Relevance of C1 and C2 epitopes for hemopoietic stem cell transplantation: role for sequential acquisition of HLA-C-specific inhibitory killer Ig-like receptor

Reconstitution of natural killer cell receptors influences natural killer activity and relapse rate after haploidentical transplantation of T- and B-cell depleted grafts in children

BACKGROUND

Natural killer cells have been demonstrated to exert remarkable graft-versus-leukemia effects after haploidentical transplantation. Acquisition of both, inhibiting and activating, receptors on developing natural killer cells is an important step in their functional maturation. Here, we report on the reconstitution of natural killer receptors after haploidentical transplantation of T-and B-cell (CD3/CD19) depleted grafts with co-transfusion of natural killer cells in children and its influence on natural killer cell activity and clinical outcome.

DESIGN AND METHODS

We analyzed reconstitution patterns of natural killer receptors at different time intervals after haploidentical transplantation by multi-color flow cytometry. Natural killer cell activity and antibody-dependent cellular cytotoxicity was tested against cell lines and leukemic blasts in vitro. Survival was analyzed using Kaplan-Meier estimates.

RESULTS

Recovery of CD56(+)/CD16(+) cells was fast with high cytolytic activity against K562 and strong antibody-dependent cellular cytotoxicity activity against neuroblastoma and leukemic blasts as early as day 14 posttransplant. KIR reconstitution showed a predominance of KIR negative natural killer cells early after transplantation and an early reconstitution of CD158b compared to CD158a and CD158e. These differences were independent of presence or absence of the corresponding KIR ligands in donors or recipients. This reconstitution pattern was associated with a higher relapse probability of patients homozygous for HLA-C1-alleles compared to patients homozygous or even heterozygous for HLA-C2-alleles.

CONCLUSIONS

Our results indicate a fast recovery of functional and alloreactive natural killer cells with a constant KIR pattern after haploidentical transplantation with T- and B-cell depleted grafts. Moreover, these natural killer cells can mediate antibody-dependent cellular cytotoxicity and therefore may allow for an early use of antibodies against residual malignant cells.

Sequential recovery of NK cell receptor repertoire after allogeneic hematopoietic SCT

Alloreactivity of natural killer (NK) cells contributes to the GVL reaction after allogeneic hematopoietic SCT (allo-HSCT). However, various procedure-related factors may affect NK cell maturation and their ability to recognize and kill leukemic cells. In this study, we prospectively evaluated expression of NK cell inhibitory receptors in 83 adults treated with myeloablative, killer cell Ig-like receptor (KIR)-ligand-matched allo-HSCT. NK cell maturation was evaluated by comparing the phenotypic patterns after allo-HSCT with the donor ones. The frequencies of KIR3DL1 were comparable to the donor ones on day +28, while they decreased significantly starting from day +100. The expression of KIR2DL2/3 was significantly lower in patients compared with donors up to day +100. The expression of KIR2DL1, despite continues growth, remained significantly decreased for 1 year after allo-HSCT. NKG2A was over-expressed up to day +180. Within 1 year after allo-HSCT, the NK cell phenotypic pattern tended to recapitulate the donor type. The process was disturbed by the use of steroids with significant differences observed on days +56 (P=0.01) and +100 (P=0.04). Up to day +100, reconstitution of NK cell receptor repertoire correlated with the absolute numbers of circulating CD3(+), CD3(+)CD4(+) and CD3(+)CD8(+) cells. Our observations should be taken into account when trying to predict potential benefit from NK cell alloreactivity.

Expansion of NKG2A-LIR1- natural killer cells in HLA-matched, killer cell immunoglobulin-like receptors/HLA-ligand mismatched patients following hematopoietic cell transplantation

The prognosis after hematopoietic cell transplantation (HCT) for the treatment of leukemia or lymphoma in humans is influenced by donor-derived natural killer (NK) cells, which enhance the graft-versus-leukemia (GVL) effect. Such alloreactive killer cells can be generated in vivo after HCT if the donor expresses killer cell immunoglobulin-like receptors (KIRs), such as KIR2DL1, KIR2DL2/3, or KIR3DL1, for which the recipient lacks HLA class I ligands. We studied effector cells from 22 KIR/HLA-ligand mismatched and 14 KIR/HLA-ligand matched, primarily HLA-matched patient-donor pairs after allogeneic HCT. A novel 8-color flow cytometry panel allowed us to characterize effector-cell populations without "broadly reactive" inhibitory receptors such as CD94/NKG2A or LIR1. The numbers of such NKG2A(-) LIR1(-) NK cells increased following HCT in patients transplanted by KIR/HLA-ligand mismatched grafts, compared to KIR/HLA-ligand matched grafts, and in patients transplanted from donors of the A/B, compared to A/A, KIR haplotypes. NKG2A(-)LIR1(-) NK cells expressing only those inhibitory KIRs for which the patient had no HLA class I ligands could be stimulated by HLA class I-deficient cells to express CD107a. Thus, NKG2A(-)LIR1(-) NK cells may be important GVL effector cells following HCT, even in patients transplanted from HLA-matched donors.

Natural killer cells and cancer: regulation by the killer cell Ig-like receptors (KIR)

Natural killer (NK) cells are innate immune effector cells that make up approximately 10-15% of the peripheral blood lymphocytes in humans and are primarily involved in immunosurveillance to eliminate transformed and virally-infected cells. They were originally defined by their ability to spontaneously eliminate rare cells lacking expression of class I major histocompatibility complex (MHC-I) self molecules, which is commonly referred to as "missing self" recognition. The molecular basis for missing self recognition emerges from the expression of MHC-I-specific inhibitory receptors on the NK cell surface that tolerize NK cells toward normal MHC-I-expressing cells. By lacking inhibitory receptor ligands, tumor cells or virus-infected cells that have down-modulated surface MHC-I expression become susceptible to attack by NK cells. Killer cell Ig-like receptors (KIR; CD158) constitute a family of MHC-I binding receptors that plays a major role in regulating the activation thresholds of NK cells and some T cells in humans. Here, we review the multiple levels of KIR diversity that contribute to the generation of a highly varied NK cell repertoire and explain how this diversity can influence susceptibility to a variety of diseases, including cancer. We further describe strategies by which KIR can be manipulated therapeutically to treat cancer, through the exploitation of KIR/MHC-I ligand mismatch to potentiate hematopoietic stem cell transplantation and the use of KIR blockade to enhance tumor cell killing.

KIR acquisition probabilities are independent of self-HLA class I ligands and increase with cellular KIR expression

Inhibitory killer cell immunoglobulin-like receptors (KIRs) preserve tolerance to self and shape the functional response of human natural killer (NK) cells. Here, we have evaluated the influence of selection processes in the formation of inhibitory KIR repertoires in a cohort of 44 donors homozygous for the group A KIR haplotype. Coexpression of multiple KIRs was more frequent than expected by the product rule that describes random association of independent events. In line with this observation, the probability of KIR acquisition increased with the cellular expression of KIRs. Three types of KIR repertoires were distinguished that differed in frequencies of KIR- and NKG2A-positive cells but showed no dependency on the number of self-HLA class I ligands. Furthermore, the distribution of self- and nonself-KIRs at the cell surface reflected a random combination of receptors rather than a selection process conferred by cognate HLA class I molecules. Finally, NKG2A was found to buffer overall functional responses in KIR repertoires characterized by low-KIR expression frequencies. The results provide new insights into the formation of inhibitory KIR repertoires on human NK cells and support a model in which variegated KIR repertoires are generated through sequential and random acquisition of KIRs in the absence of selection.

Breaking tolerance to self, circulating natural killer cells expressing inhibitory KIR for non-self HLA exhibit effector function after T cell-depleted allogeneic hematopoietic cell transplantation

Alloreactive natural killer (NK) cells are an important influence on hematopoietic stem cell transplantation (HSCT) outcome. In HLA-mismatched HSCT, alloreactivity occurs when licensed donor NK cells expressing inhibitory killer Ig-like receptors (KIR) for donor MHC class I ligands recognize the lack of the class I ligands in the mismatched recipient ("missing self"). Studies in HLA-matched HSCT, however, have also demonstrated improved outcome in patients lacking class I ligands for donor inhibitory KIR ("missing ligand"), indicating that classically nonlicensed donor NK cells expressing KIR for non-self MHC class I ligands may exhibit functional competence in HSCT. We examined NK function in 16 recipients of T cell-depleted allografts from HLA-identical or KIR-ligand matched donors after myeloablative therapy. After HSCT, nonlicensed NK cells expressing inhibitory KIR for non-self class I exhibit robust intracellular IFN-gamma and cytotoxic response to target cells lacking cognate ligand, gradually becoming tolerized to self by day 100. These findings could not be correlated with cytokine environment or phenotypic markers of NK development, nor could they be attributed to non-KIR receptors such as CD94/NKG2A. These findings confirm that NK alloreactivity can occur in HLA-matched HSCT, where tolerance to self is either acquired by the stem cell-derived NK cell after exiting the bone marrow or where tolerance to self can be temporarily overcome.

KIR genes and KIR ligands affect occurrence of acute GVHD after unrelated, 12/12 HLA matched, hematopoietic stem cell transplantation

Interactions of polymorphic killer Ig-like receptor (KIR) receptors with KIR ligands have been shown to modify the outcome of hematopoietic SCT (HSCT). The association of these genetic factors with different transplantation endpoints, however, varies substantially, depending on clinical and study setup variables. We aimed to assess whether KIR ligands, KIR genes and KIR haplotypes are associated with HSCT outcome of 124 patients with various hematological malignancies, transplanted with 12/12 HLA matched grafts from unrelated donors. For this purpose, patient and donor KIR gene and KIR ligand polymorphisms were determined and correlated with clinical data in simple and multiple models. We found that a missing HLA-C2 ligand for donor inhibitory KIR2DL1 was significantly associated with an increased risk of acute GVHD (aGVHD) (II-IV) (hazard ratio (HR)=2.23, 95% confidence interval (95% CI): 1.21-4.10, P=0.010), as were the AA KIR haplotypes in patients and donors in HLA-C1CX (HR=2.37, 95% CI: 1.16-4.84, P=0.018) and in HLA-Bw4(-) (HR=3.20, 95% CI: 1.35-7.60, P=0.008) patients. On the contrary, transplantation of HLA-C1C2 patients with KIR2DS2 positive grafts were associated with a decreased risk of aGVHD (II-IV) (HR=0.24, 95% CI: 0.07-0.85, P=0.027). Thus, our single center study provides evidence for the modification of aGVHD risk by KIRs and their ligands.

Permanent silencing of NKG2A expression for cell-based therapeutics

Natural killer (NK) and T-cell cytotoxicity is significantly reduced by signaling via CD94/NKG2A receptors. High levels of NKG2A on NK cells have been shown to compromise the graft-versus-leukemia effect in hematopoietic stem cell transplantation. We therefore evaluated the functional relevance of NKG2A silencing for the cytotoxic potential of genetically engineered NK and T cells. Lentiviral vectors containing short hairpin RNA (shRNA) sequences targeting NKG2A transcripts were used to transduce NKG2A(+) primary NK and T cells. NKG2A expression levels were measured by flow cytometry and real-time PCR. The effect of NKG2A silencing on the cytolytic potential of NK and T cells was evaluated in cytotoxicity assays using K562 and B lymphoblastoid cells as targets. Granzyme B mRNA transcript levels were detected by real-time PCR. The transduction of inducible RNAi cassettes containing the sequences for shRNAs targeting NKG2A reduced protein expression in NK and T cells by up to 95%. The cytotoxicity assays demonstrated that NKG2A silencing effectively enhanced NK and CD8+ T-cell lysis by up to 40% and 15%, respectively. However, lysis of K562 cells which lack human leukocyte antigen-E, the ligand of NKG2A, was associated with an upregulation of the natural cytotoxicity receptor NKp30 in NKG2A-silenced NK cells. Our data suggest that RNAi-mediated silencing of NKG2A in effector cells could improve the efficacy of cell-based immunotherapies but also show that indirect effects of NKG2A knockdown exist that have to be considered when designing therapeutic protocols with genetically engineered NK or T cells.

Use of natural killer cells as immunotherapy for leukaemia

Natural killer (NK) cells potentially play a significant role in eradicating residual disease following allogeneic haematopoietic cell transplantation, and have been explored as tools for adoptive immunotherapy for chemotherapy-refractory patients. NK cell cytotoxicity is modulated by multiple activating and inhibitory receptors that maintain a balance between self-tolerance and providing surveillance against pathogens and malignant transformation. The functional characteristics of NK cells are dictated by the strength of inhibitory receptor signalling. Major histocompatibility complex (MHC)-specific inhibitory receptor acquisition occurs sequentially during NK cell development, and is determined by the nature of immunological reconstitution after allogeneic haematopoietic cell transplantation. Polymorphisms of inhibitory receptors [killer immunoglobulin-like receptors (KIRs)] and their ligands (MHC) contribute to interindividual variability. As a result, the functional NK cell repertoire of individual donors has variable potential for graft-vs-leukaemia reactions. Models predicting NK cell alloreactivity, including KIR ligand mismatch and missing KIR ligand strategies, are discussed as algorithms for optimal NK cell donor selection. Future modifications to improve NK cell adoptive immunotherapy by means of increasing target recognition and reducing inhibitory signalling are being explored.

Distinct KIR/HLA compound genotypes affect the kinetics of human antiviral natural killer cell responses

Genetic studies suggest a role for killer cell immunoglobulin-like receptor/HLA (KIR/HLA) compound genotypes in the outcome of viral infections, but functional data to explain these epidemiological observations have not been reported. Using an in vitro model of infection with influenza A virus (IAV), we attribute functional differences in human NK cell activity to distinct KIR/HLA genotypes. Multicolor flow cytometry revealed that the HLA-C-inhibited NK cell subset in HLA-C1 homozygous subjects was larger and responded more rapidly in IFN-gamma secretion and CD107a degranulation assays than its counterpart in HLA-C2 homozygous subjects. The differential IFN-gamma response was also observed at the level of bulk NK cells and was independent of KIR3DL1/HLA-Bw4 interactions. Moreover, the differential response was not caused by differences in NK cell maturation status and phenotype, nor by differences in the type I IFN response of IAV-infected accessory cells between HLA-C1 and HLA-C2 homozygous subjects. These results provide functional evidence for differential NK cell responsiveness depending on KIR/HLA genotype and may provide useful insights into differential innate immune responsiveness to viral infections such as IAV.

T cell subset-specific susceptibility to aging

With increasing age, the competence of the immune system to fight infections and tumors declines. Age-dependent changes have been mostly described for human CD8 T cells, raising the question of whether the response patterns for CD4 T cells are different. Gene expression arrays of memory CD4 T cells yielded a similar age-induced fingerprint as has been described for CD8 T cells. In cross-sectional studies, the phenotypic changes were not qualitatively different for CD4 and CD8 T cells, but occurred much more frequently in CD8 T cells. Homeostatic stability partially explained this lesser age sensitivity of CD4 T cells. With aging, naïve and central memory CD8 T cells were lost at the expense of phenotypically distinct CD8 effector T cells, while effector CD4 T cells did not accumulate. However, phenotypic shifts on central memory T cells were also more pronounced in CD8 T cells. This distinct stability in cell surface marker expression can be reproduced in vitro. The data show that CD8 T cells are age sensitive by at least two partially independent mechanisms: fragile homeostatic control and gene expression instability in a large set of regulatory cell surface molecules.

Donor killer immunoglobulin-like receptor (KIR) genotype-patient cognate KIR ligand combination and antithymocyte globulin preadministration are critical factors in outcome of HLA-C-KIR ligand-mismatched T cell-replete unrelated bone marrow transplantation

We previously reported the potent adverse effects of killer immunoglobulin-like receptor (KIR) ligand mismatch (KIR-L-MM) on the outcome of T cell-replete unrelated hematopoietic stem cell transplantation (UR-HSCT) through the Japan Marrow Donor Program. Other UR-HSCT studies have yielded inconsistent results. To address this discrepancy, we evaluated candidate factors contributing to the effects of KIR-L-MM on transplantation outcomes in retrospectively selected hematologic malignancy cases with uniform graft-versus-host disease (GVHD) prophylaxis (n = 1489). KIR-L-MM in the graft-versus-host direction (KIR-L-MM-G) was associated with a higher incidence of acute GVHD (aGVHD; P < .002) and a lower overall survival (OS; P < .0001) only without the preadministration of antithymocyte globulin (ATG). Furthermore, in KIR-L-MM-G, the donor KIR2DS2 gene with the patient cognate C1 ligand was associated with a higher incidence of aGVHD (P = .012). Multivariate analysis by Cox proportional hazard models suggested that donor 2DS2 and ATG preadministration were critical factors in grade III-IV aGVHD (hazard ratio = 1.96; 95% confidence interval = 1.01-3.80; P = .045, and hazard ratio = 0.56; 95% confidence interval = 0.31-0.99; P = .047, respectively). These results indicate that the adverse effects of KIR-L-MM-G depend on combination of donor-activating KIR genotype-patient cognate KIR ligand type and no ATG preadministration, thereby suggesting the importance of these factors in UR-HSCT and in leukemia treatment using natural killer (NK) cell alloreactivity.

KIR2DS1-positive NK cells mediate alloresponse against the C2 HLA-KIR ligand group in vitro

The inhibitory 2DL1 and activating 2DS1 killer Ig-like receptors (KIR) both have shared ligand specificity for codon sequences in the C2 group HLA-Cw Ags. In this study, we have investigated NK cell activation by allogeneic target cells expressing different combinations of the HLA-KIR ligand groups C1, C2, and Bw4. We demonstrate that fresh NK cells as well as IL-2-propagated NK cells from 2DS1-positive donors that are homozygous for the C1 ligand group are activated in vitro by B lymphoblastoid cell lines expressing the C2 group. This response is, in part, due to the absence of C1 group recognition mediated by the inhibitory receptor 2DL2/3. This "missing self" alloresponse to C2, however, is rarely observed in NK cells from donors lacking 2DS1. Even in presence of 2DS1, the NK alloresponse is dramatically reduced in donors that have C2 group as "self." Analysis of selected NK clones that express 2DS1 mRNA and lack mRNA for 2DL1 demonstrates that activation by the C2 ligand and mAb cross-linking of 2DS1 in these clones induces IFN-gamma. Furthermore, this C2 group-induced activation is inhibited by Abs to both HLA class I and the receptor. Collectively, these studies demonstrate that NK cells from 2DS1-positive donors are activated by target cells that express the C2 group as an alloantigen. This leads to increased IFN-gamma-positive fresh NK cells and induces NK allocytotoxicity in IL2-propagated polyclonal NK cells and NK clones. This study also provides support for the concept that incompatibility for the HLA-KIR ligand groups C1, C2, and Bw4 dominates NK alloactivation in vitro.