PDCD1 and Erythema: PD-1−/− mice developed significantly more severe IMQ-induced psoriasis-like dermatitis (Fig. 2a) when compared to WT mice as revealed by clinical measurements including ear swelling (change from the baseline at day 7, 20.6 ± 2.6 μm vs. 7.2 ± 1.4 μm, P = 0.0014 by two-way ANOVA, Fig. 2b) and PASI score, which represents the severity of erythema, scaling and skin thickness, (7.8 ± 0.2 vs. 3.2 ± 0.2, P < 0.0001 by two-way ANOVA, Fig. 2c) at day 7.