Figure 1.

Structures of the first through fourth generations of chimeric antigen receptors. All generations of CARs have a typical structure comprised by an extracellular antigen-binding domain (single-chain fragment variable, peptides, nanobodies, cytokines or other ligands), a hinge region (CD28, CD8, IgG1 or IgG4), a transmembrane domain (CD8α, CD4, CD3ζ, CD28, or ICOS) and intracellular signaling domains. The first-generation CARs have only the CD3ζ intracellular activation domain, while an additional co-stimulatory domain was added to second-generation CARs (e.g., CD28, 4-1BB, OX40, ICOS, CD27, KIR2DS2, and MYD88-CD40). The third-generation CAR has two co-stimulatory domains in tandem. The fourth-generation of CAR-T cells, also called armored CAR-T cells, has the same structure of second or third generation CAR. However, their producing vectors were armored with the advantage to secrete some additional molecules that give anti-tumor properties, such as the release of cytokines, chemokines, enzymes, ligands, receptors, peptides or monoclonal antibodies against different therapeutic targets.