Update on the pathogenesis of Scleroderma: focus on circulating progenitor cells [version 1; referees: 2 approved]

In systemic sclerosis (SSc), the development of fibrosis seems to be a consequence of the initial ischemic process related to an endothelial injury. The initial trigger event in SSc is still unknown, but circulating progenitor cells (CPCs) might play a key role. Such cells have the ability to traffic into injury sites, exhibiting inflammatory features of macrophages, tissue remodeling properties of fibroblasts, and vasculogenesis functions of endothelial cells. The different subsets of CPCs described thus far in SSc arise from a pool of circulating monocyte precursors (CD14+ cells) and probably correspond to a different degree of differentiation of a single cell of origin. Several subsets of CPCs have been described in patients with SSc, all have a monocytic origin but may or may not express CD14, and all of these cells have the ability to give origin to endothelial cells, or collagen (Col)-producing cells, or both. We were able to identify six subsets of CPCs: pluripotent stem cells (CD14+, CD45+, and CD34+), monocyte-derived multipotential cells (MOMCs) or monocyte-derived mesenchymal progenitors (CD14+, CD45+, CD34+, Col I+, CD11b+, CD68+, CD105+, and VEGFR1+), early endothelial progenitor cells (EPCs) or monocytic pro-angiogenic hematopoietic cells or circulating hematopoietic cells (CD14+, CD45+, CD34low/−, VEGFR2+/−, CXCR4+, c-kit+, and DC117+), late EPCs (CD14−, CD133+, VEGFR2+, CD144+ [VE-cadherin+], and CD146+), fibroblast-like cells (FLCs)/circulating Col-producing monocytes (CD14+, CD45+, CD34+/−, and Col I+), and fibrocytes (CD14−, CD45+, CD34+, Col I+, and CXCR4+). It has been demonstrated that circulating CD14+ monocytes with an activated phenotype are increased in patients with SSc when compared with normal subjects. CD14+, CD34+, and Col I+ spindle-shaped cells have been found in increased numbers in lungs of SSc patients with interstitial lung disease. Elevated blood amounts of early EPCs have been found in patients with SSc by different groups of researchers and such levels correlate directly with the interstitial lung involvement. The prevalence of hematopoietic markers expressed by CPCs that migrate from blood into injury sites in SSc differs and changes according to the degree of differentiation. CXCR4 is the most commonly expressed marker, followed by CD34 and CD45 at an end stage of differentiation. Such difference also indicates a continuous process of cell differentiation that might relate to the SSc clinical phenotype (degree of fibrosis and vascular involvement). A deeper understanding of the role of each subtype of CPCs in the development of the disease will help us to better classify patients in order to offer them targeted approaches in the future