Fibrocytes in health and disease

Fibrocytes, a group of bone marrow-derived mesenchymal progenitor cells, were first described in 1994 as fibroblast-like, peripheral blood cells that migrate to regions of tissue injury. These cells are unique in their expression of extracellular matrix proteins concomitantly with markers of hematopoietic and monocyte lineage. The involvement of fibrocytes and the specific role they play in the process of wound repair has been a focus of study since their initial description. Fibrocytes contribute to the healing repertoire via several mechanisms; they produce a combination of cytokines, chemokines, and growth factors to create a milieu favorable for repair to occur; they serve as antigen presenting cells (APCs); they contribute to wound closure; and, they promote angiogenesis. Furthermore, regulatory pathways involving serum amyloid P, leukocyte-specific protein 1, and adenosine A2A receptors have emphasized the significant role that fibrocytes have in wound healing and fibrosis. The therapeutic targeting of fibrocytes holds promise for the augmentation of wound repair and the treatment of different fibrosing disorders

Fibrocytes are associated with vascular and parenchymal remodelling in patients with obliterative bronchiolitis

<p>Abstract</p> <p>Background</p> <p>The aim of the present study was to explore the occurrence of fibrocytes in tissue and to investigate whether the appearance of fibrocytes may be linked to structural changes of the parenchyme and vasculature in the lungs of patients with obliterative bronchiolitis (OB) following lung or bone marrow transplantation.</p> <p>Methods</p> <p>Identification of parenchyme, vasculature, and fibrocytes was done by histological methods in lung tissue from bone marrow or lung-transplanted patients with obliterative bronchiolitis, and from controls.</p> <p>Results</p> <p>The transplanted patients had significantly higher amounts of tissue in the alveolar parenchyme (46.5 ± 17.6%) than the controls (21.7 ± 7.6%) (p < 0.05). The patients also had significantly increased numbers of fibrocytes identified by CXCR4/prolyl4-hydroxylase, CD45R0/prolyl4-hydroxylase, and CD34/prolyl4-hydroxylase compared to the controls (p < 0.01). There was a correlation between the number of fibrocytes and the area of alveolar parenchyma; CXCR4/prolyl 4-hydroxylase (p < 0.01), CD45R0/prolyl 4-hydroxylase (p < 0.05) and CD34/prolyl 4-hydroxylase (p < 0.05). In the pulmonary vessels, there was an increase in the endothelial layer in patients (0.31 ± 0.13%) relative to the controls (0.037 ± 0.02%) (p < 0.01). There was a significant correlation between the number of fibrocytes and the total area of the endothelial layer CXCR4/prolyl 4-hydroxylase (p < 0.001), CD45R0/prolyl 4-hydroxylase (p < 0.001) and CD34/prolyl 4-hydroxylase (p < 0.01). The percent areas of the lumen of the vessels were significant (p < 0.001) enlarged in the patient with OB compared to the controls. There was also a correlation between total area of the lumen and number of fibrocytes, CXCR4/prolyl 4-hydroxylase (p < 0.01), CD45R0/prolyl 4-hydroxylase (p < 0.001) and CD34/prolyl 4-hydroxylase (p < 0.01).</p> <p>Conclusion</p> <p>Our results indicate that fibrocytes are associated with pathological remodelling processes in patients with OB and that tissue fibrocytes might be a useful biomarker in these processes.</p

Altered monocyte and fibrocyte phenotype and function in scleroderma interstitial lung disease: reversal by caveolin-1 scaffolding domain peptide

Interstitial lung disease (ILD) is a major cause of morbidity and mortality in scleroderma (systemic sclerosis, or SSc). Fibrocytes are a monocyte-derived cell population implicated in the pathogenesis of fibrosing disorders. Given the recently recognized importance of caveolin-1 in regulating function and signaling in SSc monocytes, in the present study we examined the role of caveolin-1 in the migration and/or trafficking and phenotype of monocytes and fibrocytes in fibrotic lung disease in human patients and an animal model. These studies fill a gap in our understanding of how monocytes and fibrocytes contribute to SSc-ILD pathology. We found that C-X-C chemokine receptor type 4-positive (CXCR4+)/collagen I-positive (ColI+), CD34+/ColI+ and CD45+/ColI+ cells are present in SSc-ILD lungs, but not in control lungs, with CXCR4+ cells being most prevalent. Expression of CXCR4 and its ligand, stromal cell-derived factor 1 (CXCL12), are also highly upregulated in SSc-ILD lung tissue. SSc monocytes, which lack caveolin-1 and therefore overexpress CXCR4, exhibit almost sevenfold increased migration toward CXCL12 compared to control monocytes. Restoration of caveolin-1 function by administering the caveolin scaffolding domain (CSD) peptide reverses this hypermigration. Similarly, transforming growth factor β-treated normal monocytes lose caveolin-1, overexpress CXCR4 and exhibit 15-fold increased monocyte migration that is CSD peptide-sensitive. SSc monocytes exhibit a different phenotype than normal monocytes, expressing high levels of ColI, CD14 and CD34. Because ColI+/CD14+ cells are prevalent in SSc blood, we looked for such cells in lung tissue and confirmed their presence in SSc-ILD lungs but not in normal lungs. Finally, in the bleomycin model of lung fibrosis, we show that CSD peptide diminishes fibrocyte accumulation in the lungs. Our results suggest that low caveolin-1 in SSc monocytes contributes to ILD via effects on cell migration and phenotype and that the hyperaccumulation of fibrocytes in SSc-ILD may result from the altered phenotype and migratory activity of their monocyte precursors

Extracellular ATP is a pro-angiogenic factor for pulmonary artery vasa vasorum endothelial cells

Expansion of the vasa vasorum network has been observed in a variety of systemic and pulmonary vascular diseases. We recently reported that a marked expansion of the vasa vasorum network occurs in the pulmonary artery adventitia of chronically hypoxic calves. Since hypoxia has been shown to stimulate ATP release from both vascular resident as well as circulatory blood cells, these studies were undertaken to determine if extracellular ATP exerts angiogenic effects on isolated vasa vasorum endothelial cells (VVEC) and/or if it augments the effects of other angiogenic factors (VEGF and basic FGF) known to be present in the hypoxic microenvironment. We found that extracellular ATP dramatically increases DNA synthesis, migration, and rearrangement into tube-like networks on Matrigel in VVEC, but not in pulmonary artery (MPAEC) or aortic (AOEC) endothelial cells obtained from the same animals. Extracellular ATP potentiated the effects of both VEGF and bFGF to stimulate DNA synthesis in VVEC but not in MPAEC and AOEC. Analysis of purine and pyrimidine nucleotides revealed that ATP, ADP and MeSADP were the most potent in stimulating mitogenic responses in VVEC, indicating the involvement of the family of P2Y1-like purinergic receptors. Using pharmacological inhibitors, Western blot analysis, and Phosphatidylinositol-3 kinase (PI3K) in vitro kinase assays, we found that PI3K/Akt/mTOR and ERK1/2 play a critical role in mediating the extracellular ATP-induced mitogenic and migratory responses in VVEC. However, PI3K/Akt and mTOR/p70S6K do not significantly contribute to extracellular ATP-induced tube formation on Matrigel. Our studies indicate that VVEC, isolated from the sites of active angiogenesis, exhibit distinct functional responses to ATP, compared to endothelial cells derived from large pulmonary or systemic vessels. Collectively, our data support the idea that extracellular ATP participates in the expansion of the vasa vasorum that can be observed in hypoxic conditions

Fibrocytes and the tissue niche in lung repair

Human fibrocytes are bone marrow-derived mesenchymal progenitor cells that express a variety of markers related to leukocytes, hematopoietic stem cells and a diverse set of fibroblast phenotypes. Fibrocytes can be recruited from the circulation to the tissue where they further can differentiate and proliferate into various mesenchymal cell types depending on the tissue niche. This local tissue niche is important because it modulates the fibrocytes and coordinates their role in tissue behaviour and repair. However, plasticity of a niche may be co-opted in chronic airway diseases such as asthma, idiopathic pulmonary fibrosis and obliterative bronchiolitis. This review will therefore focus on a possible role of fibrocytes in pathological tissue repair processes in those diseases

High and Low Molecular Weight Hyaluronic Acid Differentially Regulate Human Fibrocyte Differentiation

Following tissue injury, monocytes can enter the tissue and differentiate into fibroblast-like cells called fibrocytes, but little is known about what regulates this differentiation. Extracellular matrix contains high molecular weight hyaluronic acid (HMWHA; ∼2×10(6) Da). During injury, HMWHA breaks down to low molecular weight hyaluronic acid (LMWHA; ∼0.8-8×10(5) Da).In this report, we show that HMWHA potentiates the differentiation of human monocytes into fibrocytes, while LMWHA inhibits fibrocyte differentiation. Digestion of HMWHA with hyaluronidase produces small hyaluronic acid fragments, and these fragments inhibit fibrocyte differentiation. Monocytes internalize HMWHA and LMWHA equally well, suggesting that the opposing effects on fibrocyte differentiation are not due to differential internalization of HMWHA or LMWHA. Adding HMWHA to PBMC does not appear to affect the levels of the hyaluronic acid receptor CD44, whereas adding LMWHA decreases CD44 levels. The addition of anti-CD44 antibodies potentiates fibrocyte differentiation, suggesting that CD44 mediates at least some of the effect of hyaluronic acid on fibrocyte differentiation. The fibrocyte differentiation-inhibiting factor serum amyloid P (SAP) inhibits HMWHA-induced fibrocyte differentiation and potentiates LMWHA-induced inhibition. Conversely, LMWHA inhibits the ability of HMWHA, interleukin-4 (IL-4), or interleukin-13 (IL-13) to promote fibrocyte differentiation.We hypothesize that hyaluronic acid signals at least in part through CD44 to regulate fibrocyte differentiation, with a dominance hierarchy of SAP>LMWHA≥HMWHA>IL-4 or IL-13

Eur. J. Immunol.

Endowing tumor cells with costimulatory signals for T cell activation has emerged as a promising strategy for tumor immunotherapy. Costimulatory molecules were either transfected into tumor cells to generate vaccines or were fused, e.g. to antibodies against tumor-associated antigens, to achieve targeted T cell costimulation in vivo. Here we report the production and purification of rM28, a recombinant bispecific single-chain antibody directed to a melanoma-associated proteoglycan and to the costimulatory CD28 molecule on human T cells. We found that a dimer of the recombinant molecule, bound to tumor target cells, induced pronounced T cell activation in peripheral blood mononuclear cell preparations without additional TCR/CD3 stimulation being required. The lytic activity generated after 3 days of stimulation effectively prevented tumor cell growth. However, it was unspecific and predominantly mediated by non T cells. Our findings demonstrate that presentation of a CD28 antibody within a suitable recombinant, bispecific format may result in a "targeted supra- agonistic stimulation" of the CD28 molecule, which leads to effective tumor cell killing after induction of unspecifically lytic cells