NK Cell KIR Ligand Mismatches Influence Engraftment Following Combined Haploidentical and Umbilical Cord Blood (UCB) Transplantation In Patients With Severe Aplastic Anemia (SAA)

Allogeneic hematopoietic stem cell transplant (HSCT) is curative for patients (pts) with severe aplastic anemia (SAA). For SAA pts who lack HLA-identical donors, we explored a HSCT approach that co-infuses PBSCs enriched for CD34+ cells from a haplo-identical relative combined with a single umbilical cord blood unit (UCB). Although most pts undergoing this approach had early haplo-myeloid engraftment that was eventually supplanted by the UCB unit, engraftment patterns were highly variable; in some pts full cord myeloid chimerism was delayed greater than one year, while others had loss of cord engraftment with sustained full haplo-donor myeloid chimerism. Here, we investigated the impact of various patient, UCB, and haplo-donor characteristics, as well as NK cell KIR ligand mismatches between graft sources, on engraftment kinetics following haplo-cord HSCT.

Pts with SAA or SAA evolved to MDS unresponsive to immunosuppressive therapy with severe neutropenia (ANC<500), who lacked an HLA-matched donor and had an available haploidentical family member, and had at least one ≥ 4/6 matched UCB unit with a minimum TNC dose ≥1.5x10⁷ cells/kg were eligible for HSCT. Pts were conditioned with cyclophosphamide (120 mg/kg), fludarabine (125 mg/m²), equine-ATG (160 mg/kg) and 200 cGy of total body irradiation. On Day 0, pts received a CD34 selected (Miltenyi CliniMacs) G-CSF mobilized haplo-donor allograft combined with a single UCB. Tacrolimus and MMF were given for GVHD prophylaxis.

16 pts (median age: 18.9 yrs, range: 4.5-27.9) including 13 SAA and 3 with SAA evolved to MDS underwent haplo-cord transplant. Ten pts received a 4/6, and six pts received a 5/6 HLA-matched UCB unit. A median 3.6x10⁷ TNC/kg (range: 1.96-6.93) from the UCB unit, and 3.3x10⁶CD34+ cells/kg (range: 3.0-4.1) from the haplo-donor were transplanted. All 16 pts had sustained engraftment with 15/15 pts evaluable past day 100 having transfusion independence. At a median follow-up of 570 days (range: 55-1826), 14/16 pts survive for an overall survival rate of 81.8%. Two pts died at day 414 and 402 post-HSCT from viral related complications (CMV pneumonitis and limbic encephalopathy). Neutrophil and platelet recovery occurred at a median 10 (range: 9-22), and 21 (range: 10-213) days post-HSCT, respectively. By post-HSCT day 11, 15/16 pts had neutrophil recovery. Cord myeloid engraftment (cord ANC>500, calculated from chimerism data) occurred in 13/16 pts at a median 42 days. 3/16 did not achieve a cord ANC>500 but had sustained haplo-donor engraftment. The cumulative incidence of acute grade II-IV GvHD was 38.1%. Engraftment profiles were highly variable among pts; 12 achieved full cord chimerism in all cell lineages, 2 remained mixed haplo-cord chimeras, and 2 failed to have UCB engraftment but had sustained 100% haplo-donor myeloid chimerism. Higher degrees of HLA matching (out of 10 alleles) between recipient and UCB unit were associated with faster rates of full cord engraftment (p=0.006) and a higher probability of complete loss of haplo-donor chimerism (p=0.018). KIR ligand incompatibility in the haplo vs. cord direction (defined as the presence of a KIR ligand in the haplo-donor graft that is absent in the UCB unit at HLA epitopes Bw4, HLA-C Group 1 & 2, HLA-A3, and HLA-A11) negatively impacted cord myeloid engraftment. 5/5 (100%) pts who failed to achieve full cord myeloid chimerism by post-HSCT day 400 had haplo vs. cord KIR ligand incompatibility. Moreover, both pts who failed to have UCB engraftment and had sustained haplo-donor chimerism had haplo vs. cord KIR ligand incompatibility. In contrast, only 3/11 (27%) pts who achieved full cord myeloid chimerism post-HSCT by day 231 had haplo vs. cord KIR ligand incompatibility (p=0.026). KIR ligand incompatibility in the cord vs. haplo direction showed no significant effect on haplo-donor myeloid engraftment.

These results show that haplo-cord HSCT is an effective treatment option for pts with SAA who lack an HLA-matched donor. Further, these results suggest that NK cell alloreactivity, occurring as a consequence of KIR ligand mismatch between the two graft sources, may have a negative impact on cord engraftment when haplo vs. cord KIR ligand incompatibility is present. In summary, this study highlights a novel factor that should be considered during graft selection for haplo-cord transplantation.

No relevant conflicts of interest to declare.