CD4 and systemic lupus erythematosus: The co-expression of CD3+ PD1+ in CD3+ CD4+ cells also showed significant differences among the three groups, with the highest expression in the control group (MFI = 22.07 ± 2.27), and significant differences between the control and mild SLE (MFI = 16.30 ± 3.92, p = 0.010) as well as severe SLE (MFI = 16.76 ± 3.35, p = 0.001) (Figure 6d).