Immunomodulation of Interleukin-34 and its Potential Significance as a Disease Biomarker and Therapeutic Target

Abstract

Interleukin (IL)-34 is a cytokine discovered a few years ago and identified as the second colony-stimulating factor (CSF)-1 receptor (CSF-1R) ligand. Although CSF-1 and IL-34 share the same receptor through which they trigger similar effects, IL-34 also binds to receptors protein-tyrosine phosphatase (PTP)-ζ and syndecan-1. Thus, IL-34 is involved in several signaling pathways and participates in a wide array of biological actions. This review analyzes current studies on the role of IL-34 under physiological and pathological conditions, and explores its potential significance as a disease biomarker and therapeutic target. In physiological conditions, IL-34 expression is restricted to the microglia and Langerhans cells, with a fundamental role in cellular differentiation, adhesion and migration, proliferation, metabolism, and survival. It is released in response to inflammatory stimuli, such as pathogen-associated molecular patterns or pro-inflammatory cytokines, with effects over various immune cells, including monocytes, macrophages, and regulatory T cells that shape the immune microenvironment. Over the past decade, accumulating evidence has suggested a potent immune regulation of IL-34 in pathological states such as autoimmune diseases, cancer, transplant rejection, neurologic diseases, infections, and inflammatory diseases. Importantly, IL-34 may hold great promise for acting as a biomarker for monitoring disease severity and progression, and may serve as a new therapeutic target for the treatment of several diseases in clinical settings.

Introduction

Interleukin (IL)-34 is the second ligand of colony-stimulating factor-1 receptor (CSF-1R) to be discovered ¹. Figure 1 shows a timeline of major advances in IL-34 research. In 2002, a study of Dai et al. helped to predict a new ligand for CSF-1R. Deficiency of CSF-1R in mice contributed to a more severe decrease in macrophages and osteopetrosis phenotype compared with CSF-1-deficient mice, suggesting that CSF-1R could result from its CSF-1-independent activation ². In 2008, the discovery of IL-34 completed the understanding of activation of CSF-1R signaling ¹. The CSF-1/CSF-1R axis is critical for differentiation, proliferation, viability, and survival of hematopoietic cells ^2-4. As another ligand of CSF-1R, IL-34 shares functional similarity with CSF-1, acting as an important modulator of macrophage, osteoclast, and monocyte functions. At the same time, IL-34 shows differences from CSF-1 in its expression pattern, and it acts through additional signaling pathways ^5,6. Protein expression of IL-34 was especially evident in the spleen, skin, and brain. Several studies documented that IL-34 might play a crucial role in the development of the microglia and Langerhans cells as evident by a huge decline in these cells in IL-34-deficient mice ^7,8. Nakamichi et al. reported an interesting issue that IL-34 played a key role in maintaining the splenic reservoir of osteoclast precursors and their transfer to the bone in CSF-1-deficient mice ⁶. However, IL-34 mRNA was extensively expressed in tissues and its expression could be changed in various pathological contexts ⁵. For example, IL-34 expression is induced in response to pro-inflammatory cytokines, pathogen-associated molecular patterns (PAMPs), viral infection, and chemical stimuli. This process can be modulated by nuclear factor (NF)-κB signaling ^9,10. Interestingly, IL-34 secreted by regulatory T cells (Tregs) could mediate immune tolerance ¹¹. In addition, protein-tyrosine phosphatase (PTP)-ζ and syndecan-1 (CD138) were recently identified as functional receptors for IL-34. Through these receptors, IL-34 helped intracellular signaling pathways involved in migration, proliferation, motility, and clonogenicity of myeloid cells ^12,13.

Figure 1.

Timeline of major advances in IL-34 research since 2002.

In the current review, we recapitulate emerging knowledge with regard to receptor and signaling pathway of IL-34, its physiological significance and, in particular, its immune regulation in different diseases. Importantly, IL-34 has been proposed as a useful diagnostic and prognostic biomarker, as well as an attractive therapeutic target in various illnesses.

Basic structure of IL-34

IL-34 is a secreted homodimeric protein of 242 amino acids in humans and 235 amino acids in mouse, with a molecular mass of 39 kDa. The structure of IL-34 comprises two β strands, four short helices, and four long helices, with an antiparallel four-helix core homologous to that of colony-stimulating factor (CSF)-1 (Figure 2) ^14,15.

Figure 2.

Structure of IL-34.

Receptor and signaling pathway of IL-34

CSF-1R is considered the major functional receptor of IL-34. The interaction between IL-34 and CSF-1R triggers several signaling pathways involving NF-κB, phosphoinositide 3-kinase (PI3K)/AKT, Janus kinase (JAK), signal transducer and activator of transcription (STAT) 3, p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), and c-Jun N-terminal kinase (JNK) (Figure 3) ^9,10,16-18. IL-34 binding to CSF-1R can activate caspase-3/8 and induce autophagy via the AMP-activated protein kinase (AMPK)-UNC-51-like Kinase (ULK) 1 pathway ¹⁹.

Figure 3.

The signaling network of IL-34. IL-34 can be induced by various stimuli such as pro-inflammatory cytokines, pathogen-associated molecular patterns (PAMPs), viral infection, and chemical stimuli.IL-34 can bind to CSF-1 receptor (CSF-1R) and subsequently induce a series of signaling networks involving nuclear factor-kappaB (NF-κB), c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (p38 MAPK), extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), Janus kinase (JAK), signal transducer and activator of transcription (STAT)-3,phosphoinositide 3-kinase (PI3K)/AKT, caspase-3/8, as well as autophagy activated by AMP-activated protein kinase (AMPK)-UNC-51-like kinase (ULK) 1. Moreover, IL-34 can induce paxillin and focal adhesion kinase (FAK) by binding protein-tyrosine phosphatase ζ (PTP-ζ). In this way, IL-34-mediated signaling is a key modulator in various cellular processes, including cellular differentiation, motility, adhesion, metabolism, proliferation, and survival. As a novel functional receptor, syndecan-1 has been shown to regulate the activation of IL-34/CSF-1R signaling and may serve as a potent regulator of IL-34 activity. Nevertheless, IL-34/syndecan-1-induced signaling is still largely unexplored.

IL-34 might signal through additional receptors, since it was strongly expressed in brain areas where CSF-1R expression was low. Mass spectrometry identified PTP-ζ as an alternative IL-34 receptor in glial cells and neuronal progenitors in the murine brain ¹². The interaction between IL-34 and PTP-ζ can induce a series of signaling cascades that inhibit motility, clonogenicity, and proliferation of specific cell types, via tyrosine phosphorylation of paxillin and focal adhesion kinase (FAK) ¹³. Recently, IL-34 has been suggested to promote migration of myeloid cells in a syndecan-1-dependent manner, but the interaction of IL-34 with this receptor remains relatively unexplored.

Physiological significance of IL-34

Physiological functions of IL-34 have been indicated from many studies in animal models. For example, IL-34-deficient mice showed much lower frequencies and activities of F4/80⁺CD11b⁺CD45^int microglia and F4/80⁺CD11b⁺CD45⁺ Langerhans cells than wild-type mice but otherwise normal fertility, lifespan, and counts of blood monocytes as well as tissue macrophages ^7,8. IL-34 deficiency is also linked to lower numbers of CD11c⁺CD11b⁺ dendritic cells in the lung ^7,8. IL-34 is selectively expressed by microglia and Langerhans cells, and it can promote the differentiation and development of these cells. Interestingly, IL-34-mediated effects appear to be restricted to physiological conditions in Langerhans cells. During inflammation, Langerhans cells rely on neutrophil-released CSF-1 other than IL-34 ^19,20; once the inflammatory response subsides, Langerhans cells are again dependent on IL-34 ^21,22.

IL-34 plays a role in the expression pattern and the development of nervous system ^23,24. CSF-1 is constitutively expressed in the ventricular and subventricular regions of the brain, while IL-34 is restricted to the striatum and hippocampus ⁷. IL-34-deficientmice showed a reduction in microglial cells in the hippocampus and cortex, confirming its role in the development and homeostasis of microglia ²⁵. IL-34 has been detected in the cerebrospinal fluid because of its secretion from choroid plexus and ependymal cells ⁸. Taken together, these findings implicate the physiological importance of IL-34 in microglia and Langerhans cells (Figure 4). IL-34 expression is evident in the spleen, seminiferous tubule germ cells, proximal renal tubule cells, and placental syncytiotrophoblasts, while further work is needed to explore the potential function of IL-34 in these tissues ^26,27.

Figure 4.

Pathophysiological significance and potential applications of IL-34. IL-34 is constitutively expressed in the microglia and Langerhans cells under physiological conditions, where it plays a crucial role in development, function, and survival. Under pathological states, IL-34 may exert an effect on several cell types (e.g., monocytes/macrophages, tumor-associated macrophages, regulatory T cells, fibroblasts, and osteoclasts) and contribute to the pathogenesis of various diseases, including autoimmune diseases, cancer, transplant rejection, neurologic diseases, inflammatory diseases, and infections. Abbreviations: IL-34, interleukin-34; TAMs, tumor-associated macrophages; Tregs, regulatory T cells; HBV, hepatitis B virus; HCV, hepatitis C virus; HIV, human immunodeficiency virus; EIAV, equine infectious anemia virus; IBD, inflammatory bowel disease; RA, rheumatoid arthritis; SLE, systemic lupus erythematosus; RA, rheumatoid arthritis.

IL-34 in inflammatory and immune responses

IL-34 can orchestrate inflammatory and immune responses, and it is markedly induced in the inflammatory milieu via signaling NF-κB pathways ¹⁰. IL-34 can amplify the inflammatory effect by inducing the expression of chemokines, metalloproteases, and pro-inflammatory mediators ^28,29. Likewise, it has the potential to mediate the differentiation of monocytes into IL-10^highIL-12^low macrophages, which act as a master regulator in immune response ³⁰. In this context, the IL-34-differentiated macrophages are similar to tumor-associated macrophages (TAMs). IL-34-stimulated macrophages secrete membrane-associated IL-1α, which switches memory T cells into helper T cells (Th) 17 ³¹. In this way, IL-34 instructs macrophages to maintain local inflammation, which is necessary for metastasis and angiogenesis. Moreover, IL-34 and CSF-1 show distinct abilities to polarize monocytes into M1 or M2 macrophages: IL-34, but not CSF-1, generates M1 and M2 macrophages that express high levels of IL-10 and chemokine ligand 17 (CCL17) ¹⁹. IL-34-evoked macrophages can also polarize naïve T cells into Th1 cells ³⁰.

While IL-34 can stimulate the immune response, it may also mediate immune tolerance and resolution of inflammation. It inhibits Toll-like receptor (TLR) signaling and production of anti-inflammatory cytokines ^19,32. IL-34-differentiated macrophages may be polarized to the M2 phenotype and exert anti-inflammatory and immunosuppressive effects by suppressing the function of both T and natural killer (NK) cells, and expanding CD4⁺/CD8⁺ Foxp3⁺Tregs ^33-35. Therefore, IL-34 acts as a potent and pleiotropic cytokine in the regulation of inflammatory and immune processes.

Immune regulatory of IL-34 in various diseases

There is a growing body of evidence showing that IL-34-mediated immune response participates in various pathological states (summarized in Table 1). In fact, high levels of IL-34 have been observed in animal models and patients with autoimmune diseases, cancer, transplant rejection, neurologic diseases, infections, and inflammatory disorders. Furthermore, increased levels of IL-34 are closely associated with the pathogenesis, progression, and severity of these diseases. In some cases, IL-34 levels return to the physiological range after successful treatment. Thus, IL-34 may be a promising clinical biomarker for prediction, diagnosis, and responsiveness to treatment (summarized in Table 2).

Table 1.

Summary of studies concerning the clinical significance of IL-34 in different diseases.

Diseases	Years	Authors	Clinical observations	Ref.
RA	2013	Tian et al.	Elevated serum and synovial fluid levels of IL-34 were correlated with disease activity in RA patients. The level of serum IL-34 decreased after anti-TNF treatment.	38
	2017	Wang et al.	Serum IL-34 levels were increased and closely related to the diseases activity of RA. IL-34 upregulated Th17 production via increased IL-6 expression by rheumatoid fibroblast-like synoviocytes.	39
	2012	Chemel et al.	IL-34 expression was associated with synovitis severity in RA patients.	42
	2012	Hwang et al.	IL-34 produced by human fibroblast-like synovial cells in RA supports osteoclastogenesis. IL-34 elevation in plasma from RA patients was decreased after the administration of disease-modifying anti-rheumatic drugs.	43
	2013	Moon et al.	Increased levels of IL-34 in serum and synovial fluid were associated with rheumatoid factor and anticycliccitrullinated peptide antibody titers in patients of RA.	44
	2015	Chang et al.	Baseline serum IL-34 levels independently predict radiographic progression in patients with RA.	45
	2016	Garcia et al.	CSF-1R blockade reduced inflammation in human and murine models of RA.	46
	2016	Yang et al.	IL-34 upregulation contributed to the increment of microRNA 21 expression via STAT3 activation associated with disease activity in RA.	47
	2015	Ding et al.	IL-34 levels in serum predicted response to TNF-α antagonist therapy.	50
SLE	2016	Wang et al.	Serum IL-34 levels were elevated in patients with SLE.	53
	2018	Xie et al.	Elevated serum IL-34 level in patients with SLE was associated with disease activity.	54
SS	2013	Ciccia et al.	IL-34 was highly expressed in ductal epithelial cells and infiltrating mononuclear cells of the inflamed salivary gland, which correlated with pro-inflammatory cytokines and local expansion of pro-inflammatory CD14bright CD16+ monocytes.	55
Cancer	2017	Raggi et al.	High levels of serum IL-34 were observed in cholangiocarcinoma.	33
	2010	Baud'huin et al.	IL-34 expression was enhanced in giant cell tumors.	41
	2016	Zhou et al.	High levels of IL-34 were correlated with poor prognosis in hepatocellular carcinoma.	71
	2018	Han et al.	IL-34 was strongly expressed in melanoma and correlated with CD163+ cell counts in nivolumab-resistant metastatic melanoma.	69
	2018	Baghdadi et al.	IL-34 levels were correlated with tumor progression in lung cancer.	68
	2015	Rietkötter et al.	IL-34 mRNA expression was up-regulated in breast cancer and lung cancer with brain metastasis.	64
	2015	Ségaliny et al.	Heterogeneous expression of IL-34 was evident in human osteosarcomas.	65
	2017	Franzè et al.	Over-expression of IL-34 was present in sporadic colorectal cancer.	67
	2014	Cioce et al.	IL-34 was produced in malignant pleural mesothelioma cells, and it induced chemoresistance.	62
Liver transplantation	2016	San Segundo et al.	Increased levels of serum IL-34 were noticed in acute liver rejection.	73
AD	2017	Walker et al.	IL-34 expression was down-regulated in the inferior temporal gyrus in patients with AD.	79
HBV infection	2017	Cheng et al.	IL-34 levels were decreased in the serum and PBMCs of patients with chronic HBV infection, and they were negatively correlated with disease activity.	88
	2018	Wang et al.	Serum IL-34 was elevated in HBV infection and correlated with liver inflammation and fibrosis in patients with chronic HBV infection.	81
HCV infection	2014	Preisser et al.	Increased levels of serum IL-34 were correlated with advanced liver fibrosis stage.	82
Influenza A viral infection	2015	Yu et al.	High levels of IL-34 were observed in PBMCs and serum in response to influenza A infection.	83
Sepsis	2018	Lin et al.	Serum IL-34 levels were increased in sepsis.	91
IBD	2015	Franzè et al	Over-expression of IL-34 was noted in the epithelial layer and infiltrating immune cells in the inflamed mucosa.	92
Atopic dermatitis	2015	Esaki et al.	IL-34 expression was decreased in lesional epidermis.	95
Chronic apical periodontitis	2016	Ma et al.	IL-34 expression was evident in plasma cells, lymphocytes, and macrophages in chronic periapical lesions.	100
Type 2 diabetes mellitus	2011	Below et al.	Serum IL-34 predicted the risk of vascular diabetic complications.	102
	2016	Zorena et al.	IL-34 was located within risk loci associated with type 2 diabetes mellitus.	103
Heart failure	2016	Fan et al.	High levels of serum IL-34 were correlated with cardiovascular death, risk of renal dysfunction, and mortality.	99

Abbreviations: IL-34, interleukin-34; RA, Rheumatoid arthritis; SLE, Systemic lupus erythematosus; SS, Sjogren's syndrome; AD, Alzheimer's disease; IBD, inflammatory bowel disease; HBV, hepatitis B virus; HCV, Hepatitis C virus infection; PBMC, peripheral blood mononuclear cell; TNF-α, tumor necrosis factor-α; CSF-1R, Colony-stimulating factor 1 receptor.

Table 2.

Summary of studies with regard to the potential targeting IL-34 in various diseases.

Diseases	Year	Authors	Potential applications	Ref.
SS	2013	Ciccia et al.	Targeting IL-34 inhibited the expansion of CD14bright CD16+ monocytes in inflamed salivary glands.	55
RA	2013 2016 2016	Bostrom et al. Yang et al. Zhou et al.	Neutralizing IL-34 attenuated the inflammatory lesions, decreased pathogenic immune cell subsets, sensitized synovial fibroblasts to apoptosis, and alleviated cartilage destruction as well as bone erosion in rheumatoid arthritis.	36 47 48
IBD	2015	Franze et al.	Neutralizing IL-34 mitigated the inflammatory cascade by inhibiting the production of CCL20, TNF-α, and IL-6.	92
HBV infection	2017	Cheng et al.	In HBV transgenic mice, IL-34 administration suppressed the replication of HBV DNA.	88
HIV infection	2010	Chihara et al.	Blockade of IL-34 inhibited HIV replication.	16
Influenza A viral infection	2015	Yu et al.	Neutralizing IL-34 alleviated the inflammatory response in influenza A viral infection.	83
Sepsis	2018	Lin et al.	Treatment with IL-34 improved bacterial clearance and survival in a mouse model of sepsis.	91
Atopic dermatitis	2015	Esaki et al.	IL-34 suppressed propagation of the inflammatory circle in active skin lesions.	95
Acute kidney injury	2015	Baek et al.	IL-34 mediated acute kidney injury and worsened chronic kidney disease.	97

Abbreviations: IL-34, interleukin-34; SS, Sjogren's syndrome; RA, Rheumatoid arthritis; IBD, inflammatory bowel disease; CCL20, C-C motif chemokine ligand 20; HBV, hepatitis B virus; HIV, human immunodeficiency virus; TNF-α, tumor necrosis factor-α.

IL-34 in autoimmune diseases

Immunomodulation of IL-34 is critically involved in the etiology of autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Sjogren's syndrome, psoriasis, and psoriatic arthritis. RA is a systemic and chronic autoimmune disease characterized by bone destruction and progressive cartilage. In response to stimulation by inflammatory cytokines [e.g., IL-1β, tumor necrosis factor (TNF)-α, IL-6, IL-17], IL-34 is produced by synovial fibroblasts and released into the synovial fluid. High levels of IL-34 can be detected in the synovial fluids and serum of RA patients. IL-34 facilitates the secretion of pro-inflammatory cytokines, and amplifies the inflammatory cascades in the NF-κB- and JNK-dependent mechanisms ^36,37. Simultaneously, elevation of IL-34 positively correlates with numerous inflammatory parameters in RA, such as leukocyte count, C-reactive protein, erythrocyte sedimentation rate, pro-inflammatory cytokine levels, severity of synovitis, synovial hyperplasia disease activity score, and auto-antibody production (e.g., rheumatoid factor, anti-cyclic citrullinated peptide antibody) ^42-49. IL-34 can induce the differentiation, proliferation, and infiltration of macrophages in the inflamed synovia, which are the central immune cells in RA pathogenesis ^31,32,36. It also stimulates IL-17-producing Th17 cells, which are the crucial drivers of autoimmune tissue injury in RA ^38-40. Furthermore, overexpression of IL-17 is implicated in RA etiology. It activates NF-κB ligand (RANKL)-mediated osteoclastogenesis by enhancing the proliferation and adhesion of osteoclast progenitors, leading to aggravated bone erosion ⁴¹. RA treatment with TNF-α antagonist therapy or disease-modifying antirheumatic drugs can dramatically reduce serum IL-34 levels ^50,51.

SLE is characterized by formation of immune complexes and anti-nuclear antibodies as well as activation of B and T lymphocytes, resulting in multiple organ damage. Experimental data in animal models support a pathological role of IL-34 in SLE, and elevated levels of IL-34 have been observed in patients with SLE and closely correlate with clinical outcomes ^52-54. After treatment with immunosuppressive drugs or corticosteroids, IL-34 levels return to normal range. Nonetheless, the immunoregulatory mechanisms underlying IL-34 in the pathogenesis of SLE need to be investigated in future studies.

Sjogren's syndrome is a chronic inflammatory and autoimmune disorder. Infiltration of T and B cells and altered levels of pro-inflammatory mediators play essential roles in the etiology of Sjogren's syndrome. Overexpression of IL-34 was reported in inflamed salivary glands, accompanied by production of pro-inflammatory cytokines, such as IL-17, IL-1β, and TNF-α, and pro-inflammatory CD14^BrightCD16⁺ monocytes ^55,56. Thus, IL-34 may be involved in the development of salivary gland inflammation and Sjogren's syndrome. Likely, psoriasis is a chronic inflammatory and autoimmune disease, which is frequently accompanied by psoriatic arthritis. Inflammatory mediators including IL-12, IL-1β, and TNF-α were participated in the immunopathogenicity of psoriasis and psoriatic arthritis ⁵⁷. Patients with these disorders showed increased levels of serum IL-34. Clearly, psoriatic arthritis is characterized by progressive osteoclastogenesis, joint damage, and bone erosions. A strong increase in circulating IL-34 positively correlates with serum osteoclast precursors, pointing towards IL-34 as a potential clinical marker for monitoring the progression of psoriatic arthritis ⁵⁷. These findings suggest a critical role for IL-34 in the pathology of psoriasis and psoriatic arthritis.

IL-34 in cancer

TAMs act as key immune regulators in the tumor microenvironment, promoting the growth, angiogenesis, metastasis, and invasion of cancer cells, and weakening anti-tumor immunity. Clinical studies demonstrate a strong link between expansion of TAMs and poor outcomes in patients with cancers ⁵⁸. Thus, the size of the TAM population may serve as a clinical marker for stratifying the risk of tumors.

CSF-1R signaling is required for the function and survival of TAMs ^59-61. As a ligand of CSF-1R, IL-34 exhibits pro-tumorigenic effects and has gained attention for its potential utility in the diagnosis, prediction, and targeted therapy of cancers ^62,63. IL-34 recruits TAMs, which drive neo-angiogenesis and metastasis in cancers ^33,34. IL-34/CSF-1R signaling contributes to extravasation of immune cells and formation of new vessels in a paracrine manner ^62-70. IL-34 activates AKT and then triggers C/EBPβ signaling, thereby augmenting the immunosuppressive activity of TAMs in the tumor microenvironment ^22,34. In hepatocellular carcinoma, TAMs release transforming growth factor (TGF)-β and subsequently accelerate the production of IL-34, a feedback loop that is involved in metastasis ⁷¹. Consistent with a pro-tumorigenic role of IL-34, many studies have been confirmed that the functional receptors of IL-34, including CSF-1R, PTP-ζ, and syndecan-1, are obviously expressed in a variety of cancers ^12,13,59.

IL-34/CSF-1R axis can trigger survival pathways in an autocrine manner when tumor cells are treated with chemotherapy, and chemotherapy per se induces high expression of IL-34 ⁶⁰. Blockade of IL-34 or CSF-1R can improve the survival of mice with breast cancer by reducing vessel density, inhibiting proliferation of cancer cells, and enhancing CD8⁺ cell- mediated anti-tumor immunity ^57,58,62. Paradoxically, IL-34 may exert antitumor effects in several cancer types. For instance, it markedly suppresses the motility, clonogenicity, and proliferation of glioblastoma cells ^12,13. Consistently, low IL-34 levels are associated with poor outcomes in various cancers ^12,13. Therefore, immunomodulation of IL-34 in tumors is complex, and possibly context-dependent, and should be investigated in further studies.

IL-34 in transplant rejection

Immune rejection is the major cause of transplant failure, thus induction of immune tolerance is key to successful outcomes after transplantation. IL-34 may be of importance for mediating such immune tolerance and ensuring graft survival. In a study of cardiac allografts in animal models, IL-34 induced the expansion and potentiation of CD8⁺/ CD4⁺Tregs, and protected against allograft rejection via a M2 macrophage-dependent pathway ⁷². Interestingly, Tregs can release IL-34 in an autocrine manner. In a rat model of liver transplantation, IL-34 contributed to polarizing Kupffer cells to the M2 type, improving recipient survival ⁷³. In clinical settings, it was reported that patients who suffered acute rejection of liver or kidney transplants showed increased serum IL-34 levels and worse prognosis than those who did not experience rejection ^72,73. These results indicate that IL-34 appears to be a critical mediator of transplant tolerance and may be useful as a prognostic biomarker of graft survival.

IL-34 in neurological diseases

IL-34 has been suggested to exert immune protective effects on the central nervous system by impacting microglia, neurons, and endothelial cells. In physiological conditions, IL-34 expression is restricted to neurons, where it plays a role in maintenance of the immune homeostasis in the central nervous system ⁷⁴. In the development of diseases, IL-34 production triggers survival signaling in damaged neurons ^75-77, and treatment with IL-34 markedly attenuates excitotoxin-induced neuronal loss in mice ⁷⁷. Similarly, IL-34 accelerates the degradation of oligomeric amyloid as well as the production of TGF-β and heme oxygenase-1, alleviating oxidative stress ⁷⁸. In a mouse model of Alzheimer's disease, IL-34 administration significantly enhanced the clearance of soluble oligomeric amyloid-β and improved associative learning ⁷⁹. IL-34 has the potential to up-regulate tight junction molecules to maintain the integrity of the blood-brain barrier ⁸⁰. These observations support the notions that IL-34 may exert beneficial effects against neurodegeneration and brain injury, making it a possible new therapeutic approach for the management of neurological diseases.

IL-34 in infections

It is well known that cytokines regulate host immune response during infection. Pro-inflammatory cytokines and PAMPs induce IL-34 expression, which then modulates innate immune responses to infections. IL-34 contributes to the persistence of viruses by involving in immune tolerance ^20-22. In hepatitis C infection, IL-34 is generated by hepatocytes and induces monocytes to progress to macrophages with a profibrotic phenotype, thereby facilitating the progression of hepatic fibrosis ⁸¹. Patients with hepatitis C infection reveal increased levels of IL-34 ⁸²; the same has been observed in patients with influenza A infection, in whom IL-34 production depends strongly on IL-22-mediated inflammatory response ⁸³. Notably, expression of IL-34 can be induced in macrophages infected with equine infectious anemia virus, and the cytokine promotes the proliferation of monocytes/macrophages ⁸⁴. IL-34 maybe closely associated with the survival of human immunodeficiency virus (HIV) in infected microglia and macrophages by inhibiting apoptosis, thereby contributing to HIV-induced neuropathogenesis ^85-87. Consistently, IL-34-stimulated macrophages show high replication of HIV. These evidences indicate that IL-34 may help to provide a favorable microenvironment for viral replication, particularly in macrophages.

While IL-34 promotes virus survival, it can also strengthen antiviral immunity. IL-34 levels were found to correlate negatively with genome copy number of hepatitis B virus (HBV) in vitro, suggesting that IL-34 could suppress HBV replication. Similarly, serum IL-34 levels were correlated inversely with HBV DNA levels in patients with chronic HBV infection ⁸⁸. Therefore, IL-34 seems to be a helpful clinical marker to evaluate HBV infection.

More recently, it has been demonstrated that IL-34 is involved in infections of fungus, parasite, bacteria, candidemia, and sepsis ^89-91. For example, Xu et al. ³² noticed that IL-34 down-regulated Candida albicans-induced TNF-α release from M1-type macrophages by suppressing Dectin-1 and TLR2 signaling, in turn maintaining immune tolerance in the skin and mucosa. These results support a novel role for IL-34 in the development of infections, although its impact is probably context-dependent and can vary depending on the infectious agent, the site of infection, and the phase of the infectious process.

IL-34 in inflammatory disorders

IL-34 plays an essential role in inflammatory disorders both in animal models and patients, including inflammatory bowel disease (IBD), skin disease, heart failure, chronic apical periodontitis, and type II diabetes mellitus. IBD is an inflammatory and autoimmune disease that includes ulcerative colitis and Crohn's disease. CSF-1R and PTP-ζ have been detected in intestinal epithelial cells as well as in monocytes and macrophages in the gut. Clinically, overexpression of IL-34 has been noted in the inflamed colon ⁹². In the development of IBD, IL-34 amplifies the inflammatory cascade in the gut. It enhances the production of chemokines (e.g., CCL20) and pro-inflammatory cytokines (e.g., TNF-α, IL-6) via the ERK pathway ⁹³. In a recent study of ulcerative colitis in animal models, IL-34 induced Th2-type response and activated inflammatory remodeling, which was required for epithelial hyperplasia and fibrosis ⁹⁴. Therefore, IL-34 may be interesting as a novel marker to monitor IBD severity, and as a potential therapeutic target for treatment of the inflamed gut. In addition to the significance of IL-34 in the pathophysiology of IBD, it is now clear thatIL-34 is down-regulated in atopic dermatitis, which is characterized by inflammatory lesions, oozing papules, and plaques ⁹⁵. IL-34 can dampen the inflammatory cascade, thereby attenuating the active skin lesions.

Macrophage-based immune and inflammatory responses are involved in the pathogenesis of acute kidney injury (AKI). Macrophages can mediate kidney destruction or repair, thus playing dual roles in the fate of the kidney. CSF-1 contributes to the recovery of AKI via a macrophage-dependent mechanism: in a mouse model of ischemia-reperfusion induced AKI, expressions of IL-34, CSF-1R, and PTP-ζ are up-regulated in tubular epithelial cells. Local IL-34-induced macrophages augment the destruction of tubular epithelial cells and worsen AKI, while circulating IL-34 enhances the proliferation of myeloid cells and increases monocytes, thereby amplifying persistent inflammation ⁹⁵. In the clinic, high IL-34 levels are associated with severe AKI and subsequent chronic kidney disease ⁹⁶. Elevated levels of IL-34 are also found in patients who suffered kidney rejection after transplantation. In the engrafted kidney, IL-34 produced by tubular epithelial cells might trigger the expansion of Ly6G⁺ neutrophils and CD68⁺ macrophages ⁹⁷.

In addition, IL-34-mediated inflammatory and immune responses are implicated in the pathogenesis of obesity-associated complications, heart failure, and chronic apical periodontitis ^98-103. For instance, adipocyte-derived IL-34 recruits macrophages to adipose tissues, leading to chronic inflammation and insulin resistance in type II diabetes mellitus ^102,103. IL-34 may act as a potential predictor of prognosis and severity in these diseases.

Conclusions and perspectives

Despite its recent discovery, IL-34 has already been identified as a multifunctional cytokine that regulates a variety of cellular processes including adhesion, differentiation, proliferation, metabolism, angiogenesis, inflammation, and immune responses. Notably, IL-34 plays a crucial role in the functions and survival of microglia and Langerhans cells. As the second ligand of CSF-1R, IL-34 efficiently promotes the differentiation, migration, and proliferation of monocytes/macrophages, which are involved in various pathophysiological states. At the same time, IL-34 participates in several signaling pathways that do not involve CSF-1. Syndecan-1 and PTP-ζ are alternative receptors of IL-34. The interactions between IL-34 and syndecan-1 influence cellular migration, modulate the activation of IL-34/CSF-1 binding, and inhibit glioma cell function. Precise mechanisms and additional receptors/regulators that interact with IL-34 need to be investigated in the future work.

Recently, there is increasing evidence that IL-34 affects different immune cells and participates in immune and inflammatory responses, thereby influencing the onset and progression of various diseases (Figure 4). Depending on the pathological conditions, increased formation of IL-34 amplifies inflammatory cascades or, conversely, mediates immune tolerance. Therefore, IL-34 shows promise as a prognostic biomarker in many diseases, including autoimmune diseases, cancer, transplant rejection, infections, and inflammatory diseases.

IL-34 may be useful as a therapeutic target in the management of illnesses. Treatment with IL-34 can prevent various diseases such as neurological disorders, skin lesions, transplant rejection, and infections. IL-34 provides powerful neuroprotection through its effect on microglia, neurons, and endothelial cells. It can induce the differentiation of pluripotent stem cells into microglia, suggesting a potential role in neural regeneration. Likewise, IL-34 is effective in mediating immune tolerance and ameliorating inflammatory lesions by inducing the expansion of Tregs and M2-polarized macrophages. IL-34-mediated tolerance also helps the control of inflammation in skin diseases and prolongs survival of the implanted grafts. Studies on the significance of IL-34 in outcomes of kidney and liver transplantation have been limited to animal models. Further works are warranted to investigate whether IL-34 can benefit other types of organ transplantation, and to elucidate the underlying mechanisms in details. In addition, IL-34 can suppress HBV replication and improve bacterial clearance, indicating that this cytokine may be useful as an adjuvant treatment for preventing HBV and bacterial infections. Interestingly, IL-34 exerts paradoxical effects on different viral infections. In contrast to the case of HBV infection, neutralizing IL-34 can inhibit HIV replication, as well as decrease profibrotic macrophages in the setting of HCV infection.

In the process of cancer, IL-34 induces immune tolerance and triggers pro-survival signaling. Therefore, blockade of IL-34 seems to be a potential approach for the treatment of several IL-34-producing cancers, as a strategy that is currently being investigated. Beyond cancer, the inhibition of IL-34 activity may help to alleviate inflammatory response by down-regulating the expression of pro-inflammatory cytokines. In this context, targeting IL-34 is expected to be a potential therapeutic strategy in the treatment of IBD, RA, and AKI.

In summary, the immunoregulatory properties of IL-34 appear to be of significance in a wide range of diseases. Elucidating the favorable and unfavorable effects of IL-34 in the immune microenvironment may be of great importance for a comprehensive understanding of its pathophysiological implication. Therefore, future research concerning the evaluation of therapeutic potential of IL-34 in human diseases are required.

Abbreviations

AKI

acute kidney injury

AMPK

AMP-activated protein kinase

CD

Crohn's disease

CSF-1

colony- stimulating factor-1

CSF-1R

colony-stimulating factor-1 receptor

ERK1/2

extracellular signal-regulated protein kinases 1 and 2

FAK

focal adhesion kinase

HBV

hepatitis B virus

HCV

hepatitis C virus

HIV

human immunodeficiency virus

IBD

inflammatory bowel disease

IL-34

interleukin-34

JAK

Janus kinase

JNK

c-Jun N-terminal kinase

MAPK

mitogen-activated protein kinases

p38 MAPK

p38 mitogen-activated protein kinase

NF-κB

nuclear factor of kappa B

PAMP

pathogen-associated molecular pattern

PI3K

phosphoinositide 3-kinase

PTP-ζ

protein-tyrosine phosphatase ζ

RA

rheumatoid arthritis

SLE

systemic lupus erythematosus

STAT3

signal transducer and activator of transcription 3

TAMs

tumor-associated macrophages

TGF-β

transforming growth factor-β

Th

helper T cells

TLR

Toll-like receptor

TNF-α

tumor necrosis factor-α

Tregs

regulatory T cells

ULK1

UNC-51-like kinase 1

UC

ulcerative colitis