CD33 and central nervous system cancer: Of note, the majority of infiltrating leukocytes consisted of CD33+ myeloid cells (91.2, 92.2 and 85.6% in grade II, III and IV gliomas respectively), mainly composed by CD33+/HLA-DR+ macrophages (mean of 85.2% in grade II, 84.3% in grade III and 64.7% in GBM), and by a lower percentage of CD33dim/HLA-DR− polymorphonuclear cells (PMNs, 10.8% in grade II, 10.2% in grade III and 15.8% in GBM) (Fig. 1a).