SFRP2+ CAFs Hinder Abscopal Effect from Combined Radiotherapy and Immunotherapy by Forming a Hostile Perivascular Niche
Despite accumulating cases of radiotherapy-induced abscopal effect in the lung cancer with the introduction of immune checkpoint inhibitors (ICIs), the occurrence of this effect remains infrequent and unpredictable to be a therapeutic goal. Here, we showed that the combination of radiotherapy (8Gy*3F) and ICI alleviated the tumor burden at the irradiated site whereas no discernible benefit was observed in the abscopal tumors. RNA-sequencing data showed that extracellular structure organization pathways were enriched in the abscopal tumors after combined therapy, with Sfrp2 being identified as a central hub. SFRP2 expression was observed in cancer-associated fibroblasts (CAFs) and was elevated in abscopal tumors after combined therapy. Blockade of SFRP2 followed by combined radiotherapy and ICI reinvigorated infiltration and cytotoxicity of CD8+ T cells, and elicited regression of abscopal tumors, which was abrogated by CD8a depletion. Mechanistically, in vitro experiments demonstrated that SFRP2+ CAFs induced apoptosis of CD8+ T cells. The spatial transcriptome analysis showed that SFRP2+ CAFs were located in proximity to the vessels and surrounded by abundant macrophages and limited CD8Tex, thereby creating an immunosuppressive perivascular niche, which was validated in paraffin sections of human lung cancer. Lineage-tracing assays showed SFRP2+ CAFs were derived from pericytes. IGF-1 released by irradiated tumors facilitated the transition of pericytes into fibroblasts and stimulated the expression of SFRP2. In summary, SFRP2+ CAFs hijack the abscopal effect from combined radiotherapy and immunotherapy via inducing apoptosis of CD8+ T cells and orchestrating a hostile perivascular niche in the lung cancer. Targeting SFRP2+ CAF may recondition the TME and promote the abscopal effect. Overall design: RNA-sequencing of 4T1 murine tumor specimens administered with different treatments:  immune checkpoint inhibitor, or combination of immune checkpoint inhibitor and radiation. 3 replicates for each group.