PR1 peptide vaccine induces specific immunity with clinical responses in myeloid malignancies

Abstract

PR1, an HLA-A2-restricted peptide derived from both proteinase 3 and neutrophil elastase, is recognized on myeloid leukemia cells by cytotoxic T lymphocytes (CTLs) that preferentially kill leukemia and contribute to cytogenetic remission. To evaluate safety, immunogenicity and clinical activity of PR1 vaccination, a phase I/II trial was conducted. Sixty-six HLA-A2+ patients with acute myeloid leukemia (AML: 42), chronic myeloid leukemia (CML: 13) or myelodysplastic syndrome (MDS: 11) received three to six PR1 peptide vaccinations, administered subcutaneously every 3 weeks at dose levels of 0.25, 0.5 or 1.0 mg. Patients were randomized to the three dose levels after establishing the safety of the highest dose level. Primary end points were safety and immune response, assessed by doubling of PR1/HLA-A2 tetramer-specific CTL, and the secondary end point was clinical response. Immune responses were noted in 35 of 66 (53%) patients. Of the 53 evaluable patients with active disease, 12 (24%) had objective clinical responses (complete: 8; partial: 1 and hematological improvement: 3). PR1-specific immune response was seen in 9 of 25 clinical responders versus 3 of 28 clinical non-responders (P=0.03). In conclusion, PR1 peptide vaccine induces specific immunity that correlates with clinical responses, including molecular remission, in AML, CML and MDS patients.

Figure 1. PR1 –specific CTLs were detected in patient peripheral blood following PR1-peptide vaccination

(a) Gating schema for PR1-CTLs. CTLs, defined by the expression of CD8 and lack of expression of CD4, 14, 16, and 19, were analyzed for binding to PR1-HLA-A2 tetramer. PR1-CTLs were detected in peripheral blood from immune responders following vaccination with 3 (b) and 6 (d) doses of PR1-peptide vaccine, in contrast to non-responders (c) and (e) where <1% of CD8+ CTLs were PR1-specific.

Figure 2. PR1-CTLs are enriched in central memory cells following PR1-peptide vaccination

CD8⁺ T-cells and PR1-specific CTLs were stained for CCR7, CD45RO and CD28 to determine cell T- cell phenotype. (A) Representative flow cytometry staining of one patient sample showing percentages of central memory (CD45RO⁺ CCR7⁺)/(CD28⁺ CCR7⁺) and effector memory (CD45RO⁺ CCR7^-)/ (CD28⁺ CCR7-) in CD8⁺ T-cell and PR1-specific CTL compartments. (B) CD8+ T-cells and PR1-CTLs demonstrating CM phenotype were significantly increased within PR1 CTL (p=0.008) while cells with an EM phenotype were significantly decreased (p=0.007) compared to the overall CD8+ cell population. Open bars are PR1-CTL; black bars are the remaining CD8+ T cells. N, naive; CM, memory; EM, effector memory; TD, terminally differentiated.

Figure 3. PR1-peptide vaccine increases TCR avidity of PR1-CTLs

(a) PR1-HLA tetramer staining of 18 representative patient samples demonstrates increased PR1-CTL avidity following PR1-peptide vaccination. PR1-CTL avidity was significantly higher in patients with an IR and CR in comparison with patients with IR who failed to achieve a CR (P<0.01). In patients with a CR and IR, PR1-CTLs showed higher TCR avidity for PR1-HLA tetramer in comparison to patients with CR and no-IR (p=0.02). (b) Change in PR1/HLA-A2 tetramer staining of PR1-CTLs in patients according to IR and CR status. Patients with IR and CR had the greatest changes in PR1-CTL TCR avidity following vaccination, compared to PR1-CTL TCR avidity prior to vaccine administration. TCR, T-cell receptor; CTLs, cytotoxic T lymphocytes; IR, immunologic response; CR, clinical response.

Figure 4. Event-free and Overall Survival of all 66 patients treated with PR1 Peptide Vaccine

a) EFS of 66 patients who received PR1 peptide vaccine; b) OS of 66 patients who received PR1 peptide vaccine