car t cell car t cell in biology, chimeric antigen receptors (cars)—also known as chimeric immunoreceptors, chimeric t cell receptors or artificial t cell receptors—are receptor proteins that have been engineered to give t cells the new ability to target a specific antigen. the receptors are chimeric in that they combine both antigen-binding and t cell activating functions into a single receptor. chimeric antigen receptor t cell production and infusion:1. t cells are isolated from a patient's blood2. a new gene encoding a chimeric antigen receptor is incorporated into the t cells3. engineered t cells are now specific to a desired target antigen4. engineered t cells are expanded in tissue culture5. engineered t cells are infused back into the patient car t cell therapy uses t cells engineered with cars to treat cancer. the premise of car t immunotherapy is to modify t cells to recognize cancer cells in order to more effectively target and destroy them. scientists harvest t cells from people, genetically alter them, then infuse the resulting car t cells into patients to attack their tumors. car t cells can be derived either from t cells in a patient's own blood (autologously) or from the t cells of another, healthy, donor (allogeneically). once isolated from a person, these t cells are genetically engineered to express a specific car, which programs them to target an antigen that is present on the surface of tumors. for safety, car t cells are engineered to be specific to an antigen that is expressed on a tumor but is not expressed on healthy cells. after car t cells are infused into a patient, they act as a "living drug" against cancer cells. when they come in contact with their targeted antigen on a cell's surface, car t cells bind to it and become activated, then proceed to proliferate and become cytotoxic. car t cells destroy cells through several mechanisms, including extensive stimulated cell proliferation, increasing the degree to which they are toxic to other living cells (cytotoxicity), and by causing the increased secretion of factors that can affect other cells such as cytokines, interleukins and growth factors. the surface of car t cells can bear either of two types of co-receptors, cd4 and cd8. these two cell types, called cd4+ and cd8+, respectively, have different and interacting cytotoxic effects, and it appears that therapies employing a 1-to-1 ratio of the cell types provide synergistic antitumor effects. history the first chimeric receptors containing portions of an antibody and the t cell receptor was described in 1987 by yoshihisa kuwana et al. at the institute for comprehensive medical science in aichi, japan, and independently in 1989 by gideon gross and zelig eshhar at the weizmann institute in israel. originally termed "t-bodies", these early approaches combined an antibody's ability to specifically bind to diverse targets with the constant domains of the tcr-α or tcr-β proteins. in 1991, chimeric receptors containing the intracellular signaling domain of cd3ζ were shown to activate t cell signaling by arthur weiss at the university of california, san francisco. this work prompted cd3ζ intracellular domains to be added to chimeric receptors with antibody-like extracellular domains, commonly single-chain fraction variable (scfv) domains, as well as proteins such as cd4, subsequently termed first generation cars. a first generation car containing a cd4 extracellular domain and a cd3ζ intracellular domain was used in the first clinical trial of chimeric antigen receptor t cells by the biotechnology company cell genesys in the mid 1990s, allowing adoptively transferred t cells to target hiv infected cells, although it failed to show any clinical improvement. similar early clinical trials of car t cells in solid tumors in the 1990s using first generation cars targeting a solid tumor antigens such as muc1 did not show long-term persistence of the transferred t cells or result in significant remissions. in the early 2000s, co-stimulatory domains such as cd28 or 4-1bb were added to first generation car's cd3ζ intracellular domain. termed second generation cars, these constructs showed greater persistence and improved tumor clearance in pre-clinical models. clinical trials in the early 2010s using second generation cars targeting cd19, a protein expressed by normal b cells as well as b-cell leukemias and lymphomas, by investigators at the nci, university of pennsylvania, and memorial sloan kettering cancer center demonstrated the clinical efficacy of car t cell therapies and resulted in complete remissions in many heavily pre-treated patients. these trials ultimately led in the us to the fda's first two approvals of car t cells in 2017, those for tisagenlecleucel (kymriah), marketed by novartis originally for b-cell precursor acute lymphoblastic leukemia (b-all), and axicabtagene ciloleucel (yescarta), marketed by kite pharma originally for diffuse large b-cell lymphoma (dlbcl). there are now six fda-approved car t therapies. production depiction of adoptive cell transfer therapy with car-engineered t cells the first step in the production of car t-cells is the isolation of t cells from human blood. car t-cells may be manufactured either from the patient's own blood, known as an autologous treatment, or from the blood of a healthy donor, known as an allogeneic treatment. the manufacturing process is the same in both cases; only the choice of initial blood donor is different. first, leukocytes are isolated using a blood cell separator in a process known as leukocyte apheresis. peripheral blood mononuclear cells (pbmcs) are then separated and collected. the products of leukocyte apheresis are then transferred to a cell-processing center. in the cell processing center, specific t cells are stimulated so that they will actively proliferate and expand to large numbers. to drive their expansion, t cells are typically treated with the cytokine interleukin 2 (il-2) and anti-cd3 antibodies. the expanded t cells are purified and then transduced with a gene encoding the engineered car via a retroviral vector, typically either an integrating gammaretrovirus (rv) or a lentiviral (lv) vector. these vectors are very safe in modern times due to a partial deletion of the u3 region. the new gene editing tool crispr/cas9 has recently been used instead of retroviral vectors to integrate the car gene into specific sites in the genome. the patient undergoes lymphodepletion chemotherapy prior to the introduction of the engineered car t-cells. the depletion of the number of circulating leukocytes in the patient upregulates the number of cytokines that are produced and reduces competition for resources, which helps to promote the expansion of the engineered car t-cells. clinical applications as of march 2019, there were around 364 ongoing clinical trials happening globally involving car t cells. the majority of those trials target blood cancers: car t therapies account for more than half of all trials for hematological malignancies. cd19 continues to be the most popular antigen target, followed by bcma (commonly expressed in multiple myeloma). in 2016, studies began to explore the viability of other antigens, such as cd20. trials for solid tumors are less dominated by car t, with about half of cell therapy-based trials involving other platforms such as nk cells. cancer t cells are genetically engineered to express chimeric antigen receptors specifically directed toward antigens on a patient's tumor cells, then infused into the patient where they attack and kill the cancer cells. adoptive transfer of t cells expressing cars is a promising anti-cancer therapeutic, because car-modified t cells can be engineered to target potentially any tumor associated antigen. early car t cell research has focused on blood cancers. the first approved treatments use cars that target the antigen cd19, present in b-cell-derived cancers such as acute lymphoblastic leukemia (all) and diffuse large b-cell lymphoma (dlbcl). there are also efforts underway to engineer cars targeting many other blood cancer antigens, including cd30 in refractory hodgkin's lymphoma; cd33, cd123, and flt3 in acute myeloid leukemia (aml); and bcma in multiple myeloma. solid tumors have presented a more difficult target. identification of good antigens has been challenging: such antigens must be highly expressed on the majority of cancer cells, but largely absent on normal tissues. car t cells are also not trafficked efficiently into the center of solid tumor masses, and the hostile tumor microenvironment suppresses t cell activity. autoimmune disease while most car t cell studies focus on creating a car t cell that can eradicate a certain cell population (for instance, car t cells that target lymphoma cells), there are other potential uses for this technology. t cells can also mediate tolerance to antigens. a regulatory t cell outfitted with a car could have the potential to confer tolerance to a specific antigen, something that could be utilized in organ transplantation or rheumatologic diseases like lupus. fda-approved car t cell therapies fda-approved car t cell therapies car t cell brand name company approval date target antigen recognition domain intracellular signaling domain indication (targeted disease / line of therapy) bla stn drug label tisagenlecleucel kymriah novartis 08/30/2017 cd19 scfv 41bb - cd3ζ b-cell precursor all (third line) diffuse large b-cell lymphoma (third line) follicular lymphoma (third line) 125646 link axicabtagene ciloleucel yescarta kite pharma / gilead 10/18/2017 cd19 scfv cd28 - cd3ζ diffuse large b-cell lymphoma (second line) follicular lymphoma (third line) 125643 link brexucabtagene autoleucel tecartus kite pharma / gilead 07/24/2020 cd19 scfv cd28 - cd3ζ mantle cell lymphoma (third line) b-cell precursor all (third line) 125703 link lisocabtagene maraleucel breyanzi juno therapeutics / bms 02/05/2021 cd19 scfv 41bb - cd3ζ diffuse large b-cell lymphoma (third line) 125714 link idecabtagene vicleucel abecma bluebird bio / bms 03/26/2021 bcma scfv 41bb - cd3ζ multiple myeloma (fifth line) 125736 link ciltacabtagene autoleucel carvykti janssen / j&j 02/28/2022 bcma vhh 41bb - cd3ζ multiple myeloma (fifth line) 125746 link safety there are serious side effects that result from car t-cells being introduced into the body, including cytokine release syndrome and neurological toxicity. because it is a relatively new treatment, there is little data about the long-term effects of car t-cell therapy. there are still concerns about long-term patient survival, as well as pregnancy complications in female patients treated with car t-cells. anaphylaxis may be a side effect, as the car is made with a foreign monoclonal antibody, and as a result provokes an immune response. on-target/off-tumor recognition occurs when the car t-cell recognizes the correct antigen, but the antigen is expressed on healthy, non-pathogenic tissue. this results in the car t-cells attacking non-tumor tissue, such as healthy b cells that express cd19 causing b-cell aplasia. the severity of this adverse effect can vary but the combination of prior immunosuppression, lymphodepleting chemotherapy and on-target effects causing hypogammaglobulinaemia and prolonged cytopenias places patients at increased risk of serious infections. there is also the unlikely possibility that the engineered car t-cells will themselves become transformed into cancerous cells through insertional mutagenesis, due to the viral vector inserting the car gene into a tumor suppressor or oncogene in the host t cell's genome. some retroviral (rv) vectors carry a lower risk than lentiviral (lv) vectors. however, both have the potential to be oncogenic. genomic sequencing analysis of car insertion sites in t cells has been established for better understanding of car t-cell function and persistence in vivo. cytokine release syndrome main article: cytokine release syndrome the most common issue after treatment with car t-cells is cytokine release syndrome (crs), a condition in which the immune system is activated and releases an increased number of inflammatory cytokines. the clinical manifestation of this syndrome resembles sepsis with high fever, fatigue, myalgia, nausea, capillary leakages, tachycardia and other cardiac dysfunction, liver failure, and kidney impairment. crs occurs in almost all patients treated with car t-cell therapy; in fact, the presence of crs is a diagnostic marker that indicates the car t-cells are working as intended to kill the cancer cells. the severity of crs does not correlate with an increased response to the treatment, but rather higher disease burden. severe cytokine release syndrome can be managed with immunosuppressants such as corticosteroids, and with tocilizumab, an anti-il-6 monoclonal antibody. early intervention using tocilizumab was shown to reduce the frequency of severe crs in multiple studies without affecting the therapeutic effect of the treatment. immune effector cell-associated neurotoxicity neurological toxicity is also often associated with car t-cell treatment. the underlying mechanism is poorly understood, and may or may not be related to crs. clinical manifestations include delirium, the partial loss of the ability to speak coherently while still having