Adoptive Cell Transfer, a Versatile Way to Re-educate the Immune System for a Plethora of Diseases

Adoptive cell transfer (ACT) has contributed significantly in the development of cancer treatments, and its improving led researchers to technologies based on genetically engineered T lymphocytes [1]. The paradigm of ACTs are chimeric antigen receptor (CAR)-T cells that fights against CD19 B lymphocytes. CAR are synthetic constructs that consist of an extracellular domain for target cell recognition made by a single-chain variable fragment (scFv) from a monoclonal antibody and an intracellular part that includes up to several signaling motifs capable of T cell activation [2], which together can activate T cells in a major histocompatibility complex (MHC)-independent manner. In order to prepare CAR-T cells for therapeutic applications researchers need to separate T cells from patient’s peripheral blood, followed by a viral or non-viral insertion of CAR genes into the T cell genome. Finally, CAR-T cells are expanded and cells are infused back into the patient. As the number of available targets for CAR-T cells is expanding rapidly [3], major successes in oncology have led to a growing scientific and clinical interest in using CAR-Ts as a treatment for other types of diseases such as autoimmunity, allergy and asthma, infectious diseases and cardiac fibrosis. Although with different etiology, there are common features in their pathogenesis that potentially allow them to be treated with CAR-T cells.

CAAR-Ts mechanism of action. They recognize autoreactive and T cells with receptors to autoantigens, as well as autoreactive antigen presenting cells, and then execute the cytotoxic-based immune attack. Zmievskaya, E, et al. ‘Application of CAR-T Cell Therapy beyond Oncology: Autoimmune Diseases and Viral Infections’. Biomedicines 2021, 9, 59.  DOI: 10.3390/ biomedicines9010059

Pathogenesis of many autoimmune diseases (AIDs) begins when T cell tolerance to self antigens fails due to the activation of autoreactive B cell clones that produce autoantibodies promoting tissue damage, or by suppression of cytotoxic or regulatory T (Treg) cells [4]. Bearing this in mind, researchers are trying to develop anti-autoimmune CAR-T cells that could eliminate autoreactive clones of immune cells. In this regard, the chimeric autoantibody receptor (CAAR) represents a variation of modified CAR which acts as a target for autoreactive B cells and defines the selective cytotoxicity of CAAR-T cells against immune cells that carry receptors to specific autoantigen without inducing immunosuppression. This strategy could lead to the direct elimination of surface immunoglobulin memory B cells and the indirect elimination of short lived plasma cells that produce disease-causing antibodies.

Researcher are also trying to restore the immune tolerance by switching the T cell phenotype from cytotoxic to regulatory, since Tregs are usually suppressed in AIDs. This could be achieved by transducing the FOXP3 gene along with the CAR, as it controls the pathway responsible for the development and function of Treg cells [5]. Those CAR-Tregs will recognize and regulate autoimmune T lymphocytes through induction of anergy, immunological ignorance and/or clonal deletion.

CAR-Tregs mechanism of action: They identify targets in the same manner that CAAR-T cells, but they exert an immunosuppressive outcome rather than cytotoxic because of their association with cytokines and immune checkpoint proteins. Zmievskaya, E, et al. ‘Application of CAR-T Cell Therapy beyond Oncology: Autoimmune Diseases and Viral Infections’. Biomedicines 2021, 9, 59.  DOI: 10.3390/ biomedicines9010059

Allergic diseases and asthma are characterized by a strong Th2 immune response that can be modulated by Tregs, and the immunoglobulin E (IgE) produced by B cells may be targeted by CAR-T cells. Allergic reactions start when IgE bind to its receptor (FcεRI) in the surface of mast cells, eosinophils and basophils, causing degranulation and releasing inflammatory mediators that result in type I hypersensitivity reactions and allergic symptoms [6]. Targeting IgE expressing cells by CAR-Ts through the recognition of the transmembrane form of IgE (mIgE) can be done by designing CARs including the extracellular domain of FcεRI α chain for mIgE binding. FcεRIα-based low affinity CD8+ CAR-Ts can mediate potent primary T cell responses against mIgE+ target cells.

Allergic asthma is characterized by a low number of Tergs and an overall reduced immunosuppressive activity, together with a Th2 cell-dominated response to allergens that lead to airway inflammation, hyper-reactivity and reversible obstruction of airways [7]. Researchers tried to redirect Tregs to the lungs and initiate their activation by CAR that recognizes a glycoprotein presented on the surface of adeno epithelia in lungs called CEA. Anti-CEA Tregs increased the suppression effect in the lungs and reduced the expression of Th2 cytokines, as well as allergen-specific IgE, airway hyper-reactivity, eosinophilic airway inflammation and enhanced mucus production.

Regarding infectious diseases, CD8+ T cells are in charge to eliminate infected cells, making them attractive CAR-Ts for treating infectious diseases. CAR-Ts are especially valuable to treat viral infections such as hepatitis B (HBV) and C (HCV) viruses, as well as influenza virus or HIV. CAR-Ts with and S domain receptor (S-CARs) for all 3HBV envelope proteins formed an HBV surface antigen (HBsAg) that was exposed on the surface of the infected cells. After treatment with S-CARs, researchers showed a decrease in the number of hepatocytes with cytoplasmic expression of HBV core protein, virions circulating in the bloodstream and replicative forms of HBV DNA. Nevertheless, S-CAR-Ts were completely exhausted soon after the treatment and viral activity increased again. Some other researchers tried to solve these issues by deeply modifying the CAR structure [8, 9].

The major trouble with HCV cases is that a high risk of post-treatment reinfection remains, which implies a need for the development of alternative therapeutic approaches. A new strategy consists of CAR-T cells that recognize the HCV E2 glycoprotein (HCV/E2), a major target of the host immune response and one of the most variable viral protein. Anti-HCV/E2 CAR-T cells demonstrated substantial cytotoxic activity against HCV infected cells.

CAR-T cells against the conserved region of M2e Influenza A protein, which is expressed on the surface of infected cells, was selected as a target in both in vitro and in vivo experiments, resulting in a reduction of the viral titer in murine lungs.

Anti-HIV strategies: First one is based on gp120-CD4 interaction (left), and the second uses single chain variable fragments from broadly neutralizing antibodies (BNAbs) targeting gp120 and gp41 regions of Env protein (right). Zmievskaya, E, et al. ‘Application of CAR-T Cell Therapy beyond Oncology: Autoimmune Diseases and Viral Infections’. Biomedicines 2021, 9, 59.  DOI: 10.3390/ biomedicines9010059

Human immunodeficiency virus type 1 (HIV-1) replication can be successfully suppressed via combination of antiretroviral therapy (cART); however, complete elimination of the latent reservoir of infected cells still remains a major problem. Therefore, researchers tried to use CAR-Ts as HIV-1 treatment as a promising new therapeutic approach. The main target for anti-HIV-1 CAR-T therapy is the gp120 region of the virus envelope glycoprotein, expressed on the surface of HIV-infected cells. CD4+ CAR-T cells which targets gp120 were able to suppress the viral replication in HIV-infected T cells and macrophages, as well as destroy HIV-infected T cells. As structure and function of clinically tested CARs is being improved, a second generation CD4+ CAR-Ts that in vitro eliminated not only infected and HIV-producing cells but the entire latent cell population were developed [10]. Despite their limited in vivo efficiency, application of anti-HIV CAR-T has significantly intensified over past years due to the outstanding success of CAR-T therapy in cancer treatment, and a new generation of anti-HIV CARs based on HIV-1 broadly neutralizing antibodies (BNAbs) that target gp120 and gp41 motifs have been developed [11]. These BNAbs-based CAR-Ts exhibited substantial antiviral activity in vitro through the recognition and destruction of HIV-1 infected cells, while they are capable to suppress HIV-1 viral rebound after withdrawal of the antiviral treatment.

CAR-T cells were proposed as a therapy for fibrosis, a pathological process associated with the hyperactivation and expansion of fibroblast and with the deposition of extracellular matrix components. By engineering CAR-T cells that target activated heart fibroblasts through the recognition of fibroblast activation protein in mice with induced hypersensitivity cardiac injury and fibrosis, researchers significantly reduced or even completely eliminate cardiac fibrosis, also inducing a partial rescue of both systolic and diastolic cardiac functions with no adverse effects.

In addition to their potential therapeutic use, research regarding CAR-T cell action in immune dysfunction caused by autoimmune diseases or HIV can help understand mechanisms of tumor immune evasion. Exploring how CAR-T cells migrate into the tumor stroma and the lymph node HIV sanctuaries will generate mutually complementary new knowledge about the molecular mechanisms involved in both pathogenic conditions. Despite unresolved issues regarding CAR-T cells long term toxicity, they constitute a powerful tool for treating diseases whose pathology is mediate by different immune system actors.

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