Expanding the antigenic repertoire of CAR-T cells with "TCR-like" antibody specificity

The ability to manufacture increasingly humanized clinical grade monoclonal antibodies has led to a burgeoning field of successful antibody therapy for diverse malignant and non-malignant indications. In the treatment of B cell malignancies, the antiCD20 monoclonal rituximab has proved to be highly effective and is routinely incorporated into treatment schedules for lymphomas and chronic lymphocytic leukemia.While monoclonal antibodies can bind to a number of antigenic targets, such as CD19, CD20, CD22, on B lymphoid malignancies, investigators have long sought to boost their cytotoxic efficiency by linking them to cytotoxic or radioactive compounds, or to cytotoxic lymphocytes.The idea of coupling the cytotoxicity of aT cell with the avidity of an antibody targeting a surface molecule on a cancer cell to create a powerful anticancer cell therapeutic originated in 1985, but successful techniques to create clinical grade T cells engineered to carry a chimeric antigen receptor—so called CAR-T cells—took two decades to refine [1,2]. The inserted CAR gene associates an antibody withT cell receptor activation molecules. Engagement with surface molecules on malignant cells by the antibody variable region of the CART cell signals through a costimulatory molecule (usually CD28 or 4-1BB) to activate the T cell receptor CD3 zeta chain [3]. The activated cytotoxic T cell then kills the target cell it is bound to. In several concept-changing clinical studies, CAR-T cells armed with CD19 have been shown to produce complete molecular remissions in high risk patients with B cell malignancies, especially acute lymphoblastic leukemia, who have failed all other treatments (reviewed in [4]). As the field moves on to exploit CAR-T cell technology beyond these first proofs of principle in B cell malignancy, investigators are confronted with the limited availability of surface antigens on malignant cells that are adequately specific for the tumor and represent suitable antigens for antibody targeting.Desirable antigens are those expressed on malignant precursors, are not shed or readily downregulated, and are lineage-restricted. Unlike the rich diversity of antigens presented through HLA molecules to T cells through their natural TCR/MHC/peptide engagement, good antibody-targeted surface antigens on malignant cells are few. In myeloid malignancy, for example, the only antibody candidates so far include CD33 and CD 123, but concerns over their nonspecific myeloid toxicity limit the application of CAR-T cells for treating acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). In this issue of Cytotherapy, Molldrem et al. [5] present a new development in the CAR concept where the antibody component recognizes and binds to a peptide/MHC complex, acting in lieu of the natural TCR/peptide-MHC complex engagement. Signaling through CD28 and the CD3 zeta chain, the antibody activates the CAR-T cell to become cytotoxic to the target. The antigen selected—PR1 (a nonapeptide of proteinase 3) – presented through HLA-A2 is overexpressed inmanymyeloidmalignancies and some solid cancers.Molldrem first described the antigen in 1996 [6] and in this issue they report that PR1specificT cells can readily be generated from umbilical cord blood [7]. PR1 is a target of T cells after stem cell transplantation, and PR1 vaccination can induce T cell responses and reduce the level of residual disease inAML andMDS [8]. In preclinical studies,Molldrem here show that PR1/HLAA2CAR-T cells rapidly and efficiently kill AML cells in vitro. The therapeutic