Tumor-expressed immune checkpoint B7x-mediated resistance to anti-CTLA-4 therapy Immune checkpoint blockade of CTLA-4 and PD-1/PD-L1 have advanced the treatment of cancer patients. However, one of the biggest challenges is that the majority of cancer patients do not respond to these treatments. Based on our new results, our central hypothesis of this proposed revision research is that tumor-expressed immune checkpoint B7x induces resistance to anti-CTLA-4 therapy and that combination treatment of anti-B7x and anti-CTLA-4 leads to synergistic therapeutic efficacy and overcomes the resistance to anti-CTLA-4 therapy. Guided by our published clinical and basic research, and our strong preliminary data, we will pursue two specific aims: 1) Dissect the mechanisms underlying tumor-expressed B7x mediated resistance to anti-CTLA-4 therapy; and 2) Develop novel combination therapies of anti-B7x and anti-CTLA-4 to overcome the resistance. We have generated a number of novel tools for this project. The outcomes of this project will reveal new mechanisms underlying tumor-expressed B7x-mediated resistance to anti-CTLA-4 therapy and will develop into new immunotherapies overcoming the resistance.