Overview

CAR-T, Chimeric Antigen Receptor T-Cell Immunotherapy.CAR construct is transferred into the genomes of T cells using a lentivirus vector. After activation and expansion, it can selectively recognize and kill tumor cells. CAR-T Cell Immunotherapy is considered as one of the most promising approaches to cure tumors.

The overall structure of a CAR consists of antigen recognition moieties and T-cell signaling domains:an antigen-binding domain ( a single chain variable fragment (scFv), composed of the antigen-binding regions of both heavy and light chains of a monoclonal antibody); a transmembrane domain; and an intracellular signaling domain (endodomain) (T-cell-activation motif:immunoreceptors tyrosine-based activation motif(ITAM) of CD3 ,CD3zeta;co-stimulatory receptors, CD28 ICD and/or 4-1BB).Now, there are three generations of CAR, the structure as shown below:

The first generation of CAR-mediated T cell activation is accomplished by a tyrosine activation motif on the CD3 ζ chain or FcεRIγ. The CD3 ζ chain provides the signals required for T cell activation, lysis of target cells, regulation of IL-2 secretion, and antitumor activity in vivo. However, the anti-tumor activity of the first generation of CAR-modified T cells was limited in vivo, and the decrease in T cell proliferation ultimately led to the apoptosis of T cells.

The second generation of CAR added a new costimulatory signal (transmembrane costimulatory signal) in the cell. Experiments have shown that this has led to the expansion of the "signal 1" derived from the TCR/CD3 complex. Compared with the first-generation CAR, many studies have shown that the second-generation CAR equipped with "Signal 2" has no antigen specificity, T cell proliferation, increased cytokine secretion, increased secretion of anti-apoptotic proteins, and delayed cell death. A commonly used costimulatory molecule is CD28.

To further improve the design of CAR, many research groups began to focus on the development of third-generation CAR, including not only "Signal 1", "Signal 2", but also additional co-stimulatory signals. Some studies have reported that recombinant T cells expressing the third generation of CAR have significantly improved antitumor activity, survival cycle and cytokine release; commonly used costimulatory molecules are 4-1BB.

The mechanism of CAR-T cells killing tumor cells in vivo is as follows：