Aberrantly expressed neutrophil elastase (ELA2) in the nucleus and cytoplasm of acute lymphocytic leukemia (ALL) cells cleaves cyclin E (CCNE) into low-molecular-weight forms (LMWFs) yielding novel HLA-A2 restricted determinants (50.28)

We reported that CCNE-specific CTL elicited from healthy donors (HDs) kill ALL and CML cells, which aberrantly express CCNE. Further, 5 LMWFs of CCNE1 are present only in malignant cells and constitutively active to promote cell division. In addition, we showed ELA2-specific CTL also kill AML and CML, and ELA2 is normally expressed only in myeloid cells. To determine whether ELA2 might be aberrantly expressed in ALL and whether ELA2 cleaves CCNE to LMWF’s, we first found CCNE of LMWF to be overexpressed by western blot of cell lysates from ALL, but not from ALL in remission or HD B-cells. ELA2 added to ALL lysates increased LMWF’s and was inhibited by the ELA2 inhibitor elafin. Subcellular fractionation and co-immunoprecipitation showed ELA2 to be aberrantly associated with CCNE in nucleus, cytoplasm, and membrane-bound organelles in ALL, but not normal bone marrow cells. Because determinants recognized by CCNE-CTL are also found in the LMWFs and lysis of leukemia correlates with target protein overexpression, we studied the effect of induced CCNE expression on target susceptibility to cytolysis. While normal PBMC were not killed, PHA or α-CD3/28 stimulated PBMC were killed by CCNE-CTL, although non-specifically. We conclude that in ALL, aberrant expression of the myeloid protein ELA2 results in mistrafficking of ELA2 to cytoplasm and nucleus, thus mediating CCNE cleavage to LMWF’s and increasing susceptibility to CCNE-CTL killing. This shows novel tumor antigen interaction that may cooperatively reverse CTL tolerance.