Tumor-expressed immune checkpoint B7x promotes cancer progression and antigen-specific CD8 T cell exhaustion and suppressive innate immune cells

// Kim C. Ohaegbulam 1 , Weifeng Liu 2 , Hyungjun Jeon 1 , Steven C. Almo 2, 3 and Xingxing Zang 1, 4, 5 1 Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, NY, USA 2 Department of Biochemistry, Albert Einstein College of Medicine, Bronx, NY, USA 3 Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY, USA 4 Department of Medicine, Albert Einstein College of Medicine, Bronx, NY, USA 5 Department of Urology, Albert Einstein College of Medicine, Bronx, NY, USA Correspondence to: Xingxing Zang, email: xingxing.zang@einstein.yu.edu Keywords: B7x, immune checkpoint, pulmonary metastases, CD8 T cells, innate cells Received: July 12, 2017      Accepted: August 29, 2017      Published: September 20, 2017 ABSTRACT B7x (B7-H4 or B7S1) is a coinhibitory member of the B7 immune checkpoint ligand family that regulates immune function following ligation with its unknown cognate receptors. B7x has limited expression on normal tissues, but is up-regulated on solid human tumors to inhibit anti-tumor immunity and associates with poor clinical prognosis. We assessed the contribution of cytokine stimuli to induce surface B7x expression on cancer cells and the role of tumor-expressed B7x in a murine pulmonary metastasis model, and finally evaluated the potential interaction between B7x and Neuropilin-1, a suggested potential cognate receptor. We showed that pro-inflammatory and anti-inflammatory cytokines IFNγ, TNFα, and IL-10 did not induce expression of B7x on human or murine cancer cells. Following i.v. injection of CT26, a murine colon cancer cell line in the BALB/c background, we observed a significant increase in tumor burden in the lung of B7x-expressing CT26 mice compared to B7x-negative parental CT26 control mice. This was marked by a significant increase in M2 tumor associated macrophages and antigen-specific CD8 T cell exhaustion. Finally, we found through multiple systems that there was no evidence for B7x and Neuropilin-1 direct interaction. Thus, the B7x pathway has an essential role in modulating the innate and adaptive immune cell infiltrate in the tumor microenvironment with its currently unknown cognate receptor(s).