People with plaque psoriasis start out with low vitamin D and high levels of the immune peptide LL‑37. After a course of narrow‑band UVB light treatment, both vitamin D and LL‑37 in the blood go up a lot. This suggests the light therapy may help psoriasis partly by boosting these molecules.

Narrowband ultraviolet B (NB-UVB) has been widely used in the treatment of psoriasis. It has been shown that vitamin D is a major regulator of the expression of human antimicrobial peptide cathelicidin LL-37, which has a critical role in inflammatory cascade in psoriasis. To evaluate the effect of NB-UVB therapy on serum levels of cathelicidin LL-37 and 25-hydroxyvitamin D [25(OH)D] in psoriasis patients. Ninety-three psoriasis patients and 50 controls were included in the study. For psoriasis patients, serum levels of 25(OH)D and cathelicidin LL-37 were estimated before and after NB-UVB therapy. Before treatment, serum 25(OH)D levels were significantly lower in psoriasis patients (31.5 ± 14.41 nmol/L) compared to controls (53.5 ± 19.6 nmol/L), p  =  .015. In contrast, serum LL-37 was significantly higher in psoriasis patients (13.24 ± 3.2 ng/mL) than in controls (7.92 ± 5.33 ng/mL), p < .001. After NB-UVB treatment, there was a highly significant elevation of serum 25(OH)D to reach 56.85 ± 5.2 nmol/L (p < .001) and further elevation of serum LL-37 to reach 29.4 ± 4.2 (p  =  .02). The elevation of serum 25(OH)D and cathelicidin LL-37 could be an additional possible mechanism of action of NB-UVB therapy in the treatment of psoriasis.