Impact of Graft-Versus-Graft Natural Killer Cell Alloreactivity on Single Unit Dominance After Double Umbilical Cord Blood Transplantation

Background Natural killer (NK) cell alloreactivity is favored after double umbilical cord blood transplantation (dUCBT) in which cord blood (UCB) units and patients are often HLA class I mismatched. Generally, only 1 UCB unit persists after dUCBT. We hypothesize, that NK cell alloreactivity mediated by killer cell immunoglobulin-like receptor (KIR)-HLA interactions may explain the dominance of 1UCB unit over the other after dUCBT. Methods We investigated the impact of KIR+ NK cell alloreactivities on the dominance of 1 full UCB unit in 50 dUCBT. We analyzed the effects of the KIR/HLA genetic incompatibilities and studied cord blood cells at both the phenotypic and functional levels. Results The genetic combination of KIR3DL1(+) loser UCB unit/Bw4(-) winner UCB unit determined both the dominance of 1 UCB unit (hazards ratio, 2.88 [1.32-6.27], P = 0.0077) and correlated with an increased incidence of relapse (hazards ratio, 4.91 [1.39-17.3], P = 0.0134). It is interesting to note that cord blood cells exhibited extremely low HLA class I expression. Moreover, resting cord blood KIR3DL1(+) NK cells exhibited a basal alloreactivity against Bw4(-) target cells that increased upon activation, thus triggering death by apoptosis. Conclusions Our unicentric study suggests, for the first time, the significant impact of KIR+ NK cell alloreactivity in the determination of which UCB unit will dominate in dUCBT.