Car T Cells in Solid Tumors: Overcoming Obstacles

Abstract

Chimeric antigen receptor T cell (CAR T cell) therapy has emerged as a prominent adoptive cell therapy and a therapeutic approach of great interest in the fight against cancer. This approach has shown notorious efficacy in refractory hematological neoplasm, which has bolstered its exploration in the field of solid cancers. However, successfully managing solid tumors presents considerable intrinsic challenges, which include the necessity of guiding the modified cells toward the tumoral region, assuring their penetration and survival in adverse microenvironments, and addressing the complexity of identifying the specific antigens for each type of cancer. This review focuses on outlining the challenges faced by CAR T cell therapy when used in the treatment of solid tumors, as well as presenting optimizations and emergent approaches directed at improving its efficacy in this particular context. From precise localization to the modulation of the tumoral microenvironment and the adaptation of antigen recognition strategies, diverse pathways will be examined to overcome the current limitations and buttress the therapeutic potential of CAR T cells in the fight against solid tumors.

Keywords: cancer; immunotherapy; solid tumors.

Figure 1

CAR T cell synthesis: 1. recognize specific antigens expressed on tumor cells. Leukapheresis: blood is extracted from the patient, and both CD4+ and CD8+ T cells are selected. 2. Reprogramming: the isolated T cells are cultured and then exposed to antibody-coated beads to activate them. CAR genes are introduced through different viral vectors, which results in permanent genome modification and CAR expression. 3. CAR T cells: now, the T cells are capable of recognizing and attacking specific tumor antigens. 4 Expansion: with the goal of generating a sufficient number of CAR T cells for therapy, the cells go through several rounds of divisions in different culture media. They are then cleansed and concentrated, and a sample is taken for quality control. 5. Administration: prior to the reintroduction of CAR T cells, the patient undergoes “preconditioning” chemotherapy for lymphodepletion, which reduces immunosuppressive cells that could slow the expansion of CAR T cells; in addition to releasing proinflammatory cytokines that promote the activation of CAR T cells, once infused, they are able to initiate their destruction.

Figure 2

Mechanism of action of CAR T cells: CARs target the CD19 antigen, a molecule expressed on the surface of B lymphocytes. After antigen recognition, CARs transmit cascading signals that promote activation, expansion, persistence, and acquisition of effector functions. These cells then secrete cytokines and inflammatory chemokines that mediate the elimination of target cells through (1) the secretion of perforin and granzymes, and (2) the Fas/Fas-ligand signaling pathway. In the presence of Ca, perforins are activated and are able to be incorporated into the cell membrane of the target cell. Consequently, the perforins and granzymes are internalized through endocytosis. Once inside the vesicle, the perforins form pores that allow the granzymes entry into the cytosol, where they initiate apoptosis. On the other hand, TNF binding to Fas results in the formation of a proapoptotic signaling complex that causes the activation of a cell death effector protease known as caspase 8, which in turn cleaves and activates a caspase cascade that ultimately leads to cell death.

Figure 3

Antigen expression in healthy tissues: in the case of solid tumors, the selection of surface antigens that are exclusive to transformed cells represents a challenge due to their paucity in epithelial cells. Furthermore, these antigens tend to be heterogeneous in distribution and intensity. Such is the case with Her2/Neu, which is expressed in the lungs and the heart, and EGFR, which is found in the skin, digestive tract, kidneys, among others.

Figure 4

CAR T cell efficiency: liquid vs. solid malignancies: 1. Antigens: while the target antigens in liquid malignancies such as leukemia and lymphoma are efficient, solid tumors pose a challenge due to their scarce and heterogenous expression of antigens; various antigens like CAIX, EGFR, Her2/neu, and PMSA, among others, have been tested; however, the results have not been completely satisfactory, particularly in contrast with CD-19 antigen, used to treat B cell malignancies, the arrows pointing down in solid tumors represent the scarcity of antigens in these malignancies and, in contrast, the arrow pointing up in liquid neoplasia reflects the availability of these surface molecules in these cancers. 2. Immunosuppressive TME: the hostile tumoral microenvironment (TME) in solid tumors has hampered the development of successful CAR T cell therapy since this environment not only promotes tumoral development but also has the ability to deactivate immune cells, therefore obstructing the labor of CAR T cells; in contrast, liquid neoplasia does not present these issues as the cells are “fluid” and circulate within blood or lymphatic vessels. 3. Lymphocyte homing: finding the target antigen in leukemias and lymphomas is not a particularly challenging task for T cells since these cells can be found circulating in the bloodstream; however, in solid tumors, CAR T cells must be trafficked to specific sites, penetrate into the tumoral stroma, and evade multiple suppressing molecules to reach their target.

Figure 5

Novel strategies to improve CAR T cell efficacy in solid tumors: currently, a variety of techniques focused on increasing specificity and restricting tumor escape are being implemented. Chimeric receptor models with multiple targets have been proposed, such as (a) dual CAR T cells, (b) TanCARs, (c) iCARs, (d) SynNotch CARs, (e) switch CARs, and (f) CARs with suicide genes. In addition, modifications that improve TME penetration have been introduced. This class includes the co-stimulatory molecules (g) CARs 4-1BB and CD28. Likewise, CARs that are activated or inactivated through signaling molecules have been designed, e.g., (h) CARs switch receptors and (i) remote controlled CARs. To resist the harsh microenvironment, T cells have been modified into (j) CARs TRUCKS to produce and secrete proinflammatory cytokines such as IL-15 and IL-18. (k) CARs that combine anti-immune therapy checkpoints have also been designed, for example, the addition of CARs combined with anti-PD-1 molecules or the combination of an endodomain with a CD28 molecule and a PD-1 exodomain. Moreover, the use of the CRISPR/Cas9 system has been proposed in order to eliminate genes responsible for the expression of immune checkpoints such as PD-1 or CTLA-4. Finally, to improve delivery and infiltration into the neoplastic area, therapies have been developed that use (l) oncolytic viruses for their ability to be deposited in the tumor area.