Psoriatic arthritis in Japan: difference in clinical features and approach to precision medicine.

Psoriatic arthritis (PsA) is a chronic and progressive inflammatory arthritis that is common among patients with psoriasis, often resulting in permanent damage of joints and spines. Recent report indicates that the prevalence of PsA among Japanese patients with psoriasis is 14.3%, which is similar or slightly less than that of PsA in Caucasian, 6-42%. Skin disorders precede arthritis in 60-80% of Japanese patients with PsA and oligoarthritis or polyarthritis is the dominant pattern of them. The genotypic backgrounds appear different among Japanese and Caucasians. Biological DMARDs (bDMARDs) targeting cytokines IL-12/IL-23, TNF and IL-17 involved in the pathogenesis of PsA, have been emerging for the treatment. Although background characteristics are various among studies, anti-IL-17 seemed to be slightly better in Japanese than in global, whereas anti-IL-12/23 and anti-TNF tended to be better in global than in Japanese. Because PsA is a clinically heterogeneous disorder, we have tried to classify PsA by phenotypic differences of peripheral lymphocyte using 8-color flow cytometry and found that PsA can be divided to four types, activated Th17-dominant, Th1-dominant, both of them and neither of them. We currently try to treat patients with different bDMARDs based on the difference of lymphocyte phenotype, which may lead to precision medicine of PsA.