Inducing Humoral Immunity across Cancer Types with Dendritic Cell Reprogramming Uncovers Biomarkers of Response

Effective immune responses, within the tumor microenvironment (TME), including T and B cell activation, are key for successful cancer immunotherapies. Tertiary lymphoid structures(TLS), which generate persistent T and B cell responses, correlate with improved outcomes. However, tools to therapeutically induce TLS across cancers are lacking. We previously developed an approach to reprogram tumor cells in vivo into type 1 conventional dendritic-like (cDC1-like) cells, triggering T cell responses and TLS formation in melanoma. Here, we hypothesize that dendritic cell (DC) reprogramming can induce TLS and humoral responses across tumor types. In vivo cDC1 reprogramming led to consistent synthetic TLS (synTLS) formation and tumor-agnostic anti-tumor immunity. DC reprogramming induced complete responses (CR) and outperformed anti-PD-1 therapy, across subcutaneous and orthotopic models, including ICB-resistant YUMM1.7 (60% vs. 0% CR), MC38 (80% vs. 40% CR), and CT26 (100% vs. 0% CR). Orthotopic models such as 4T1 (100% vs. 25% CR), B16 (20% vs. 0%) and LLC, also responded, confirming location-independent efficacy. Strikingly, we even observed 50% CR in the orthotopic immunosuppressive SB28 glioblastoma model, which has so far not been shown treatable with immunotherapy modalities. SynTLS formed within 9 days, containing CD4+ and CD8+ T cells, CD19+ B cells and BCL6+ germinal centers and gradually disappeared with tumor regression. Formation occurred even in BATF3KO mice lacking endogenous cDC1s. B cells contributed to anti-tumor immunity, as 30% of B-cell-depleted mice showed impaired tumor growth control despite treatment with reprogramming. Serum binding assays and serum transfer between animals demonstrated tumor- specific antibody binding of tumor tissue and when transplanted tumor growth delay suggesting role in anti-tumor immunity. Furthermore, mice treated with DC reprogramming developed circulatory tumor-specific antibodies which could serve as blood-based biomarkers of therapeutic response. Overall, cDC1 reprogramming acts as a tumor-agnostic immunotherapy that drives TLS formation, elicits humoral immunity, and generates circulating biomarkers for treatment monitoring, supporting future clinical settings of DC reprogramming.