The human leukemia cell line K562 represents an attractive platform for creating an artificial antigen presenting cell (AAPC): it is readily expandable, does not express HLA class I and II and can be stably transduced with various genes. To generate an AAPC to expand CMV antigen-specific T cells for adoptive immunotherapy, we stably transduced K562 with HLA-A2 in combination with 4-1BB ligand, or CD64 or HLA-DR15. In preliminary experiments, irradiated K562 cells expressing HLA-A2 and 4-1BB ligand pulsed with CMV pp65 and IE-1 HLA-A2 specific peptides failed to elicit antigen-specific CD8+ T cells in HLA-A2+ peripheral blood mononuclear cells (PBMC) or isolated T cells. Since CMV peptides added directly to the PBMC readily expanded antigen-specific CD8+ T cells, we concluded that K562 AAPC inhibited the T cell response. We found that both parental K562 cells and AAPC strongly inhibited T cell proliferation to the bacterial superantigen staphylococcus enterotoxin B (SEB), anti-CD3 stimulation with OKT3, and in MLR. The inhibitory effect of K562 appeared to be T cell specific since K562 cells did not suppress EBV-transformed B cells.
Transwell experiments demonstrated preservation of the inhibition, suggesting that suppression was mediated by a soluble factor. MLR inhibition was not reversed by neutralizing anti-TGFβ antibody or PGE2 inhibitors. Finally, the full abrogation of the suppressive activity of K562 was achieved by a brief fixation of cells with formaldehyde at concentrations as low as 0.1%: The MLR was restored when K562 was fixed, and donor T cell response to SEB- and OKT3-loaded K562 AAPC was significantly higher when using fixed K562 cells compared to unfixed cells. Moreover, fixed pp65 and IE-1 peptide-loaded HLA A2+ AAPC expressing 4-1BB ligand induced robust (3–5 fold improved) expansion of CMV-specific T cells from all tested HLA-A2+ donors when compared with irradiated AAPC control. Thus, fixed K562 cell constructs efficiently presented antigen and stimulated T cells.
Overall, we demonstrate that K562 line can serve as a source of AAPC for cell therapy approaches after abrogation of their suppressive activity using formaldehyde. However, our results also revealed a previously unappreciated feature of K562 biology, clearly indicating that these commonly used cells are potent inhibitors of peptide antigen-, superantigen-, and OKT3- driven T cell proliferation. Thus K562 line displays a myeloid-derived suppressor cell-like functionality. Our findings have implications for broader understanding of the immune evasion mechanisms used by leukemias and other tumors.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.