Abstract 1643: BTN1A1: a novel immune checkpoint for cancer immunotherapy beyond the PD-1/PD-L1 axis
Authors: Chung Ezra M. , Bong Yong-Sik , Kim Young-Seung , Park Andrew , You Young-Ok , Sharma Amrish , Lin Steven H. , Lee Young-Joon , Jung Hyunjin , Yoo Stephen S. 
Published: July 2021

Abstract
Cancer immunotherapy is an effective treatment against individuals with late-stage cancer forms. The PD-1/PD-LI axis is a main therapeutic target used in clinical settings, but only 15-20% of cancer patients are responsive. Thus, there is an urgent unmet need to identify other immuno-therapeutic targets to overcome this limitation. Butyrophilin (BTN) belongs to the B7 family, peripheral membrane proteins containing an immunoglobulin domain found in PD-L1 and CTLA4. Among the BTN family, BTN2A2 and BTN3A1 are shown as alternative co-inhibitory or co-stimulatory cancer immune checkpoint(s) in the effector T cell compartment. However, the immune-modulatory roles of BTN1A1 are largely unknown, especially in vivo. Here, we report the immune-modulatory role of BTN1A1 in T cell proliferation and activation in vitro and in vivo. We have found that BTN1A1 inhibits the proliferation of T cells that are activated by anti-CD3 and anti-CD28 antibodies in vitro. The overexpression of BTN1A1 in PC3 cells also inhibits T cell-mediated cancer cell killing. BTN1A1-overexpressing B16-Ova melanoma cancer cell lines also accelerate tumor growth compared to B16-Ova wild type in a syngeneic mouse model. The effect of BTN1A1 was fully attenuated in an immune-compromised SCID mouse. BTN1A1-deficient MC38 colorectal cancer cell lines also grew significantly slower than wild-type MC38 cell lines when they were inoculated subcutaneously into a syngeneic mouse model. This result strongly supports BTN1A1's role in immune evasion. BTN1A1 is expressed on immune cells. These cells include macrophages, B cells or activated CD8 T cells. We found that BTN1A1 is expressed in human tumors, with an expression that was mutually exclusive to PD-L1. We have developed the monoclonal antibody (STC810) against the human BTN1A1. STC810 has anti-tumor activity in a human immune environment in vitro and ex vivo. Since STC810 has no cross-reactivity with mouse BTN1A1 proteins, we generated mouse surrogate antibodies to study the effects of blocking BTN1A1 in vivo in mice tumor models. The anti-BTN1A1 surrogate antibody not only exhibits a single agent but has a synergistic effect in combination with the anti-PD-L1 antibody. Overall, using mouse models, genetic ablation, and antibody tests ex vivo we demonstrate that BTN1A1 is a bona fide immune checkpoint inhibitor. Late-stage preclinical studies of humanized STC810 are underway. The Phase I clinical trial is set to start in late 2021.
Citation Format: Ezra M. Chung, Yong-Sik Bong, Young-Seung Kim, Andrew Park, Young-Ok You, Amrish Sharma, Steven H. Lin, Young-Joon Lee, Hyunjin Jung, Stephen S. Yoo. BTN1A1: a novel immune checkpoint for cancer immunotherapy beyond the PD-1/PD-L1 axis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1643.