Proteinase 3 (P3) and neutrophil elastase (NE) are abnormally expressed in the cytoplasm of myeloid leukemia, and we have shown that the HLA-A2-restricted peptide PR1 is processed and cross-presented from both proteins by antigen-presenting cells (APC) to cytotoxic T lymphocytes (CTL). PR1-specific CTL kills leukemia cells and contributes to cytogenetic remission in patients with chronic myelogenous leukemia. Both exogenous P3 and NE can bind to monocytes and P3 promotes DC maturation in vitro. Since high levels of circulating P3 were found in patients with the ANCA-associated systemic vasculitis Wegener’s granulomatosis, modulation of DC by P3 has been implicated in reversal of tolerance. The role of extracellular P3 and NE in reversing tolerance in patients with myeloid leukemia is unknown. We used an ELISA assay to assess P3 and NE level in the sera of patients with AML and CML. Serum NE level was highly increased in all 15 AML patients studied (1045±149 ng/ml, n = 15) and in all 15 CML patients (1013±128, n = 15), compared to a control group of healthy donors (54±13 ng/ml, n = 4; p = 0.0038 and p = 0.0014, respectively). In contrast, serum P3 was increased in only 5 out of 18 (28%) AML patients (47 ± 18, n = 18 ng/ml) and in 8 out of 28 (29%) CML patients (95 ± 44 ng/ml, n = 28) compared to a group of 14 healthy donors (10 ± 2.2 ng/ml; p = 0.09 and p = 0.06, respectively). There was no relationship between elevated serum NE and P3 levels (r=0.16, n=35). P3 level in untreated patients and healthy donors did not change within at least 13 months, although P3 decreased after allogeneic BMT in 5 out of 5 patients (p < 0.05) with high serum levels of P3 pre-BMT. Because P3 is expressed by myeloid progenitor cells in the bone marrow, we studied the concentration of P3 in bone marrow-derived serum (BMS). To our surprise, BMS P3 concentration in patients with myeloid leukemia (23 ± 9.5 ng/ml, n = 9) was similar to that of a control group of patients with non-myeloid hematological malignancies (27 ± 11.8, n = 5) (p = 0.08), and BMS P3 concentration was similar to sera P3 concentration in the same patient (p = 0.52). Interestingly, BMS P3 level correlated with the level of BMS GM-CSF (quantified by ELISA) in a small group of patients with AML and CML (r = 0.94, n = 9), suggesting that P3 secretion by leukemia cells may be downstream of GM-CSF. We also analyzed membrane expression of P3 and NE in 5 AML patients using flow cytometry and found that P3, but not NE, was expressed on circulating cells in 3 of 5 patients. This unique expression of P3 and NE suggests that circulating NE might be the predominant source of cross-presented PR1 antigen by APC or myeloid leukemia cells.

Disclosures: The Vaccine Company.; The Vaccine Company.

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