Reduced CLEC9A expression in synovial tissue of psoriatic arthritis patients after adalimumab therapy

We aimed to investigate the early changes in expression of C-type lectin domain family 9, member A (CLEC9A), a C-type lectin that is specifically expressed by the CD141(+) dendritic cell subset that is involved in cross-presentation to CD8(+) T cells, by evaluating gene and/or protein expression in three different compartments [skin, synovial tissue (ST) and serum] after short-term adalimumab treatment in PsA patients compared with placebo.

Patients with active PsA and psoriasis were randomized to receive adalimumab or placebo for 4 weeks. Synovial and skin biopsies were obtained before and after 4 weeks of treatment and serum samples 4 weeks, 12 weeks and 1 year after treatment. Skin and serum from healthy donors were used as control. CLEC9A expression was assessed by immunohistochemistry, double immunofluorescence using terminal deoxynucleotidyl transferase 2'-deoxyuridine 5'-triphosphate nick-end labelling (TUNEL), quantitative PCR and ELISA.

CLEC9A expression was significantly higher in psoriatic skin compared with healthy donor. In psoriatic skin and PsA ST, CLEC9A(+) cells were in close proximity to TUNEL(+) cells. SF CLEC9A levels were significantly lower compared with paired PsA serum. Adalimumab treatment did not affect CLEC9A serum level and skin expression. However, ST CLEC9A protein expression was significantly decreased after adalimumab treatment compared with the placebo group while CLEC9A gene expression remained unchanged. There was a positive correlation between T cell numbers and ST CLEC9A protein expression. CD141(+) cell numbers and chemokine (C motif) receptor 1 expression were not affected with adalimumab treatment.

Altogether, the present study suggests that the downregulation of synovial CLEC9A might be associated with a novel mechanism by which anti-TNF therapy might reduce CD8-mediated inflammation in PsA patients.