CD33 and glioblastoma: In glioblastoma patients, different MDSC populations are present, with the major population being polymorphonuclear CD15+CD33+HLADR- (PMN-MDSCs) accounting for 82%, followed by lineage-negative (E-MDSCs) at 15% and monocytic (CD14+CD33+HLADR-; M-MDSCs) at 3% (61, 65).