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.

The immune milieu in the first few months post-stem cell transplantation (SCT) is favorable for graft-versus-leukemia (GVL) responses. A homeostatic drive caused by lymphopenia stimulates expansions of transplanted donor T-cells responding to diverse antigenic stimuli in the recipient. Human leukemia antigens such as proteinase 3 (PR3) and elastase (ELA2) are self-antigens which induce low but detectable frequencies of autoreactive T cells in normal individuals. PR1, an HLA-A*0201 restricted peptide shared by PR3 and ELA2, is expressed in normal neutrophils and overexpressed in myeloid (but not lymphoid) leukemias. T-cell responses against PR1 have been linked to GVL. We studied patients early post SCT to look for early induction of T cell responses to PR1, and correlate them with ELA2 and PR3 expression and GVL effects. Using PR1/HLA-A*0201 tetramers and flow cytometry for intracellular IFN-gamma, we analyzed PBMC for CD8+ T-cell responses against PR1 on days 30, 60, 90 and 120 following a T-depleted SCT in 28 patients (13 CML, 10 ALL, 5 solid tumor). Ten patients with CML, 6 with ALL and 3 with solid tumors had detectable PR1 responses post-SCT. PR1 specific CD8+ T cells had a predominantly effector-memory phenotype (CD45RO+CD27−CD57+). PR3 and ELA2 gene expression in these samples was assessed by RQ-PCR and found to be significantly correlated (P < 0.001). There was a strong association between the expression of PR3 and ELA2 and the emergence of PR1 specific CD8+ T-cell responses. Conversely, reduction or disappearance of PR3 and ELA-2 expression from blood coincided with reduction or disappearance of PR1 specific CD8+ T-cell responses (P <0.001) (as depicted below). The in-vivo anti-leukemia effect of the PR1 response was assessed in CML patients by BCR-ABL transcript numbers at day 90 post-SCT. Eight of ten patients with significant PR1 responses post-SCT were BCR-ABL negative at day 90 compared to 1 of 3 without PR1 responses (P <0.001). This GVL association was restricted to CML patients: in ALL using WT1 gene expression as a measure of minimal residual disease (MRD) 2 of 5 patients with PR1 responses and 3 of 5 patients without were MRD positive on day 90 post-SCT (P =0.36). Since PR1 responses were not restricted to CML and because these transplant approaches usually induce 100% donor myeloid chimerism by day 30 post-SCT, the recovering donor marrow is the likely antigenic source of PR3 and ELA2 driving the PR1 response. Our findings suggest that the post-SCT milieu is favorable for exaggerating weak autoimmune responses to self antigens such as PR1 causing antigen-specific T-cell proliferation. GVL effects may follow if the self antigen is expressed on the leukemia as occurs in CML. These results suggest that vaccination in conjunction with induction of T cell homeostatic proliferation is likely to enhance the anti-leukemia response and effectiveness of a transplant procedure.

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