High Anti-Tumor Activity of Natural Killer Cells after Non-Myeloablative Conditioning and Allogeneic Stem Cell Transplantation.

Background: It is hypothesized that enhanced graft-versus-leukemia activity of natural killer (NK) cells contributes to the clinical efficacy of non-myeloablative allogeneic hemotopoietic stem cell transplantation (HSCT). We compared NK cell cytotoxic activity after transplantation with full and reduced dose regimens with normal values obtained from healthy volunteers.

Methods&Material: Using a flow cytometric assay that detects CD107a expression after coincubation with leukemia cell lines as a marker for NK cell degranulation we prospectively quantified and characterized NK cells mediating anti-tumor activity in patients after HSCT. Mononuclear cells (MNC) were isolated from peripheral blood of 17 healthy individuals and 31 patients transplanted with G-CSF mobilized peripheral blood stem cells at day +30. MNC were incubated with leukemia cell lines HL60 and K562 (effector/target ratio 1:1) and expression of CD107a was measured after 3 hours. The absolute number of degranulating (CD107a⁺) NK cells was calculated.

Results: Thirty one patients (five with ALL, four with NHL, one with Aplastic Anemia and 21 with AML) were enrolled, of whome 22 were transplanted from a matched related donor, seven from a matched unrelated donor, and two from a haploidentical donor. Nine patients had been transplanted after conditioning with 2 Gy TBI only, whereas 22 patients had received various conventional dose regimens. NK cell counts at day +30 after HSCT with conventional conditioning were comparable to normal values, but percentage and number of degranulating cells were significantly reduced (2.4% and 7/μl vs. 6.2% and 18/μl; p<0.0001 and p=0.0003). In contrast, in patients after 2 Gy TBI the percentage of degranulating NK cells was comparable to healthy donors (7.6%; p=0.9), and interestingly, absolute numbers of all and of degranulating NK cells (41/μl; p=0.004) were higher than normal values and consequently higher than in patients with conventional dose conditioning.

Conclusions: Using this new method we were able to exactly quantify tumor-reactive NK cells after HSCT in a feasible way. We found a high cytotoxic activity of NK cells towards leukemia cell lines in patients undergoing non-myeloablative conditioning with 2Gy TBI. Further studies to determine the clinical impact of these findings are needed.