Irradiation-induced calreticulin secretion normalizes tumor vasculature to enhance immunotherapy

The enhanced efficacy of immune checkpoint blockade (ICB) when combined with radiotherapy (RT) depends on effective immune cell infiltration into tumors through a normalized vasculature, rather than through the typically disorganized and dysfunctional tumor blood vessels. However, the molecular factors driving RT-induced vascular remodeling remain unclear. In this study, we demonstrated that RT promotes tumor vascular normalization, enhances T cell infiltration, and improves the long-term therapeutic efficacy of anti-PD-L1 immunotherapy in murine models. Proteomic profiling of the RT-induced tumor cell secretome identified calreticulin (CRT) as a key mediator that induces genes associated with endothelial maturation in vitro. Furthermore, melanoma and lung tumors in which Calr was stably silenced failed to exhibit radiation-induced vascular normalization within the tumor microenvironment and showed diminished responsiveness to RT. Notably, CRT deficiency abrogated the robust tumor growth suppression achieved by the triple combination of RT, anti-VEGFR2, and anti-PD-L1 therapy. These findings reveal CRT as a critical regulator of RT-induced vascular remodeling and provide mechanistic insight into how RT optimizes anti-tumor immunity. This work supports the strategic integration of RT with antiangiogenic and ICB therapies to enhance therapeutic outcomes.