Interleukin-9 serves as a key link between systemic inflammation and angiogenesis in psoriasis.

Background: Psoriasis is a T helper cell-mediated chronic immune-mediated inflammatory disease affecting mainly the skin, although systemic pathological effects are also observed. Cytokine-mediated interaction between T lymphocytes and keratinocytes lead to excessive proliferation of keratinocytes, which in turn leads to formation of a proinflammatory milieu and finally to psoriatic plaque formation.

Aim: To measure interleukin (IL)-9, IL-17 and vascular endothelial growth factor (VEGF) levels in patients with psoriasis compared with controls, and to evaluate the effect of methotrexate (MTX) monotherapy on the aforesaid cytokine levels in psoriasis.

Methods: This cohort study included 54 patients with psoriasis and 54 age- and sex-matched healthy controls (HCs). IL-9, IL-17 and VEGF levels were measured by using commercially available ELISA kits. Patients with psoriasis who were on MTX monotherapy were followed up for a period of 3 months.

Results: Patients with psoriasis had increased levels of IL-9, IL-17 and VEGF at baseline, compared with the HC group. After 3 months of MTX monotherapy, Psoriasis Area Severity Index (PASI), Dermatology Life Quality Index (DLQI) and levels of cytokines (IL-9, IL-17 and VEGF) were significantly decreased compared with baseline. PASI and DLQI at baseline also showed a positive correlation with IL-9, IL-17 and VEGF.

Conclusion: Our results suggest the existence of a proinflammatory milieu in psoriasis, with increased levels of IL-9, IL-17 and the proangiogenic growth factor VEGF, showing an increasing trend with increasing disease severity and impaired quality of life (QoL). MTX treatment helps to reduce levels of IL-9, IL-17 and VEGF, thereby limiting disease progression and improving QoL in psoriasis.