Biology and genetics of hybrid resistance

Murine models to study human NK cells in human solid tumors

Since the first studies, the mouse models have provided crucial support for the most important discoveries on NK cells, on their development, function, and circulation within normal and tumor tissues. Murine tumor models were initially set to study murine NK cells, then, ever more sophisticated human-in-mice models have been developed to investigate the behavior of human NK cells and minimize the interferences from the murine environment. This review presents an overview of the models that have been used along time to study NK cells, focusing on the most popular NOG and NSG models, which work as recipients for the preparation of human-in-mice tumor models, the study of transferred human NK cells, and the evaluation of various enhancers of human NK cell function, including cytokines and chimeric molecules. Finally, an overview of the next generation humanized mice is also provided along with a discussion on how traditional and innovative in-vivo and in-vitro approaches could be integrated to optimize effective pre-clinical studies.

Can we make a better match or mismatch with KIR genotyping?

Natural killer (NK) cell function is regulated by a fine balance between numerous activating and inhibitory receptors, of which killer-cell immunoglobulin-like receptors (KIRs) are among the most polymorphic and comprehensively studied. KIRs allow NK cells to recognize downregulation or the absence of HLA class I molecules on target cells (known as missing-self), a phenomenon that is commonly observed in virally infected cells or cancer cells. Because KIR and HLA genes are located on different chromosomes, in an allogeneic environment such as after hematopoietic stem cell transplantation, donor NK cells that express an inhibitory KIR for an HLA class I molecule that is absent on recipient targets (KIR/KIR-ligand mismatch), can recognize and react to this missing self and mediate cytotoxicity. Accumulating data indicate that epistatic interactions between KIR and HLA influence outcomes in several clinical conditions. Herein, we discuss the genetic and functional features of KIR/KIR-ligand interactions in hematopoietic stem cell transplantation and how these data can guide donor selection. We will also review clinical studies of adoptive NK cell therapy in leukemia and emerging data on the use of genetically modified NK cells that could broaden the scope of cancer immunotherapy.

Haploidentical Hematopoietic Stem Cell Transplantation as a Platform for Post-Transplantation Cellular Therapy

Haploidentical transplantation can extend the opportunity for transplantation to almost all patients who lack an HLA-matched donor. Advances in the field of haploidentical transplantation have led to a marked decrease in treatment-related mortality, allowing investigators to focus on developing rationale pre- and peri-remission therapies aimed at preventing disease relapse after transplantation. Because of widespread availability, low treatment-related mortality, and cost, haploidentical donors may become the preferred "alternative" donors for allogeneic hematopoietic stem cell transplantation. One of the major advantages of using a related donor is the possibility of collecting or generating additional cellular products from the same immediately available donor, which will not be rejected. Infusion of these cells in the peri-transplantation period, derived from the same immune system, is opening the possibility of markedly enhancing the antitumor effects of the graft and hastening immunologic reconstitution after transplantation.

Genetic investigation of MHC-independent missing-self recognition by mouse NK cells using an in vivo bone marrow transplantation model

MHC-I-specific receptors play a vital role in NK cell-mediated "missing-self" recognition, which contributes to NK cell activation. In contrast, MHC-independent NK recognition mechanisms are less well characterized. In this study, we investigated the role of NKR-P1B:Clr-b (Klrb1:Clec2d) interactions in determining the outcome of murine hematopoietic cell transplantation in vivo. Using a competitive transplant assay, we show that Clr-b(-/-) bone marrow (BM) cells were selectively rejected by wild-type B6 recipients, to a similar extent as H-2D(b-/-) MHC-I-deficient BM cells. Selective rejection of Clr-b(-/-) BM cells was mitigated by NK depletion of recipient mice. Competitive rejection of Clr-b(-/-) BM cells also occurred in allogeneic transplant recipients, where it was reversed by selective depletion of NKR-P1B(hi) NK cells, leaving the remaining NKR-P1B(lo) NK subset and MHC-I-dependent missing-self recognition intact. Moreover, competitive rejection of Clr-b(-/-) hematopoietic cells was abrogated in Nkrp1b-deficient recipients, which lack the receptor for Clr-b. Of interest, similar to MHC-I-deficient NK cells, Clr-b(-/-) NK cells were hyporesponsive to both NK1.1 (NKR-P1C)-stimulated and IL-12/18 cytokine-primed IFN-γ production. These findings support a unique and nonredundant role for NKR-P1B:Clr-b interactions in missing-self recognition of normal hematopoietic cells and suggest that optimal BM transplant success relies on MHC-independent tolerance mechanisms. These findings provide a model for human NKR-P1A:LLT1 (KLRB1:CLEC2D) interactions in human hematopoietic cell transplants.

Recent advances in cytomegalovirus: an update on pharmacologic and cellular therapies

The 2015 Tandem American Society for Blood and Marrow Transplantation/Center for International Blood and Marrow Transplant Meetings provide an opportunity to review the current status and future perspectives on therapy for cytomegalovirus (CMV) infection in the setting of hematopoietic stem cell transplantation (HSCT). After many years during which we have seen few tangible advances in terms of new antiviral drugs, we are now experiencing an exciting period of late-stage drug development, characterized by a series of phase III trials incorporating a variety of novel agents. These trials have the potential to shift our current standard therapeutic strategies, which generally involve pre-emptive therapy based on sensitive molecular surveillance, towards the prophylactic approaches we see more generally with other herpes viruses such as herpes simplex and varicella zoster. This comes at a time when the promise of extensive preclinical research has been translated into encouraging clinical responses with several cellular immunotherapy strategies, which have also been moved towards definitive late-stage clinical trials. How these approaches will be integrated with the new wave of antiviral drugs remains open to conjecture. Although most of the focus of these cellular immunotherapy studies has been on adaptive immunity, and in particular T cells, an increasing awareness of the possible role of other cellular subsets in controlling CMV infection has developed. In particular, the role of natural killer (NK) cells is being revisited, along with that of γδ T cells. Depletion of NK cells in mice results in higher titers of murine CMV in tissues and increased mortality, whereas NK cell deficiency in humans has been linked to severe CMV disease. We will review recent progress in these areas.

At the Bench: Preclinical rationale for exploiting NK cells and γδ T lymphocytes for the treatment of high-risk leukemias

NK cells and γδ T lymphocytes display potent cytolytic activity against leukemias and CMV-infected cells and are thus, promising immune effector cells in the context of allo-HSCT. NK cells express HLA class I-specific inhibitory receptors and preferentially kill HLA class I(low) tumors or virus-infected cells. Killing occurs upon engagement of activating NKRs with ligands that are up-regulated on tumors and infected cells. A similar activating receptor/ligand interaction strategy is used by γδ T cells, which in addition, use their TCRs for recognition of phosphorylated antigens and still largely undefined ligands on tumor cells. In the haploidentical allo-HSCT setting, alloreactive NK cells, derived from donor HSCs, can exert potent antileukemia activity and kill residual patient DCs and T cells, thus preventing GvHD and graft rejection. However, generation of KIR(+) alloreactive NK cells from HSCs requires many weeks, during which leukemia relapses, and life-threatening infections may occur. Importantly, mature NK cells and γδ T cells can control certain infectious agents efficiently, in particular, limit CMV reactivation, and infusion of such donor cells at the time of HSCT has been implemented. Development of novel, cell-based immunotherapies, allowing improved trafficking and better targeting, will endow NK cells and γδ T lymphocytes with enhanced anti-tumor activity, also making them key reagents for therapies against solid tumors. The clinical aspects of using NK cells and γδ T lymphocytes against hematological malignancies, including the allo-HSCT context, are reviewed in the related side-by-side paper by Locatelli and colleagues [1].

Advantages and clinical applications of natural killer cells in cancer immunotherapy

The past decade has witnessed a burgeoning of research and further insight into the biology and clinical applications of natural killer (NK) cells. Once thought to be simple innate cells important only as cytotoxic effector cells, our understanding of NK cells has grown to include memory-like responses, the guidance of adaptive responses, tissue repair, and a delicate paradigm for how NK cells become activated now termed "licensing" or "arming." Although these cells were initially discovered and named for their spontaneous ability to kill tumor cells, manipulating NK cells in therapeutic settings has proved difficult and complex in part due to our emerging understanding of their biology. Therapies involving NK cells may either activate endogenous NK cells or involve transfers of exogenous cells by hematopoietic stem cell transplantation or adoptive cell therapy. Here, we review the basic biology of NK cells, highlighting characteristics which make NK cells particularly useful in cancer therapies. We also explore current treatment strategies that have been used for cancer as well as discuss potential future directions for the field.

Cellular and molecular basis of haploidentical hematopoietic stem cell transplantation in the successful treatment of high-risk leukemias: role of alloreactive NK cells

Natural killer (NK) cells are involved in innate immune responses and play a major role in tumor surveillance and in defense against viruses. Human NK cells recognize human leukocyte antigen (HLA) class I molecules via surface receptors [killer immunoglobulin-like receptor (KIR) and NKG2A] delivering signals that inhibit NK cell function and kill HLA class I-deficient target cells, a frequent event in tumors or virus-infected cells. NK cell triggering is mediated by activating receptors that recognize ligands expressed primarily on tumors or virus-infected cells. NK cells play also a key role in the cure of high-risk leukemias. Thus, donor-derived “alloreactive” NK cells are fundamental effectors in adult acute myeloid leukemia and in pediatric acute lymphoblastic leukemia patients undergoing haploidentical hematopoietic stem cell transplantation (HSCT). Alloreactive NK cells mediate killing of leukemia cells and patient’s dendritic cell, thus preventing respectively leukemic relapses and graft-vs-host responses. Cytofluorimetric analysis of KIRs expressed by NK cells allows to define the size of the alloreactive NK subset and the selection of the best potential donor. Recently, it has been shown that also the expression of activating KIRs, in particular the (C2-specific) KIR2DS1, may contribute to donor NK alloreactivity. It has also been established a correlation between the size of the alloreactive NK cell population and the clinical outcome. Notably, the alloreactive NK cells derived from donor’s hematopoietic stem cells are generated and persist in patients over time. The high survival rates of patients undergoing haploidentical HSCT highlight an important new reality in the setting of allograft performed to cure otherwise fatal leukemias. Novel approaches are in progress to further improve the clinical outcome based on the infusion of donor alloreactive NK cells either as a component of the transplanted cell population or as in vitro expanded NK cells.

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.

Human NK receptors: from the molecules to the therapy of high risk leukemias

Natural killer cells are important players of the innate immunity. In humans, they express HLA-class I-specific inhibitory receptors including the allotypic-specific KIR and various activating receptors. In most instances, in an autologous setting NK cells do not kill self cells. In contrast, in an allogeneic setting as the haploidentical hematopoietic stem cell transplantation to cure high risk leukemias, donor-derived NK cells may express inhibitory KIR that are not engaged by the HLA-class I alleles (KIR ligands) expressed by recipient cells. Such "alloreactive" NK cells may be responsible for the eradication of leukemia blasts escaping the preparative regimen, residual host dendritic cells and T lymphocytes, thus preventing leukemia relapse, GvHD and graft rejection, respectively. These NK-mediated effects result in a sharp improvement of the estimated 5 years survival.

Killer Ig–like receptor-mediated control of natural killer cell alloreactivity in haploidentical hematopoietic stem cell transplantation

Natural killer (NK) cells are key members of the innate immune system. In a self-environment, they sense and kill target cells lacking major histocompatibility complex class I molecules and release various cytokines on activation. The discovery of human leukocyte antigen (HLA) class I specific inhibitory receptors (including the allotype-specific killer immunoglobulin-like receptors), and of various activating receptors and their ligands, provided the basis for understanding the molecular mechanism of NK-cell activation and function, mainly resulting from the balance between activating and inhibitory signals. In an allogeneic setting, such as T cell–depleted haploidentical hematopoietic stem cell transplantation, NK cells may express inhibitory killer immunoglobulin-like receptors that are not engaged by any of the HLA class I alleles present on allogeneic cells. Such “alloreactive” NK cells greatly contribute both to eradication of leukemia blasts escaping the preparative regimen and to clearance of residual host dendritic cells and T lymphocytes (thus preventing graft-versus-host disease and graft rejection, respectively). Improved prevention of graft-versus-host disease might be achieved by redirecting to lymph nodes adoptively transferred, alloreactive NK cells by inducing CCR7-uptake in vitro. Recent studies suggested that, after immune-suppressive therapy, alloreactive NK cells from an HLA-haploidentical donor may prevent leukemia recurrence also in patients who have not received allogeneic hematopoietic stem cell transplantation.

Extending killer Ig-like receptor function: from HLA class I recognition to sensors of microbial products

Killer Ig-like receptors (KIRs) are human natural killer (NK) receptors that recognize allotypic determinants of human leukocyte antigen (HLA) class I. Inhibitory KIRs discriminate normal cells from tumour or virus-infected cells that have lost or reduced HLA class I expression. Donor NK cell "alloeffector" responses are exploited in haploidentical haematopoietic stem cell transplantation to treat leukaemia. NK cells also express several toll-like receptors (TLRs) that increase NK cell cytotoxicity and cytokine release in response to ligands. Surprisingly, KIR3DL2 binds the TLR ligand CpG-oligodexynucleotides, and together, they are co-internalized and translocated to TLR9-rich early endosomes. This novel KIR-associated function offers clues to understanding the NK cell response to microbial infection, and extends the role played by KIRs in immune defence.

Transplantation and innate immunity: the lesson of natural killer cells

Natural killer cells have been demonstrated to play a major role in mediating an anti-leukemia effect in patients given a T-cell depleted allogeneic hematopoietic stem cell transplantation from an HLA-haploidentical family donor. In particular, donor-derived natural killer cells, which are alloreactive (i.e. KIR/HLA mismatched) towards recipient cells, significantly contribute to the eradication of leukemia blasts escaping the preparative regimen to transplantation. A recent study on high-risk pediatric acute lymphoblastic leukemia refractory to chemotherapy further highlighted the importance of donors with alloreactive natural killer cells in haploidentical hematopoietic stem cell transplantation, as it demonstrated that these cells can emerge starting from the fourth-fifth month after the allograft and persist for many months. This study represents a major breakthrough in the cure of otherwise fatal leukemias, providing information on the best criteria for choosing the optimal donor.

Natural killer alloeffector responses in haploidentical hemopoietic stem cell transplantation to treat high-risk leukemias

Natural killer (NK) cells, a major cell type of the innate immunity, express surface receptors that regulate potent effector functions such as cytolytic activity and release of cytokines playing a central role in inflammatory response and immunoregulation. In this contribution, we briefly outline the major steps from the discovery of human leukocyte antigen (HLA)-class I-specific inhibitory receptors in humans to recent successful clinical applications in the cure of high-risk leukemias both in adults and in pediatric patients. A central role is played by 'alloreactive' NK cells originated from donor's CD 34(+) cells in eradicating leukemic cells in the setting of T-cell-depleted haploidentical hemopoietic stem cell transplantation. Because alloreactive NK cells play a central role also in preventing graft rejection and graft-vs-host disease, they may represent an ideal tool to treat patients affected by acute high-risk leukemias.

Activating and inhibitory killer immunoglobulin-like receptors (KIR) in haploidentical haemopoietic stem cell transplantation to cure high-risk leukaemias

A number of experimental studies have shown that natural killer (NK) cells can eliminate cancer cells and the mechanisms involved in this effect have been uncovered during the last two decades. Clinical data from haploidentical haematopoietic stem cell transplantation (haplo-HSCT) revealed that NK cells were responsible for remarkably favourable effects in both adult and paediatric high-risk leukaemias. NK receptors specific for major histocompatibility complex (MHC) class I molecules, including killer immunoglobulin (Ig)-like receptors (KIR) and CD94/NKG2A, play a major role in the anti-leukaemia effect (mediating either inhibitory or activating signals). Haplo- HSCT requires a heavy conditioning regimen for the patient and the use of large numbers of T cell-depleted HSC to be grafted. After transplantation, natural killer cells develop from HSC shortly after engraftment and may include 'alloreactive' NK cells that kill leukaemic cells and prevent graft-versus-host disease (GvHD). Alloreactive NK cells are characterized by the expression of KIR that are not engaged by any of the human leucocyte antigen (HLA) class I alleles expressed by the patient. Their generation is dependent upon the existence of a KIR/HLA class I mismatch between donor and recipient. Novel important information on the function and specificity of different KIR has been obtained recently by the analysis of donor-derived alloreactive NK cells in a cohort of paediatric patients given haplo-HSCT to cure acute, high-risk leukaemias.

Haploidentical hemopoietic stem cell transplantation for the treatment of high-risk leukemias: how NK cells make the difference

T-cell-depleted hematopoietic stem cell (HSC) transplantation from an HLA-haploidentical relative (Haplo HSCT) may represent a suitable and effective transplant option, as it is capable of rescuing not only adult patients with high-risk acute myeloid leukemias (AML) but also children with relapsed acute lymphoblastic leukemia (ALL), as shown by the two representative cases presented in this study. In Haplo HSCT, the anti-leukemia effect is mediated by "alloreactive" (i.e. KIR/HLA-mismatched) NK cells originated from donor HSCs. The availability of suitable KIR-specific monoclonal antibodies allows the prompt identification of alloreactive NK cell subsets as well as their quantification. This is important for selection of the most suitable donor and evaluation of the generation and persistence of these alloreactive NK cells after transplantation. In view of the favorable clinical outcome of children with chemo-resistant ALL, Haplo HSCT from an NK-alloreactive relative could become a first option in these high-risk leukemia patients.

NK cell tolerance and the maternal-fetal interface

Natural killer (NK) cells play a fundamental role in the innate immune response through their ability to secrete cytokines and kill target cells without prior sensitization. These effector functions are central to NK cell anti-viral and anti-tumor abilities. Due to their cytotoxic nature, it is vital that NK cells have the capacity to recognize normal self-tissue and thus prevent their destruction. In addition to their role in host defense, NK cells accumulate at the maternal-fetal interface and are thought to play a critical role during pregnancy. The close proximity of uterine NK (uNK) cells to fetal trophoblast cells of the placenta would seemingly lead to catastrophic consequences, as the trophoblast cells are semi-allogeneic. A fundamental enigma of pregnancy is that the fetal cells constitute an allograft but, in normal pregnancies, they are in effect not perceived as foreign and are not rejected by the maternal immune system. Although the mechanisms involved in achieving NK cell tolerance are becoming increasingly well-defined, further clarification is required, given the clinical implications of this work in the areas of infection, transplantation, cancer and pregnancy. Herein, we discuss several mechanisms of NK cell tolerance and speculate as to how they may apply to uNK cells at the maternal-fetal interface.

Allogeneic bone marrow transplantation in models of experimental autoimmune encephalomyelitis: evidence for a graft-versus-autoimmunity effect

Autologous hematopoietic stem cell transplantation (HSCT) is being explored in the treatment of severe multiple sclerosis (MS), and is based on the concept of "resetting" the immune system. The use of allogeneic HSCT may offer additional advantages, such as the replacement of the autoreactive immune compartment by healthy allogeneic cells and development of a graft-versus-autoimmunity (GVA) effect. However, in clinical practice, the genetic susceptibility to MS of allogeneic stem cell donors is generally unknown, and GVA may therefore be an important mechanism of action. Experimental autoimmune encephalomyelitis (EAE)-susceptible and -resistant mouse strains were used to determine the roles of genetic susceptibility, level of donor-chimerism, and alloreactivity in the therapeutic potential of syngeneic versus allogeneic bone marrow transplant (BMT) for EAE. After transplantation and EAE induction, animals were evaluated for clinical EAE and ex vivo myelin oligodendrocyte glycoprotein-specific proliferation. Early after BMT, both syngeneic and allogeneic chimeras were protected from EAE development. On the longer term, allogeneic but not syngeneic BMT conferred protection, but this required high-level donor-chimerism from EAE-resistant donors. Importantly, when EAE-susceptible donors were used, robust protection from EAE was obtained when active alloreactivity, induced by donor lymphocyte infusions, was provided. Our findings indicate the requirement of a sufficient level of donor-chimerism from a nonsusceptible donor in the therapeutic effect of allogeneic BMT. Importantly, the data indicate that, independently of genetic susceptibility, active alloreactivity is associated with a GVA effect, thereby providing new evidence to support the potential role of allogeneic BMT in the treatment of MS.

Efficacy and Limitations of Natural Killer Cell Depletion in Cyclophosphamide-Induced Tolerance

PURPOSE

We previously developed a cyclophosphamide (CP)-induced tolerance protocol, consisting of an intravenous injection of 1 x 10(8) donor spleen cells (SC) given on day 0 and an intraperitoneal injection of 200 mg/kg CP given on day 2. In the present study, we modified this protocol with natural killer cell (NK) depletion in recipient mice, and evaluated the efficacy of tolerance induction.

METHODS

We used B10.D2 (H-2d; IE+) and B10 (H-2b; IE-) mice as both donors and recipients. The recipient mice were treated with donor SC, CP, and donor bone marrow cells (BMCs) with or without NK depletion.

RESULTS

A higher level of mixed chimerism was achieved in the NK-depleted recipients. Survival of both the skin and heart donor grafts was significantly prolonged in the NK-depleted recipients. Donor reactive Vbeta11+ T cells were found at the same level as in untreated control mice. Pretreatment with recipient NK cell depletion was effective in inducing higher levels of donor mixed chimerism; however, permanent engraftment of donor bone marrow was not achieved.

CONCLUSION

Survival of donor grafts was remarkably prolonged in the NK cell-depleted group, but transplantation tolerance could not be induced. Our results suggest that NK cell depletion in CP-induced tolerance conditioning has some effect on the induction of donor-specific tolerance.

Licensing of natural killer cells by self-major histocompatibility complex class I

Natural killer (NK) cells have potent capacities to immediately kill cellular targets and produce cytokines that may potentially damage normal self-tissues unless they are kept in check. Such tolerance mechanisms are incompletely understood. Here we discuss recent studies suggesting that NK cells undergo a host major histocompatibility complex (MHC) class I-dependent functional maturation process, termed 'licensing'. Ironically, licensing directly involves inhibitory receptors that recognize target cell MHC class I molecules and block activation of NK cells in effector responses. This process results in two types of tolerant NK cells: functionally competent (licensed) NK cells, whose effector responses are inhibited by self-MHC class I molecules through the same receptors that conferred licensing, and functionally incompetent (unlicensed) NK cells.

How do natural killer cells find self to achieve tolerance?

Natural killer (NK) cells provide innate defense against tumors and infections by virtue of potent capacities to immediately kill cellular targets and produce cytokines. These effector functions may potentially damage normal self-tissues unless they are kept in check by tolerance mechanisms that need clarification. Here, we discuss recent studies indicating that the NK cells acquire functional competence directly through engagement of their MHC-specific receptors by self-MHC. Ironically, these receptors were first identified in terms of recognizing target cell MHC class I molecules and inhibiting NK cells in effector responses. Other studies of NK cell tolerance are also discussed. Although these studies begin to clarify the means by which NK cell tolerance is achieved, much more investigation is needed because NK cell tolerance is relevant to clinical observations in patients with infections and cancer.

NK Cell Receptors as Tools in Cancer Immunotherapy

Natural killer (NK) cells were identified 30 years ago based on their ability to "spontaneously" kill tumor cells. The basis for NK cell recognition and activation is due to a variety of receptors that bind to specific ligands on tumor cells and normal cells. Some of these receptors have the ability to inhibit NK cell function, and other receptors activate NK cell function. Therapeutic strategies for cancer therapy are being developed based on preventing NK cell inhibition or using NK cell receptors to activate NK cells or T cells. There are intriguing clinical data from studies of bone marrow transplantation that support the idea that preventing NK cell inhibition by human leukocyte antigen (HLA) class I molecules can be a means to promote graft-versus-leukemia (GvL) effects and limit graft-versus-host disease (GvHD) in acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) patients. Experimental findings also support the blockade of NK cell inhibitory receptors as a way to protect against leukemia relapse. It may be possible to use our knowledge of NK cell activating receptors and their ligands to immunize patients with modified tumor cells to promote beneficial NK cell responses and development of host antitumor cytotoxic T lymphocytes (CTLs). Finally, new data support the idea of using modified NK cell receptors as a means to target patients' T cells against their own tumor cells and induce long-term immunity against them. Tumors are essentially tissues that have overcome normal regulation mechanisms, and therefore the ability to distinguish normal cells from abnormal cells is a key part of selectively attacking tumor cells. NK cells have various receptor systems designed to recognize infected and abnormal cells. Understanding NK cell receptors and their recognition mechanisms provides new tools for the development of immunotherapies against cancer.

NK cells play a critical role in the regulation of class I-deficient hemopoietic stem cell engraftment: evidence for NK tolerance correlates with receptor editing

The role that NK cells play in the rejection of hemopoietic stem cell (HSC) and tolerance induction has remained controversial. In this study, we examined whether NK cells play a direct role in the rejection of HSC. Purified HSC from MHC class II-deficient mice engrafted readily in congenic mice, while HSC from class I-deficient donors (beta(2)-microglobulin(-/-) (beta(2)m(-/-))) failed to engraft. Recipient mice lacking CD8(+), CD4(+), or T cells also rejected HSC from class I-deficient donors, pointing directly to NK cells as the effector in rejection of HSC. Recipients, deficient in or depleted of NK cells, engrafted readily with beta(2)m(-/-) HSC. Expression of the activating Ly-49D and inhibitory Ly-49G2 receptors on recipient NK cells was significantly decreased in these beta(2)m(-/-)-->B6 chimeras, and the proportion of donor NK cells expressing Ly-49D was also significantly decreased. Notably, beta(2)m(-/-) chimeras accepted beta(2)m(-/-) HSC in second transplants, demonstrating that NK cells in the chimeras had been tolerized to beta(2)m(-/-). Taken together, our data demonstrate that NK cells play a direct role in the regulation of HSC engraftment, and down-regulation and/or deletion of specific NK subsets in mixed chimeras can contribute to the induction of NK cell tolerance in vivo. Moreover, our data show that bone marrow-derived elements significantly contribute to NK cell development and tolerance.

The regulatory functions of Ly-49A, Ly-49D and Ly-49G2 on NK cells in the recognition and rejection of the alloantigen in vivo

Ly-49A is an inhibitory receptor that binds H-2Dd and H-2Dk. The downregulation of Ly-49A is thought to mediate NK self tolerance in vivo. In this study, we analyzed the regulation of Ly-49A, D and G2 on NK cells in an in vivo rejection model. After injection with 1 x 10(8) B10.D2 spleen cells (SC) into B 10 mice, we found Ly-49A downregulated within 3 h on NK cells of B10 mice, whereas expressions of Ly-49D and G2 were augmented. To investigate effects of different expression patterns of Ly-49 receptors on NK cells, Ly-49A, D or G2-depleted B10 mice were inoculated with B10.D2 SC. NK cells from SC of Ly-49A-depleted and B10.D2 SC-injected B10 mice showed enhanced cytotoxicity to Dd-positive targets in vitro. Furthermore, reduced numbers of B10.D2 SC were observed in Ly-49A or G2-depleted B10 mice, whereas increased numbers of B10.D2 SC were observed in Ly-49D-depleted B10 mice after inoculation with B10.D2 SC in vivo. These findings indicated that the downregulation of Ly-49A and the augmentation of Ly-49D expression may mediate NK cells to recognize and kill Dd antigen efficiently. In conclusion, each Ly-49 isoform may play independent roles in the regulation of activation or inhibition on NK cells.

Influence of Maternal-Fetal Histocompatibility and MHC Zygosity on Maternal Microchimerism

To investigate the relationship between maternal-fetal histocompatibility and maternal microchimerism, we developed a sensitive quantitative PCR assay for the neomycin resistance gene (neoR), and, in a mouse model system, used neoR as a noninherited maternal allele marker of maternal cells to detect and quantitate maternal microchimerism in tissues of neoR(-/-) N2 backcross progeny of (neoR(+/-))F(1) females mated with neoR(-/-) males. Using this approach, we obtained evidence for the presence of chimeric maternal cells in the brain, spleen, and thymus of all weanling and adult mice so tested. The numbers of chimeric maternal cells present in the spleen did not differ significantly from those in the thymus regardless of age or maternal-fetal histocompatibility. At all ages, brain tissue had higher level of maternal microchimerism than lymphoid tissue in mice MHC identical with their mothers, but the levels were similar in mice MHC disparate with their mothers. The levels of chimeric maternal cells in both brain and lymphoid tissue of mice with homozygous syngenicity and maternal allogenicity were similar, and tended to be higher than tissue-specific levels in mice with either combined maternal-fetal allogenicity or heterozygous syngenicity. Thus, MHC homozygous progeny had higher levels of maternal microchimerism than MHC heterozygous progeny. We conclude that normal mice possess small numbers of maternal cells in spleen, thymus, brain, and probably most other tissues, and that maternal-fetal histocompatibility influences the levels of these cells by mechanisms related to MHC zygosity of the progeny.

Killer immunoglobulin-like receptors

Killer Ig-like receptors (KIRs) are surface inhibitory receptors specific for allelic forms of human leukocyte antigen (HLA) class I molecules, which are expressed by natural killer (NK) cells and a subset of T lymphocytes. Upon engagement with HLA class I molecules, KIRs block NK cell activation and function. Cells lacking HLA class I molecules are promptly killed by NK cells because of the predominant effect of several activating NK receptors. The NK-mediated killing of these cells might represent an important defence mechanism, antagonizing spreading of pathogens and tumours. Evidence has been accumulated that KIR-encoding genes have evolved and diversified rapidly in primates and in humans. Similar to HLA loci, KIR sequences are highly polymorphic and, moreover, KIR haplotypes greatly vary in the number of the type of genes they contain. KIR gene expression is regulated by mechanisms of DNA methylation. As recently shown, the HLA class I regulated control of NK cell function can be exploited in an allogeneic bone marrow transplantation setting to eradicate acute myeloid leukaemias.

Cytokines and cytotoxic pathways in engraftment resistance to purified allogeneic hematopoietic stem cells

The way that allogeneic hematopoietic cells are rejected is not completely understood. Regimen-resistant populations, including natural killer (NK) cells and lymphocytes, are thought to mediate the allograft barrier. In this report, the mechanism by which recipient cell populations resist engraftment of purified allogeneic hematopoietic stem cells (HSCs) was examined in mice. To define the immunoregulatory pathways involved in allogeneic hematopoietic cell resistance, HSC transplantations were performed in immune-defective recipients. Recipients were wild-type mice treated with alpha-NK cell antibodies or knockout strain mice lacking expression of CD8, perforin, Fas ligand, or 1 of the following cytokines: tumor necrosis factor alpha, transforming growth factor beta, interferon gamma, interleukin 4, or interleukin 10. Elimination of a single cytotoxic pathway was ineffective in reducing engraftment resistance, although mice treated with a polyclonal antibody that recognizes NK-cell determinants or CD8 expression showed a profound reduction in the engraftment barrier. Posttransplantation chimerism analysis revealed regeneration of host hematopoiesis in some experimental groups. These studies show, for the first time, that elimination of selected cytokines does not alter allogeneic hematopoietic resistance. Furthermore, the chimerism data reinforce the importance of competition for HSC niches in conjunction with immune mechanisms in resistance to long-term HSC engraftment.

Dietary dehydroepiandrosterone inhibits bone marrow and leukemia cell transplants: role of food restriction

Dietary dehydroepiandrosterone (DHEA) inhibits the proliferation of syngeneic bone marrow cells (BMC) infused into lethally irradiated mice. Potential mechanisms for suppression of hematopoiesis were evaluated and the findings were as follows: (i) depletion of NK, T, B or macrophage cells failed to reverse suppression by DHEA; (ii) stem cell stimulation by erythropoietin, growth hormone, interleukin-2, Friend leukemia virus, or cyclophosphamide failed to reverse suppression; (iii) supplementation of fatty acids, mevalonate, or deoxyribonucleotides, which are dependent upon glucose-6-phosphate dehydrogenase function, did not enhance BMC growth in mice fed DHEA; (iv) DHEA downstream metabolites 4-androstenedione and 17beta-estradiol, as well as the synthetic steroid, 16alpha-chloroepiandrosterone (but not testosterone or 5-androstene-3beta,17beta-diol), also inhibited BMC growth. Tamoxifen antagonized the effects of 17beta-estradiol but not DHEA; (v) dietary DHEA causes hypothermia, but housing of DHEA-fed mice at 34 degrees C to maintain normal body temperature did not reverse suppression; (vi) DHEA leads to a decrease in food intake in rodents. Pair-feeding control diet to mice fed DHEA mimicked the effects of dietary DHEA; (vii) adrenalectomy and orchiectomy decrease the levels of stress and sex hormones, respectively. Neither procedure affected the ability of food restriction or DHEA feeding to inhibit hematopoiesis; (viii) growth of GR-3 NM pre-B leukemia cells in unirradiated mice was also suppressed by DHEA or food restriction. We conclude that DHEA, by reducing food intake in mice, inhibits bone marrow and leukemia cell growth. The precise mechanism(s) by which reduced food intake per se inhibits hematopoiesis is not known, but may involve an increased rate of cellular apoptosis.

Low Number of H-2Dd-Negative Haematopoietic Cells in Mixed Bone Marrow Chimeras Convey In Vivo Tolerance to H-2Dd-Negative Cells But Fail to Prevent Resistance to H-2Dd-Negative Leukaemia

Natural killer (NK) cells kill cells lacking self major histocompatibility complex (MHC) class I. This missing self reactivity is beneficial in haploidentical bone marrow transplantations to cure leukaemia, in which donor-derived NK cells reject MHC disparate leukaemia cells and prevent relapse. To understand the role of NK cells in transplantation, we have studied NK cell tolerance in mice receiving mixed bone marrow transplants with limiting number of the MHC disparate component. Using an MHC class I (Dd) transgenic mouse model, we generated bone marrow chimeras carrying mixtures of Dd-positive and -negative cells. NK reactivity against Dd-negative cells (missing self) was assayed by outgrowth of lymphoma cells, stability of the chimerism in vivo and killing of Concanavalin A blasts in vitro. Up to 20% Dd-negative haematopoietic cells reduced, but did not abrogate, rejection of Dd-negative tumours and killing of Dd-negative T-cell blasts. In contrast, the ratios between Dd-positive and -negative cells were stable in vivo, suggesting tolerance to normal cells. Our data suggest that NK cell tolerance to normal cells and tumours in mixed MHC environments is differentially regulated, tolerance to normal cells being more easily induced. These results are important in relation to the role of NK cells in antileukaemic reactions after bone marrow transplantation.

The immunobiology of natural killer cells and bone marrow allograft rejection

Natural killer (NK) cells mediate the acute rejection of bone marrow cell (BMC) allografts, but not solid tissue grafts, in lethally irradiated mice. However, the mechanisms underlying this capability for rejecting BMC remain unclear. NK cells express (1) inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules and (2) activating receptors with diverse specificities. Inhibitory NK receptors confer to NK cells the ability to discriminate between MHC class I-positive and -negative target cells and are therefore involved in the control of NK cell tolerance to self, as well as in the elimination of cells that have downregulation of MHC class I molecules. Preclinical studies in mice have provided good evidence that subsets of NK cells that bear different combinations of both inhibitory and activating Ly49 receptors can interact with each other and target specific BMC rejection, as well as NK cell responses toward tumor cells. Recent clinical studies have also shown that the use of killer cell immunoglobulin-like receptor ligand incompatibility in patients with leukemia who received hematopoietic stem cell transplants correlated not only with the elimination of graft rejection, but also with eradication of tumor and prevention of graft-versus-host disease; this offers a significant advantage for survival. In this review, we attempt to bring together literature regarding the biology of NK cells and discuss the current issues in bone marrow transplantation and the potential clinical role of NK cell alloreactivity in the efficacy of this procedure for immunotherapy of cancer and infectious states.

Alloreactive killer cells: hindrance and help for haematopoietic transplants

Haematopoietic-cell transplantation is a treatment for leukaemia and lymphoma. To reduce the incidence of graft-versus-host disease (GVHD) caused by transplanted T cells, donors and recipients are HLA matched. For patients for whom a matched donor is not available, one option is transplantation from an HLA-mismatched relative who shares one HLA haplotype. This procedure is distinguished by the use of a stronger conditioning regimen for the patient and of a T-cell-depleted graft containing numerous stem cells. After transplantation, natural killer cells are prevalent, and they can include alloreactive cells that kill tumour cells and prevent GVHD. The alloreactions seem to be determined by the mismatched HLA class I ligands and their killer-cell immunoglobulin-like receptors.

Analysis of donor NK and T cells infused in patients undergoing MHC-matched allogeneic hematopoietic transplantation

We retrospectively analyzed the percentages and absolute numbers of T cells, natural killer (NK) cells and NK cell subsets in cryopreserved samples of either bone marrow or blood non-T cell-depleted allogeneic MHC-matched hematopoietic grafts. Using flow cytometry, we found higher numbers of NK cells in aphereses than in bone marrow collections. We further investigated the distribution of NK cell subsets, defined by the cell surface expression of MHC class I-specific receptors, in these allogeneic grafts. The distribution of NK cell subsets from the two different origins were similar, with the exception of the CD158a/h(+) NK cell subset, whose size appeared to be smaller in bone marrow. The search for relations between the numbers of infused cells and post-transplantation events demonstrated that increasing numbers of infused T cells but not NK cells are related with decreased overall survival. Our study highlights the toxicity of infused T cells but not NK cells in allogeneic MHC-matched hematopoietic grafts. These data pave the way for further trials to investigate the effect of NK cell infusion in MHC-matched allogeneic transplantation, and in particular whether ex vivo NK cell expansion and activation may enhance the anti-tumoral effect of the procedure and decrease its morbidity.

CD8(+), alphabeta-TCR(+), and gammadelta-TCR(+) cells in the recipient hematopoietic environment mediate resistance to engraftment of allogeneic donor bone marrow

Historically, conditioning for engraftment of hematopoietic stem cells has been nonspecific. In the present study, we characterized which cells in the recipient hematopoietic microenvironment prevent allogeneic marrow engraftment. Mice defective in production of alphabeta-TCR(+), gammadelta-TCR(+), alphabeta- plus gammadelta-TCR(+), CD8(+), or CD4(+) cells were transplanted with MHC-disparate allogeneic bone marrow. Conditioning with 500 cGy total body irradiation (TBI) plus a single dose of cyclophosphamide (CyP) on day +2 establishes chimerism in normal recipients. When mice were conditioned with 300 cGy TBI plus a single dose of CyP on day +2, all engrafted, except wild-type controls and those defective in production of CD4(+) T cells. Mice lacking both alphabeta- and gammadelta-TCR(+) cells engrafted without conditioning, suggesting that both alphabeta- and gammadelta-TCR T cells in the host play critical and nonredundant roles in preventing engraftment of allogeneic bone marrow. CD8 knockout (KO) mice engrafted without TBI, but only if they received CyP on day +2 relative to the marrow infusion, showing that a CD8(-) cell was targeted by the CyP conditioning. The CD8(+) cell effector function is mechanistically different from that for conventional T cells, and independent of CD4(+) T helper cells because CD4 KO mice require substantially higher levels of conditioning than the other KO phenotypes. These results suggest that a number of cell populations with different mechanisms of action mediate resistance to engraftment of allogeneic marrow. Targeting of specific recipient cellular populations may permit conditioning approaches to allow mixed chimerism with minimal morbidity and could potentially avoid the requirement for myelotoxic agents altogether.

Oxygen as a regulator of cellular phenotypes in pregnancy and cancer

Cellular phenotype is determined by genetic and microenvironmental factors. There is evidence that tissue oxygenation status is one of the microenvironmental factors regulating cellular behaviour. Both normal and pathological processes such as blastocyst implantation in the uterus, placentation, and rapidly growing tumours occur under conditions characterized by relatively low oxygen levels. In this review, we address the effects of low oxygen concentrations on the phenotype of trophoblast and cancer cells. We provide evidence that oxygenation levels play an important role in the regulation of normal and pathological cellular invasiveness as it occurs during trophoblast invasion of the uterus and in tumour progression and metastasis, drug resistance in cancer, and antitumour activity of natural killer cells of the immune system.

Regulation of the natural killer cell receptor repertoire

Natural killer cells express inhibitory receptors specific for MHC class I proteins and stimulatory receptors with diverse specificities. The MHC-specific receptors discriminate among different MHC class I alleles and are expressed in a variegated, overlapping fashion, such that each NK cell expresses several inhibitory and stimulatory receptors. Evidence suggests that individual developing NK cells initiate expression of inhibitory receptor genes in a sequential, cumulative, and stochastic fashion. Superimposed on the receptor acquisition process are multiple education mechanisms, which act to coordinate the stimulatory and inhibitory specificities of developing NK cells. One process influences the complement of receptors expressed by individual NK cells. Other mechanisms may prevent NK cell autoaggression even when the developing NK cell fails to express self-MHC-specific inhibitory receptors. Together, these mechanisms ensure a self-tolerant and maximally discriminating NK cell population. Like NK cells, a fraction of memory phenotype CD8(+) T cells, as well as other T cell subsets, express inhibitory class I--specific receptors in a variegated, overlapping fashion. The characteristics of these cells suggest that inhibitory receptor expression may be a response to prior antigenic stimulation as well as to poorly defined additional signals. A unifying hypothesis is that both NK cells and certain T cell subsets initiate expression of inhibitory receptors in response to stimulation.

Cellular immunotherapy of malignancies using the clonal natural killer cell line NK-92

For years activated natural killer (A-NK) cells have been explored with respect to their efficacy in anticancer therapy, but, except for some anectdotal reports, no clear clinical benefit has been shown. However, as the understanding about the interactions of NK cells and tumor cells advances, the use of A-NK cells might be revisited with more sophisticated approaches that pay tribute to mechanisms which allow tumor cells to escape immune surveillance. Here the highly cytotoxic NK cell line NK-92 seems to be an attractive alternative for use in adoptive immunotherapy, because it was shown to exhibit substantial antitumor activity against a wide range of malignancies in vitro as well as in xenografted SCID mice. NK-92 cells are characterized by an almost complete lack of killer cell immunglobulin-like receptors (KIRs) yet conserved ability to perforin and granzyme B-mediated cytolytic activity, which make them unique among the few established NK and T cell-like cell lines. NK-92 is the only natural killer cell line that has entered clinical trials. Here we discuss the current status of development of this cell line for adoptive immunotherapy (AIT) of malignancies and review our first clinical experience in patients with advanced cancer who have received repeated transfusions of irradiated NK-92 in a phase I/II trial. Also we discuss issues that address safety aspects of immunotherapy with clonal cell lines and describe further manipulations, which hold the potential of significantly improving the clinical outcome of AIT with NK-92.

Long-term donor chimerism after MHC (RT1) mismatched bone marrow transplantation in the rat: the role of host alloreactive NK cells

We have investigated the role of major histocompatibility complex (MHC) (RT1) disparities in the engraftment of bone marrow (BM) cells after whole body irradiation of rats. Mononuclear BM cells from PVG.RT7.2 (RT1c) rats were injected i.v. into sublethally (10Gy) whole body irradiated PVG (RT1c) rats and RT1 congenic and recombinant PVG rats. Repopulation of the BM, spleen, and blood with donor cells was assessed by FACS analysis of cells labelled with the fluorescein isothiocyanate (FITC)-labelled HIS41 monoclonal antibody (MoAb) against the RT7.2 marker. In RT1 matched (PVG.RT7.2 --> PVG) and RT1-mismatched combinations (PVG.RT7.2 --> PVG.1AV1), where radioresistant host natural killer (NK) cells could not recognize the BM inoculum as foreign, a donor chimerism close to 100% was observed after 6-8 weeks. However, in rat strain combinations where host NK cells could recognize an RT1 mismatch, almost no donor cells survived, and the rats were repopulated with leukocytes of host origin. In intra-MHC recombinant rat strains the element determining rejection or acceptance of the allograft mapped to the RT1-B/D-C/E/M region in PVG.R8 and PVG.R23 rats, in accordance with the patterns of NK alloreactivity in these strain combinations. NK cells may therefore be a primary obstacle to successful allogeneic BM engraftment in this model.

Expression of Ly49E and CD94/NKG2 on fetal and adult NK cells

Murine NK cells express inhibitory receptors belonging to the Ly49 and CD94/NKG2 family. Ly49E and CD94 are the only NK cell receptor transcripts detectable in fetal NK cells. Still unproved is the surface expression of Ly49E on NK cells. Here we generated two novel mAbs, a mAb recognizing Ly49E with cross-reactivity to Ly49C, and a mAb against NKG2A/C/E. Ly49E was immunoprecipitated as a disulfide-linked homodimer with 46-kDa subunits. Removal of N-linked carbohydrates revealed a 31-kDa protein backbone. NKG2A was immunoprecipitated as a 38-kDa protein. Although the frequency of fetal NK cells expressing Ly49E was higher than 25%, it decreased drastically from 2 wk after birth. Phenotypic analysis showed that approximately 90% of fetal NK cells and approximately 50% of adult NK cells express high levels of CD94/NKG2. The remaining 50% of adult NK cells expressed low surface levels of CD94/NKG2. Expression of Ly49E and CD94/NKG2 was not restricted to NK cells, but was also observed on NK T and memory T cells. Functional analysis showed that sorted Ly49E(+) and CD94/NKG2(+) fetal NK cells could discriminate between MHC class I-positive and MHC class I-negative tumor cells. We also demonstrated that Ly49E becomes phosphorylated following pervanadate stimulation of fetal NK cells. The expression levels of Ly49E and CD94/NKG2 were similar in wild-type compared with beta(2)-microglobulin(-/-) mice. In conclusion, generation of mAbs against Ly49E and NKG2 extended the phenotypic and functional characterization of NK cells.

Transplantation of a fetus with paternal Thy-1(+)CD34(+)cells for chronic granulomatous disease

A fetus diagnosed with X-linked chronic granulomatous disease was transplanted with Thy-1(+)CD34(+) cells of paternal origin. The transplant was performed at 14 weeks gestation by ultrasound guided injection into the peritoneal cavity. The fetus was delivered at 38 weeks gestation after an otherwise uneventful pregnancy. Umbilical cord blood was collected and used to determine the level of peripheral blood chimerism as well as levels of functional engrafted cells. Flow cytometry was used to detect donor leukocytes identified as HLA-A2(-)B7(+) cells, whereas recipient cells were identified as HLA-A2(+)B7(-) cells. No evidence of donor cell engraftment above a level of 0.01% was found. PCR was used to detect HLA-DRB1*15(+) donor cells among the recipient's HLA-DRB1*15(-) cells, but no engraftment was seen with a sensitivity of 1:1000. The presence of functional, donor-derived neutrophils was assessed by flow cytometry using two different fluorescent dyes that measure reactive oxygen species generated by the phagocyte NADPH oxidase. No evidence of paternal-derived functional neutrophils above a level of 0.15% was observed. Peripheral blood and bone marrow samples were collected at 6 months of age. Neither sample showed engraftment by HLA typing using both flow cytometry and PCR. Functional phagocytes were also not observed. Furthermore, no indication of immunological tolerance specific for the donor cells was indicated by a mixed lymphocyte reaction assay performed at 6 months of age. While there appears to be no engraftment of the donor stem cells, the transplant caused no harm to the fetus and the child was healthy at 6 months of age. Analyses of fetal tissues, obtained from elective abortions, revealed that CD3(+) T cells and CD56(+)CD3(-) NK cells are present in the liver at 8 weeks gestation and in the blood by 9 weeks gestation. The presence of these lymphocytes may contribute to the lack of donor cell engraftment in the human fetus.

NK-mediated elimination of mutant lymphocytes that have lost expression of MHC class I molecules

Mutant cells generated in vivo can be eliminated when mutated gene products are presented as altered MHC/peptide complexes and recognized by T cells. Diminished expression of MHC/peptide complexes enables mutant cells to escape recognition by T cells. In the present study, we tested the hypothesis that mutant lymphocytes lacking expression of MHC class I molecules are eliminated by autologous NK cells. In H-2b/k F1 mice, the frequency of H-2Kb-negative T cells was higher than that of H-2Kk-negative T cells. The frequency of H-2K-deficient T cells increased transiently after total body irradiation. During recovery from irradiation, H-2Kk-negative T cells disappeared more rapidly than H-2Kb-negative T cells. The disappearance of H-2K-deficient T cells was inhibited by administration of Ab against asialo-GM1. H-2Kk-negative T cells showed higher sensitivity to autologous NK cells in vitro than H-2Kb/k heterozygous or H-2Kb-negative T cells. Adding syngeneic NK cells to in vitro cultures prevented emergence of mutant cells lacking H-2Kk expression but had little effect on the emergence of mutant cells lacking H-2Kb expression. Results in the H-2b/k F1 strain correspond with the sensitivity of parental H-2-homozygous cells in models of marrow graft rejection. In H-2b/d F1 mice, there was no significant difference between the frequencies of H-2Kb-negative and H-2Kd-negative T cells, although the frequencies of mutant cells were different after radiation exposure among the strains examined. H-2b/d F1 mice also showed rapid disappearance of the mutant T cells after irradiation, and administration of Ab against asialo-GM1 inhibited the disappearance of H-2K-deficient T cells in H-2b/d F1 mice. Our results provide direct evidence that autologous NK cells eliminate mutant cell populations that have lost expression of self-MHC class I molecules.

Vigorous allograft rejection in the absence of danger

Tolerance to self is a necessary attribute of the immune system. It is thought that most autoreactive T cells are deleted in the thymus during the process of negative selection. However, peripheral tolerance mechanisms also exist to prevent development of autoimmune diseases against peripheral self-Ags. It has been proposed that T cells develop tolerance to peripheral self-Ags encountered in the absence of inflammation or "danger" signals. We have used immunodeficient Rag 1-/- mice to study the response of T cells to neo-self peripheral Ags in the form of well-healed skin and vascularized cardiac allografts. In this paper we report that skin and cardiac allografts without evidence of inflammation are vigorously rejected by transferred T cells or when recipients are reconstituted with T cells at a physiologic rate by nude bone graft transplantation. These results provide new insights into the role of inflammation or "danger" in the initiation of T cell-dependent immune responses. These findings also have profound implications in organ transplantation and suggest that in the absence of central deletional tolerance, peripheral tolerance mechanisms are not sufficient to inhibit alloimmune responses even in the absence of inflammation or danger.

Ly49I NK cell receptor transgene inhibition of rejection of H2b mouse bone marrow transplants

The Ly49 family of genes encode NK cell receptors that bind class I MHC Ags and transmit negative signals if the cytoplasmic domains have immunoregulatory tyrosine-based inhibitory motifs (ITIMs). 5E6 mAbs recognize Ly49C and Ly49I receptors and depletion of 5E6+ NK cells prevents rejection of allogeneic or parental-strain H2d bone marrow cell (BMC) grafts. To determine the function of the Ly49I gene in the rejection of BMC grafts, we transfected fertilized eggs of FVB mice with a vector containing DNA for B6 strain Ly49I (Ly49IB6). Ly49IB6 is ITIM+ and is recognized by 5E6 as well as Ly49I-specific 8H7 mAbs. Normal FVB H2q mice reject H2b but not H2d BMC allografts, and the rejection of H2b BMC was inhibited partially by anti-NK1.1 and completely by anti-asialo GM1, but not by anti-CD8, Abs. In FVB mice, NK1.1 is expressed on only 60% NK cells. FVB. Ly49IB6 hosts failed to reject H2d or H2b BMC, but did reject class I-deficient TAP-1-/- BMC, indicating that NK cells were functional. Nondepleting doses of anti-Ly49I Abs reversed the acceptance of H2b BMC by FVB.Ly49IB6 mice. FVB.Ly49IB6+/- mice were crossed and back-crossed with 129 mice-H2b, 5E6-, poor responders to H2d BMC grafts. While transgene-negative H2b/q F1 or first-generation back-crossed mice rejected H2b marrow grafts (hybrid resistance), transgene-positive mice did not. Thus B6 strain Ly49I receptors transmit inhibitory signals from H2b MHC class I molecules. Moreover, Ly49IB6 has no positive influence on the rejection of H2d allografts.