IMMU-85. Uncovering differences in local and systemic immunity in human primary and metastatic brain tumors

Abstract While therapies targeting T cell immune checkpoint receptors have demonstrated clinical efficacy for patients with brain metastases, adults with high-grade gliomas fail to benefit. We comprehensively profiled intratumoral and circulating T cells in patients with untreated brain tumors to identify immunological features which may account for this discrepancy. Tumor and blood samples were collected from 22 patients with newly diagnosed brain tumors (12 high-grade glioma and 10 brain metastases from non-small cell lung cancer [NSCLC]). We performed paired single-cell RNA and T cell receptor (TCR) sequencing and multiplexed flow cytometry to deeply profile intratumoral and circulating T cells. We used a high-throughput T cell antigen identification method to screen intratumoral TCRs against 3,726 putative neoantigens and 146 tumor-associated antigens. Publicly available bulk RNA sequencing datasets were analyzed for biomarker validation. In total, we examined 159,043 high-quality T cells. Tumor-infiltrating T cells in metastases were more abundant, more clonally expanded, and had a less diverse clonal repertoire. While circulating effector memory CD8+ T cells were differentially abundant in patients with brain metastases, no differences in the composition of CD8+ T cell subsets were observed in the tumor. In contrast, a clonally expanded, tumor-enriched population of CXCL13+CD4+ T cells were differentially abundant in metastases. CXCL13+CD4+ T cells highly expressed co-inhibitory receptors, effector cytokines, and interferon-stimulated genes. Of 44 TCRs derived from CXCL13+CD4+ T cells screened, 27.3% were reactive against tumor antigens (10 neoantigens and 2 tumor-associated antigens). Higher expression of the CXCL13+CD4+ T cell signature correlated with improved overall survival in NSCLC patients treated with immune checkpoint therapy. In summary, our study highlights distinct differences in both local and systemic adaptive immunity in patients with newly diagnosed brain tumors and identifies tumor antigen-reactive CD4+ T cells in immunogenic metastases as a potential predictive biomarker for immune checkpoint therapy response.