Glioblastoma (GBM) is a lethal brain cancer known for its potent immunosuppressive effects. Loss of Methylthioadenosine Phosphorylase (MTAP) expression, via gene deletion or epigenetic silencing, is one of the most common alterations in GBM. Here, we show that MTAP loss in GBM cells is correlated with differential expression of immune regulatory genes. In silico analysis of gene expression profiles in GBM samples revealed that low MTAP expression is correlated with reduced proportions of γδT cells, fewer activated CD4 cells, and an increased proportion of M2 macrophages. Using in vitro macrophage models, we found that methylthioadenosine (MTA), the metabolite that accumulates as a result of MTAP loss in GBM cells, promotes the immunosuppressive alternative activation (M2) of macrophages. We show that this effect of MTA on macrophages is independent of IL4/IL3 signaling, is mediated by the adenosine A 2B receptor, and can be pharmacologically reversed. This study suggests that MTAP loss in GBM cells contributes to the immunosuppressive microenvironment, and that MTAP status should be a factor for consideration in understanding GBM immune states and devising immunotherapy-based approaches for treating MTAPnull GBM. laboratory for the LC-MS/MS metabolite analysis, and Heather Hemric, Laura-Leigh Rowlette, and Holly Dressman of the Duke Sequencing and Genomic Technologies Shared Resource for the Affymetrix array processing service.