PDCD1 and systemic lupus erythematosus: In CD3+ CD4+ cells, PD-1 expression showed significant differences, with the highest expression in mild SLE (MFI = 0.99 ± 0.41), and significant differences between mild SLE and severe SLE (MFI = 0.56 ± 0.10, p = 0.001) as well as between severe SLE and control (p = 0.007) (Figure 5d).