019 KLK6-PAR1 signaling drives psoriasiform manifestations in skin and bone

Kallikrein-related peptidase 6 (KLK6) is a serine protease hypothesized to promote inflammation via cleavage of protease-activated receptors PAR1 and PAR2. KLK6 and PAR1/2 are expressed in skin, however their biological roles remain unclear. RNAseq of lesional skin from psoriasis (Ps) patients identified KLK6 as the most-increased KLK family member (n=6; 13-fold; P<0.01). Ps patients treated with etanercept show rapid KLK6 decreases that correspond with disease improvement (n=9; P<0.05). As KLK6 protein localized largely to keratinocytes (KCs), we engineered a tet-repressible KC-KLK6 overexpressing mouse (KLK6+) to study KLK6 function. KLK6+ mice spontaneously develop a skin phenotype at ∼7wks of age characterized by a ∼5-fold increase in acanthosis (n=19-28; P<0.001), increased Ki67+ and phospho-Stat3+, a dense inflammatory infiltrate containing CD4/8+ T cells, CD11c+ DCs, F4/80+ macrophages and micro-abscesses containing Gr1+ neutrophils. RNAseq of KLK6+ mouse skin revealed correspondence with human Ps (r=0.38) and identified increased levels (>300-fold) of Il22 and Il17a/f. ELISA confirmed 285- and 28-fold increases in skin IL-23 and IL-17A (n=7-16; P<0.001). By ∼10wks of age, KLK6+ mice develop a psoriatic arthritis (PsA)-like phenotype, including forelimb dactylitis, and decreased bone mineral density, periarticular osteopenia with fusion and erosive changes in the SI joint and pubic symphysis. Gene repression for 4wks using doxycycline in KLK6+ mice with established disease reversed the Ps- and PsA-like phenotypes. To study KLK6-PAR1/2 signaling, KLK6+ mice were mated with PAR1- or PAR2-KO mice. KLK6+PAR2KO mice looked similar to KLK6+ mice. In contrast, KLK6+PAR1KO mice had attenuated skin inflammation and a delay in PsA-like outcomes. Our results identify a critical role for KLK6 in promoting Ps-like inflammation via PAR1 signaling and suggest targeting PAR1 may provide a cytokine-independent approach for treating psoriasis.