[Purpose] Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) is an interferon α (IFN)-induced, apoptosis-inducing molecule. TRAIL is one of the promising reagents for therapeutic use in combination with imatinib in chronic myeloid leukemia (CML). A previous report showed that IFN-induced TRAIL is only weakly expressed on cell surface but is mainly secreted as a soluble form (sTRAIL) by polymorphonuclear neutrophils (PMNs) (and also by monocytes) in healthy individuals. Up to now, however, whether sTRAIL is actually released into serum in CML patients and whether its serum level is up-regulated during in vivo IFN therapy are unsolved issues. To assess the significance of sTRAIL, we examined serum sTRAIL levels by ELISA in total 40 (36 in CP, 4 in BP) CML patients either prior to or undergoing IFN therapy.
[Results] Serum sTRAIL were detectable in untreated CML patients (1258±560 pg/ml, n=25), which levels were substantially comparable to those in healthy donors (1159±343, n=10). Serum sTRAIL levels significantly increased in patients during IFN therapy (1801±712, n=18) compared to those in untreated patients (P=0.009). This IFN-induced up-regulation of serum sTRAIL was further confirmed in all seven patients whom we could analyze both before and during the therapies (before 1228±396 vs. during 1956±914, P=0.037). As for initial sTRAIL levels, among the restricted patients group who later showed any cytogenetic response (n=15), there was a tendency that initially higher serum sTRAIL levels accompanied better cytogenetic responses (CCyR: 1873±719, PCyR: 1130±204, minor CyR: 830±267, P=0.03), indicating that the initial sTRAIL level may have some relation with the later response to IFN therapy. Next, to investigate the source of the sTRAIL secretion into serum, we assessed TRAIL mRNA expressions in PMNs in the peripheral blood by RQ-PCR. Again, TRAIL mRNA expressions in patients during IFN therapy (n=22) significantly increased compared to those in untreated patients (n=16) (TRAIL/GAPDH: before 1.4 vs. during 8.0, P=0.005). This IFN-induced up-regulation of TRAIL mRNA expression in PMNs was confirmed in all 16 patients whom we could analyze both before and during IFN therapies (before 3.7 vs. during 21.3, P=0.018). In vitro IFN stimulation of PMNs in five untreated patients showed marked induction (10 to 30 fold) of TRAIL mRNA at 12h, while induction of FAS mRNA stayed rather low (1 to 2 fold). Furthermore, IFN concentration-dependent accumulations of sTRAIL into the culture supernatants were observed for up to 48 h. Next, we assessed TRAIL mRNA expressions in purified CD34-positive cells in the bone marrow. TRAIL mRNA expressions were also detectable, and the mRNA levels significantly increased in patients during IFN therapy (n=37), compared to those in untreated patients (n=17) (P=0.02). This IFN-induced up-regulation of TRAIL mRNA was confirmed in nine patients whom we could analyze both before and during the therapies (P=0.01), indicating that even immature CML elements are responsive to IFN and may contribute to sTRAIL release in the microenvironment.
[Conclusion] This is the first evidence showing that IFN, in vivo, provokes the release of sTRAIL when administered systematically in CML patients. The main source of the serum sTRAIL is probably PMNs in CML patients. We suspect that one of the mechanisms of anti-proliferative action of IFN on CML is sTRAIL secreted into serum. Our findings also give rationale for the use of IFN (or recombinant sTRAIL) in combination with imatinib in CML patients.