Single-cell and spatial profiling identify three response trajectories to pembrolizumab and radiation therapy in triple negative breast cancer

Cancer immunotherapy trials have produced encouraging results, but resistance remains a problem necessitating strategies to identify patients that benefit from immunotherapy alone or who require additional combinations like chemotherapy or radiotherapy. Here we employ single-cell transcriptomics and spatial proteomics to profile triple negative breast cancer biopsies taken before and after one cycle of pembrolizumab and after a second cycle of pembrolizumab given with radiotherapy. Non-responders lack immune infiltrate before and after therapy and exhibit minimal therapy-induced immune changes. Responding tumors form two groups that are distinguishable by a classifier prior to therapy, with one showing high MHC expression, evidence of tertiary lymphoid structures and displaying anti-tumor immunity before treatment. The other responder group resembles non-responders at baseline and mounts a maximal immune response after the combination therapy, which is characterized by cytotoxic T cell and antigen presenting myeloid cell interactions and is mirrored in murine breast tumors only after radiation.