Author(s): Milena Kalaitsidou aff1 , Gray Kueberuwa aff1 , Antje Schütt aff1 , David Edward Gilham [*] aff1
adoptive cell therapy; CAR T cells; cytokine storm; preclinical models; toxicity
Chimeric antigen receptor (CAR, also known as chimeric immune receptor [CIR] and T body) T-cell technology has developed rapidly over the last 20 years. The initial concept featured the fusion of T-cell signaling proteins, including the T-cell receptor [beta] chain, CD3[gamma] and CD3ζ with target-binding domains. When expressed in a T cell, these fusion proteins result in T-cell effector functions being controlled by the ligand binding specificity of the target-binding domain [1 ]. Over time, this linear arrangement of targeting domain fused to one signaling domain has been termed 'first-generation' CARs. Upon this basic structure, a diverse range of CARs have been developed and shown to function. The target-binding domain most commonly used involves single-chain antibody variable fragments (scFvs) derived from monoclonal antibody hybridomas or by phage display technology. Though less common, intact extracellular protein domains and receptor ligands have also been successfully used in the CAR context [2,3 ]. The explosion of interest in CARs is highlighted by the wide number of excellent recent reviews detailing CAR structures and issues relating to the clinical delivery of this therapy [4-10 ]. In brief here, to provide an overview of the diversity of CARs, the modular nature of CARs has encouraged the incorporation of multiple signaling domains to form hybrids that increase downstream signaling potency (Figure 1). Hence, CARs containing two signaling domains fused together are termed 'second generation,' while those bearing three are listed as 'third generation.' The combinations of signaling domains vary, however, for the majority, the cytosolic sequence from CD3ζ protein is included to provide the main T-cell activating signal. Recently developed 'fourth-generation' CARs (or TRUCKs) involve two separate transgenes with a first, second- or third-generation CAR expressed alongside a T-cell activation responsive promoter linked to a cytokine such as IL-12. Consequently, upon T-cell activation, high levels of IL-12 are produced that can modulate the local microenvironment and aid CAR T-cell function [ 11 ].
Against this background of CAR structural diversity, there is no current consensus on the optimal combination of signaling domains and also no obvious rules that define the extracellular domains upon which target binding by scFv is dependent. Consequently, clinical studies arising from this diversity have proceeded in a largely empirical manner, with individual research groups using locally developed CARs with differing targeting moieties against varying diseases, associated with different levels of patient preconditioning and using T cells generated using different ex vivo systems [12 ]. Despite this, objective clinical responses have been reported in several centers, predominantly targeting B-cell leukemia through the CD19 antigen (see [8 ] for recent review), and this is driving a major surge of interest and activity in CAR T-cell therapy [13 ].
Few, if any, therapies deliver clinical responses in the absence of some form of toxicity and CAR T-cell therapy is no exception to this. The clinical success of targeting the...