Case Report: Mature tertiary lymphoid structures in a metastatic urothelial carcinoma patient with an exceptional response to sequential immune checkpoint inhibitor and antibody-drug conjugate therapy.

BACKGROUND: Biomarkers that predict durable benefit from immune checkpoint inhibitors (ICIs) and antibody-drug conjugates (ADCs) in metastatic urothelial carcinoma (mUC) remain limited. Tertiary lymphoid structures (TLS), particularly mature TLS (mTLS) characterized by follicular dendritic cell networks and germinal centers, may reflect a highly compartmentalized antitumor immune niche. CASE PRESENTATION: A 77-year-old man with bladder cancer and lung metastases experienced disease progression after two cycles of gemcitabine/cisplatin. Pembrolizumab was initiated but discontinued after a single dose due to destructive thyroiditis; nevertheless, his lung metastases subsequently regressed. Because residual urothelial disease persisted in the bladder, enfortumab vedotin (EV) was initiated. A complete response (CR) was achieved after three cycles and maintained through 10 cycles, at which point treatment was stopped due to fatigue. The patient has remained recurrence-free for over 4 years since the initiation of pembrolizumab. METHODS AND RESULTS: Immunohistochemistry of the pre-treatment transurethral resection of bladder tumor (TUR-Bt) specimen (CD3, CD4, CD8, CD20, CD21, BCL6) revealed numerous TLS adjacent to the tumor, many of which met the criteria for mTLS (CD21+ FDC networks and BCL6+ germinal center B cells). A HALO-based digital pathology spatial analysis demonstrated a highly enriched B- and T-cell microenvironment, with the vast majority of these lymphocytes tightly compartmentalized within the mTLS regions. In an exploratory cohort of six additional sequentially treated mUC cases evaluated using the same methodology, mTLS were not detected, even among cases exhibiting a generalized T-cell infiltration. CONCLUSION: Pre-existing mTLS identified in routine TUR specimens reflect a highly compartmentalized immune niche-distinct from a simple "hot tumor" phenotype-that may help predict which patients are likely to achieve a durable benefit from sequential ICI and ADC therapy, potentially informing treatment selection and sequencing in mUC.