Cytotoxic T Lymphocyte Responses to PR1 Peptide in Chronic Myeloid Leukemia Patients Inversely Correlate with Proteinase 3 and Elastase Expression but Donor PR1 Responses Determine Survival after Stem Cell Transplantation.

The primary granule proteins (PGP) neutrophil elastase (ELA2) and proteinase 3 (PR3) both contain the nonapeptide PR1 which can induce cytotoxic T lymphocyte (CTL) responses in chronic myeloid leukemia (CML). The relative contribution of PR3 and ELA2 to PR1 expression is not known. We previously found that higher levels of PR3 and ELA2 gene expression in CD34+ progenitor cells were associated with longer survival in CML patients. Eradication of leukemia depends on the elimination of leukemic stem cells which are thought to reside within the CD34+ progenitor cell pool. We therefore studied PGP expression and T cell response to PR1 in 23 CML patients and their HLA-identical family donors prior to T-cell depleted allogeneic stem cell transplantation (SCT) with T-cell add-back on day 45–100 post-SCT. CD8+ T cells and CD34+ progenitor cells were purified from mononuclear cells (MNC) of cryopreserved leukapheresis products from HLA-A*0201+ CML patients. Following reverse transcription of RNA from MNC and CD34+ cells, ELA2 and PR3 gene expression was measured using real-time quantitative polymerase chain reaction (RQ-PCR). To assess PR1-CTL responses, T2 cells were loaded with PR1 peptide (VLQELNVTV) at 0.1, 1, and 10μM for 2h, irradiated and subsequently co-cultured with CD8+ T cells for 3h. PR1-CTL response was measured as interferon-γ mRNA expression (relative to CD8) obtained by RQ-PCR. PR1-specific CTL responses were detected in 5/23 CML patients and 6/16 HLA-identical donors. In CML patients, pre-SCT expression of both PR3 and ELA2 in MNC was strongly correlated with the expression in CD34+ cells (p=0.009 and p=0.0006 respectively). There was an inverse relationship between PR1-CTL response in CML patients pre-SCT and PR3 or ELA2 expression (p=0.02 and p=0.01 respectively). This data suggest that both PR3 and ELA2 expression in CD34+ cells and their progeny are a potential source of PR1. However, high expression of these proteins may result in selective deletion of PR1 T cell clones. The presence of PR1-responses in HLA-identical donors was associated with an improved overall survival post-SCT. However, there was no impact of PR1-response in either CML patients pre-SCT or donors, on time to achieve molecular remission post-SCT but a greater proportion of patients whose donors did not have a PR1-response succumbed to fatal graft-versus-host disease. These findings support post-transplant vaccination strategies with PR1 peptide to eradicate minimal residual disease but in patients who are not eligible for SCT, the lower PR1 response in the presence of high PGP expression suggests that there is a risk that vaccines given to patients with minimal residual disease could induce tolerance.

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