Understanding the synaptic interactions through cell avidity measurements
An immune (or immunological) synapse is the interface between a cancer and a T cell, which is formed in a highly stable, organized manner. This interface can also be formed between cancer cells and other effector cells, such as natural killer cells. The immune synapse is composed of all intercellular interactions taking place at the interface between the interacting pair, including TCR clustering, co-receptor binding, cell-cell adhesion proteins and even orientations and valencies (figure 1).
Proper formation of the immune synaptic interface is required to initiate T cell activation, achieve sustained proliferation and increase cytokine secretion. The essential aspects that shape it are the intrinsic potency of the T cell receptor (TCR) and the molecules present at the binding site, thus making the synaptic structure an important indicator of T cell function and T cell efficacy.
Explanatory illustration showing the interactions taking place at the interface of a cancer cell and an immune cell.
Immune synapse formation
2. Immunological synapse formation is triggered by TCR engagement. A bullseye structure is displayed at a TCR-mediated immune synapse (left) and bsAb-mediated immune synapse (right), while a nonclassical structure is formed in a CAR-mediated immune synapse (middle). The functional consequence of the CAR-mediated immune synapse is rapid signaling and expeditious cytotoxicity on tumor cells.
Immune synapse formation mediated by T cells
Under physiological conditions where the cancer cells are recognized by T cells, T cell movement is suppressed and immune synapse formation starts upon TCR engagement (Alcover et al., 2016). In a mature TCR-mediated immune synapse, the endosomal compartment, cytoskeleton, and signaling network are finely tuned to achieve proper T cell activation and effective immune responses (Soares et al., 2013). A proper TCR-mediated immune synapse displays a well-organized bullseye structure (figure 2, left), where the cytoskeleton filaments mediate the centripetal movement of TCR microclusters to achieve an adequate T cell activation (Martín-Cófreces et al., 2014). During T cell effector functions, lytic granules or cytokines are released to the synaptic cleft by cytotoxic or helper T cells, respectively (Huse et al., 2006; Stinchcombe et al., 2006). For a sustained T cell activation and secured cell-cell adhesion, the immunological synapse requires actin clearance, polarization of the microtubule fibbers as well as integrin activity (Martín-Cófreces et al., 2018).
Immune synapse formed in engineered T cells
When it comes to engineered T cells, the situation can be different. While the classical bullseye structure is maintained in bsAb-mediated synapse, CAR-mediated synapse displays a nonclassical structure. Here, the actin cytoskeleton is not completely cleared from the center, and CAR microclusters and signaling molecules are dispersed at the interface (figure 2, middle) (Davenport et al., 2018; Watanabe et al., 2018). In the CAR-mediated immune synapse, the lytic granule secretion is rapidly triggered upon CAR engagement as an immediate response to antigen recognition even before the microtubule polarization takes place. Compared to conventional TCR-mediated immune synapse, CAR-mediated immune synapse drives rapid cytotoxicity, but has shorter activating signals and shorter cell-cell interactions that may therefore reduce cell persistence (Bertrand et al., 2013).
Investigating cell-cell interactions across the immune synapse could predict cell therapy efficacy
Studying the immune synaptic interface is a crucial step to accelerate cell therapy development. The purpose of creating artificial construct engraftment strategies using TCR T cells, CAR T cells, and bispecific antibodies is to redirect T cells. However to properly achieve this, scientists need to get a proper understanding of the molecular mechanisms that take place at the cell-cell interface between cancer and immune cells during T cell activation. As we’ve seen, structural or procedural differences in CAR-mediated and bsAb-mediated immune synapse from the conventional TCR-mediated immune synapse will likely alter the nature of the resulting signaling and the eventual therapeutic T cell responses.
Looking into the binding events across the immunological synapse helps researchers to predict the effectiveness of T cells (Xiong et al, 2018). Furthermore, prompt T cell activation upon the synaptic formation allows to correlate binding events across the synapse with T cell functionality (Davenport et al, 2018). Therefore, investigating the immune synapse not only provides a better understanding of T cell mechanisms behind the observed immune response, but also serves as a powerful indicator of T cell functionality.
Having a solution that gives insight into the activities beyond the single-receptor interaction across the immune synapse will help to uncover the biological complexity that is inherently linked to T cell function. Cell avidity analysis can be a potential tool to study the immune synapse, as it unveils cell-cell interactions, and measures the total intercellular binding strength between cancer cells and T cells, including the ones across the synaptic interface. Cell avidity can not only help researchers to understand the processes underlying T cell response, but also to identify effective T cell candidates at an early stage. This early selection can improve the clinical success rate, and reduce costs and save time by reducing the use of in vivo mouse models.
The z-Movi® Cell Avidity Analyzer is a solution for researchers to determine cell avidity, which has been notoriously difficult to measure until now. Through avidity measurements, the z-Movi can help researchers investigate cell interaction properties that correspond to immune cell response in a predictive, reproducible, and fast manner. All this at a high-throughput and single-cell level, without compromising cell viability.
Being able to measure these interactions provides researchers valuable information and enables them to select best candidates at an early stage. This informed selection from the start can improve their success rate dramatically.