PDCD1 and systemic lupus erythematosus: The overall expression of PD-1 in all lymphocytes differed significantly among the three groups, with the lowest expression in severe SLE (MFI = 0.49 ± 0.05) and significant differences between severe SLE and control (MFI = 0.78 ± 0.11, p = 0.001) as well as between severe SLE and mild SLE (MFI = 0.82 ± 0.21, p = 0.001) (Figure 5a).