Cell-Kinetic Evidence for Increased Recruitment of Cycling Epidermal Cells in Psoriasis: The Ratio of Histone and Ki-67 Antigen Expression Is Constant

BACKGROUND: One of the hallmarks of the psoriatic plaque is increased epidermal proliferation. Whether this is the result of an increased recruitment of cycling epidermal cells or a decrease in cell cycle time has been a matter of debate for years. OBJECTIVE: Calculating cell-kinetic information from the number of S phase cells in psoriasis by in situ hybridisation using a histone probe and the number of cycling epidermal cells by immunohistochemistry using the MIB-1 antibody. METHODS: Immunohistochemistry and non-isotopic in situ hybridisation were performed on serial sections of 33 untreated psoriatic samples and 14 tacalcitol-treated samples. RESULTS: The labelling index (number of cells in S phase/number cycling cells per millimetre length of section) in psoriatic untreated as well as in treated plaques is 16%. The amount of S phase cells in our experiment is equal compared with the number of cells in S phase as determined by BrdU incorporation. CONCLUSION: Using this direct approach to study cell-kinetic behaviour of psoriatic skin, we reconfirm that the psoriatic abnormality is due to a defect in the G(0)-G(1) recruitment mechanism (by increased recruitment of G(0) cells), a decrease in apoptosis or an increase in the number of cell divisions in the transit-amplifying compartment, rather than a reduction in the cell cycle time.