Natural Killer Cell Expression of Natural Cytotoxicity Receptors Determines Relapse Risk in Elderly Patients with Acute Myeloid Leukemia Receiving Immunotherapy with Histamine Dihydrochloride and Interleukin-2

Background: The insufficiency of natural killer (NK) cells in acute myeloid leukemia (AML) and the purported role of NK cells for prognosis have inspired the use of NK cell-based immunotherapies for relapse prevention, including the administration of NK cell-activating cytokines. A previous phase III trial using immunotherapy with histamine dihydrochloride and low-dose interleukin-2 (HDC/IL-2) was shown to significantly prevent relapse in patients who had achieved complete remission (CR) and completed the phase of consolidation chemotherapy (Brune et al., Blood108:88). Details regarding the mechanisms of relapse prevention are only partly understood.

Trial design and methods: In a single-armed multicenter phase IV study (Re:Mission Trial, NCT01347996) we aimed at assessing the immunomodulatory effects of HDC/IL-2 with a focus on NK cell biology and to correlate NK cell markers with relapse risk. Eighty-four patients (age 18-79, median 61.2) with AML in first CR received post-consolidation immunotherapy with HDC/IL-2 for 18 months in ten 3-week cycles with 3 (cycle 1-6) or 6 week (cycle 7-10) intervals between cycles. NK cell counts in blood and natural cytotoxicity receptor (NCR) expression were determined by flow cytometry before and after cycles 1 and 3. The trial protocol specified separate analyses in younger (<60 years old, n=37) and elderly patients (>60 years old, n=47).

Results: During cycle 1, CD56^bright (CD3^–/16^–/56^bright) and CD16⁺ (CD3^–/16⁺/56⁺) NK cell counts increased in the blood (P=3x10^-8 and P=2x10^-5, respectively, paired t-test, n=47, all evaluable patients) along with an increased expression intensity of the NCRs NKp30 and NKp46 in the cytotoxic CD16⁺ subset (P=2x10^-7 and P=6x10^-8, respectively, paired t-test n=47). NK cell counts declined between the end of treatment cycle 1 and the start of cycle 3. CD56^bright NK cell counts typically returned to pre-treatment levels whereas CD16⁺ NK cell counts remained modestly elevated at the onset of cycle 3 (P=.003, paired t-test; n=46). A renewed accumulation of CD56^bright and CD16⁺ NK cells in the blood was observed during treatment cycle 3 (P=2x10^-6 and P=.007, respectively, paired t-test, n=40). Young and elderly patients showed similar fluctuations in counts of CD56^bright and CD16⁺ NK cell within and between treatment cycles. Also, significant induction of the expression intensity of NKp30 and NKp46 on CD16⁺ NK was observed in young and elderly patients.

Among elderly patients, expression of NKp46 in CD16⁺ NK cells after the first treatment cycle was associated with favorable outcome (RFS for above-median NKp46 expression in CD16⁺ NK cells; P=.002, HR=4.1, CI: 1.7-10.0; logrank test). Similar results, with increased likelihood of RFS, were obtained in elderly patients with NK cells showing above-median expression of NKp30. The expression intensity of NKp30 and NKp46 did not predict relapse in younger patients.

Conclusion: These results highlight the impact of NK cells and NCRs for AML prognosis and suggest that NK cell expression of NCRs may determine the efficacy of immunotherapy for remission maintenance in elderly patients.