MHC-independent Surface Antigens
Many tumors have downregulated presentation of MHC molecules and/or MHC-dependent antigens to escape immune recognition, and this will confer resistance in order to immunotherapy with MHC-restricted T cells. Indeed, loss of tumor expression of the entire mismatched MHC haplotype has been demonstrated to be a common mechanism responsible for immune escape and relapse after haploidentical HCT. These data suggest that novel approaches that do not rely solely on MHC-restricted T cells may be needed to prevent post-HCT relapse. CARs, whose specificity is governed by a single-chain-variable fragment comprising the heavy and light chains of a tumor-reactive antibody linked to an intracellular T-cell signaling moiety, were developed to overcome downregulation of MHC expression on tumors and to allow targeting of antigens regardless of the HLA type of the T-cell donor or recipient. Introduction of a transgene encoding a CAR into T cells can redirect their specificity to protein, glycolipid, carbohydrate and other cell surface antigens, and the inclusion of an 'in-line' activation domain from one or more costimulatory molecules in the CAR allows T cells to respond in the absence of costimulatory ligands on the tumor.
CARs have been constructed to target several distinct surface molecules expressed on hematopoietic malignancies (Table 1). CAR-modified T cells have not yet been used in the allogeneic HCT setting, but impressive results in a small number of patients with refractory CLL treated with autologous T cells modified with a CD19-specific CAR incorporating a 4-1BB costimulatory domain illustrates the potential to incorporate CAR-modified T cells as an effective post-HCT therapy to prevent or treat relapse.[78,79] Just as with the introduction of TCR genes into allogeneic donor T cells, it will be essential with CAR gene transfer to know the specificity of the endogenous TCR in modified cells to avoid infusing potentially alloreactive T cells and causing GVHD. The strategy that the authors' laboratory has pursued involves selectively transducing donor cytomegalovirus (CMV) or EBV-specific T cells to express the tumor-reactive CAR, since virus-specific donor T cells have been adoptively transferred without causing GVHD (Figure 1). Clinical trials of CAR-modified T cells following allogeneic HCT are expected to begin accruing in the near future and should assist in defining both the potential for efficacy and limitations of this approach.
Strategy for genetically retargeting donor virus-specific central memory T cells for treatment of CD19+ malignancies after allogeneic hematopoietic stem cell transplantation. (A) CD8+ TCM cells were selected from donor PBMC using a two-step immunomagnetic selection on the CliniMACS device (picture used with permission from Miltenyi Biotec). Clinical grade monoclonal antibody bead conjugates that are specific for CD45RA, CD4 and CD14 were used first for depleting cells bearing these markers, followed by positive selection of CD62L+ cells. (B & C) TCM-enriched cells are stimulated with immunogenic Epstein–Barr virus or cytomegalovirus peptides and exposed to the CD19 CAR lentivirus to induce selective proliferation and transduction of virus-specific T cells. (D) CAR-specific T cells are expanded by stimulation with irradiated CD19+ antigen-presenting cells. (E) Virus-specific T cells are then purified by selection with MHC class I streptamers to minimize potential contamination with alloreactive T cells in the T-cell product. (F) Final expansion of the T-cell product for infusion. Total time for cell manufacturing is approximately 30 days.CAR: Chimeric antigen receptor; HCT: Hematopoietic stem cell transplantation; LCL: Lymphoblastoid cell line; PBMC: Peripheral blood mononuclear cell; TCM: Central memory T cell; TCR: T-cell receptor.
Expert Rev Hematol. 2012;5(4):409-425. © 2012 Expert Reviews Ltd.