Microbial Peptides Activate Tumor-Infiltrating Lymphocytes in Glioblastoma

Microbial organisms play key roles in numerous physiological processes in the human body and have recently been shown to modify the response to cancer radiotherapy and immune checkpoint inhibitors. Here, we demonstrate that HLA molecules of both glioblastoma tissues as well as tumor cell lines present bacteria-specific peptides. This finding prompted us to examine whether tumor-infiltrating lymphocytes (TILs) recognize intratumoral microbiota. Indeed, bacterial peptides eluted from HLA-DR II molecules are recognized by both TILs, albeit weakly, and peripheral blood-derived memory CD4+ T cells, which, upon stimulation with bacterial peptides, also recognize several tumor antigens. Furthermore, using an unbiased antigen discovery approach for a TIL CD4+ T cell clone (TCC) we show that it recognizes a broad spectrum of peptides from pathogenic bacteria, commensal gut microbiota and also glioblastoma-related tumor antigens. These peptides were strongly stimulatory for bulk TILs and peripheral blood memory cells. Our data hint at how bacterial pathogens and bacterial gut microbiota can be involved in specific immune recognition of tumor antigens.