Checkpoint Molecules on Tumor Cells: A Paradigm Shift in Cancer Therapy

Over the past two decades, immune checkpoint pathways have fundamentally reshaped the oncology therapeutic landscape of oncology. Molecules such as programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte antigen 4 (CTLA-4), lymphocyte activation gene 3 (LAG-3), T-cell immunoreceptor with Ig and ITIM domains (TIGIT), T-cell immunoglobulin and mucin domain-3 (TIM-3), and V-domain Ig suppressor of T-cell activation (VISTA) are classically viewed as key regulators of adaptive immunity, particularly controlling the activation, exhaustion, and tolerance of T cells.[1,2] Their clinical targeting through immune checkpoint blockade (ICB) has yielded unprecedented survival benefits in multiple malignancies, positioning checkpoint modulation as a cornerstone of modern immunotherapy. However, recent studies have revealed a paradigm shift in this conceptual framework: tumor cells themselves can express immune checkpoint molecules, including both canonical (e.g., PD-L1, PD-1) and noncanonical forms. This tumor-intrinsic expression challenges the traditional immune-centric view of checkpoint biology. Rather than functioning solely as ligands for immune receptors, these molecules can engage in autocrine and paracrine signaling, shaping the tumor microenvironment (TME) and conferring selective growth and survival advantages to malignant cells. Importantly, tumor-intrinsic checkpoint expression has been documented in a broad spectrum of cancers, including melanoma, pancreatic ductal adenocarcinoma, triple-negative breast cancer, glioblastoma, colorectal cancer, and non-small cell lung cancer. This emerging evidence suggests that immune checkpoint expression by tumor cells is not a mere epiphenomenon but a strategic adaptation that contributes to immune evasion, tumor progression, and therapeutic resistance. It adds a new dimension to our understanding of tumor-immune interactions and has profound implications for current therapeutic strategies targeting checkpoint pathways. These findings suggest that immune checkpoint expression by tumor cells is not merely a passive mimicry of immune regulation but an active driver of tumor fitness, plasticity, and therapeutic resistance.