients with PHTS. Immune dysregulation in patients with PHTS included lymphopenia, CD4+ T-cell reduction, and changes in T- and B-cell subsets. Although total CD4+FOXP3+ Treg cell numbers are reduced, frequencies are maintained in the blood and intestine. Despite pathogenic PTEN mutations, the FOXP3+ T cells are phenotypically normal. We show that the phosphatase PH domain leucine-rich repeat protein phosphatase (PHLPP) downstream of PTEN is highly expressed in normal human Treg cells and provides complementary phosphatase activity. PHLPP is indispensable for the differentiation of induced Treg cells in vitro and Treg cell mitochondrial fitness. PTEN and PHLPP form a phosphatase network that is polarized at the immunologic synapse.ConclusionHeterozygous loss of function of PTEN in human subjects has a significant effect on T- and B-cell immunity. Assembly of the PTEN-PHLPP phosphatase network allows coordinated phosphatase activities at the site of T-cell receptor activation, which is important for limiting PI3K hyperactivation in Treg cells despite PTEN haploinsufficiency.