CART: Overcoming Challenges against Solid Tumors
Chimeric antigen receptor (CAR) T cells are engineered T cells having synthetic receptors that direct T cells to recognize antigens and promote subsequent eradication of targeted cells. To date, CAR T-cell therapy have demonstrated its effectiveness in eradicating hematological cancers with compelling evidence primarily with the pioneer CART T-cell against CD19 in leukemias (Ye et al. 2018). However, this success is not yet extrapolated in other types of cancers especially for solid tumors. This is due to the difference in cancer types; their nature, accessibility and their microenvironment which posed challenges for CAR T-cells hampering their effectivity. Solid tumor is a mass of cancer cells that grow in a particular organ, tissue or gland. If this type of tumor is benign it could stay in their place of origin and can be generally removed by surgical means. However, if solid tumors acquired properties making it aggressive, it could spread or metastasize by entering the bloodstream and migrating to other parts of the body (Chaddah, 2013).
The nature of the cells in the solid tumors differs from hematologic cancers in such a way that the presented antigens and cell surface receptor are different. Malignant solid tumors such as sarcomas and carcinomas have different specific tumor antigens which are more difficult to identify due to the heterogeneity of the present cells in the tumor site. Unlike in the case of CD19 CARs, where the antigen CD19 is expressed constantly on only dispensable B cells, there are many antigens in solid tumors which require profiling before designing CARs. Currently, there are roughly 30 tumor antigens under evaluation, analyzing their potential as targets for genetically engineer T cells (Newick et al. 2016). These tumor antigens require profiling because each of them varies due to the heterogeneity of the cancer cells in the tumor, some of which are neoantigens or those having mutated sequences of the wildtype antigens. Antigens with mutated sequences are expressed as variants which makes targeting via the previously designed CARs ineffective and might result to additional abnormalities. Other antigens are oncofetal or developmental in nature, which are only expressed on specific stages of tumor progression. Most of the exploited targets are tumor-selective antigens which are highly expressed in tumors or neoplastic cancer cells but low in the normal cells. Enriched expression allows better chance of targeting cancer cells but might affect other normal cells depending on several factors such as concentration and tolerance (Karlos, 2018). To target solid tumors, CARs are constructed to allow genetically modified T cells to target the mutated receptors or antigen. Mutations lead to generation of variants or isoforms of the receptors or antigens thus, profiling the solid tumor is required in order to properly design the CARs based from the mutated sequences.
Another problem in targeting solid tumors lies in poor trafficking of CAR T-cells after injection on the patient. The physical barrier of reaching the target cells in solid tumors posed difficulty in accessibility rendering CAR T-cell therapy ineffective. This is because solid tumors tend to develop fibrotic extracellular matrix (ECM) which impedes efficient T cell penetration. The ability of the CART T-cells to localize on tumor sites and infiltrate the solid mass of cancer cells depends on different factors and stimulus. The expression of adherent receptors on T cells and tumor endothelium as well as receptor-ligand matching via chemokine receptor on CAR T-cells and chemokines secreted by tumor cells plays a critical role in the trafficking (Newick et al. 2016). To overcome poor trafficking of CAR T-cells, they can be enhanced by co-expression of appropriate chemokine receptor based on the solid tumor chemokine secretion profile which can be determined by quantitative analyses. Expression of CCR2b in CAR T-cells, the receptor for CCL2 chemokine secreted by carcinomas have been found to enhance intratumoral migration after transfection in mice models. In addition to expression of appropriate signaling and recognition receptors, site specific administration of CAR T-cells in tumor sites via surgical efforts can be explored to increase effectivity.
Aside from the difficulty of target antigens or receptors and poor trafficking, the hostile tumor microenvironment also presents various problems for the CAR T-cells which can be of physical or metabolic in nature. Since solid tumor is comprised of high density mass of different cancer cells, the site is often under high tissue pressure which prevents extravasation and are inaccessible to CAR T-cells. To address this problem, CAR T-cell can be engineered to reduce tumor fibroblast numbers and other stromal cancer cells. CAR T-cells can be enhanced to secrete enzymes that could degrade matrix to allow T-cell penetration (Yu et al. 2017).
In addition, the microenvironment of solid tumors is stressful and normally inhospitable for T cells because of the metabolic including hypoxia, nutrient starvation and acidosis. There is also a lack of glucose and other metabolites as well as inhibition of T-cell proliferation and cytokine production. Because the tumor site is in this stressful condition, the stress response is activated. Any cells which are not adapted to the condition will suffer from stress response mechanism resulting to autophagy and cell death. Solid tumor site is hypoxic because it outgrowths its blood supply during cancer progression which led to deprivation of oxygen and nutrients. The hypoxic condition also induces metabolic shift that results in elevated generation of lactate which led to acidosis. In order for engineered T cells to survive the stressful microenvironments, CAR T cells are enhanced by implicating key cellular regulators of specific proteins involved (Newick et al. 2016). For example, to evade immediate authophagy, CAR T cell can be armed to block recognition by autophagosomes.
Tumor-derived soluble factors and cytokines also prevent efficient targeting of CAR T-cells in solid tumors. One soluble factor that is found in tissue extracts from solid tumors is prostaglandin 2 (PGE2) which produced by tumor cells and macrophages. This immunosuppressive factor has been found not only prevented the recognition by T cell, but also prevented T cell proliferation, sequestration of immune cells and subversion of T cell differentiation (Wang & Dubois, 2006). It works by signaling through G-coupled receptors via protein kinase A activation. CAR T-cells have been found to be effective against tumors when protein A kinase activity is inhibited (Li et al. 2018), thus genetic modification in the CAR T-cell by inhibition of implicated signaling proteins could enhance their anti-tumor efficacy.
The success of CAR T cell therapy on hematological malignancies urges researchers to explore its application on solid tumors. Because of the difference in nature of both malignancies, the efficacy and sensitivity of CAR T-cells on targeting cells on these cancer types varies significantly. There are still problems and barriers that are needed to be addressed in order for CAR T-cell technology be adapted with full potential to target solid tumors. Because of the advances in genetic engineering, enhancing the current CAR T-cell can be done to overcome challenges of heterogenous tumor cell antigens, physical and chemokine immunosuppressive factors as well as hostile tumor environment with the consideration on the adverse effects of attacking normal tissues.
Chaddah, M. 2013. Cancers: Solid Tumor. Stem Cell Network
Ye, B., Stary, M., Li, M., Gao, Q., Kang, C., Xiong, X. 2018. Engineering chimeric antigen receptor-T cells for cancer treatment. Molecular Cancer 17(1): 32
Karlos, M. 2018. CART targeting of solid tumors: More pieces to the puzzle. Clinical Cancer Research.
Yu, S., Li, A., Liu, Q., Li, T., Yuan, X., Han, X. and Wu, K. 2017. Chimeric antigen receptor T cells: a novel therapy for solid tumors. Journal of Hematology and Oncology.
Li, J., Li, W., Huang, K., Zhang, Y., Kupfer, G. and Zhao, Q. 2018. Chimeric antigen receptor T cell (CAR-T) immunotherapy for solid tumors: lessons learned and strategies for moving forward. Journal of Hematolohgy and Oncology, 11:22
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