Abstract: NI Li-yan, WANG Qiang. Department of Dermatology, Zhongshan Hospital, Fudan University, Shanghai 200032, China Corresponding author: WANG Qiang, Email: wang.qiang@zs-hospital.sh.cn 【Abstract】 Objective To evaluate the effect of a specific cathepsin B inhibitor, CA-074 Me, on the expression of cathepsin B in coxsackievirus B1-induced polymyositis in guinea pigs, and to elucidate the protective mechanisms of CA-074 Me on muscle fibers. Methods Polymyositis model was established in 32 guinea pigs by infection with coxsackievirus B1, which were then equally divided into 4 groups: γ-interferon group treated with intraperitoneal γ-interferon （150 000 IU per kilogram per day） from week 5 to week 8, polymyositis model group receiving no treatment, pseudo-intervention group treated with intraperitoneal sodium chloride physiological solution, CA-074 Me group treated with intraperitoneal CA-074 Me （4 mg per kilogram per day） for 7 days, after the infection with coxsackievirus B1. Eight guinea pigs receiving no infection or treatment served as the healthy control group. Blood samples and muscle tissue samples were obtained from the guinea pigs in the γ-interferon group on week 8 and in the other 4 groups on week 5. The serum level of muscle enzymes including creatine kinase （CK）, CK-MM, aspartic transaminase （AST） and l-lactate dehydrogenase （LDH） were determined. Muscle tissue samples were studied by hematoxylin and eosin （HE） staining, and Envision two-step method was used to quantify the expression of cathepsin B and to numerate CD8+ T cells. The apoptosis in muscle cells was detected by terminal deoxynucleotide transferase-mediated dUTP-biotin nick end labeling （TUNEL）. One-way analysis of variance（ANOVA） was conducted to compare the serum level of muscle enzymes, inflammation score of muscle and apoptosis index of muscular cells, and Pearson chi-square test to compare the count of CD8+ T cells and cathepsin B expression, among these groups. Results Polymyositis model was successfully established by infection with coxsackievirus B1 in the 32 guinea pigs with a marked increase in the serum level of the tested enzymes, especially in that of CK. In detail, the serum level of CK was （410.7 ± 167.9） U/L in the healthy control group, significantly lower than that in the polymyositis model group （（3537.3 ± 2141.6） U/L, P < 0.05）, pseudo-intervention group （（2222.0 ± 226.9） U/L, P < 0.05）, γ-interferon group （（973.8 ± 423.2） U/L, P < 0.05） and CA-074 Me group （（814.0 ± 268.4） U/L, P < 0.05）. Compared with the pseudo-intervention group, the γ-interferon group and CA-074 Me group showed a slight increase in the serum level of all the four enzymes （all P < 0.05）. There was a significant elevation in the inflammation score of skeletal muscles in the polymyositis model group, pseudo-intervention group, γ-interferon group and CA-074 Me group compared with the healthy control group （1.75 ± 0.50, 1.40 ± 0.55, 2.38 ± 0.74 and 1.20 ± 0.45 vs. 0.00 ± 0.00, all P < 0.05）, with the most intense infiltration of inflammatory cells observed in the γ-interferon group. Moreover, the number of CD8+ T cells, cathepsin B expression and muscular cell apoptosis index were all significantly higher in the polymyositis model group, pseudo-intervention group, γ-interferon group and CA-074 Me group than in the healthy control group （all P < 0.05）. Compared with the pseudo-intervention group, the CA-074 Me group showed less CD8+ T cells （42.3 ± 27.4 vs. 68.0 ± 13.2, P < 0.05） and lower expression of cathepsin B （31.3 ± 6.7 vs. 37.5 ± 9.2, P < 0.05）, whereas the γ-interferon group exhibited elevated cathepsin B expression （49.3 ± 17.0 vs. 37.5 ± 9.2, P < 0.05） and apoptosis index （40.1 ± 6.7 vs. 25.4 ± 5.0, P < 0.05）. Conclusions Cathepsin B is highly expressed in the guinea pig model of polymyositis, while CA-074 Me may protect muscle tissue in this model by downregulating the expression of cathepsin B and attenuating the inflammation and apoptosis induced by cathepsin B. 【Key words】 Polymyositis; Cathepsin B; Apoptosis; CD8-positive T-lymphocytes

Key words: Polymyositis, Cathepsin B