The neutrophil-osteogenic cell axis promotes bone destruction in periodontitis

The immune-stromal cell interactions play a key role in health and diseases. In periodontitis, the most prevalent infectious disease in humans, immune cells accumulate in the oral mucosa and promote bone destruction by inducing receptor activator of nuclear factor-κB ligand (RANKL) expression in osteogenic cells such as osteoblasts and periodontal ligament cells. However, the detailed mechanism underlying immune-bone cell interactions in periodontitis is not fully understood. Here, we performed single-cell RNA-sequencing analysis on mouse periodontal lesions and showed that neutrophil-osteogenic cell crosstalk is involved in periodontitis-induced bone loss. The periodontal lesions displayed marked infiltration of neutrophils, and in silico analyses suggested that the neutrophils interacted with osteogenic cells through cytokine production. Among the cytokines expressed in the periodontal neutrophils, oncostatin M (OSM) potently induced RANKL expression in the primary osteoblasts, and deletion of the OSM receptor in osteogenic cells significantly ameliorated periodontitis-induced bone loss. Epigenomic data analyses identified the OSM-regulated RANKL enhancer region in osteogenic cells, and mice lacking this enhancer showed decreased periodontal bone loss while maintaining physiological bone metabolism. These findings shed light on the role of neutrophils in bone regulation during bacterial infection, highlighting the novel mechanism underlying osteoimmune crosstalk.

Effect of JAK inhibitors on the three forms of bone damage in autoimmune arthritis: joint erosion, periarticular osteopenia, and systemic bone loss

BACKGROUND

The types of bone damage in rheumatoid arthritis (RA) include joint erosion, periarticular osteoporosis, and systemic osteoporosis. Janus kinase (JAK) inhibitors ameliorate inflammation and joint erosion in RA, but their effect on the three types of bone loss have not been reportedly explored in depth. We aimed to clarify how JAK inhibitors influence the various types of bone loss in arthritis by modulating osteoclastic bone resorption and/or osteoblastic bone formation.

METHODS

Collagen-induced arthritis (CIA) mice were treated with a JAK inhibitor after the onset of arthritis. Micro-computed tomography (μCT) and histological analyses (bone morphometric analyses) on the erosive calcaneocuboid joint, periarticular bone (distal femur or proximal tibia), and vertebrae were performed. The effect of four different JAK inhibitors on osteoclastogenesis under various conditions was examined in vitro.

RESULTS

The JAK inhibitor ameliorated joint erosion, periarticular osteopenia and systemic bone loss. It reduced the osteoclast number in all the three types of bone damage. The JAK inhibitor enhanced osteoblastic bone formation in the calcaneus distal to inflammatory synovium in the calcaneocuboid joints, periarticular region of the tibia and vertebrae, but not the inflamed calcaneocuboid joint. All the JAK inhibitors suppressed osteoclastogenesis in vitro to a similar extent in the presence of osteoblastic cells. Most of the JAK inhibitors abrogated the suppressive effect of Th1 cells on osteoclastogenesis by inhibiting IFN-γ signaling in osteoclast precursor cells, while a JAK inhibitor did not affect this effect due to less ability to inhibit IFN-γ signaling.

CONCLUSIONS

The JAK inhibitor suppressed joint erosion mainly by inhibiting osteoclastogenesis, while it ameliorated periarticular osteopenia and systemic bone loss by both inhibiting osteoclastogenesis and promoting osteoblastogenesis. These results indicate that the effect of JAK inhibitors on osteoclastogenesis and osteoblastogenesis depends on the bone damage type and the affected bone area. In vitro studies suggest that while JAK inhibitors inhibit osteoclastic bone resorption, their effects on osteoclastogenesis in inflammatory environments vary depending on the cytokine milieu, JAK selectivity and cytokine signaling specificity. The findings reported here should contribute to the strategic use of antirheumatic drugs against structural damages in RA.

Extracellular aaRSs drive autoimmune and inflammatory responses in rheumatoid arthritis via the release of cytokines and PAD4

OBJECTIVES

Recent studies demonstrate that extracellular-released aminoacyl-tRNA synthetases (aaRSs) play unique roles in immune responses and diseases. This study aimed to understand the role of extracellular aaRSs in the pathogenesis of rheumatoid arthritis (RA).

METHODS

Primary macrophages and fibroblast-like synoviocytes were cultured with aaRSs. aaRS-induced cytokine production including IL-6 and TNF-α was detected by ELISA. Transcriptomic features of aaRS-stimulated macrophages were examined using RNA-sequencing. Serum and synovial fluid (SF) aaRS levels in patients with RA were assessed using ELISA. Peptidyl arginine deiminase (PAD) 4 release from macrophages stimulated with aaRSs was detected by ELISA. Citrullination of aaRSs by themselves was examined by immunoprecipitation and western blotting. Furthermore, aaRS inhibitory peptides were used for inhibition of arthritis in two mouse RA models, collagen-induced arthritis and collagen antibody-induced arthritis.

RESULTS

All 20 aaRSs functioned as alarmin; they induced pro-inflammatory cytokines through the CD14-MD2-TLR4 axis. Stimulation of macrophages with aaRSs displayed persistent innate inflammatory responses. Serum and SF levels of many aaRSs increased in patients with RA compared with control subjects. Furthermore, aaRSs released PAD4 from living macrophages, leading to their citrullination. We demonstrate that aaRS inhibitory peptides suppress cytokine production and PAD4 release by aaRSs and alleviate arthritic symptoms in a mouse RA model.

CONCLUSIONS

Our findings uncovered the significant role of aaRSs as a novel alarmin in RA pathogenesis, indicating that their blocking agents are potent antirheumatic drugs.

Identification of an intronic enhancer regulating RANKL expression in osteocytic cells

The bony skeleton is continuously renewed throughout adult life by the bone remodeling process, in which old or damaged bone is removed by osteoclasts via largely unknown mechanisms. Osteocytes regulate bone remodeling by producing the osteoclast differentiation factor RANKL (encoded by the TNFSF11 gene). However, the precise mechanisms underlying RANKL expression in osteocytes are still elusive. Here, we explored the epigenomic landscape of osteocytic cells and identified a hitherto-undescribed osteocytic cell-specific intronic enhancer in the TNFSF11 gene locus. Bioinformatics analyses showed that transcription factors involved in cell death and senescence act on this intronic enhancer region. Single-cell transcriptomic data analysis demonstrated that cell death signaling increased RANKL expression in osteocytic cells. Genetic deletion of the intronic enhancer led to a high-bone-mass phenotype with decreased levels of RANKL in osteocytic cells and osteoclastogenesis in the adult stage, while RANKL expression was not affected in osteoblasts or lymphocytes. These data suggest that osteocytes may utilize a specialized regulatory element to facilitate osteoclast formation at the bone surface to be resorbed by linking signals from cellular senescence/death and RANKL expression.

Identification of BST2 as a conjunctival epithelial stem/progenitor cell marker

The conjunctival epithelium consists of conjunctival epithelial cells and goblet cells derived from conjunctival epithelial stem/progenitor cells. However, the source of these cells is not well known because no specific markers for conjunctival epithelial stem/progenitor cells have been discovered. Therefore, to identify conjunctival epithelial stem/progenitor cell markers, we performed single-cell RNA sequencing of a conjunctival epithelial cell population derived from human-induced pluripotent stem cells (hiPSCs). The following conjunctival epithelial markers were identified: BST2, SLC2A3, AGR2, TMEM54, OLR1, and TRIM29. Notably, BST2 was strongly positive in the basal conjunctival epithelium, which is thought to be rich in stem/progenitor cells. Moreover, BST2 was able to sort conjunctival epithelial stem/progenitor cells from hiPSC-derived ocular surface epithelial cell populations. BST2-positive cells were highly proliferative and capable of successfully generating conjunctival epithelial sheets containing goblet cells. In conclusion, BST2 has been identified as a specific marker of conjunctival epithelial stem/progenitor cells.

Effect of T cells on bone

Bone and immune systems mutually influence each other by sharing a variety of regulatory molecules and the tissue microenvironment. The interdisciplinary research field "osteoimmunology" has illuminated the complex and dynamic interactions between the two systems in the maintenance of tissue homeostasis as well as in the development of immune and skeletal disorders. T cells play a central role in the immune response by secreting various immune factors and stimulating other immune cells and structural cells such as fibroblasts and epithelial cells, thereby contributing to pathogen elimination and pathogenesis of immune diseases. The finding on regulation of osteoclastic bone resorption by activated CD4⁺ T cells in rheumatoid arthritis was one of the driving forces for the development of osteoimmunology. With advances in research on helper T cell subsets and rare lymphoid cells such as γδ T cells in the immunology field, it is becoming clear that various types of T cells exert multiple effects on bone metabolism depending on immune context. Understanding the diverse effects of T cells on bone is essential for deciphering the osteoimmune regulatory network in various biological settings.

Spleen tyrosine kinase mediates the γδTCR signaling required for γδT cell commitment and γδT17 differentiation

The γδT cells that produce IL-17 (γδT17 cells) play a key role in various pathophysiologic processes in host defense and homeostasis. The development of γδT cells in the thymus requires γδT cell receptor (γδTCR) signaling mediated by the spleen tyrosine kinase (Syk) family proteins, Syk and Zap70. Here, we show a critical role of Syk in the early phase of γδT cell development using mice deficient for Syk specifically in lymphoid lineage cells (Syk-conditional knockout (cKO) mice). The development of γδT cells in the Syk-cKO mice was arrested at the precursor stage where the expression of Rag genes and αβT-lineage-associated genes were retained, indicating that Syk is required for γδT-cell lineage commitment. Loss of Syk in γδT cells weakened TCR signal-induced phosphorylation of Erk and Akt, which is mandatory for the thymic development of γδT17 cells. Syk-cKO mice exhibited a loss of γδT17 cells in the thymus as well as throughout the body, and thereby are protected from γδT17-dependent psoriasis-like skin inflammation. Collectively, our results indicate that Syk is a key player in the lineage commitment of γδT cells and the priming of γδT17 cell differentiation.

ETS1 governs pathological tissue-remodeling programs in disease-associated fibroblasts

Fibroblasts, the most abundant structural cells, exert homeostatic functions but also drive disease pathogenesis. Single-cell technologies have illuminated the shared characteristics of pathogenic fibroblasts in multiple diseases including autoimmune arthritis, cancer and inflammatory colitis. However, the molecular mechanisms underlying the disease-associated fibroblast phenotypes remain largely unclear. Here, we identify ETS1 as the key transcription factor governing the pathological tissue-remodeling programs in fibroblasts. In arthritis, ETS1 drives polarization toward tissue-destructive fibroblasts by orchestrating hitherto undescribed regulatory elements of the osteoclast differentiation factor receptor activator of nuclear factor-κB ligand (RANKL) as well as matrix metalloproteinases. Fibroblast-specific ETS1 deletion resulted in ameliorated bone and cartilage damage under arthritic conditions without affecting the inflammation level. Cross-tissue fibroblast single-cell data analyses and genetic loss-of-function experiments lent support to the notion that ETS1 defines the perturbation-specific fibroblasts shared among various disease settings. These findings provide a mechanistic basis for pathogenic fibroblast polarization and have important therapeutic implications.

Periosteal stem cells control growth plate stem cells during postnatal skeletal growth

The ontogeny and fate of stem cells have been extensively investigated by lineage-tracing approaches. At distinct anatomical sites, bone tissue harbors multiple types of skeletal stem cells, which may independently supply osteogenic cells in a site-specific manner. Periosteal stem cells (PSCs) and growth plate resting zone stem cells (RZSCs) critically contribute to intramembranous and endochondral bone formation, respectively. However, it remains unclear whether there is functional crosstalk between these two types of skeletal stem cells. Here we show PSCs are not only required for intramembranous bone formation, but also for the growth plate maintenance and prolonged longitudinal bone growth. Mice deficient in PSCs display progressive defects in intramembranous and endochondral bone formation, the latter of which is caused by a deficiency in PSC-derived Indian hedgehog (Ihh). PSC-specific deletion of Ihh impairs the maintenance of the RZSCs, leading to a severe defect in endochondral bone formation in postnatal life. Thus, crosstalk between periosteal and growth plate stem cells is essential for post-developmental skeletal growth.

Intramembranous and endochondral bone formation have been considered to be independent processes mediated by independent stem cells. Here the authors show that periosteal stem cells participate in both types of bone formation, supporting endochondral formation by producing Ihh.

Reduced dynamic loads due to hip dislocation induce acetabular cartilage degeneration by IL-6 and MMP3 via the STAT3/periostin/NF-κB axis

Developmental dysplasia of the hip (DDH) is characterized by anatomical abnormalities of the hip joint, ranging from mild acetabular dysplasia to hip subluxation and eventually dislocation. The mechanism underlying the cartilage degeneration of the hip joints exposed to reduced dynamic loads due to hip dislocation remains unknown. We established a rodent hip dislocation (disarticulation; DA) model of DDH (DA-DDH rats and mice) by swaddling. Expression levels of periostin (Postn) and catabolic factors, such as interleukin-6 (IL-6) and matrix metalloproteinase 3 (Mmp3), increased and those of chondrogenic markers decreased in the acetabular cartilage of the DA-DDH models. Postn induced IL-6 and Mmp3 expression in chondrocytes through integrin αVβ3, focal adhesion kinase, Src, and nuclear factor-κB (NF-κB) signaling. The microgravity environment created by a random positioning machine induced Postn expression in chondrocytes through signal transducer and activator of transcription 3 (STAT3) signaling. IL-6 stimulated Postn expression via STAT3 signaling. Furthermore, cartilage degeneration was suppressed in the acetabulum of Postn^−/− DA-DDH mice compared with that in the acetabulum of wild type DA-DDH mice. In summary, reduced dynamic loads due to hip dislocation induced acetabular cartilage degeneration via IL-6 and MMP3 through STAT3/periostin/NF-κB signaling in the rodent DA-DDH models.

Mechanisms of joint destruction in rheumatoid arthritis - immune cell-fibroblast-bone interactions

Rheumatoid arthritis (RA) is characterized by inflammation and destruction of bone and cartilage in affected joints. Autoimmune responses lead to increased osteoclastic bone resorption and impaired osteoblastic bone formation, the imbalance of which underlies bone loss in RA, which includes bone erosion, periarticular bone loss and systemic osteoporosis. The crucial role of osteoclasts in bone erosion has been demonstrated in basic studies as well as by the clinical efficacy of antibodies targeting RANKL, an important mediator of osteoclastogenesis. Synovial fibroblasts contribute to joint damage by stimulating both pro-inflammatory and tissue-destructive pathways. New technologies, such as single-cell RNA sequencing, have revealed the heterogeneity of synovial fibroblasts and of immune cells including T cells and macrophages. To understand the mechanisms of bone damage in RA, it is important to clarify how the immune system promotes the tissue-destructive properties of synovial fibroblasts and influences bone cells. The interaction between immune cells and fibroblasts underlies the imbalance between regulatory T cells and T helper 17 cells, which in turn exacerbates not only inflammation but also bone destruction, mainly by promoting RANKL expression on synovial fibroblasts. An improved understanding of the immune mechanisms underlying joint damage and the interplay between the immune system, synovial fibroblasts and bone will contribute to the identification of novel therapeutic targets in RA.

Long-term survival in non-human primates of stem cell-derived, MHC-unmatched corneal epithelial cell sheets

When corneal epithelial stem cells residing in the corneal limbus become dysfunctional, called a limbal stem cell deficiency (LSCD), corneal transparency is decreased, causing severe vision loss. Transplantation of corneal epithelial cell sheets (CEPS) derived from stem cells, including induced pluripotent stem cells, is a promising treatment for LSCD. However, the potential effect of human leukocyte antigen (HLA) concordance on CEPS transplantation has not been addressed. Here, we show that there is no difference in the immune response to CEPS between HLA-matched and -unmatched peripheral blood mononuclear cells in mixed lymphocyte reactions. CEPS transplantation in cynomolgus monkeys revealed that the immune response to major histocompatibility-unmatched CEPS was not strong and could be controlled by local steroid administration. Furthermore, programmed death ligand 1 was identified as an immunosuppressive molecule in CEPS under inflammatory conditions in vitro. Our results indicate that corneal epithelium has low immunogenicity and allogeneic CEPS transplantation requires mild immunosuppression.

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There is no difference in the immune response to CEPS owing to HLA conformity in MLR

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The immune response to MHC-unmatched CEPS is not strong after transplantation

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Local steroid administration could control the immune response to MHC-unmatched CEPS

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PD-L1 was identified as an immunosuppressive molecule in CEPS

The Transcription Factor Sox4 is Required for Thymic Tuft Cell Development

Medullary thymic epithelial cells (mTECs) help shape the thymic microenvironment for T cell development by expressing a variety of peripheral tissue-restricted antigens (TRAs). The self-tolerance of T cells is established by negative selection of autoreactive T cells that bind to TRAs. To increase the diversity of TRAs, a fraction of mTECs terminally differentiate into distinct subsets resembling atypical types of epithelial cells in specific peripheral tissues. As such, thymic tuft cells that express peripheral tuft cell genes have recently emerged. Here we show that the transcription factor Sox4 is highly expressed in mTECs and is essential for the development of thymic tuft cells. Mice lacking Sox4 specifically in TECs had a significantly reduced number of thymic tuft cells with no effect on the differentiation of other mTEC subsets, including Aire + and Ccl21a + mTECs. Furthermore, Sox4 expression was diminished in mice deficient in TEC-specific lymphotoxin β receptor (LTβR), indicating a role for the LTβR-Sox4 axis in the differentiation of thymic tuft cells. Given that Sox4 promotes differentiation of peripheral tuft cells, our findings suggest that mTECs employ the same transcriptional program as peripheral epithelial cells. This mechanism may explain how mTECs diversify peripheral antigen expression to project an immunological self within the thymic medulla.

Osteoimmunology as an intrinsic part of immunology

Osteoimmunology has emerged as a field linking immunology and bone biology, but it has yet to be recognized as belonging to mainstream immunology. However, the extent of the research fields immunology actually covers has been enormously widened, and it is now ready to include such an interdisciplinary subject. One of the most obvious examples of an interaction between the immune and bone systems is the pathogenesis of rheumatoid arthritis, where bone resorption is increased by the autoimmune response. Moreover, the regulation of the immune system by bone cells has been clearly demonstrated by the finding that osteoprogenitor cells contribute to hematopoietic stem cell maintenance as well as the suppression of hematopoietic malignancy. Thus, the bidirectional dialogue has been established and inevitably will lead to the union of bone and immunity. Here I summarize the history and concept of osteoimmunology, providing a perspective on the future of immunology.

Cytokine profile in patients with chronic non-bacterial osteomyelitis, juvenile idiopathic arthritis, and insulin-dependent diabetes mellitus

OBJECTIVES

Our study aimed to evaluate the cytokine levels in pediatric chronic non-bacterial osteomyelitis (CNO) patients and compare these with other immune-mediated diseases and healthy controls.

METHODS

In this prospective study, we included 42 children with CNO, 28 patients with non-systemic juvenile idiopathic arthritis (JIA), 17 children with insulin-dependent diabetes mellitus (IDDM), and 30 healthy age-matched controls. In each of the CNO patients and comparison groups, the levels of 14-3-3-η protein, S100A8/A9 protein, interleukin-4 (IL-4), interleukin-17 (IL-17), interleukin-18 (IL-18), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) were measured by ELISA assay.

RESULTS

All studied cytokines in the CNO patients were significantly higher than controls, and IDDM, 14-3-3-η protein, IL-18, IL-4, IL-17, IL-1β, and TNF-α were less than in JIA patients. In the discriminant analysis, ESR, 14-3-3 protein, S100A8/A9, IL-18, IL-4, and TNF-α can discriminate CNO from JIA, and 14-3-3 protein, S100A8/A9, IL-18, IL-17, IL-4, and TNF-α can distinguish CNO from other diseases and HC.

CONCLUSION

The increased level of pro-inflammatory cytokines confirms the role of monocyte-driven inflammation in CNO patients. Cytokines may prove valuable as biomarkers and potential therapeutic targets for CNO.

The fibroblast: An emerging key player in thymic T cell selection

Fibroblasts have recently attracted attention as a key stromal component that controls the immune responses in lymphoid tissues. The thymus has a unique microenvironment comprised of a variety of stromal cells, including fibroblasts and thymic epithelial cells (TECs), the latter of which is known to be important for T cell development because of their ability to express self‐antigens. Thymic fibroblasts contribute to thymus organogenesis during embryogenesis and form the capsule and medullary reticular network in the adult thymus. However, the immunological significance of thymic fibroblasts has thus far only been poorly elucidated. In this review, we will summarize the current views on the development and functions of thymic fibroblasts as revealed by new technologies such as multicolor flow cytometry and single cell–based transcriptome profiling. Furthermore, the recently discovered role of medullary fibroblasts in the establishment of T cell tolerance by producing a unique set of self‐antigens will be highlighted.

OPG Production Matters Where It Happened

Osteoprotegerin (OPG) is a circulating decoy receptor for RANKL, a multifunctional cytokine essential for the differentiation of tissue-specific cells in bone and immune systems such as osteoclasts, medullary thymic epithelial cells (mTECs), and intestinal microfold cells (M cells). However, it is unknown whether OPG functions only at the production site or circulates to other tissues acting in an endocrine fashion. Here we explore the cellular source of OPG by generating OPG-floxed mice and show that locally produced OPG, rather than circulating OPG, is crucial for bone and immune homeostasis. Deletion of OPG in osteoblastic cells leads to severe osteopenia without affecting serum OPG. Deletion of locally produced OPG increases mTEC and M cell numbers while retaining the normal serum OPG level. This study shows that OPG limits its functions within the tissue where it was produced, illuminating the importance of local regulation of the RANKL system.

Plasma cells promote osteoclastogenesis and periarticular bone loss in autoimmune arthritis

In rheumatoid arthritis (RA), osteoclastic bone resorption causes structural joint damage as well as periarticular and systemic bone loss. Periarticular bone loss is one of the earliest indices of RA, often preceding the onset of clinical symptoms via largely unknown mechanisms. Excessive osteoclastogenesis induced by receptor activator of NF-κB ligand (RANKL) expressed by synovial fibroblasts causes joint erosion, whereas the role of RANKL expressed by lymphocytes in various types of bone damage has yet to be elucidated. In the bone marrow of arthritic mice, we found an increase in the number of RANKL-expressing plasma cells, which displayed an ability to induce osteoclastogenesis in vitro. Genetic ablation of RANKL in B-lineage cells resulted in amelioration of periarticular bone loss, but not of articular erosion or systemic bone loss, in autoimmune arthritis. We also show conclusive evidence for the critical contribution of synovial fibroblast RANKL to joint erosion in collagen-induced arthritis on the arthritogenic DBA/1J background. This study highlights the importance of plasma-cell RANKL in periarticular bone loss in arthritis and provides mechanistic insight into the early manifestation of bone lesion induced by autoimmunity.

Expressions of conventional vitellogenin and vitellogenin-like A in worker brains are associated with a nursing task in a ponerine ant

In eusocial insect colonies, non-reproductive workers often perform different tasks. Tasks of an individual worker are shifted depending on various factors, e.g., age and colony demography. Although a vitellogenin (Vg) gene play regulatory roles in both reproductive and non-reproductive division of labours in a honeybee, it has been shown that the insect Vg underwent multiple gene duplications and sub-functionalisation, especially in apical ant lineages. The regulatory roles of duplicated Vgs were suggested to change evolutionarily among ants, whereas such roles in phylogenetically basal ants remain unclear. Here, we examined the expression patterns of conventional Vg (CVg), Vg-like A, Vg-like B and Vg-like C, as well as Vg receptor, during the task shift in an age-dependent manner and under experimental manipulation of colony demography in a primitive ant Diacamma sp. Expressions of CVg and Vg-like A in a brain were associated with a nursing task. It is suggested that associations of brain expressions of these Vgs with worker tasks were acquired in the basal ant lineage, and that such Vg functions could have sub-functionalised in the derived ant lineage.

Non-Epithelial Thymic Stromal Cells: Unsung Heroes in Thymus Organogenesis and T Cell Development

The stromal microenvironment in the thymus is essential for generating a functional T cell repertoire. Thymic epithelial cells (TECs) are numerically and phenotypically one of the most prominent stromal cell types in the thymus, and have been recognized as one of most unusual cell types in the body by virtue of their unique functions in the course of the positive and negative selection of developing T cells. In addition to TECs, there are other stromal cell types of mesenchymal origin, such as fibroblasts and endothelial cells. These mesenchymal stromal cells are not only components of the parenchymal and vascular architecture, but also have a pivotal role in controlling TEC development, although their functions have been less extensively explored than TECs. Here, we review both the historical studies on and recent advances in our understanding of the contribution of such non-TEC stromal cells to thymic organogenesis and T cell development. In particular, we highlight the recently discovered functional effect of thymic fibroblasts on T cell repertoire selection.

RANKL as the master regulator of osteoclast differentiation

RANKL, the essential cue for osteoclast differentiation, is the membrane-bound factor expressed by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes. In vivo evidence indicates that RANKL functions as the indispensable and irreplaceable in the program of osteoclast differentiation. The reason why RANKL plays a critical role in osteoclastogenesis is discussed from the viewpoint of the distinct signaling pathways mediated by co-stimulatory receptors and the key transcription factor NFATc1.

Stepwise cell fate decision pathways during osteoclastogenesis at single-cell resolution

Osteoclasts are the exclusive bone-resorbing cells, playing a central role in bone metabolism, as well as the bone damage that occurs under pathological conditions^1,2. In postnatal life, haematopoietic stem-cell-derived precursors give rise to osteoclasts in response to stimulation with macrophage colony-stimulating factor and receptor activator of nuclear factor-κB ligand, both of which are produced by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes^1-3. However, the precise mechanisms underlying cell fate specification during osteoclast differentiation remain unclear. Here, we report the transcriptional profiling of 7,228 murine cells undergoing in vitro osteoclastogenesis, describing the stepwise events that take place during the osteoclast fate decision process. Based on our single-cell transcriptomic dataset, we find that osteoclast precursor cells transiently express CD11c, and deletion of receptor activator of nuclear factor-κB specifically in CD11c-expressing cells inhibited osteoclast formation in vivo and in vitro. Furthermore, we identify Cbp/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 2 (Cited2) as the molecular switch triggering terminal differentiation of osteoclasts, and deletion of Cited2 in osteoclast precursors in vivo resulted in a failure to commit to osteoclast fate. Together, the results of this study provide a detailed molecular road map of the osteoclast differentiation process, refining and expanding our understanding of the molecular mechanisms underlying osteoclastogenesis.

Chd4 choreographs self-antigen expression for central immune tolerance

Autoreactive T cells are eliminated in the thymus to prevent autoimmunity by promiscuous expression of tissue-restricted self-antigens in medullary thymic epithelial cells. This expression is dependent on the transcription factor Fezf2, as well as the transcriptional regulator Aire, but the entire picture of the transcriptional program has been obscure. Here, we found that the chromatin remodeler Chd4, also called Mi-2β, plays a key role in the self-antigen expression in medullary thymic epithelial cells. To maximize the diversity of self-antigen expression, Fezf2 and Aire utilized completely distinct transcriptional mechanisms, both of which were under the control of Chd4. Chd4 organized the promoter regions of Fezf2-dependent genes, while contributing to the Aire-mediated induction of self-antigens via super-enhancers. Mice deficient in Chd4 specifically in thymic epithelial cells exhibited autoimmune phenotypes, including T cell infiltration. Thus, Chd4 plays a critical role in integrating Fezf2- and Aire-mediated gene induction to establish central immune tolerance.

Butyrophilin-like proteins display combinatorial diversity in selecting and maintaining signature intraepithelial γδ T cell compartments

Butyrophilin-like (Btnl) genes are emerging as major epithelial determinants of tissue-associated γδ T cell compartments. Thus, the development of signature, murine TCRγδ⁺ intraepithelial lymphocytes (IEL) in gut and skin depends on Btnl family members, Btnl1 and Skint1, respectively. In seeking mechanisms underlying these profound effects, we now show that normal gut and skin γδ IEL development additionally requires Btnl6 and Skint2, respectively, and furthermore that different Btnl heteromers can seemingly shape different intestinal γδ⁺ IEL repertoires. This formal genetic evidence for the importance of Btnl heteromers also applied to the steady-state, since sustained Btnl expression is required to maintain the signature TCR.Vγ7⁺ IEL phenotype, including specific responsiveness to Btnl proteins. In sum, Btnl proteins are required to select and to maintain the phenotypes of tissue-protective γδ IEL compartments, with combinatorially diverse heteromers having differential impacts on different IEL subsets.

Microbiota-derived butyrate limits the autoimmune response by promoting the differentiation of follicular regulatory T cells

Background

Rheumatoid arthritis (RA) is a chronic debilitating autoimmune disorder with a high prevalence, especially in industrialized countries. Dysbiosis of the intestinal microbiota has been observed in RA patients. For instance, new-onset untreated RA (NORA) is associated with the underrepresentation of the Clostridium cluster XIVa, including Lachnospiraceae, which are major butyrate producers, although the pathological relevance has remained obscure. Follicular regulatory T (T_FR) cells play critical regulatory roles in the pathogenesis of autoimmune diseases, including RA. Reduced number of circulating T_FR cells has been associated with the elevation of autoantibodies and disease severity in RA. However, the contribution of commensal microbe-derived butyrate in controlling T_FR cell differentiation remains unknown.

Methods

We examined the contribution of microbe-derived butyrate in controlling autoimmune arthritis using collagen-induced arthritis (CIA) and SKG arthritis models. We phenotyped autoimmune responses in the gut-associated lymphoid tissues (GALT) in the colon and joint-draining lymph nodes in the CIA model. We developed an in vitro CXCR5⁺Bcl-6⁺Foxp3⁺ T_FR (iT_FR) cell culture system and examined whether butyrate promotes the differentiation of iT_FR cells.

Findings

Microbe-derived butyrate suppressed the development of autoimmune arthritis. The immunization of type II collagen (CII) caused hypertrophy of the GALT in the colon by amplifying the GC reaction prior to the onset of the CIA. Butyrate mitigated these pathological events by promoting T_FR cell differentiation. Butyrate directly induced the differentiation of functional T_FR cells in vitro by enhancing histone acetylation in T_FR cell marker genes. This effect was attributed to histone deacetylase (HDAC) inhibition by butyrate, leading to histone hyperacetylation in the promoter region of the T_FR-cell marker genes. The adoptive transfer of the butyrate-treated iT_FR cells reduced CII-specific autoantibody production and thus ameliorated the symptoms of arthritis.

Interpretation

Accordingly, microbiota-derived butyrate serves as an environmental cue to enhance T_FR cells, which suppress autoantibody production in the systemic lymphoid tissue, eventually ameliorating RA. Our findings provide mechanistic insights into the link between the gut environment and RA risk.

Funding

This work was supported by 10.13039/100009619AMED-Crest (16gm1010004h0101, 17gm1010004h0102, 18gm1010004h0103, and 19gm1010004s0104 to KH), the Japan Society for the Promotion of Science (JP17KT0055, JP16H01369, and JP18H04680 to KH; JP17K15734 to DT), Keio University Special Grant-in-Aid for Innovative Collaborative Research Projects (KH), Keio Gijuku Fukuzawa Memorial Fund for the Advancement of Education and Research (DT), the SECOM Science and Technology Foundation (KH), the Cell Science Research Foundation (KH), the Mochida Memorial Foundation for Medical and Pharmaceutical Research (DT), the Suzuken Memorial Foundation (KH and DT), the Takeda Science Foundation (KH and DT), The Science Research Promotion Fund, and The Promotion and Mutual Aid Corporation for Private Schools of Japan (KH).

Multicentre descriptive cross-sectional study of Japanese home visit patients: reasons for encounter, health problems and multimorbidity

BACKGROUND

GP in Japan are encouraged to conduct home visits for older adults. However, most previous studies on home visits were based on secondary analyses of billing data that did not include reasons for the encounter.

OBJECTIVES

This study aimed to describe home visit care by GP in Japan, including reasons for encounter, health problems, episodes of care, comprehensiveness and multimorbidity.

METHODS

This multicentre descriptive cross-sectional study used the International Classification of Primary Care, second edition, and was conducted in Japan from 1 October 2016 to 31 March 2017. Participants were patients who received home visits from 10 enrolled GPs working in urban and rural areas across Japan. The main outcome measures were reasons for encounter, health problems and multimorbidity.

RESULTS

Of 253 potential patient participants, 250 were included in this analysis; 92.4% were aged 65 years and older. We registered 1,278 regular home visits and 110 emergency home visits. The top three reasons for encounters home visits were associated with cardiovascular and gastrointestinal disorders: prescriptions for cardiovascular diseases (n = 796), medical examination/health evaluation for cardiovascular diseases (n = 758) and prescriptions for gastrointestinal problems (n = 554). About 50% of patients had multimorbidity. Cardiovascular, endocrine and neuropsychological diseases were the most frequent problems in patients with multimorbidity.

CONCLUSIONS

The main reasons for encounter were prescriptions for chronic conditions. Emergency visits accounted for 8% of all visits. Around half of the patients had multimorbidity. This information may help GPs and policy makers to better assess home visit patients' needs.

Effects of continuous exposure to low concentration of ClO2 gas on the growth, viability, and maintenance of undifferentiated MSCs in long-term cultures

Introduction

Hygienic management is more important in the manufacturing of cell products than in the production of chemical agents, because cell material and final product cannot be decontaminated. On the other hand, especially in the selection of hygienic agent, the adverse effects on the cells must be considered as well as the decontamination effect. ClO₂ is a potent disinfectant, which is now expected as a safe and effective hygienic agent in the field of cell production. In this study, we investigated the effects of low dose ClO₂ gas in the atmosphere of CO₂ incubator on the characteristics of MSCs cultured in it.

Methods

First, we installed a ClO₂ generator to a CO₂ incubator for cell culture in which a constant level of ClO₂ can be maintained. After culturing human cord derived MSCs in the CO₂ incubator, the characteristics of cells were analyzed.

Results

Continuous exposure to 0.05 ppmv of ClO₂ gas did not affect cell proliferation until at least 8th passage. In the FACS analysis, antigens usually expressed on MSCs, CD105, CD90, CD44, CD73 and CD29, were positively observed, but differentiation markers, CD11b and CD34, were little expressed on the MSCs exposed to 0.05 ppmv or 0.1 ppmv of ClO₂ gas just as on the control cells. Also in the investigation for cell death, 0.05 ppmv and 0.1 ppmv of ClO₂ gas little affected the viability, apoptosis or necrosis of MSCs. Furthermore, we assessed senescence using SA-β-gal staining. Although the frequency of stained cells cultured in 0.1 ppmv of ClO₂ gas was significantly increased than that of not exposed cells, the stained cells in 0.05 ppmv were rare and their frequency was almost the same as that in control.

Conclusions

All these results indicate that, although excessive concentration of ClO₂ gas induces senescence but neither apoptosis nor cell differentiation, exposure to 0.05 ppmv of ClO₂ gas little affected the characteristics of MSCs. In this study we demonstrate that continuous exposure to appropriate dose of ClO₂ gas can be safely used as decontamination agent in cell processing facilities.

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Continuous exposure to low concentration of ClO₂ gas little affected to of MSCs.

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Higher concentration of ClO₂ gas induced senescence to MSCs.

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The most suitable concentration for the continuous of ClO₂ gas exposure during the culture of MSCs was identified.

Endoplasmic reticulum mediates mitochondrial transfer within the osteocyte dendritic network

Mitochondrial transfer plays a crucial role in the regulation of tissue homeostasis and resistance to cancer chemotherapy. Osteocytes have interconnecting dendritic networks and are a model to investigate its mechanism. We have demonstrated, in primary murine osteocytes with photoactivatable mitochondria (PhAM)^floxed and in MLO-Y4 cells, mitochondrial transfer in the dendritic networks visualized by high-resolution confocal imaging. Normal osteocytes transferred mitochondria to adjacent metabolically stressed osteocytes and restored their metabolic function. The coordinated movement and transfer of mitochondria within the dendritic network rely on contact between the endoplasmic reticulum (ER) and mitochondria. Mitofusin 2 (Mfn2), a GTPase that tethers ER to mitochondria, predominantly mediates the transfer. A decline in Mfn2 expression with age occurs concomitantly with both impaired mitochondrial distribution and transfer in the osteocyte dendritic network. These data show a previously unknown function of ER-mitochondrial contact in mediating mitochondrial transfer and provide a mechanism to explain the homeostasis of osteocytes.

Soluble RANKL is physiologically dispensable but accelerates tumour metastasis to bone

Receptor activator of NF-κB ligand (RANKL) is a multifunctional cytokine known to affect immune and skeletal systems, as well as oncogenesis and metastasis^1-4. RANKL is synthesized as a membrane-bound molecule, and cleaved into its soluble form by proteases^5-7. As the soluble form of RANKL does not contribute greatly to bone remodelling or ovariectomy-induced bone loss⁸, whether soluble RANKL has a role in pathological settings remains unclear. Here we show that soluble RANKL promotes the formation of tumour metastases in bone. Mice that selectively lack soluble RANKL (Tnfsf11^ΔS/ΔS)^5-7,9 have normal bone homoeostasis and develop a normal immune system but display markedly reduced numbers of bone metastases after intracardiac injection of RANK-expressing melanoma and breast cancer cells. Deletion of soluble RANKL does not affect osteoclast numbers in metastatic lesions or tumour metastasis to non-skeletal tissues. Therefore, soluble RANKL is dispensable for physiological regulation of bone and immune systems, but has a distinct and pivotal role in the promotion of bone metastases.

Mice lacking all of the Skint family genes

γδT cells develop in the thymus and play important roles in protection against infection and tumor development, but the mechanisms by which the thymic microenvironment supports γδT cell differentiation remain largely unclear. Skint1, a B7-related protein expressed in thymic epithelial cells, was shown to be essential for the development of mouse Vγ5Vδ1 γδT cells. The Skint family in mouse consists of 11 members, Skint1-11. Here we generated mutant mice lacking the entire genomic region that contains all of the Skint genes. These mice exhibited a marked reduction of Vγ5Vδ1 γδT cells in the thymus and skin, but surprisingly, had normal development of other γδT cell subsets and leukocytes including αβT, B and myeloid cells. This phenotype is essentially identical to that of Skint1-deficient mice. These results indicate that the Skint family exerts an exclusive function in regulating the development of Vγ5Vδ1 γδT cells and is dispensable for development of other leukocytes.

PRMT5-mediated arginine methylation controls the strength of signaling via γc-family cytokines required for T cell maintenance

Protein arginine methylation is a post-translational modification which is involved in a wide range of biological processes such as transcriptional control and pre-mRNA splicing. Various cytokines, including IL-2 and IL-7, play crucial roles in T cell development and activation. These cytokines utilize the common cytokine receptor γ-chain (γc, encoded by Il2rg) and its associated kinase JAK3 for signal transduction, but the regulatory mechanism underlying γc and JAK3 expression remains to be elucidated. Here we show that protein arginine methyltransferase 5 (PRMT5) has a pivotal role in the maintenance of invariant NKT (iNKT), CD4+T and CD8+T cells. T-cell specific deletion of the Prmt5 gene resulted in a severe loss of iNKT cells as well as a decreased number of CD4+T and CD8+T cells. PRMT5-mediated arginine methylation is essential for the expression of γc and JAK3 required for iNKT cell development and the proliferation and survival of peripheral T cells. PRMT5 stimulates the symmetric dimethylation of the Sm proteins that facilitates the splicing of the Il2rg and Jak3 premRNA in T cells. Taken together, our findings show that PRMT5-mediated arginine methylation is a crucial determinant of the strength of signaling via γc-family cytokines and T cell maintenance.

Stromal Interaction Molecule Deficiency in T Cells Promotes Spontaneous Follicular Helper T Cell Development and Causes Type 2 Immune Disorders

Appropriate T cell responses are controlled by strict balance between activatory and inhibitory pathways downstream of TCR. Although mice or humans with impaired TCR signaling develop autoimmunity, the precise molecular mechanisms linking reduced TCR signaling to autoimmunity are not fully understood. Engagement of TCR activates Ca²⁺ signaling mainly through store-operated Ca²⁺ entry activated by stromal interaction molecule (Stim) 1 and Stim2. Despite defective T cell activation, mice deficient in both Stim1 and Stim2 in T cells (conditional double knockout [cDKO]) developed lymphoproliferative disorders and skin inflammation with a concomitant increase in serum IgG1 and IgE levels. In cDKO mice, follicular helper T (Tfh) cells were dramatically increased in number, and they produced IL-4 spontaneously. These inflammatory symptoms were abolished by the deletion of IL-4 in cDKO mice. Tfh development and inflammatory symptoms in cDKO mice were abrogated by further deletion of NFAT2 in T cells. These findings suggest that Tfh cells spontaneously developed in the absence of Ca²⁺ signaling and caused unregulated type 2 responses.

Autoregulation of Osteocyte Sema3A Orchestrates Estrogen Action and Counteracts Bone Aging

Osteocyte survival is key to bone homeostasis and is perturbed in menopause and aging. However, it remains unknown how osteocyte-mediated maintenance of the skeleton is regulated by the osteoprotective factor semaphorin 3A (Sema3A), a secreted protein that is known to reduce bone resorption and enhance bone formation. Here, we show that estrogen induces osteocyte expression of Sema3A, which acts on its receptor on osteocytes to promote their survival and maintain bone homeostasis. Postnatal global and conditional deletion of Sema3a in osteoblastic cells resulted in a severe osteoporotic phenotype marked by fewer osteocytes. This phenotype was recapitulated by osteocyte-specific deficiency of either Sema3A or its receptor component neuropilin-1 (Nrp1). A stimulator of soluble guanylate cyclase-cGMP signaling mimicked Sema3A action and ameliorated bone loss after ovariectomy. We further show that serum levels of SEMA3A decreased with age or after menopause in humans. Thus, we provide a mechanistic insight into the estrogen action and a promising therapeutic approach to protect against bone-related aging.

Suppression of hematopoietic cell kinase ameliorates the bone destruction associated with inflammation

Objectives: To investigate the role of non-receptor tyrosine kinases (NRTKs) in inflammation-induced osteoclastogenesis.Methods: Microarray analyses of global mRNA expression during receptor activator of NF-κB ligand (RANKL) and RANKL plus tumor necrosis factor (TNF)-α-induced osteoclast differentiation were performed. The inhibitory effect on TNF-α-induced osteoclast differentiation of A-419259, a potent inhibitor of hematopoietic cell kinase (Hck), was examined. The in vivo therapeutic effect of A-419259 treatment on lipopolysaccharide (LPS)-induced inflammatory bone destruction was evaluated.Results: We confirmed that Hck expression was selectively increased among the NRTKs during the osteoclast differentiation induced by RANKL and TNF-α, but not by RANKL alone. RANKL and TNF-α-induced osteoclast differentiation and they were dose-dependently inhibited by A-419259 treatment through inhibition of the expression of key regulators of osteoclastogenesis, including Prdm1 and Nfatc1. Notably, LPS-induced inflammatory bone loss in murine calvarial bones was ameliorated by the administration of A-419259.Conclusions: Our results demonstrate that the administration of A-419259 is effective for the inhibition of osteoclast differentiation induced by TNF-α in the presence of RANKL. Therefore, an inhibitor of Hck may be useful as a potent anti-osteoclastogenic agent for the treatment of inflammatory bone destruction.

Osteoimmunology

Bone is a crucial element of the skeletal-locomotor system, but also functions as an immunological organ that harbors hematopoietic stem cells (HSCs) and immune progenitor cells. Additionally, the skeletal and immune systems share a number of regulatory molecules, including cytokines and signaling molecules. Osteoimmunology was created as an interdisciplinary field to explore the shared molecules and interactions between the skeletal and immune systems. In particular, the importance of an inseparable link between the two systems has been highlighted by studies on the pathogenesis of rheumatoid arthritis (RA), in which pathogenic helper T cells induce the progressive destruction of multiple joints through aberrant expression of receptor activator of nuclear factor (NF)-κB ligand (RANKL). The conceptual bridge of osteoimmunology provides not only a novel framework for understanding these biological systems but also a molecular basis for the development of therapeutic approaches for diseases of bone and/or the immune system.

Immune-bone interplay in the structural damage in rheumatoid arthritis

The immune and bone systems maintain homeostasis by interacting closely with each other. Rheumatoid arthritis is a pathological consequence of their interplay, as activated T cell immune responses result in osteoclast-mediated bone erosion. An imbalance between forkhead box protein 3 (Foxp3)⁺ regulatory T (T_reg ) cells and T helper type 17 (Th17) cells is often linked with autoimmune diseases, including arthritis. Th17 cells contribute to the bone destruction in arthritis by up-regulating receptor activator of nuclear factor kappa-Β ligand (RANKL) on synovial fibroblasts as well as inducing local inflammation. Studies on the origin of Th17 cells in inflammation have shed light on the pathogenic conversion of Foxp3⁺ T cells. Th17 cells converted from Foxp3⁺ T cells (exFoxp3 Th17 cells) comprise the most potent osteoclastogenic T cell subset in inflammatory bone loss. It has been suggested that osteoclastogenic T cells may have developed originally to stop local infection in periodontitis by inducing tooth loss. In addition, Th17 cells also contribute to the pathogenesis of arthritis by modulating antibody function. Antibodies and immune complexes have attracted considerable attention for their direct role in osteoclastogenesis, and a specific T cell subset in joints was shown to be involved in B cell antibody production. Here we summarize the recent advances in our understanding of the immune-bone interplay in the context of the bone destruction in arthritis.

Roles of Enhancer RNAs in RANKL-induced Osteoclast Differentiation Identified by Genome-wide Cap-analysis of Gene Expression using CRISPR/Cas9

Bidirectional transcription has been proposed to play a role associated with enhancer activity. Transcripts called enhancer RNAs (eRNAs) play important roles in gene regulation; however, their roles in osteoclasts are unknown. To analyse eRNAs in osteoclasts comprehensively, we used cap-analysis of gene expression (CAGE) to detect adjacent transcription start sites (TSSs) that were distant from promoters for protein-coding gene expression. When comparing bidirectional TSSs between osteoclast precursors and osteoclasts, we found that bidirectional TSSs were located in the 5′-flanking regions of the Nrp2 and Dcstamp genes. We also detected bidirectional TSSs in the intron region of the Nfatc1 gene. To investigate the role of bidirectional transcription in osteoclasts, we performed loss of function analyses using the CRISPR/Cas9 system. Targeted deletion of the DNA regions between the bidirectional TSSs led to decreased expression of the bidirectional transcripts, as well as the protein-coding RNAs of Nrp2, Dcstamp, and Nfatc1, suggesting that these transcripts act as eRNAs. Furthermore, osteoclast differentiation was impaired by targeted deletion of bidirectional eRNA regions. The combined results show that eRNAs play important roles in osteoclastogenic gene regulation, and may therefore provide novel insights to elucidate the transcriptional mechanisms that control osteoclast differentiation.

Overview of Osteoimmunology

Aberrant or prolonged immune responses often affect bone metabolism. The investigation on bone destruction observed in autoimmune arthritis contributed to the development of research area on effect of the immune system on bone. A number of reports on bone phenotypes of immunocompromised mice indicate that the immune and skeletal systems share various molecules, including transcription factors, signaling molecules, and membrane receptors, suggesting the interplay between the two systems. Furthermore, much attention has been paid to the modulation of immune cells, including hematopoietic progenitor cells, by bone cells in the bone marrow. Thus, osteoimmunology which deals with the crosstalk and shared mechanisms of the bone and immune systems became the conceptual framework fundamental to a proper understanding of both systems and the development of new therapeutic strategies.

Glucocorticoid impairs cell-cell communication by autophagy-mediated degradation of connexin 43 in osteocytes

Osteocytes comprising over 90% of the bone cell population are highly susceptible to the adverse effects of glucocorticoids (GC) administration. Here we observed that Dexamethasone (Dex) induces a robust cytoskeleton rearrangement and decreases Cx43 protein expression in osteocyte-like MLO-Y4 cells. Using a Dmp1Cre-mT/mG osteocyte ex vivo culture system, we found significant shortening of dendritic processes in primary osteocytes following Dex administration. Loss of dendritic processes is a consequence of reduced Cx43 connectivity upon Dex induced autophagy in both RFP-GFP-LC3B transfected MLO-Y4 cells and primary calvarial osteocytes from LC3GFP transgenic mice. Upon the induction of autophagy by Dex, Cx43 was internalized into autophagosome/autolysosomes and degraded by autophagy. The degradation was attenuated following lysosomal inhibition using chloroquine (CLQ) and suppression of autophagy by Atg5 silencing. Inhibition Akt-mTORC1 signaling by Dex induces autophagy subsequently resulting in Cx43 degradation.Activation of Akt phosphorylation by IGF-1 attenuated Dex induced autophagy and degradation of Cx43. Together, we demonstrated that GC impair osteocyte cell-cell connectivity via autophagy mediated degradation of Cx43 through inhibition of the Akt-mTORC1 signaling. This may account for the deleterious effect of GC-induced bone loss.

γδTCR recruits the Syk/PI3K axis to drive proinflammatory differentiation program

γδT cells produce inflammatory cytokines and have been implicated in the pathogenesis of cancer, infectious diseases, and autoimmunity. The T cell receptor (TCR) signal transduction that specifically regulates the development of IL-17-producing γδT (γδT17) cells largely remains unclear. Here, we showed that the receptor proximal tyrosine kinase Syk is essential for γδTCR signal transduction and development of γδT17 in the mouse thymus. Zap70, another tyrosine kinase essential for the development of αβT cells, failed to functionally substitute for Syk in the development of γδT17. Syk induced the activation of the PI3K/Akt pathway upon γδTCR stimulation. Mice deficient in PI3K signaling exhibited a complete loss of γδT17, without impaired development of IFN-γ-producing γδT cells. Moreover, γδT17-dependent skin inflammation was ameliorated in mice deficient in RhoH, an adaptor known to recruit Syk. Thus, we deciphered lineage-specific TCR signaling and identified the Syk/PI3K pathway as a critical determinant of proinflammatory γδT cell differentiation.

RANK rewires energy homeostasis in lung cancer cells and drives primary lung cancer

Rao et al. report that RANK, the key regulator of osteoclastogenesis, is frequently expressed in primary lung tumors, and clonal genetic inactivation of RANK in mouse lung epithelial cells markedly impairs the progression of KRas^G12D-driven lung cancer. RANK rewires energy homeostasis in human and murine lung cancer cells and promotes expansion of lung cancer stem-like cells.

Lung cancer is the leading cause of cancer deaths. Besides smoking, epidemiological studies have linked female sex hormones to lung cancer in women; however, the underlying mechanisms remain unclear. Here we report that the receptor activator of nuclear factor-kB (RANK), the key regulator of osteoclastogenesis, is frequently expressed in primary lung tumors, an active RANK pathway correlates with decreased survival, and pharmacologic RANK inhibition reduces tumor growth in patient-derived lung cancer xenografts. Clonal genetic inactivation of KRas^G12D in mouse lung epithelial cells markedly impairs the progression of KRas^G12D-driven lung cancer, resulting in a significant survival advantage. Mechanistically, RANK rewires energy homeostasis in human and murine lung cancer cells and promotes expansion of lung cancer stem-like cells, which is blocked by inhibiting mitochondrial respiration. Our data also indicate survival differences in KRas^G12D-driven lung cancer between male and female mice, and we show that female sex hormones can promote lung cancer progression via the RANK pathway. These data uncover a direct role for RANK in lung cancer and may explain why female sex hormones accelerate lung cancer development. Inhibition of RANK using the approved drug denosumab may be a therapeutic drug candidate for primary lung cancer.

The Mechanisms of T Cell Selection in the Thymus

T cells undergo positive and negative selection in the thymic cortex and medulla, respectively. A promiscuous expression of a wide array of self-antigens in the thymus is essential for the negative selection of self-reactive T cells and the establishment of central tolerance. Aire was originally thought to be the exclusive factor regulating the expression of tissue-restricted antigens, but Fezf2 recently emerged as a critical transcription factor in this regulatory activity. Fezf2 is selectively expressed in thymic medullary epithelial cells, regulates a large number of tissue-restricted antigens and suppresses the onset of autoimmune responses. Here, we discuss novel findings on the transcriptional mechanisms of tissue restricted-antigen expression in the medullary thymic epithelial cells and its effects on T cell selection.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.

Osteoimmunology in Bone Fracture Healing

PURPOSE OF REVIEW

In the process of bone fracture healing, inflammation is thought to be an essential process that precedes bone formation and remodeling. We review recent studies on bone fracture healing from an osteoimmunological point of view.

RECENT FINDINGS

Based on previous observations that many types of immune cells infiltrate into the bone injury site and release a variety of molecules, recent studies have addressed the roles of specific immune cell subsets. Macrophages and interleukin (IL)-17-producing γδ T cells enhance bone healing, whereas CD8⁺ T cells impair bone repair. Additionally, IL-10-producing B cells may contribute to bone healing by suppressing excessive and/or prolonged inflammation. Although the involvement of other cells and molecules has been suggested, the precise underlying mechanisms remain elusive. Accumulating evidence has begun to reveal the deeper picture of bone fracture healing. Further studies are required for the development of novel therapeutic strategies for bone fracture.

Human thymoproteasome variations influence CD8 T cell selection

The proteasome is a multi-subunit protease complex essential for housekeeping protein degradation and the production of the major histocompatibility complex (MHC) class I-bound antigen peptides that are essential for recognition by CD8 T cells. MHC variations dramatically contribute to T cell selection and autoimmunity, but genetic variations of peptide processing machinery including proteasome genes have been poorly explored in this context. In the computational analysis of human proteasome gene variation, we documented that PSMB11 was highly enriched for nucleotide changes that interfere with protein function. This gene encodes β5t, a thymus-specific catalytic subunit that regulates positive selection of CD8 T cells by producing a distinct set of MHC class I-bound peptides. The introduction of PSMB11 variations into the mouse genome by genome-editing revealed that these variations impaired the development of CD8 T cells in vivo. One of the PSMB11 polymorphisms altered the CD8 T cell repertoire in mice and was associated with a higher risk of an autoimmune disease in humans. Our findings suggest that, in addition to the MHC haplotype, proteasome variations influence T cell repertoire selection and may contribute to the difference in individual susceptibility to autoimmunity.

Intestinal homeostasis governed by mesenchymal stromal cells

The mammalian intestine harbors numerous bacteria and contains a large number of immune cells. A continuous dialogue between immune cells and luminal microbes is critical for the maintenance of the intestinal homeostasis. The intestinal epithelium is a single-layered cell sheet that separates numerous microbes and the immune system. The appropriate development and maintenance of the epithelium are essential for the clearance of pathogens and the prevention of excess immune responses against commensal microbes. Microfold (M) cells are epithelial cells specialized for bacterial sampling and initiate germinal center formation and IgA production in the gut associated lymphoid tissues (GALTs). Here, we show that previously unrecognized mesenchymal stromal cells regulate an epithelial function and induce IgA production in the GALTs. The impairment of these mesenchymal stromal cells caused a reduction in IgA production against commensal bacteria, resulting in the altered composition of the gut microbiota. These results suggest that these mesenchymal stromal cells contribute to the maintenance of the intestinal homeostasis.

LOX Fails to Substitute for RANKL in Osteoclastogenesis

Osteoclasts are the exclusive bone-resorbing cells that have a central role in bone homeostasis as well as bone destruction in cancer and autoimmune disease. Both mouse and human genetic studies have clearly proven that receptor activator of NF-κB ligand (RANKL; encoded by the Tnfsf11 gene) and its receptor RANK are essential for osteoclastogenesis. Although there have been several reports on RANKL-independent osteoclastogenesis, previous studies have never provided in vivo evidence showing RANKL can be substituted by other molecules using RANKL- or RANK-deficient genetic backgrounds. Thus, to date, there is no clear evidence of RANKL-independent osteoclastogenesis and no molecule has ever been proven capable of inducing osteoclast differentiation more efficiently than RANKL. Recently, lysyl oxidase (LOX), the enzyme that mediates collagen cross-linking, has been shown to induce human osteoclasts in the absence of RANKL and has a stronger osteoclastogenic activity than RANKL. Here, we investigated the effect of LOX on osteoclast differentiation using RANKL- and RANK-deficient cells to strictly explore RANKL-independent osteoclastogenesis. CD14⁺ human peripheral blood cells as well as osteoclast precursor cells derived from wild-type, RANKL- and RANK-deficient mice were treated with RANKL and/or LOX in short-term (3 days) or long-term (3 weeks) experimental settings. LOX treatment alone did not result in the formation of tartrate-resistant acid phosphatase (TRAP)⁺ cells or resorption pits in either short-term or long-term culture. In combination with RANKL, long-term treatment with LOX synergistically promoted osteoclastogenesis in cells derived from wild-type mice; however, this was abrogated in RANKL-deficient cells. Long-term treatment with LOX stimulated RANKL expression in mouse bone marrow stromal cells via the production of reactive oxygen species (ROS). Furthermore, LOX injection failed to rescue the phenotype of RANKL-deficient mice. These results suggest that LOX has the ability to induce RANKL expression on stromal cells; however, it fails to substitute for RANKL in osteoclastogenesis. © 2016 American Society for Bone and Mineral Research.

[Osteoimmunology-overview]

Bone serves not only as a locomotive organ but also as an organ instrumental in many other functions such as hematopoiesis. To this end, bone undergoes renewal by sequential resorption and formation. Bone resorption and formation are regulated by many systems including the endocrine, the nervous and the immune systems. The skeletal and the immune systems are closely related because they share many regulatory molecules such as cytokines and transcription factors. Studies on such shared molecules and inflammatory bone destruction in rheumatoid arthritis have fostered a novel interdisciplinary field, "osteoimmunology". Advances in osteoimmunology may lead to the novel therapeutic strategies in bone and immune diseases.