Fibrocytes
Highlights:
Circulating bone marrow–derived fibrocytes are recruited to the wound site after injury.
They contribute to wound healing and fibrosis by producing ECM, secreting cytokines including TGF-β, and presenting antigen to T cells.
They regulate fibroblasts and promote myofibroblast differentiation (Fig. 6-3).
Previously, Bucala has identified an adherent and proliferating population of cells with a fibroblast-like morphology that expresses a variety of biomarkers including some hematopoietic cell markers, mesenchymal cell markers, chemokine receptors, antigen- presenting cell markers, and others.74–77 They make up 0.5% of peripheral blood leukocytes, but can constitute 10% of cells infiltrating acute wounds.76 The chemokines CCL12 and CXCL12 and secondary lymphoid chemokines and their receptors CCR2, CXCR4, and CCR7 are involved in fibrocyte migration in vivo.78 These migrating fibrocytes are capable of synthesizing ECM proteins, proteases including collagenase, and growth factors such as TGF-β1, tumor necrosis factor alpha (TNF-α), interleukin (IL)-6 and IL-10, but can also present antigens and thereby prime naïve T lymphocytes.79,80 It has been demonstrated that fibrocytes differentiate into fibroblasts and myofibroblasts via in vivo and in vitro experiments.81–82
Fibrocytes have been identified in burn patients from the peripheral blood mononuclear cells (PBMCs), where the percentage of type I collagen–positive fibrocytes was significantly higher (up to 10% of PBMCs) than for control individuals (normal level <0.5%) and correlated with serum levels of TGF-β.83,84 In vitro, fibrocytes can be cultured from CD14+ PBMCs, but required TGF-β in the conditioned media from CD14− PBMCs for differentiation.85 Leukocyte-specific protein 1 (LSP-1) is a unique marker for fibrocytes and is upregulated in burn patients, remaining stable through differentiation.84,85 Double staining with antibodies to LSP-1 and the C- propeptide of type 1 collagen (COL-I) has identified a 300% increase in fibrocytes in HTS tissue, located primarily in deeper layers of the papillary dermis (Fig. 6-4).84 Characteristic morphologic alterations in fibrocytes occur in vitro after exposure to
endotoxin, which are corrected by treatment with IFN-α2b.85–87
From serial analysis of burn patients with HTS, increased numbers of fibrocytes are present in HTS tissues compared to mature scars and normal skin.86–88 Quantitatively, fibrocytes produce less collagen than HTS fibroblasts; however, fibrocytes from burn patients differ from that of normal individuals because of their paracrine effects that include stimulating dermal fibroblasts to proliferate, production and contraction of the ECM, and producing TGF-β and its downstream effector, CTGF.87,88 These findings resemble others,89–92 where the principal source of collagen in other fibrosis models appears to be local fibroblasts. However, bone marrow–derived immune cells resembling fibrocytes persist in the matrix, suggesting an important paracrine role of fibrocytes in HTS and other FPD. It is possible to antagonize many of these fibrogenic effects of fibrocytes in vitro with IFN-α; significantly decreased numbers of fibrocytes were also found in the tissues of burn patients in response to systemic IFN treatment in vivo and were associated with a concomitant resolution of fibrosis and scar remodeling.88 In addition, increased angiogenesis associated with increased vascular endothelial growth factor (VEGF) in HTS is reduced by IFN-α, in part because of suppression of endothelial cell proliferation and tubule formation through reduction in VEGF receptor expression in endothelial cells.87 Coexpression of VEGF mRNA with the stromal cell–derived factor 1 (SDF-1) mRNA further implicates fibrocytes in the pathophysiology of idiopathic pulmonary fibrosis and other fibroses.92
FIGURE 6-3 Biomarkers and bioactivities of fibrocytes. Fibrocytes express a variety of biomarkers including hemotopoietic cell markers, mesenchymal cell markers, chemokine receptors, antigen-presenting cell markers, and others. After injuries, circulating fibrocytes are recruited to the wound site and are involved in wound healing or scar formation by producing ECM, secreting cytokines, presenting antigen to T cells, regulating local fibroblasts, or directly differentiating to fibroblasts and myofibroblasts.
FIGURE 6-4 Dual immunofluorescent labeling of fibrocytes in scar tissue. Cryosections of (A–C) hypertrophic scar (A–C) and mature scar (D–F) were stained for type I collagen (A and D in red color) and leukocyte-specific protein 1 (LSP-1) (B and E in green color). The colocalization of the two molecules displayed a yellow-colored outline of the fibrocytes (C and F, arrows). Original magnifications × 100. (From Yang L, Jiao H, Shankowsky HA, et al. Identification of fibrocytes in post-burn hypertrophic scar. Wound Repair Regen. 2005;13(4):398– 404.)
Heterotopic ossification (HO) is a clinical condition where mature lamellar bone is formed in nondamaged tissues such as muscle, tendon, and fascia, particularly after burns and traumatic injuries.93,94 HO can lead to skin breakdown, significant soft tissue deformity, joint ankylosis, and chronic pain that can prolong rehabilitation. In burn patients, the incidence of HO varies between 0.2% and 4%,93 and is more frequent in patients with extensive burns (>20% TBSA). Although HO may occur in joints unrelated to burn injuries, lesions may develop under areas of deep burns complicated by HTS, especially in the elbow,95,96 and it is associated with prolonged loss of consciousness, mechanical ventilation, long-term immobilization, burn wound infection and/or delayed closure, loss of skin grafts, and recurring local trauma including passive range of motion.95–98 Therapeutic strategies for the prevention of HO are of limited success and include local radiation, but concerns of long-term side effects of radiation, including the development of secondary malignancies, stress the need for better animal models to develop and adequately test novel therapies before application to patients.99
Recently, a large population of fibrocytes (LSP-1+ COL-I+) have been identified within HO specimens as distinctive blood-borne cells that traffic to injured and apparently noninjured tissues and interact with resident cells.100 Fibrocytes have the potential to differentiate into osteoblasts and chondrocytes101 and can be reprogrammed into antifibrotic cells stimulating MMP-1 production in dermal fibroblasts, collagen breakdown, and scar remodeling.102 Thus, HO and FPD such as HTS have common features and appear to be causally related. In this concept, after significant initial local tissue injury generates a systemic inflammatory response, unique PBMCs including fibrocytes contribute to the development of fibrosis and osteogenic matrix in injured and noninjured tissues in as yet unidentified mechanisms.
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