Plasticity of Non-Apoptotic Residual Tumor Cells After Neoadjuvant Immunochemotherapy: Epigenetic and Microenvironmental Determinants.

Neoadjuvant immunochemotherapy (NICT), mainly anti-PD-1/PD-L1 therapy combined with cytotoxic chemotherapy, significantly improved perioperative outcomes for resectable solid tumors such as lung cancer and breast cancer. But a large number of patients still had residual lesions and eventually relapsed. Residual tumor cells are not simply unremoved cellular debris, but represent a therapy-selected and therapy-amplified subset of a pre-existing heterogeneous and plastic tumor ecosystem. To avoid implying that therapy generates a new form of tumor plasticity de novo, we use the term "plasticity of non-apoptotic residual tumor cells" to describe the plastic behavior of viable malignant cells that survive treatment-induced cytotoxicity rather than entering apoptosis. In this review, we define the plasticity of non-apoptotic residual tumor cells as the capacity of residual malignant cells to preserve, switch, or re-enter phenotypic states such as dormancy, hybrid EMT, stem-like regeneration, and immune evasion under the combined influence of intrinsic tumor heterogeneity, systemic therapy pressure, and microenvironmental protection. Before the NICT-specific discussion, we outline general theoretical frameworks including therapeutic stress response, apoptosis-induced regeneration, genetic and non-genetic heterogeneity, as well as spatial heterogeneity of involved lymph nodes, so as to provide a more robust interpretation of residual lesion biology under NICT. Also, this review proposes that residual disease may be reconceptualized as a treatment-shaped plastic niche, whose biological behavior is jointly shaped by clonal selection, reversible phenotypic transformation, and microenvironmental ecological protection. We summarize several key states of residual tumor cells: persistent-like/resting state, hybrid EMT/invasive plasticity state, stem-like/regenerative state, and immune escape state, and elucidate the underlying epigenetic basis, including DNA methylation, histone modification, chromatin remodeling, and non-coding RNA network reprogramming. Meanwhile, niche factors such as immune stress, CAF/TAM enrichment, fibrotic matrix, hypoxia, and metabolic stress can further stabilize these states and promote the survival of relapse seeds. Based on this, we propose that future postoperative assessments should be upgraded from residual volume to a stratified residual state, and dynamically identified by combining single-cell omics, spatial pathology, and ctDNA/MRD monitoring. Furthermore, treatment strategies should shift from simply shrinking tumors to plasticity-locking therapy, that is, identifying, classifying, and blocking the plasticity escape pathways of residual lesions before they evolve into recurrence.