CD4 and systemic lupus erythematosus: The expression of CD28 in CD3+ CD4+ cells was highest in severe SLE (MFI = 3.83 ± 1.12), with significant differences compared to control (MFI = 3.11 ± 0.37, p = 0.039) and mild SLE (MFI = 2.56 ± 0.58, p = 0.048).