Mechanistic Dissection and Therapeutic Targeting of B7x in Cancer

PROJECT SUMMARY/ABSTRACT Immune checkpoints are proteins that regulate the body?s immune system via inhibition of immune cells. In diseases such as cancer, these same pathways may be commandeered in order to inappropriately inhibit immune responses. One approach to overcome this excessive inhibition and restore normal immune function is to physically block these inhibitory proteins using monoclonal antibodies (mAbs), a strategy known as immune checkpoint blockade. Our lab discovered the protein B7x (VTCN1/B7S1/B7-H4), an immune checkpoint whose regulation and mechanism of action are still being elucidated. B7x is expressed on a wide variety of cancers and is associated with poor clinical outcomes. It has been shown to inhibit effector T cell functions such as cytokine production and proliferation, and promotes T cell exhaustion; it has also been associated with tumor infiltration of immunosuppressive cell populations such as myeloid derived suppressor cells (MDSCs). Intriguingly, it is not typically co-expressed with the well-studied immune checkpoint PD-L1, and blocking B7x using anti-B7x mAbs improves anti-tumor responses and survival in mouse models. This suggests that B7x holds a non-overlapping but key role in cancer immune evasion. For these reasons I hypothesize that B7x mediates immune evasion through inhibiting effector cell functions while simultaneously promoting immunosuppressive cells and is therefore a promising target for cancer immunotherapy. Thus, we propose the following two aims: 1) Examine the mechanisms that regulate B7x expression and its role on MDSCs; and 2) Develop and characterize a new anti-B7x immunotherapy. In aim 1 I will examine the how hypoxia and other tumor microenvironment-associated cytokines regulate B7x expression. In addition, I will explore how B7x affects the immunosuppressive MDSC cell population generation and survival. In aim 2 I will test the anti-tumor efficacy of newly generated anti-B7x mAb-based immune checkpoint blockade in vivo using metastasis and spontaneous models of lung cancer. We will also characterize how this new immunotherapy alters T cell phenotypes using flow cytometry and single-cell RNA sequencing. Finally, we will explore if combining our anti-B7x mAb with anti-TIM3 and anti-PD1 mAbs confers superior anti-tumor efficacy than anti-B7x monotherapy in vivo. Together, these studies will allow us to broadly explore the role of B7x in tumorigenesis and therapeutic potential of anti-B7x immunotherapy, and combined with a personalized training plan and supportive research environment at Einstein, cultivate the skills required of a physician-scientist.