Search

Detection of mismatched human leukocyte antigens by adaptive immune cells is considered as the main cause of transplant rejection, leading to either T-cell mediated rejection or antibody-mediated rejection. This canonical view guided the successful development of immunosuppressive therapies and shaped the diagnostic Banff classification for kidney transplant rejection that is used in clinics worldwide. However, several observations have recently emerged that question this dichotomization between T-cell mediated rejection and antibody-mediated rejection, related to heterogeneity in the serology, histology, and prognosis of the rejection phenotypes. In parallel, novel insights were obtained concerning the dynamics of donor-specific anti-human leukocyte antigen antibodies, the immunogenicity of donor-recipient non-human leukocyte antigen mismatches, and the autoreactivity against self-antigens. Moreover, the potential of innate allorecognition was uncovered, as exemplified by natural killer cell-mediated microvascular inflammation through missing self, and by the emerging evidence on monocyte-driven allorecognition. In this review, we highlight the gaps in the current classification of rejection, provide an overview of the expanding insights into the mechanisms of allorecognition, and critically appraise how these could improve our understanding and clinical approach to kidney transplant rejection. We argue that consideration of the complex interplay of various allorecognition mechanisms can foster a more integrated view of kidney transplant rejection and can lead to improved risk stratification, targeted therapies, and better outcome after kidney transplantation. ispartof: KIDNEY INTERNATIONAL vol:101 issue:4 pages:692-710 ispartof: location:United States status: published

International audience

This article initially argues that Dickens's pivotal paragraph uniquely requires and inspires 24 pages of text to explore its 158 words in the novel. Esther's “looking two ways” paragraph describes, often through ironic forms of comparison and contrast, her twilight perspectives and projections as she looks through an inn window (or windows) during her and Bucket's search for her mother, Lady Dedlock. The paragraph remarkably looks before and after in the text as it contextually explores the “middle way” dynamics of four major clarifying approaches to Esther's self-development: the via negativa, Holy Grail myths, liminal motifs, and the Freudian uncanny. These four approaches sometimes support one another and sometimes conflict with, if not challenge, one another. Throughout, the article not only discusses and documents Esther's problematic self-development and Dickens's profound artistry in detailing that development, but it also examines the extent of reader involvement in evaluating both Esther and Dickens.

Natural killer (NK) cells are innate immune cells that contribute to host defense against virus infections. NK cells respond to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in vitro and are activated in patients with acute coronavirus disease 2019 (COVID-19). However, by which mechanisms NK cells detect SARS-CoV-2-infected cells remains largely unknown. Here, we show that the Non-structural protein 13 of SARS-CoV-2 encodes for a peptide that is presented by human leukocyte antigen E (HLA-E). In contrast with self-peptides, the viral peptide prevents binding of HLA-E to the inhibitory receptor NKG2A, thereby rendering target cells susceptible to NK cell attack. In line with these observations, NKG2A-expressing NK cells are particularly activated in patients with COVID-19 and proficiently limit SARS-CoV-2 replication in infected lung epithelial cells in vitro. Thus, these data suggest that a viral peptide presented by HLA-E abrogates inhibition of NKG2A+ NK cells, resulting in missing self-recognition.

BACKGROUND: Binding of donor-specific antibodies (DSAs) to kidney allograft endothelial cells that does not activate the classic complement cascade can trigger the recruitment of innate immune effectors, including NK cells. Activated NK cells contribute to microvascular inflammation leading to chronic antibody-mediated rejection (AMR). Recipient NK cells can also trigger antibody-independent microvascular inflammation by sensing the absence of self HLA class I molecules (“missing self”) on allograft endothelial cells. This translational study investigated whether the condition of missing self amplifies DSA-dependent NK cell activation to worsen chronic AMR. METHODS AND RESULTS: Among 1682 kidney transplant recipients who underwent an allograft biopsy at Lyon University Hospital between 2004 and 2017, 135 fulfilled the diagnostic criteria for AMR and were enrolled in the study. Patients with complement-fixing DSAs identified by a positive C3d binding assay (n=73, 54%) had a higher risk of transplant failure (P=0.002). Among the remaining patients with complement-independent chronic AMR (n=62, 46%), those in whom missing self was identified through donor and recipient genotyping exhibited worse allograft survival (P=0.02). In multivariable analysis, only proteinuria (HR: 7.24; P=0.01) and the presence of missing self (HR: 3.57; P=0.04) were independent predictors for transplant failure following diagnosis of chronic AMR. Cocultures of human NK cells and endothelial cells confirmed that addition of missing self to DSA-induced NK cell activation increased endothelial damage. CONCLUSIONS: The assessment of missing self at the time of diagnosis of chronic AMR identifies patients at higher risk for kidney transplant failure.

BACKGROUND: Circulating anti-HLA donor-specific antibodies (HLA-DSA) are often absent in kidney transplant recipients with microvascular inflammation (MVI). Missing self, the inability of donor endothelial cells to provide HLA I-mediated signals to inhibitory killer cell Ig-like receptors (KIRs) on recipient natural killer cells, can cause endothelial damage in vitro, and has been associated with HLA-DSA-negative MVI. However, missing self's clinical importance as a nonhumoral trigger of allograft rejection remains unclear. METHODS: In a population-based study of 924 consecutive kidney transplantations between March 2004 and February 2013, we performed high-resolution donor and recipient HLA typing and recipient KIR genotyping. Missing self was defined as the absence of A3/A11, Bw4, C1, or C2 donor genotype, with the presence of the corresponding educated recipient inhibitory KIR gene. RESULTS: We identified missing self in 399 of 924 transplantations. Co-occurrence of missing self types had an additive effect in increasing MVI risk, with a threshold at two concurrent types (hazard ratio [HR], 1.78; 95% confidence interval [95% CI], 1.26 to 2.53), independent of HLA-DSA (HR, 5.65; 95% CI, 4.01 to 7.96). Missing self and lesions of cellular rejection were not associated. No HLA-DSAs were detectable in 146 of 222 recipients with MVI; 28 of the 146 had at least two missing self types. Missing self associated with transplant glomerulopathy after MVI (HR, 2.51; 95% CI, 1.12 to 5.62), although allograft survival was better than with HLA-DSA-associated MVI. CONCLUSION: Missing self specifically and cumulatively increases MVI risk after kidney transplantation, independent of HLA-DSA. Systematic evaluation of missing self improves understanding of HLA-DSA-negative MVI and might be relevant for improved diagnostic classification and patient risk stratification. ispartof: pages:8-8 ispartof: JOURNAL OF THE AMERICAN SOCIETY OF NEPHROLOGY vol:32 issue:8 pages:2070-2082 ispartof: location:United States status: published

BACKGROUND: Binding of donor-specific antibodies (DSAs) to kidney allograft endothelial cells that does not activate the classic complement cascade can trigger the recruitment of innate immune effectors, including NK cells. Activated NK cells contribute to microvascular inflammation leading to chronic antibody-mediated rejection (AMR). Recipient NK cells can also trigger antibody-independent microvascular inflammation by sensing the absence of self HLA class I molecules ("missing self") on allograft endothelial cells. This translational study investigated whether the condition of missing self amplifies DSA-dependent NK cell activation to worsen chronic AMR. METHODS AND RESULTS: Among 1682 kidney transplant recipients who underwent an allograft biopsy at Lyon University Hospital between 2004 and 2017, 135 fulfilled the diagnostic criteria for AMR and were enrolled in the study. Patients with complement-fixing DSAs identified by a positive C3d binding assay (n=73, 54%) had a higher risk of transplant failure (P=0.002). Among the remaining patients with complement-independent chronic AMR (n=62, 46%), those in whom missing self was identified through donor and recipient genotyping exhibited worse allograft survival (P=0.02). In multivariable analysis, only proteinuria (HR: 7.24; P=0.01) and the presence of missing self (HR: 3.57; P=0.04) were independent predictors for transplant failure following diagnosis of chronic AMR. Cocultures of human NK cells and endothelial cells confirmed that addition of missing self to DSA-induced NK cell activation increased endothelial damage. CONCLUSIONS: The assessment of missing self at the time of diagnosis of chronic AMR identifies patients at higher risk for kidney transplant failure. ispartof: JOURNAL OF THE AMERICAN SOCIETY OF NEPHROLOGY vol:32 issue:2 pages:479-494 ispartof: location:United States status: published

Resistance to parental bone marrow (BM) grafts in F1 hybrid recipients is due to natural killer (NK) cell–mediated rejection triggered through “missing self” recognition. “Hybrid resistance” has usually been investigated in lethally irradiated F1 recipients in conjunction with pharmacological activation of NK cells. Here, we investigated BM‐directed NK‐cell alloreactivity in settings of reduced conditioning. Nonlethally irradiated (1‐3 Gy) or nonirradiated F1 (C57BL6 × BALB/c) recipient mice received titrated doses (5‐20 x 106) of unseparated parental BALB/c BM without pharmacological NK cell activation. BM successfully engrafted in all mice and multilineage donor chimerism persisted long‐term (24 weeks), even in the absence of irradiation. Chimerism was associated with the rearrangement of the NK‐cell receptor repertoire suggestive of reduced reactivity to BALB/c. Chimerism levels were lower after transplantation with parental BALB/c than with syngeneic F1 BM, indicating partial NK‐mediated rejection of parental BM. Activation of NK cells with polyinosinic–polycytidylic acid sodium salt poly(I:C), reduced parental chimerism in nonirradiated BM recipients but did not prevent hematopoietic stem cell engraftment. In contrast, equal numbers of parental lymph node cells were completely rejected. Hence, hybrid resistance leads to incomplete rejection of parental BM under reduced conditioning settings., Modifying conditioning in the hybrid resistance model reveals that lasting chimerism ensues after nonmyeloablative irradiation, even when no recipient irradiation is given, indicating that natural killer cell–mediated bone marrow rejection is incomplete in reduced conditioning settings.

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.

Viruses have evolved strategies that highlight critical, intertwined host immune mechanisms. As postulated by the missing-self hypothesis, natural killer (NK) cells and major histocompatibility complex class I (MHC-I)-restricted cytotoxic T lymphocytes (CTLs) have opposing requirements for ubiquitously expressed MHC-I molecules. Since NK cell MHC-I-specific Ly49 inhibitory receptors prevent killing of cells with normal MHC-I, viruses evading CTLs by down-regulating MHC-I should be vulnerable to NK cells. However, definitive integrated in vivo evidence for this interplay has been lacking, in part due to receptor polymorphism and a proposed second function of Ly49 receptors in licensing NK cells via self-MHC-I. Here we generated mice lacking specific Ly49 inhibitory receptors to show their essential role in licensing and controlling murine cytomegalovirus (MCMV) infection in vivo in an MHC-restricted manner. When MCMV cannot down-regulate MHC-I, NK cells cannot control infection that instead is mediated by CTLs, as predicted by the missing-self hypothesis.

Natural killer (NK) cells are attractive candidates for allogeneic cell-based immunotherapy due to their potent antitumor effector function and good safety profile. NK cells express killer immunoglobulin-like receptors (KIRs) and the NKG2A receptor important for NK cells education as well as providing inhibitory signals upon encountering HLA-expressing target cells. Multiple myeloma (MM) is an example of a tumor expressing relatively high levels of HLA molecules. In this review, we discuss the functional relevance of inhibitory KIRs and NKG2A for NK cells anti-MM response and strategies to lower these inhibitory signaling to enhance clinical efficacy of allogeneic NK cells in MM.

Natural killer (NK) cells are effectors of the innate immune system and recognize cells transformed by viruses or neoplasia. Their response to "missing self" signals was described 3 decades ago, but the recent discovery of a panoply of activating receptors has made it clear that NK cell reactivity arises from a combination of inhibitory and activating signals. Successful clinical exploitation of NK cell reactivity was demonstrated in allogeneic transplantation for acute myelogenous leukemia from HLA-haploidentical donors when matched donors were not available. Multiple clinical studies have since attempted to use NK reactivity in the setting of both HLA-matched and -mismatched transplantation, with varying results. This review summarizes the heterogeneous clinical results and explains them based on a succinct description of NK cell biology.

The transplantation of cardiomyocytes derived from embryonic stem (ES) cells into infarcted heart has been shown to improve heart function in animal models. However, immune rejection of transplanted cells may hamper the clinical application of this approach. Natural killer (NK) cells could play an important role in this process in both autologous and allogeneic settings by eliminating cells expressing low levels of major histocompatibility complex (MHC) class I molecules. Here we characterize embryonic stem cell-derived cardiomyocytes (ESCM) in terms of their sensitivity to NK cells. We show that despite expression of very low levels of MHC class I molecules, murine ESCM were neither recognized nor lysed by activated syngeneic NK cells in vitro. In contrast, undifferentiated ES cells expressing similarly low levels of MHC class I molecules as ESCM were recognized and lysed by NK cells. This differential susceptibility results from the differential expression of ligands for the major activating natural killer cell receptor natural-killer group 2 member D (NKG2D) and intercellular adhesion molecule 1 (ICAM-1) on ES cells versus ESCM. NKG2D ligands and ICAM-1 were expressed on ES cells but were absent from ESCM. Undifferentiated ES cells were lysed by NK cells in a perforin-dependent manner. However, simultaneous blockade of NKG2D and ICAM-1 by antibodies inhibited this killing. These data suggest that in the course of differentiation ESCM acquire resistance to NK cell-mediated lysis by downregulating the expression of ligands required for activation of NK cell cytotoxicity.

The idea that NK cells can distinguish aberrant cells by recognizing 'absence of the expected', rather than 'presence of the unexpected' emerged more than 25 years ago. Klas Karre recapitulates how the idea took shape, and the first five years of experimental work to test its general predictions.

MHC-I–specific receptors play a vital role in NK cell–mediated “missing-self” recognition, which contributes to NK cell activation. In contrast, MHC-independent NK recognition mechanisms are less well characterized. In this study, we investigated the role of NKR-P1B:Clr-b (Klrb1:Clec2d) interactions in determining the outcome of murine hematopoietic cell transplantation in vivo. Using a competitive transplant assay, we show that Clr-b−/− bone marrow (BM) cells were selectively rejected by wild-type B6 recipients, to a similar extent as H-2Db−/− MHC-I–deficient BM cells. Selective rejection of Clr-b−/− BM cells was mitigated by NK depletion of recipient mice. Competitive rejection of Clr-b−/− BM cells also occurred in allogeneic transplant recipients, where it was reversed by selective depletion of NKR-P1Bhi NK cells, leaving the remaining NKR-P1Blo NK subset and MHC-I–dependent missing-self recognition intact. Moreover, competitive rejection of Clr-b−/− hematopoietic cells was abrogated in Nkrp1b-deficient recipients, which lack the receptor for Clr-b. Of interest, similar to MHC-I–deficient NK cells, Clr-b−/− NK cells were hyporesponsive to both NK1.1 (NKR-P1C)–stimulated and IL-12/18 cytokine–primed IFN-γ production. These findings support a unique and nonredundant role for NKR-P1B:Clr-b interactions in missing-self recognition of normal hematopoietic cells and suggest that optimal BM transplant success relies on MHC-independent tolerance mechanisms. These findings provide a model for human NKR-P1A:LLT1 (KLRB1:CLEC2D) interactions in human hematopoietic cell transplants.

Much modern research is quite focused, attempting to understand aspects of systems that are already revealed in broad outline. Researching a problem at its birth is an opportunity that many will never experience, a lonely exercise that many may avoid. In these situations, pre-existing findings from

This undoubted faith in the integrity of the photograph is often rudely shaken; for while photographs may not lie, liars may photograph. (Hine 1909, 3) In “Ban the Sunset? Nonpropositional Content ...

0143–4004/03/$-see front matter and almost every volume of Trophoblast Research has contained his reviews. Yung Wai Loke (known to everyone as Charlie) was born in Penang, an island off the West Coast of Malaysia. His family fled to Singapore at the time of the Japanese invasion and he remembers the dog fights overhead before Singapore was captured in 1941. The remainder of the war was spent under the Japanese occupation in Kuala Lumpur living on a diet of brown rice. After the war he was despatched to England, a journey taking three days by air, and was educated at the Leys School and then King’s College in Cambridge. He then moved to St Thomas’s Hospital and qualified as a doctor in 1960 but, after spending a year as a houseman, he left England to return to Malaya where he worked as a Pathologist. It was there that his life was influenced in two important ways; he reduced his golf handicap to four and developed an interest in hydatidiform moles and choriocarcinomas, both common tumours in the Far East. There were strong pulls to settle permanently in Malaya and join the family business (his grandfather had emigrated from Southern China and founded the tin and rubber industries in Malaya), but in 1967 Charlie felt he should return to Cambridge. He was appointed as a Demonstrator in the Department of Pathology where he stayed for the next 35 years. In one of his earliest publications, arising from work performed in Malaya, he counted Barr bodies in placental tumours and found they were all XX in chromosome constitution and inferred (naively as it now turns out) that female placentae are more prone to the development of trophoblast tumours than males (Loke, 1969). Charlie’s long interest in trophoblast had begun. He became fascinated by how the allogeneic nature of choriocarcinomas might influence their behaviour (Loke, 1975) and the extension of this question to the normal placenta became his main intellectual preoccupation. Is the placenta discerned by the mother as self, nonself, missing self or surrogate self? How does this happen? Trophoblast cells are extraembryonic, arising from trophectoderm as the first differentiated cell of the embryo. The placenta is neither mother nor fetus, but provides an interface, a crucial intermediary, a no man’s land between these two genetically different individuals. There is an analogy with this concept in Charlie himself, buffered between England and Malaysia yet a To whom correspondence should be addressed at: Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QP, UK. Tel.: 44 1223 333727; Fax: 44 1223 765065; E-mail: am485@cam.ac.uk Figure 1. Charlie Loke, Melbourne, Australia, October 2002.

In this article, I outline the studies that led to the discovery of Ly49A as the first ‘missing-self ’ receptor on natural killer cells. It is a more detailed and formal version of my acceptance speech at the Novartis Award ceremony at the International Congress of Immunology in Stockholm. Yet, it is impossible to mention my gratitude to all of my previous mentors and colleagues or to credit all the scientists and immunologists who preceded us by making contributions, large and small, to knowledge upon which our findings were based. It is therefore humbling to accept the 2001 Novartis Award for Basic Immunology along with Professors Klas Karre and Lorenzo Moretta.

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.

Natural killer (NK) cells represent a first line of defense against pathogens and tumor cells. The activation of NK cells is regulated by the integration of signals deriving from activating and inhibitory receptors expressed on their surface. However, different NK cells respond differently to the same stimulus, be it target cells or agents that crosslink activating receptors. The processes that determine the level of NK cell responsiveness have been referred to collectively as NK cell education. NK cell education plays an important role in steady state conditions, where potentially auto-reactive NK cells are rendered tolerant to the surrounding environment. According to the "tuning" concept, the responsiveness of each NK cell is quantitatively adjusted to ensure self tolerance while at the same time ensuring useful reactivity against potential threats. MHC-specific inhibitory receptors displayed by NK cells play a major role in tuning NK cell responsiveness, but recent studies indicate that signaling from activating receptors is also important, suggesting that the critical determinant is an integrated signal from both types of receptors. An important and still unresolved question is whether NK cell education involves interactions with a specific cell population in the environment. Whether hematopoietic and/or non-hematopoietic cells play a role is still under debate. Recent results demonstrated that NK cell tuning exhibits plasticity in steady state conditions, meaning that it can be re-set if the MHC environment changes. Other evidence suggests, however, that inflammatory conditions accompanying infections may favor high responsiveness, indicating that inflammatory agents can over-ride the natural tendency of NK cells to adjust to the steady state environment. These findings raise many questions such as whether viruses and tumor cells manipulate NK cell responsiveness to evade immune-recognition. As knowledge of the underlying processes grows, the possibility of modulating NK cell responsiveness for therapeutic purposes is becoming increasingly attractive, and is now under serious investigation in clinical studies. © 2014 Elsevier Ltd.

Natural killer (NK) cells have different roles in the host response against Plasmodium-induced malaria depending on the stage of infection. Liver NK cells have a protective role during the initial hepatic stage of infection by production of the TH1-type cytokines IFN-γ and TNF-α. In the subsequent erythrocytic stage of infection, NK cells also induce protection through Th1-type cytokines but, in addition, may also promote development of cerebral malaria via CXCR3-induction on CD8(+) T cells resulting in migration of these cells to the brain. We have recently shown that the regulatory Ly49E NK receptor is expressed on liver NK cells in particular. The main objective of this study was therefore to examine the role of Ly49E expression in the immune response upon Plasmodium berghei ANKA infection, for which we compared wild type (WT) to Ly49E knockout (KO) mice. We show that the parasitemia was higher at the early stage, i.e. at days 6-7 of Plasmodium berghei ANKA infection in Ly49E KO mice, which correlated with lower induction of CD69, IFN-γ and TNF-α in DX5(-) liver NK cells at day 5 post-infection. At later stages, these differences faded. There was also no difference in the kinetics and the percentage of cerebral malaria development and in lymphocyte CXCR3 expression in WT versus Ly49E KO mice. Collectively, we show that the immune response against Plasmodium berghei ANKA infection is not drastically affected in Ly49E KO mice. Although NK cells play a crucial role in Plasmodium infection and Ly49E is highly expressed on liver NK cells, the Ly49E NK receptor only has a temporarily role in the immune control of this parasite. ispartof: PLoS One vol:9 issue:1 ispartof: location:United States status: published

We appreciate the interest of Drs Behrendt and Zaia in our recent study of allogeneic killer cell immunoglobulin-like receptor (KIR)/HLA combinations in HIV-1 transmission,[1][1] and we are grateful for the opportunity to respond to their comments. Behrendt and Zaia reanalyzed our data by testing

The elusive task of defining the character of γδ T cells has been an evolving process for immunologists since stumbling upon their existence during the molecular characterization of the α and β T cell receptor genes of their better understood brethren. Defying the categorical rules used to distinctly characterize lymphocytes as either innate or adaptive in nature, γδ T cells inhabit a hybrid world of their own. At opposing ends of the simplified spectrum of modes of antigen recognition used by lymphocytes, natural killer and αβ T cells are particularly well equipped to respond to the ‘missing self' and the ‘dangerous non-self', respectively. However, between these two reductive extremes, we are chronically faced with the challenge of making peace with the ‘safe non-self' and dealing with the inevitable ‘distressed self', and it is within this more complex realm γδ T cells excel thanks to their highly empathetic nature. This review gives an overview of the latest insights revealing the unfolding story of human γδ T cells, providing a biographical sketch of these unique lymphocytes in an attempt to capture the essence of their fundamental nature and events that influence their life trajectory. What hangs in their balance is their nuanced ability to differentiate the friends from the foe and the pathological from the benign to help us adapt swiftly and efficiently to life's many stresses.

Balance of signals coming from activating and inhibitory receptors expressed on the surface of NK cells regulates their response during acute mouse cytomegalovirus (MCMV) infection. In order to sidestep recognition by CD8+ T cells, CMVs encode proteins that downmodulate expression of MHC I on the surface of infected cells. Although this process should make infected cells prone to ‘missing-self’-mediated killing, in most laboratory mouse strains MCMV efficiently avoids early NK cell control. MCMV encodes three proteins involved in the regulation of MHC I expression. Whereas m152 and m06 downregulate MHC I, m04 instead binds to these proteins in the ER, forming complexes that reach the cell surface. We hypothesized that this mechanism evolved to prevent NK cell activation and killing by restoring the ‘self’ signature and allowing the engagement of inhibitory Ly49 receptors by their natural ligands. We have shown that indeed, the ligation of an inhibitory Ly49A NK cell receptor leads to a protection of infected cells from specific lysis, impaired proliferation of NK cells and loss of control of viral titer in mice of H-2d and H-2k haplotype, all during the first 3 days of infection (Babić et al, J Exp Med, 2010). In addition, we have shown that m04 is essential for the recognition of infected cells by the activating Ly49 receptors (Ly49P, Ly49P1, Ly49D2 and Ly49L), which restrictively recognize MHC I, together with m04 and another, unidentified viral component. Our results show that the recognition of MHC I/m04 complex by Ly49L+ NK cells in BALB.K mice leads to a specific expansion of this NK cell subset and increased IFN-γ production resulting in the faster clearance of infectious virus from the organ of these mice when compared to BALB/c or BALB.B mice (days 6-10 pi ; Pyzik et al, J Exp Med, 2011). Our current efforts are aimed to define the mechanisms leading to activation of NK cells in the context of the above described results.

In order to sidestep antigen presentation and recognition by CD8+T cells, cytomegaloviruses (CMVs) encode immunoevasion proteins that downmodulate the expression of MHC class I on the surface of infected cells. However, by lowering interactions and triggering through inhibitory Ly49 receptors, this process makes infected cells prone to ‘missing-self’ -mediated killing by NK cells. The aim of this study was to explain the mechanisms by which MCMV avoids NK cell control, in spite of the early and strong downregulation of MHC-I molecules on the surface of infected cells. MCMV encodes three proteins involved in the regulation of MHC-I expression. The m152-encoded gp40 glycoprotein arrests MHC-I at the level of the ERGIC/cis-Golgi compartment, whereas m06-encoded gp48 redirects MHC-I complexes to lysosomes for degradation. The third MCMV regulator of MHC-I expression is gp34, encoded by the m04 gene. Unlike the other two, m04/gp34 does not prevent the surface expression of MHC-I but instead binds to these proteins in the ER, forming complexes that can reach the cell surface. It has been suggested that m04/gp34 might inhibit NK cell activation through its ability to escort MHC-I to the cell surface and serve as an NK cell decoy. Our hypothesis was that this mechanism evolved to prevent NK cell activation and killing by restoring the ‘self’ signature and allowing the engagement of inhibitory Ly49 receptors by their natural ligands. In addition, we have evidence that m04/gp34 is essential for the recognition of infected cells by the activating Ly49P and Ly49L receptors, which both recognize MHC-I molecule (H-2Dk or H-2Dd, respectively), together with m04 product and another, so far unidentified viral component. Here we show that the target cells infected with the virus lacking m04 (m04) were unable to activate a reporter cell line expressing inhibitory Ly49A receptor. Additionally, m04 MCMV was attenuated in vivo in an NK cell- and MHC-I-dependent manner. Such NK cell control of the infection was dependent on the presence of NK cell subsets expressing different inhibitory Ly49 receptors. Therefore, here we provide the first evidence that NK cell recognition of the ‘missing-self’ is relevant in the recognition and control of a viral pathogen in vivo. However, our preliminary data show that in addition, also other MCMV encoded MHC-I-like molecules might serve to engage inhibitory Ly49 receptors and it remains to be determined how these contribute to compromise ‘missing-self’ mediated recognition of infected cells.

Regulation of the NK cell response in acute mouse cytomegalovirus (MCMV) infection is dependent on the fine balance of signals coming from activating and inhibitory receptors expressed on their cell surface. Here, we provide evidence that MCMV protein, m04/gp34, is essential for the recognition of infected cells by several activating Ly49 receptors, which recognize MHC I molecule, together with m04 product and another, so far unidentified viral component. Our results show that the recognition of MHC I/m04 complex by Ly49L+ NK cells in BALB.K mice leads to a specific expansion of this NK cell subset and their dominant IFN-γ production resulting in faster clearance of infectious virus from the organ of these mice when compared to BALB/c or BALB.B mice (day 6-10) (Pyzik M et al, J Exp Med, 2011). However, in the early days of infection (day 1-4), mice of H-2d and H-2k haplotype are unable to restrain viral replication in spite of the early and strong downregulation of MHC I molecules from the surface of infected cells which should lead to a 'missing self'-mediated recognition by NK cells. It has been suggested that m04/gp34 might inhibit NK cell activation through its ability to escort MHC I to the cell surface and serve as an NK cell decoy. Our hypothesis was that this mechanism evolved to prevent NK cell activation and killing by restoring the 'self' signature and allowing the engagement of inhibitory Ly49 receptors by their natural ligands. Our results indeed show that the ligation of an inhibitory Ly49A NK cell receptor leads to the protection of infected cells from specific lysis, impaired proliferation of NK cells and loss of control of viral replication in vivo in mice of H-2d and H-2k haplotype, all during the first 3 days of infection (Babić M et al, J Exp Med, 2010). In addition, we here suggest that the strong 'missing self'-mediated control of Δm04 MCMV infection by NK cells does not impair the later ongoing CD8+ T cell response. Our results implicate that this is most likely due to the preserved number and activity of plasmacytoid DCs and maintainance of conventional DCs, when compared to WT MCMV infection.

Cytomegaloviruses (CMVs) are renowned for interfering with the immune system of their hosts. To sidestep antigen presentation and destruction by CD8+ T cells, these viruses reduce expression of major histocompatibility complex class I (MHC I) molecules. However, this process sensitizes the virus-infected cells to natural killer (NK) cell–mediated killing via the “missing self” axis. Mouse cytomegalovirus (MCMV) uses m152 and m06 encoded proteins to inhibit surface expression of MHC I molecules. In addition, it encodes another protein, m04, which forms complexes with MHC I and escorts them to the cell surface. This mechanism is believed to prevent NK cell activation and killing by restoring the “self” signature and allowing the engagement of inhibitory Ly49 receptors on NK cells. Here we show that MCMV lacking m04 was attenuated in an NK cell– and MHC I–dependent manner. NK cell–mediated control of the infection was dependent on the presence of NK cell subsets expressing different inhibitory Ly49 receptors. In addition to providing evidence for immunoevasion strategies used by CMVs to avoid NK cell control via the missing-self pathway, our study is the first to demonstrate that missing self–dependent NK cell activation is biologically relevant in the protection against viral infection in vivo.

In this issue of Immunity, Voigt et al. (2007) and Lu et al. (2007) extend the understanding of the consequences of missing-self recognition in immune evasion and immune cell regulation via NKRP-1 and CD94-NKG2A inhibitory receptors, respectively.

In 1986, Joe Phillips and I published a paper in The Journal of Immunology identifying the CD56bright+, CD16−, and CD56+, CD16+ NK cell subsets in humans that began, “Natural killer (NK) cells are a population of lymphocytes derived from an unimmunized host that lyse certain tumor cell lines and

The past ten years have witnessed dramatic progress in our understanding of how natural killer (NK) cells function, their role in innate defenses and their possible exploitation in therapy. This article traces the major advances in these formerly mysterious cells, from the ‘missing self' hypothesis and the first discovery of HLA-class I-specific inhibitory receptors to a recent major breakthrough that highlighted important perspectives and major expectations regarding the cure of life-threatening leukemias. The key role of ‘alloreactive' NK cells in eradicating acute myeloid leukemias and in preventing both graft rejection and graft-versus-host disease, might lead to a true revolution in bone marrow transplantation. Thus, it might now be possible to search for appropriate HLA class I mismatches to set NK cells in action.