Interleukin-23 Regulates Inflammatory Osteoclastogenesis via Activation of CLEC5A(+) Osteoclast Precursors

What are mice teaching us about psoriatic arthritis?

PURPOSE OF REVIEW

This review summarizes important mouse models of psoriatic arthritis (PsA), shedding light on their advantages and disadvantages in modeling human disease.

RECENT FINDINGS

Two newly created mouse models of PsA validate NF-κB signaling as disease-causing and identify pathogenic roles for CD8 + and CD4 + FoxP3 + T cells in the development of specific PsA phenotypes. The IkbkbGoF/GoF model demonstrates that homozygosity for a gain-of-function mutation in Ikbkb results in expansion of FoxP3 + CD25 + IL-17A + Tregs that lead to the development of dactylitis, spondylitis and PsA-like changes to the nails and skin, and when transferred to wildtype mice, reproduce these outcomes. The humanized mouse PsA model (Hu-PsA) establishes that introduction of PsA patient sera and PBMCs into NSG-SGM3 mice has the capacity to elicit distinct subtypes of PsA and identifies a critical role for CD8 + IL-32 + CXCL14 + T cells and immunoglobulins in disease development.

SUMMARY

Mouse models of PsA are powerful research tools for elucidating pathogenesis of disease, biomarker identification and may assist in the discovery of a cure.

Epigenetic upregulation of CLEC5A contributes to monocyte/macrophage dysfunction in coronary artery disease

Inflammatory activation and dysfunction of immune cells are essential events in coronary artery disease (CAD) pathogenesis. C-type lectin domain family 5 member A (CLEC5A) has recently been regarded as a potent modulator of inflammation, while its contribution to CAD remains undefined. This study aims to clarify the involvement of CLEC5A in atherosclerosis and explore its epigenetic regulatory mechanisms. Integrated methylome and transcriptome analyses identified CLEC5A as a DNA methylation-driven gene in CAD. Functional studies revealed that DNMT1 overexpression suppresses CLEC5A expression in THP-1 cells, with subsequent identification of the cg06744540 CpG site as the critical regulatory locus. Clinical correlation analyses demonstrated that elevated CLEC5A expression is inversely associated with hypomethylation at cg06744540 in CAD patients. Furthermore, CLEC5A overexpression significantly enhanced monocyte inflammation, migration, and adhesion, promoted macrophage polarization and lipid accumulation, and inhibited apoptosis. Mechanistic investigations revealed that CLEC5A exacerbates inflammation in monocyte/macrophage by activating the NF-κB signaling pathway. Conversely, CLEC5A knockdown resulted in opposite effects. Notably, treatment of cells with folic acid, a methylation-enhancing factor, significantly increased DNMT1 expression and declined CLEC5A expression. Consistently, folic acid reversed high-fat-induced CLEC5A expression and inflammation and suppressed the formation of atherosclerotic plaques in vivo. In conclusion, the epigenetic upregulation of CLEC5A through DNMT1-mediated cg06744540 demethylation contributed to monocyte/macrophage dysfunction, thus accelerating CAD progression.

Nephrectomy Induces Severe Bone Loss in Mice Expressing Constitutively Active TGFβ Receptor Type I

Transforming growth factor beta (TGF-β), a master regulator of renal fibrosis, is the hallmark of chronic kidney disease (CKD) progression, and CKD worsens bone remodeling. However, the effects of the dysregulation of TGF-β signaling on bone remodeling during CKD have not been investigated. Here, we determined the effects of TGF-β receptor I (TβRI) overexpression under the control of Mx1-Cre on bone remodeling in CKD mice (Mx1;TβRICA-CKD mice). Our results demonstrated that kidney fibrosis and serum urea nitrogen levels were elevated in Mx1;TβRICA-CKD mice compared to WT-CKD, indicating that TβRI overexpression exacerbated renal injury during CKD. Serum calcium was decreased, while PTH was enhanced, in Mx1;TβRICA-CKD mice. Mx1;TβRICA-CKD mice displayed severe osteopenia as assessed by uCT in both femurs and mandibles. An histomorphometric analysis showed that tibial cancellous bone volume was decreased in Mx1;TβRICA-CKD. Likewise, mRNA expression levels of an osteoclastogenesis marker, Tnfsf11/Tnfrsf11b, was increased, and osteoblast marker genes Runx2 and Sp7 were decreased in Mx1;TβRICA-CKD mice. Mx1;TβRICA-CKD mice displayed increased inflammatory cytokines levels. Together, our results indicated that in the setting of CKD, TβRI overexpression induced both CKD progression and the dysregulation of bone remodeling, leading to severe bone loss. As such, these data provide an avenue for the future development of therapeutics for CKD-induced osteoporosis.

Novel insights and recent progress in osteoimmunology

Osteoimmunology is an interdisciplinary branch of immunology studying bidirectional interactions between the immune and skeletal systems. Bone marrow is vital for the production of immune cells and is implicated in multiple diseases across all immunology disciplines. Here, we briefly discuss recent progress from the past 5 years in the field and how it impacts our current understanding of health and disease.

Pathogenesis of psoriatic arthritis: new insights from a bone marrow perspective

PURPOSE OF REVIEW

Psoriatic arthritis is an immune-mediated disease that primarily affects the skin and joints. It falls under the umbrella term of rheumatic diseases, which describes a group of closely related yet distinct disorders with many common underlying molecular pathways. Despite the distinct clinical manifestation of each disorder, the shared therapeutic strategies attest to the commonality of cellular and molecular underpinnings. Herein we provide a concise yet comprehensive overview of the interleukin (IL)-23/IL-17 axis and its involvement in mechanistic pathways leading to the pathogenesis of this dual skin and joint clinical manifestation which is characteristic of psoriatic arthritis and other rheumatic diseases.

RECENT FINDINGS

The interconnection between activated innate immune cells and adaptive immunity has transformed current thinking to include other organs such as the bone marrow as potential tissue of disease origin. A plethora of animal models and genetic studies converge on the critical role of IL-23/IL-17 axis, and highlight the importance of myeloid cell activation as common pathways between autoinflammatory and autoimmune diseases and chronic inflammation. These findings underscore the intricate immune mechanisms involved in inflammatory arthritis and highlight molecular mechanisms in disease pathogenesis.

SUMMARY

These insights pave the way for the development of novel diagnostic and therapeutic strategies, with a focus on translating these findings into improved clinical practice.

Dectin-2 depletion alleviates osteoclast-induced bone loss in periodontitis via Syk/NOX2/ROS signaling

Periodontitis is the sixth most common disease worldwide and is closely associated with various systemic diseases, impacting overall health. It is characterized by the over-differentiation and activity of osteoclasts, leading to increased bone resorption and subsequent bone loss. Current treatments for bone loss are not ideal, highlighting the need for new targeted therapeutic strategies. Dectin-2, a member of the C-type lectin receptor (CLR) family, has recently been reported to play an important role in immune regulation, but its role in osteoclastogenesis has not been documented. This study identified a significant upregulation of Dectin-2 expression during osteoclast differentiation through single-cell sequencing and transcriptomic analysis. Knocking down Dectin-2 significantly inhibits the differentiation of RAW264.7 cells and bone marrow-derived macrophages (BMDMs) into osteoclasts, while overexpressing Dectin-2 enhances osteoclast differentiation and function. Mechanistically, transcriptomic analysis indicates that Dectin-2 deficiency disrupts redox homeostasis and affects the MAPK signaling pathway. Furthermore, the study demonstrates that Dectin-2 promotes osteoclastogenesis via the Syk/NOX2/ROS/MAPK signaling axis. In vivo, Dectin-2 knockout mice show reduced osteoclast numbers and decreased alveolar bone resorption in a periodontitis model. In conclusion, these findings suggest that Dectin-2 is a key regulatory factor in osteoclast-mediated bone resorption and may serve as a promising therapeutic target for bone diseases characterized by osteoclast overactivity, such as periodontitis.

Innate immune training of osteoclastogenesis promotes inflammatory bone loss in mice

We previously demonstrated that long-term trained immunity (TRIM) involves adaptations that imprint innate immune memory in long-lived myelopoiesis precursors and their progeny, monocytes/macrophages and neutrophils, which thereby acquire enhanced responsiveness to future challenges. Here, we show that a distinct component of myeloid biology, osteoclastogenesis, can also undergo innate immune training. Indeed, β-glucan-induced TRIM was associated with an increased osteoclastogenesis bias in the bone marrow and an expansion of monocytes/osteoclast progenitors in the periphery, resulting in aggravated severity of experimental periodontitis and arthritis. In the setting of trained inflammatory osteoclastogenesis, we observed transcriptomic rewiring in synovial myeloid cells of arthritic mice, featuring prominent upregulation of the transcription factor melanogenesis-associated transcription factor (MITF). Adoptive transfer of splenic monocytes from β-glucan-trained mice to naive recipients exacerbated arthritis in the latter in a strictly MITF-dependent manner. Our findings establish trained osteoclastogenesis as a maladaptive component of TRIM and potentially provide therapeutic targets in inflammatory bone loss disorders.

IL-23 regulation of myeloid cell biology during inflammation

Interleukin (IL)-23 is implicated in the pathogenesis of several inflammatory diseases and is usually linked with helper T cell (Th17) biology. However, there is some data linking IL-23 with innate immune biology in such diseases. We therefore examined the effects of IL-23p19 genetic deletion and/or neutralization on in vitro macrophage activation and in an innate immune-driven peritonitis model. We report that endogenous IL-23 was required for maximal macrophage activation by zymosan as determined by pro-inflammatory cytokine production, including a dramatic upregulation of granulocyte-colony stimulating factor (G-CSF). Furthermore, both IL-23p19 genetic deletion and neutralization in zymosan-induced peritonitis (ZIP) led to a specific reduction in the neutrophil numbers, as well as a reduction in the G-CSF levels in exudate fluids. We conclude that endogenous IL-23 can contribute significantly to macrophage activation during an inflammatory response, mostly likely via an autocrine/paracrine mechanism; of note, endogenous IL-23 can directly up-regulate macrophage G-CSF expression, which in turn is likely to contribute to the regulation of IL-23-dependent neutrophil number and function during an inflammatory response, with potential significance for IL-23 targeting particularly in neutrophil-associated inflammatory diseases.

IL-23 induces CLEC5A+ IL-17A+ neutrophils and elicit skin inflammation associated with psoriatic arthritis

IL-23-activation of IL-17 producing T cells is involved in many rheumatic diseases. Herein, we investigate the role of IL-23 in the activation of myeloid cell subsets that contribute to skin inflammation in mice and man. IL-23 gene transfer in WT, IL-23RGFP reporter mice and subsequent analysis with spectral cytometry show that IL-23 regulates early innate immune events by inducing the expansion of a myeloid MDL1+CD11b+Ly6G+ population that dictates epidermal hyperplasia, acanthosis, and parakeratosis; hallmark pathologic features of psoriasis. Genetic ablation of MDL-1, a major PU.1 transcriptional target during myeloid differentiation exclusively expressed in myeloid cells, completely prevents IL-23-pathology. Moreover, we show that IL-23-induced myeloid subsets are also capable of producing IL-17A and IL-23R+MDL1+ cells are present in the involved skin of psoriasis patients and gene expression correlations between IL-23 and MDL-1 have been validated in multiple patient cohorts. Collectively, our data demonstrate a novel role of IL-23 in MDL-1-myelopoiesis that is responsible for skin inflammation and related pathologies. Our data open a new avenue of investigations regarding the role of IL-23 in the activation of myeloid immunoreceptors and their role in autoimmunity.

Innate immune memory in inflammatory arthritis

The concept of immunological memory was demonstrated in antiquity when protection against re-exposure to pathogens was observed during the plague of Athens. Immunological memory has been linked with the adaptive features of T and B cells; however, in the past decade, evidence has demonstrated that innate immune cells can exhibit memory, a phenomenon called 'innate immune memory' or 'trained immunity'. Innate immune memory is currently being defined and is transforming our understanding of chronic inflammation and autoimmunity. In this Review, we provide an up-to-date overview of the memory-like features of innate immune cells in inflammatory arthritis and the crosstalk between chronic inflammatory milieu and cell reprogramming. Aberrant pro-inflammatory signalling, including cytokines, regulates the metabolic and epigenetic reprogramming of haematopoietic progenitors, leading to exacerbated inflammatory responses and osteoclast differentiation, in turn leading to bone destruction. Moreover, imprinted memory on mature cells including terminally differentiated osteoclasts alters responsiveness to therapies and modifies disease outcomes, commonly manifested by persistent inflammatory flares and relapse following medication withdrawal.

IL-27 attenuates IL-23 mediated inflammatory arthritis

Interleukin 27 has both pro-inflammatory and anti-inflammatory properties in autoimmunity. The anti-inflammatory effects of IL-27 are linked with inhibition of Th17 differentiation but the IL-27 effect on myeloid cells is less studied. Herein we demonstrate that IL-27 inhibits IL-23-induced inflammation associated not only with Th17 cells but also with myeloid cell infiltration in the joints and splenic myeloid populations of CD11b+ GR1+ and CD3-CD11b+CD11c-GR1- cells. The IL-27 anti-inflammatory response was associated with reduced levels of myeloid cells in the spleen and bone marrow. Overall, our data demonstrate that IL-27 has an immunosuppressive role that affects IL-23-dependent myelopoiesis in the bone marrow and its progression to inflammatory arthritis and plays a crucial role in controlling IL-23 driven joint inflammation by negatively regulating the expansion of myeloid cell subsets.