The co-expression of CD4+ PD1+ was also significantly higher in the control group (MFI = 18.71 ± 2.17) compared to severe SLE (MFI = 15.66 ± 1.90, p = 0.009) and mild SLE (MFI = 14.78 ± 2.50, p = 0.003) (Figure 5b). Here, PDCD1 is linked to systemic lupus erythematosus.