PDCD1 and glioblastoma: The spearman correlations between TREM2 gene and immunosuppressive checkpoint-related genes in GBM microenvironment based on TCGA database were further analyzed, and the results shown in Fig. 3E suggested that TREM2 was positively correlated with HAVCR2, KLRB1, CD273, CTLA4, TIGIT and CD279, and the correlation between TREM2 and HAVCR2 which encoding TIM-3 protein was strongest with the Spearman’s rank correlation coefficient of 0.863.