The twin cytokines interleukin-34 and CSF-1: masterful conductors of macrophage homeostasis

Abstract

Macrophages are specialized cells that control tissue homeostasis. They include non-resident and tissue-resident macrophage populations which are characterized by the expression of particular cell surface markers and the secretion of molecules with a wide range of biological functions. The differentiation and polarization of macrophages relies on specific growth factors and their receptors. Macrophage-colony stimulating factor (CSF-1) and interleukine-34 (IL-34), also known as "twin" cytokines, are part of this regluatory landscape. CSF-1 and IL-34 share a common receptor, the macrophage-colony stimulating factor receptor (CSF-1R), which is activated in a similar way by both factors and turns on identical signaling pathways. However, there is some discrete differential activation leading to specific activities. In this review, we disscuss recent progress in understanding of the role of the twin cytokines in macrophage differentiation, from their interaction with CSF-1R and the activation of signaling pathways, to their implication in macrophage polarization of non-resident and tissue-resident macrophages. A special focus on IL-34, its involvement in pathophsyiological contexts, and its potential as a theranostic target for macrophage therapy will be proposed.

Keywords: IL-34; inflammation; macrophage differentiation; theranostics; tumor.

Figure 1

Macrophage ontogeny and the implications of IL-34 during macrophage differentiation. Depending on their origin, macrophages are divided into two different populations: tissue-resident macrophages and non-resident macrophages. Tissue-resident macrophages originate in the embryonic yolk sac, fetal liver, and bone marrow. Tissue-resident macrophages are capable of self-renewal of their own population (round arrows). However, in pathogenic situations, non-resident macrophages can migrate into the affected tissues and replenish the local populations by acquiring tissue specificities. Depending on the tissue, IL-34 drives macrophage differentiation, proliferation, maintenance, migration, and adhesion. Non-resident macrophages originate in the bone marrow and spleen. Circulating monocytes can extravasate and migrate to different tissues where, through the actions of different growth factors, they induce their polarization into M1 or M2 subtypes. M1 macrophages detect pathogenic particles or inflammatory molecules such as LPS or INT-γ and display pro-inflammatory functions by secreting pro-inflammatory factors such as TNF-α, IL-6 and Il-12. M2 macrophages are sensitive to molecules such as IL-4 or IL-13 and display an anti-inflammatory profile by producing soluble factors such as IL-10. IL-34 mainly induces the polarization of monocytes into an M2 subset. In pathological situations such as bacterial or viral infection, or inflammation, IL-34 can act as a pro- or anti-viral/inflammatory agent. In cancer, IL-34 behaves in a pro- or anti-tumor manner. IL-34 also induces macrophage differentiation into tumor-associated macrophages (TAMs), which are characterized by an M2 phenotype that promotes tumor proliferation, angiogenesis, and metastasis. The capacity of IL-34 to act in a positive or negative direction is tissue- and microenvironment-dependent.

Figure 2

Molecular modeling of CSF-1R binding to its ligands. Molecular modeling was generated as described in A) representation of the three-dimensional crystal structure of the CSF-1/CSF-1R complex. In red and orange: monomers of CSF-1; and in blue and purple: monomers of CSF-1R. B) Representation of the three-dimensional crystal structure of the IL-34/CSF-1R complex. In green and light green: monomers of IL-34; and in blue and purple: monomers of CSF-1R.

Figure 3

IL-34 signaling pathways involved in macrophage differentiation and non-monocyte cells. A) Various stimuli, such as bacterial or viral infections, pro-inflammatory cytokines, DNA damage, or chemical molecules, modulate IL-34 expression. IL-34 binds to CSF-1R or to syndecan-1 receptors expressed at the cell surface of monocytes/macrophages. The binding of IL-34 to CSF-1R induces activation of CSF-1R through auto-phosphorylation of the different tyrosines present in the cytosolic domain of CSF-1R. Compared to CSF-1, IL-34 induces strong and transient activation of CSF-1R, as well as rapid downregulation of CSF-1R. These differences between the two cytokines imply differential activation of downstream signaling pathways that result in a diversity of macrophage biological processes such as differentiation, proliferation, survival, or migration. The binding of IL-34 to the chondroitin chains of syndecan-1 results in in vitro phosphorylations of the tyrosines Y708 and Y723 of CSF-1R, suggesting that the complex IL-34/syndecan-1 acts as a regulator of CSF-1R activity. Moreover, IL-34/syndecan-1 interaction regulates macrophage migration. B) IL-34 expression in the microenvironment of epithelial cells, fibroblasts and tumor cells can induce, via CD155, activation of the different signaling pathways implicated in biological functions such as cell proliferation, migration, survival, and cytokines. IL-34 also binds to RPTP-ζ, and controls inhibition of migration and proliferation of tumor cells lines such as glioblastoma U251.

Figure 4

IL-34 is a pro-M2 macrophage differentiation factor. Macrophage isolation and treatment were performed as described in A) IL-34 treatment induced macrophage differentiation with an M2 phenotype, alone or in combination with IL-4 and IL-10. Macrophages were treated with IL-34 (50 ng/ml) or in combination with IFN-γ (50 ng/ml; pro M1), IL-4 (50 ng/ml; pro M2a), and IL-10 (50 ng/ml; pro M2c), for 2 days and cells were analyzed by means of flow cytometry using specific antibodies for both M1-like macrophages (CD14, CD86 and CD64) and M2-like macrophages (CD163, CD200R and CD206). B) Comparison of macrophage differentiation after treatment with the cytokines CSF-2 (20 ng/ml), CSF-1 (50 ng/ml) and IL-34 (50 ng/ml) alone or in combination with IFN-γ, IL-4 or IL-10 as performed in A. The three cytokines in combination with IFN-γ were able to induce M1 macrophage differentiation as shown by the increase in CD64, an M1 marker. IL-34 modulates M2 markers (CD163, CD200R and CD206) alone or in combination with IL-4 and IL-10. No effect of IL-34 was observed in CD14 expression. Overall, IL-34 was able to induce M1 and M2 macrophage differentiation with a specific increase in CD163, an M2 marker.

Figure 5

Macrophages as a theranostic tool. Macrophages can be used as drug delivery systems and as therapeutic targets for anti-inflammatory or anti-tumor therapy. Due to their ability to infiltrate the tumor microenvironment, macrophages can be used as drug vehicles for tumor therapy. Various types of theranostic nanoparticles were developed to simultaneously combine imaging and therapeutic approaches. Different types of electromagnetic radiations were used for functional and anatomical imaging. To favor the phagocytosis of nanoparticles, various methods of coating were proposed, including nanoparticle repolarization. Once macrophages were located in the regions of interest (inflammatory or tumor regions), these irradiation sources were also used to activate nanoparticle elements for PTT (e.g. WO, AuNRs) and delivery of chemotherapy agents (e.g. siRNA, oncolytic viruses). As macrophages markedly contribute to the development of tumors or inflammatory diseases, specific targeting of macrophages leading to their elimination or repolarization was proposed as theranostic tools. Macrophage ablation is based on PTT, ROS therapies or blocking antibodies. Macrophage reprogramming methods are based on the generation of nanoparticles with bacterial or viral elements that activate macrophage TRL signaling and induce the production of pro-inflammatory molecules which reprogram TAMs into a M1 subtype.