Abrogation of Myeloid Derived Suppressor Cell-Like Inhibitory Activity of K562 Restores Antigen Presenting Cell Functions

Abstract 4118 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. Disclosures: No relevant conflicts of interest to declare.