CD4 and systemic lupus erythematosus: The results have shown that SLEDAI scores were significantly related to upregulated PD-1 expression on CD4+ T cells, CD8+ T cells, CD56+ T cells, and increased PD-1 mRNA expression levels in PBMCs from PB samples of SLE patients (Figure 2), which indicates that upregulated PD-1 expression may be involved in the pathogenesis of SLE.