NK Cell KIR Reconstitution after Allogeneic Hematopoietic Cell Transplant (HCT) Is Affected by T Cell Number and Function.

NK cell KIR interactions are among the variables known to affect clinical outcomes including relapse, graft versus host disease (GVHD) and survival after HCT. We hypothesized that T cells in graft sources available for HCT may affect KIR recovery and the therapeutic potential of KIR alloreactivity. We studied KIR reconstitution (the percentage of KIR⁺ NK cells measured by flow cytometry) in blood collected from recipients at day +100 after T cell deplete (TCD-BMT) and unmanipulated (U-BMT) unrelated BM transplants. We found that KIR reconstitution was suppressed compared to the healthy donors, significantly more so after U-BMT transplants (donor: 48.42 ± 2.35% KIR⁺ NK cells versus recipient: 26.74 ± 1.94, n = 36; P < .001) than after TCD-BMT transplants (donor: 53.34 ± 3.25% versus recipient: 42.68 ± 3.32%, n = 38; P = .017), with P = .001 between the recipient groups. Additionally, multivariate Cox proportional hazards models showed that improved KIR recovery independently correlated with improved survival and that higher NK cell IFN-γ production independently correlated with more frequent acute GVHD in that patient cohort. These data suggested that T cell number in the graft affects KIR reconstitution and transplant outcome. We next examined other sources of hematopoietic cells in which T cell function may be suppressed either by growth factor mobilization (sibling donor unmanipulated peripheral blood: SibU-PB) or the innate naivety of the T cells (umbilical cord blood: UCB). KIR⁺ NK reconstitution on recovering cells at day +100 after all HCT graft types was significantly less than that on normal donor cells (normals 55.33 ± 1.73%, n = 124; all P < .0006). U-BMT recipients had significantly lower KIR⁺ NK recovery (27.31 ± 2.06%, n = 36 vs. SibU-PB: 37.58 ± 3.29%, n = 29; TCD-BMT: 42.68 ± 3.32%, n = 38; or UCB, 37.99 ± 2.54%, n = 49) when compared to all other transplant types. The highest absolute T cell inoculum, found in SibU-PB, showed KIR reconstitution similar to that of TCD-BMT, which had the lowest T cell content (p=0.29), perhaps due to the lower alloreactivity of the Sib grafts and to the G-CSF-priming which preferentially mobilizes T cells with a suppressive phenotype. Similarly, KIR reconstitution was better after UCB compared to U-BMT (P = .0027), possibly due to the more permissive interactions with naive T cells. These results suggest that reduced T cell number after T cell depletion, suppressed T cells found after growth factor mobilization, or naive T cells present in UCB grafts enhance in vivo KIR reconstitution after allogeneic HCT when compared to unmanipulated marrow grafts. Such enhanced KIR reconstitution may have clinical consequences. Graft T cells may directly compete for cytokines and growth factors, or may be a surrogate marker for other transplant factors such as the development of GVHD and the requirement for intensive post-transplant immunosuppression. Understanding these interactions will allow judicious selection of hematopoietic cell source to select for enhanced KIR recovery. For example, among unrelated unmanipulated donor grafts, KIR⁺ NK recovery was significantly better using UCB than adult donors and further investigation may show that this is advantageous to improve clinical outcomes.