C3 and systemic lupus erythematosus: We also evaluated the correlation between disease severity and TIGIT expression in Treg cells of SLE patients and found that complement 3 (C3) and C4 were negatively correlated with TIGIT expression in the Tregs (Fig. 1E), suggesting that a higher proportion of Tregs exhibited the Th1 phenotype in SLE patients compared to HDs, with low TIGIT expression and disrupted immune suppression function, which is related to the severity of SLE disease.