Background/Purpose: Stem cell factor (SCF) is a potential driving factor in the development of systemic sclerosis (SSc) and a possible therapeutic target. SCF is a cytokine which acts via c-Kit, a tyrosine kinase receptor, present on the surface of progenitor cells, mast cells and melanocytes. This relationship with mast cells potentially drives pruritus, an under-reported but significant problem in SSc, which can suggest disease activity. The greatly altered pigmentation seen in SSc may also reflect altered SCF/c-Kit signalling affecting melanocytes. Previous work from our laboratory has demonstrated increased levels of SCF and c-kit in SSc fibroblasts compared to healthy controls Our aim was to measure the activity of SCF/c-Kit pathway in Systemic sclerosis.

Methods: Blister fluid, tissue and plasma samples were harvested from healthy controls (HC) and SSc patients. The SCF and c-Kit protein levels in SSc and HC lung fibroblasts were determined by western blotting. SCF (soluble and membrane bound) and c-kit gene expression was measured using quantitative PCR (qPCR) from control and SSc lung fibroblasts and epidermal sheet. The levels of SCF and c-Kit in SSc and HC were assessed by ELISA. The expression of CD117 positive lung fibroblasts was analysed by FACS.

Results: The soluble SCF mRNA expression was enhanced in SSc lung fibroblast samples by qPCR compared to healthy controls (soluble transcript: 23 versus 10 copy number respectively p=0.008 and membrane bound: 1.1 versus 0.7 p=NS). C-kit was expressed at low levels in both SSc and HC fibroblasts (0.31 versus 0.28 copy number, p=NS). Western blotting showed increased SCF protein levels in SSc lung fibroblasts compared to controls with relative density scan of 2.25 and 0.76 respectively (p=0.009) while the level of c-Kit protein expression was low in both SSc and control fibroblasts (0.16 vs 0.25) p=NS. Furthermore, measuring the epidermal sheet for SCF gene expression in SSc and HC showed that the soluble SCF isoform had 537 and 366 copy number respectively, while the membrane bound isoform showed gene copy numbers of 106 for SSc and 134 for HC. Using ELISA, SCF plasma levels were found to be lower in SSc patients (1272pg/ml) than the HC group (1425pg/ml) p=0.04, as were the plasma c-Kit levels (13ng/ml versus 16ng/ml respectively) p=0.028. Levels of SCF in conditioned media of cultured lung fibroblasts were higher in SSc (150ng/ml) vs HC (130ng/ml) p=0.002 but lower in blister fluid 1223pg/ml vs 1414 pg/ml respectively. C-Kit was undetectable in conditioned media and blister fluid.  A subpopulation of CD117 positive cells was found in both SSc and HC lung fibroblasts (1.26% SSc cells, 2.3% HC).

Conclusion: We demonstrated that the full length soluble SCF mRNA is found at higher levels in the epidermis and in the lung fibroblasts of SSc subject and that SCF appears at slightly higher levels in SSc fibroblast conditioned media. However, when measured in plasma or blister fluid SCF was lower or unaltered in SSc when compared to controls. Cultured fibroblasts were heterogeneous and only a minority were positive for c-Kit.  If SCF is important in SSc pathogenesis then it might be acting locally on a small subpopulation of c-Kit positive cells.