Impact of natural killer cell dose and donor killer-cell immunoglobulin-like receptor (KIR) genotype on outcome following human leucocyte antigen-identical haematopoietic stem cell transplantation

Clinical impact of KIR haplotypes in 10/10 HLA-matched unrelated donor-recipient pairs undergoing allogeneic hematopoietic stem cell transplantation

To evaluate the impact of killer immunoglobulin-like receptor (KIR) genotyping in allogeneic hematopoietic stem cell transplantation for myeloid disorders at our institution, retrospective KIR genotyping was performed on 77 patients and their 10/10 matched unrelated donors. In a multivariate model including donor age, HLA-DPB1 permissiveness, and presence of donor KIR B/x, an association with overall survival was observed (p = .047). Within the model, increasing donor age increased risk (RR 1.03 [1.00-1.06]/year, p = .046), while donor KIR and HLA-DPB1 permissiveness were not associated with risk (RR 0.51 [0.26-1.03] and RR 0.68 [0.34-1.36]). Grouping recipients by conditioning regimen or limiting the analysis to recipients of peripheral blood stem cells, no association between donor KIR and survival or relapse was identified. No significant associations were observed between overall survival, relapse, grade III-IV acute, or chronic graft versus host disease and presence of KIR B (B/x), quantity of donor KIR B haplotype motifs, or centromeric KIR type (all p > .05).

The Role of NK Cells and Their Exosomes in Graft Versus Host Disease and Graft Versus Leukemia

Natural killer (NK) cells are one of the innate immune cells that play an important role in preventing and controlling tumors and viral diseases, but their role in hematopoietic stem cell transplantation (HCT) is not yet fully understood. However, according to some research, these cells can prevent infections and tumor relapse without causing graft versus host disease (GVHD). In addition to NK cells, several studies are about the anti-leukemia effects of NK cell-derived exosomes that can highlight their roles in graft-versus-leukemia (GVL). In this paper, we intend to investigate the results of various articles on the role of NK cells in allogeneic hematopoietic cell transplantation and also their exosomes in GVL. Also, we have discussed the antiviral effects of these cells in post-HCT cytomegalovirus infection.

Impact of natural killer cells on outcomes after allogeneic hematopoietic stem cell transplantation: A systematic review and meta-analysis

Background

Natural killer (NK) cells play a vital role in early immune reconstitution following allogeneic hematopoietic stem cell transplantation (HSCT).

Methods

A literature search was performed on PubMed, Cochrane, and Clinical trials.gov through April 20, 2022. We included 21 studies reporting data on the impact of NK cells on outcomes after HSCT. Data was extracted following the PRISMA guidelines. Pooled analysis was done using the meta-package (Schwarzer et al.). Proportions with 95% confidence intervals (CI) were computed.

Results

We included 1785 patients from 21 studies investigating the impact of NK cell reconstitution post-HSCT (8 studies/1455 patients), stem cell graft NK cell content (4 studies/185 patients), therapeutic NK cell infusions post-HSCT (5 studies/74 patients), and pre-emptive/prophylactic NK cell infusions post-HSCT (4 studies/77 patients). Higher NK cell reconstitution was associated with a better 2-year overall survival (OS) (high: 77%, 95%CI 0.73-0.82 vs low: 55%, 95%CI 0.37-0.72; n=899), however, pooled analysis for relapse rate (RR) or graft versus host disease (GVHD) could not be performed due to insufficient data. Higher graft NK cell content demonstrated a trend towards a better pooled OS (high: 65.2%, 95%CI 0.47-0.81 vs low: 46.5%, 95%CI 0.24-0.70; n=157), lower RR (high: 16.9%, 95%CI 0.10-0.25 vs low: 33%, 95%CI 0.04-0.72; n=157), and lower acute GVHD incidence (high: 27.6%, 95%CI 0.20-0.36 vs low: 49.7%, 95%CI 0.26-0.74; n=157). Therapeutic NK or cytokine-induced killer (CIK) cell infusions for hematologic relapse post-HSCT reported an overall response rate (ORR) and complete response (CR) of 48.9% and 11% with CIK cell infusions and 82.8% and 44.8% with NK cell infusions, respectively. RR, acute GVHD, and chronic GVHD were observed in 55.6% and 51.7%, 34.5% and 20%, and 20.7% and 11.1% of patients with CIK and NK cell infusions, respectively. Pre-emptive donor-derived NK cell infusions to prevent relapse post-HSCT had promising outcomes with 1-year OS of 69%, CR rate of 42%, ORR of 77%, RR of 28%, and acute and chronic GVHD rates of 24.9% and 3.7%, respectively.

Conclusion

NK cells have a favorable impact on outcomes after HSCT. The optimal use of NK cell infusions post-HSCT may be in a pre-emptive fashion to prevent disease relapse.

Recent Advances of Human Leukocyte Antigen (HLA) Typing Technology Based on High-Throughput Sequencing

The major histocompatibility complex (MHC) in humans is a genetic region consisting of cell surface proteins located on the short arm of chromosome 6. This is also known as the human leukocyte antigen (HLA) region. The HLA region consists of genes that exhibit complex genetic polymorphisms, and are extensively involved in immune responses. Each individual has a unique set of HLAs. Donor-recipient HLA allele matching is an important factor for organ transplantation. Therefore, an established rapid and accurate HLA typing technology is instrumental to preventing graft-verses-host disease (GVHD) in organ recipients. As of recent, high-throughput sequencing has allowed for an increase read length and higher accuracy and throughput, thus achieving complete and high-resolution full-length typing. With more advanced nanotechnology used in high-throughput sequencing, HLA typing is more widely used in third-generation single-molecule sequencing. This review article summarizes some of the most widely used sequencing typing platforms and evaluates the latest developments in HLA typing kits and their clinical applications.

Increased donor inhibitory KIR with known HLA interactions provide protection from relapse following HLA matched unrelated donor HCT for AML

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 cell transplant (HCT) for AML. Accounting for known KIR-KIRL interactions may identify donors with optimal NK cell-mediated alloreactivity and GVL. A retrospective study of 2359 donor-recipient pairs (DRP) who underwent unrelated donor (URD) HCT for AML was performed. KIR-KIRL combinations were determined and associations with clinical outcomes examined. Relapse risk was reduced in DRP with both higher inhibitory KIR-KIRL (iKIR) and missing KIRL (mKIR) scores, with HR 0.86 (P = 0.01) & HR 0.84 (P = 0.02) respectively. The iKIR and mKIR score components were summed to give a maximal inhibitory KIR ligand (IM-KIR) score for each donor, which if it was 5, as opposed to <5, was also associated with a lower relapse risk, SHR 0.8 (P = 0.004). All IM = 5 donors possess KIR Haplotype B/x. Transplant-related mortality was increased among those with IM-KIR = 5, HR, 1.32 (P = 0.01). In a subset analysis of those transplanted with 8/8 HLA-matched DRP, anti-thymocyte globulin recipients with IM-KIR = 5, had a lower relapse rate HR, 0.61 (p = 0.001). This study demonstrates that HLA-matched unrelated donors with the highest inhibitory KIR content confer relapse protection, albeit with increased TRM. These donors all have KIR haplotype B. Clinical trials utilizing donors with a higher iKIR content in conjunction with novel strategies to reduce TRM should be considered for URD HCT in recipients with AML to optimize clinical outcomes.

Peripheral blood stem cell grafts in allogeneic hematopoietic cell transplantation: It is not all about the CD34+ cell dose

Allogeneic Hematopoietic Cell Transplantation is a curative approach in various malignant and non-malignant disorders. The majority of adult transplants in the current era are performed using mobilized stem cells, harvested from the peripheral blood by leukapheresis. Peripheral blood stem cell (PBSC) collections are designed to target a dose of stem cells that will result in safe engraftment and hematopoietic recovery; however, 99 % of the cells contained in a PBSC graft are not stem cells and a growing number of studies attempt to characterize the associations between graft composition and transplant outcomes. A better understanding of the impact of the quantity and quality of various cell types in PBSC grafts may lead to development of novel collection strategies or improved donor selection algorithms. Here we review relevant findings from recent studies in this area.

Influence of KIR and NK Cell Reconstitution in the Outcomes of Hematopoietic Stem Cell Transplantation

Natural killer (NK) cells play a significant role in immune tolerance and immune surveillance. Killer immunoglobin-like receptors (KIRs), which recognize human leukocyte antigen (HLA) class I molecules, are particularly important for NK cell functions. Previous studies have suggested that, in the setting of hematopoietic stem cell transplantation (HSCT), alloreactive NK cells from the donor could efficiently eliminate recipient tumor cells and the residual immune cells. Subsequently, several clinical models were established to determine the optimal donors who would exhibit a graft-vs. -leukemia (GVL) effect without developing graft-vs. -host disease (GVHD). In addition, hypotheses about specific beneficial receptor-ligand pairs and KIR genes have been raised and the favorable effects of alloreactive NK cells are being investigated. Moreover, with a deeper understanding of the process of NK cell reconstitution post-HSCT, new factors involved in this process and the defects of previous models have been observed. In this review, we summarize the most relevant literatures about the impact of NK cell alloreactivity on transplant outcomes and the factors affecting NK cell reconstitution.

Improved Relapse-Free Survival in Patients With High Natural Killer Cell Doses in Grafts and During Early Immune Reconstitution After Allogeneic Stem Cell Transplantation

Mature immunocompetent cells from the stem cell graft as well as early robust immune reconstitution are essential for the graft-vs. -tumor (GVT) effect to eliminate residual malignant cells after allogeneic hematopoietic stem cell transplantation (HSCT). In this prospective study we characterized graft composition of T- and NK cell subsets in 88 recipients of peripheral blood stem cell grafts with multicolor flowcytometry. Our primary aim was to analyze the impact of graft composition on immune reconstitution and clinical outcomes after transplantation. Patients transplanted with graft NK cell doses above the median value of 27 × 10⁶/kg had significantly increased relapse-free-survival compared to patients transplanted with lower doses, HR 2.12 (95% CI 1.01-4.45, p = 0.04) Peripheral blood concentrations of NK cells obtained from donors before G-CSF mobilization were significantly correlated to graft NK cell doses (Spearman's ρ 0.53, p = 0.03). The dose of transplanted NK cells/kg correlated significantly with NK cell concentrations in patients early after transplantation (Spearman's ρ 0.26, p = 0.02, and ρ = 0.35, p = 0.001 for days 28 and 56, respectively). Early immune reconstitution above median values of NK cells was significantly associated with improved relapse-free survival (HR 2.84 [95% CI 1.29-6.28], p = 0.01, and HR 4.19 [95% CI 1.68-10.4], p = 0.002, for day 28 and 56, respectively). Early concentrations above the median value of the mature effector CD56dim NK cell subset were significantly associated with decreased relapse incidences at 1 year, 7% (95% CI 1.8-17) vs. 28% (95% CI 15-42), p = 0.04, and 7% (95% CI 1.8-18) vs. 26% (95% CI 14-40) %, p = 0.03, for days 28 and 56, respectively. The results suggest a protective effect of high doses of NK cells in grafts and during early immune reconstitution and support the perception of NK cells as innate effector cells with anti-tumor effects in the setting of allogeneic stem cell transplantation.

Killer Immunoglobulin-Like Receptor-Ligand Interactions Predict Clinical Outcomes following Unrelated Donor Transplantations

Killer immunoglobulin-like receptor (KIR) and KIR ligand (KIRL) interactions play an important role in natural killer (NK) cell-mediated graft-versus-leukemia effect following hematopoietic cell transplantation (HCT). However, there is considerable heterogeneity in the KIR gene and KIRL content in individuals, making it difficult to estimate the full clinical impact of NK cell reconstitution following HCT. Here we present a novel adaptive mathematical model designed to quantify these interactions to better assess the influence of NK cell-mediated alloreactivity on transplant outcomes. Ninety-eight HLA- matched unrelated donor (URD) HCT recipients were studied retrospectively. The KIR-KIRL interactions were quantified using a system of matrix equations. Unit values were ascribed to each KIR-KIRL interaction, and the directionality of interactions was denoted by either a positive (activating) or negative (inhibition) symbol; these interactions were then summed. The absolute values of both the missing KIRL and inhibitory KIR-KIRL interactions were significantly associated with overall survival and relapse. These score components were initially used to develop a weighted score (w-KIR score) and subsequently a simplified, nonweighted KIR-KIRL interaction score (IM-KIR score). Increased w-KIR score and IM-KIR score were predictive of all-cause mortality and relapse (w-KIR score: hazard ratio [HR], .37 [P = .001] and .44 [P = .044], respectively; IM-KIR score: HR, .5 [P = .049] and .44 [P = .002], respectively). IM-KIR score was also associated with NK cell reconstitution post-HCT. KIR-KIRL interactions as reflected by the w-KIR and IM-KIR scores influence both relapse risk and survival in recipients of HLA-matched URD HCT with hematologic malignancies.

NK Cells in the Treatment of Hematological Malignancies

Natural killer (NK) cells have the innate ability to kill cancer cells, however, tumor cells may acquire the capability of evading the immune response, thereby leading to malignancies. Restoring or potentiation of this natural antitumor activity of NK cells has become a relevant therapeutic approach in cancer and, particularly, in hematological cancers. The use of tumor-specific antibodies that promote antibody-dependent cell-mediated cytotoxicity (ADCC) through the ligation of CD16 receptor on NK cells has become standard for many hematologic malignancies. Hematopoietic stem cell transplantation is another key therapeutic strategy that harnesses the alloreactivity of NK cells against cancer cells. This strategy may be refined by adoptive transfer of NK cells that may be previously expanded, activated, or redirected (chimeric antigen receptor (CAR)-NK cells) against cancer cells. The antitumor activity of NK cells can also be boosted by cytokines or immunostimulatory drugs such as lenalidomide or pomalidomide. Finally, targeting immunosubversive mechanisms developed by hematological cancers and, in particular, using antibodies that block NK cell inhibitory receptors and checkpoint proteins are novel promising therapeutic approaches in these malignant diseases.

Natural Killer Cells as Allogeneic Effectors in Adoptive Cancer Immunotherapy

Natural killer (NK) cells are attractive within adoptive transfer settings in cancer immunotherapy due to their potential for allogeneic use; their alloreactivity is enhanced under conditions of killer immunoglobulin-like receptor (KIR) mismatch with human leukocyte antigen (HLA) ligands on cancer cells. In addition to this, NK cells are platforms for genetic modification, and proliferate in vivo for a shorter time relative to T cells, limiting off-target activation. Current clinical studies have demonstrated the safety and efficacy of allogeneic NK cell adoptive transfer therapies as a means for treatment of hematologic malignancies and, to a lesser extent, solid tumors. However, challenges associated with sourcing allogeneic NK cells have given rise to controversy over the contribution of NK cells to graft-versus-host disease (GvHD). Specifically, blood-derived NK cell infusions contain contaminating T cells, whose activation with NK-stimulating cytokines has been known to lead to heightened release of proinflammatory cytokines and trigger the onset of GvHD in vivo. NK cells sourced from cell lines and stem cells lack contaminating T cells, but can also lack many phenotypic characteristics of mature NK cells. Here, we discuss the available published evidence for the varying roles of NK cells in GvHD and, more broadly, their use in allogeneic adoptive transfer settings to treat various cancers.

Homozygous HLA-C1 is Associated with Reduced Risk of Relapse after HLA-Matched Transplantation in Patients with Myeloid Leukemia

Natural killer (NK) cells assume graft-versus-leukemia alloreactivity after hematopoietic stem cell transplantation (HSCT) through their inhibitory killer cell immunoglobulin-like receptors (KIRs). KIR2D family members recognize HLA-C alleles with Asn80 (HLA-C1) or Lys80 (HLA-C2). The predominance of HLA-C1 over HLA-C2 and the frequent presence of KIR2DL1 are characteristic of Japanese people. We compared clinical outcomes among homozygous HLA-C1 (HLA-C1/C1) patients and heterozygous HLA-C1/C2 patients who underwent HLA-matched HSCT for hematologic malignancies by assessing the data of 10,638 patients from the Japanese national registry. HLA-C1/C1 recipients had a lower rate of relapse than HLA-C1/C2 recipients after transplantation for acute myelogenous leukemia (AML) (hazard ratio [HR], .79; P = .006) and chronic myelogenous leukemia (CML) (HR, .48; P = .025), but not for acute lymphoblastic leukemia (HR, 1.36), lymphoma (HR, .97), or low-grade myelodysplastic syndrome (HR, 1.40). We then grouped AML and CML patients together and divided them into several subgroups. Advantages of HLA-C1/C1 recipients over HLA-C1/C2 recipients regarding relapse were observed irrespective of donor relation (related: HR, .79, P = .069; unrelated: HR, .77, P = .022), preparative regimen (myeloablative: HR, .79, P = .014; reduced intensity: HR, .73, P = .084), and occurrence of acute graft-versus-host disease (yes: HR, .70, P = .122; no, HR .71, P = .026) or cytomegalovirus reactivation (reactivated: HR .67,P = .054; nonreactivated: HR .71, P = .033); however, these advantages were not observed in recipients with a delay in achieving complete chimerism (HR, 1.06). The advantage of decreasing relapse and extending relapse-free survival of C1/1 over C1/2 KIR-ligand status was most pronounced in T cell-depleted HSCT (HR, .27; P < .001 and HR, .30; P = .002, respectively) and in children age <15 years (HR, .29; P < .001 and HR .31; P < .001, respectively). Our findings represent an important mechanism responsible for the immunity against HLA-C2-negative myeloid leukemia cells after HLA-matched transplantation.

NK cell therapy after hematopoietic stem cell transplantation: can we improve anti-tumor effect?

After decades since the discovery of natural killer (NK) cells as potential effector cells fighting malignantly transformed and virally infected cells, little progress has been made in their clinical application. This yet unrealized therapeutic effect is presumably, at least in part, due to low numbers of functional NK cells that could be obtained from the peripheral blood relative to tumor burden. Our group hypothesized that a relatively small NK cell number to targeted malignant cells is the cause of a lack of clinical effect. We pursued obtaining large numbers of NK cells via ex vivo expansion using feeder cells that express membrane-bound IL-21. Early clinical studies demonstrate safety of administration of ex vivo expanded NK cells after transplantation using this method and suggest a therapeutic benefit in terms on decreasing relapse rate and possible control of viral infections post-transplant can be achieved. Successful application of NK cells after hematopoietic stem cell transplantation opens the possibility to effectively enhance the anti-tumor effect and decrease relapse rate post-transplant. Moreover, high doses of NK cells could prove more efficacious in enhancing anti-tumor effects, not only in hematological malignancies, with our without transplantation, but also in solid tumor oncology.

The effect of killer cell immunoglobulin-like receptor genotype on outcome of hematopoietic stem cell transplantation from matched sibling

The alloreactivity of natural killer (NK) cell after allogeneic hematopoietic stem cell transplantation (AHSCT) is regulated by the interaction between donor killer immunoglobulin-like receptors (KIRs) and recipient human leukocyte antigen (HLA)-class I molecules. The aim was to identify KIR genes, haplotypes and their HLA-class I ligands and to investigate their association with transplantation outcome. The study included 65 patient/donor pairs who received AHSCT from HLA-matched identical siblings. KIR genotyping was done for donors using reverse sequence specific oligonucleotide probes (rSSO) coupled with luminex technology, while HLA-C genotyping was performed in patients using rSSO strip assay. In multivariate analysis, KIR2DS4 was associated with significant reduced incidence of relapse (p = .002). A trend towards reduced incidence of relapse was also observed with more than two KIR B motifs (p = .09), whereas a significant increased relapse was associated with homozygous HLA-C2 ligand compared to combined C1/C2 and C1/C1 (p = .04). Activating KIR2DS3 was associated with rapid leukocyte engraftment (p = .02). While, KIR 2DL5 was associated with decreased CMV infection (p = .03) and better platelets engraftment (p = .05). KIR genes, haplotypes and HLA-C alleles have an impact on HSCT outcome. Better selection of donors with favorable KIR genotype can improve HLA-matched sibling HSCT outcome especially for AML patients.

HLA-C KIR-Ligands Determine the Impact of Anti-Thymocyte Globulin (ATG) on Graft versus Host and Graft versus Leukemia Effects Following Hematopoietic Stem Cell Transplantation

Rabbit anti-thymocyte globulins (ATGs) are widely used for the prevention of acute and chronic graft versus host disease (aGVHD, cGVHD) following allogeneic hematopoietic stem cell transplantation (HSCT). However, most prospective and retrospective studies did not reveal an overall survival (OS) benefit associated with ATG. Homozygosity for human leukocyte antigen (HLA)-C group 1 killer-cell immunoglobulin-like receptor ligands (KIR-L), i.e. C1/1 KIR-L status, was recently shown to be a risk factor for severe aGVHD. Congruously, we have previously reported favorable outcomes in C1/1 recipients after ATG-based transplants in a monocentric analysis. Here, within an extended cohort, we test the hypothesis that incorporation of ATG for GVHD prophylaxis may improve survival particularly in HSCT recipients with at least one C1 KIR-ligand. Retrospectively, 775 consecutive allogeneic (excluding haploidentical) HSCTs were analyzed, including peripheral blood and bone marrow grafts for adults with hematological diseases at two Austrian HSCT centers. ATG-Fresenius/Grafalon, Thymoglobuline, and alemtuzumab were applied in 256, 87, and 7 transplants, respectively (subsequently summarized as "ATG"), while 425 HSCT were performed without ATG. Median follow-up of surviving patients is 48 months. Adjusted for age, disease-risk, HLA-match, donor and graft type, sex match, cytomegalovirus serostatus, conditioning intensity, and type of post-grafting GVHD prophylaxis, Cox regression analysis of the entire cohort (n = 775) revealed a significant association of ATG with decreased non-relapse mortality (NRM) (risk ratio (RR), 0.57; p = 0.001), and overall mortality (RR, 0.71; p = 0.014). Upon stratification for HLA-C KIR-L, the greatest benefit for ATG emerged in C1/1 recipients (n = 291), by reduction of non-relapse (RR, 0.34; p = 0.0002) and overall mortality (RR, 0.50; p = 0.003). Less pronounced, ATG decreased NRM (RR, 0.60; p = 0.036) in HLA-C group 1/2 recipients (n = 364), without significantly influencing overall mortality (RR, 0.70; p = 0.065). After exclusion of higher-dose ATG-based transplants, serotherapy significantly improved both NRM (RR, 0.54; p = 0.019; n = 322) and overall mortality (RR, 0.60; p = 0.018) in C1/2 recipients as well. In both, C1/1 (RR, 1.70; p = 0.10) and particularly in C1/2 recipients (RR, 0.94; p = 0.81), there was no statistically significant impact of ATG on relapse incidence. By contrast, in C2/2 recipients (n = 121), ATG neither reduced NRM (RR, 1.10; p = 0.82) nor overall mortality (RR, 1.50; p = 0.17), but increased the risk for relapse (RR, 4.38; p = 0.02). These retrospective findings suggest ATG may provide a survival benefit in recipients with at least one C1 group KIR-L, by reducing NRM without significantly increasing the relapse risk.

Early Natural Killer Cell Reconstitution Predicts Overall Survival in T Cell-Replete Allogeneic Hematopoietic Stem Cell Transplantation

Early immune reconstitution plays a critical role in clinical outcome after allogeneic hematopoietic stem cell transplantation (HSCT). Natural killer (NK) cells are the first lymphocytes to recover after transplantation and are considered powerful effector cells in HSCT. We aimed to evaluate the clinical impact of early NK cell recovery in T cell-replete transplant recipients. Immune reconstitution was studied in 298 adult patients undergoing HSCT for acute myeloid leukemia, acute lymphoblastic leukemia, and myelodysplastic syndrome from 2005 to 2013. In multivariate analysis NK cell numbers on day 30 (NK30) > 150 cells/µL were independently associated with superior overall survival (hazard ratio, .79; 95% confidence interval, .66 to .95; P = .01). Cumulative incidence analyses showed that patients with NK30 > 150 cells/µL had significantly less transplant-related mortality (TRM), P = .01. Patients with NK30 > 150 cells/µL experienced significantly lower numbers of life-threatening bacterial infections as well as viral infections, including cytomegalovirus. No association was observed in relation to relapse. These results suggest an independent protective effect of high early NK cell reconstitution on TRM that translates into improved overall survival after T cell-replete HSCT.

Natural killer cells in malignant hematology: A primer for the non-immunologist

Natural killer cells were first described over 40years ago, but the last 15years has shown tremendous progress in our understanding of their biology and our ability to manipulate them for clinical therapeutic effect. Despite the increased understanding by clinicians and scientists investigating these cells, their biology remains a confusing subject for many because of the wide array of receptors, complex interactions, multiple models of predicting function, and contradictory data in the literature. While they are microscopically indistinguishable from T cells and share many of the same effector functions, their mechanisms of target recognition are completely distinct from yet complimentary to T cells. In this review we provide a basic understanding of NK cell biology and HLA recognition as compared and contrasted to T cells using a metaphor of border patrol and passports. We conclude with a summary of the evidence for NK cell effects in hematologic malignancies and describe new advances in NK cell immunotherapy aimed at improving these effects.

Synergistic effect of KIR ligands missing and cytomegalovirus reactivation in improving outcomes of haematopoietic stem cell transplantation from HLA-matched sibling donor for treatment of myeloid malignancies

The goal of this study was to evaluate the influence of KIR-HLA genotypes on the outcome of patients undergoing treatment for haematological malignancies by non-T-depleted lymphocyte haematopoietic stem cell transplantation (HSCT) from HLA-matched sibling donors. The prospective study was conducted at the Center of Hematology, University of Campinas, and 50 patients and their donors were followed up from 2008 to 2014. KIR and HLA class I genes were genotyped and patients grouped based on the presence of KIR ligands combined with KIR genotype of their respective donors. Patients with all KIR ligands present (n=13) had a significantly higher (p=0.04) incidence of acute graft-versus-host-disease (GVHD) than patients with one or more KIR ligands missing (n=37). The overall survival following transplantation of patients with myeloid malignancies (n=27) was significantly higher (p=0.035) in the group with one or more KIR ligands missing (n=18) than in the group with all ligands present (n=9). Presence of KIR2DS2 was associated with a worsening of HSCT outcome while reactivation of cytomegalovirus (CMV) infection improved the outcome of patients with one or more KIR ligands missing. Our results indicate that KIR-HLA interactions affect the outcome of the HLA-matched transplantation, particularly in patients with myeloid malignancies.

Donor-Recipient Matching for KIR Genotypes Reduces Chronic GVHD and Missing Inhibitory KIR Ligands Protect against Relapse after Myeloablative, HLA Matched Hematopoietic Cell Transplantation

BACKGROUND

Allogeneic hematopoietic cell transplantation (HCT) can be curative for many hematologic diseases. However, complications such as graft-versus-host disease (GVHD) and relapse of primary malignancy remain significant and are the leading causes of morbidity and mortality. Effects of killer Ig-like receptors (KIR)-influenced NK cells on HCT outcomes have been extensively pursued over the last decade. However, the relevance of the reported algorithms on HLA matched myeloablative HCT with rabbit antithymocyte globulin (ATG) is used for GVHD prophylaxis remains elusive. Here we examined the role of KIR and KIR-ligands of donor-recipient pairs in modifying the outcomes of ATG conditioned HLA matched sibling and unrelated donor HCT.

METHODS AND FINDINGS

The study cohort consisted of 281 HLA matched sibling and unrelated donor-recipient pairs of first allogeneic marrow or blood stem cell transplantation allocated into 'discovery' (135 pairs) and 'validation' (146 pairs) cohorts. High resolution HLA typing was obtained from the medical charts and KIR gene repertoires were obtained by a Luminex® based SSO method. All surviving patients were followed-up for a minimum of two years. KIR and HLA class I distributions of HCT pairs were stratified as per applicable definitions and were tested for their association with cause specific outcomes [acute GVHD grade II-IV (aGVHD), chronic GVHD needing systemic therapy (cGVHD) and relapse] using a multivariate competing risks regression model as well as with survival outcomes [relapse-free survival (RFS), cGVHD & relapse free survival (cGRFS) and overall survival (OS)] by multivariate Cox proportional hazards regression model. A significant association between KIR genotype mismatching (KIR-B/x donor into KIR-AA recipient or vice versa) and cGVHD was found in both discovery (p = 0.001; SHR = 2.78; 95%CI: 1.50-5.17) and validation cohorts (p = 0.005; SHR = 2.61; 95%CI: 1.33-5.11). High incidence of cGVHD associated with KIR genotype mismatching was applicable to both sibling and unrelated donors and was specific to recipients who had one or two C1 bearing HLA-C epitopes (HLA-C1/x, p = 0.001; SHR = 2.40; 95%CI: 1.42-4.06). When compared with KIR genotype mismatched transplants, HLA-C1/x patients receiving grafts from KIR genotype matched donors had a significantly improved cGRFS (p = 0.013; HR = 1.62; 95%CI: 1.11-2.39). Although there was no effect of KIR genotype matching on survival outcomes, a significantly reduced incidence of relapse (p = 0.001; SHR = 0.22; 95%CI: 0.10-0.54) and improved relapse-free survival (p = 0.038; HR = 0.40; 95%CI: 0.17-0.95) was observed with one or more missing ligands for donor inhibitory KIR among the recipients of unrelated donor transplants.

CONCLUSIONS

The present study for the first time presents the beneficial effects of KIR genotype matching in reducing cGVHD in myeloablative transplant setting using HLA matched (sibling and unrelated) donors. The findings offer a clinically applicable donor selection strategy that can help control cGVHD without affecting the risk of relapse and/or identify patients at a high risk of developing cGVHD as potential candidates for preemptive therapy. The findings also affirm the beneficial effect of one or more missing inhibitory KIR ligands in the recipient in reducing relapse and improving a relapse free survival in unrelated donor transplants.

The effect of missing KIR ligands, activating KIR genotype and haplotype on the outcome of T-cell-replete hematopoietic stem cell transplantation from HLA-identical siblings in Thai patients

This study was a retrospective analysis of Thai patients undergoing T-replete hematopoietic stem cell transplant from human leukocyte antigen (HLA)-identical sibling donors. We investigated 66 patients, including 40 patients with acute myeloid leukemia (AML), 12 patients with acute lymphoblastic leukemia and 14 patients with chronic myeloid leukemia. Killer cell immunoglobulin-like receptor (KIR) genes and HLA ligands were typed by polymerase chain reaction-sequence specific oligonucleotide probes. We analyzed the effect of the number of missing KIR ligands (Bw4, C1 and C2) on clinical outcomes. A beneficial effect of missing KIR ligand was not observed in both univariate and multivariate analysis. When we analyzed the effect of specific missing KIR ligand on clinical outcomes, there was a trend that patients with missing A11 ligand had lower relapse rate (P = 0.076). Therefore, we also conducted the analysis by including the group with missing KIR ligands of Bw4, C1, C2 and A11. Patients with two or more than two missing KIR ligands had a trend for better clinical outcome including reduced relapse (P = 055) and statistically significant in terms of reduced acute graft-vs-host disease (aGVHD) rate (P = 0.013). In multivariate analysis, patients with two or more than two missing KIR ligands had a statistically significant better clinical outcome in terms of reduced aGVHD rate (HR = 0.155, 95%CI = 0.040-0.605, P = 0.007). The association between clinical outcome with KIR haplotypes, centromeric B haplotype and activating KIR was not observed here. Although the sample size in this study is rather limited, these data can later be subjected to meta-analysis to help reach the conclusion of the usefulness of this additional promising KIR genotyping in various hematopoietic stem cell transplantation types.

Cryopreservation has no effect on function of natural killer cells differentiated in vitro from umbilical cord blood CD34(+) cells

BACKGROUND AIMS

Natural killer (NK) cells offer the potential for a powerful cellular immunotherapy because they can target malignant cells without being direct effectors of graft-versus-host disease. We have previously shown that high numbers of functional NK cells can be differentiated in vitro from umbilical cord blood (CB) CD34(+) cells. To develop a readily available, off-the-shelf cellular product, it is essential that NK cells differentiated in vitro can be frozen and thawed while maintaining the same phenotype and functions.

METHODS

We evaluated the phenotype and function of fresh and frozen NK cells differentiated in vitro. We also assessed whether the concentration of NK cells at the time of freezing had an impact on cell viability.

RESULTS

We found that cell concentration of NK cells at the time of freezing did not have an impact on their viability and on cell recovery post-thaw. Moreover, freezing of differentiated NK cells in vitro did not affect their phenotype, cytotoxicity and degranulation capacity toward K562 cells, cytokine production and proliferation.

CONCLUSIONS

We are therefore able to generate large numbers of functional NK cells from CB CD34(+) cells that maintain the same phenotype and function post-cryopreservation, which will allow for multiple infusions of a highly cytotoxic NK cell product.

[Genes of killer cell immunoglobulin-like receptors and their HLA ligands after allogeneic hematopoietic stem cell transplantation in myeloid leukemia patients]

AIM

To study the impact of the genes of donor killer cell immunoglobulin-like receptors (KIR) and HLA-KIR ligands on overall (OS) and event-free survival (EFS) rates in patients with myeloid leukemia after transplantation with allogeneic hematopoietic stem cells (allo-HSCT) from HLA-identical related and HLA-compatible unrelated donors.

SUBJECTS AND METHODS

The investigation enrolled 29 patients who had undergone allo-HSCT from KIR-genotyped donors at the Department of Bone Marrow Transplantation, Hematology Research Center (see symbol) in 2010-2013. OS and EFS rates after allo-HSCT were calculated using the Kaplan-Meier method.

RESULTS

The main predictor of recurrence and survival in patients after allo-HSCT was a recurrence-risk group the patient belonged to before transplantation. The standard-risk group patients whose donors had telomeric gene-content motifs of KIR-B haplotypes had higher EFS rates than those whose donors lacked these genes. The standard-risk patients homozygous for HLA-1 alleles (i.e. without HLA-C2 ligand) tended to have higher EFS rates, so did the patients without HLA-Bw4 ligand.

CONCLUSION

The donors having telomeric gene-content motifs of KIR-B haplotypes are more preferred for allo-HSCT for patients with myeloid leukemia as the presence of donor telomeric KIR-B genes increases EFS rates in standard-risk patients.

Enhanced cytotoxic function of natural killer and CD3+CD56+ cells in cord blood after culture

Rate of immune reconstitution directly correlates with the number of hematopoietic stem cells infused and is particularly delayed in patients undergoing cord blood (CB) transplantation (CBT). Methods to increase the number of CB natural killer (NK) cells have the potential to improve immune reconstitution after CBT. NK cells are the first lymphocyte population to recover after hematopoietic stem cells transplantation and are central to preventing early relapse and infection. CB NK cells are low in number and are known to be incomplete in maturation and require activation for effective function. Here, we report a clinically relevant ex vivo expansion method that increases the number of activated CB NK cells. We report a multilog increase in NK cell number when CB mononuclear cells are cocultured with IL-2 and IL-15. Furthermore, NK cells expressing activating receptors and adhesion molecules responsible for cytotoxicity increased throughout culture, whereas inhibitory receptor expression remained low. Additionally, cytotoxic function against various malignancies was significantly enhanced in cultured NK cells but not CD3(+)CD56(+) cells. These data suggest that ex vivo expansion and activation of CB NK cells is a clinically feasible and relevant approach to prevent early infection and relapse after CBT.

Aerosol interleukin-2 induces natural killer cell proliferation in the lung and combination therapy improves the survival of mice with osteosarcoma lung metastasis

BACKGROUND

We have previously shown that aerosol interleukin-2 (IL-2) increased the number of intravenously injected human natural killer (NK) cells in the lungs. In this study we investigated whether this increase was secondary to NK cell proliferation and determined the site of the proliferation.

MATERIALS AND METHODS

Nude mice with osteosarcoma lung metastases were injected with NK cells and treated with aerosol IL-2 or aerosol PBS. BrdU was injected prior to euthanasia to identify proliferating NK cells. The percentage of proliferating NK cells in the lung, bone marrow, spleen, and liver was determined using flow cytometry. Survival studies for mice with osteosarcoma lung metastasis treated with aerosol PBS, aerosol IL-2 alone, aerosol PBS plus NK cells, and aerosol IL-2 plus NK cells were also performed.

RESULTS

Treatment with aerosol IL-2 induced the proliferation of injected NK cells in the lung. Aerosol IL-2 did not increase the proliferation of NK cells in the spleen and liver. Treatment with aerosol IL-2 and aerosol IL-2 plus NK cells increased the overall survival of mice with osteosarcoma lung metastasis.

CONCLUSION

Aerosol IL-2 increases lung NK cell numbers by stimulating local NK cell proliferation. Aerosol IL-2's effect on NK cell proliferation is organ specific, which makes it ideal for the specific targeting of lung metastasis. Aerosol IL-2 plus NK cell therapy induced metastatic regression and increased overall survival demonstrating the potential of this therapeutic approach for patients with osteosarcoma.

Opportunities and limitations of natural killer cells as adoptive therapy for malignant disease

Although natural killer (NK) cells can be readily generated for adoptive therapy with current techniques, their optimal application to treat malignant diseases requires an appreciation of the dynamic balance between signals that either synergize with or antagonize each other. Individuals display wide differences in NK function that determine their therapeutic efficacy. The ability of NK cells to kill target cells or produce cytokines depends on the balance between signals from activating and inhibitory cell-surface receptors. The selection of NK cells with a predominant activating profile is critical for delivering successful anti-tumor activity. This can be achieved through selection of killer immunoglobulin-like receptor-mismatched NK donors and by use of blocking molecules against inhibitory pathways. Optimum NK cytotoxicity may require licensing or priming with tumor cells. Recent discoveries in the molecular and cellular biology of NK cells inform in the design of new strategies, including adjuvant therapies, to maximize the cytotoxic potential of NK cells for adoptive transfer to treat human malignancies.

Improving the outcome of leukemia by natural killer cell-based immunotherapeutic strategies

Blurring the boundary between innate and adaptive immune system, natural killer (NK) cells are widely recognized as potent anti-leukemia mediators. Alloreactive donor NK cells have been shown to improve the outcome of allogeneic stem-cell transplantation for leukemia. In addition, in vivo transfer of NK cells may soon reveal an important therapeutic tool for leukemia, if tolerance to NK-mediated anti-leukemia effects is overcome. This will require, at a minimum, the ex vivo generation of a clinically safe NK cell product containing adequate numbers of NK cells with robust anti-leukemia potential. Ideally, ex vivo generated NK cells should also have similar anti-leukemia potential in different patients, and be easy to obtain for convenient clinical scale-up. Moreover, optimal clinical protocols for NK therapy in leukemia and other cancers are still lacking. These and other issues are being currently addressed by multiple research groups. This review will first describe current laboratory NK cell expansion and differentiation techniques by separately addressing different NK cell sources. Subsequently, it will address the mechanisms known to be responsible for NK cell alloreactivity, as well as their clinical impact in the hematopoietic stem cells transplantation setting. Finally, it will briefly provide insight on past NK-based clinical trials.

Frozen cord blood hematopoietic stem cells differentiate into higher numbers of functional natural killer cells in vitro than mobilized hematopoietic stem cells or freshly isolated cord blood hematopoietic stem cells

Adoptive natural killer (NK) cell therapy relies on the acquisition of large numbers of NK cells that are cytotoxic but not exhausted. NK cell differentiation from hematopoietic stem cells (HSC) has become an alluring option for NK cell therapy, with umbilical cord blood (UCB) and mobilized peripheral blood (PBCD34⁺) being the most accessible HSC sources as collection procedures are less invasive. In this study we compared the capacity of frozen or freshly isolated UCB hematopoietic stem cells (CBCD34⁺) and frozen PBCD34⁺ to generate NK cells in vitro. By modifying a previously published protocol, we showed that frozen CBCD34⁺ cultures generated higher NK cell numbers without loss of function compared to fresh CBCD34⁺ cultures. NK cells generated from CBCD34⁺ and PBCD34⁺ expressed low levels of killer-cell immunoglobulin-like receptors but high levels of activating receptors and of the myeloid marker CD33. However, blocking studies showed that CD33 expression did not impact on the functions of the generated cells. CBCD34⁺-NK cells exhibited increased capacity to secrete IFN-γ and kill K562 in vitro and in vivo as compared to PBCD34⁺-NK cells. Moreover, K562 killing by the generated NK cells could be further enhanced by IL-12 stimulation. Our data indicate that the use of frozen CBCD34⁺ for the production of NK cells in vitro results in higher cell numbers than PBCD34⁺, without jeopardizing their functionality, rendering them suitable for NK cell immunotherapy. The results presented here provide an optimal strategy to generate NK cells in vitro for immunotherapy that exhibit enhanced effector function when compared to alternate sources of HSC.

Donor killer immunoglobulin-like receptor genes and reactivation of cytomegalovirus after HLA-matched hematopoietic stem-cell transplantation: HLA-C allotype is an essential cofactor

Natural killer (NK) cells whose killer immunoglobulin-like receptors (KIRs) recognize human leukocyte antigen (HLA) ligand are “licensed” for activity. In contrast, non-licensed NK cells display KIRs for which ligand is absent from the self genotype and are usually hyporesponsive. Surprisingly, non-licensed cells are active in tumor control after hematopoietic stem-cell transplantation (HSCT) and dominate NK response to murine cytomegalovirus (CMV) infection. From those reports, we hypothesized that control of human CMV early after HSCT is influenced by donor KIR genes whose HLA ligand is absent-from-genotype of HLA-matched donor and recipient. To investigate, we studied CMV reactivation through Day 100 after grafts involving CMV-seropositive donor and/or recipient. A multivariate proportional rates model controlled for variability in surveillance and established covariates including acute graft-versus-host disease; statistical significance was adjusted for testing of multiple KIRs with identified HLA class I ligand (2DL1, 2DL2/3, 2DS1, 2DS2, full-length 2DS4, 3DL1/3DS1, 3DL2). Among HSCT recipients (n = 286), CMV reactivation-free survival time varied with individual donor KIR genes evolutionarily specific for HLA-C: when ligand was absent from the donor/recipient genotype, inhibitory KIRs 2DL2 (P < 0.0001) and 2DL1 (P = 0.015) each predicted inferior outcome, and activating KIRs 2DS2 (P < 0.0001), 2DS1 (P = 0.016), and 2DS4 (P = 0.016) each predicted superior outcome. Otherwise, with ligand present-in-genotype, donor KIR genes had no effect. In conclusion, early after HLA-matched HSCT, individual inhibitory and activating KIR genes have qualitatively different effects on risk of CMV reactivation; unexpectedly, absence of HLA-C ligand from the donor/recipient genotype constitutes an essential cofactor in these associations. Being KIR- and HLA-C-specific, these findings are independent of licensing via alternate NK cell receptors (NKG2A, NKG2C) that recognize HLA-E.

The role of KIR genes and ligands in leukemia surveillance

The antileukemic potential of natural killer (NK) cells has been of rising interest in recent years. Interactions between inhibitory killer cell immunoglobulin-like receptors (KIR) and HLA class I ligands seem to be critically involved in the immunosurveillance process. It is also well established that mismatching of HLA class I-encoded KIR ligands in the setting of hematopoietic stem cell transplantation leads to allorecognition of leukemic cells by NK cells, which is in line with the concept of missing-self recognition. Recent data now suggest that KIR gene polymorphism constitutes another important parameter that needs to be taken into account for selection of suitable stem cell donors. Moreover, the role of KIR gene polymorphism for predisposition to leukemia is a current matter of debate. Here, we would like to review the role of KIR function and genetic polymorphism for recognition of leukemia and discuss the impact of these findings for developing novel concepts for NK cell-based immunotherapy strategies.

Role of HLA in Hematopoietic Stem Cell Transplantation

The selection of hematopoietic stem cell transplantation (HSCT) donors includes a rigorous assessment of the availability and human leukocyte antigen (HLA) match status of donors. HLA plays a critical role in HSCT, but its involvement in HSCT is constantly in flux because of changing technologies and variations in clinical transplantation results. The increased availability of HSCT through the use of HLA-mismatched related and unrelated donors is feasible with a more complete understanding of permissible HLA mismatches and the role of killer-cell immunoglobulin-like receptor (KIR) genes in HSCT. The influence of nongenetic factors on the tolerability of HLA mismatching has recently become evident, demonstrating a need for the integration of both genetic and nongenetic variables in donor selection.

Targeting natural killer cells and natural killer T cells in cancer

Natural killer (NK) cells and natural killer T (NKT) cells are subsets of lymphocytes that share some phenotypical and functional similarities. Both cell types can rapidly respond to the presence of tumour cells and participate in antitumour immune responses. This has prompted interest in the development of innovative cancer therapies that are based on the manipulation of NK and NKT cells. Recent studies have highlighted how the immune reactivity of NK and NKT cells is shaped by the environment in which they develop. The rational use of these cells in cancer immunotherapies awaits a better understanding of their effector functions, migratory patterns and survival properties in humans.

Prevention and treatment of acute myeloid leukemia relapse after allogeneic stem cell transplantation

PURPOSE OF REVIEW

Relapse remains a major cause of treatment failure for acute myeloid leukemia (AML) patients treated with allogeneic hematopoietic stem cell transplantation (allo-HCT). Most patients that recur will perish due to low treatment efficacy, toxicity, or frailty issues. This review summarizes recent developments in clinical research and therapeutic applications for prevention and treatment of this complication of transplantation.

RECENT FINDINGS

Several groups have demonstrated that monitoring minimal residual disease (MRD) after allo-HCT is feasible and is predictive of impending hematologic recurrence. The introduction of novel antileukemia agents in the preparative regimen, maintenance of remission treatment posttransplant, and early MRD-based therapeutic interventions all have the potential to improve outcomes.

SUMMARY

Innovative basic and clinical investigation is urgently needed to improve treatment and prevention of AML recurrence after allogeneic transplantation.

Cellular therapy to treat haematological and other malignancies: progress and pitfalls

The recent Food and Drug Administration (FDA) approval of a cellular therapy to treat castration resistant prostate cancer has reinforced the potential of cellular therapy to consolidate current pharmacological approaches to treating cancer. The emergence of the cell manufacturing facility to facilitate clinical translation of these new methodologies allows greater access to these novel therapies. Here we review different strategies currently being explored to treat haematological malignancies with a focus on adoptive allogeneic or autologous transfer of antigen specific T cells, NK cells or dendritic cells. These approaches all aim to generate immunological responses against overexpressed tissue antigens, mismatched minor histocompatability antigens or tumour associated antigens. Current successes and limitations of these different approaches will be discussed with an emphasis on challenges encountered in generating long term engraftment, antigen selection and implementation as well as therapeutic immune monitoring of clinical responses, with examples from recent clinical trials.

Overview of the killer cell immunoglobulin-like receptor system

Natural killer (NK) cells are more than simple killers and have been implicated in control and clearance of malignant and virally infected cells, regulation of adaptive immune responses, rejection of bone marrow transplants, and autoimmunity and the maintenance of pregnancy. Human NK cells largely use a family of germ-line encoded killer cell immunoglobulin-like receptors (KIR) to respond to the perturbations from self-HLA class I molecules present on infected, malignant, or HLA-disparate fetal or allogenic transplants. Genes encoding KIR receptors and HLA class I ligands are located on different chromosomes, and both feature extraordinary diversity in the number and type of genes. The independent segregation of KIR and HLA gene families produce diversity in the number and type of KIR-HLA gene combinations inherited in individuals, which may determine their immunity and susceptibility to diseases. This chapter provides an overview of NK cells and their unprecedented phenotypic and functional diversity within and between individuals.

Human diversity of killer cell immunoglobulin-like receptors and disease

Natural Killer (NK) cells are the third population of lymphocyte in the mononuclear cell compartment that triggers first-line of defense against viral infection and tumor transformation. Historically, NK cells were thought of as components of innate immunity based on their intrinsic ability to spontaneously kill target cells independent of HLA antigen restriction. However, it is now clear that NK cells are quite sophisticated and use a highly specific and complex target cell recognition receptor system arbitrated via a multitude of inhibitory and activating receptors. Killer cell immunoglobulin-like receptors (KIR) are the key receptors of human NK cells development and function. To date, fourteen distinct KIRs have been identified: eight are inhibitory types, and six are activating types. The number and type of KIR genes present varies substantially between individuals. Inhibitory KIRs recognize distinct motifs of polymorphic HLA class I molecules. Upon engagement of their specific HLA class I ligands, inhibitory KIR dampen NK cell reactivity. In contrast, activating KIRs are believed to stimulate NK cell reactivity when they sense their ligands (unknown). KIR and HLA gene families map to different human chromosomes (19 and 6, respectively), and their independent segregation produces a wide diversity in the number and type of inherited KIR-HLA combinations, likely contributing to overall immune competency. Consistent with this hypothesis, certain combinations of KIR-HLA variants have been correlated with susceptibility to diseases as diverse as autoimmunity, viral infections, and cancer. This review summarizes our emerging understanding of KIR-HLA diversity in human health and disease.

Mouse NK cell-mediated rejection of bone marrow allografts exhibits patterns consistent with Ly49 subset licensing

Natural killer (NK) cells can mediate the rejection of bone marrow allografts and exist as subsets based on expression of inhibitory/activating receptors that can bind MHC. In vitro data have shown that NK subsets bearing Ly49 receptors for self-MHC class I have intrinsically higher effector function, supporting the hypothesis that NK cells undergo a host MHC-dependent functional education. These subsets also play a role in bone marrow cell (BMC) allograft rejection. Thus far, little in vivo evidence for this preferential licensing across mouse strains with different MHC haplotypes has been shown. We assessed the intrinsic response potential of the different Ly49(+) subsets in BMC rejection by using β2-microglobulin deficient (β2m(-/-)) mice as donors. Using congenic and allogeneic mice as recipients and depleting the different Ly49 subsets, we found that NK subsets bearing Ly49s, which bind "self-MHC" were found to be the dominant subset responsible for β2m(-/-) BMC rejection. This provides in vivo evidence for host MHC class I-dependent functional education. Interestingly, all H2(d) strain mice regardless of background were able to resist significantly greater amounts of β2m(-/-), but not wild-type BMC than H2(b) mice, providing evidence that the rheostat hypothesis regarding Ly49 affinities for MHC and NK-cell function impacts BMC rejection capability.

Clinical options after failure of allogeneic hematopoietic stem cell transplantation in patients with hematologic malignancies

Disease recurrence is the single most common cause of death after allogeneic or autologous hematopoietic stem cell transplantation (HSCT). Disease status and chemosensitivity at the time of transplantation, as well as the development of graft-versus-host disease (GVHD), are factors known to influence the risk of relapse post-HSCT. Both acute and chronic GVHD have been associated with decreased relapse rates; however, owing to toxicity, overall survival is not consistently improved in these patients. Furthermore, there is a transient period of immunodeficiency after HSCT, which may permit residual malignant cells to proliferate early in the post-transplant course, before the donor immune system can establish a graft-versus-tumor response. Patients who fail an initial HSCT have an extremely poor outcome; therefore, maneuvers to prevent, identify and treat recurrent disease as early as possible in these situations are necessary. Strategies to distinguish graft-versus-tumor from GVHD, to enhance both general and disease-specific immune reconstitution after transplantation, and to increase donor-mediated anti-host immune reactions are being investigated in clinical trials. Single agent nontoxic post-HSCT chemotherapy, cellular therapies and second allogeneic HSCT using reduced intensity regimens are among the modalities under investigation.

Cytotoxic function of umbilical cord blood natural killer cells: relevance to adoptive immunotherapy

Decreased graft-versus-host disease (GVHD), ease of accessibility, and sustained engraftment encourage the use of umbilical cord blood (UCB) as an alternative source to bone marrow for immune reconstitution in children with leukemia. Natural killer (NK) cells rapidly expand after stem cell transplantation and are important for regulating GVHD and providing graft-versus-leukemia (GVL) effects. This review highlights the phenotypic and functional differences between UCB NK cells and adult peripheral blood (APB) NK cells, and discusses the possible therapeutic benefit of using UCB NK cells for adoptive immunotherapy in leukemia. Alloreactive NK cells show potent cytotoxic activities against human leukocyte antigen (HLA)-nonidentical leukemic cells and reduce leukemia relapses. The higher numbers of NK progenitors in UCB makes it a convenient source for ex vivo expansion of UCB NK cells for posttransplant treatment. UCB NK cells readily respond to interleukin-15, which may greatly enhance their antitumor effect. Activation and expansion protocols for UCB NK cells are currently being developed.

Bone marrow may be the preferable graft source in recipients homozygous for HLA-C group 2 ligands for inhibitory killer Ig-like receptors

HLA class I molecules participate in natural killer cell regulation by acting as ligands for inhibitory killer cell Ig-like receptors (KIRs). One individual may express one or more inhibitory KIR lacking the corresponding HLA ligand. The role of this 'missing KIR ligand' constellation in hematopoietic SCT (HSCT) remains controversial and depends on incompletely defined transplant variables. We have retrospectively analyzed the effects of missing HLA-C group 1/2 and Bw4 KIR ligands in the recipients on the outcome in 382 HSCT, comparing 118 BMT to 264 PBSC transplants (PBSCT). In the multivariate Cox analysis of PBSCT, poor PFS was observed in homozygous HLA-C group 2 (C2/2) recipients (risk ratio (RR), 1.59; P=0.026). In contrast, C2 homozygosity was not unfavorable after BMT (RR, 0.68; P=0.16). C2 homozygous recipients (n=68) had better PFS after BMT than after PBSCT (RR, 0.17; P=0.001), due to fewer relapses (RR, 0.27; P=0.018). Missing Bw4 favorably influenced PFS after BMT (RR, 0.56; P=0.04), but not after PBSCT. These data suggest opposite effects of missing KIR ligands in BMT vs PBSCT. Larger studies are required to reassess whether BMT should be preferred to PBSCT as an option for C2/C2 recipients.

KIR2DS1 genotype predicts for complete cytogenetic response and survival in newly diagnosed chronic myeloid leukemia patients treated with imatinib

Natural killer (NK) cells are expanded in chronic myeloid leukemia (CML) patients on tyrosine kinase inhibitors (TKI) and exert cytotoxicity. The inherited repertoire of killer immunoglobulin-like receptors (KIR) may influence response to TKI. We investigated the impact of KIR-genotype on outcome in 166 chronic phase CML patients on first-line imatinib treatment. We validated our findings in an independent patient group. On multivariate analysis, KIR2DS1 genotype (RR=1.51, P=0.03) and Sokal risk score (low-risk RR=1, intermediate-risk RR=1.53, P=0.04, high-risk RR=1.69, P=0.034) were the only independent predictors for failure to achieve complete cytogenetic response (CCyR). Furthermore, KIR2DS1 was the only factor predicting shorter progression-free (PFS) (RR=3.1, P=0.03) and overall survival (OS) (RR=2.6, P=0.04). The association between KIR2DS1 and CCyR, PFS and OS was validated by KIR genotyping in 174 CML patients on first-line imatinib in the UK multi-center SPIRIT-1 trial; in this cohort, KIR2DS1(+) patients had significantly lower 2-year probabilities of achieving CCyR (76.9 vs 87.9%, P=0.003), PFS (85.3 vs 98.1%, P=0.007) and OS (94.4 vs 100%, P=0.015) than KIR2DS1(-) patients. The impact of KIR2DS1 on CCyR was greatest when the ligand for the corresponding inhibitory receptor, KIR2DL1, was absent (P=0.00006). Our data suggest a novel role for KIR-HLA immunogenetics in CML patients on TKI.

Re-educating natural killer cells

The development and function of natural killer (NK) cells is dictated by signals received through activating and inhibitory receptors expressed on the cell surface. During their maturation in the bone marrow, NK cells undergo an education process that ensures they are tolerant to healthy peripheral tissues. Several recent studies advance our understanding of self-tolerance mechanisms at work in NK cells. These studies demonstrate that the developmental programming in NK cells is not fixed, and that perturbations to the peripheral environment (via transplantation or viral infection, for example) greatly influence the ability of mature NK cells to mount an effector response. This newfound ability of mature NK cells to be "re-educated" may be clinically applicable in the immunotherapeutic use of NK cells against infection and cancer.

Natural killer cell licensing during viral infection

Natural Killer (NK) cell functionality is controlled by inhibitory receptors that recognize self-MHC class I. NK cells that do not interact with self-MHC class I are hypo-responsive to many stimuli and fail to reject MHC class I-deficient cells. Thus, although the mechanisms are unknown, interactions with MHC class I "licensed" NK cells respond efficiently. Surprisingly, these licensed NK cells fail to control viral infection. During mouse cytomegalovirus (MCMV) infection, SHP-1 signaling downstream of inhibitory receptors for MHC class I limits NK cell proliferation. Interactions with MHC class I prevent licensed NK cells from controlling of MCMV replication and pathogenesis; rather, it is the unlicensed NK cells that are not inhibited by self-MHC class I that efficiently control MCMV infection. Therefore, the licensing hypothesis is not sufficient to explain NK cell functionality during viral infection.

NCI First International Workshop on the Biology, Prevention, and Treatment of Relapse after Allogeneic Hematopoietic Stem Cell Transplantation: Report from the Committee on Treatment of Relapse after Allogeneic Hematopoietic Stem Cell Transplantation

Relapse is a major cause of treatment failure after allogeneic hematopoietic stem cell transplantation (alloHSCT). Treatment options for relapse have been inadequate, and the majority of patients ultimately die of their disease. There is no standard approach to treating relapse after alloHSCT. Withdrawal of immune suppression and donor lymphocyte infusions are commonly used for all diseases; although these interventions are remarkably effective for relapsed chronic myelogenous leukemia, they have limited efficacy in other hematologic malignancies. Conventional and novel chemotherapy, monoclonal antibody therapy, targeted therapies, and second transplants have been utilized in a variety of relapsed diseases, but reports on these therapies are generally anecdotal and retrospective. As such, there is an immediate need for well-designed, disease-specific trials for treatment of relapse after alloHSCT. This report summarizes current treatment options under investigation for relapse after alloHSCT in a disease-specific manner. In addition, recommendations are provided for specific areas of research necessary in the treatment of relapse after alloHSCT.

Natural killer cell education and tolerance

Natural killer (NK) cells play a key role in the immune response to certain infections and malignancies by direct cytolysis of infected or transformed cells and by secretion of potent immune mediators. NK cells express an array of activating receptors that recognize self-molecules. If not restrained by inhibitory receptors recognizing major histocompatibility complex (MHC) class I proteins on the surface of self cells, NK cells are able to kill normal, healthy cells. Not all NK cells express inhibitory receptors for self-MHC class I; thus, other tolerance mechanisms are necessary to prevent NK cell-mediated autoimmunity. Here we review the major mechanisms of NK cell education and tolerance.

Alloreactive natural killer cells in hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) for leukemia can play a major role in reducing the risk of relapse by inducing a graft versus leukemia (GVL) effect. Here, we review the effectiveness of mismatching inhibitory killer-cell-immunoglobulin-like receptors (KIR) on donor natural killer (NK) cells as a mechanism for GVL. We review the range of KIR and the importance of T cell and NK cell content of the graft, together with considerations of the graft source. Further understanding of conditioning and mechanisms to reduce graft versus host disease (GVHD) will improve our ability to manipulate NK cells in HSCT.

Interaction between KIR3DS1 and HLA-Bw4 predicts for progression-free survival after autologous stem cell transplantation in patients with multiple myeloma

Natural killer (NK) cells exert antimyeloma cytotoxicity. The balance between inhibition and activation of NK-cells played by the inherited repertoire of killer immunoglobulin-like receptor (KIR) genes therefore may influence prognosis. One hundred eighty-two patients with multiple myeloma (MM) were analyzed for KIR repertoire. Multivariate analysis showed that progression-free survival (PFS) after autologous stem cell transplantation (ASCT) was significantly shorter for patients who are KIR3DS1(+) (P = .01). This was most evident for patients in complete or partial remission (good risk; GR) at ASCT. The relative risk (RR) of progression or death for patients with KIR3DS1(+) compared with KIR3DS1(-) was 1.9 (95% CI, 1.3-3.1; P = .002). The most significant difference in PFS was observed in patients with GR KIR3DS1(+) in whom HLA-Bw4, the ligand for the corresponding inhibitory receptor KIR3DL1, was missing. Patients with KIR3DS1(+) KIR3DL1(+) HLA-Bw4(-) had a significantly shorter PFS than patients who were KIR3DS1(-), translating to a difference in median PFS of 12 months (12.2 vs 24 months; P = .002). Our data show that KIR-human leukocyte antigen immunogenetics represent a novel prognostic tool for patients with myeloma, shown here in the context of ASCT, and that KIR3DS1 positivity may identify patients at greater risk of progression.

Decreased infections in recipients of unrelated donor hematopoietic cell transplantation from donors with an activating KIR genotype

Infectious complications following allogeneic hematopoietic cell transplantation (HCT) from unrelated donors (URD) result in significant morbidity. We hypothesized that recipients of a URD with an activating natural killer cell immunoglobulin-like receptor (KIR) (B/x) genotype would have decreased infectious complications because of enhanced natural killer (NK) cell function. We compared the infectious complications in 116 recipients of a graft from a donor with an A/A KIR (n = 44) genotype and a B/x KIR (n = 72) genotype. All recipients participated in the prospective National Marrow Donor Program infection project collecting infection data from conditioning until 6 months posttransplant. The cohort with a B/x donor had fewer initial bacterial infections by day 180 (A/A: 86%; 95% confidence interval [CI], 75-95; B/x: 68%; 95% CI, 57-78; P = .02). There was no difference in the incidence of viral or fungal infections. When accounting for multiple infections, fewer bacterial infections were seen in the B/x cohort (A/A: 3.55/patient; B/x: 2.63/patient; P = .09). During the study period, only 19 patients had no infections; of these, 15 had received cells from a B/x KIR donor. The role of donor KIR genotype on infection complications is intriguing and warrants further investigation.

'Unlicensed' natural killer cells dominate the response to cytomegalovirus infection

Natural killer (NK) cells expressing inhibitory receptors that bind to self-MHC class I are “licensed” or rendered functionally more responsive to stimulation, whereas “unlicensed” NK cells lacking receptors for self-MHC class I are hyporesponsive. Here we show that, contrary to the licensing hypothesis, unlicensed NK cells were the primary mediators of NK cell-mediated control of mouse cytomegalovirus infection in vivo. Depletion of unlicensed, but not licensed, NK cells impaired control of viral titers. Transfer of unlicensed NK cells was more protective than licensed NK cells. SHP-1 signaling limited proliferation of licensed, but not unlicensed NK cells during infection. Thus, “unlicensed” NK cells are critical for protection against viral infection.