CD28 and systemic lupus erythematosus: The expression of CD4+ CD28+ cells in all lymphocytes was significantly higher in control (MFI = 21.63 ± 1.25) compared to mild SLE (MFI = 15.39 ± 1.88, p = 0.001) and severe SLE (MFI = 15.63 ± 3.39, p = 0.001) (Figure 3c).