Decoding the Cardiac Immune Microenvironment and Fibroblast Crosstalk in Radiotherapy Combined with Immunotherapy‐Induced Cardiac Fibrosis Based on Single‐Cell Transcriptomic Analysis

ABSTRACT Purpose: The combination of radiotherapy and immunotherapy (radioimmunotherapy) shows promising antitumor efficacy but raises cardiotoxicity concerns. The underlying mechanisms remain unclear. Methods: Preclinical models were used to assess cardiac function at day 28, 3 months, and 5 months post radioimmunotherapy intervention. The scRNA‐seq and molecular experiments were conducted. IL‐6 knockout mice and tocilizumab (IL‐6R inhibitor) were used for targeted interventions. Results: Radioimmunotherapy exacerbated cardiac fibrosis and enhanced fibroblast‐immune cell crosstalk, accompanied by robust activation of IL‐6 signaling predominantly derived from fibroblasts. Elevated serum IL‐6 levels were also observed in patients receiving combined thoracic radiotherapy and immunotherapy. Both IL‐6 knockout and tocilizumab treatment effectively alleviated acute cardiac injury, inflammation, and fibrosis. Notably, tocilizumab likely inhibits the IL‐6+ fibroblasts‐mediated activation of themselves and CCR2+ macrophages, which these subsets exhibit enhanced pro‐fibrotic scores in radioimmunotherapy‐induced cardiac damage. Moreover, alterations in immune checkpoint molecules were observed in the cardiac microenvironment following radioimmunotherapy. Macrophages with high IL‐6 signaling activity exhibited elevated CD86 expression, which was reduced upon tocilizumab treatment. Conclusions: Our study identifies fibroblast‐immune cell interactions, particularly IL‐6‐mediated fibroblast‐macrophage crosstalk, as a key mechanism in radioimmunotherapy‐induced cardiac fibrosis. Tocilizumab, an IL‐6R inhibitor, demonstrates therapeutic potential to attenuate this cardiotoxicity.