8-Methoxypsoralen Plus Ultraviolet A Reduces the Psoriatic Response to Imiquimod in a Murine Model

The effects of 8-Methoxypsoralen plus ultraviolet A (PUVA) or ultraviolet B (UVB) alone on imiquimod-induced psoriasis were examined in a mouse model. Mouse skin was treated with repetitive sub-phototoxic doses of PUVA or UVB before or during the induction of toll-like receptor 7/8 activation and psoriasis through the application of imiquimod. PUVA, to a greater degree than UVB, suppressed the established imiquimod-induced psoriatic phenotype, but pretreatment with PUVA prior to administration of imiquimod also reduced the susceptibility of murine skin to respond to imiquimod to a greater degree than did pretreatment with UVB. PUVA downregulated baseline levels of miRNA27a and 29a, as well as interferon-γ, interleukin-17 and -9, cytokines, which drive psoriatic inflammation. Microarray analysis showed enrichment of senescence pathway genes linked to upregulation of p16/p21 proteins after PUVA pretreatment. However, the anti-psoriatic effect of PUVA was lost when there was an interval of 7 days between final exposure to PUVA and the start of administration of imiquimod. This indicated that (UVB and) PUVA diminished imiquimod-induced established psoriatic inflammation, but also primed the skin in favour of a reduced responsiveness to toll-like receptor activation.