Microbial-induced trained immunity for cancer immunotherapy

Myeloid innate immune cells, including macrophages, neutrophils, myeloid-derived suppressor cells, and dendritic cells, represent major components of the tumor microenvironment (TME), exhibiting remarkable plasticity and dual roles in cancer progression and immune regulation. In recent years, microbial-induced innate immune memory, also termed "trained immunity" (TRIM), has emerged as a novel strategy to reprogram myeloid cells into an immunostimulatory, antitumor state. In this review, we explore the intricate landscape of myeloid cells in cancer and examine how microbial ligands, such as the Bacillus Calmette-Guérin vaccine and β-glucan, reprogram both bone marrow progenitors and tissue-resident myeloid cells to enhance inflammatory and antitumor responses. Notable findings include the hematopoietic stem and progenitor cell reprogramming by Bacillus Calmette-Guérin for sustained anticancer immunity, and the enhanced granulopoiesis and neutrophil-mediated tumor killing mediated by β-glucan-induced TRIM. These mechanisms synergize with immunotherapies, such as immune checkpoint inhibitors, by reshaping the immunosuppressive TME and enhancing adaptive immunity. However, challenges remain, including the structural complexity of microbial products, the lack of predictive biomarkers, and the need for optimized dosing and delivery strategies. Addressing these gaps by introducing precise characterization of microbial-derived ligands, biomarker-driven patient selection through large-scale clinical trials, as well as the development of novel approaches for targeted therapy will be essential to harness the full potential of microbial-induced TRIM, ultimately paving the way for more effective and durable cancer immunotherapies. SIGNIFICANCE STATEMENT: Tumor-promoting myeloid cells within the tumor microenvironment remain a major barrier to effective cancer immunotherapy. Microbial-induced trained immunity offers a novel strategy to reprogram myeloid cells into an antitumor state. This review provides a comprehensive overview of myeloid cell populations in cancer and the mechanisms underlying microbial-induced trained immunity. It also highlights preclinical and clinical evidence demonstrating the efficacy of microbial-based strategies in overcoming immunosuppression and synergizing with existing immunotherapies, offering a promising approach to improve cancer treatment outcomes.