CD4 and systemic lupus erythematosus: This lower expression may be due to reduced CTLA-4 expression in specific subsets, as supported by Zhao et al.’s study [40], which found that the CTLA-4 levels of CD4+ CD25+ T cells and CD4+ CD25+ FoxP3+ Treg cells were significantly reduced in SLE patients and negatively correlated with the disease activity and severity of SLE.