PR1-Specific T Cell Responses In The First Months Following T-Cell Depleted Allogeneic Stem Cell Transplantation Occur In Both Myeloid And Non-Myeloid Malignancies But Are Only Associated With A GVL Effect In Myeloid Leukemias

Lymphopenia-driven homeostasis post-SCT allows expansions of donor T-cells against antigens (Ag). Leukemia-associated-Ag proteinase 3 (PR3) & elastase (ELA2) are self-Ag which induce low frequency autoreactive T-cells in normal controls. PR1, an HLA-A∗0201 epitope shared by PR3 & ELA2, is expressed in normal neutrophils & overexpressed in myeloid (not lymphoid) leukemias. T-cells against PR1 have been linked to GVL. We looked for PR1-CD8+T-cells in 28 patients (13 CML, 10 ALL, 5 solid tumors) at day +30-120 following T-cell depleted SCT, using PR1/HLA-A2 tetramers & IC-IFN-γ staining & correlated them with ELA2 & PR3 expression (using qRT-PCR) & GVL. Ten CML, 6 ALL & 3 solid tumor patients had PR1 responses post-SCT. PR3 & ELA2 expression was strongly associated with emergence of PR1-T-cells. Reduction in PR3 & ELA-2 expression coincided with disappearance of PR1-T-cells (P<0.001). In-vivo anti-leukemia effect of the PR1 response was assessed in CML patients by BCR-ABL qRT-PCR: 9/10 patients with early PR1 responses were BCR-ABL negative at day +90 compared to 0/3 without (P<0.001).This GVL association was restricted to CML: in ALL, using WT1 qRT-PCR to measure MRD, 2/5 patients with PR1 responses & 3/5 patients without were MRD+ on day 90 (P=0.36). Since PR1 responses were not CML restricted and all patients had 100% donor myeloid chimerism by day 30, the recovering myeloid hematopoiesis from the donor is the likely antigenic source of PR3 & ELA2 driving the PR1 response. Based on this hypothesis we initiated a clinical trial in patients with relapsed AML to receive weekly vaccination with the WT1-126 peptide admixed with Montanide adjuvant plus GM-CSF following an immunotherapy approach of lymphodepletion by Fludarabine & infusion of autologous lymphocytes collected prior to chemotherapy. All 3 patients enrolled so far had detectable WT1-CD8+T cells following vaccination. Interestingly, PR1-CD8+T-cells were also detected in 3/3 evaluable patients during hematopoietic recovery, associated with increases in PR3 & ELA2 expression. These data suggest the induction of an ‘auto-vaccination’ process by the recovering marrow & that the lymphopenic milieu permits exaggerated weak autoimmune responses to normal self-Ag such as PR1. This immune response may result in GVL if the self-antigen is also expressed on the leukemia as in CML. Vaccination together with induction of T-cell homeostatic proliferation is likely to enhance the anti-leukemia response of SCT.