A novel human anti-TIGIT monoclonal antibody with excellent function in eliciting NK cell-mediated antitumor immunity
Introduction
Checkpoint blockade immunotherapies targeting the PD-1/PD-L1 axis and/or CTLA-4 have achieved dramatic clinical benefits in multiple malignancies by enhancing effector T cell function. However, some challenges still exist, such as the small proportion of patients exhibiting remarkable and durable responses and a subset of patients showing drug resistance, suggesting that some mechanisms must be limiting the antitumor immunity of T cells and that supplementary novel strategies and therapeutic targets urgently need to be explored.
The inhibitory receptor TIGIT is an emerging novel checkpoint target that is expressed on both T cells and NK cells [1]. Mechanistic studies suggest that TIGIT induces the suppression of immune cells through the binding of CD155, generally observed on antigen-presenting cells or target cells; CD155 is a ligand shared by TIGIT, CD226 and CD96 and shows the highest affinity for TIGIT [[2], [3], [4]]. The role of TIGIT/CD155 signaling in tumor immunotolerance is analogous to that of the PD-1/PD-L1 axis. In recent years, the association between tumor progression and elevated TIGIT expression in tumor-infiltrating immune cells, especially cytotoxic T lymphocytes, has been extensively demonstrated in preclinical models and tumor-bearing patients [[4], [5], [6]]. Most recently, work by researchers further revealed the critical role of NK cells in the therapeutic effects of TIGIT blockade and highlighted NK cells as emerging targets for checkpoint inhibition [7,8]. Their data demonstrated that blockade of TIGIT reverses the exhaustion of tumor-infiltrating NK cells and boosts NK cell–mediated antitumor immunity. Moreover, unleashing NK cells via TIGIT blockade could prevent the exhaustion of CD8+ T cells and improve the therapeutic effects of PD-1/PD-L1 antibodies. Thus, with respect to immunotherapy targeting TIGIT, its regulation of the function and activity of NK cells may underpin an additional mode of action and simultaneously unleash both innate (NK cell) and adaptive (T cell) antitumor immunity.
Several investigational antibodies against human TIGIT, as a monotherapy or in combination with PD-1/PD-L1 antibodies, have demonstrated appreciable and excellent efficacy in some cancers, which makes the blockade of TIGIT a promising therapeutic approach. Among these agents, tiragolumab, developed by Roche/Genentech, is relatively advanced in clinical progression (phase III) and exerts notable antitumor effects in patients with nonsmall cell lung cancer when combined with atezolizumab (anti-PD-1 antibody) [9]. MK7684, a humanized antibody from MSD. that is in a phase II clinical trial, is the only candidate showing favorable efficacy as a monotherapy, displaying a 35% disease control rate in patients with advanced solid tumors [10]. However, given the limited knowledge about the pivotal role of NK cells in the early stage, almost all of the research on these investigational anti-TIGIT antibodies has focused on the immune responses of cytotoxic T cells within the tumor microenvironment.
Here, we describe a novel anti-TIGIT monoclonal antibody, named AET2010, acquired from a phage-displayed human single-chain antibody library through a cell panning strategy. With emphasis on its regulation of NK cells, we confirmed the excellent ex vivo and in vivo antitumor immunity of AET2010 mediated by the NK-92MI cell line, indicating a promising therapeutic application. Intriguingly, our work also revealed that AET2010 displays a lower affinity but parallel avidity and activity relative to MK7684, implying a reasonable balance of potency and potential toxicity for AET2010.
Section snippets
Construction and identification of a stable cell line with high TIGIT expression (CHO-TIGIT)
A human TIGIT cDNA ORF clone (SinoBiological, Beijing, China) was transfected into CHO cells (maintained in our lab) using TransIntro™ Transfection Reagent (TransGen, Beijing, China). Subsequently, the cells were cultured in RPMI-1640 medium (Gibco, NY, USA) containing 10% FBS (Gibco, NY, USA) and 200 μg/ml hygromycin-B (WWR Chemicals, CA, USA) for selection, which was renewed every 3–5 days. Single clone cells were resuspended and cultured on new cell plates until reaching approximately 90%
Generation of anti-human TIGIT antibody by using the phage-displayed human antibody library and cell panning
The recombinant TIGIT ectodomain protein has a small molecular weight (approximately 14 kDa) and shows unstable and heterogeneous characteristics, which makes it difficult to use to select antibodies from our phage library in precoated immunotubes, a commonly used high-throughput compatible method [11,12]. Instead, a cell panning strategy was established to obtain phage-displayed specific antibodies against TIGIT in this study. Due to their lack of endogenous expression of TIGIT and low
Discussion
NK cells are essential antitumor innate lymphocytes and suffer functional exhaustion in tumor microenvironments [7,15]. Unleashing tumor-infiltrating NK cells might provide a direct attack on escaped tumor cells with downregulated expression of MHC class I and prevent the exhaustion of CD8+ T cells, which is considered beneficial for the therapeutic effects of PD-1/PD-L1 antibodies [7,8]. As a promising checkpoint target expressed on both NK cells and T cells, TIGIT has been proven to be
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Dr. Shuang Wang (Beijing Kohnoor Science & Technology Co., Ltd., Beijing, China) for the gift of nivolumab. We are also grateful to Prof. Junjie Yue for his support of computational design workstation. We further thank Prof. Zhiwei Sun, Qiang Sun, Weiyi Qiu and Yunzhou Yu for their expert technical help.
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These authors contribute equally.