CART Immunotherapy: Development, Success, and Translation to Malignant Gliomas and Other Solid Tumors

Abstract

T cell chimeric antigen receptor (CAR) technology has allowed for the introduction of a high degree of tumor selectivity into adoptive cell transfer therapies. Evolution of this technology has produced a robust antitumor immunotherapeutic strategy that has resulted in dramatic outcomes in liquid cancers. CAR-expressing T-cells (CARTs) targeting CD19 and CD20 have been successfully used in the treatment of hematologic malignancies, producing sustained tumor regressions in a majority of treated patients. These encouraging results have led to a historic and unprecedented FDA approval of CTL019, Novartis' CAR T-cell therapy for the treatment of children and young adults with relapsed or refractory B-cell acute lymphoblastic leukemia (ALL). However, the translation of this technology to solid tumors, like malignant gliomas (MG), has thus far been unsuccessful. This review provides a timely analysis of the factors leading to the success of CART immunotherapy in the setting of hematologic malignancies, barriers limiting its success in the treatment of solid tumors, and approaches to overcome these challenges and allow the application of CART immunotherapy as a treatment modality for refractory tumors, like malignant gliomas, that are in desperate need of effective therapies.

Keywords: CAR T-cells; chimeric antigen receptor; glioblastoma; immunotherapy; malignant glioma.

Figure 1

Immune-mediated interactions in solid tumors and rationale for CART immunotherapy. (A) Release of cell debris and tumor antigens from malignant cells activates a cascade of host antitumor immune responses, initiated by innate immune cells that release pro-inflammatory cytokines and contribute to tumor cell destruction. Among these cells are dendritic cells, which capture tumor antigens, mature in response to the pro-inflammatory cytokines in the environment, and travel to lymphoid tissues to stimulate T-cell proliferation and activation of antigen-specific adaptive immune responses leading to tumor death. (B). Tumors often develop adaptations to evade detection and destruction by the host immune system. Through the recruitment of suppressive leukocytes and elaboration of immunosuppressive cytokines, tumors inhibit the function of infiltrating immune cells, including dendritic cells. Incompletely matured DCs are unable to effectively activate naïve T cells, instead inducing T-cell anergy, apoptosis, or tolerance to tumor-associated antigens. Downregulation of antigen-presenting machinery and the development of antigen-loss variants enable tumor cells to escape detection by infiltrating immune cells. (C) CAR T-cells, which recognize antigens via a mechanism distinct from TCR stimulation, bypass the need for DC antigen presentation and are unaffected by MHC downregulation. CAR structure and culture conditions can also be optimized to create CART populations with superior cytotoxicity and resistance to tumor-induced suppressive influences.

Figure 2

CAR structure. CARs are comprised of an antigen-recognition ectodomain derived from the single-chain variable fragment (scFv) of a monoclonal antibody connected by a flexible hinge and transmembrane segment to an intracellular endodomain. Originally derived from the CD3ζ domain of the classical TCR in (A) first generation CARs, this intracellular signaling component may contain (B) one (second generation) or (C) two (third generation) additional costimulatory domains that enhance the proliferation, persistence, and efficacy of adoptively transferred cells.

Figure 3

Development of CART immunotherapy. Following development of the first chimeric T-cell receptor in 1989, early preclinical studies of the first CARTs demonstrated the ability to selectively identify and destroy antigen-expressing tumor cells (5, 6). However, upon adoptive transfer into live patients, T-cells expressing these first-generation CARs displayed limited persistence and were often rendered anergic due to the absence of costimulatory signals within the tumor microenvironment (TME) (26). With the introduction of costimulatory domains to provide these necessary activating signals, CART immunotherapy experienced a dramatic improvement in therapeutic efficacy (22). Optimization of CAR structure and ex vivo culture conditions to improve CART persistence, cytotoxicity, and resistance to tumor-induced immunosuppression remains an area of continued research. Evaluated in a variety of tumor types, CART immunotherapy has been markedly successful in the eradication of liquid tumors, culminating in the FDA approval of CART immunotherapy for the treatment of relapsed or refractory B-cell ALL in 2017.

Figure 4

Barriers to successful CART immunotherapy in solid tumors. (A) Factors within the tumor microenvironment: Solid tumors contain an abundance of immunosuppressive leukocytes, immune checkpoint molecules, and suppressive cytokines. The tumor cells themselves are highly heterogeneous, preventing the identification of uniformly expressed targets for CAR design; antigen selection is further limited by an inability of CARTs to target intracellularly derived antigens. (B) Barriers to CART migration and entry into tumor sites: In contrast to the disseminated nature of hematologic cancers, solid tumors are often found in isolated locations that are difficult to access, like the brain. (a) following adoptive cancer, CARTs have been shown to preferentially accumulate in organs such as the lungs, liver, and spleen, with limited natural trafficking to tumor sites. (b) downregulated expression of ICAMs and other adhesion molecules on tumor vasculature limits lymphocyte extravasation, (c) reduced release of lymphocyte-attracting chemokines such as CXCL9-11 precludes CART homing to tumor sites, (d) in addition to supporting the growth and persistence of malignant cells, tumor-associated stroma provides both a physical and immunologic barrier to CART immunotherapy, (e) release of angiogenic factors such as VEGF promotes the develop of abnormal, tortuous, high-pressure vasculature that impedes lymphocyte entry.(C) Toxicity secondary to off-target effects: the use of overexpressed self-antigens as CAR targets introduces the risk of significant toxicity associated with CART identification and destruction of normal, healthy cells expressing these antigens.