Abstract 917: Functional blocking of CD5 on T cells to enhance the efficacy of therapeutic PD-1 blockade in treatment of mouse 4T1 breast tumors

Blockade of programmed cell death protein 1 (PD-1) is approved for treatment of multiple human cancers and is the focus of multiple studies due to its key role in T cell function. However, only patients with ‘9hot99 tumors (i.e. those with increased numbers of tumor-infiltrating CD8+ T cells) respond well to the blockade. This suggests that new strategies to increase infiltration of CD8+ T cells into tumors will increase the number of tumor types and patients benefiting from anti-PD-1 therapy. CD5, a member of the scavenger receptor cysteine-rich (SRCR) superfamily, is expressed on T cells and a subset of B cells (B1a). It can attenuate TCR signaling and impair cytotoxic T lymphocyte (CTL) activation. CD5 knockout mice have increased anti-tumor immunity and reduced homograft tumor growth: reducing CD5 on CTLs may be therapeutically beneficial to enhance the anti-tumor response and increase tumor-infiltrating CD8+ T cells. We report that in vivo administration of anti-CD5 blocking MAb treatment increased primary T cell activation in response to 4T1 tumor cell homografts and ex vivo activation as measured by increased CD69, Fas, Fas ligand, IFNγ, PD-1, and apoptosis. Further, anti-CD5 treatment enhanced the capacity of primary T cells to kill 4T1 tumor cells ex vivo. Mice receiving anti-CD5 and anti-PD-1 in combination exhibited increased overall survival compared to mice treated with either agent alone. These data support the potential of blockade of CD5 function to enhance T cell-mediated anti-tumor immunity and PD-1 blockade treatment. Citation Format: Faizah Alotaibi, Mateusz Rytelewski, Rene Figueredo, Ronak Zareardalan1, Saman Maleki Vareki, Xiufen Zheng, Meng Zhang, Peter Ferguson, Mikal El-Hajjar, Yousef Najajreh, Wei-ping Min, James Koropatnick. Functional blocking of CD5 on T cells to enhance the efficacy of therapeutic PD-1 blockade in treatment of mouse 4T1 breast tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 917.