Are All Chimeric Antigen Receptors Created Equal

In the report accompanying this article, Kochenderfer et al discuss the efficacy of autologous T cells expressing a CD19-specific chimeric antigen receptor (CAR) in patients with relapsed diffuse large B-cell lymphoma. Although previous reports have demonstrated significant antitumor activity in low-grade B-cell malignancies and B-cell acute lymphoblastic leukemia (ALL), this is the first report to our knowledge to demonstrate the efficacy of CD19-targeted CAR T cells in patients with diffuse large B-cell lymphoma. A CAR is a recombinant receptor constructs composed of an extracellular single-chain variable fragment (scFv) derived from an antibody, linked to a transmembrane domain, which is further linked to one or more intracellular T-cell signaling domains (Fig 1). Because CARs combine the binding domain of an antibody and T-cell signaling moieties, they can redirect T-cell specificity to the tumor in an HLA-independent fashion and activate the CAR-modified T lymphocytes to lyse targeted tumor cells. Initial first-generation CARs were constructed through the fusion of an scFv-based antigen binding domain to an inert CD8 transmembrane domain, linked to a cytoplasmic signaling domain derived from the CD3or Fc receptor chains (Fig 1). Although T cells expressing first-generation CARs effectively engaged the target antigens, limited T-cell proliferation and diminished cytolytic activities were observed because of the lack of T-cell costimulation. For optimal activation and proliferation, T cells require both T-cell receptor engagement and signaling (termed signal 1), as well as costimulatory signaling through costimulatory receptors (ie, CD28, 4-1BB, OX-40) on T cells binding to cognate ligands (ie, CD80/86, 4-1BBL, OX-40L) expressed either by the targeted tumor cell or professional antigen-presenting cells (termed signal 2). To overcome the lack of T-cell costimulation (signal 2), firstgeneration CARs were further modified by incorporating the cytoplasmic signaling domains of T-cell costimulatory receptors. These second-generation CARs (Fig 1) enhanced signaling strength and persistence of the modified T cells, leading to superior antitumor activity. However, it remains unclear whether any particular second-generation CAR design is superior to the other. In one study by Carpenito et al, both CD28and 4-1BB–based CAR-modified T cells had the same antitumor activity, but the 4-1BB–based CAR enhanced in vivo persistence of T cells. In contrast, another study failed to demonstrate any significant difference between CD28and 4-1BB–based second-generation CARs with regard to proliferation, potency, antitumor efficacy, and persistence of the T cells. More recently, third-generation CARs containing tandem cytoplasmic signaling domains from two costimulatory receptors (Fig 1) were tested in certain mouse models, but clinical experience so far has been limited. Initial clinical results with first-generation CD19-targeted CAR-modified T cells were disappointing, but second-generation CAR-modified T cells displayed significantly enhanced expansion and persistence in patients with B-cell lymphoma. Using either CD28or 4-1BB–based second-generation CD19-specific CARs, investigators at Memorial Sloan Kettering Cancer Center (MSKCC), the University of Pennsylvania, and the National Cancer Institute published initial promising results of phase I clinical trials, reporting durable antitumor efficacy in patients with indolent B-cell malignancies, including chronic lymphocytic leukemia (CLL), and more recently reporting dramatic clinical responses in patients with relapsed B-cell ALL. These trials varied significantly with respect to infused T-cell product; CAR design, including scFv and costimulatory domains (CD28 v 4-1BB); means of gene transfer (retrovirus v lentivirus); ex vivo T-cell expansion; T-cell dose; intensity of conditioning chemotherapy; degree of tumor burden at the time of therapy; and age of the treated patient population. All of these variables make it difficult to compare results from one trial with those of another and the efficacy of one CAR T cell with that of another. Hinge First-generation CAR Second-generation CAR Third-generation CAR