Low-grade glioma harbors few CD8 T cells, which is accompanied by decreased expression of chemo-attractants, not immunogenic antigens

In multiple tumor types, prediction of response to immune therapies relates to the presence, distribution and activation state of tumor infiltrating lymphocytes (TILs). Although such therapies are, to date, unsuccessful in gliomas, little is known on the immune contexture of TILs in these tumors. We assessed whether low and high-grade glioma (LGG and HGG, grade II and IV respectively) differ with respect to number, location and tumor reactivity of TILs; as well as expression of molecules involved in the trafficking and activation of T cells. Intra-tumoral CD8 T cells were quantified by flow cytometry (LGG: n = 12; HGG: n = 8) and immunofluorescence (LGG: n = 28; HGG: n = 28). Neoantigen load and expression of Cancer Germline Antigens (CGAs) were assessed using whole exome sequencing and RNA-seq. TIL-derived DNA was sequenced and the variable domain of the TCRβ chain was classified according to IMGT nomenclature. QPCR was used to determine expression of T cell-related genes. CD8 T cell numbers were significantly lower in LGG and, in contrast to HGG, mainly remained in close vicinity to blood vessels. This was accompanied by lower expression of chemo-attractants CXCL9, CXCL10 and adhesion molecule ICAM1. We did not observe a difference in the number of expressed neoantigens or CGAs, nor in diversity of TCR-Vβ gene usage. In summary, LGG have lower numbers of intra-tumoral CD8 T cells compared to HGG, potentially linked to decreased T cell trafficking. We have found no evidence for distinct tumor reactivity of T cells in either tumor type. The near absence of TILs in LGG suggest that, at present, checkpoint inhibitors are unlikely to have clinical efficacy in this tumor type.