Evidence for a similar or common mechanism for natural killer cell activity and resistance to hemopoietic grafts

Comprehensive Insight into the Functional Roles of NK and NKT Cells in HTLV-1-Associated Diseases and Asymptomatic Carriers

Human T cell leukemia virus type 1 (HTLV-1) is the first human oncogenic retrovirus to be discovered and causes two major diseases: a progressive neuro-inflammatory disease, termed HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP), and an aggressive malignancy of T lymphocytes known as adult T cell leukemia (ATL). Innate and acquired immune responses play pivotal roles in controlling the status of HTLV-1-infected cells and such, the outcome of HTLV-1 infection. Natural killer cells (NKCs) are the effector cells of the innate immune system and are involved in controlling viral infections and several types of cancers. The ability of NKCs to trigger cytotoxicity to provide surveillance against viruses and cancer depends on the balance between the inhibitory and activating signals. In this review, we will discuss NKC function and the alterations in the frequency of these cells in HTLV-1 infection.

Five decades of natural killer cell discovery

The first descriptions of "non-specific" killing of tumor cells by lymphocytes were reported in 1973, and subsequently, the mediators of the activity were named "natural killer" (NK) cells by Rolf Kiessling and colleagues at the Karolinska Institute in 1975. The activity was detected in mice, rats, and humans that had no prior exposure to the tumors, major histocompatibility complex (MHC) antigen matching of the effectors and tumor cells was not required, and the cells responsible were distinct from MHC-restricted, antigen-specific T cells. In the ensuing five decades, research by many labs has extended knowledge of NK cells beyond an in vitro curiosity to demonstrate their in vivo relevance in host defense against tumors and microbial pathogens and their role in regulation of the immune system. This brief Perspective highlights a timeline of a few selected advancements in NK cell biology from a personal perspective of being involved in this quest.

Advances in immune therapies in hematological malignancies

Immunotherapy in cancer takes advantage of the exquisite specificity, potency, and flexibility of the immune system to eliminate alien tumor cells. It involves strategies to activate the entire immune defense, by unlocking mechanisms developed by tumor cells to escape from surrounding immune cells, as well as engineered antibody and cellular therapies. What is important to note is that these are therapeutics with curative potential. The earliest example of immune therapy is allogeneic stem cell transplantation, introduced in 1957, which is still an important modality in hematology, most notably in myeloid malignancies. In this review, we discuss developmental trends of immunotherapy in hematological malignancies, focusing on some of the strategies that we believe will have the most impact on future clinical practice in this field. In particular, we delineate novel developments for therapies that have already been introduced into the clinic, such as immune checkpoint inhibition and chimeric antigen receptor T-cell therapies. Finally, we discuss the therapeutic potential of emerging strategies based on T-cell receptors and adoptive transfer of allogeneic natural killer cells.

Engineered cellular immunotherapies in cancer and beyond

This year marks the tenth anniversary of cell therapy with chimeric antigen receptor (CAR)-modified T cells for refractory leukemia. The widespread commercial approval of genetically engineered T cells for a variety of blood cancers offers hope for patients with other types of cancer, and the convergence of human genome engineering and cell therapy technology holds great potential for generation of a new class of cellular therapeutics. In this Review, we discuss the goals of cellular immunotherapy in cancer, key challenges facing the field and exciting strategies that are emerging to overcome these obstacles. Finally, we outline how developments in the cancer field are paving the way for cellular immunotherapeutics in other diseases.

A NK Cell Odyssey: From Bench to Therapeutics Against Hematological Malignancies

In 1975 two independent groups noticed the presence of immune cells with a unique ability to recognize and eliminate transformed hematopoietic cells without any prior sensitization or expansion of specific clones. Since then, NK cells have been the axis of thousands of studies that have resulted until June 2021, in more than 70 000 publications indexed in PubMed. As result of this work, which include approaches in vitro, in vivo, and in natura, it has been possible to appreciate the role played by the NK cells, not only as effectors against specific pathogens, but also as regulators of the immune response. Recent advances have revealed previous unidentified attributes of NK cells including the ability to adapt to new conditions under the context of chronic infections, or their ability to develop some memory-like characteristics. In this review, we will discuss significant findings that have rule our understanding of the NK cell biology, the developing of these findings into new concepts in immunology, and how these conceptual platforms are being used in the design of strategies for cancer immunotherapy.

NK Cell-Mediated Processing Of Chlamydia psittaci Drives Potent Anti-Bacterial Th1 Immunity

Natural killer (NK) cells are innate immune cells critically involved in the early immune response against various pathogens including chlamydia. Here, we demonstrate that chlamydia-infected NK cells prevent the intracellular establishment and growth of the bacteria. Upon infection, they display functional maturation characterized by enhanced IFN-γ secretion, CD146 induction, PKCϴ activation, and granule secretion. Eventually, chlamydia are released in a non-infectious, highly immunogenic form driving a potent Th1 immune response. Further, anti-chlamydial antibodies generated during immunization neutralize the infection of epithelial cells. The release of chlamydia from NK cells requires PKCϴ function and active degranulation, while granule-associated granzyme B drives the loss of chlamydial infectivity. Cellular infection and bacterial release can be undergone repeatedly and do not affect NK cell function. Strikingly, NK cells passing through such an infection cycle significantly improve their cytotoxicity. Thus, NK cells not only protect themselves against productive chlamydial infections but also actively trigger potent anti-bacterial responses.

Immunity to the Dual Threat of Silica Exposure and Mycobacterium tuberculosis

Exposure to silica and the consequent development of silicosis are well-known health problems in countries with mining and other dust producing industries. Apart from its direct fibrotic effect on lung tissue, chronic and immunomodulatory character of silica causes susceptibility to tuberculosis (TB) leading to a significantly higher TB incidence in silica-exposed populations. The presence of silica particles in the lung and silicosis may facilitate initiation of tuberculous infection and progression to active TB, and exacerbate the course and outcome of TB, including prognosis and survival. However, the exact mechanisms of the involvement of silica in the pathological processes during mycobacterial infection are not yet fully understood. In this review, we focus on the host's immunological response to both silica and Mycobacterium tuberculosis, on agents of innate and adaptive immunity, and particularly on silica-induced immunological modifications in co-exposure that influence disease pathogenesis. We review what is known about the impact of silica and Mycobacterium tuberculosis or their co-exposure on the host's immune system, especially an impact that goes beyond an exclusive focus on macrophages as the first line of the defense. In both silicosis and TB, acquired immunity plays a major role in the restriction and/or elimination of pathogenic agents. Further research is needed to determine the effects of silica in adaptive immunity and in the pathogenesis of TB.

Immune selection during tumor checkpoint inhibition therapy paves way for NK-cell "missing self" recognition

The ability of NK cells to specifically recognize cells lacking expression of self-MHC class I molecules was discovered over 30 years ago. It provided the foundation for the "missing self" hypothesis. Research in the two past decades has contributed to a detailed understanding of the molecular mechanisms that determine the specificity and strength of NK cell-mediated "missing self" responses to tumor cells. However, in light of the recent remarkable breakthroughs in clinical cancer immunotherapy, the cytolytic potential of NK cells still remains largely untapped in clinical settings. There is abundant evidence demonstrating partial or complete loss of HLA class I expression in a wide spectrum of human tumor types. Such loss may result from immune selection of escape variants by tumor-specific CD8 T cells and has more recently also been linked to acquired resistance to checkpoint inhibition therapy. In the present review, we discuss the early predictions of the "missing self" hypothesis, its molecular basis and outline the potential for NK cell-based adoptive immunotherapy to convert checkpoint inhibitor therapy-resistant patients into clinical responders.

Immunological Basis of Bone Marrow Failure after Allogeneic Hematopoietic Stem Cell Transplantation

Bone marrow failure (BMF) syndromes are severe complications of allogeneic hematopoietic stem cell transplantation (allo-HSCT). In this paper, we distinguish two different entities, the graft failure (GF) and the poor graft function (PGF), and we review the current understanding of the interactions between the immune and hematopoietic compartments in these conditions. We first discuss how GF occurs as the result of classical alloreactive immune responses mediated by residual host cellular and humoral immunity persisting after conditioning and prevented by host and donor regulatory T cells. We next summarize the current knowledge about the contribution of inflammatory mediators to the development of PGF. In situations of chronic inflammation complicating allo-HSCT, such as graft-versus-host disease or infections, PGF seems to be essentially the result of a sustained impairment of hematopoietic stem cells (HSC) self-renewal and proliferation caused by inflammatory mediators, such as interferon-γ (IFN-γ) and tumor necrosis factor-α, and of induction of apoptosis through the Fas/Fas ligand pathway. Interestingly, the production of inflammatory molecules leads to a non-MHC restricted, bystander inhibition of hematopoiesis, therefore, representing a promising target for immunological interventions. Finally, we discuss immune-mediated impairment of bone marrow microenvironment as a potential mechanism hampering hematopoietic recovery. Better understanding of immunological mechanisms responsible for BMF syndromes after allo-HSCT may lead to the development of more efficient immunotherapeutic interventions.

Selection, tuning, and adaptation in mouse NK cell education

Natural killer (NK) cells recognize transformed cells with an array of germline-encoded inhibitory and activating receptors. Inhibitory Ly49 receptors bind major histocompatibility complex class I (MHC-I) molecules, providing a mechanism by which NK cells maintain self-tolerance yet eliminate cells expressing reduced levels of MHC-I. Additionally, MHC-I molecules are required for NK cell education, a process in which NK cells acquire responsiveness. In this review, we discuss three facets of MHC class I-dependent education of mouse NK cells: skewing of the inhibitory receptor repertoire, induction of functional responsiveness, and tuning in response to changes in MHC-I expression. We discuss prevailing models for education such as licensing and disarming and propose a model for positive selection of 'useful' NK cell subsets. Furthermore, we argue that both repertoire skewing and functional NK cell education may be altered in mature NK cells subject to changes in MHC-I input and suggest that this process may provide increased dynamics to the NK cell system.

Frequency and Risk Factors Associated with Cord Graft Failure after Transplant with Single-Unit Umbilical Cord Cells Supplemented by Haploidentical Cells with Reduced-Intensity Conditioning

Delayed engraftment and cord graft failure (CGF) are serious complications after unrelated cord blood (UCB) hematopoietic stem cell transplantation (HSCT), particularly when using low-cell-dose UCB units. The haplo-cord HSCT approach allows the use of a lower dose single UCB unit by co-infusion of a CD34(+) selected haploidentical graft, which provides early transient engraftment while awaiting durable UCB engraftment. We describe the frequency, complications, and risk factors of CGF after reduced-intensity conditioning haplo-cord HSCT. Among 107 patients who underwent haplo-cord HSCT, 94 were assessable for CGF, defined as <5% cord blood chimerism at day 60 in the myeloid and CD3 compartments, irrespective of neutrophil and platelet counts. CGF occurred in 14 of 94 assessable patients (15%). Median survival after CGF was 12.7 months with haploidentical or mixed haploidentical-autologous hematopoiesis persisting in the 7 surviving. Median progression-free survival after CGF was 7.7 months and was not statistically different from those without CGF (10.47 months; P = .18). In univariate analyses, no UCB factors were associated with CGF, including cell dose, cell viability, recipient major ABO mismatch against the UCB unit, or degree of HLA match. We also found no association of CGF with recipient cytomegalovirus serostatus, haploidentical donor age, or day 30 haploidentical chimerism. However, higher haploidentical total nucleated and CD34(+) cell doses and day 30 UCB chimerism < 5% in either the myeloid or CD3 compartments were associated with greater risk of CGF. We conclude that assessing chimerism at day 30 may foretell impending CGF, and avoidance of high haploidentical cell doses may reduce risk of CGF after haplo-cord HSCT. However, long-term survival is possible after CGF because of predominant haploidentical or mixed chimerism and hematopoietic function.

Donor Specific Anti-HLA Antibody and Risk of Graft Failure in Haploidentical Stem Cell Transplantation

Outcomes of allogeneic hematopoietic stem cell transplantation (AHSCT) using HLA-half matched related donors (haploidentical) have recently improved due to better control of alloreactive reactions in both graft-versus-host and host-versus-graft directions. The recognition of the role of humoral rejection in the development of primary graft failure in this setting has broadened our understanding about causes of engraftment failure in these patients, helped us better select donors for patients in need of AHSCT, and developed rational therapeutic measures for HLA sensitized patients to prevent this unfortunate event, which is usually associated with a very high mortality rate. With these recent advances the rate of graft failure in haploidentical transplantation has decreased to less than 5%.

Primary graft failure after myeloablative allogeneic hematopoietic cell transplantation for hematologic malignancies

Clinical outcomes after primary graft failure (PGF) remain poor. Here we present a large retrospective analysis (n=23,272) which investigates means to prevent PGF and early detection of patients at high risk. In patients with hematologic malignancies, who underwent their first myeloablative allogeneic hematopoietic cell transplantation, PGF was reported in 1,278 (5.5%), and there was a marked difference in PGFs using peripheral blood stem cell compared to bone marrow grafts (2.5 vs. 7.3%; P<0.001). A 4-fold increase of PGF was observed in myeloproliferative disorders compared to acute leukemia (P<0.001). Other risk factors for PGF included recipient age below 30, HLA-mismatch, male recipients of female donor grafts, ABO-incompatibility, busulfan/cyclophosphamide conditioning, and cryopreservation. In bone marrow transplants, total nucleated cell doses ≤2.4 × 10⁸/kg were associated with PGF (OR 1.39; P<0.001). The use of tacrolimus-based immunosuppression and granulocyte colony-stimulating factor were associated with decreased PGF risk. These data, allow clinicians to do more informed choices with respect to graft source, donor selection, conditioning and immunosuppressive regimens to reduce the risk of PGF. Moreover, a novel risk score determined on day 21 post-transplant may provide the rationale for an early request for additional hematopoietic stem cells.

The Past, Present, and Future of NK Cells in Hematopoietic Cell Transplantation and Adoptive Transfer

Hematopoietic cell transplantation (HCT) has been used as a part of cancer therapy for over half a decade. Beyond the necessity for donor-derived cells to reconstitute hematopoiesis after radiation and chemotherapy, immunologic reconstitution from allogeneic cells is important for the elimination of residual tumor cells. Natural killer (NK) cells are first among lymphocytes to reconstitute post-transplant and protect against cancer relapse. In this review, we provide a historical perspective on the role of NK cells in cancer control in the transplant setting and focus on current research aimed at improving NK cell responses for therapeutic benefit.

Antigen-specific expansion and differentiation of natural killer cells by alloantigen stimulation

Nabekura and Lanier provide evidence that alloantigen stimulation of mouse NK cells promotes the in vivo expansion and generation of memory-like NK cells. NK cells expressing the activating Ly49D receptor preferentially expand and differentiate when challenged with allogeneic cells in an inflammatory environment, but this can be suppressed if NK cells also express the inhibitory Ly49A receptor which recognizes the same ligand. Recall responses were driven by expression of activating Ly49 receptors and regulated by inhibitory MHC I receptors.

Natural killer (NK) cells provide important host defense against microbial pathogens and can generate a population of long-lived memory NK cells after infection or immunization. Here, we addressed whether NK cells can expand and differentiate after alloantigen stimulation, which may be important in hematopoietic stem cell and solid tissue transplantation. A subset of NK cell in C57BL/6 mice expresses the activating Ly49D receptor that is specific for H-2D^d. These Ly49D⁺ NK cells can preferentially expand and differentiate when challenged with allogeneic H-2D^d cells in the context of an inflammatory environment. H-2D^d is also recognized by the inhibitory Ly49A receptor, which, when coexpressed on Ly49D⁺ NK cells, suppresses the expansion of Ly49D⁺ NK cells. Specificity of the secondary response of alloantigen-primed NK cells was defined by the expression of activating Ly49 receptors and regulated by the inhibitory receptors for MHC class I. Thus, the summation of signals through a repertoire of Ly49 receptors controls the adaptive immune features of NK cells responding to allogeneic cells.

The early days of NK cells: an example of how a phenomenon led to detection of a novel immune receptor system - lessons from a rat model

In this review, I summarize some of the early research on NK cell biology and function that led to the discovery of a totally new receptor system for polymorphic MHC class I molecules. That NK cells both could recognize and kill tumor cells but also normal hematopoietic cells through expression of MHC class I molecules found a unifying explanation in the “missing self” hypothesis. This initiated a whole new area of leukocyte receptor research. The common underlying mechanism was that NK cells expressed receptors that were inhibited by recognition of unmodified “self” MHC-I molecules. This could explain both the killing of tumor cells with poor expression of MHC-I molecules and hybrid resistance, i.e., that F1 hybrid mice sometimes could reject parental bone marrow cells. However, a contrasting phenomenon termed allogeneic lymphocyte cytotoxicity in rats gave strong evidence that some of these receptors were activated rather than inhibited by recognition of polymorphic MHC-I. This was soon followed by molecular identification of both inhibitory and stimulatory Ly49 receptors in mice and rats and killer cell immunoglobulin-like receptors in humans that could be either inhibited or activated when recognizing their cognate MHC-I ligand. Since most of these receptors now have been molecularly characterized, their ligands and the intracellular pathways leading to activation or inhibition identified, we still lack a more complete understanding of how the repertoire of activating and inhibitory receptors is formed and how interactions between these receptors for MHC-I molecules on a single NK cell are integrated to generate a productive immune response. Although several NK receptor systems have been characterized that recognize MHC-I or MHC-like molecules, I here concentrate on the repertoires of NK receptors encoded by the natural killer cell gene complex and designed to recognize polymorphic MHC-I molecules in rodents, i.e., Ly49 (KLRA) receptors.

Uterine NK cells: active regulators at the maternal-fetal interface

Pregnancy presents an immunological conundrum because two genetically different individuals coexist. The maternal lymphocytes at the uterine maternal-fetal interface that can recognize mismatched placental cells are T cells and abundant distinctive uterine NK (uNK) cells. Multiple mechanisms exist that avoid damaging T cell responses to the fetus, whereas activation of uNK cells is probably physiological. Indeed, genetic epidemiological data suggest that the variability of NK cell receptors and their MHC ligands define pregnancy success; however, exactly how uNK cells function in normal and pathological pregnancy is still unclear, and any therapies aimed at suppressing NK cells must be viewed with caution. Allorecognition of fetal placental cells by uNK cells is emerging as the key maternal-fetal immune mechanism that regulates placentation.

Allospecific rejection of MHC class I-deficient bone marrow by CD8 T cells

Avoidance of long-term immunosuppression is a desired goal in organ transplantation. Mixed chimerism offers a promising approach to tolerance induction, and we have aimed to develop low-toxicity, nonimmunodepleting approaches to achieve this outcome. In a mouse model achieving fully MHC-mismatched allogeneic bone marrow engraftment with minimal conditioning (3 Gy total body irradiation followed by anti-CD154 and T cell-depleted allogeneic bone marrow cells), CD4 T cells in the recipient are required to promote tolerance of preexisting alloreactive recipient CD8 T cells and thereby permit chimerism induction. We now demonstrate that mice devoid of CD4 T cells and NK cells reject MHC Class I-deficient and Class I/Class II-deficient marrow in a CD8 T cell-dependent manner. This rejection is specific for donor alloantigens, since recipient hematopoiesis is not affected by donor marrow rejection and MHC Class I-deficient bone marrow that is syngeneic to the recipient is not rejected. Recipient CD8 T cells are activated and develop cytotoxicity against MHC Class I-deficient donor cells in association with rejection. These data implicate a novel CD8 T cell-dependent bone marrow rejection pathway, wherein recipient CD8 T cells indirectly activated by donor alloantigens promote direct killing, in a T cell receptor-independent manner, of Class I-deficient donor cells.

Blockade of individual Notch ligands and receptors controls graft-versus-host disease

Graft-versus-host disease (GVHD) is the main complication of allogeneic bone marrow transplantation. Current strategies to control GVHD rely on global immunosuppression. These strategies are incompletely effective and decrease the anticancer activity of the allogeneic graft. We previously identified Notch signaling in T cells as a new therapeutic target for preventing GVHD. Notch-deprived T cells showed markedly decreased production of inflammatory cytokines, but normal in vivo proliferation, increased accumulation of regulatory T cells, and preserved anticancer effects. Here, we report that γ-secretase inhibitors can block all Notch signals in alloreactive T cells, but lead to severe on-target intestinal toxicity. Using newly developed humanized antibodies and conditional genetic models, we demonstrate that Notch1/Notch2 receptors and the Notch ligands Delta-like1/4 mediate all the effects of Notch signaling in T cells during GVHD, with dominant roles for Notch1 and Delta-like4. Notch1 inhibition controlled GVHD, but led to treatment-limiting toxicity. In contrast, Delta-like1/4 inhibition blocked GVHD without limiting adverse effects while preserving substantial anticancer activity. Transient blockade in the peritransplant period provided durable protection. These findings open new perspectives for selective and safe targeting of individual Notch pathway components in GVHD and other T cell-mediated human disorders.

NK cells controlling virus-specific T cells: Rheostats for acute vs. persistent infections

Viral infections characteristically induce a cytokine-driven activated natural killer (NK) cell response that precedes an antigen-driven T cell response. These NK cells can restrain some but not all viral infections by attacking virus-infected cells and can thereby provide time for an effective T cell response to mobilize. Recent studies have revealed an additional immunoregulatory role for the NK cells, where they inhibit the size and functionality of the T cell response, regardless of whether the viruses are themselves sensitive to NK cells. This subsequent change in T cell dynamics can alter patterns of immunopathology and persistence and implicates NK cells as rheostat-like regulators of persistent infections.

Graft failure in the modern era of allogeneic hematopoietic SCT

Graft failure may contribute to increased morbidity and mortality after allogeneic hematopoietic SCT (allo-HSCT). Here, we present risk factors for graft failure in all first allo-HSCTs performed at our center from 1995 to mid-2010 (n=967). Graft failure was defined as >95% recipient cells any time after engraftment with no signs of relapse, or re-transplantation because of primary or secondary neutropenia (<0.5 × 10(9)/L) and/or thrombocytopenia (<30 × 10(9)/L). Fifty-four patients (5.6%) experienced graft failure. The majority were because of autologous reconstitution (n=43), and only a few patients underwent re-transplantation because of primary (n=6) or secondary (n=5) graft failures. In non-malignant disorders, graft failure had no effect on survival, whereas in malignant disease graft failure was associated with reduced 5-year survival (22 vs 53%, P<0.01). In multivariate analysis, ex vivo T-cell depletion (relative risk (RR) 8.82, P<0.001), HLA-mismatched grafts (RR 7.64, P<0.001), non-malignant disorders (RR 3.32, P<0.01) and reduced-intensity conditioning (RR 2.58, P<0.01) increased the risk for graft failure, whereas graft failures were prevented by total nucleated cell doses of ≥ 2.5 × 10(8)/kg (RR 0.36, P<0.01). In conclusion, graft failure was only associated with inferior survival in malignant disease. Non-malignant disorders, HLA match, conditioning intensity, immunosuppression regimen and cell dose all influenced graft failure risk.

Differential effects of stimulatory factors on natural killer cell activities of young and aged mice

Age-associated influences on natural killer (NK) cell functions following cytokine stimulation were examined in splenocytes from C57BL/6 mice. NK cells of both young and aged mice exhibited significantly increased: interferon-γ production after interleukin (IL)-12 or IL-15 alone or any combination of IL-12, IL-18, and IL-2; cytotoxicity after IL-2 or IL-15; and granzyme B expression after IL-15. The only significant age-associated differences were observed in interferon-γ production after IL-15 or IL-12 + 18 + 2 and in granzyme B expression following IL-2 or IL-15. Perforin expression did not increase following stimulation; however, NK cells from aged mice expressed significantly higher levels than young mice. These results underscore the complexity of the cytokine-induced functional activities of NK cells and illustrate the differential response of NK cells from young and aged mice to cytokine stimulation.

Current perspectives of natural killer cell education by MHC class I molecules

From the early days of natural killer (NK) cell research, it was clear that MHC genes controlled the specificity of mouse NK cell-dependent responses, such as the ability to reject transplanted allogeneic bone marrow and to kill tumour cells. Although several mechanisms that are involved in this 'education' process have been clarified, most of the mechanisms have still to be identified. Here, we review the current understanding of the processes that are involved in NK cell education, including how the host MHC class I molecules regulate responsiveness and receptor repertoire formation in NK cells and the signalling pathways that are involved.

Graft failure after allogeneic hematopoietic cell transplantation

Graft failure is a significant complication following allogeneic hematopoietic cell transplantation (alloHCT). It may result from rejection caused by recipient T cells, natural killer (NK) cells, or antibodies. It is increased in HLA-mismatched grafts, unrelated grafts, T cell-replete transplants, sensitized patients, and in patients treated with reduced-intensity conditioning (RIC). In recipients of unrelated grafts, graft failure is increased in patients receiving major AB0 blood group mismatched transplants (P = .008). Recent data also suggest that donor-specific antibodies to CD34(+)/VEGFR-2(+) cells may be involved in graft failure after alloHCT. Graft failure may be overcome by more intensified conditioning, increased cell dose, or more effective immunosuppression. With more frequent use of RIC, cord blood grafts and other HLA-mismatched transplants, graft failure is an increasing problem after alloHCT.

Production of protective gamma interferon by natural killer cells during early mouse hepatitis virus infection

Gamma interferon (IFN-gamma) plays a major role in the protection against lethal infection with mouse hepatitis virus A59. IFN-gamma production reaches a maximum level 2 days after viral inoculation, especially in liver immune cells. Among these cells, natural killer cells are the major producers of this cytokine. Transfer experiments indicated that the protective role of IFN-gamma is mediated through a direct effect on cells targeted by the virus rather than through indirect activation of T lymphocytes.

Regulation of NK cell responsiveness to achieve self-tolerance and maximal responses to diseased target cells

Inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules govern the capacity of natural killer (NK) cells to attack class I-deficient cells ('missing-self recognition'). These receptors are expressed stochastically, such that the panel of expressed receptors varies between NK cells. This review addresses how the activity of NK cells is coordinated in the face of this variation to achieve a repertoire that is self-tolerant and optimally reactive with diseased cells. Recent studies show that NK cells arise in normal animals or humans that lack any known inhibitory receptors specific for self-MHC class I. These NK cells exhibit self-tolerance and exhibit functional hyporesponsiveness to stimulation through various activating receptors. Evidence suggests that hyporesponsiveness is induced because these NK cells cannot engage inhibitory MHC class I molecules and are therefore persistently over-stimulated by normal cells in the environment. Finally, we discuss evidence that hyporesponsiveness is a quantitative trait that varies depending on the balance of signals encountered by developing NK cells. Thus, a tuning process determines the functional set-point of NK cells, providing a basis for discriminating self from missing-self, and at the same time endowing each NK cell with the highest inherent responsiveness compatible with self-tolerance.

The number of activating KIR genes inversely correlates with the rate of CMV infection/reactivation in kidney transplant recipients

Viral infection is a common complication after kidney transplantation. The role of natural killer cells (NK cells) in this setting remains unknown. NK cells express activating and inhibitory killer cell immunoglobulin-like receptors (KIR). We analyzed whether activating KIR genes carried by kidney transplant-recipients influence the rate of viral infection during the first year after transplantation. In patients with a KIR A/A genotype (n = 40, KIR2DS4 only activating KIR) the rate of cytomegalovirus (CMV) infection and reactivation was 36%, as compared to 20% in transplant recipients with more than one activating KIR gene (KIR B/X genotype, n = 82, p = 0.04). Adjusting for other risk factors in Cox regression, the relative risk of B versus A genotype patients was 0.34 (95% CI 0.15-0.76, p = 0.009). The degree of protection increased with the number of activating KIR genes. Symptomatic CMV disease was only observed in four individuals, all carrying a KIR A/A genotype. As for viral infections other than CMV, and for bacterial infections, no KIR-linked protective effect could be detected. Also, graft function and the rate-rejection episodes were similar in KIR A/A and KIR B/X genotype individuals. This study supports a role for activating KIR in the control of CMV infection after kidney transplantation.

Xenogeneic β2-Microglobulin Substitution Alters NK Cell Function

Recently, it has been shown that human beta(2)-microglobulin (h-beta(2)m) blocks the association between the NK cell inhibitory receptor Ly49C and H-2K(b). Given this finding, we therefore sought to assess the immunobiology of NK cells derived from C57BL/6 (H-2(b)) mice expressing exclusively h-beta(2)m. Initial analysis revealed that the Ly49C expression profile of NK cells from h-beta(2)m(+) mice was modified, despite the fact that H-2K(b) expression was normal in these mice. Moreover, the NK cells were not anergic in that IL-2 treatment of h-beta(2)m(+) NK cells in vitro enabled efficient lysis of prototypic tumor cell lines as well as of syngeneic h-beta(2)m(+) lymphoblasts. This loss of self-tolerance appeared to correlate with the activation status of h-beta(2)m(+) NK cells because quiescent h-beta(2)m(+) transplant recipients maintained h-beta(2)m(+) grafts but polyinosine:polycytidylic acid-treated recipients acutely rejected h-beta(2)m(+) grafts. NK cell reactivity toward h-beta(2)m(+) targets was attributed to defective Ly49C interactions with h-beta(2)m:H-2K(b) molecules. With regard to NK cell regulatory mechanisms, we observed that h-beta(2)m:H-2K(b) complexes in the cis-configuration were inefficient at regulating Ly49C and, furthermore, that receptor-mediated uptake of h-beta(2)m:H-2K(b) by Ly49C was impaired compared with uptake of mouse beta(2)m:H-2K(b). Thus, we conclude that transgenic expression of h-beta(2)m alters self-MHC class I in such a way that it modulates the NK cell phenotype and interferes with regulatory mechanisms, which in turn causes in vitro-expanded and polyinosine:polycytidylic acid-activated NK cells to be partially self-reactive similar to what is seen with NK cells derived from MHC class I-deficient mice.

Establishment of a nude mice model of human monocytic leukemia with CNS and multiorgan extramedullary infiltration

OBJECT

To establish a human monocytic leukemia model with central nervous system infiltration in BALB/c nude mice.

METHODS

BALB/c nu/nu mice, pretreated by splenectomy, cytoxan intraperitoneal injection, and sublethal irradiation, were transplanted intravenously with human monocytic leukemic cell line SHI-1. The leukemic cells engrafted in the mice were traced by RT-PCR, histopathological examination, immunohistochemistry and FCM.

RESULT

After engraftment of SHI-1 cells in SCI-nu/nu mice, multiple organs were involved and green solid neoplasms were formed in some organs. Paralysis was developed in some mice at both of the rear legs. Histopathological examination found that vertebral and skull bone marrow were replaced by leukemic cells. Leukemic cells penetrated to the surface of vertebrae, forming neoplasm, and entering to the subdural space, but seldom involving in the spinal parenchyma. In the brain, leukemic cells filled in the subdural space and pia-arachnoid, covered the surface of cerebrum, cerebellum and along the virchow-robin space on the surface of pia mater, eventually invading into the brain parenchyma.

CONCLUSIONS

SHI-1 cells could engraft in the SCI-nu/nu mice, creating an efficient and reproducible experimental model of central nervous system leukemia (CNSL) and multiorgan infiltration. This experimental model may be useful for studies on the pathogenesis of CNSL.

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.

NKG2D in NK and T Cell-Mediated Immunity

One of the best characterized NK cell receptors is NKG2D, a highly conserved C-type lectin-like membrane glycoprotein expressed on essentially all NK cells, as well as on gammadelta-TcR+ T cells and alphabeta-TcR+ CD8+ T cells, in humans and mice. Here we review recent studies implicating NKG2D in T cell and NK cell-mediated immunity to viruses and tumors, and its potential role in autoimmune diseases and allogeneic bone marrow transplantation.

Function of NKG2D in natural killer cell-mediated rejection of mouse bone marrow grafts

Irradiation-resistant natural killer (NK) cells in an F(1) recipient can reject parental bone marrow, and host NK cells can also prevent engraftment of allogeneic bone marrow. We show here that repopulating bone marrow cells in certain mouse strains expressed retinoic acid early inducible 1 proteins, which are ligands for the activating NKG2D NK cell receptor. Treatment with a neutralizing antibody to NKG2D prevented rejection of parental BALB/c bone marrow in (C57BL/6 x BALB/c) F(1) recipients and allowed engraftment of allogeneic BALB.B bone marrow in C57BL/6 recipients. Additionally, bone marrow from C57BL/6 mice transgenic for retinoic acid early inducible 1epsilon was rejected by syngeneic mice but was accepted after treatment with antibody to NKG2D. If other stem cells or tissues upregulate expression of NKG2D ligands after transplantation, NKG2D may contribute to graft rejection in immunocompetent hosts.

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.

Anti-NK cell treatment induces stable mixed chimerism in MHC-mismatched, T cell-depleted, nonmyeloablative bone marrow transplantation

OBJECTIVE

To clarify natural killer (NK) cell-mediated resistance under cytoreductive conditioning and T cell-depleted bone marrow transplantation, we investigated the effects of host NK cell depletion on engraftment and induction of stable mixed chimerism.

METHODS

BALB/c mice (H-2kd) were injected intraperitoneally with anti-asialoGM1 antibody (anti-NK Ab) on day -1. On day 0, they received total body irradiation (TBI) at a dose of 500 cGy, followed by intravenous infusion of 2 x 10(7) T cell-depleted (TCD) bone marrow cells from C57BL/6 mice (H-2kb). Early engraftment and chimerism were determined by the relative ratio of peripheral blood (PB) lymphocytes expressing either H-2kd or H-2kb on day +21. Long-term engraftment and chimerism were evaluated on PB and spleen by multicolor flow cytometry.

RESULTS

Although no recipients treated with TBI alone showed engraftment, all the recipients conditioned with anti-NK Ab and TBI showed successful engraftment as well as a donor-dominant pattern of mixed chimerism in both PB and spleen. Spleen cells from recipients with mixed chimerism showed specific tolerance to both host and donor strains, but not to a third party (C3H/He). None of the reconstituted mice showed signs of graft vs host disease, and all survived up to day +330.

CONCLUSION

These observations indicate that host NK cell depletion may be used to reduce the intensity of conditioning regimens for engraftment of TCD grafts, and can contribute to establishment of stable mixed chimerism in major histocompatibility complex-mismatched nonmyeloablative transplantation.

Rapid tumor death model for evaluation of new therapeutic agents for adult T-cell leukemia

Adult T-cell leukemia/lymphoma (ATL) is an aggressive T-cell neoplasm. The health of ATL patients rapidly deteriorates resulting in death; however, the induction of death in a small animal model due to tumor has not yet been reported. SCID mice, 5 weeks old, younger than those previously used, which were inoculated with ATL cells, eliminated NK cell activity and showed rapid tumor formation resulting in death. Age is the crucial factor influencing tumor formation and death in the SCID mice model for cancer.

NK cells, MHC class I molecules and the missing self

This article is based on a lecture presented at the Novartis Prize ceremony at the International Congress of Immunology in July 2001. It gives a personal and historical perspective on the research performed by the author and his colleagues during the development and pursuit of the model of 'missing-self recognition' for natural killer (NK) cells. This model is based on the idea that one important function of NK cells is to detect and eliminate cells because they fail to express normal self markers. Further mechanistic models predicted the existence of inhibitory major histocompatibility complex (MHC) class I specific receptors, later identified by the fellow Novartis laureates contributing in this issue. The article covers the first decade (1980-1990) of research on this concept. It discusses factors contributing to the formulation of a hypothesis, the use of predictions and experimental test models, the importance of international collaborations and reagent exchange, and several other aspects that allowed the progression of this research project. Finally, the perspective of today's knowledge is used to discuss some surprising findings where the missing-self hypothesis made the wrong predictions, or at least failed to make the correct ones.

Critical role of NK but not NKT cells in acute rejection of parental bone marrow cells in F1 hybrid mice

F1 hybrid mice vigorously reject transplanted parental bone marrow (BM) cells, which is a phenomenon called "hybrid resistance (HR)". Since NK1.1(+) cells play crucial role in HR, both NK1.1(+)CD3(+) NKT cells and NK1.1(+)CD3(-) NK cells have been possible candidates of effector cells. To elucidate the major effector cells in HR, we employed Rag-2(-/-) mice devoid of T, B, and NKT cells and cytokine receptor common gamma subunit and Rag-2 double-deficient (gamma(c)(-/-(y))-Rag-2(-/-)) mice lacking all lymphoid cells including NK cells. Rag-2(-/-) F1 hybrid mice rejected parental BM cells to the extent similar to wild-type (WT) F1 hybrids. In contrast, male gamma(c)(-/y)-Rag-2(-/-) F1 hybrid mice were unable to reject parental BM cells. After reconstitution with NK but not NKT cells, male gamma(c)(-/y)-Rag-2(-/-) F1 hybrid mice restored the ability to reject parental BM cells. Collectively, it is concluded that NKT cells play little role, if any, and NK cells are the only cells involved in HR.

The effect of in vivo depletion of NKR-P1+ or CD8+ lymphocytes on the acute rejection of allogeneic lymphocytes (ALC) in the rat

We have depleted lymphocyte subsets in PVG and AO rats with MoAbs 3.2.3 (against NKR-P1 on NK and NK/T cells) and OX-8 (against CD8 on CTL and NK cells), and examined the effect on the killing of YAC-1 target cells in vitro and the effect on the acute rejection of small allogeneic lymphocytes in vivo (allogeneic lymphocyte cytotoxicity, ALC). While 3.2.3 treatment led to only a partial depletion of 3.2.3-positive cells in PVG rats, this treatment drastically reduced the number of NKR-P1+ cells in AO rats, abolished splenic NK activity against the NK-sensitive tumour target YAC-1, and markedly diminished the ALC response. Rats treated with OX-8 for 1 day showed a similar loss of NK cell function in vivo and in vitro. However, in rats treated with OX-8 for 3 days a 3.2.3+ and OX-8- population consisting of NK cells appeared, restoring ALC. The results demonstrate that NK cell responses can be greatly diminished after in vivo treatment with these MoAbs. Furthermore, they demonstrate that ALC is not necessarily linked to expression of the CD8 molecule.