Accumulating data support the concept that ionizing radiation therapy (RT) has the potential to convert the tumor into an in situ individualized vaccine; however this potential is rarely realized by RT alone. Transforming growth factor xce xb2 (TGF xce xb2) is an immunosuppressive cytokine that is activated by RT and inhibits the antigen-presenting function of dendritic cells and the differentiation of effector CD8+ T cells. Here we tested the hypothesis that TGF xce xb2 hinders the ability of RT to promote anti-tumor immunity. Development of tumor-specific immunity was examined in a pre-clinical model of metastatic breast cancer. Mice bearing established 4T1 mouse mammary carcinoma treated with pan-isoform specific TGF xce xb2 neutralizing antibody 1D11 showed significantly improved control of the irradiated tumor and non-irradiated metastases but no effect in the absence of RT. Notably whole tumor transcriptional analysis demonstrated the selective upregulation of genes associated with immune-mediated rejection only in tumors of mice treated with RT+TGF xce xb2 blockade. Mice treated with RT+TGF xce xb2 blockade exhibited cross-priming of CD8+ T cells producing IFN xce xb3 in response to three tumor-specific antigens in tumor-draining lymph nodes which was not evident for single modality treatment. Analysis of the immune infiltrate in mouse tumors showed a significant increase in CD4+ and CD8+ T cells only in mice treated with the combination of RT+TGF xce xb2 blockade. Depletion of CD4+ or CD8+ T cells abrogated the therapeutic benefit of RT+TGF xce xb2 blockade. These data identify TGF xce xb2 as a master inhibitor of the ability of RT to generate an in situ tumor vaccine which supports testing inhibition of TGF xce xb2 during radiotherapy to promote therapeutically effective anti-tumor immunity. We used genome-wide microarray to depict main biological processes responsibles for the therapeutic benefit of the combination ofTGF-beta blockade and local radiotherapy. To gain a more comprehensice protrait of the effects of RT and TGFbeta blockade on gene expressionin tumors we collected 4T1 tumors 4 days after completion of RT. Three tumors from each group were then subjected to RNA extraction and hybridization on affymetrix array.