CD4 and systemic lupus erythematosus: At the resting state, in CD3+ CD4+ cells, the overall expression level of CTLA-4 showed significant differences among the three groups, with mild SLE having the highest CTLA-4 expression (MFI = 0.48 ± 0.17), significantly different from severe SLE (MFI = 0.34 ± 0.02, p = 0.007) (Figure 4a).