NKG2D in innate and adaptive immunity

Bidirectional crosstalk between therapeutic cancer vaccines and the tumor microenvironment: Beyond tumor antigens

Immunotherapy has rejuvenated cancer therapy, especially after anti-PD-(L)1 came onto the scene. Among the many therapeutic options, therapeutic cancer vaccines are one of the most essential players. Although great progress has been made in research on tumor antigen vaccines, few phase III trials have shown clinical benefits. One of the reasons lies in obstruction from the tumor microenvironment (TME). Meanwhile, the therapeutic cancer vaccine reshapes the TME in an ambivalent way, leading to immune stimulation or immune escape. In this review, we summarize recent progress on the interaction between therapeutic cancer vaccines and the TME. With respect to vaccine resistance, innate immunosuppressive TME components and acquired resistance caused by vaccination are both involved. Understanding the underlying mechanism of this crosstalk provides insight into the treatment of cancer by directly targeting the TME or synergizing with other therapeutics.

NKG2D Ligands-Critical Targets for Cancer Immune Escape and Therapy

DNA damage, oncogene activation and excessive proliferation, chromatin modulations or oxidative stress are all important hallmarks of cancer. Interestingly, all of these abnormalities also induce a cellular stress response. By upregulating “stress-induced ligands,” damaged or transformed cells can be recognized by immune cells and cleared. The human genome encodes eight functional “stress-induced ligands”: MICA, MICB, and ULBP1-6. All of them are recognized by a single receptor, NKG2D, which is expressed on natural killer (NK) cells, cytotoxic T cells and other T cell subsets. The NKG2D ligand/NKG2D-axis is well-recognized as an important mediator of anti-tumor activity; however, patient data about the role of NKG2D ligands in immune surveillance and escape appears conflicting. As these ligands are often actively transcribed, tumor cells are urged to manipulate the expression of these ligands on post-transcriptional or post-translational level. Although our knowledge on the regulation of NKG2D ligand expression remains fragmentary, research of the past years revealed multiple cellular mechanisms that are adopted by tumor cells to reduce the expression of “stress-induced ligands” and therefore escape immune recognition. Here, we review the post-transcriptional and post-translational mechanisms by which NKG2D ligands are modulated in cancer cells and their impact on patient prognosis.We discuss controversies and approaches to apply our understanding of the NKG2D ligand/NKG2D-axis for cancer therapy.

Axitinib induces DNA damage response leading to senescence, mitotic catastrophe, and increased NK cell recognition in human renal carcinoma cells

Tyrosine kinase inhibitors (TKIs) including axitinib have been introduced in the treatment of renal cell carcinoma (RCC) because of their anti-angiogenic properties. However, no evidence are presently available on a direct cytotoxic anti-tumor activity of axitinib in RCC.

Herein we reported by western blot analysis that axitinib treatment induces a DNA damage response (DDR) initially characterized by γ-H2AX phosphorylation and Chk1 kinase activation and at later time points by p21 overexpression in A-498 and Caki-2 RCC cells although with a different potency. Analysis by immunocytochemistry for the presence of 8-oxo-7,8-dihydro-2′-deoxyguanosine in cellular DNA and flow cytometry using the redox-sensitive fluorescent dye DCFDA, demonstrated that DDR response is accompanied by the presence of oxidative DNA damage and reactive oxygen species (ROS) generation. This response leads to G2/M cell cycle arrest and induces a senescent-like phenotype accompanied by enlargement of cells and increased senescence-associated β-galactosidase activity, which are abrogated by N-acetyl cysteine (NAC) pre-treatment. In addition, axitinib-treated cells undergo to cell death through mitotic catastrophe characterized by micronucleation and abnormal microtubule assembly as assessed by fluorescence microscopy.

On the other hand, axitinib, through the DDR induction, is also able to increase the surface NKG2D ligand expression. Accordingly, drug treatment promotes NK cell recognition and degranulation in A-498 RCC cells in a ROS-dependent manner.

Collectively, our results indicate that both cytotoxic and immunomodulatory effects on RCC cells can contribute to axitinib anti-tumor activity.

Genetics, genomics, and evolutionary biology of NKG2D ligands

Human and mouse NKG2D ligands (NKG2DLs) are absent or only poorly expressed by most normal cells but are upregulated by cell stress, hence, alerting the immune system in case of malignancy or infection. Although these ligands are numerous and highly variable (at genetic, genomic, structural, and biochemical levels), they all belong to the major histocompatibility complex class I gene superfamily and bind to a single, invariant, receptor: NKG2D. NKG2D (CD314) is an activating receptor expressed on NK cells and subsets of T cells that have a key role in the recognition and lysis of infected and tumor cells. Here, we review the molecular diversity of NKG2DLs, discuss the increasing appreciation of their roles in a variety of medical conditions, and propose several explanations for the evolutionary force(s) that seem to drive the multiplicity and diversity of NKG2DLs while maintaining their interaction with a single invariant receptor.

NKG2D expression by CD8+ T cells contributes to GVHD and GVT effects in a murine model of allogeneic HSCT

In allogeneic hematopoietic stem cell transplantation (HSCT), controlling graft-versus-host disease (GVHD) while maintaining graft-versus-tumor (GVT) responses is of critical importance. Using a mouse model of allogeneic HSCT, we hereby demonstrate that NKG2D expression by CD8(+) T cells plays a major role in mediating GVHD and GVT effects by promoting the survival and cytotoxic function of CD8(+) T cells. The expression of NKG2D ligands was not induced persistently on normal tissues of allogeneic HSCT-recipient mice treated with anti-NKG2D antibody, suggesting that transient NKG2D blockade might be sufficient to attenuate GVHD and allow CD8(+) T cells to regain their GVT function. Indeed, short-term treatment with anti-NKG2D antibody restored GVT effects while maintaining an attenuated GVHD state. NKG2D expression was also detected on CD8(+) T cells from allogeneic HSCT patients and trended to be higher in those with active GVHD. Together, these data support a novel role for NKG2D expression by CD8(+) T cells during allogeneic HSCT, which could be potentially therapeutically exploited to separate GVHD from GVT effects.

Enhanced prevalence of plasmatic soluble MHC class I chain-related molecule in vascular pregnancy diseases

The major histocompatibility complex class I related chain (MIC) is a stress-inducible protein modulating the function of immune natural killer (NK) cells, a major leukocyte subset involved in proper trophoblast invasion and spiral artery remodeling. Aim of the study was to evaluate whether upregulation of soluble MIC (sMIC) may reflect immune disorders associated to vascular pregnancy diseases (VPD). sMIC was more frequently detected in the plasma of women with a diagnostic of VPD (32%) than in normal term-matched pregnancies (1.6%, P < 0.0001), with highest prevalence in intrauterine fetal death (IUDF, 44%) and vascular intrauterine growth restriction (IUGR, 39%). sMIC levels were higher in preeclampsia (PE) than in IUFD (P < 0.01) and vascular IUGR (P < 0.05). sMIC detection was associated with bilateral early diastolic uterine notches (P = 0.037), thrombocytopenia (P = 0.03), and high proteinuria (P = 0.03) in PE and with the vascular etiology of IUGR (P = 0.0038). Incubation of sMIC-positive PE plasma resulted in downregulation of NKG2D expression and NK cell-mediated IFN-γ production in vitro. Our work thus suggests that detection of sMIC molecule in maternal plasma may constitute a hallmark of altered maternal immune functions that contributes to vascular disorders that complicate pregnancy, notably by impairing NK-cell mediated production of IFN-γ, an essential cytokine favoring vascular modeling.

Differential expression of natural killer activating and inhibitory receptors in patients with newly diagnosed systemic lupus erythematosus

AIM

Systemic lupus erythematosus (SLE) presents as the abnormal activation and over-proliferation of immune competent cells. Few studies have characterized the role of natural killer (NK) and NK T (NKT) cells in the pathogenesis of SLE, and therefore a consensus has not been reached as yet.

METHOD

Thirty-two patients with new-onset SLE and 15 healthy controls were recruited. Activated and inhibitory NK and NKT cells in peripheral blood were quantified by flow cytometry. The proportions of spontaneous and stimulated interferon (IFN)-γ(+) NK and NKT cells and CD107a(+) NK cells was examined. Finally, the potential relationship between the cell subsets and clinical indexes was analyzed.

RESULTS

The proportions of NK and NKT cells (P = 0.002 and 0.004, respectively) as well as the proportions of NKG2C(+) NK cells, inhibitory NK and NKT cell subsets (P = 0.016, P = 0.019, P = 0.049, and P = 0.028, respectively) in SLE patients were significantly lower than those in controls. In contrast, the proportions of activated NK cells and NKT cell subsets were significantly higher (P = 0.036, P = 0.034, P = 0.005, and P = 0.007, respectively). Moreover, the proportions of stimulated IFN-γ(+) NKT cells were significantly higher than in the controls, and the proportions of stimulated CD107a(+) NKT cells in SLE patients were significantly lower than in the controls (P = 0.032 and P = 0.02, respectively).

CONCLUSION

Lower proportions of NK and NKT cells, higher proportions of activated NK cells and activated NKT cells, lower proportions of inhibitory NK and NKT cells, higher NKT cell activity, and lower NKT cell degranulation may induce the autoimmune reaction involved in the pathogenesis of SLE.

Innate-like behavior of human invariant natural killer T cells during herpes simplex virus infection

Invariant natural killer T (iNKT) cells, CD1d restricted T cells, are involved in the immune responses against various infection agents. Here we describe their behavior during reactivation of human herpes simplex virus (HSV). iNKT cells exhibit only discrete changes, which however, reached statistically significant level due to the relatively large patient group. Higher percentage of iNKT cells express NKG2D. iNKT cells down-regulate NKG2A in a subset of patients. Finally, iNKT cells enhance their capacity to produce TNF-α. Our data suggests that iNKT cells are involved in the immune response against HSV and contribute mainly to its early, innate phase.

Phenotype and function of natural killer cells in systemic lupus erythematosus: excess interferon-γ production in patients with active disease

OBJECTIVE

To determine the phenotype and the functionality of natural killer (NK) cells in patients with systemic lupus erythematosus (SLE).

METHODS

A total of 94 patients with SLE (91 women and 3 men) were compared with 26 healthy controls. Active SLE was defined by an SLE Disease Activity Index score≥4. Immunologic tests were performed using nonactivated and/or interleukin-2 (IL-2)-activated peripheral blood mononuclear cells. NK cell phenotype was determined by flow cytometry. NK cell natural cytotoxicity and antibody-dependent cellular cytotoxicity (ADCC) were determined by 51Cr release and CD107a degranulation experiments. Intracellular interferon-γ (IFNγ) production by NK cells was evaluated after overnight stimulation with IL-12 and IL-18. IFNα levels were assessed using an antiviral cytopathic bioassay.

RESULTS

The absolute NK cell count was decreased in patients with active SLE, but the relative frequencies of total CD3-CD56bright NK cells and CD3-CD56dim NK cells were unaffected. The CD3-CD56dim NK cells in patients with active SLE displayed unique phenotypic characteristics, including significant increases in CD69 and NKG2A and decreased expression of Fcγ receptor type IIIa/CD16, CD8α, and the killer cell immunoglobulin-like receptor (KIR) KIR2DL1/KIR2DS1. Concomitant with these findings, NK cells from SLE patients had lower cytotoxicity but a normal level of ADCC compared with NK cells from healthy controls. There was a significant positive correlation between the increased level of IFNα in the serum and the enhanced frequency of IFNγ+ cells in patients with active SLE (r=0.370, P=0.04).

CONCLUSION

NK cells in patients with active SLE display phenotypic and functional features associated with activation. Furthermore, NK cells from patients with active SLE have the capacity to produce large amounts of IFNγ. This could contribute to the dysregulation of the link between innate and adaptive immunity seen in SLE.

Thy1+ NK [corrected] cells from vaccinia virus-primed mice confer protection against vaccinia virus challenge in the absence of adaptive lymphocytes

While immunological memory has long been considered the province of T- and B- lymphocytes, it has recently been reported that innate cell populations are capable of mediating memory responses. We now show that an innate memory immune response is generated in mice following infection with vaccinia virus, a poxvirus for which no cognate germline-encoded receptor has been identified. This immune response results in viral clearance in the absence of classical adaptive T and B lymphocyte populations, and is mediated by a Thy1⁺ subset of natural killer (NK) cells. We demonstrate that immune protection against infection from a lethal dose of virus can be adoptively transferred with memory hepatic Thy1⁺ NK cells that were primed with live virus. Our results also indicate that, like classical immunological memory, stronger innate memory responses form in response to priming with live virus than a highly attenuated vector. These results demonstrate that a defined innate memory cell population alone can provide host protection against a lethal systemic infection through viral clearance.

Immunological memory is a hallmark of adaptive immunity and provides the basis for our ability to become ‘immune’ to pathogens to which we have previously been exposed, and provides the basis for vaccination. For decades, the paradigm held that only the classical adaptive lymphocytes were capable of forming and maintaining protective immunological memory. Recently, several papers have shown the capacity of an innate cell population, a subset of natural killer (NK) cells, to exhibit certain aspects of immunological memory. Here we show that innate memory forms in response to infection with vaccinia virus and resides in a discrete subset of NK cells. We further demonstrate that this innate memory provides significant host protection against a subsequent systemic infection with a lethal dose of vaccinia virus, in some cases resulting in the complete clearance of detectable virus. We also demonstrate that priming with live, replicating virus stimulates innate memory more robustly than a highly attenuated vector. These findings shed new light on this emergent area of immunology, and hold significant implications for harnessing innate memory as part of creating novel vaccination strategies.

Reduced frequencies of NKp30+NKp46+, CD161+, and NKG2D+ NK cells in acute HCV infection may predict viral clearance

BACKGROUND & AIMS

While the majority of HCV-infected patients progress to chronic hepatitis, a small fraction of individuals are able to clear the virus. Resolution of infection occurs within the first few weeks to months of infection, suggesting that innate immune functions may be critical for early control. Epidemiologic data support a role for particular NK cell receptor bearing populations in this control, yet the mechanism by which NK cells respond to HCV early in infection is unknown.

METHODS

Changes in the phenotype and function of NK cells were investigated in a cohort of 43 individuals identified during various stages of HCV infection with different clinical outcomes.

RESULTS

Acute, chronic, and resolved HCV infections were characterized by an expansion of CD56(neg) NK cells. Furthermore, increased levels of HLA-C-binding KIR(+) NK cells were observed in HCV resolvers, while all stages of HCV infection were associated with reduced percentages of NKG2D(+), NKp30(+), and NKp46(+) NK cells, and a slight increase in the ability of NK cells to respond to target cells bearing the ligands for these receptors. In contrast, NKG2A(+) and CD94(+) NK cells were elevated in acute and chronic HCV infection, but not in resolved infection. Most importantly, in acute infection, lower frequencies of NKp30(+), NKp46(+), CD161(+), and NKG2D(+) NK cells were observed in patients who were subsequently able to clear HCV infection than in those becoming chronically infected.

CONCLUSIONS

These data implicate particular populations of NK cells in the early control and clearance of HCV infection.

Low numbers and altered phenotype of invariant natural killer T cells in recurrent varicella zoster virus infection

Invariant natural killer T (iNKT) cells play an important role in the immune response against various infectious agents. In this study we investigated their role in human defense against the varicella zoster virus. We observed decreased numbers of iNKT cells in patients who failed to control latent varicella zoster virus infection, e.g. underwent several reactivations of the virus. The residual population of iNKT cells expressed significantly higher levels of inhibitory receptor CD158a that was further up-regulated in the course of acute viral infection. Both of these abnormalities might contribute to impaired control of varicella zoster virus in human.

Recombinant mouse cytomegalovirus expressing a ligand for the NKG2D receptor is attenuated and has improved vaccine properties

Human CMV (HCMV) is a major cause of morbidity and mortality in both congenitally infected and immunocompromised individuals. Development of an effective HCMV vaccine would help protect these vulnerable groups. NK group 2, member D (NKG2D) is a potent activating receptor expressed by cells of the innate and adaptive immune systems. Its importance in HCMV immune surveillance is indicated by the elaborative evasion mechanisms evolved by the virus to avoid NKG2D. In order to study this signaling pathway, we engineered a recombinant mouse CMV expressing the high-affinity NKG2D ligand RAE-1γ (RAE-1γMCMV). Expression of RAE-1γ by MCMV resulted in profound virus attenuation in vivo and lower latent viral DNA loads. RAE-1γMCMV infection was efficiently controlled by immunodeficient hosts, including mice lacking type I interferon receptors or immunosuppressed by sublethal γ-irradiation. Features of MCMV infection in neonates were also diminished. Despite tight innate immune control, RAE-1γMCMV infection elicited strong and long-lasting protective immunity. Maternal RAE-1γMCMV immunization protected neonatal mice from MCMV disease via placental transfer of antiviral Abs. Despite strong selective pressure, the RAE-1γ transgene did not exhibit sequence variation following infection. Together, our results indicate that use of a recombinant virus encoding the ligand for an activating NK cell receptor could be a powerful approach to developing a safe and immunogenic HCMV vaccine.

Sialylation of 3-methylcholanthrene-induced fibrosarcoma determines antitumor immune responses during immunoediting

Sialylation of tumor cells is involved in various aspects of their malignancy (proliferation, motility, invasion, and metastasis); however, its effect on the process of immunoediting that affects tumor cell immunogenicity has not been studied. We have shown that in mice with impaired immunoediting, such as in IL-1α(-/-) and IFNγ(-/-) mice, 3-methylcholanthrene-induced fibrosarcoma cells are immunogenic and concomitantly bear low levels of surface sialylation, whereas tumor cells derived from wild type mice are nonimmunogenic and bear higher levels of surface sialylation. To study immune mechanisms whose interaction with tumor cells involves surface sialic acid residues, we used highly sialylated 3-methylcholanthrene-induced nonimmunogenic fibrosarcoma cell lines from wild type mice, which were treated with sialidase to mimic immunogenic tumor cell variants. In vivo and in vitro experiments revealed that desialylation of tumor cells reduced their growth and induced cytotoxicity by NK cells. Moreover, sialidase-treated tumor cells better activated NK cells for IFN-γ secretion. The NKG2D-activating receptor on NK cells was shown to be involved in interactions with desialylated ligands on tumor cells, the nature of which is still not known. Thus, the degree of sialylation on tumor cells, which is selected during the process of immunoediting, has possibly evolved as an important mechanism of tumor cells with low intrinsic immunogenicity or select for tumor cells that can evade the immune system or subvert its function. When immunoediting is impaired, such as in IFN-γ(-/-) and IL-1α(-/-) mice, the overt tumor consists of desialylayed tumor cells that interact better with immunosurveillance cells.

Copy number variation in the mouse genome: implications for the mouse as a model organism for human disease

Individuals within a species have genetic differences which ultimately result in the spectrum of phenotypic variation that we observe. Genetic variation exists at the nucleotide level in the form of single nucleotide polymorphisms (SNPs), and at a structural level as inversions, deletions and amplifications of larger stretches of nucleotides. Profiling of human and mouse genomes has identified numerous genomic segmental copy number variations (CNVs) throughout these genomes. Since inbred mice are widely used laboratory models for the study of both normal and disease biology, it is crucial that we understand the full scope of genetic variation, including CNVs, within these animals. These genetic differences can inform us about the history of a population or species, enlighten us on gene function, and guide our selection of a model system for the study of human disease.

Two novel NKG2D ligands of the mouse H60 family with differential expression patterns and binding affinities to NKG2D

H60, originally described as a dominant minor histocompatibility Ag, is an MHC class I-like molecule that serves as a ligand for the NKG2D receptor. In the present study, we identified two novel mouse chromosome 10-encoded NKG2D ligands structurally resembling H60. These ligands, which we named H60b and H60c, encode MHC class I-like molecules with two extracellular domains. Whereas H60b has a transmembrane region, H60c is a GPI-anchored protein. Recombinant soluble H60b and H60c proteins bound to NKG2D with affinities typical of cell-cell recognition receptors (K(d) = 310 nM for H60b and K(d) = 8.7 muM for H60c). Furthermore, expression of H60b or H60c rendered Ba/F3 cells susceptible to lysis by NK cells, thereby establishing H60b and H60c as functional ligands for NKG2D. H60b and H60c transcripts were detected only at low levels in tissues of healthy adult mice. Whereas H60b transcripts were detectable in various tissues, H60c transcripts were detected mainly in the skin. Infection of mouse embryonic fibroblasts with murine cytomegalovirus induced expression of H60b, but not H60c or the previously known H60 gene, indicating that transcriptional activation of the three types of H60 genes is differentially regulated. The present study adds two new members to the current list of NKG2D ligands.

Immunological aspects of axon injury in multiple sclerosis

The role of immune-mediated axonal injury in the induction of nonremitting functional deficits associated with multiple sclerosis is an area of active research that promises to substantially alter our understanding of the pathogenesis of this disease and modify or change our therapeutic focus. This review summarizes the current state of research regarding changes in axonal function during demyelination, provides evidence of axonal dysmorphia and degeneration associated with demyelination, and identifies the cellular and molecular effectors of immune-mediated axonal injury. Finally, a unifying hypothesis that links neuronal stress associated with demyelination-induced axonal dysfunction to immune recognition and immunopathology is provided in an effort to shape future experimentation.

IFN-gamma production dominates the early human natural killer cell response to Coxsackievirus infection

Coxsackieviruses (CV) are important human pathogens that have been implicated in the pathogenesis of several diseases, including myocarditis and pancreatitis. How the human immune system recognizes and controls CV infections is not well understood. Studies in mice suggest that natural killer (NK) cells play a critical role in viral clearance and host survival, but the mechanism(s) by which human NK cells may contribute to the host anti-CV defence has not been investigated. Here we show that CVB3 infection markedly reduces HLA class I cell surface expression but does not increase the expression of the activating NK cell receptor ligands MICA/B and ULBP1-3 on human cells. We also demonstrate that the lowered target cell HLA class I surface expression does not correlate with an increased susceptibility to NK cell-mediated killing. However, NK cells responded with a robust production of interferon gamma (IFN-gamma) when peripheral blood mononuclear cells were cocultured with infected cells. In summary, this study shows that CVB3 interferes with the expression of NK cell receptor ligands on infected cells and indicates that IFN-gamma production, rather than cytotoxicity, marks the early human NK cell response to CVB3 infection.

NK cells rapidly reject allogeneic bone marrow in the spleen through a perforin- and Ly49D-dependent, but NKG2D-independent mechanism

We have used a sensitive and specific in vivo killing assay to monitor the kinetics, anatomic location and mechanisms controlling NK-mediated rejection of Balb/c bone marrow by C57BL/6 natural killer (NK) cells. We find that NK killing of fully allogeneic bone marrow is a rapid, highly efficient process, leading to substantial rejection of transplanted marrow within 6 h of transplant and elimination of 85% of the transplanted cells within 2 days. NK-mediated rejection occurred predominantly in the spleen, with sparing of rejection in the bone marrow and lymph nodes. Rejection was dependent on Perforin gene function, but was independent of interferon-gamma. Finally, rejection of Balb/c bone marrow by B6 NK cells required signaling through the Ly49D receptor, but occurred despite blockade of NKG2D, which distinguishes these results from previous studies using semiallogeneic transplant pairs. These results identify NK cells as highly active mediators of bone marrow rejection, and suggest that inhibiting NK function early during transplantation may increase the efficiency of engraftment and allow successful engraftment of limiting doses of donor bone marrow.

Decidual NK cells regulate key developmental processes at the human fetal-maternal interface

Human CD56(bright) NK cells accumulate in the maternal decidua during pregnancy and are found in direct contact with fetal trophoblasts. Several mechanisms have been proposed to explain the inability of NK cells to kill the semiallogeneic fetal cells. However, the actual functions of decidual NK (dNK) cells during pregnancy are mostly unknown. Here we show that dNK cells, but not peripheral blood-derived NK subsets, regulate trophoblast invasion both in vitro and in vivo by production of the interleukin-8 and interferon-inducible protein-10 chemokines. Furthermore, dNK cells are potent secretors of an array of angiogenic factors and induce vascular growth in the decidua. Notably, such functions are regulated by specific interactions between dNK-activating and dNK-inhibitory receptors and their ligands, uniquely expressed at the fetal-maternal interface. The overall results support a 'peaceful' model for reproductive immunology, in which elements of innate immunity have been incorporated in a constructive manner to support reproductive tissue development.

NKG2D-mediated activation of cytotoxic lymphocytes: unique signaling pathways and distinct functional outcomes

NKG2D is an important immunosurveillance receptor that triggers a unique signal transduction pathway resulting in a variety of functional outcomes. NKG2D couples to the non-ITAM-containing DAP10 and initiates at least two signaling branches that are both required for cytotoxicity. Transformed, infected, or healthy cells can express NKG2D ligands, and NKG2D(+) lymphocytes can be unaffected, costimulated, or fully activated by the NKG2D-ligand interaction. The NKG2D-mediated response can be modulated by factors such as cytokine milieu and possibly the particular ligand expressed. Thus, this nontraditional NKG2D-DAP10 initiated signal pathway enables lymphocytes to differentially respond to ligands based on the cellular environment.

How TCRs bind MHCs, peptides, and coreceptors

Since the first crystal structure determinations of alphabeta T cell receptors (TCRs) bound to class I MHC-peptide (pMHC) antigens in 1996, a sizable database of 24 class I and class II TCR/pMHC complexes has been accumulated that now defines a substantial degree of structural variability in TCR/pMHC recognition. Recent determination of free and bound gammadelta TCR structures has enabled comparisons of the modes of antigen recognition by alphabeta and gammadelta T cells and antibodies. Crystal structures of TCR accessory (CD4, CD8) and coreceptor molecules (CD3epsilondelta, CD3epsilongamma) have further advanced our structural understanding of most of the components that constitute the TCR signaling complex. Despite all these efforts, the structural basis for MHC restriction and signaling remains elusive as no structural features that define a common binding mode or signaling mechanism have yet been gleaned from the current set of TCR/pMHC complexes. Notwithstanding, the impressive array of self, foreign (microbial), and autoimmune TCR complexes have uncovered the diverse ways in which antigens can be specifically recognized by TCRs.

NKG2D splice variants: a reexamination of adaptor molecule associations

NKG2D is a homodimeric C-type lectin-related receptor expressed on natural killer (NK) cells and T cells. In mice, alternative deoxyribonucleic acid (DNA) splicing generates two isoforms of NKG2D that differ in the length of their cytoplasmic domains. Their ability to induce cellular activation is mediated via association with two membrane-bound, signaling adaptor molecules, DAP10 and DAP12. It has been reported that the long form of NKG2D associates exclusively with DAP10, whereas the short variant can interact with either adaptor. The short isoform was reported to be almost undetectable in naive NK cells. Using two distinct cell types, we demonstrate that like the short isoform, the long variant of NKG2D also associates not only with DAP10 but also with DAP12. Using reporter cells (70Z/3), we demonstrate that DAP12 can compete equally with DAP10 for association with both variants of NKG2D when DAP10 and DAP12 are coexpressed. Cross-linking either isoform of NKG2D induces a calcium flux when associated exclusively with DAP10 or DAP12. Moreover, using quantitative polymerase chain reaction (PCR), we also show that the short isoform of NKG2D is expressed in naive NK cells. Our data suggest that signaling via mouse NKG2D isoforms is more complex than originally presented.

MIC and other NKG2D ligands: from none to too many

NKG2D, a prime activatory receptor on human NK, CD8(+) alphabeta and gammadelta cells, has a variety of ligands, which, despite sharing membership of the MHC class I structural club, display an array of unique features. Chronologically, human MIC molecules were the first NKG2D ligands to be identified. Then came RAET1 (ULBP) molecules, which were identified in both man and mouse, as well as H60 and MULT1, which have no counterparts in man to date. The question remains as to why, more than how, the evolutionary conserved, apparently monomorphic, single copy, NKG2D, can/should adapt to this variety of ligands, and when it does, what is the evolutionary advantage of this profusion of ligands for a single receptor?