<p>Radioimmunotherapy for lung cancer is effective but could cause cardiac damage. Our previous research indicated that combining radiotherapy with PD-1 inhibitors leads to myocardial injury, marked by increased HMGB1 and pyroptosis-related proteins in heart tissue. Pyroptosis, a pro-inflammatory form of programmed death, releases abundant inflammatory cytokines that further amplify tissue damage. Notably, in addition to its pivotal role in cardiac injury, HMGB1 exerts context-dependent, dual effects within the tumor microenvironment—either tumor-promoting or tumor-suppressive—whose net impact remains undefined. Therefore, this study proposes to specifically block HMGB1 in a lung cancer model to determine whether this intervention can suppress pyroptosis, alleviate cardiac damage, and to concurrently evaluate its potential impact on the tumor control, thereby providing a novel therapeutic strategy to mitigate the cardiotoxicity of radiation–immunotherapy. We established a cardiac injury model in tumor-bearing mice using radiation and PD-1 inhibitors to assess the impact of HMGB1 blockade on heart and tumor treatment. We evaluated cardiac injury and fibrosis with HE and Masson staining, assessed cardiac function via echocardiography and detect the level of cytokine in heart by ELISA. Lymphocyte infiltration was analyzed by flow cytometry, while immunofluorescence, immunohistochemistry, Western blotting, and PCR examined changes of HMGB1 and pyroptosis pathways. Additionally, we monitored tumor growth and necrosis following HMGB1 blockade. Recent research demonstrates that the combination of radiotherapy and PD-1 inhibitors significantly exacerbates myocardial injury compared to radiotherapy alone. This exacerbation is evidenced by elevated levels of inflammatory cytokines, including HMGB1, IL-1β, and IL-18, within the myocardium, along with increased fibrosis and pyroptosis. Additionally, there is an upregulation of the pyroptosis pathway, specifically the HMGB1-Caspase-1-GSDMD axis. The incorporation of an HMGB1 neutralizing antibody into the combined treatment regimen has been shown to down-regulate HMGB1 and proteins associated with pyroptosis, thereby substantially reducing cardiotoxicity and myocardial injury. Notably, the administration of the antibody does not compromise the anti-tumor efficacy of the combined regimen, as indicated by comparable tumor growth, necrosis levels, and peripheral blood lymphocyte distribution relative to the group receiving the combined treatment without the antibody. HMGB1 blockade attenuates cardiac injury caused by radiotherapy combined with PD-1 inhibitor while maintaining the anti-tumor efficacy. This demonstrates that in the context of tumor treatment, targeting HMGB1 represents a promising option for alleviating the cardiac damage caused by combined therapy.</p>