Genetically engineered fixed K562 cells: potent "off-the-shelf" antigen-presenting cells for generating virus-specific T cells

Background aims. The human leukemia cell line K562 represents an attractive platform for creating artificial antigen-presenting cells (aAPC). It is readily expandable, does not express human leukocyte antigen (HLA) class I and II and can be stably transduced with various genes. Methods. In order to generate cytomegalovirus (CMV) antigen-specific T cells for adoptive immunotherapy, we transduced K562 with HLA-A*0201 in combination with co-stimulatory molecules. Results. In preliminary experiments, irradiated K562 expressing HLA-A*0201 and 4-1BBL pulsed with CMV pp65 and IE-1 peptide libraries failed to elicit antigen-specific CD8(+) T cells in HLA-A*0201(+) peripheral blood mononuclear cells (PBMC) or isolated T cells. Both wild-type K62 and aAPC strongly inhibited T cell proliferation to the bacterial superantigen staphylococcal enterotoxin B (SEB) and OKT3 and in mixed lymphocyte reaction (MLR). Transwell experiments suggested that suppression was mediated by a soluble factor; however, MLR inhibition was not reversed using transforming growth factor-beta blocking antibody or prostaglandin E-2 inhibitors. Full abrogation of the suppressive activity of K562 on MLR, SEB and OKT3 stimulation was only achieved by brief fixation with 0.1% formaldehyde. Fixed, pp65 and IE-1 peptide-loaded aAPC induced robust expansion of CMV-specific T cells. Conclusions. Fixed gene-modified K562 can serve as effective aAPC to expand CMV-specific cytotoxic T lymphocytes for therapeutic use in patients after stem cell transplantation. Our findings have implications for broader understanding of the immune evasion mechanisms used by leukemia and other tumors.