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

[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.