Plaque psoriasis vs. atopic dermatitis and lichen planus: a comparison for lesional T‐cell subsets, epidermal proliferation and differentiation

Summary Background  T‐cell infiltration in plaque psoriasis has recently been an important subject of investigation. Interestingly, comparative analyses of the disease‐specific composition of the lesional T‐cell infiltrate in plaque psoriasis and other inflammatory dermatoses have only sparsely been performed. Objectives  To compare plaque psoriasis vs. atopic dermatitis and lichen ruber planus with respect to T‐cell subsets, epidermal proliferation and keratinization. Patients and methods  Biopsies were taken from untreated lesional skin of patients, six with psoriasis, six with atopic dermatitis and six with lichen planus. T‐cell subsets (CD4+, CD8+, CD45RO+, CD45RA+, CD2+, CD25+), an epidermal proliferation (Ki‐67) and a keratinization marker (K10) were stained immunohistochemically and quantified using image analysis. Results  The high number of CD8+ T cells (52 ± 13 cells mm −1 ) found in the psoriatic epidermis was not found in the epidermis of atopic dermatitis (9 ± 4), nor in the epidermis of lichen planus (34 ± 10). The other T‐cell subsets in the epidermis and dermis showed no statistically significant differences between psoriasis and atopic dermatitis. In contrast to the limited presence of CD4+, CD8+ and CD2+ in the psoriatic dermis (110 ± 19, 27 ± 9, 127 ± 41, cells mm −1 , respectively), more impressive numbers of these cells were observed in the dermis of lichen planus (300 ± 53, 144 ± 38, 272 ± 48, respectively). CD45RO+ memory effector T‐cell counts were significantly higher in the epidermis of lichen planus (39 ± 10) than in psoriasis (19 ± 5). Psoriatic epidermis proved to have major keratinocyte hyperproliferation (247 ± 26 cells mm −1 lamina basalis), as compared with atopic dermatitis (134 ± 15) and lichen planus (128 ± 20). Furthermore, a marked decreased expression of keratin 10 was observed in psoriasis (41% of epidermal area) contrary to atopic dermatitis (70%). Conclusions  Psoriatic epidermis exhibits a pronounced CD8+ epidermotropism with accompanying epidermal hyperproliferation and abnormal keratinization, which changes are only minimally expressed in atopic dermatitis and lichen planus. In plaque psoriasis, substantially fewer activated CD4+ and CD8+ T cells in the dermis and less CD45RO+ T cells in the epidermis are present in comparison with lichen ruber planus.