Increased Inhibitory KIR-Ligand Interactions Confer Relapse Protection Following HLA Matched Unrelated Donor HCT for AML
Authors: Krieger Elizabeth , Qayyum Rehan , Toor Amir Ahmed 
Published: November 2020

Killer immunoglobulin-like receptor (KIR) and KIR-ligand (KIRL) interactions play an important role in natural killer cell-mediated graft versus leukemia effect (GVL) after hematopoietic stem cell transplant (HCT) for AML. There is considerable heterogeneity in the KIR gene and KIRL content of individuals, making it difficult to estimate the full clinical impact of NK cell alloreactivity following HCT. Herein, we validate a mathematical model accounting for KIR-KIRL interactions identifying donors with optimal NK cell-mediated alloreactivity and GVL.
This retrospective study was performed on de-identified donor and recipient demographic and clinical outcomes data provided by the Center for International Blood and Marrow Transplant Research (CIBMTR). Donor recipient pairs (DRP) who underwent unrelated donor (URD) HCT for early and intermediate AML were included. KIR-KIRL interaction values were assigned as follows; if an inhibitory KIR (iKIR) on the NK cell encounters a ligand on its target, this will give the NK cell an inhibitory signal and this is scored as a single interaction(Figure 1b), as is the case, if there is no ligand for an inhibitory KIR, i.e., missing KIRL (mKIRL) (Figure 1c). Finally, activating KIR (aKIR) interacting with its ligands is similarly scored(Figure 1a). The absolute values of the iKIR and mKIR scores were summed to calculate the composite inhibitory-missing ligand (IM)-KIR score (Figure 1d).
The study cohort was comprised of 2365 donor-recipient pairs (DRP) who underwent URD HCT for early or intermediate AML. Mean age was 53 years; 85% of DRPs were high-resolution 8/8 HLA-matched for HLA-A, -B, -C, and -DRB1. All patients received T cell replete grafts; 42% (n=996) received in vivo T cell depletion, 937 (94%) with anti-thymocyte globulin (ATG); 86% received a graft of mobilized peripheral blood stem cells (PBSC), 59% received myeloablative conditioning. This cohort was primarily of Caucasian descent (89%).
When adjusted for recipient age, donor age, CMV status, KPS, GVHD prophylaxis, cytogenetics, disease status, conditioning regimen, in vivo T cell depletion, graft source, and sex match, relapse risk was significantly reduced in donor-recipient pairs (DRP) with higher inhibitory KIR-KIRL interaction and missing KIRL (mKIR) scores, with HR=0.86 (P=0.01) & HR=0.84 (P=0.02) respectively. This effect was not observed with activating KIR-KIRL interactions. Chronic GVHD and TRM were not significantly affected by iKIR, mKIR or aKIR. Given the significant individual impact of iKIR and mKIR, the summed inhibitory-missing ligand (IM-KIR) score was next assessed, and when this score was 5 (as opposed to &lt;5), the IM-KIR score was also associated with lower relapse risk, HR 0.8 (P=0.004) (Figure 2a). Acute and chronic graft vs. host disease (GVHD), survival, GRFS, and relapse-free survival were not significantly different, likely due to increased TRM among these patients, HR 1.32 (P=0.01). Interaction analysis indicated that amongst the HLA matched DRP, ATG recipients with IM-KIR=5, had a lower relapse rate compared to those with an IM-KIR&lt;5, HR 0.61 (p=0.001) (Figure 2b), among the cohort who did not receive ATG there was no significant difference in relapse among IM-KIR=5 and IM-KIR&lt;5; thus, the use of ATG significantly modified the effect of IM KIR score in an interaction analysis (p=0.049), suggesting higher NK cell magnitude of KIR-KIRL interaction may compensate for the general increase of relapse in those who receive in vivo T cell depletion. Nevertheless, TRM was also increased in these patients, HR 1.49 (p=0.034), likely abrogating survival advantage from a lower relapse risk.
This large international study confirms that unrelated DRPs with greater magnitude of inhibitory KIR-KIRL interactions confer significant relapse protection after MUD HCT in standard-risk AML. This challenges the notion that KIR are irrelevant to donor selection. Future clinical trials evaluating donor selection for URD HCT should include this measure to evaluate its value prospectively in uniformly treated patient cohorts, with adequate GVHD and antiviral prophylaxis to mitigate TRM.

Disclosures
No relevant conflicts of interest to declare.