IMMU-01. CHARACTERIZATION OF TUMOR-INFILTRATING CD8+ T CELLS IN BRAIN METASTASES

Abstract

Abstract BACKGROUND Use of immune checkpoint blockade (ICB) therapy has prolonged overall survival in patients with metastatic cancer. One potential strategy to improve the effectiveness of ICB is to target additional inhibitory receptors on exhausted CD8+ T cells, which may promote rescue of CD8+ T cells that do not respond to PD-1 pathway blockade alone. This therapeutic strategy requires knowledge of the inhibitory molecules expressed on tumor-specific CD8+ T cells. Toward achieving this goal, we characterized the phenotype of CD8+ T cells infiltrating brain metastases. METHODS We performed flow cytometry on 45 brain metastases samples, single cell RNA sequencing with T cell receptor (TCR) sequencing on 5 samples, and spatially-resolved transcriptomics on 8 samples. RESULTS Analysis of our scRNA-seq data revealed 4 populations of PD-1+ CD8+ T cells infiltrating brain metastases. One of these populations (cluster A) has a terminal effector exhausted phenotype suggesting that this population contains tumor-specific CD8+ T cells. Two of the other populations (clusters B and C) have a transcriptional profile that suggests they may contain stem-like CD8+ T cells. TCR sequencing shows that cells in cluster A do not express the same TCRs as cells in clusters B and C, suggesting that stem-like cells in clusters B and C are not the progenitors of the terminal effector cells in cluster A. Bystander cells expressing TCRs specific for viral antigens are found predominantly in clusters B and C, further supporting the hypothesis that cluster A contains tumor-specific cells. Spatial transcriptomics reveals that cluster A cells are infiltrating the tumor parenchyma while cluster B and C cells are predominantly in peri-tumoral inflammatory tissue. CONCLUSIONS Brain metastases are infiltrated by a population of terminally-differentiated effector CD8+ T cells which express co-inhibitory molecules that may be potential therapeutic targets to improve control of metastatic disease in the brain.