The anti-B7-H3-4Ig antibody TES7 recognizes cancer stem cell lines, modulates angiogenic factor secretion, and exhibits potent anti-tumor activity in vivo

2555 B7-H3 is a member of the B7 super-family of immunoregulatory proteins and is the only member existing in two isoforms. A two-domain structure, as shared by all members of the family, has been detected in mice and hamsters, while a four-domain structure, known as B7-H3b or B7-H3-4Ig, was found exclusively in humans and monkeys. B7-H3-4Ig is believed to originate from gene duplication - both IgV-IgC domains share very high sequence similarity. Although its function is still unclear, both co-stimulatory and co-inhibitory responses have been reported for B7-H3, suggesting B7-H3 may interact with multiple receptors or have diverging activities depending on biological context. The monoclonal antibody TES7 was raised using Raven’s cancer therapy platform and recognizes the B7-H3-4Ig antigen, but not a B7-H3-2Ig variant. ELISA-based binding studies with recombinant B7-H3 indicate TES7 requires a specific cell surface presentation of B7-H3-4Ig and TES7 binding is sensitive to antigen density and possibly conformation. Immunohistochemistry studies showed that TES7 exhibits very little binding to normal human tissues, while it binds to select fetal tissues and multiple human tumors, including prostate, colon, breast and lung adenocarcinomas, consistent with B7-H3-4Ig being an oncofetal antigen. Contrary to other B7-H3 antibodies, TES7 showed only very restricted staining on normal tissue and strong anti-tumor activity in vivo experiments. In addition, TES7 recognizes an antigen expressed on a broad variety of Raven-derived cancer stem cell lines from solid tumor explants, including colon, pancreas, prostate, lung, and breast cancers. Thus TES7 may recognize an epitope on B7-H3-4Ig that is unique relative to other anti-B7-H3 antibodies. The anti-tumor activity of TES7 was assessed in subrenal capsule (SRC) and subcutenous (SC) xenograft models. Treatment of established Hs700T pancreatic adenocarcinoma SC xenografts with TES7 led to a significant reduction in tumor size up 50% after 4 weeks of treatment. Further, treatment of prostate tumor stem cell SRC xenografts, with TES7 at 50 mg/kg/dose twice weekly for 3 weeks, led to a ~60% reduction in tumor size. Since tumor progression and development are influenced by intrinsic and micro-environmental factors, we investigated the cytokine profile of cancer stem cells in response to TES7. Importantly, TES7 significantly reduced the secretion of VEGF up to 60% in a subset of the tumor stem cell lines examined. This is the first report of regulation of angiogenic factors by tumor stem cells with an antibody that we are aware of, and suggests that regulation of VEGF may contribute to the anti-tumor activity of TES7 observed in vivo.