Fibrocytes are a subpopulation of mesenchymal progenitor cells, which were first described in 1994 as fibroblast-like, peripheral blood cells that enter sites of tissue invasion or injury. Fibrocytes are unique in their expression of extracellular matrix proteins (collagens) together with myeloid (CD45) and hematopoietic progenitor markers (CD34). Fibrocytes contribute to the innate response to injury and tissue remodeling. They serve as antigen-presenting and immunomodulatory cells; they produce a distinct profile of cytokines, chemokines and growth factors that contributes to repair and tissue remodeling; and they mediate fibrogenesis in a number of systemic and organ-specific fibrosing disorders. Regulatory networks involving serum amyloid P (SAP), immune cytokines and chemokines and T cells emphasize the role of fibrocytes in an integrated host response to injury. The therapeutic targeting of fibrocytes now holds promise for the augmentation of wound repair and the treatment of different fibrosing disorders.
Fibrocytes were first identified in a murine model of wound repair that relied on the sampling of exudate cells in wound chambers surgically implanted into subcutaneous tissues.1 A flow cytometric examination of peripheral blood cells within 2 days of wound chamber implantation revealed a unique population of CD34+ and collagen I (Col I)+ cells. The coincident expression of the hematopoietic progenitor marker, CD34, together with connective tissue collagen thus led to the naming of these circulating, spindle-shaped cells as ‘fibrocytes’. An abundance of information regarding fibrocyte biology has since been acquired from both experimental murine and human clinical investigations. Fibrocytes now are appreciated as participating in normal and aberrant wound repair, including hypertrophic scars and keloids, airway remodeling in asthma, interstitial pulmonary fibroses, the systemic disorders scleroderma and nephrogenic systemic fibrosis, atherosclerosis, the stromal response to tumor invasion and Graves ophthalmopathy.2,3
Fibrocytes express a unique cytokine and chemokine profile distinct from monocytes, dendritic cells, skin Langerhans cells, T lymphocytes, fibroblasts, endothelial cells and epithelial cells. By scanning electron microscopy, fibrocytes also exhibit unique cytoplasmic extensions that are intermediate in size between microvilli and pseudopodia. Fibrocytes comprise ∼0.5% of leukocytes in the peripheral blood, and they can be observed to differentiate from CD14+ monocytes in culture into a phenotype with wound-healing potential.4
In model studies, fibrocytes migrate to wounds in response to secondary lymphoid chemokine (SLC), which is the ligand for CCR7.4 Expression profiling also has revealed other chemokine receptors on the surface of fibrocytes, such as CCR2, CCR3, CCR5, CCR7 and CXCR4, which can effect specific trafficking responses in the context of different mouse models of disease or acute injury.5–7
The CD34 hematopoietic marker expressed by fibrocytes in circulation and during near-term culture is down-regulated over time. This decrease in CD34 occurs in concert with the increased expression of prolyl-4-hydroxylase, an enzyme that is necessary for the stabilization of the collagen triple helix.8 The leukocyte common antigen (CD45) shows more robust expression than CD34 but it also down-regulated in situ as fibrocytes differentiate and mature.9 The pro-fibrotic cytokine TGF-β1 accelerates fibrocyte differentiation into cells that appear phenotoypically similar to mature fibroblasts and myofibroblasts.4,10
Fibrocytes express surface proteins required for antigen presentation, including the class II major histocompatability complex molecules and the costimulatory molecules CD80 and CD86 that are capable of activating naïve T cells.11,12 Fibrocytes exhibit a potent antigen-presenting capability, indicating their likely role in the initiation of immunity during injury, wound repair and in fibrotic responses associated with inflammation, such as granulomas and scleroderma.
Wound closure is an essential part of wound healing in which myofibroblasts play an active role. Fibrocytes have been shown to express alpha smooth muscle actin (α-SMA) and to contract collagen gels in vitro, revealing their potential to differentiate into myofibroblasts and contribute to wound contraction.4 Indeed, fibrocytes also were found to differentiate into a myofibroblast phenotype in vivo and express α-SMA, thereby confirming that these circulating cells contribute to a subset of myofibroblasts in wounds and are integral mediators of wound healing.9
The regulation of fibrocytes is a dynamic process that appears to be influenced by many soluble and cell-based mediators. The profibrotic cytokines, IL-4 and IL-13, along with platelet-derived growth factor (PDGF) promote the differentiation of CD14+ precursors to fibrocytes.13 Furthermore, the pro-inflammatory cytokines IL-1β, IL-12 and IFN-γ, along with SAP immune complexes, inhibit the maturation of CD14+ precursors into fibrocytes.14–16 The differentiation of fibrocytes to more mature connective tissue cells is not only stimulated by TGF-β1 but also with endothelin-1 (ET-1), a peptide that plays an important role in vascular homeostasis.10
Recent studies implicating SAP, leukocyte-specific protein 1 (LSP1) and adenosine A2A receptors in the regulation of fibrocyte have also provided promising therapeutic potential.15,17,18 SAP, which is a member of the pentraxin family of proteins, inhibits fibrocyte differentiation from monocyte precursors by a mechanism that may involve ITIM signaling, and it is under active clinical evaluation for the treatment of fibrosis.19 In a recent study, it has been shown that the administration of exogenous SAP, either locally or systemically, compromises dermal wound healing.20 SAP-treated wounds showed a decreased rate of wound closure and a decreased number of myofibroblasts at 7 days post wounding.
First evidence for a role for circulating fibrocytes in the tissue remodeling response of the lung was provided by the group of Sabrina Mattoli, who found evidence for the presence of CD34+, collagen-producing cells in the subepithelial areas of the bronchial mucosa of patients with allergic asthma.10 By tracking labeled fibrocytes in a mouse model of this disease, fibrocytes were observed to be recruited into the bronchial tissue following allergen exposure and to differentiate into contractile myofibroblasts. Further work in models of lung injury also have shown the entry of fibrocytes in response to discrete chemokine signals into areas of inflamed lung, with persistent collagen production and further differentiation into myofibroblasts.5–7
Strieter and colleagues explored the potential contribution of fibrocytes to human fibrotic interstitial lung disease by examining pulmonary tissue and peripheral blood from patients with usual interstitial pneumonia (UIP) and fibrotic, non-specific interstitial pneumonia (NSIP) for the expression of the fibrocyte-attracting chemokine, CXCL12, and for circulating CXCR12 receptor-positive fibrocytes.21 Enhanced expression of CXCL12 in both the lungs and plasma of patients with fibrotic lung disease was observed. Notably, elevated CXCL12 levels were associated with higher numbers of fibrocytes in the peripheral blood of these patients. Most of the circulating fibrocytes were negative for the myofibroblast marker α-SMA, suggesting a relatively undifferentiated phenotype. These observations provided strong evidence that fibrocytes are involved in the pathogenesis of human lung fibrosis and may represent a novel circulating biomarker.
This concept was confirmed and extended by Moeller et al.,22 who quantified fibrocytes by co-expression of CD45 and collagen-I in patients with idiopathic pulmonary fibrosis during acute exacerbation of the disease. Fibrocytes were significantly elevated in patients with stable IPF when compared to controls, with a further increase noted during acute disease exacerbation. Fibrocyte numbers were not correlated with lung function or radiologic severity scores, but they were an independent predictor of early mortality. The mean survival of patients with fibrocyte levels that were higher than 5% of the total blood leukocytes was 7.5 months compared with 27 months for patients with <5%. These data support the conclusion that circulating fibrocytes are an indicator of disease activity in IPF and might be useful as a clinical marker for disease progression. An elevation in the circulating level of fibrocytes also has been shown in scleroderma patients with interstitial lung disease.23 Intriguingly, a relative elevation in the circulating concentration of fibrocytes was observed in aged vs. healthy controls, suggesting that these cells may exert a physiological role in aging, perhaps in the maintenance of tissue homeostasis.
Fibrosis of the lung, kidney and other organs can occur as an inexorable sequela of several autoimmune diseases. In this regard, the role of T cells in the immunopathogenesis of fibrosis is of high clinical interest. That T cells might have a role in fibrocyte maintenance or differentiation has long been suspected by the observation that during in vitro culture, peripheral blood T cells may persist for several days in close association and cell–cell contact.4 Niedermeier et al.24 recently reported that the development of fibrocytes from a CD11b+ CD155+ Gr1+ monocyte subpopulation is dependent on the control of CD4+ T cells. Immunologic activation of CD4+ T cells induced the release of IL-2, TNFα, IFN-γ and IL-4, which prevented the differentiation and outgrowth of fibrocytes. Using a clacineurin inhibitor, activation of CD4+ T cells led to significantly increased outgrowth of fibrocytes and renal deposition of collagen I. Thus, the differentiation of fibrocytes may critically be dependent on the activation state of CD4+ T cells, which may either support or block the development and differentiation of fibrocytes. These data also suggest potential immunopharmacological approaches for ameliorating the pathologic progression of interstitial lung disease, glomerulonephritis and scleroderma.
Integrated scheme for the influence of different mediators on fibrocyte differentiation and trafficking. Information compiled from various references.13–16,24–26
In summary, a more detailed understanding of how fibrocytes differentiate and mobilize to tissues, and the trafficking signals they utilize is clearly warranted. There are currently no clinically effective therapies for fibrosing diseases, and the therapeutic targeting of fibrocytes or their progenitors may prove useful not only for preventing pathologic fibrosis but for augmenting wound repair.