BSBM-18 SINGLE-CELL PROFILING TUMOR-INFILTRATING IMMUNE CELLS REVEALS CXCL13(+) FOLLICULAR HELPER-LIKE CD4(+) T CELLS IN HUMAN BRAIN TUMORS

While therapies enhancing antitumor T cell function have clinical efficacy for patients with brain metastases, patients with high-grade gliomas fail to benefit. How the brain tumor microenvironment differentially shapes T cell function remains unknown. We performed droplet-based single-cell RNA and T cell receptor sequencing on immune cells from tumor and blood of patients with newly diagnosed non-small cell lung cancer brain metastases (n=10) and high-grade glioma (n=12). In total, we examined 220,049 high-quality immune cells, including 159,043 T cells. Tumor-infiltrating T cells in metastases were more abundant (p=0.005), more clonally expanded (p=0.039), and had a less diverse clonal repertoire (p=0.033). While no differences were observed among CD8(+) T cells, a population of CXCL13(+)CD4(+) T cells was more abundant in metastases (6.03% vs. 1.82%, p=0.004). This population expressed genes encoding transcriptional factors (BCL6, MAF), co-inhibitory receptors (PDCD1, LAG3, TIGIT), and effector cytokines (IL21, IL4, IFNG) characteristic of follicular helper T cells though notably lacked expression for the canonical chemokine receptor CXCR5. CXCL13(+)CD4(+) T cells were highly expanded, had minimal clonal overlap with other populations, largely confined to the tumor, associated with germinal center B cell and plasma cell signatures, and resembled T cell populations predictive of anti-PD-1 therapy response in extracranial disease. Based on the expression of potential ligand-receptors and downstream signaling targets, CXCL13(+)CD4(+) T cells were predicted to interact with B cells and antigen-presenting cells, as seen in tertiary lymphoid structures. To investigate the relevance of CXCL13(+)CD4(+) T cells in glioblastoma, we analyzed bulk RNA sequencing data from The Cancer Genome Atlas (n=160) and observed a trend towards longer overall survival in patients with high follicular helper-like expression. In summary, we identified a follicular helper-like CD4(+) T cell population that is preferentially found in brain metastases and may be a feature of productive antitumor immune responses in human brain tumors.