CD26/dipeptidyl-peptidase IV in psoriatic skin: upregulation and topographical changes

Summary Background Psoriasis is known to affect 2–3% of the population and can be considered an organ‐specific autoimmune disease. CD26/dipeptidyl‐peptidase IV (DPP‐IV) is a membrane‐bound protease with diverse properties. In theory, the expression of CD26/DPP‐IV has common grounds with three principal key players of the psoriatic pathogenesis: keratinocytes, T cells and cytokines. Objectives To assess CD26/DPP‐IV expression in psoriasis in order to expand on the search for complementary biomarkers related to inflammation and proliferation in psoriasis. Methods The pattern of expression of CD26/DPP‐IV was investigated on the mRNA‐, protein‐ and enzyme‐functionality level using immunohistochemical, immunofluorescent and enzyme activity labelling techniques. Results An 11‐fold significant increase of CD26/DPP‐IV on the mRNA level was demonstrated in psoriatic epidermal sheets compared with normal skin. Immunohistochemistry on psoriatic sections showed a distinct patchy honeycomb‐like CD26/DPP‐IV staining in the suprapapillary layers. Moreover, a clearly distinguishable column‐like staining pattern throughout the suprabasal compartment along the rete ridges was seen, whereas in normal skin these patterns were absent. Strikingly, CD26/DPP‐IV enzyme activity correlated with this immunohistochemical reactivity pattern for the CD26/DPP‐IV protein. The T‐cell bound expression of CD26/DPP‐IV in psoriatic skin was explicitly present, albeit in small quantities. Conclusions Our data provide clear evidence for a versatile upregulation of CD26/DPP‐IV expression in psoriatic (epi)dermis. Although the exact functional contribution remains speculative, the topographical distribution of this complex multifunctional protein suggests a suitable role as a complementary biomarker in psoriasis.