Donor-Derived NK Cell Infusion Following Haploidentical Hematopoietic Stem Cell Transplant for High-Risk and Relapsed/Refractory Pediatric Leukemia

Background

Natural killer (NK) lymphocytes are cells of innate immunity, involved in early defense against tumor and infections. Their function and activity are regulated by a balance between activator and inhibitory signals through surface receptors, such as the inhibitory killer-cell immunoglobulin-like receptors (iKIRs), that allow NK cells to discriminate between healthy self and to mediate cytolytic activity against malignant or virally infected cells. In haploidentical hematopoietic stem cell transplantation (haplo-HSCT), NK cells play a key role in preventing early relapse and virus reactivation and exert an efficient graft-versus-leukemia (GvL) effect, without increasing graft-versus-host disease (GvHD) incidence.

After haplo-HSCT, donor NK cells derived from CD34⁺ stem cells are immature cells with decreased cytolytic activity (CD56^bright). Progression toward more mature and potent CD56^dim NK cells is characterized by acquisition of KIRs, CD16 and CD57 and is completed in 4-6 weeks. Infusion of mature donor NK cells could be useful in patients undergoing haplo-HSCT, ensuring control of disease and viral reactivations, before and during engraftment in the recipient.

Patients and Methods

We evaluated the outcome of 5 pediatric patients with high-risk leukemia who were planned to receive haplo-HSCT with donor NK cells infusion following one of two different conditioning platforms, Post-Transplant Cyclophosphamide (PTCy) and T-αβ/B cell-depleted. Four patients were male. Median age was 1.2 years (0.7-18). Three patients out of five had infant leukemia, 2 with a refractory disease and 1 relapsed after HSCT, and the remaining 2 patients had relapsed-refractory disease. All patients received a median of 3 lines of chemotherapy and 1 received prior allo-HSCT. Donor was one of the parents for all patients, selected based on genetic, immunological and clinical criteria. Four patients received T αβ/B cell-depleted haplo-HSCT; 1 patient received PTCy haplo-HSCT. Conditioning regimen consisted of Busulfan/Treosulfan + Fludarabine and Thiotepa backbone for 4 patients; 1 patient received TBI. The median CD34⁺, CD3⁺ TCRαβ⁺, CD3⁺ TCRγδ⁺ dose was 14.06x10⁶/Kg (7.16-19.8), 0.3125x10⁶/Kg (0.089-0.239) and 16.19 x 10⁶/Kg (6.3-19.81), respectively. The TNC dose was 5.8x10⁸/Kg for patient undergone PTCy aplo-HSCT. Patients who underwent T αβ/B cells depletion haplo-HSCT received rituximab and letermovir for EBV and CMV prophylaxis, respectively. NK counts were monitored on day +7, +14, +21, +28 after transplantation and on day +7, +14, +21, +28 after NK infusion. All patients, except one because of the onset of hepatic acute GvHD, received donor NK infusion at median of 35 days after transplantation (31-62), triggered by the decrease of NK count. Median number of NK cells infused was 2 x 10⁷/kg (0.67-2.6).

Results

The median time to neutrophils and platelets engraftment was 14 (13-20) and 10 (8-14) days, respectively. None of the patients developed CMV or EBV reactivation while ADV reactivation was observed in three patients but only one required antiviral treatment. Three patients experienced Grade 1-2 acute GvHD and 1 patient experienced chronic GvHD. All patients are in complete molecular remission (OS and EFS 100%) with full donor chimerism at a median follow up of 25 months (13-32).

Conclusion

Donor NK cell infusion after haplo-HSCT could represent an effective option to improve transplant outcome and prevent virus reactivation and early relapse in pediatric patients with very high-risk or relapsed/refractory leukemia. Mild, steroid-responsive acute GvHD was observed and limited chronic GvHD. These limited but very promising preliminary results need to be confirmed in larger cohorts.

Wierda:National Comprehensive Care Center (NCCN): Other: Financial relationship (Chair, CLL); AstraZeneca: Research Funding; Gilead Sciences: Research Funding; Janssen: Research Funding; Eli Lilly: Research Funding; BMS: Research Funding; Juno Therapeutics: Research Funding; Pharmacyclics LLC, an AbbVie Company: Research Funding; Kite: Research Funding; Oncternal Therapeutics: Research Funding; Novartis: Research Funding; Genentech, Inc.: Research Funding; Nurix Therapeutics: Research Funding; GSK: Research Funding; Oncternal Therapeutics: Research Funding; Acerta Pharma: Research Funding; F. Hoffmann-La Roche Ltd.: Research Funding; Cyclacel Pharmaceuticals Inc: Research Funding; Loxo Oncology: Research Funding; Accutar Biotechnology: Research Funding; Numab Therapeutics: Research Funding; AbbVie: Research Funding. Tambaro:Jazz Pharma: Membership on an entity's Board of Directors or advisory committees, Other: travel expense; Amgen: Other: travel expense, Speakers Bureau; Novartis: Other: travel support; Neovii: Other: travel support ; Gilead: Membership on an entity's Board of Directors or advisory committees; Medac: Other: travel support , Speakers Bureau.