Rejection of class I MHC-deficient haemopoietic cells by irradiated MHC-matched mice

A genetic defect in mice that impairs missing self recognition despite evidence for normal maturation and MHC class I-dependent education of NK cells

In studies of a CD1d1-deficient mouse strain, we unexpectedly observed a severely impaired capacity for NK cell-mediated rejection of MHC class I-deficient (spleen or tumor) cells. Studies of another CD1-defective strain, as well as intercrosses with C57BL/6 mice, indicated that the impaired missing self rejection (IMSR) NK cell defect was a recessive trait, independent from the targeted CD1 locus. Studies with mixed bone marrow chimeras indicated that the defect is intrinsic to NK cells. The IMSR mice had normal proportions of NK cells, displaying a typical cell surface phenotype, as evaluated using a panel of Abs to developmental markers and known receptors. The impaired missing self recognition could not be overcome through cytokine stimulation. There was also an impaired capacity with respect to NKG2D-dependent cytotoxicity, whereas the mice exhibited normal Ly49D/DAP12-dependent responses in vivo and in vitro. The NK cell system of IMSR mice showed two hallmarks of MHC-dependent education: skewing of the Ly49 receptor repertoire and differential in vitro responsiveness between NK cells with and without inhibitory receptors for self-MHC ("licensing"). We conclude that these mice have a recessive trait that perturbs the missing self reaction, as well as NKG2D-dependent responses, whereas other aspects of the NK system, such as development, capacity to sense MHC molecules during education, and Ly49D/DAP12-dependent responses, are largely intact.

Recognition of tumors by the innate immune system and natural killer cells

In recent years, roles of the immune system in immune surveillance of cancer have been explored using a variety of approaches. The roles of the adaptive immune system have been a major emphasis, but increasing evidence supports a role for innate immune effector cells such as natural killer (NK) cells in tumor surveillance. Here, we discuss some of the evidence for roles in tumor surveillance of innate immune cells. In particular, we focus on NK cells and other immune cells that express germline-encoded receptors, often labeled NK receptors. The impact of these receptors and the cells that express them on tumor suppression is summarized. We discuss in detail some of the pathways and events in tumor cells that induce or upregulate cell-surface expression of the ligands for these receptors, and the logic of how those pathways serve to identify malignant, or potentially malignant cells. How tumors often evade tumor suppression mediated by innate killer cells is another major subject of the review. We end with a discussion on some of the implications of the various findings with respect to possible therapeutic approaches.

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.

Cutting edge: NK cell licensing modulates adhesion to target cells

Binding of NK cell inhibitory receptors to MHC class I (MHC-I) confers increased responsiveness to NK cells by a process known as NK cell licensing/education. Reduced MHC-I expression or a lack of inhibitory receptors for MHC-I results in diminished NK cell responsiveness. In this study, we evaluated the effect of human and mouse NK cell licensing on early stages of natural cytotoxicity. Unlicensed NK cells did not form as many stable conjugates with target cells. The reduction of NK cell conjugation to target cells was not attributed to altered β2 integrin LFA-1 properties but was instead due to reduced inside-out signaling to LFA-1 by activating receptors. For those unlicensed NK cells that did form conjugates, LFA-1-dependent granule polarization was similar to that in licensed NK cells. Thus, licensing controls signals as proximal as inside-out signaling by activating receptors but not integrin outside-in signaling for granule polarization.

Contrasting effects of anti-Ly49A due to MHC class I cis binding on NK cell-mediated allogeneic bone marrow cell resistance

NK subsets have activating and inhibitory receptors that bind MHC-I. Ly49A is a mouse inhibitory receptor that binds with high affinity to H2(d) in both a cis- and trans-manner. Ly49A cis-associations limit trans-interactions with H2(d)-expressing targets as well as mAb binding. We demonstrate that cis-interactions affect mAb effector functions. In vivo administration of anti-Ly49A depleted NK cells in H2(b) but not H2(d) mice. Despite lack of depletion, in vivo treatment with anti-Ly49A reduced NK killing capabilities and inhibited activation, partially due to its agonistic effect. These data explain the previously described in vivo effects on bone marrow allograft rejection observed with anti-Ly49A treatment in H2(d)-haplotype mice. However, prior treatment of mice with poly(I:C) or mouse CMV infection resulted in increased Ly49A expression and Ly49A(+) NK cell depletion in H2(d) mice. These data indicate that, although Ly49 mAbs can exert similar in vivo effects in mice with different MHC haplotypes, these effects are mediated via different mechanisms of action correlating with Ly49A expression levels and can be altered within the same strain contingent on stimuli. This illustrates the marked diversity of mAb effector functions due to the regulation of the level of expression of target Ags and responses by stimulatory incidents such as infection.

HLA engineering of human pluripotent stem cells

The clinical use of human pluripotent stem cells and their derivatives is limited by the rejection of transplanted cells due to differences in their human leukocyte antigen (HLA) genes. This has led to the proposed use of histocompatible, patient-specific stem cells; however, the preparation of many different stem cell lines for clinical use is a daunting task. Here, we develop two distinct genetic engineering approaches that address this problem. First, we use a combination of gene targeting and mitotic recombination to derive HLA-homozygous embryonic stem cell (ESC) subclones from an HLA-heterozygous parental line. A small bank of HLA-homozygous stem cells with common haplotypes would match a significant proportion of the population. Second, we derive HLA class I-negative cells by targeted disruption of both alleles of the Beta-2 Microglobulin (B2M) gene in ESCs. Mixed leukocyte reactions and peptide-specific HLA-restricted CD8(+) T cell responses were reduced in class I-negative cells that had undergone differentiation in embryoid bodies. These B2M(-/-) ESCs could act as universal donor cells in applications where the transplanted cells do not express HLA class II genes. Both approaches used adeno-associated virus (AAV) vectors for efficient gene targeting in the absence of potentially genotoxic nucleases, and produced pluripotent, transgene-free cell lines.

The TNF receptor-ligands 4-1BB-4-1BBL and GITR-GITRL in NK cell responses

Interactions between several tumor necrosis factor (TNF)-TNF receptor (TNFR) superfamily members that are expressed by T cells and natural killer (NK) cells and various other cell types modulate immune responses. This review summarizes the current understanding of how the TNF ligand-TNFR interactions 4-1BBL with 4-1BB, and GITRL with glucocorticoid-induced TNFR-related (GITR) regulate NK cell mediated antitumor responses and discuss its therapeutic implications.

When NK cells overcome their lack of education

Cells of the immune system have evolved various molecular mechanisms to sense their environment and react to alterations of self. NK cells are lymphocytes with effector and regulatory functions, which are remarkably adaptable to changes in self. In a study published in this issue of the JCI, Tarek and colleagues report the clinical benefits of manipulating NK cell adaptation to self in an innovative mAb-based therapy against neuroblastoma (NB). This novel therapeutic strategy should stimulate further research on NK cell therapies.

Natural killer cells and their receptors in multiple sclerosis

The immune system has crucial roles in the pathogenesis of multiple sclerosis. While the adaptive immune cell subsets, T and B cells, have been the main focus of immunological research in multiple sclerosis, it is now important to realize that the innate immune system also has a key involvement in regulating autoimmune responses in the central nervous system. Natural killer cells are innate lymphocytes that play vital roles in a diverse range of infections. There is evidence that they influence a number of autoimmune conditions. Recent studies in multiple sclerosis and its murine model, experimental autoimmune encephalomyelitis, are starting to provide some understanding of the role of natural killer cells in regulating inflammation in the central nervous system. Natural killer cells express a diverse range of polymorphic cell surface receptors, which interact with polymorphic ligands; this interaction controls the function and the activation status of the natural killer cell. In this review, we discuss evidence for the role of natural killer cells in multiple sclerosis and experimental autoimmune encephalomyelitis. We consider how a change in the balance of signals received by the natural killer cell influences its involvement in the ensuing immune response, in relation to multiple sclerosis.

Natural killer cell tolerance: control by self or self-control?

A major challenge for the immune system is to control pathogens and stressed cells, such as infected or tumors cells, while sparing healthy self-cells. To achieve this tolerance to self, immune cells must recognize and differentiate "self" versus "nonself" and "self" versus "altered self." In the absence of self-tolerance, cells of the adaptive immune system attack healthy cells and cause autoimmune diseases such as lupus, psoriasis, and type I diabetes. Mechanisms at work to ensure tolerance in the innate immune system are still poorly understood. Natural killer cells are innate immune lymphocytes, which have the capacity to kill cellular targets and produce cytokines without prior specific sensitization. Because of these intrinsic effector capacities, tolerance mechanisms must exist to prevent autoreactivity. Herein, we will review the present knowledge on NK cell tolerance.

Overview of the killer cell immunoglobulin-like receptor system

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

Immunoproteasome-deficiency has no effects on NK cell education, but confers lymphocytes into targets for NK cells in infected wild-type mice

Natural killer (NK) cells are part of the innate immune system and contribute to the eradication of virus infected cells and tumors. NK cells express inhibitory and activating receptors and their decision to kill a target cell is based on the balance of signals received through these receptors. MHC class I molecules are recognized by inhibitory receptors, and their presence during NK cell education influences the responsiveness of peripheral NK cells. We here demonstrate that mice with reduced MHC class I cell surface expression, due to deficiency of immunoproteasomes, have responsive NK cells in the periphery, indicating that the lower MHC class I levels do not alter NK cell education. Following adoptive transfer into wild-type (wt) recipients, immunoproteasome-deficient splenocytes are tolerated in naive but rejected in virus-infected recipients, in an NK cell dependent fashion. These results indicate that the relatively low MHC class I levels are sufficient to protect these cells from rejection by wt NK cells, but that this tolerance is broken in infection, inducing an NK cell-dependent rejection of immunoproteasome-deficient cells.

Unifying concepts of MHC-dependent natural killer cell education

Natural killer (NK) cells, like B and T lymphocytes, are potent effector cells that are crucial for immunity to tumors and infections. These effector responses must be controlled to avoid inadvertent attack against normal self. Yet, the mechanisms that guide NK cell tolerance differ from those guiding T and B cell tolerance. Here, we discuss how NK cells are licensed by self-MHC class I molecules through their inhibitory receptors which results in NK cell functional competence to be triggered through their activation receptors. We discuss recent data with respect to issues related to licensing, thereby providing a framework for unifying concepts on NK cell education.

The innate immune system in transplantation

The vertebrate innate immune system consists of inflammatory cells and soluble mediators that comprise the first line of defense against microbial infection and, importantly, trigger antigen-specific T and B cell responses that lead to lasting immunity. The molecular mechanisms responsible for microbial non-self recognition by the innate immune system have been elucidated for a large number of pathogens. How the innate immune system recognizes non-microbial non-self, such as organ transplants, is less clear. In this review, we approach this question by describing the principal mechanisms of non-self, or 'damaged' self, recognition by the innate immune system (pattern recognition receptors, the missing self theory, and the danger hypothesis) and discussing whether and how these mechanisms apply to allograft rejection.

Self MHC class I-licensed NK cells enhance adaptive CD8 T-cell viral immunity

MHC class I (MHC I) is essential to NK- and T-cell effector and surveillance functions. However, it is unknown whether MHC I polymorphism influences adaptive immunity through NK cells. Previously, we found that MHC I D(k), a cognate ligand for the Ly49G2 inhibitory receptor, was essential to NK control of murine (M)CMV infection. Here we assessed the significance of NK inhibitory receptor recognition of MCMV on CD8 T cells in genetically defined MHC I D(k) disparate mice. We observed that D(k)-licensed Ly49G2⁺ NK cells stabilized and then enhanced conventional dendritic cells (cDCs) recovery after infection. Furthermore, licensed NK support of cDC recovery was essential to enhance the tempo, magnitude, and effector activity of virus-specific CD8 T cells. Minimal cDC and CD8 T-cell number differences after low-dose MCMV in D(k) disparate animals further implied that licensed NK recognition of MCMV imparted qualitative cDC changes to enhance CD8 T-cell priming.

Increased killer immunoglobulin-like receptor expression and functional defects in natural killer cells in lung cancer

Frequencies of natural killer (NK) cells from patients with non-small cell lung cancer (NSCLC) or small cell lung cancer (SCLC) did not differ from healthy controls. A higher proportion of NK cells from NSCLC patients expressed the killer immunoglobulin-like receptor (KIR) CD158b than in controls (P = 0.0004), in the presence or absence of its ligand, HLA-C1. A similar result was obtained for CD158e in the presence of its ligand HLA-Bw4 in NSCLC patients (P = 0.003); this was entirely attributable to the Bw4I group of alleles in the presence of which a fivefold higher percentage of CD158e(+) NK cells was found in NSCLC patients than controls. Proportions of CD158b(+) NK cells declined with advancing disease in NSCLC patients. Expression of NKp46, CD25 and perforin A, and production of interferon-γ following stimulation with interleukin-12 and interleukin-18, were all significantly lower in NK cells from NSCLC patients than in controls. Both NK cell cytotoxicity and granzyme B expression were also reduced in lung cancer patients. Increased inhibitory KIR expression would decrease NK cell cytotoxic function against tumour cells retaining class I HLA expression. Furthermore, the reduced ability to produce interferon-γ would restrict the ability of NK cells to stimulate T-cell responses in patients with lung cancer.

The non-conventional MHC class I MR1 molecule controls infection by Klebsiella pneumoniae in mice

As opposed to the well established role of MHC-linked, polymorphic, class I (MHC-I) genes in adaptive immunity, a universal role for non-conventional MHC-I is unknown, thus requiring a case-by-case study. The MHC unlinked, monomorphic, but β₂microglobulin (β₂m)-associated "MHC class I related" MR1 molecule interacts with a semi-invariant TCR. The pathophysiology of this interaction or more importantly of this peculiar MHC-I remains mostly unknown. Recently it was shown that β₂m deficient mice were more susceptible to infection by Klebsiella pneumoniae, a widely spread Gram-negative bacteria that causes diverse and often severe ailments in man. Here we demonstrate, using both an in vivo imaging system and survival tests, the increased susceptibility to K. pneumoniae (but not to several other Gram negative bacteria) of MR1 deficient mice. This is accompanied by a consequent change in body temperature and systemic cytokine profile. Hence MR1 controls K. pneumoniae infection in vivo.

Development and function of CD94-deficient natural killer cells

The CD94 transmembrane-anchored glycoprotein forms disulfide-bonded heterodimers with the NKG2A subunit to form an inhibitory receptor or with the NKG2C or NKG2E subunits to assemble a receptor complex with activating DAP12 signaling proteins. CD94 receptors expressed on human and mouse NK cells and T cells have been proposed to be important in NK cell tolerance to self, play an important role in NK cell development, and contribute to NK cell-mediated immunity to certain infections including human cytomegalovirus. We generated a gene-targeted CD94-deficient mouse to understand the role of CD94 receptors in NK cell biology. CD94-deficient NK cells develop normally and efficiently kill NK cell-susceptible targets. Lack of these CD94 receptors does not alter control of mouse cytomegalovirus, lymphocytic choriomeningitis virus, vaccinia virus, or Listeria monocytogenes. Thus, the expression of CD94 and its associated NKG2A, NKG2C, and NKG2E subunits is dispensable for NK cell development, education, and many NK cell functions.

Major histocompatibility complex-I expression on embryonic stem cell-derived vascular progenitor cells is critical for syngeneic transplant survival

Donor-recipient cell interactions are essential for functional engraftment after nonautologous cell transplantation. During this process, transplant engraftment is characterized and defined by interactions between transplanted cells with local and recruited inflammatory cells. The outcome of these interactions determines donor cell fate. Here, we provide evidence that lineage-committed embryonic stem cell (ESC)-derived vascular progenitor cells are the target of major histocompatibility complex (MHC) class I-dependent, natural killer (NK) cell-mediated elimination in vitro and in vivo. Treatment with interferon γ was found to significantly upregulate MHC class I expression on ESC-derived vascular progenitor cells, rendering them less susceptible to syngeneic NK cell-mediated killing in vitro and enhancing their survival and differentiation potential in vivo. Furthermore, in vivo ablation of NK cells led to enhanced progenitor cell survival after transplantation into a syngeneic murine ischemic hindlimb model, providing additional evidence that NK cells mediate ESC-derived progenitor cell transplant rejection. These data highlight the importance of recipient immune-donor cell interactions, and indicate a functional role for MHC-I antigen expression during successful ESC-derived syngeneic transplant engraftment.

Natural killer cell education and tolerance

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

Effect of Ly49 haplotype variance on NK cell function and education

The class I MHC-specific receptors expressed by murine NK cells exhibit remarkable variation. Specific activating killer Ig-related receptor/Ly49 have major effects on autoimmune and infectious disease induction and outcome in humans and mice. However, these studies are greatly affected by individual background genetics. Furthermore, the educational impact of variable inhibitory KIR/Ly49 gene numbers on NK cell development and the subsequent ability to survey for MHC class I (MHC-I) expression remain unknown. To address these questions, Ly49 congenic mice were generated that maintain a 129-derived Ly49 gene cluster on a C57BL/6 genetic background (B6.Ly49(129) mice), and the in vitro and in vivo NK cell function of these mice was compared with their inbred parental 129S1 and C57BL/6 counterparts. Notably, target cell recognition directed by activating Ly49 receptors was profoundly affected by allelic variation in B6.Ly49(129) congenic cells versus C57BL/6 NK cells. Furthermore, when assessing NK cell function based on education and subsequent recognition of the C57BL/6 MHC-I haplotype by inhibitory Ly49 receptors, B6.Ly49(129) congenic mice exhibited robust NK cell activity, demonstrating efficient NK cell education by the 129S1 Ly49 cluster during development. The responsiveness of NK cells expressing 129S1 Ly49 was shown to be mediated by subsets expressing one or more self-MHC receptors, including Ly49I, Ly49O, Ly49V, and NKG2A. These findings demonstrate that the genetically segregating and diverse MHC-I and Ly49 loci in mice exhibit independent and epistatic effects on NK cell education that can be uncoupled during the intercrossing of inbred strains.

MHC class I-deficient natural killer cells acquire a licensed phenotype after transfer into an MHC class I-sufficient environment

In MHC class I-deficient hosts, natural killer (NK) cells are hyporesponsive to cross-linking of activation receptors. Functional competence requires engagement of a self-major histocompatability complex (MHC) class I-specific inhibitory receptor, a process referred to as "licensing." We previously suggested that licensing is developmentally determined in the bone marrow. In this study, we find that unlicensed mature MHC class I-deficient splenic NK cells show gain-of-function and acquire a licensed phenotype after adoptive transfer into wild-type (WT) hosts. Transferred NK cells produce WT levels of interferon-γ after engagement of multiple activation receptors, and degranulate at levels equivalent to WT NK cells upon coincubation with target cells. Only NK cells expressing an inhibitory Ly49 receptor specific for a cognate host MHC class I molecule show this gain-of-function. Therefore, these findings, which may be relevant to clinical bone marrow transplantation, suggest that neither exposure to MHC class I ligands during NK development in the BM nor endogenous MHC class I expression by NK cells themselves is absolutely required for licensing.

Mature natural killer cells reset their responsiveness when exposed to an altered MHC environment

Some mature natural killer (NK) cells cannot be inhibited by major histocompatibility complex (MHC) I molecules, either because they lack corresponding inhibitory receptors or because the host lacks the corresponding MHC I ligands for the receptors. Such NK cells nevertheless remain self-tolerant and exhibit a generalized hyporesponsiveness to stimulation through activating receptors. To address whether NK cell responsiveness is set only during the NK cell differentiation process, we transferred mature NK cells from wild-type (WT) to MHC I–deficient hosts or vice versa. Remarkably, mature responsive NK cells from WT mice became hyporesponsive after transfer to MHC I–deficient mice, whereas mature hyporesponsive NK cells from MHC I–deficient mice became responsive after transfer to WT mice. Altered responsiveness was evident among mature NK cells that had not divided in the recipient animals, indicating that the cells were mature before transfer and that alterations in activity did not require cell division. Furthermore, the percentages of NK cells expressing KLRG1, CD11b, CD27, and Ly49 receptors specific for H-2^b were not markedly altered after transfer. Thus, the functional activity of mature NK cells can be reset when the cells are exposed to a changed MHC environment. These findings have important implications for how NK cell functions may be curtailed or enhanced in the context of disease.

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.

The requirement for NKG2D in NK cell-mediated rejection of parental bone marrow grafts is determined by MHC class I expressed by the graft recipient

Natural killer (NK) cells provide a unique barrier to semiallogeneic bone marrow (BM) transplantation. In the setting where the parents donate to the F1 offspring, rejection of parental bone marrow occurs. This "hybrid resistance" is completely NK cell dependent, as T cells in the F1 recipient tolerate parental grafts. Previously, we demonstrated that rejection of BALB/c parental BM by (BALB/c × C57BL/6) F1-recipient NK cells is dependent on the NKG2D-activating receptor, whereas rejection of parental C57BL/6 BM does not require NKG2D. BALB/c and B6 mice possess different NKG2D ligand genes and express these ligands differently on reconstituting BM cells. Herein, we show that the requirement for NKG2D in rejection depends on the major histocompatibility complex haplotype of donor cells and not the differences in the expression of NKG2D ligands. NKG2D stimulation of NK cell-mediated rejection was required to overcome inhibition induced by H-2D(d) when it engaged an inhibitory Ly49 receptor, whereas rejection of parental BM expressing the ligand, H-2K(b), did not require NKG2D. Thus, interactions between the inhibitory receptors on F1 NK cells and parental major histocompatibility complex class I ligands determine whether activation via NKG2D is required to achieve the threshold for rejection of parental BM grafts.

Cell-dose-dependent increases in circulating levels of immune effector cells in rhesus macaques following intracranial injection of allogeneic MSCs

OBJECTIVE

Mesenchymal stem cells (MSCs) possess potent immunomodulatory activity, but whether they evade immune surveillance in an allogeneic transplant setting remains controversial. Herein we evaluated whether administration of major histocompatibility complex (MHC) class I-mismatched MSCs induce an immune response in rhesus macaques.

MATERIALS AND METHODS

MSCs from a male donor were injected intracranially at two different doses into eight immunocompetent female infant rhesus macaques. Blood cell counts and circulating levels of lymphocyte subpopulations were quantified prior to surgery and at 10, 30, and 90 to 180 days postsurgery by flow cytometry. Immunoreactivity of recipient peripheral blood mononuclear cells to donor MSCs was evaluated in vitro and alloantibody production in vivo was determined by enzyme-linked immunosorbent assay and flow cytometry.

RESULTS

MSC transplantation induced transient but significant increases in circulating white blood cells, lymphocytes, and neutrophils in most transplant recipients, but not sham-operated control animals. Flow cytometric analysis revealed a strong correlation between expansion of CD8(+), CD16(+), and CD8(+)/CD16(+) lymphocyte subpopulations in peripheral blood, the dose of administered MSCs, and degree of antigenic mismatch between donor and recipient. MSC-specific alloantibodies were also detected in several transplant recipients. However, peripheral blood mononuclear cells harvested from transplant recipients postsurgery exhibited no lytic activity against donor MSCs in vitro upon rechallenge.

CONCLUSIONS

MSCs induced an allograft response in rhesus macaques that involved principally CD8(+), CD16(+), and CD8(+)/CD16(+) lymphocyte subpopulations and was cell-dose- and haplotype-dependent. This study demonstrates that MSCs are weakly immunogenic in vivo when transplanted across MHC class I barriers.

Associations between genes for killer immunoglobulin-like receptors and their ligands in patients with solid tumors

Killer immunoglobulin-like receptor (KIR) and human leukocyte antigen (HLA) genotypes were analyzed from panels of lung (non-small-cell lung cancer [NSCLC] and small-cell lung cancer [SCLC]), colon, and kidney cancer patients and compared with normal control subjects. No significant differences were noted between KIR gene frequencies in patients compared with normal subjects. When combinations of KIR genes and their HLA ligands were considered, there were significant decreases in frequencies of both KIR2DL2 and KIR2DL3 in homozygotes for their ligand HLA-C1, and an increase in the frequency of KIR3DL1 and its ligand HLA-Bw4 in kidney cancer patients compared with controls. Both associations were partly attributable to changes in ligand frequencies alone. NSCLC patients showed a significant increase in the frequency of KIR2DL1 and its ligand HLA-C2 and a corresponding decrease in frequency of KIR2DL3 and its ligand HLA-C1 in homozygotes. In NSCLC, the Ile80 form of HLA-Bw4 was decreased in KIR3DL1+ HLA-Bw4+ patients, whereas in SCLC the Ile80 form was increased and the Thr80 form decreased in KIR3DS1+ HLA-Bw4+ patients. These findings are consistent with increased co-expression of high-affinity inhibitory KIRs and their ligands, potentially resulting in decreased natural killer cell function, and hence with natural killer cells having a protective role in lung and kidney cancer but not colon cancer.

Human uterine stem/progenitor cells: their possible role in uterine physiology and pathology

The human uterus mainly consists of the endometrium and the outer smooth muscle layer termed the myometrium. The uterus harbours the exceptional and remarkable regenerative ability responsible for cyclical regeneration and remodelling throughout the reproductive life. The uterus must swiftly and cooperatively enlarge to hold the growing foetus during pregnancy. Furthermore, the endometrium, in particular the functionalis layer, must also regenerate, differentiate and regress with each menstrual cycle under hormonal control. Endometrial regeneration from the basal layer is thought to contribute to replacement of the functionalis layer followed by its slough off during menses and parturition. These morphological and functional features of human endometrium can be reproduced in murine models in which severely immunodeficient mice are xenotransplanted with dispersed human endometrial cells under the kidney capsule. The uterine myometrium possesses the similar plasticity of the endometrium. This is demonstrated by multiple cycles of pregnancy-induced enlargement and regression after parturition. It is likely that regeneration and remodelling in the female reproductive tract are achieved presumably through endometrial and myometrial stem cell systems. Recent evidence now supports the existence of these stem cell systems in humans. Here, we will review our current understanding of uterine stem/progenitor cells. We also propose a novel hypothetical model in which stem cell activities explain the physiological remodelling and regeneration of the human uterus and the pathogenesis of gynaecological diseases such as endometriosis.

In vivo tumor cell rejection induced by NK cell inhibitory receptor blockade: maintained tolerance to normal cells even in the presence of IL-2

Missing-self-reactivity can be mimicked by blocking self-specific inhibitory receptors on NK cells, leading to increased rejection of syngeneic tumor cells. Using a mouse model, we investigated whether Ab-mediated blocking of inhibitory receptors, to a degree where NK cells rejected syngeneic tumor cells, would still allow self-tolerance toward normal syngeneic cells. Ly49C/I inhibitory receptors on C57BL/6 (H-2(b)) NK cells were blocked with F(ab')(2) fragments of the mAb 5E6. Inhibitory receptor blockade in vivo caused rejection of i.v. inoculated fluorescence-labeled syngeneic lymphoma line cells but not of syngeneic spleen cells, BM cells or lymphoblasts. The selective rejection of tumor cells was NK cell-dependent and specifically induced by Ly49C/I blockade. Moreover, selective tumor rejection was maintained after treatment with 5E6 F(ab')(2) for 9 wk, arguing against the induction of NK cell anergy or autoreactivity during this time. Combination therapy using 5E6 F(ab')(2) together with high dose IL-2 treatment further increased lymphoma cell rejection. In addition, combination therapy reduced growth of melanoma cell line tumors established by s.c. inoculation 3 days before start of treatment. Our results demonstrate that inhibitory receptor blockade does not result in attack on normal cells, despite potent reactivity against MHC class I-expressing tumors.

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

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

An innate response to allogeneic nonself mediated by monocytes

The mammalian innate immune system has evolved diverse strategies to distinguish self from microbial nonself. How the innate immune system distinguishes self-tissues from those of other members of the same species (allogeneic nonself) is less clear. To address this question, we studied the cutaneous hypersensitivity response of lymphocyte-deficient RAG(-/-) mice to spleen cells transplanted from either allogeneic or syngeneic RAG(-/-) donors. We found that RAG(-/-) mice mount a specific response to allogeneic cells characterized by swelling and infiltration of the skin with host monocytes/macrophages and neutrophils. The response required prior priming with allogeneic splenocytes or skin grafts and exhibited features of memory as it could be elicited at least 4 wk after immunization. Neither depletion of host NK cells nor rechallenging immunized mice with F(1) hybrid splenocytes inhibited the response, indicating that the response is not mediated by NK cells. Depletion of host monocytes/macrophages or neutrophils at the time of rechallenge significantly diminished the response and, importantly, the adoptive transfer of monocytes from alloimmunized RAG(-/-) mice conferred alloimmunity to naive RAG(-/-) hosts. Unlike NK- and T cell-dependent alloresponses, monocyte-mediated alloimmunity could be elicited only when donor and responder mice differed at non-MHC loci. These observations indicate that monocytes mount a response to allogeneic nonself, a function not previously attributed to them, and suggest the existence of mammalian innate allorecognition strategies distinct from detection of missing self-MHC molecules by NK cells.

Inhibitory Ly49 receptors on mouse natural killer cells

The Ly49 receptors, which are expressed in a stochastic manner on subsets of murine natural killer (NK) cells, T cells, and other cells, are encoded by the Klra gene family and include receptors with either inhibitory or activating function. All of the inhibitory Ly49 receptors are characterized by an immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic domain, which upon phosphorylation recruits tyrosine or lipid phosphatases to dampen signals transmitted through other activating receptors. Most of the inhibitory Ly49 receptors recognize polymorphic epitopes on major histocompatibility complex (MHC) class I proteins as ligands. Here, we review the polymorphism, ligand specificity, and signaling capacity of the inhibitory Ly49 receptors and discuss how these molecules regulate NK cell development and function.

Development of functional human NK cells in an immunodeficient mouse model with the ability to provide protection against tumor challenge

Studies of human NK cells and their role in tumor suppression have largely been restricted to in vitro experiments which lack the complexity of whole organisms, or mouse models which differ significantly from humans. In this study we showed that, in contrast to C57BL/6 Rag2^−/−/γ_c^−/− and NOD/Scid mice, newborn BALB/c Rag2^−/−/γ_c^−/− mice can support the development of human NK cells and CD56+ T cells after intrahepatic injection with hematopoietic stem cells. The human CD56⁺ cells in BALB/c Rag2^−/−/γ_c^−/− mice were able to produce IFN-γ in response to human IL-15 and polyI:C. NK cells from reconstituted Rag2^−/−/γ_c^−/− mice were also able to kill and inhibit the growth of K562 cells in vitro and were able to produce IFN-γ in response to stimulation with K562 cells. In vivo, reconstituted Rag2^−/−/γ_c^−/− mice had higher survival rates after K562 challenge compared to non-reconstituted Rag2^−/−/γ_c^−/− mice and were able to control tumor burden in various organs. Reconstituted Rag2^−/−/γ_c^−/− mice represent a model in which functional human NK and CD56+ T cells can develop from stem cells and can thus be used to study human disease in a more clinically relevant environment.

A Mage3/Heat Shock Protein70 DNA vaccine induces both innate and adaptive immune responses for the antitumor activity

Heat shock proteins (HSPs) are highly effective and versatile molecules in promoting antitumor immune responses. We tested whether a HSP-based DNA vaccine can induce effective immune response against Mage3, a cancer testis (CT) antigen frequently expressed in many human tumors, thereby controlling the Mage3-expressing tumor. The vaccine was constructed by linking human inducible HSP70 to the C-terminus of a modified Mage3 gene (sMage3) that was attached at its N-terminus with the signal leader sequence of the human RANTES for releasing the expressed fusion protein from the transduced cells. Intramuscular injection of sMage3Hsp DNA induced CD4(+)/CD8(+) T cell and antibody responses. Vaccination with sMage3Hsp DNA was more effective in inhibiting Mage3-expressing TC-1 tumors. When we dissected the antitumor activity of CD4(+) and CD8(+) T cells by immunizing CD4(+) and CD8(+) knockout mice with sMage3Hsp DNA, we found that both CD8(+) T and CD4(+) T cells played a role in control of inoculated tumor, but did not constitute the whole of immune protection in the prophylactic immunization. Instead, depletion of natural killer (NK) cells led to a major loss of antitumor activity in the immunized mice. These results indicate that the HSP-based Mage3 DNA vaccine can more effectively inhibit tumor growth by inducing both the innate immune responses and Mage3-specific adaptive immune responses via the Hsp-associated adjuvant function.

Genetic and antibody-mediated reprogramming of natural killer cell missing-self recognition in vivo

Natural killer (NK) cells are lymphocytes of the innate immune system able to recognize and kill tumors lacking self-MHC class I molecules. This "missing-self" recognition is mediated by the lack of engagement of MHC class I-specific inhibitory NK cell receptors that include the killer cell Ig-like receptors (KIR) in humans and Ly49 molecules in mice. A promising immunotherapeutic strategy against MHC class I(+) cancer cells is to block NK cell inhibitory receptors using monoclonal antibodies (mAb). However, interactions between MHC class I molecules and their inhibitory receptors are also required for the acquisition of NK cell functional competence, a process referred as to "education." In addition, inhibitory receptors are involved in self-tolerance on educated NK cells. Here, we developed a preclinical mouse model in which all NK cells are educated by a single transgenic inhibitory receptor, human KIR2DL3, through the engagement with its HLA-Cw3 ligand. This approach revealed that NK cells could be reprogrammed to control the development of mouse syngenic tumors in vivo. Moreover, in vivo anti-KIR mAb treatment induced the killing of HLA(+) target cells without breaking self-tolerance. Finally, the long-term infusion of anti-KIR mAb neither abolished NK cell education nor tumor cell recognition. Therefore, these results strongly support the use of inhibitory receptor blockade in cancer patients.

Probing natural killer cell education by Ly49 receptor expression analysis and computational modelling in single MHC class I mice

Murine natural killer (NK) cells express inhibitory Ly49 receptors for MHC class I molecules, which allows for “missing self” recognition of cells that downregulate MHC class I expression. During murine NK cell development, host MHC class I molecules impose an “educating impact” on the NK cell pool. As a result, mice with different MHC class I expression display different frequency distributions of Ly49 receptor combinations on NK cells. Two models have been put forward to explain this impact. The two-step selection model proposes a stochastic Ly49 receptor expression followed by selection for NK cells expressing appropriate receptor combinations. The sequential model, on the other hand, proposes that each NK cell sequentially expresses Ly49 receptors until an interaction of sufficient magnitude with self-class I MHC is reached for the NK cell to mature. With the aim to clarify which one of these models is most likely to reflect the actual biological process, we simulated the two educational schemes by mathematical modelling, and fitted the results to Ly49 expression patterns, which were analyzed in mice expressing single MHC class I molecules. Our results favour the two-step selection model over the sequential model. Furthermore, the MHC class I environment favoured maturation of NK cells expressing one or a few self receptors, suggesting a possible step of positive selection in NK cell education. Based on the predicted Ly49 binding preferences revealed by the model, we also propose, that Ly49 receptors are more promiscuous than previously thought in their interactions with MHC class I molecules, which was supported by functional studies of NK cell subsets expressing individual Ly49 receptors.

NK cell responsiveness is tuned commensurate with the number of inhibitory receptors for self-MHC class I: the rheostat model

Inhibitory receptors that engage self-MHC class I molecules enable NK cells to detect disease-associated loss of MHC class I on surrounding cells. Previous studies showed that some NK cells lack all receptors for self-MHC class I, yet fail to exhibit autoimmunity because they are generally hyporesponsive to stimulation. We asked whether NK cells exist in only two states, responsive and hyporesponsive, corresponding to cells that express or fail to express inhibitory receptors for self-MHC class I. The alternative model is that NK cells vary continuously in their responsiveness, based on variations in the number of different inhibitory and stimulatory receptors they express, which is known to vary. In this study, we show in the murine system that NK cell responsiveness increases quantitatively with each added self-MHC-specific inhibitory receptor. Genetic analysis demonstrated that interactions of each of the receptors with self-MHC class I were necessary to observe augmented responsiveness. These findings suggest that NK cell responsiveness is comparable to a rheostat: it is tuned to an optimal set point depending on the inhibitory and stimulatory interactions encountered in the normal environment, so as to ensure self-tolerance and yet optimize sensitivity to changes in normal cells.

NK cell education: not an on-off switch but a tunable rheostat

Heterogeneity in the natural killer (NK) cell population is determined by variegated expression of polygenic and polymorphic receptors and distinct phenotypes and functions of NK cell subsets. Here, we outline an additional heterogeneity at the level of MHC-dependent education of NK cells. Based on data obtained using polychromatic flow cytometry and mice with single MHC class I alleles, we argue that NK cell responsiveness is tuned along a continuum determined by the strength of the inhibitory input received by the individual NK cell during education. This model has implications for the use of NK cells in therapeutic settings and affects interpretations of how NK cells control virus infections and regulate autoimmunity.

Beyond licensing and disarming: a quantitative view on NK-cell education

In this article, we will move beyond the current controversy of different models for NK-cell education and instead discuss the properties characterizing MHC class I-guided functional development of NK cells. We will argue that this development, which represents the endpoint of NK-cell education, is a quantitative and dynamic, and perhaps even reversible, process.

Cutting edge: viral infection breaks NK cell tolerance to "missing self"

NK cells attack cells lacking MHC class I, yet MHC class I-deficient mice have normal numbers of NK cells with intact, albeit diminished, functions. Moreover, wild-type NK cells are tolerant of MHC class I-deficient cells in mixed bone marrow chimeras. In this study, we investigated how the absence of MHC class I affects NK cells. NK cells from beta(2)-microglobulin-deficient (B2m(-/-)) and wild-type mice exhibit similar phenotypic and functional characteristics. Both B2m(-/-) and wild-type Ly49H(+) NK cells proliferated robustly and produced IFN-gamma after infection with mouse CMV. NK cells in mixed wild-type:B2m(-/-) chimeric mice were initially tolerant of MHC class I-deficient host cells. However, this tolerance was gradually lost over time and after mouse CMV infection was rapidly broken, with a pronounced rejection of host B2m(-/-) hematopoietic cells. Thus, although NK cells can be held in check against "missing self," acute inflammation driven by infection can rapidly break established self-tolerance.

Immunological tolerance and tumor rejection in embryo-aggregated chimeric mice - lessons for tumor immunity

Background

Rejection of transplanted tumors by the immune system is a rare event in syngeneic hosts, and is considered to be dependent on the local interaction of defensive immune reactions and tumor tolerance mechanisms. Here, we have enlisted the aid of a unique set of embryo-aggregated lineage chimeric mice derived from C57/BL6 and FVB donors to study the interplay between local and systemic tumor immunity and tolerance in rejection of mouse B16 melanoma cells, syngeneic to the C57/BL6 donor strain.

Methods

Two variants of embryo-aggregated chimeric mice with either variable or no contribution of C57-derived cells to their skin were generated by the fusion of different ratios of morula stage blastomers. Chimeric mice were analyzed for s.c. growth of B16 tumors in comparison to their respective donor strains as well as normal F1 hybrids, and the relative frequencies of cellular components of the immune system by FACS analysis of peripheral blood or lymph node cells.

Results

B16 tumors grew significantly faster in mice with full chimerism in their skin as compared to syngeneic C57 or semi-syngeneic C57 × FVB F1 hosts. In contrast, s.c. tumor growth was either absent or significantly reduced in chimeric mice lacking C57-derived cells in their skin, but tolerant to C57 tissue in other organs. Comparison of the relative frequencies of various immune cells in the periphery via FACS-analysis did not reveal any significant differences between the two types of chimeric mice with respect to their donor strains.

Conclusion

Our data suggest a complex interplay between mechanisms of local peripheral tolerance and innate antitumor mechanisms possibly involving NK cell allorecognition as a basis for the differential growth or rejection of B16 tumors in these unique chimeric mice, which we suggest to constitute a valuable new model system for the study of immune-mediated tumor rejection.

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.

NK cell receptors in rodents and cattle

Natural killer (NK) cells discriminate between normal syngeneic cells and infected, neoplastic or MHC-disparate allogeneic cells. The reactivity of NK cells appears to be regulated by a balance between activating receptors that recognize non-self or altered self, and inhibitory receptors recognizing normal, self-encoded MHC class I molecules. Subfamilies of NK receptors undergo rapid evolution, and appear to co-evolve with the MHC. We here review present views on the evolution and function of NK cell receptors, with an emphasis on knowledge gained in cattle and rodents.

Influence of a non-NK complex region of chromosome 6 on CD4+ invariant NK T cell homeostasis

The number and function of immunoregulatory invariant NKT (iNKT) cells are genetically controlled. A defect of iNKT cell ontogeny and function has been implicated as one causal factor of NOD mouse susceptibility to type 1 diabetes. Other factors of diabetes susceptibility, such as a decrease of regulatory T cell function or an increase in TLR1 expression, are corrected in diabetes-resistant Idd6 NOD.C3H 6.VIII congenic mice. Thus, we surmised that the iNKT cell defects found in NOD mice may also be rescued in congenic mice. Unexpectedly, we found, in both the thymus and the periphery, a 50% reduction in iNKT cell number in NOD.C3H 6.VIII mice as compared with NOD mice. This reduction only affected CD4(+) iNKT cells, and left the double negative iNKT cells unchanged. In parallel, the production of IL-4 and IFN-gamma following alpha-GalCer stimulation was proportionally reduced. Using three subcongenic strains, we have narrowed down the region controlling iNKT development within Idd6 (5.8 Mb) to Idd6.2 region (2.5 Mb). Idd6 region had no effect on NK cell number and in vivo cytotoxic activity. These results indicate that the role of iNKT cells in diabetes development is equivocal and more complex than initially considered. In addition, they bring strong evidence that the regulation of CD4(+) iNKT cell production is independent from that of DN iNKT cells, and involves genes of the Idd6 locus.