CAR-T immunotherapy and cancer therapy

Chimeric antigen receptor (CAR)-modified T cells (CAR-T) are T cells genetically engineered to express CAR (Fig. 2A) [2,3], which can specifically recognize their target antigen via the scFv binding domain of CAR, resulting in T cell activation to specifically target and destroy tumor cells [3-5]. To date, four generations of CAR have been developed according to the structure of the endodomain (Fig. 2B) [3]. Due to little ability to generate enough interleukin-2 (IL-2), the 1st generation CAR-T cells (such as Ag-specific CD3ζ (MFEζ)-CAR-T cells, alpha-folate receptor (FR) -CAR-T cells, CE7R-CAR-T cells, scFv(G250)-CAR-T cells, GD2- CAR-T cells and CD10- CAR-T cells) benefitted substantially from the combination of cytokines [6], and were used for the treatment of different tumors [7-10]. However, most of the studies using the 1st generation CAR-T cells did not show very satisfactory results because of the inadequate proliferation, cytotoxicity, and insufficient secreted cytokines in vivo. To overcome these shortcomings, the 2nd generation CAR-T cells were designed by adding intracellular signaling domains from various co-stimulatory protein receptors to the cytoplasmic tail of the CARs, [11-13]. The 2nd generation CAR-T cells, such as scFvCD19-CD137-CD3-CAR-T cells, MOv19-BBζ-CAR-T cells and scFvCD19-CD28-CD3ζ-CAR-T cells displayed better curative effects on B cell malignancies [12,14]. On the basis of the 2nd generation CAR-T cells, the 3rd generation CAR-T cells were produced with the addition of multiple signaling domains, such as CD3ζ-CD28-OX40 or CD3ζ-CD28-41BB, to promote cytokine production and killing ability. The 3rd generation CAR-T cells, such as CD20-CD28-CD137-CD3ζ-CAR-T cells and HER2-CAR-T cells were applied for the treatment of lymphoma and colon cancer, but with no better outcomes than the 2nd generation CAR-T cells, and the reasons need to be further studied [15,16]. Based on the 2nd generation CAR-T cells, the 4th generation CAR-T cells were obtained with the addition of IL-12, and can mediate T cell redirected for universal cytokine-mediated killing (TRUCKs). TRUCK T cells show great outcomes in disease therapy by augmenting T-cell activation, and also activating and recruiting innate immune cells to destroy the antigen-negative cancer cells in the targeted lesion, and can also be used for the therapy of viral infections, metabolic disorders and auto-immune diseases [17]. Overall, these successive generations of CAR-T cells have yielded remarkable efficacy in several types of cancer or tumor therapy, and some of them have been translated into clinical trials with few side effects (Table 1) [18].