CTLA4 blockade promotes vessel normalization in breast tumors via the accumulation of eosinophils

Immune checkpoint blockade (ICB) has shown long‐term survival benefits, but only in a small fraction of cancer patients. Recent studies suggest that improved vessel perfusion by ICB positively correlates with its therapeutic outcomes. However, the underlying mechanism of such a process remains unclear. Here, we show that anti‐cytotoxic T‐lymphocyte‐associated protein 4 (CTLA4) treatment‐induced tumor vessel normalization was accompanied by an increased infiltration of eosinophils into breast tumors. Eosinophil accumulation was positively correlated with the responsiveness of a breast tumor to anti‐CTLA4 therapy. Depletion of eosinophils subsequently negated vessel normalization, reduced antitumor immunity and attenuated tumor growth inhibition by anti‐CTLA4 therapy. Moreover, intratumoral accumulation of eosinophils relied on T lymphocytes and interferon γ production. Together, these results suggest that eosinophils partially mediate the antitumor effects of CTLA4 blockade through vascular remodeling. Our findings uncover an unidentified role of eosinophils in anti‐CTLA4 therapy, providing a potential new target to improve ICB therapy and to predict its efficacy.