Cancer Stem Cells Decide the Fate of Cancer Immunotherapy by Remodeling Tumor Microenvironment.

Success of cancer immunotherapy (CIT) is intricately influenced by the tumor microenvironment (TME), a complex ecosystem that encompasses immune cells, stromal elements, and extracellular components. Despite the clinical breakthroughs of immune-checkpoint inhibitors (ICIs), adoptive cell therapies, cancer vaccines, and other immunotherapeutic interventions, many patients fail to respond and eventually die. Emerging evidence points to cancer stem cells (CSCs) as critical drivers of immune evasion, therapy-resistance, and tumor relapse. CSCs modulate the TME by secreting immune-suppressive factors, recruiting regulatory immune cells, and inducing phenotype-switching of anti-tumor TME subsets, thereby creating a protective niche that hinders immune surveillance. Conversely, the TME protects CSCs through hypoxia, altered metabolism, and immuno-suppressive cell populations. This bi-directional crosstalk supports tumor progression and provides resistance to immunotherapeutic strategies mainly by: (i) escaping immune-recognition and inhibiting active T cells via high immune-checkpoint molecule expression, (ii) creating immunosuppressive pro-tumor environment, and (iii) evading immune-mediated apoptosis of CSCs along with therapy-induced enrichment of their pool. Targeting CSCs in concert with reprogramming the TME via CSC-directed agents, metabolic modulators, or combinatorial immunotherapies, therefore, offers a promising avenue to overcome immunotherapy-resistance and achieve durable clinical responses. This review discusses the deeper mechanistic understanding of CSC-TME interactions, in light of designing next-generation immunotherapies with broader efficacy across diverse tumor types.