Syndecan-1 regulates the biological activities of interleukin-34

Cytokines in the pathogenesis of rheumatoid arthritis: new players and therapeutic targets

In recent years, the landscape of pro- and anti-inflammatory cytokines has rapidly expanded with the identification of new members proven to be involved at different extent in the pathogenesis of chronic immune mediated inflammatory diseases including rheumatoid arthritis (RA). The advance of our understanding of mediators involved in the pathogenesis of RA and in consequence, the development of novel targeted therapies is necessary to provide patients not responding to currently available strategies with novel compounds. The aim of this review article is to provide an overview on recently identified cytokines, emphasizing their pathogenic role and therapeutic potential in RA. A systematic literature review was performed to retrieve articles related to every cytokine discussed in the review. In some cases, evidence from animal models and RA patients is already consistent to move forward into drug development. In others, conflicting observation and the paucity of data require further investigations.Forty years after the discovery of IL-1, the landscape of cytokines is continuously expanding with increasing possibilities to develop novel therapeutic strategies in RA.

Interleukin 34, from pathogenesis to clinical applications

Interleukin-34 (IL-34) is a hematopoietic cytokine that was described for the first time in 2008 as a second ligand of CSF1R in addition to M-CSF. IL-34 and M-CSF share no sequence homology, but have similar functions, affecting the biology of myeloid cell lineage. In contrast to M-CSF, IL-34 shows unique signaling and expression patterns. Physiologically, IL-34 expression is restricted to epidermis and CNS, acting as a regulator of Langerhans cells and microglia, respectively. However, IL-34 expression can be induced and regulated by NF-κB under pathological conditions. Importantly, growing evidence indicates a correlation between IL-34 and disease severity, chronicity and progression. In addition to its promising roles as a novel diagnostic and prognostic biomarker of disease, IL-34 may also serve as a powerful target for therapeutic intervention. Here, we review the current knowledge regarding the emerging roles of IL-34 in disease, and focus on the clinical applications of IL-34 in medicine.

Immunoregulatory properties of the cytokine IL-34

Interleukin-34 is a cytokine with only partially understood functions, described for the first time in 2008. Although IL-34 shares very little homology with CSF-1 (CSF1, M-CSF), they share a common receptor CSF-1R (CSF-1R) and IL-34 has also two distinct receptors (PTP-ζ) and CD138 (syndecan-1). To make the situation more complex, IL-34 has also been shown as pairing with CSF-1 to form a heterodimer. Until now, studies have demonstrated that this cytokine is released by some tissues that differ to those where CSF-1 is expressed and is involved in the differentiation and survival of macrophages, monocytes, and dendritic cells in response to inflammation. The involvement of IL-34 has been shown in areas as diverse as neuronal protection, autoimmune diseases, infection, cancer, and transplantation. Our recent work has demonstrated a new and possible therapeutic role for IL-34 as a Foxp3+ Treg-secreted cytokine mediator of transplant tolerance. In this review, we recapitulate most recent findings on IL-34 and its controversial effects on immune responses and address its immunoregulatory properties and the potential of targeting this cytokine in human.

From top to bottom: midkine and pleiotrophin as emerging players in immune regulation

Cytokines are pivotal in the generation and resolution of the inflammatory response. The midkine/pleiotrophin (MK/PTN) family of cytokines, composed of just two members, was discovered as heparin-binding neurite outgrowth-promoting factors. Since their discovery, expression of this cytokine family has been reported in a wide array of inflammatory diseases and cancer. In this minireview, we will discuss the emerging appreciation of the functions of the MK/PTN family in the immune system, which include promoting lymphocyte survival, sculpting myeloid cell phenotype, driving immune cell chemotaxis, and maintaining hematopoiesis.

Serum Interleukin-34 Levels Are Elevated in Patients with Systemic Lupus Erythematosus

Interleukin-34 (IL-34) was initially identified as an alternative ligand for the colony-stimulating factor-1 receptor (CSF-1R) to mediate the biology of mononuclear phagocytic cells. Recently, IL-34 was found to be associated with chronic inflammation, such as in rheumatoid arthritis (RA). Both RA and systemic lupus erythematosus (SLE) are multifactorial autoimmune diseases and are characterized by excessive immune and inflammatory responses. Thus, we investigated whether IL-34 is involved in the pathogenesis of SLE. In all, 78 SLE patients and 53 healthy controls were enrolled in the research. Enzyme-linked immunosorbent assay (ELISA) was employed to measure the concentrations of serological IL-34. Then serum IL-34 levels between the SLE group and healthy controls were analyzed by the Mann-Whitney U test. Meanwhile, the correlations between the serum IL-34 levels and disease activity indexes and other established serum markers were assessed. Furthermore, the serum IL-34 levels of 20 active SLE patients were reevaluated when diseases were in the remission stage from corticosteroids or immunosuppressive drugs. Serum IL-34 levels were significantly higher in SLE patients compared to healthy controls. Their levels were remarkably associated with accumulation of the clinical features of SLE. Additionally, IL-34 titers were positively correlated with the SLE disease activity indexes, anti-double-stranded DNA antibody (anti-dsDNA) titers and C-reactive protein (CRP) levels, and inversely with complement3 (C3) levels. Moreover, serum IL-34 levels were significantly decreased after successful treatment of SLE. Serum IL-34 could be a candidate biomarker for SLE as there are elevated serum levels in treatment-naive SLE patients and we saw a significant decrease after effective treatment.

Regulation of Embryonic and Postnatal Development by the CSF-1 Receptor

Macrophages are found in all tissues and regulate tissue morphogenesis during development through trophic and scavenger functions. The colony stimulating factor-1 (CSF-1) receptor (CSF-1R) is the major regulator of tissue macrophage development and maintenance. In combination with receptor activator of nuclear factor κB (RANK), the CSF-1R also regulates the differentiation of the bone-resorbing osteoclast and controls bone remodeling during embryonic and early postnatal development. CSF-1R-regulated macrophages play trophic and remodeling roles in development. Outside the mononuclear phagocytic system, the CSF-1R directly regulates neuronal survival and differentiation, the development of intestinal Paneth cells and of preimplantation embryos, as well as trophoblast innate immune function. Consistent with the pleiotropic roles of the receptor during development, CSF-1R deficiency in most mouse strains causes embryonic or perinatal death and the surviving mice exhibit multiple developmental and functional deficits. The CSF-1R is activated by two dimeric glycoprotein ligands, CSF-1, and interleukin-34 (IL-34). Homozygous Csf1-null mutations phenocopy most of the deficits of Csf1r-null mice. In contrast, Il34-null mice have no gross phenotype, except for decreased numbers of Langerhans cells and microglia, indicating that CSF-1 plays the major developmental role. Homozygous inactivating mutations of the Csf1r or its ligands have not been reported in man. However, heterozygous inactivating mutations in the Csf1r lead to a dominantly inherited adult-onset progressive dementia, highlighting the importance of CSF-1R signaling in the brain.

Receptor-Type Protein-Tyrosine Phosphatase ζ and Colony Stimulating Factor-1 Receptor in the Intestine: Cellular Expression and Cytokine- and Chemokine Responses by Interleukin-34 and Colony Stimulating Factor-1

Differential intestinal expression of the macrophage growth factors colony stimulating factor-1 (CSF-1), interleukin (IL)-34, and their shared CSF-1 receptor (CSF-1R) in inflammatory bowel disease (IBD) has been shown. Diverse expression between CSF-1 and IL-34, suggest that IL-34 may signal via an alternate receptor. Receptor-type protein-tyrosine phosphatase ζ (PTPRZ1, RPTP-ζ), an additional IL-34 receptor, was recently identified. Here, we aimed to assess PTPRZ1 expression in IBD and non-IBD intestinal biopsies. Further, we aimed to investigate cellular PTPRZ1 and CSF-1R expression, and cytokine- and chemokine responses by IL-34 and CSF-1. The expression of PTPRZ1 was higher in non-IBD colon compared to ileum. PTPRZ1 expression was not altered with inflammation in IBD, however, correlated to IL34, CSF1, and CSF1R. The expression patterns of PTPRZ1 and CSF-1R differed in peripheral blood mononuclear cells (PBMCs), monocytes, macrophages, and intestinal epithelial cell line. PBMCs and monocytes of the same donors responded differently to IL-34 and CSF-1 with altered expression of tumor-necrosis factor α (TNF-α), IL-1β, interferon γ (IFN-γ), IL-13, IL-8, and monocyte chemotactic protein-1 (MCP-1) levels. This study shows that PTPRZ1 was expressed in bowel tissue. Furthermore, CSF-1R protein was detected in an intestinal epithelial cell line and donor dependently in primary PBMCs, monocytes, and macrophages, and first hints also suggest an expression in these cells for PTPRZ1, which may mediate IL-34 and CSF-1 actions.

Functions of interleukin-34 and its emerging association with rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic, synovial inflammation affecting multiple joints, finally leading to extra-articular lesions for which limited effective treatment options are currently available. Interleukin-34 (IL-34), recently discovered as the second colony-stimulating factor-1 receptor (CSF-1R) ligand, is a newly discovered cytokine. Accumulating evidence has disclosed crucial roles of IL-34 in the proliferation and differentiation of mononuclear phagocyte lineage cells, osteoclastogenesis and inflammation. Recently, IL-34 was detected at high levels in patients with active RA and in experimental models of inflammatory arthritis. Blockade of functional IL-34 with a specific monoclonal antibody can reduce the severity of inflammatory arthritis, suggesting that targeting IL-34 or its receptors may constitute a novel therapeutic strategy for autoimmune diseases such as RA. Here, we have comprehensively discussed the structure and biological functions of IL-34, and reviewed recent advances in our understanding of the emerging role of IL-34 in the development of RA as well as its potential utility as a therapeutic target.

Interleukin 34 Upregulation Contributes to the Increment of MicroRNA 21 Expression through STAT3 Activation Associated with Disease Activity in Rheumatoid Arthritis

OBJECTIVE

Interleukin 34 (IL-34) and microRNA 21 (miR-21) were found to be involved in the pathological process of rheumatoid arthritis (RA), but the details were unclear. In this study, we aimed to clarify the relationship between IL-34 and miR-21 in RA.

METHODS

IL-34 concentrations in serum and synovial fluid (SF) of patients with RA were measured by ELISA. Fibroblast-like synovial cells (FLS) were cultured for evaluation of STAT3 activation, miR-21, and Bax/Bcl-2 expression by Western blot and real-time PCR. Correlations were analyzed between clinical features and detectable variables including SF IL-34 levels and miR-21 expression.

RESULTS

SF IL-34 levels were significantly higher in patients with RA who had a high 28-joint Disease Activity Score (DAS28 ≥ 3.2) than in those with a lower DAS28 (DAS28 < 3.2). DAS28 scores and miR-21 expression in FLS had a significant positive correlation with the SF IL-34 levels. In addition, IL-34 stimulation strengthened the activation of p-STAT3, resulting in the increment of miR-21 expression. Inhibiting of miR-21 expression contributed to decreased Bcl-2/Bax ratio, suggesting that miR-21 was involved in the resistance to apoptosis. With the blocking of the colony-stimulating factor-1 receptor (CSF1R), decreased protein expressions including CSF1R, p-STAT3/STAT3, and Bcl-2/Bax were shown, suggesting that CSF1R participated in the biological functions of IL-34 in RA.

CONCLUSION

The IL-34/STAT3/miR-21 pathway is crucial for the survival of synovial fibroblasts in RA, which might be candidate therapeutic targets for RA treatment.

Chronicity following ischaemia-reperfusion injury depends on tubular-macrophage crosstalk involving two tubular cell-derived CSF-1R activators: CSF-1 and IL-34

Two structurally unrelated ligands activate the macrophage colony stimulating factor receptor (CSF-1R, c-fms, CD115): M-CSF/CSF-1 and interleukin-34 (IL-34). Both ligands promote macrophage proliferation, survival and differentiation. IL-34 also activates the protein-tyrosine phosphatase ζ receptor (PTP-ζ, PTPRZ1). Both receptors and cytokines are increased during acute kidney injury. While tubular cell-derived CSF-1 is required for kidney repair, Baek et al (J Clin Invest 2015; 125: 3198-3214) have now identified tubular epithelial cell-derived IL-34 as a promoter of kidney neutrophil and macrophage infiltration and tubular cell destruction during experimental kidney ischaemia-reperfusion, leading to chronic injury. IL-34 promoted proliferation of both intrarenal macrophages and bone marrow cells, increasing circulating neutrophils and monocytes and their kidney recruitment. Thus, injured tubular cells release two CSF-1R activators, one (CSF-1) that promotes tubular cell survival and kidney repair and another (IL-34) that promotes chronic kidney damage. These results hold promise for the development of IL-34-targeting strategies to prevent ischaemia-reperfusion kidney injury in contexts such as kidney transplantation. However, careful consideration should be given to the recent characterization by Bezie et al. (J Clin Invest 2015; 125: 3952-3964) of IL-34 as a T regulatory cell (Treg) cytokine that modulates macrophage responses so that IL-34-primed macrophages potentiate the immune suppressive capacity of Tregs and promote graft tolerance.