CARTs to the rescue: therapeutic potential of chimeric antigen receptor T-cells in patients with B-cell malignancies

The curative potential of allogeneic stem cell transplant in hematologic malignancies exemplifies how the immune system can control tumors where chemotherapy has failed [1]. However, allogeneic stem cell transplant is limited by the availability of suitable donors, and the toxicity of the non-specific immunity resulting in varying grades of graft-versus-host disease, which still remains a serious issue for patients after transplant. In principle, the “holy grail” of anti-cancer immune therapy aims at generating effector cells that are: (1) independent of the requirement for a histocompatible donor; (2) have specificity for tumor targets without any unnecessary spillover toxicity on normal tissues; and (3) persist in the body, thereby providing long-term protection from recurrent tumor. The recent innovative development of “designer” chimeric antigen receptor T-cells (CARTs) promises to fulfill all of these requirements.

CARTs are T-cells which have been manipulated to have their specificity redirected against a tumor antigen, by the insertion (transduction) of a chimeric receptor complex comprising an antigen recognition site and a signaling molecule (usually the CD3ζ chain) [2]. First-generation CARTs were limited by little anti-tumor activity in human trials, and the inability of the infused T-cells to survive for more than a few days [2]. Second-generation CARTs included a second signal (co-stimulation), most commonly CD28 [3,4] or CD137 (4-1BB) [5]. These cells survive for longer periods of time with medium-term persistence in the body, being detectable up to several months after infusion. These T-cells are associated with promising anti-cancer activities, including the occurrence of acute cytokine-release syndromes and clinical regression of tumor masses [5,6]. Further novel refinements have included the generation of CARTs that incorporate both the CD28 and CD137 signaling domains [7], CARTs which secrete stimulatory cytokines such as interleukin-12 (IL-12) [8] or IL-21 [9] and CARTs that carry “suicide genes,” permitting elective deletion when their efficacy and usefulness expires [10].

Independent of the actual nature of the infused cells, early phase cellular immunotherapy trials in humans have shown that how the cells are given is also an important determinant of therapeutic success. In this issue of the journal [11], Xu and colleagues review the characteristics and results of anti-CD19 CART therapy for B-cell malignancies conducted by the five US centers that pioneered this treatment (City of Hope, National Cancer Institute, University of Pennsylvania, Baylor College of Medicine and Memorial Sloan-Kettering) [2,4,11]. The primary aim of their review is to compare across clinical trials and gain insight into factors important for successful CART therapy in the clinic.

In total, 26 patients with relapsed B-cell malignancies received a total of 27 CART infusions (one patient was retreated following disease relapse after the first infusion) [11]. The majority had relapsed chronic lymphocytic leukemia or non-Hodgkin lymphoma, and most patients received second-generation CARTs. There were considerable variations in the technique and dosage of infusions: CART doses ranged between 10⁷ and 10⁹ cells, lymphodepletive conditioning regimens ranged from none [4] to high-dose cyclophosphamide [12], and two centers used post-infusion IL-2 supplementation [2,6]. In this overall review, factors found to be important for successful disease control were application of lymphodepletive chemotherapy (p < 0.001), IL-2 supplementation (p = 0.028), persistence of CART cells ≥ 4 weeks (p = 0.014), peripheral blood CART peak ≥ 0.1% (p = 0.004) and peak interferon-γ ≥ 200 pg/mL (p < 0.001) [11]. The dose and composition (CD4:8) of the infused T-cells did not reach significance. There were insufficient numbers of patients treated with first-generation CARTs to draw firm conclusions about efficacy, but second-generation CARTs were known to be capable of substantially longer persistence in vivo (an essential determinant of treatment success) [2,4]. Key individual examples of dramatic responses underscore the impressive potential of these cells to eradicate and control resistant tumor [12].

What does all the above tell us about CART therapy? This comprehensive analysis essentially confirms everything that we suspected would be important in cellular immunotherapy: success is critically dependent on the ability of the infused cells to expand and persist in vivo, which in turn is assisted by lymphodepleting chemotherapy, debulking of the disease burden to a minimum, and use of limited post-infusion cytokine supplementation. Indeed, the new generation of cytokine secreting CARTs [8] may overcome many of the elements, such as chemotherapy and IL-2, currently required for successful engraftment of second-generation CARTs.

Now, where do we go in the future after the initial pioneering steps have been successfully taken? First, simpler transduction techniques (e.g. the “sleeping beauty” system [13]) will greatly assist the more widespread dissemination of this technology outside of specialized centers. Second, manufacture of “off-the-shelf” generic CARTs requires removal of the intrinsic T-cell receptor complex, which is already possible through the use of zinc finger nucleases [14]. Third, exploration of alternative tumor cell targets – such as the ROR1 antigen in CLL cells [15] – will allow generation of a “panel” of cells to effectively target the tumor from multiple angles, thus cutting off potential immune surveillance escape routes by selective antigen loss. Lastly, the insertion of “suicide genes” will allow better control and the deletion of CART cells in the event of unexpected toxicity, or once their mission to eradicate cancer is accomplished [10]. Indeed, the arrival of CARTs may herald the era of selective immune therapy that we have all been waiting for.

Potential conflict of interest

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