Eosinophils in lung cancer: from inflammatory cytokines to immunotherapeutic biomarkers

Eosinophils, traditionally viewed as effector cells in allergic and parasitic responses, have emerged as multifaceted regulators within the tumor microenvironment (TME). In lung cancer, eosinophils demonstrate complex and context-dependent functions, shaped by chemokines, cytokines, and tumor-derived signals such as CCL11 and IL-33. Recent studies indicate that eosinophils may either promote anti-tumor immunity, by enhancing CD8⁺ T cell infiltration, secreting cytotoxic granules, and cooperating with IL-33, or facilitate tumor progression through recruitment of regulatory T cells, immune suppression, and expression of immunoregulatory enzymes like IDO. Moreover, eosinophil abundance in tumor tissues and peripheral blood has been associated with both favorable and unfavorable prognostic outcomes in lung cancer patients. Notably, elevated eosinophil counts correlate with improved responses to immune checkpoint inhibitors (ICIs), positioning them as potential biomarkers for immunotherapy efficacy. However, distinctions between tumor-infiltrating and circulating eosinophils, as well as their dualistic roles in metastasis and immune modulation, remain incompletely understood. This review summarizes current advances in understanding eosinophil biology in lung cancer and underscores their promise as diagnostic and therapeutic targets in precision immuno-oncology.