Expression of immune checkpoint molecules on tumor infiltrating lymphocytes in glioblastoma multiforme

Abstract Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults and is manifested with poor prognosis. GBM has long been recognized as an immunosuppressive neoplasm characterized by activation of various immune escape mechanisms. Recently, immune checkpoint inhibitors have shown clinically relevant improvements with high response rates and durable tumor remissions in several cancer types, and anti-PD-1, PD-L1 and CTLA-4 blockade are currently tested in clinical trials for GBM. However, the expression and principal role of the checkpoint molecules in GBM have not been well studied. The present study assessed the expression of immune checkpoint molecules in patients with GBM by flow cytometry. Overall levels of PD-1, CTLA-4, Tim-3 and LAG-3 on CD8+ T cells were significantly higher in tumor infiltrating lymphocytes (TILs) compared to those detected on the T cells in peripheral blood mononuclear cells (PBMCs). In the case of CD4+ TILs, PD-1, CTLA-4, Tim-3, LAG-3 and TIGIT were significantly increased. Furthermore, co-expression of the checkpoint molecules was prevalently found in TILs. PD-1+CTLA-4+, PD-1+Tim-3+, and PD-1+LAG-3+ CD8+ T cells and PD-1+CTLA-4+, PD-1+CTLA-4+Tim-3+, and PD-1+CTLA-4+TIGIT+ CD4+T cells were relatively enriched in TILs compared to PBMCs. However, in detail, checkpoint molecules in each individual patient showed various expression patterns. Collectively, our results indicate that GBM induces expression of multiple immune checkpoint molecules on TILs. Further studies are needed to determine whether immune checkpoint blockers can functionally restore these exhausted T cells and which combination of the blockers can optimally restore T cell function in GBM.