High dose-rate brachytherapy of localized prostate cancer converts tumors from cold to hot

Abstract Prostate cancer (PCa) has a profoundly immunosuppressive microenvironment, we hypothesized that radiation therapy would break this immune suppression. To investigate this, we performed high-throughput immune cell subset analysis, and gene expression profiling of pre-versus post-radiation tissue in a cohort of patients with localized disease that received high dose-rate brachytherapy (HDRBT). We resolved tumor and non-tumor zones in our spatial analysis to explore what drives the immunological response. Nanostring immune profiling revealed numerous immune checkpoint molecules were stimulated in response to HDRBT (e.g. B7-H3, CTLA4, PDL1 and PDL2). A published 16-gene tumor inflammation signature (TIS) gene profiling of immune activation states (high: hot , intermediate and low: cold ) showed that most tissues possessed a low TIS pre-HDRBT. Crucially, HDRBT converted 80% of these ‘cold’-phenotype tumors into an ‘intermediate’ or ‘hot’ class. We used digital spatial profiling to show these HDRBT-induced changes in prostate TIS scores were derived from the non-tumor regions. Furthermore, these changes in TIS were also associated with pervasive changes in immune cell density and spatial relationships – in particularly between T cell subsets and antigen presenting cells. We identified increased density of CD4 + FOXP3 + T cells, CD68 + macrophages and CD68 + CD11c + dendritic cells in response to HDRBT. The only subset change in tumor zones was PDL1 + macrophages. While these immune responses were heterogeneous, HDRBT induced significant changes in immune cell associations, including a gained T cell and HMWCK + PDL1 + interaction in tumor zones. In conclusion, we showed HDRBT has a clear impact in converting “cold” prostate tumors into more immunologically activated “hot” tissues, with accompanying spatially-organized immune infiltrates and signaling changes. Understanding and potentially harnessing these changes will have widespread implications for the future treatment of localized PCa and the possible use of combination immunotherapies.