Role of Macrophages in Promoting Inflammation and Fibrosis in Systemic Sclerosis

Abstract

Systemic sclerosis (SSc) is a severe autoimmune disease characterized by chronic inflammation, vascular damage and fibrosis. The hallmark clinical manifestation is fibrotic skin thickening; however, the clinical outcome is determined by the extent of internal organ fibrosis. Macrophages, integral to the innate immune system, play a crucial role in phagocytosing invading pathogens and efferocytosis of apoptotic cells, while also contributing significantly to tissue homeostasis and repair. These highly adaptable cells, particularly in the M2-like polarization state, have been associated with a pro-fibrotic environment, implicated in various fibrotic disorders as well as cancer invasion. In SSc, these cells may be dysfunctional, having the potential to produce inflammatory and pro-fibrotic cytokines, recruit other inflammatory cells and stimulate fibroblast differentiation into myofibroblast, thus promoting extracellular matrix (ECM) deposition and fibrosis. Accordingly, we hypothesize that abnormally activated macrophages have a central role in SSc, promoting inflammation and fibrosis, and driving the disease process.

1. Introduction

Systemic sclerosis (SSc) is a severe condition of uncertain etiology, in which dysregulated immune responses are somehow linked to vascular damage, inflammation, autoantibody production, and persisting fibroblast activation, leading to skin and organ-based fibrosis [1]. In SSc, the characteristic clinical manifestation is fibrotic thickening of the skin, which starts at the extremities and then spreads proximally to varying extents and is linked in severity to outcome and survival [2]. Moreover, the clinical outcome is strongly associated with the pattern of internal organ involvement, particularly the development of lung fibrosis (interstitial lung disease, ILD), now the leading cause of excess mortality in SSc [1].

SSc is classified into three main clinical subsets: limited cutaneous SSc (lcSSc), diffuse cutaneous SSc (dcSSc) and SSc sine scleroderma (ssSSc). This classification system is based on the pattern of fibrotic skin involvement. In lcSSc, skin thickening is confined to the extremities and face; this pattern accounting for 50–60% of cases. In contrast, in dcSSc, the thickening of the skin spreads more proximally to involve the upper arms, the torso and potentially affects skin throughout the body. This pattern of disease accounts for 30–40% of cases and is the more severe form, showing early development of visceral organ complications and rapid disease progression [3]. Finally, ssSSc is characterized by the absence of cutaneous manifestations and is identified by typical SSc vascular or organ involvement with positive SSc-associated autoantibody testing [4].

1.1 Epidemiology

The prevalence and incidence of SSc have been reported to vary widely across the globe, showing a pooled prevalence of 176 per million, with a high prevalence in North America (259 per million) and the lowest in Asia (68 per million). Incidence and prevalence are higher in women, with a 5 to 1 female-to-male ratio [5]. The incidence pooled was 14 per million person-years. A study in the United Kingdom showed a prevalence of 307 per million, age being a contributing factor and an annual incidence of 19 per million person-years, 9.7 higher in women. The reported mortality rate was 43.6 per thousand person-years, with risk factors for higher mortality being male sex and age [6].

The standardized mortality ratio for patients suffering from SSc has been reported to be 2.72–5.73. The cumulative survival rates at 5 and 10 years from diagnosis were 85.9 and 71.7% respectively. For the lcSSc subtype, the cumulative survival rates at 5 and 10 years from disease onset are estimated to be 90.9 and 78.2%, respectively. Cumulative survival is significantly lower for dcSSc, with a survival rate of 69.6 and 55.6% at 5 and 10 years [7]. Taken as a whole, these data indicate SSc as a significant healthcare burden which affects more than 2 million individuals worldwide and is linked to significant mortality and disability [6].

1.2 Current therapies

Treatment for SSc is not curative but instead focuses on managing active organ-specific complications. When treating vascular manifestations such as SSc-related pulmonary arterial hypertension (PAH), the objective is to reduce pulmonary arterial vascular resistance through the use of medications like prostacyclin analogues [8], endothelin receptor antagonists [9], and phosphodiesterase-5 inhibitors [10]; treatments which combine vasodilator activity with anti-proliferative effects against vascular smooth muscle cells [11]. Raynaud’s phenomenon (RP) and the associated digital ulcers are addressed with vasodilators such as calcium channel blockers, prostacyclin infusions and topical nitroglycerin. Gastrointestinal issues are managed with proton-pump inhibitors, H2 blockers, and prokinetic agents to ameliorate acid reflux and ulcers [12]. Renal disease is commonly treated with angiotensin-converting enzyme inhibitors to control blood pressure and improve renal plasma flow [13]. Cardiac complications, which may be associated with immune cell infiltration, fibrosis and vasculopathy, are managed using strong immunosuppressive treatments such as cyclophosphamide or rituximab, combined with supportive cardiac care such as anti-arrhythmics and diuretics [14]. Individual treatment plans may vary based on the specific manifestations and severity of the disease.

Aside from organ-specific treatments, immunosuppression has been attempted and demonstrates some effectiveness in reducing skin involvement as measured by the modified Rodnan skin score (mRSS) [15] and lung fibrosis in SSc patients [16]. Commonly used immunosuppressive therapies as a first line of defense include cytotoxic drugs like mycophenolate mofetil, and cyclophosphamide. Recently, biologic agents targeting specific cells or pathways have shown promise in improving skin scores and managing organ complications. Biologics such as rituximab [17] (anti-CD20), tocilizumab [18] (anti-IL-6 receptor), and abatacept [19] (T-cell co-stimulatory modulator) fall into this category. For severe SSc cases with poor prognosis and non-responsive to traditional immunosuppression and other treatments, autologous hematopoietic stem cell transplantation (AHSCT) has been employed [20]. AHSCT has shown significant improvement in skin fibrosis and quality of life. However, it comes with a higher treatment-related mortality rate of 5–10% and an association with early and late malignancies. Therefore, careful consideration and evaluation of risks and benefits are necessary when considering AHSCT as a treatment option for severe SSc cases [21].

2. Macrophages in SSc

Macrophage infiltration in SSc was first observed by Ishikawa and Ishikawa in 1992 [22]. They observed phagocytic cells located between collagen bundles and around the skin adnexa and vessels. Later in 2009, Higashi-Kuwata et al. found an elevated number of CD68, CD163, and CD204 positive cells in SSc skin biopsies [23]. The same group found that the population of CD14+, a marker for monocytes, was significantly increased in PBMC from SSc patients compared to healthy controls [24]. These monocytes co-expressed CD204+ and CD163+, markers also used for macrophage identification, indicating a possible monocyte to macrophage differentiation. Furthermore, CD16+ monocytes were found to be increased in SSc and correlated with the severity of skin fibrosis and pulmonary function [25]. Classical monocytes, expressing CD14+CD16–, derived from hematopoietic stem and progenitor cells, appear to actively migrate into the lung tissue, and differentiate into alveolar macrophages [26], whereas non classical, CD14loCD16+, monocytes are confined to the lung vasculature and preferentially differentiate into intravascular macrophages [27, 28].

In summary, the evidence supports the notion of monocyte differentiation into macrophages in SSc, proposing that monocyte-derived macrophages might play a pivotal role in the pathogenesis of SSc. This highlights a dysregulation in monocyte-macrophage differentiation, potentially contributing to an aberrant macrophage polarization and a possible increase in migration to the fibrotic tissue in SSc. Macrophages are the bridge between innate and adaptive response, these are highly heterogeneous, and have been shown to potentially contribute to all facets of SSc pathogenesis. These cells can be of embryonic origin as well as monocyte-derived [29]. Therefore, in the context of the disease it is thought that monocytes are activated producing an initial inflammatory response, which develops to fibrosis, contributing to the disease. In the following sections we will discuss the involvement of macrophages and monocytes in promoting inflammation and fibrosis in SSc as well as crosstalk to the microenvironment.

2.1 Autoimmunity

SSc patients exhibit a wide range of autoantibodies, with antinuclear antibodies being reported in 85–99% of cases. These antibodies encompass anti-topoisomerase I (ATA), anti-RNA polymerase (ARA), anti-centromere antibodies (ACA), anti-fibrillarin (anti-U3RNP), and anti-ribonucleoprotein (anti-Th/To). The frequency of antinuclear antibodies differs among subsets of SSc and is linked to different clinical manifestations, as seen in Table 1. Other autoantibodies, targeting non-nuclear autoantigens, have been less studied in the context of the disease with poor clinical characterization; these includes anti-endothelial cell antibodies (AECAs) [30], anti-fibroblast, anti-platelet-derived growth factor receptor (PDGFR), anti-fibroblast antibodies, anti-fibrillin-1 antibodies, antibodies against matrix metalloproteinases (MMPs) antibodies, and anti-epithelium antibodies (AEpCA) [31] each of which may contribute to disease pathogenesis. Their mechanisms of action involve inflammation induction, fibroblast activation, collagen accumulation induction, and endothelial cell activation [32].

Autoantibodies	Frequency (%)	Disease subset	Clinical association
Anti-topoisomerase I (Topo I, ATA, SCl-70)	15–24	dcSSc	Interstitial lung diseases; Cardiomyopathy; Renal crisis; Digital ulcers
Anti-RNA polymerase (anti-RNP)	4–20	dcSSc	Esophageal involvement; Renal crisis; Calcinosis cutis; Malignancy; Myositis
Anti-centromere antibodies (ACA)	20–57.8	lcSSc	Long-standing Raynaud’s phenomenon; Pulmonary arterial hypertension
Anti-fibrillarin (anti-U3NP)	4–10	dcSSc	Multiple skeletal manifestations; Renal crisis and cardiac involvement
Anti-Th/To ribonucleoprotein	1–13	lcSSc	Pulmonary fibrosis

Table 1.

Autoantibodies linked to Systemic Sclerosis and clinical association.

It is unclear how autoantibodies contribute to pathogens and their role with macrophages. However, the initial insult to the vascular system might be related to the antigen presenting capabilities in SSc. In other rheumatic diseases such as Systemic Lupus Erythromatosis (SLE), the capabilities of antigen presenting cells including macrophages and dendritic cells have been shown to play a role in the production of autoantibodies [33]. Efferocytosis is involved in the uptake of apoptotic bodies from the environment. In macrophages efferocytosis has been shown to trigger the change from pro-inflammatory macrophages to a wound healing repair subset [34]. Failure of efferocytosis could then lead to apoptotic bodies in the extracellular space and may lead to the release of DNA and DNA associated proteins into the environment, which induce inflammation and also provide a substrate for autoantibody development [35]. In SSc, monocyte-derived macrophages have proven to be defective at clearing T cells, which might lead to the survival of autoreactive T cells [36]. This might be due to failure to trigger adequate response to apoptotic cells, as expression of mer tyrosine kinase (MERTK), a receptor that binds to apoptotic cells, has been shown to be upregulated in SSc compared to controls [37]. Failure to remove apoptotic cells could also take place in germinal zones where macrophages might fail to clear autoreactive B cells. Furthermore, polymorphisms in MHC class genes have been reported widely in SSc [38]. It is possible that certain MHC class II variants lead to an increased affinity for self-antigens, which together with macrophage activation due to vascular damage leads to exposure of self-antigens, activating the adaptive immune system [39].

Another possible explanation is that vascular damage initiates macrophage recruitment and activation. Macrophages secrete ROS and other pro-inflammatory cytokines that could trigger oxidations of autoantigens, making them more susceptible to binding to MHC class II or creating neoepitopes and triggering activation of self-reactive T-cells and B-cells [40]. In addition, SSc plasma has elevated IL-6 and other inflammatory mediators [41, 42] and SSc monocytes have been shown to secrete elevated levels of B activator factor [43], which might combine to induce B and T cell activation, altogether suggesting a role of macrophages in contributing towards autoimmunity through T cell activation and promoting autoantibody formation in SSc.

2.2 Vasculopathy

The most frequent clinical manifestation of vascular damage in systemic sclerosis is Raynaud’s phenomenon, which is one of the major criteria for the diagnosis of the disease, alongside renal crisis and other vascular disease. It is thought that an initial insult such as viral infection or exposure to toxins, triggers endothelial cell activation, initiating the disease in genetically susceptible individuals. This, coupled to autoantibody driven endothelial cell dysfunction, leads to a vascular profile characteristic of limited systemic sclerosis or in some patients triggering an early inflammatory profile which can progress towards diffuse cutaneous system sclerosis [44]. Endothelial cell apoptosis and altered expression of adhesion molecules and cytokines might lead to the release of reactive oxygen species (ROS), resulting in continuous microvessel damage [45]. Serum levels of vascular dysfunction markers have been shown to correlate with the early stages of SSc [46]. ROS production and microvasculature damage promote inflammation, leading to the recruitment of immune cells.

One possible explanation for monocyte activation mediated by autoantibodies could be the release of topoisomerase I from apoptotic endothelial cells. This release might be triggered by the initial vascular damage and inflammation characteristic of the disease. Once released, topoisomerase I binds to heparin sulphate proteoglycans and CCR7 receptors expressed on the surface of fibroblasts. This binding event recruits ATA, forming immunocomplexes (IC) that subsequently trigger monocyte adhesion and activation [47]. Accumulation of apoptotic bodies, and ICs feeds inflammation which in turn polarizes macrophages towards a pro-inflammatory phenotype. In SSc, FCGR3A positive monocytes and macrophages have been shown to be expanded [48, 49]. These cells were reported to express higher inflammatory markers (ERE, S100A8, and S100A9), and to be enriched for interferon signature genes. In an SSc-related ILD study, a higher percentage of mixed M1/M2 macrophage subset was found; specifically two populations expressing CD163+, D206+, TLR4+, CD80+, CD86+ and either CD204+ or CD14+ were found in ATA positive SSc patients [50]. Further experiments need to be performed to assess whether the single cell populations and those previously reported using flow cytometry are the same, however, it suggest a link between activated macrophages and organ fibrosis in the disease.

Monocytes patrol the blood vessels and transvasate to the tissue of injury where they differentiate to monocyte-derived macrophages [51]. In SSc, CD52, an antiadhesion molecule, has been found to be decreased in monocytes. Decreased CD52 expression was linked to enhancement of β2 integrin CD11b and CD18 complex which was observed to be upregulated in SSc. Furthermore, an increased level of VEGF165b, a VEGF isoform generated by alternative splicing in VEGF mRNA, has been shown to be elevated in SSc skin [52]. In macrophages, binding of VEGF165b to VEGFR-2 induces an antiangiogenic M1-like phenotype that directly impairs angiogenesis [53]. It is possible that vascular damage and pro-inflammatory macrophage activation occurs in a positive feedback loop failing to repair vasculature, leading vascular leakage and ultimately to vascular remodeling. M1 macrophages thus feed inflammation, and vascular damage. In an attempt to resolve inflammation, M2 macrophages secrete remodeling factors, including transforming growth factor beta (TGF-β), CCL2, IL-10 and clear debris. In SSc, genes for M1 and M2 macrophages have been shown to be enriched, including CD68, COL6A1, CXCL1, CXCL2, HLA–DRB4, IL6ST, PLAU, S100A8, SERPINE2, SERPINH1, and WDFY4 [54]. This dual macrophage signature, could be due to the presence of both M1 and M2 populations, failing in homeostasis, or a mixed M1/M2 signature where macrophages fail to polarize towards a resolving phenotype. The mixture of macrophage factors will eventually lead to activation of fibroblasts and fibrosis.

2.3 Fibrosis

Organ fibrosis, reviewed in [55], is the distinguishing hallmark of SSc. Successful wound repair that leads to localized scarring, is a transient, non-chronic process that terminates via a resolution phase, leading to the rapid restoration of local tissue integrity. However, continuing insult or inflammation results in persistently high levels of cytokines, growth factors and locally destructive enzymes such as collagenases. This process leads to persistent fibrosis and the replacement of normal tissue architecture with compact, mechanically stressed, and rigid connective tissue. Fibrogenesis is characterized by excessive ECM deposition, defective crosslinking, and poor ECM degradation. Fibroblasts and myofibroblasts produce ECM components: collagen, fibronectin, elastin, and matrix glycoproteins. These molecules interlink with each other, giving the ECM its chemical and mechanical properties. In SSc, genetic and epigenetic variations affecting fibroblast function and ECM protein expression, altering normal connective tissue structure [56]. This is exacerbated by fibroblast to myofibroblast differentiation introduced through ischaemia and local cytokine tissue environments. Additionally, the new pathogenic ECM in SSc is characterized by increased stiffness, which triggers fibroblasts to myofibroblast differentiation in a positive feedback loop via mechano-transduction [57].

As described previously, macrophages play a key role in promoting fibrosis; they feed inflammation, leading to vascular leakage and promoting immune cell recruitment, secrete pro-fibrotic mediators, activating fibroblasts, and respond to tissue stiffness in a positive feedback loop. TGFβ is a pleiotropic cytokine known to activate fibroblasts, induce endothelial-to-mesenchymal transition and enhance stem cell recruitment. Wound-healing macrophages secrete growth factors such as TGFβ, platelet derived growth factor (PDGF), fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF) and insulin-like growth factor 1 (IGF-1), promoting cellular proliferation, fibroblast differentiation to myofibroblast and angiogenesis [58]. Nintedanib, a treatment licensed for interstitial lung disease in SSc, targets tyrosine kinases inhibiting PDGF, FGF, VEGF andCSF1R, which in macrophages has been shown to decrease pro-inflammatory markers including CD204, CD206, CD163 and MERTK and to reduce TGFβ secretion. Overall these data are indicating a potential role for macrophage-fibroblast activation through the secretion of TGFβ, thus promoting fibrosis [37].

Macrophages regulate wound healing by producing MMPs and tissue inhibitor of metalloproteinases (TIMPs) controlling ECM turnover; they engulf and digest dead cells, debris, and various ECM components. Activation by TGFβ leads to the heterodimerization of TGFβRII with TGFβRI, which is also known as activin receptor-like kinase 5 (ALK5). This interaction enables downstream canonical TGFβ signaling. The process involves the phosphorylation of SMAD2/3 and the translocation of phosphorylated SMAD2/3 to the nucleus, where it activates gene transcription. Research has shown that the deletion of Alk5 in monocytes inhibits proinflammatory markers while promoting anti-inflammatory macrophage markers.

Furthermore, macrophages engage in crosstalk with T and B cells to modulate immune responses. Adaptive immune cells are linked to autoimmunity in SSc, T cells and B cells interact with macrophages to exacerbate fibrosis. In mice, B cell depletion using Rituximab, has been shown to decrease M2 polarization and fibrosis and to be linked to decreased IL-6 production by B cells. T cells from SSc have been shown to secrete elevated IL-4, which polarizes macrophages towards an M2 pro-fibrotic phenotype. Similarly, T cell derived IL-4 has been shown to increase GM-CSF secretion from B cells [59]. Furthermore, Th2 cytokines including IL-4 and IL-13 have been shown to have increased levels in the serum of SSc patients compared to controls [59]. These cytokines bind to the IL4Rα and IL13Rα1 heterodimer Type II receptor. Receptor activation leads to downstream phosphorylation of STAT6 and subsequent transcription to the nucleus. Both STAT6 and SMAD2/3 signaling lead to M2 macrophage polarization and subsequent fibroblast activation. Although further research is needed to elucidate crosstalk with the adaptive immune system, there is evidence implicating this crosstalk in the pathogenesis of the disease.

Under normal wound healing conditions, macrophages orchestrate clearance of apoptotic cells and the regulation of fibroblast activation. In fact, a 3D co-culture experiment using primary human macrophages and myofibroblast has shown a decrease in myofibroblast activity dependent on the number of co-cultured macrophages [60], highlighting the essential role of healthy macrophages in modulating fibrosis. In SSc, fibroblast differentiation into myofibroblast and the consequent production of ECM products leads to tissue loss of function and tissue stiffness [1]. Macrophage activation in SSc contributes to fibrosis through the secretion of profibrotic mediators such as TGFβ and matrix remodeling proteins [61]. Additionally, CXCL10, CCL19 and CCL7 (MCP3) chemokines have been observed to be upregulated in SSc, with CCL19 being expressed adjacent to CD163 macrophages [39]. In addition, SSc macrophages have been shown to have an increased level of CCL2, TGF-β, and IL-6 [54]. This increase is further exacerbated by autologous plasma stimulation, indicating a possible systemic macrophage activation that might lead to continuing macrophage induction. Furthermore, a higher proportion of CD163 and CD204 positive macrophages have been reported in blood and skin biopsies from SSc patients. This provides evidence for the recruitment of macrophages and monocytes to the tissue in SSc. The phenotype of these macrophages seems to be disrupted; this disruption is probably the end result of genetic and epigenetic modifications which lead to silencing or overexpression of proteins mediating fibrosis. FLI1, a member of the Ets family of transcription factors, is expressed in endothelial cells, fibroblasts and immune cells and has been reported to be decreased in SSc skin. FLI1 has been identified as one of the factors orchestrating changes seen in SSc myeloid cells. Depletion of FLI1 in myeloid cells has shown to induce CCL2 and CCL7 gene expression [62]. The depletion was also correlated with the upregulation of CXCL10, and CXC11 chemokines and MMP12 and HMOX1, stress inducible protein. Furthermore, macrophages from silenced FLI1 induced collagen production in vitro.

Accordingly, macrophage-directed fibroblast activation might be the result of different pathways signaling cascades. A recent study has shown that CXCL4 macrophage stimulation leads to an increase secretion of PDGF-B protein, which has been shown to act on fibroblasts, inducing the expression of pro fibrotic and pro-inflammatory cytokines. Furthermore, fibroblasts treated with macrophage culture supernatants after CXCL4 activation have been shown to have an increased production of collagen and fibronectin. Furthermore, co-culture of SSc macrophages and fibroblast increased macrophage TNF/NFκB signaling and increased the expression of interferon response and αSMA in fibroblasts [54]. Additionally, SSc macrophages-fibroblast cocultures have been shown to induce expression of CCL2, IL-6, MMP1, CCL5, CXCL10, G-CSF, and IL-1Ra [63]. In SSc, this regulation seems to be disrupted, since macrophage secretion of IL-10 seemed to act in a paracrine way, inducing myofibroblast differentiation. Fibroblast activation can feedback and further drive macrophage activation. Exosomes from SSc fibroblast, have been shown to activate macrophages increasing levels of IL-6, IL-10, IL-12p40 and TNF. This further exacerbates fibrosis by activating fibroblasts and inducing collagen and fibronectin deposition. In addition to paracrine stimulation, macrophage and fibroblasts crosstalk might be due to cell-to cell contact. Cadherin, a transmembrane adhesion molecule expressed in SSc skin macrophages, has been shown to create a contact bridge between macrophages and fibroblasts and induce TGFβ signaling between latent TGFβ-producing macrophages and myofibroblasts [64].

As a consequence of the alteration to matrix composition and myofibroblast hyperactivation in SSc, stiffness of ECM increases dramatically at fibrotic niches. The young’s modulus of the skin increases from 4 to 14 KPa in healthy controls to 50–80 KPa in SSc patients in the forearm, chest and abdomen [65]. During wound healing, matrix stiffness also increases due to the deposition of the provisional ECM. In rat models of tissue fibrosis, the Young’s modulus of skin tissue increases from 1 to 4 KPa up to 20 KPa in granulation tissue. Taken together, these data implicate increased ECM stiffness as a contributory factor in the failure of resolution in the pathogenesis of SSc [66]. Mechanical properties of the ECM have been shown to affect cellular function. Increased expression of MRTF signaling network associated with mechanosensing, has been reported in skin from SSc patients [67]. Macrophages express MRTF-A and are able to sense stiffness and adapt to the mechanical properties [68]. The response to mechanical stimuli has been shown to play a role in cancer. A similar role of macrophages and mechanosensing might feedback in SSc in response to fibrosis [69]. In vitro mechanosensing properties of macrophages have been studied using hydrogels, water-swollen fibrillary three-dimensional (3D) networks where collagen type I is the major component. The adhesion of macrophages to soft ECM hydrogels has been shown to inhibit inflammatory activation compared to adhesion to stiff glass or polystyrene culture dishes (~GPa) and is dependent on RhoA kinase signaling [68]. MRTF-A is a transcription factor downstream of RhoA signaling; its inhibition could affect stiffness mediated macrophage pathogenesis in fibrosis and cancer. In fact, in macrophages, MRTF-A, MRTF-B, and SRF regulate cytoskeletal gene expression programs and appear to promote macrophage function, with specificity for proinflammatory macrophages. MRTF-A has also been found to recruit members of the H3K4 methylation complex of inflammatory promoters and activate pro-inflammatory transcription [70].

3. CD206 macrophages as therapeutic targets

CD206 is a mannose scavenger receptor member of the group 6 C-type lectin receptor family. It has eight cysteine-rich domains, which allow it to bind to glycans, monosaccharides and fibronectin type II. It is expressed on the surface of monocytes, macrophages, endothelial and dendritic cells and is encoded by the MRC1 gene. It functions as a pattern recognition receptor, triggering immunity, although it is not clear which domains or ligands trigger this response. The short cytoplasmic C-terminal domain has a di-aromatic motif that regulates its clathrin-dependent internalization and recycling [71]. Macrophages expressing CD206 have been shown to produce high levels of IL-10, IL-6, TNFα and TGFβ1 and to induce endocytosis. Genetic ablation of CD206 in mice leads to the accumulation of multiple glycoproteins in plasma. In SSc, cells positive for CD86, CD163 and CD206 have been shown to be increased in skin, heart and lung biopsy specimens, and CD206 to be overexpressed in SSc monocytes [54, 72, 73]. A recent study using RP832c, a therapeutic 10 amino acid peptide which targets CD206, has been shown to repolarize macrophages from M2 to M1 signature, suggesting a role of CD206 in the profibrotic macrophage signature [74]. Using the bleomycin mouse model, researchers showed a decrease in fibrosis when injecting RP-832c subcutaneously [74]. Altogether, there is evidence of a potential therapeutic approach targeting macrophages for the treatment of the disease. Further research during clinical trials is needed to evaluate the safety and effectiveness of these approaches in modulating fibrosis in patients (Figure 1).

Figure 1.

Schematic summary of proposed role for activated macrophages in promoting inflammatory fibrosis in SSc.

4. Concluding remarks

This chapter has examined the intricate pathophysiology of SSc, a disease marked by severe fibrosis, immune dysregulation, and extensive vascular damage leading to significant clinical heterogeneity and morbidity. We have delineated the clinical subsets of SSc, its global epidemiology, and the current therapeutic approaches, emphasizing the complexity of its management. Particularly noteworthy is the role of macrophages in SSc progression, suggesting that targeting macrophage differentiation and function could offer new therapeutic avenues. The discussion on autoimmunity and vasculopathy in SSc provides further insight into the potential mechanisms driving the disease, presenting opportunities for innovative research aimed at disrupting these processes.

As we look to the future, it is clear that advancing our understanding of the cellular and molecular mechanisms underpinning SSc, especially the roles of macrophages and the specific autoantibodies involved, will be crucial. This could lead to the development of more effective, targeted therapies that not only alleviate symptoms but also potentially alter the disease course. Continued interdisciplinary research and collaboration will be essential in tackling the complexities of SSc, with the ultimate goal of enhancing patient outcomes and providing a framework for more personalized treatment strategies.

Acronyms and abbreviations

ACA	anti-centromere antibody
AECAs	anti-endothelial antibodies
AHSCT	autologous hematopoietic stem cell transplantation
ALK5	activin receptor-like kinase 5
anti-Th/To	anti-RNAse p and mitochondrial RNAse ribonucleoprotein complexes antibodies
anti-U3NP	anti-U3-RNP/fibrillarin antibody
ARA	anti-RNA polymerase III antibody
ATA	anti-topoisomerase antibody
dcSSc	difuse cutaneous systemic sclerosis
ECM	exctracellular matrix
FGF	fibroblast growth factor
IC	immune complex
IGF-1	insulin-like growth factor 1
ILD	interstitial lung disease
lcSSc	localized cutaneous systemic sclerosis
MCP3	monocyte chemotactic protein 3
MERTK	mer tyrosine kinase
MMPs	matrix metalloproteinases
mRSS	modified Rodnan skin score
MRTF-A	myocardin related transcriptor factor a
PAH	pulmonary arterial hypertension
PDGF	platelet derived growth factor
PDGFR	platelet derived growth factor receptor
ROS	reactive oxygen species
RP	Raynaud’s phenomenon
SLE	systemic lupus erythematosus
SSc	systemic sclerosis
ssSSc	sine systemic sclerosis
TGF-β	transforming growth factor beta
TIMPs	tissue inhibitor of metalloproteinases
VEGF	vascular endothelial growth factor