Background: Ionizing radiation is still an important part of multimodal cancer treatment strategies. Late-time adverse cardiovascular events, which often occur years or decades after radiation therapy, become more evident with rising number of long-term cancer survivors. Recent experimental and clinical data indicate that endothelial cell damage leading to micro- and macrovascular changes plays an important role in the development of radiation-induced cardiac damage. Cardiovascular symptoms in patients after radiation therapy are often associated with heart failure with preserved ejection fraction. Patho-mechanisms remain incompletely understood.

Methods: Mice received a single dose of whole thorax irradiation (12.5 Gy) and were sacrificed at 1 and 3 days or 3, 6, 12, 16, 20 and 25-30 weeks. Endothelial cells and immune cells at different time points were quantified using flow cytometry (FACS). Structural changes and localization of endothelial cell damage was imaged using light-sheet fluorescence microscopy (LSFM) with CD31 staining. Development of fibrosis was determined using qRT-PCR (fibronectin and TGFβ),western blot (collagen-1,α-smooth muscle) and (immune-)histological analyses. Functional analyses were conducted using echocardiography and pressure-volume-(PV-)catheterization.

Results: Endothelial damage was determined by significant reduction of CD31 expression in mouse hearts 6 weeks after irradiation compared to sham-treated control mice using FACS analyses. LSFM showed structural changes especially in the edge zone of left ventricle presented as less densely CD31 stained regions. Additionally, we investigated cardiac immune cell response regarding innate and adaptive immunity, showing specific response to tissue damage at different time points. Highest level of monocytes and macrophages was measured at 12 weeks. Regarding cardiac long-term damage, myocardial fibrosis was detected on RNA- and protein-level as well as in histological analyses with significant changes 20 weeks after chest irradiation. This could be correlated with echocardiographic parameters for diastolic dysfunction (elevated isovolumic relaxation time/mitral valve deceleration time). Also functional reserve of irradiated mice was reduced, investigated by measurement of cardiac output and stroke volume after dobutamine injection in PV-catheterization.

Conclusion: We described a novel time-dependent endothelial cell damage and immune cell response after thoracic irradiation in mice, which could also be imaged using LSFM. Characterization of long-term damage showed cardiac fibrosis correlating with diastolic dysfunction and reduced contractile reserve. Furthermore, the established mouse model presents new starting points for therapeutic strategies to reduce radiation-induced cardiac damage.