Understanding the Spatial Topology of Artificial Immunological Synapses Assembled in T Cell-Redirecting Strategies: A Major Issue in Cancer Immunotherapy (2020)

T cell-redirection strategies aim to selectively eliminate cancer cells by physically linking T lymphocytes with cancer cells using tumor-targeted cell-cell bridging (CCB) molecules, such as membrane-anchored chimeric antigen receptors (CARs) or soluble bispecific antibodies (bsAbs) that specifically recognize a cell-surface tumor-associated antigen (TAA) (Blanco et al., 2019). In the CAR approach, a TAA-specific antibody is genetically fused to intracellular T cell signaling domains. CARs have evolved greatly since their initial description, as single-chain antibody fragment (scFv)-based receptors containing the signaling domain of the CD3ζ chain (CD247) of the T cell receptor (TCR) (Eshhar et al., 1993). Subsequently, constructs incorporating signaling domains of costimulatory molecules (e.g., CD28 or 4-1BB) in tandem with the CD3ζ signaling domain were generated (Finney et al., 1998). Engrafting T-cells with such receptors, termed second-generation CARs, enables sustained proliferation and increased cytokine secretion. Third-generation CARs contain two costimulatory domains, in addition to the CD3ζ signaling domain (Carpenito et al., 2009; Milone et al., 2009). Current CAR-T cell therapy involves the isolation of autologous T cells using leukapheresis, followed by in vitro stimulation, genetic modification to express the TAA-specific CAR, and expansion to infuse back into the patient (Blanco et al., 2019). The bsAbs are designed to simultaneously bind to the TAA in the surface of tumor cells and the CD3e chain of the TCR/CD3 complex in the surface of T cells (Blanco et al., 2019). More than a 100 different bsAb formats have been reported, including small bsAbs composed only by two antigen-binding sites, IgG-like bsAbs and larger and non-IgG bsAbs formed by different antigen-binding moieties, often combined with oligomerization modules (Nunez-Prado et al., 2015; Brinkmann and Kontermann, 2017). By connecting CD3 signaling molecules with a recognition process independent of the TCR variable domains, T cells can be hot-wired to recognize a user-defined cell-surface TAA that is not associated with the major histocompatibility complex (MHC) to activate effector cell responses and kill cancer cells (Blanco et al., 2019). Nonetheless, the precise molecular mechanisms by which T cells are activated through these CCB molecules are poorly understood.

The administration of bsAbs and CAR-T cells has achieved remarkable clinical outcomes in hematological tumors, and several products have been approved by regulatory agencies for clinical use. Blinatumomab, an anti-CD19xanti-CD3 bsAb designed in the BiTE (bispecific T cell-engager) format, was approved by the US Food and Drug Administration (FDA) for the treatment of relapsed or refractory B cell acute lymphoblastic leukemia (B-ALL) (Przepiorka et al., 2015). Two CD19-specific second generation CAR-T cell products, tisagenlecleucel and axicabtagene ciloleucel (axi-cel), have been approved by the US FDA for the treatment of pediatric and young adult patients with relapsed or refractory B-ALL (Maude et al., 2018) and adult patients with relapsed or refractory diffuse large B cell lymphomas (Neelapu et al., 2017), respectively. However, the utility of these approaches in the treatment of solid tumors targeting TAAs has been limited by organ toxicities related to activation of T cell effector functions by non-tumor cells expressing low levels of the TAA, as well as systemic cytokine-associated toxicities (Alonso-Camino et al., 2016).