PDCD1 and systemic lupus erythematosus: The co-expression of CD3+ PD1+ in CD3+ CD4+ cells was significantly higher in the control group (MFI = 19.70 ± 2.46) compared to mild SLE (MFI = 14.48 ± 2.67, p = 0.010) and severe SLE (MFI = 16.07 ± 1.74, p = 0.001) (Figure 5e).