Possible contribution of stem cell factor in psoriasis vulgaris

It is suggested that mast cell is implicated to play a role in the pathogenesis of psoriasis. In this study, to determine the role of stem cell factor (SCF), which is a growth factor of mast cells, we have examined the immunohistochemical localization and serum level of SCF in patients with psoriasis vulgaris. Immunohistochemical analysis revealed diffuse staining for SCF on keratinocytes in acanthotic epidermis in psoriasis, along with endothelial cells and fibroblasts. Serum SCF level, which was measured by enzyme-linked immunosorbent assay (ELISA), was significantly increased in patients with psoriasis vulgaris (1033+/-334 pg/ml) (n=24) than that of normal subjects (666+/-196 pg/ml) (n=15) (P<0.05). However, serum SCF did not show a correlation with the disease severity assessed by psoriasis activity and severity index (PASI) score. As patients with psoriasis vulgaris occasionally complain itching, next we divided 20 patients into two groups, those with itching (Group I) (n=8) and those without (Group II) (n=12), and compared the mast cell number located in the papillary dermis between thickened psoriatic epidermis, serum SCF and plasma histamine levels. Results showed that mast cell numbers (56.3+/-22.3/mm(2) in Group I vs 31.5+/-10. 3/mm(2) in Group II, P<0.05) and plasma histamine level (1.5+/-0.59 ng/ml vs 0.39+/-0.15 ng/ml, P<0.01) were significantly higher in patients of Group I than those of patients of Group II, however, the difference of serum SCF level (1132+/-368 pg/ml vs 890+/-373 pg/ml) did not reach a statistical significance. Finally, in a separate experiment, we examined whether exogenous SCF is capable of inducing psoriatic architecture on the transplanted uninvolved psoriatic skin onto severe combined immunodeficient (SCID) mice. SCF injection for 2 weeks could not induce a psoriasiform architecture such as acanthosis on the transplanted uninvolved psoriatic skin, although mast cells were increased in number. These results raised a possibility that keratinocyte-derived SCF plays a role, in part, in the increased number of mast cells in the papillary dermis of psoriasis, which may lead pruritus associated with psoriasis. Elevated serum SCF level may also be responsible for increment of mast cells in psoriasis vulgaris. Mast cell-derived factor stimulated by exogenous SCF could not induce psoriatic epidermis, suggesting that other factors such as activated lymphocytes or macrophages are further required for the development of psoriatic lesions.