Impaired GvL Potential of Natural Killer Cells Early Reconstituting Following Haploidentical HSCT.

Alloreactive NK cells have been suggested to be important functional players in GvL activity after haploidentical HSCT for high risk leukemia. In this study we have characterized NK cells differentiating from purified haploidentical CD34⁺ cells after transplantation into 16 patients who did (n=8) or did not (n=8) suffer acute leukemia relapse in a long term follow-up (median 208 days). The incidence of relapse in these patients was not correlated with the presence (n=9) or absence (n=7) of predicted donor NK alloreactivity (p=0.94). NK cells in the first month after transplantation were, regardless of the occurence of relapse, NKG2A⁺ (>95%) and KIR⁻ (13%), thus resembling CD56^bright NK cells from healthy donors. However, in contrast to mature CD56^bright cells, the patients’ NK cells expressed heterogeneous intensities of CD56, were only partly positive for the lymph node homing markers CD62L and CCR7, and expressed a higher amount of Fcγ receptor III (CD16). Importantly, in contrast to mature CD56^bright cells, which constitrutively express the high affinity αβγ IL-2 receptor, thus releasing γ-IFN in response to low dose IL2, the patients’ NK cells lacked IL-R α (CD25) and did not release cytokines in response to low-dose IL2, nor, most importantly, when challenged with leukemic blasts. γ-IFN release induced by leukemic blasts could be restored by inhibition of NKG2A while cytotoxicity, which was consistently lower as compared to that of mature CD56⁺ cells, could not. Our data suggest that NK cells differentiating in patients from CD34⁺ progenitors after haploidentical HSCT have important phenotipical and functional differences from both subsets of mature NK cells, accounting for an impaired in vivo GvL potential.