T cells are part of the immune system that protects the body from infection by reacting against foreign or self-antigens via T cell receptor (TCR) signaling pathways.
(CAR). CARs are genetically engineered synthetic receptors that This cellular mechanism is the foundation of chimeric antigen receptor T cell (CAR T cell) therapy that uses chimeric antigen receptors function as immune effector cells, such as lymphocyte T cells.1 The modified cells identify and kill cancer cells, revolutionizing cancer treatment.
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Development of CAR T Cell Therapy
CAR T cells have been categorized into two types, namely, autologous and allogeneic. Autologous CAR-T cells are developed from the patient’s blood, while allogenic CAR-T cells are generated from the blood of a healthy donor.2
In autologous CAR T cells, white blood cells, including T cells, are separated from the patient’s blood through a process called leukapheresis.3 Subsequently, T cells are modified in the laboratory to express a specific CAR that enables the identification and elimination of cancer cells. After a successful modification, the T cells are expanded in number and reinfused into the patent.
At present, all commercial CAR-T cell products use autologous sources of T cells.4 Even though this cell type is highly effective, this approach is highly expensive and time-consuming because of the immunosuppressive effects of cancer cells.
Normally, a patient’s lymphocytes are anergic, dysfunctional, or could be in an exhausted state, making them less likely to have the desired anti-tumor potency.
Allogeneic CAR T cells are isolated from healthy donors’ blood and bioengineered to express specific CARs.5 These modified cells are expanded and transferred to a patient. This method's key advantage is the opportunity to provide 'off-the-shelf' therapy for cancer treatment.
A successful CAR T cell therapy depends on the cells' ability to expand and persist after infusion. For instance, CAR T cells for the treatment of solid tumors exhibit a low persistence when infused within the tumor mass, which decreases therapeutic efficacy.
CAR T-Cell Therapy: How Does It Work?
Important Cellular Pathways in CAR T Cell Therapy
Unlike conventional adaptive T cell therapy, CAR T cells can bind to targeted malignant cells, independent of the major histocompatibility complex (MHC) receptor. CAR T cells enable T cells to bind with the target cell surface antigen via a single-chain variable fragment (scFv) recognition domain.6
After binding, it induces a non-classical immune synapse (IS), which is essential for effector function. These cells activate anti-tumoral effects via three pathways: cytolytic perforin and granzyme synthesis, which mediate target cell apoptosis; cytokines secretion, which sensitizes the tumor stroma; and the Fas and Fas ligand (FasL) pathways, which mediate cytotoxicity.
The perforin and granzyme pathways are essential for rapid, specific, and effective CAR T cell-induced target cell lysis. The Fas and FasL pathway is based on trimerization of the Fas receptor by the Fas ligand, which subsequently activates caspase 8 via the adapter protein Fas-associated death domain (FADD) and pro-caspase 8. Caspase 8 activation mediates cell death through multiple cellular cleavages.
Interestingly, FasL can induce lytic action even when degranulation is poor, indicating that a combination of perforin-granzyme and FasL pathways can efficiently eliminate the tumor completely.
It must be noted that the effectiveness of CAR T cell therapy is primarily dependent on the receptor’s individual components, such as scFv, hinge or spacer region, transmembrane domain, and costimulatory domains.7
The programmed cell death protein 1 (PD-1) and programmed cell death ligand 1 (PD-L1) pathways regulate the immune system by enhancing the regulatory T cells (Tregs) function and inhibiting T cells activity.
Cancer cells use the PD-1/PD-L1 axis to escape immune detection and promote tumor growth. CAR-T therapies are designed to inhibit the PD-1/PD-L1 pathway for treating solid tumors.8
Go Back to the Basics: What are CAR T cells?
CAR T Cell Therapy Applications and Emerging Insights
In the last decade, the implementation of CAR T cell therapy has significantly improved the clinical prognosis of patients with hematological malignancies, including multiple myeloma, lymphomas, and leukemia. In 2017, the U.S. Food and Drug Administration (FDA) approved CAR T cell therapy for cancer treatment.9
At present, the FDA has approved several CAR-T cell products for the treatment of multiple myeloma (MM), chronic lymphocytic leukemia (CLL), acute lymphoblastic leukemia (ALL), and different lymphoma forms including diffuse large B-cell lymphoma (DLBCL).10
A clinical study revealed that children and adult patients with refractory/relapsed (rr) ALL when treated with pre-conditioning chemotherapy (fludarabine and cyclophosphamide), followed by autologous CD19-specific CAR T-cells (CTL019) underwent durable complete remission (CR).
The FDA has approved tisagenlecleucel, manufactured by Kymriah, for the treatment of high-risk r/r ALL in pediatric and young adult patients. A clinical study indicated that r/r CLL patients treated with CD-19-targeting CAR T-cells exhibit a long-term CR.
The FDA also approved Kite pharma manufactured Axicabtagene ciloleucel (Axi-cel), which is a CD19-directed CAR-T cell to treat patients with follicular lymphoma, who did not respond to systemic chemotherapy.
The FDA has approved a second-generation autologous anti-BCMA CAR-T cell construct to treat patients with rr MM. Idecabtagene vicleucel, manufactured by Abecma, is commercially available to treat MM patients.
Conclusion
In sum, CAR T cell therapy has shown remarkable effectiveness in treating patients with aggressive hematological malignancies that did not respond to conventional chemotherapies. However, CAR-T cell therapy has not been as effective in treating solid tumors.
This has been attributed to poor CAR-T cell infiltration in these tumors due to its unique tumor microenvironment (TME), which is characterized by dense extracellular matrix, abnormal vasculature, interstitial fluid pressure, and the presence of immunosuppressive cells.
Furthermore, solid tumors contain tumor-associated antigens (TAAs), a self-antigen that is abnormally expressed in tumor cells and at low levels in certain normal cells.
Scientists are continually working to develop effective CAR T cell therapy to treat solid tumors. A preclinical study revealed that heregulin-1β (HRG1β), an endogenous ligand for HER3/HER4-based CAR-T cells, can effectively inhibit HER3-positive breast cancer cell growth in vitro and in vivo.
Furthermore, a phase I trial indicated the safety, efficacy, and clinical benefit of HER2-specific, CAR-modified virus-specific T-cells in treating patients with progressive glioblastoma.
Are CAR-T Cells the Future of Infectious Diseases Treatment?
References
- Rafiq S, et al. Engineering strategies to overcome the current roadblocks in CAR T cell therapy. Nat Rev Clin Oncol. 2020; 17,147–167. doi.org/10.1038/s41571-019-0297-y
- Mansoori S, et al. Recent updates on allogeneic CAR-T cells in hematological malignancies. Cancer Cell Int. 2024; 24, 304. doi.org/10.1186/s12935-024-03479-y
- Pessach I, Nagler A. Leukapheresis for CAR-T cell production and therapy. Transfus Apher Sci. 2023;62(6):103828. doi: 10.1016/j.transci.2023.103828.
- Abou-El-Enein M, et al. Scalable Manufacturing of CAR T cells for Cancer Immunotherapy. Blood Cancer Discov. 2021;2(5):408-422. doi: 10.1158/2643-3230.
- Lonez C, Breman E. Allogeneic CAR-T Therapy Technologies: Has the Promise Been Met? Cells. 2024; 13(2):146. doi.org/10.3390/cells13020146
- Dagar G, et al. Harnessing the potential of CAR-T cell therapy: progress, challenges, and future directions in hematological and solid tumor treatments. J Transl Med.2023; 21, 449. doi.org/10.1186/s12967-023-04292-3
- Sterner RC, Sterner RM. CAR-T cell therapy: current limitations and potential strategies. Blood Cancer J. 2021; 11, 69. doi.org/10.1038/s41408-021-00459-7
- Andreu-Saumell I, et al. CAR affinity modulates the sensitivity of CAR-T cells to PD-1/PD-L1-mediated inhibition. Nat Commun. 2024; 15, 3552. doi.org/10.1038/s41467-024-47799-z
- Uscanga-Palomeque AC, et al. CAR-T Cell Therapy: From the Shop to Cancer Therapy. Int J Mol Sci. 2023 8;24(21):15688. doi: 10.3390/ijms242115688.
- Cappell KM, Kochenderfer JN. Long-term outcomes following CAR T cell therapy: what we know so far. Nat Rev Clin Oncol. 2023; 20, 359–371. doi.org/10.1038/s41571-023-00754-1
Further Reading