The Immunologically Cold Prostate Cancer Microenvironment: How Lymphatic Dysfunction Sustains Immune Evasion

Abstract

Prostate cancer is the prototypical immunologically cold solid tumor, with objective response rates of only 3–5% to immune checkpoint inhibitors in unselected metastatic castration-resistant disease and an estimated 89.8% of tumors classified as immunologically ignorant. Three convergent features sustain this phenotype: low tumor mutational burden with defective Major Histocompatibility Complex class I antigen presentation; an immunosuppressive microenvironment dominated by regulatory T-cells, myeloid-derived suppressor cells, and M2 macrophages; and dense stromal and vascular barriers that exclude effector lymphocytes. Across these mechanisms, one compartment has received disproportionately little attention: the lymphatic system. Tumor-associated lymphatic remodeling driven by vascular endothelial growth factor C and D produces structurally abnormal vessels that impair antigen and dendritic cell trafficking to tumor-draining lymph nodes; the lymph nodes themselves are reprogrammed to a tolerogenic state by lymphatic endothelial cells expressing programmed death-ligand 1 and lacking costimulation and by regulatory T-cells that suppress effector egress. This review synthesizes evidence that in prostate cancer, immune coldness reflects not merely a problem of checkpoint engagement with the tumor but a failure of antigen trafficking at the lymphatic interface. We discuss therapeutic strategies that target this axis, such as lymphangiogenesis-inducing vaccines, lymph-node-directed checkpoint delivery, induction of intratumoral tertiary lymphoid structures and stromal reprogramming, as complements to existing immunotherapy. Targeting trafficking, not only checkpoints, may be the prerequisite for converting PCa from cold to hot.