PDCD1 and psoriasis: Histological analyses also revealed that MR16-1-treated PD-1−/− mice had less severe psoriasis than IgG Ctrl-treated PD-1−/− mice with reduced epidermal hyperplasia (38.8 ± 2.0 μm vs. 55.8 ± 1.8 μm, P = 0.01 by Mann–Whitney U test, Fig. 5d, e) and reduced numbers of epidermal neutrophilic micro-abscesses (3.2 ± 0.4 vs. 6.2 ± 0.6, P = 0.008 by Mann–Whitney U test), which was the same as IgG Ctrl-treated WT mice (34.8 ± 2.5 μm, P = 0.06; and 2.4 ± 0.4, P = 0.32 by Mann–Whitney U test, respectively, Fig. 5f).