e20509 Background: At present, most preclinical studies explore the cardiotoxicity caused by radiotherapy combined with Anti-PD1s in tumor-free mice. we assessed the overall cardiotoxicity of radiotherapy combined with Anti-PD1, the possibility that blocking HMGB1 does not affect the anti-tumor effect, and the possibility that radiotherapy combined with Anti-PD1 reduces heart damage in tumor-bearing mice. Methods: 25 tumor-bearing C57BL/6 mice on both hind limbs were treated with anti-PD1 or PBS or Anti-HMGB1 with or without thoracic irradiation 20gy and right hind limb tumor 8gy×3F irradiation. HE staining was used to observe the cardiac structure and morphological changes. Flow cytometry was used to observe the distribution of CD3+, CD4+, CD8+T cells in cardiac and peripheral blood. Cardiac function was examined by echocardiography. The expression of myocardial injury marker LDH in peripheral blood was detected by Elisa. Results: Five group was established:A = Control;B = anti-PD1,C = RT,D = RT+Anti-PD1,E = RT+Anti-PD1+Anti-HMGB1.HE showed the myocardial injury in A(without any treatment) was the most severe,and and B and D was slightly severe than C group, administration of Anti-HMGB1 partially attenuated myocardial injury in D. Elisa showed the LDH of A was highest, B were consistent with D in LDH expression, RT was lower. Cardiac function showed no difference in ejection fraction and fractional shortening in five groups. Peripheral blood flow cytometry showed the absolute counts of CD3+T-cell and cd8+T cells were greatest in B group, followed by A, C, D, and E groups. Absolute CD4+ T cell values were highest in the B. The results of myocardial tissue infiltrating lymphocytes showed that the absolute CD3+T-cell and cd8+T cells counts were biggest in the B group, followed by A, C, D, and Egroups. Absolute CD4+T cell counts were highest in B group. C, D had the best anti-tumor effect, and the use of Anti-HMGB1 partially attenuated the cardiac injury caused by D group. Conclusions: (1)Without any treatment, the cancer cachexia induced by tumor will cause serious damage to myocardial. (2) The overall cardiotoxicity of radiotherapy combined with Anti-PD1 group was only slightly increased compared to RT or anti-PD1 alone. Maybe radiotherapy killed part of the Anti-PD1-activated lymphocytes, only resulting in a slight increase in cardiac infiltrating lymphocytes. Anti-HMGB1 maintain the anti-tumor effect of RT+Anti-PD1 and partially attenuated the cardiac damage. [Table: see text]