Abstract： Objective To detect expression levels of complement regulatory proteins CD55 and CD59 in the peripheral blood of patients with Stevens-Johnson syndrome(SJS)/toxic epidermal necrolysis(TEN),and to preliminarily analyze their potential roles in the occurrence of SJS/TEN.Methods Hospitalized patients with SJS/TEN(SJS/TEN group)were collected from the Department of Dermatology of the First Affiliated Hospital of Anhui Medical University and the Second Affiliated Hospital of Anhui Medical University from December 2017 to December 2022.Meanwhile,patients with maculopapular exanthema(MPE)and healthy physical examinees were also collected and served as the mild group and healthy control group,respectively.Flow cytometry was performed to determine the proportions of CD4+T lymphocytes and CD8+T lymphocytes in peripheral blood mononuclear cells(PBMCs).The expression levels of inflammatory cytokines tumor necrosis factor(TNF)-α,interleukin(IL)-6,IL-17,IL-10,interferon(IFN)-γ,IL-2,and IL-4 were detected using flow cytometric bead array technology.The mRNA expression levels of CD55 and CD59 in PBMCs were detected by real-time fluorescence-based quantitative PCR(qRT-PCR).Flow cytometry was also performed to determine the protein expression of CD55 and CD59 on the surface of CD8+T lymphocytes.Statistical analyses were carried out using one-way analysis of variance and Tukey's test.Results Totally,13 patients with SJS/TEN,27 patients with MPE,and 40 healthy controls were collected.Among the SJS/TEN patients,there were 8 males and 5 females,with their age being 18 to 84(47.15±19.99)years,and disease duration being 7.74±2.63 days.No significant differences were observed in the gender distribution or age among the 3 groups(both P＞0.05).The proportions of CD4+T lymphocytes did not differ among the 3 groups(F=3.84,P=0.051).The proportions of CD8+T lymphocytes in the peripheral blood were significantly higher in the SJS/TEN group(25.60％±4.57％)than in the healthy control group(16.20％±6.28％;q=4.59,P=0.018).The expression levels of inflammatory cytokines TNF-α,IL-6,IL-10,IL-17,and IFN-γ were significantly higher in the SJS/TEN group than in the healthy control group and mild group(all P＜0.001).In addition,the mRNA expression of CD55(F=9.46,P＜0.001)and CD59 in PBMCs(F=15.14,P＜0.001)was significantly lower in the SJS/TEN group than in the mild group and healthy control group.The protein expression levels of CD55(F=51.51,P＜0.001)and CD59(F=31.59,P＜0.001)on the surface of CD8+T lymphocytes were also significantly lower in the SJS/TEN group than in the other two groups and the healthy control group,respectively.Conclusion Complement regulatory proteins CD55 and CD59 were downregulated in SJS/TEN patients,which may be associated with the activation of CD8+T lymphocytes and excessive inflammatory responses.