Aberrant Subcellular Localization of Azurophil Granule Proteins in Myeloid Leukemia Favors Peptide Antigen Presentation on MHC-I and Susceptibility to Killing by Cytotoxic T Lymphocytes.

The recognition of leukemia-associated antigens (LAA) by immune cells is essential for generating anti-leukemia responses. Whether elicited by vaccine or hematopoietic stem cell transplantation (HSCT), anti-leukemia immunity requires that cytotoxic T lymphocytes (CTL) recognize and mount an effective response against leukemia cells. PR1 is a nonomeric HLA-A2-restricted peptide (VLQELNVTV) derived from the myeloid-restricted serine proteases neutrophil elastase (ELA2) and proteinase-3 (PRTN3), recognized azurophil granule proteins. PR1-specific cytotoxic T lymphocytes (PR1-CTL) have been shown to preferentially recognize and kill myeloid leukemia cells. We hypothesized that aberrant protein expression in target leukemia cells predisposes to CTL killing. To determine the subcellular localization of ELA2 and PRTN3 in leukemia and in healthy donor (HD) neutrophils, high-speed ultracentrifugation of leukapheresed cells from patients with AML and CML in blast crises yielded 4 fractions: cytoplasmic, nuclear, membrane and golgi/endoplasmic reticulum (ER). Western blot analyses identified higher levels of ELA2 and PRTN3 in cytoplasmic, nuclear, and golgi/ER fractions in leukemia, compared to HD granulocytes. Confocal microscopy of leukapheresed myeloid blasts and HD granulocytes confirmed these findings. We further hypothesized that cross-presentation of PR1 is necessary for priming anti-leukemia immunity. Confocal microscopy of the human B-cell line HMy.CIR-A2, co-incubated with either purified or recombinant ELA2 and PRTN3 demonstrated co-localization of both proteins with LAMP-2, a marker of lysosomes and early endosomes, and small distribution in cytoplasm after 2 hours. Uptake of ELA2 and PRTN3 by HMy.CIR-A2 cells was additionally confirmed by Western blots. Priming of naive PR1-CTL by co-culture of HD lymphocytes with PRTN3 or ELA2-transfected HMy.CIR-A2 cells was confirmed by cell proliferation assays using BrDU. This could be abrogated by addition of lactacystin and cyclohexamide to cell culture. Together, these results demonstrate that ELA2 and PRTN3 are aberrantly localized preferentially in gogli/ER and cytoplasmic fractions in leukemia, rather than azurophil granules, which are often absent or deficient in myeloid leukemia. Similar forced subcellular localization of these proteins in B cells leads to their susceptibility to PR1-CTL killing. Therefore, we conclude that the natural mistrafficking of PRTN3 and ELA2 in myeloid leukemia is necessary to render cells susceptible to CTL attack and that cross-presentation of ELA2 and PRTN3 contributes to priming of a PR1-CTL response.