IL-34 and protein-tyrosine phosphatase receptor type-zeta-dependent mechanisms limit arthritis in mice

Ptprz Signaling, Tubule-Mediated and Macrophage-Mediated Kidney Injury, and Subsequent CKD

Background:

Macrophages and tubular epithelial cell interactions are integral in kidney ischemia-incited interstitial inflammation leading to acute kidney injury. Ischemia/reperfusion injury riggers tubular epithelial cells to express IL-34, a macrophage growth factor, that promotes acute kidney injury and subsequent chronic kidney disease. IL-34 engages the cognate receptor, c-FMS, expressed by macrophages, and the recently discovered Protein-Tyrosine Phosphatase ζ (Ptprz). Ptprz, binds to multiple ligands other than IL-34 that progressively increase their expression in kidneys after ischemia/reperfusion injury.

Methods:

We tested the hypothesis that signaling through Ptprz promotes macrophage-mediated acute kidney injury and subsequent chronic kidney disease, by comparing Ptprz knockout with wild-type mice after ischemia/reperfusion injury.

Results:

Ptprz was expressed by leukocytes and in tubular epithelial cells after ischemia/reperfusion injury in mice. Using Ptprz knockout mice we determined that during acute kidney injury and chronic kidney disease kidney pathology, and loss of kidney function were ameliorated. Ptprz-dependent mechanisms mediated: (i) tubular epithelial cell expression of chemokines that fostered macrophage and T cell rich renal inflammation, and (ii) tubule injury and apoptosis, that resulted in the loss of tubules and interstitial fibrosis during chronic kidney disease . Mechanistically, Ptprz dependent tubule epithelial cells released mediators that:(i) promoted tubule cytotoxicity, and thereby, shortened tubule survival, and (ii) stimulated Ptprz expressing macrophages to generate mediators that induce kidney destruction. These findings are translational, as after ischemia reperfusion injury in human kidney transplants, PTPRZ and PTPRZ ligands were upregulated and expressed by the same cell populations as in mice. Moreover, PTPRZ levels in sera were elevated in kidney transplant patients.

Conclusions:

Intra-renal Ptprz-dependent macrophage and tubular epithelial cell mediated mechanisms promote acute kidney injury and subsequent chronic kidney disease.

Bromodomain-Containing 4 Is a Positive Regulator of Interleukin-34 Production in the Gut

Experimental evidence suggests that, in the inflamed gut of inflammatory bowel disease (IBD) patients, interleukin-34 (IL-34) triggers detrimental signaling pathways. Factors/mechanisms regulating IL-34 production in IBD remain poorly characterized. Bromodomain-containing 4 (BRD4), a transcriptional and epigenetic regulator, is over-expressed in IBD, and studies in cancer cells suggest that BRD4 might positively control IL-34 expression. This study aimed to assess whether, in IBD, BRD4 regulates IL-34 expression. In IBD, there was an up-regulation of both IL-34 and BRD4 compared to the controls, and the two proteins co-localized in both lamina propria mononuclear cells (LPMCs) and epithelial cells. Flow cytometry analysis of CD45+ LPMCs confirmed that the percentages of IL-34- and BRD4-co-expressing cells were significantly higher in IBD than in the controls and showed that more than 80% of the IL-34-positive CD45-LPMCs expressed BRD4. IL-34 and BRD4 were mainly expressed by T cells and macrophages. IL-34 expression was reduced in IBD LPMCs transfected with BRD4 antisense oligonucleotide and in the colons of mice with dextran sulfate sodium-induced colitis treated with JQ1, a pharmacological inhibitor of BRD4. These data indicate that BRD4 is a positive regulator of IL-34 in IBD, further supporting the pathogenic role of BRD4 in IBD-associated mucosal inflammation.

Role of IL-34 and its receptors in inflammatory diseases

In recent years, IL-34 has been widely discussed as a novel cytokine. IL-34 is a pro-inflammatory cytokine binding four distinct receptors, namely CSF-1R, syndecan-1, PTP-ζ and TREM2. Previous studies have shown that IL-34 and its receptors play important roles in the development and progression of various inflammatory diseases. Therefore, IL-34 has the potential to be a biomarker and therapeutic target for inflammatory diseases. However, further study is still needed to identify the specific mechanism through which IL-34 contributes to illness. In this article, we review the recent advances in the biological roles of IL-34 and its receptors as well as their roles in the development and therapeutic application of inflammatory diseases.

Significance of IL-34 and SDC-1 in the pathogenesis of RA cells and preclinical models

IL-34 shares a common receptor with M-CSF, while it can bind to other distinct receptors including protein-tyrosine phosphatase zeta (PTPζ), and syndecan1 (SDC-1). In physiological conditions, IL-34 has a critical role in the maintenance and development of Langerhans and microglial cells in part through PTPζ ligation. Conversely, in autoimmune diseases such as rheumatoid arthritis (RA), SDC-1-induced phosphorylation of M-CSFR was responsible for the pathological effect of IL-34 in patient cells and/or preclinical models. Intriguingly, enrichment of IL-34 is strongly linked to rheumatoid factor (RF), disease activity score (DAS)28, erythrocyte sedimentation rate (ESR), c-reactive protein (CRP), and radiographic progression. In parallel, IL-34-induced naïve cell reprogramming into glycolytic RA CD14+CD86+GLUT1+ macrophage was dysregulated via M-CSFR or SDC-1 antibody therapy. Moreover, the inflammatory and erosive imprints of IL-34 arthritic mice were mitigated by glucose uptake inhibition and SDC-1, or RAG deficiency through nullifying macrophage metabolic rewiring and their ability to advance Th1/Th17 cell polarization. Consistently, IL-34-/- and SDC-1-/- mice could effectively impair CIA joint inflammation, osteoclast formation, and neovascularization by restraining monocyte infiltration as well as suppressing the inflammatory macrophage and T effector cell reconfiguration via metabolic deactivation. In conclusion, targeting IL-34/SDC-1 signaling, or its interconnected metabolites can uniquely intercept the crosstalk between glycolytic RA myeloid and lymphoid cells and their ability to trigger arthritis.

Interleukin-34-regulated T-cell responses in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease with a multifaceted etiology, which primarily affects and results in the deterioration of the synovium of patients. While the exact etiology of RA is still largely unknown, there is growing interest in the cytokine interleukin-34 (IL-34) as a driver or modulator of RA pathogenesis on the grounds that IL-34 is drastically increased in the serum and synovium of RA patients. Several studies have so far revealed the relationship between IL-34 levels and RA disease progression. Nevertheless, the significance and role of IL-34 in RA have remained ambiguous, as illustrated by two most recent studies, which reported contrasting effects of genetic IL-34 deletion in RA. Of note, IL-34 is a macrophage growth factor and is increasingly perceived as a master regulator of T-cell responses in RA via macrophage-dependent as well as T cell-intrinsic mechanisms. In this regard, several studies have demonstrated that IL-34 potentiates helper T-cell (Th) responses in RA, whereas studies also suggested that IL-34 alleviates synovial inflammation, potentially by inducing regulatory T-cells (Treg). Herein, we provide an overview of the current understanding of IL-34 involvement in RA and outline IL-34-mediated mechanisms in regulating T-cell responses in RA.