PDCD1 and psoriasis: Moreover, pathological analysis, as shown in Fig. 2d, of epidermal thickness (61.5 ± 7.9 μm vs. 35.6 ± 3.2 μm, P = 0.008 by Mann–Whitney U test, Fig. 2e) and the number of epidermal neutrophilic micro-abscesses (3.2 ± 0.58/ear slide vs. WT 0.6 ± 0.24/ear slide, P = 0.008 by Mann–Whitney U test, Fig. 2f) at day 7 further indicates the protective role of PD-1 in IMQ-induced psoriasis-like dermatitis.