cells Review The Quest for the Best: How TCR Affinity, Avidity, and Functional Avidity Affect TCR-Engineered T-Cell Antitumor Responses Diana Campillo-Davo 1,*, Donovan Flumens 1 and Eva Lion 1,2 1 Tumor Immunology Group, Laboratory of Experimental Hematology, Vaccine & Infectious Disease Institute (VAXINFECTIO), Faculty of Medicine and Health Sciences, University of Antwerp, 2610 Antwerp, Belgium; [email protected] (D.F.); [email protected] (E.L.) 2 Center for Cell Therapy & Regenerative Medicine, Antwerp University Hospital, 2650 Edegem, Belgium * Correspondence: [email protected]; Tel.: +32-(0)38215254 Received: 16 June 2020; Accepted: 15 July 2020; Published: 18 July 2020 Abstract: Over the past decades, adoptive transfer of T cells has revolutionized cancer immunotherapy. In particular, T-cell receptor (TCR) engineering of T cells has marked important milestones in developing more precise and personalized cancer immunotherapies. However, to get the most benefit out of this approach, understanding the role that TCR affinity, avidity, and functional avidity play on how TCRs and T cells function in the context of tumor-associated antigen (TAA) recognition is vital to keep generating improved adoptive T-cell therapies. Aside from TCR-related parameters, other critical factors that govern T-cell activation are the effect of TCR co-receptors on TCR–peptide-major histocompatibility complex (pMHC) stabilization and TCR signaling, tumor epitope density, and TCR expression levels in TCR-engineered T cells. In this review, we describe the key aspects governing TCR specificity, T-cell activation, and how these concepts can be applied to cancer-specific TCR redirection of T cells. Keywords: T-cell receptor; TCR affinity; TCR avidity; TCR engineering; T-cell fitness; affinity-enhanced; cancer immunotherapy 1. Introduction to TCR Affinity, Avidity, and Functional Avidity From tumor infiltrating lymphocytes to T-cell receptor (TCR) and chimeric antigen receptor (CAR) T-cell engineering, T cells have marked important milestones in cancer immunotherapy [1]. T cells recognize short peptide epitopes in the context of the major histocompatibility complex (MHC) thanks to their TCR. This receptor is a heterodimer of the immunoglobulin gene superfamily composed of two different alpha and beta polypeptides in conventional αβ T cells. The extracellular domain, which is involved in antigen recognition, comprises a variable region, a constant region, and a hinge, where a disulfide bridge is located to stabilize the interaction between the TCR chains. It continues into the transmembrane region and the intracellular domain, which interacts noncovalently with CD3 γ, δ, ", and ζ proteins to form the TCR–CD3 complex. When a TCR correctly identifies the cognate peptide-MHC complex (pMHC), including the correct matching between MHC type and CD4/CD8 co-receptor, the TCR undergoes a series of conformational changes that lead to a first activation signal [2]. Three different TCR parameters are the major players governing this pMHC recognition and posterior T-cell activation, namely, TCR affinity, avidity, and functional avidity (Figure1). TCR affinity is a key factor in controlling the sensitivity of the T cells towards the antigen and it is defined as the strength of the interaction between a single TCR and the pMHC ligand [3]. It is usually determined by the association (kon) and dissociation rates (koff) and represented as the equilibrium Cells 2020, 9, 1720; doi:10.3390/cells9071720 www.mdpi.com/journal/cells Cells 2020, 9, 1720 2 of 18 Cells 2020, 9, x 2 of 18 dissociation constant (KD)[3]. If TCR affinity relates to single receptors, TCR avidity measures the strengthmeasures of multiplethe strength TCR–pMHC of multiple engagements TCR–pMHC and considers engagements the eff ectand of considers other molecules the effect such of as TCRother co-receptorsmolecules such in the as interaction, TCR co-receptors whereas in functional the interaction, avidity whereas represents functional the T-cell avidity fitness represents and activity theat T- dicellfferent fitness concentrations and activity at of different peptide epitope.concentrations The mean of peptide functional epitope. avidity The is mean usually functional described avidity as an is EC50usually concentration, described as representing an EC50 concentration the peptide, representing dose at which the a peptide half-maximal dose at activation which a half of the-maximal T-cell populationactivation isof reached the T-cell [4]. population Although physiologicalis reached [4] TCR. Although affinities physiological can range from TCR 1 µ affinitiesM to 100 canµM[ range5,6], severalfrom 1studies µM to 100 have µM marked [5,6], theseveral threshold studies of have affinity marked for maximal the threshold T-cell activity,of affinity including for maximal antitumor T-cell T-cellactivity, responses, including at 5–10antitumorµM of T peptide-cell responses, epitope [at3, 56–108]. µM In a of comparative peptide epitope study [3,6 of– native8]. 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