The B7x pathway in the tumor microenvironment During the first five years of the R01 funding period (April 2014 – March 2019), we produced 37 publications and received 3 granted patents. We made major discoveries on function and structure of B7x, a much less studied B7 family member that was originally discovered by us. Immune checkpoint blockade of PD-1/PD-L1 and CTLA-4 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. Clearly, new strategies targeting additional immune checkpoints are needed to improve the immunotherapy of human cancers. Our basic and clinical studies and crystal structure analysis suggest B7x immune checkpoint has very different mechanisms and provides an excellent target to develop new immunotherapies. Furthermore, we recently discovered HHLA2 as a homolog of B7x and a new member of the B7 family, which provides a unique opportunity to study a new human immune checkpoint. Thus, our central hypothesis is that B7x and HHLA2, two less-studied members of the B7 family originally discovered by us, are critical immune evasion pathways within the tumor microenvironment and are therapeutic targets for new cancer immunotherapies. This hypothesis will be tested by pursuing three aims: 1) Dissect molecular and cellular mechanisms by which tumor-expressed B7x induces immunosuppression within the tumor microenvironment; 2) Develop new immune checkpoint blockade targeting B7x: Combination therapies and mechanisms; and 3) Elucidate the HHLA2 pathway: A new homology of B7x. We have generated a number of novel tools which provides us with unique opportunities to address challenges and realize goals. The outcomes of this project will reveal new immune evasion mechanisms in the tumor microenvironment and will establish the foundation for clinical design of new immunotherapies, which could potentially be effective in tumors that resist current PD-1/PD-L1 and CTLA-4 targeted therapies.