Olga V. Naidenko

Recognition of a virus-encoded ligand by a natural killer cell activation receptor

Natural killer (NK) cells express inhibitory and activation receptors that recognize MHC class I-like molecules on target cells. These receptors may be involved in the critical role of NK cells in controlling initial phases of certain viral infections. Indeed, the Ly49H NK cell activation receptor confers in vivo genetic resistance to murine cytomegalovirus (MCMV) infections, but its ligand was previously unknown. Herein, we use heterologous reporter cells to demonstrate that Ly49H recognizes MCMV-infected cells and a ligand encoded by MCMV itself. Exploiting a bioinformatics approach to the MCMV genome, we find at least 11 ORFs for molecules with previously unrecognized features of predicted MHC-like folds and limited MHC sequence homology. We identify one of these, m157, as the ligand for Ly49H. m157 triggers Ly49H-mediated cytotoxicity, and cytokine and chemokine production by freshly isolated NK cells. We hypothesize that the other ORFs with predicted MHC-like folds may be involved in immune evasion or interactions with other NK cell receptors.

Activation of natural killer T cells by α -galactosylceramide treatment prevents the onset and recurrence of autoimmune Type 1 diabetes

Type 1 diabetes (T1D) in non-obese diabetic (NOD) mice may be favored by immune dysregulation leading to the hyporesponsiveness of regulatory T cells and activation of effector T-helper type 1 (Th1) cells. The immunoregulatory activity of natural killer T (NKT) cells is well documented, and both interleukin (IL)-4 and IL-10 secreted by NKT cells have important roles in mediating this activity. NKT cells are less frequent and display deficient IL-4 responses in both NOD mice and individuals at risk for T1D (ref. 8), and this deficiency may lead to T1D (refs. 1,6–9). Thus, given that NKT cells respond to the α-galactosylceramide (α-GalCer) glycolipid in a CD1d-restricted manner by secretion of Th2 cytokines, we reasoned that activation of NKT cells by α-GalCer might prevent the onset and/or recurrence of T1D. Here we show that α-GalCer treatment, even when initiated after the onset of insulitis, protects female NOD mice from T1D and prolongs the survival of pancreatic islets transplanted into newly diabetic NOD mice. In addition, when administered after the onset of insulitis, α-GalCer and IL-7 displayed synergistic effects, possibly via the ability of IL-7 to render NKT cells fully responsive to α-GalCer. Protection from T1D by α-GalCer was associated with the suppression of both T- and B-cell autoimmunity to islet β cells and with a polarized Th2-like response in spleen and pancreas of these mice. These findings raise the possibility thatα-GalCer treatment might be used therapeutically to prevent the onset and recurrence of human T1D.

The natural killer T-cell ligand α-galactosylceramide prevents autoimmune diabetes in non-obese diabetic mice

Diabetes in non-obese diabetic (NOD) mice is mediated by pathogenic T-helper type 1 (Th1) cells that arise because of a deficiency in regulatory or suppressor T cells. Vα14–Jα15 natural killer T (NKT) cells recognize lipid antigens presented by the major histocompatibility complex class I-like protein CD1d (refs. 3,4). We have previously shown that in vivo activation of Vα14 NKT cells by α-galactosylceramide (α-GalCer) and CD1d potentiates Th2-mediated adaptive immune responses. Here we show that α-GalCer prevents development of diabetes in wild-type but not CD1d-deficient NOD mice. Disease prevention correlated with the ability of α-GalCer to suppress interferon-γ but not interleukin-4 production by NKT cells, to increase serum immunoglobulin E levels, and to promote the generation of islet autoantigen-specific Th2 cells. Because α-GalCer recognition by NKT cells is conserved among mice and humans, these findings indicate that α-GalCer might be useful for therapeutic intervention in human diseases characterized by Th1-mediated pathology such as Type 1 diabetes.

Human NKT Cells Mediate Antitumor Cytotoxicity Directly by Recognizing Target Cell CD1d with Bound Ligand or Indirectly by Producing IL-2 to Activate NK Cells

α-Galactosylceramide (αGalCer) stimulates NKT cells and has antitumor activity in mice. Murine NKT cells may directly kill tumor cells and induce NK cell cytotoxicity, but the mechanisms are not well defined. Newly developed human CD1d/αGalCer tetrameric complexes were used to obtain highly purified human αGalCer-reactive NKT cell lines (>99%), and the mechanisms of NKT cell cytotoxicity and activation of NK cells were investigated. Human NKT cells were cytotoxic against CD1d− neuroblastoma cells only when they were rendered CD1d+ by transfection and pulsed with αGalCer. Four other CD1d− tumor cell lines of diverse origin were resistant to NKT cells, whereas Jurkat and U937 leukemia cell lines, which are constitutively CD1d+, were killed. Killing of the latter was greatly augmented in the presence of αGalCer. Upon human CD1d/αGalCer recognition, NKT cells induced potent cytotoxicity of NK cells against CD1d− neuroblastoma cell lines that were not killed directly by NKT cells. NK cell activation depended upon NKT cell production of IL-2, and was enhanced by secretion of IFN-γ. These data demonstrate that cytotoxicity of human NKT cells can be CD1d and ligand dependent, and that TCR-stimulated NKT cells produce IL-2 that is required to induce NK cell cytotoxicity. Thus, NKT cells can mediate potent antitumor activity both directly by targeting CD1d and indirectly by activating NK cells.

Cutting Edge: Murine UL16-Binding Protein-Like Transcript 1: A Newly Described Transcript Encoding a High-Affinity Ligand for Murine NKG2D

Murine NKG2D is known to recognize H60 and five RAE1 variants. The human homologue recognizes both inducible MHC class I chain-related gene and constitutive (UL16-binding protein (ULBP)) ligands. Widely expressed, the latter are thought to mark transformed or infected cells for destruction by NK cells in the context of down-regulated cell surface class I (i.e., the “missing self”-response). Unlike MIC and ULBP however, mRNA for the murine ligands appears only in very limited contexts in the mature animal. In this study, we describe a NKG2D ligand termed “murine ULBP-like transcript 1 (MULT1) whose mRNA appears to be widely expressed in adult parenchyma. This molecule possesses MHC class I-like α1 and α2 domains as well as a large cytoplasmic domain. Recombinant MULT1 binds NKG2D with relatively high affinity (KD ≈ 6 nM) and low koff (∼0.006s−1). Expression of MULT1 by normally resistant RMA cells results in their susceptibility to lysis by C57BL/6 splenocytes.

CD1d-Restricted NKT Cells: An Interstrain Comparison

CD1d-restricted Vα14-Jα281 invariant αβTCR+ (NKT) cells are well defined in the C57BL/6 mouse strain, but they remain poorly characterized in non-NK1.1-expressing strains. Surrogate markers for NKT cells such as αβTCR+CD4−CD8− and DX5+CD3+ have been used in many studies, although their effectiveness in defining this lineage remains to be verified. Here, we compare NKT cells among C57BL/6, NK1.1-congenic BALB/c, and NK1.1-congenic nonobese diabetic mice. NKT cells were identified and compared using a range of approaches: NK1.1 expression, surrogate phenotypes used in previous studies, labeling with CD1d/α-galactosylceramide tetramers, and cytokine production. Our results demonstrate that NKT cells and their CD4/CD8-defined subsets are present in all three strains, and confirm that nonobese diabetic mice have a numerical and functional deficiency in these cells. We also highlight the hazards of using surrogate phenotypes, none of which accurately identify NKT cells, and one in particular (DX5+CD3+) actually excludes these cells. Finally, our results support the concept that NK1.1 expression may not be an ideal marker for CD1d-restricted NKT cells, many of which are NK1.1-negative, especially within the CD4+ subset and particularly in NK1.1-congenic BALB/c mice.

The Vα14 NKT Cell TCR Exhibits High-Affinity Binding to a Glycolipid/CD1d Complex

Most CD1d-dependent NKT cells in mice have a canonical Vα14Jα18 TCR rearrangement. However, relatively little is known concerning the molecular basis for their reactivity to glycolipid Ags presented by CD1d. Using glycolipid Ags, soluble forms of a Vα14 NKT cell-derived TCR, and mutant and wild-type CD1d molecules, we probed the TCR/CD1d interaction by surface plasmon resonance, tetramer equilibrium staining, and tetramer staining decay experiments. By these methods, several CD1d α-helical amino acids could be defined that do not greatly alter lipid binding, but that affect the interaction with the TCR. Binding of the Vα14+ TCR to CD1d requires the agonist α-galactosylceramide (α-GalCer), as opposed to the nonantigenic β-galactosylceramide, although both Ags bind to CD1d, indicating that the carbohydrate moiety of the CD1d-bound Ag plays a major role in the TCR interaction. The TCR has a relatively high-affinity binding to the α-GalCer/CD1d complex, with a particularly slow off rate. These unique properties are consistent with the coreceptor-independent action of the Vα14 TCR and may be related to the intense response to α-GalCer by NKT cells in vivo.

Costimulation through NKG2D Enhances Murine CD8+ CTL Function: Similarities and Differences between NKG2D and CD28 Costimulation

Multiple studies have demonstrated that the NK cell activating receptor NKG2D can function as a costimulatory receptor for both mouse and human CD8+ T cells. However, it has recently been suggested that stimulation through NKG2D is insufficient for costimulation of CD8+ T cells. To aid in the delineation of NKG2D function in CTL responses, we investigated whether stimulation of NKG2D by the natural ligand RAE1ε was able to costimulate effector functions of a murine CTL line generated from DUC18 TCR transgenic mice. We found that NKG2D was able to costimulate DUC CTL responses and did so in a manner similar to CD28 costimulation. The T cells exhibited increased proliferation, IFN-γ release, and cytotoxicity when presented antigenic peptide by P815 cells expressing RAE1ε or B7-1 compared with untransfected P815. In addition, both RAE1ε and B7-1 enhanced Ag-independent IFN-γ secretion in response to IL-12 and IL-18 by DUC CTL. However, only costimulation through CD28 allowed for DUC CTL survival upon secondary stimulation, whereas ligation of NKG2D, but not CD28, induced DUC CTL to form an immune synapse with target cells in the absence of TCR stimulation. Understanding the outcomes of these differences may allow for a better understanding of T cell costimulation in general.

Natural killer T cells reactive to a single glycolipid exhibit a highly diverse T cell receptor β repertoire and small clone size

CD1d-restricted natural killer (NK) T cells reactive with the glycolipid α-galactosylceramide (α-GalCer) are a distinct lymphocyte sublineage. They express an invariant Vα14-Jα18 T cell receptor (TcR), but the role of the β chain has been controversial. Here, we have used CD1d tetramers to identify and isolate NK T cells based on their antigen specificity. In mice lacking germline Vβ8, most of the α-GalCer-reactive T cells express either Vβ2 or Vβ7, strong Vβ selection being revealed by the lack of an increase in other Vβ regions. By contrast to the selection for complementarity determining region (CDR) 3β sequences in some anti-peptide responses, α-GalCer-reactive T cells have polyclonal CDR3β sequences. There is little CDR3β sequence redundancy between organs or individual mice, and, surprisingly, there also is no evidence for organ-specific CDR3β sequence motifs. These data argue against a T cell receptor-mediated self-reactivity for tissue-specific CD1d-bound ligands. Each NKT clone is represented by only 5–10 cells. This clone size is similar to naive conventional T cells, and much lower than that reported for memory T cells, although NK T cells have an activated/memory phenotype.

Genetic Control of NKT Cell Numbers Maps to Major Diabetes and Lupus Loci

Natural killer T cells are an immunoregulatory population of lymphocytes that plays a critical role in controlling the adaptive immune system and contributes to the regulation of autoimmune responses. We have previously reported deficiencies in the numbers and function of NKT cells in the nonobese diabetic (NOD) mouse strain, a well-validated model of type 1 diabetes and systemic lupus erythematosus. In this study, we report the results of a genetic linkage analysis of the genes controlling NKT cell numbers in a first backcross (BC1) from C57BL/6 to NOD.Nkrp1b mice. The numbers of thymic NKT cells of 320 BC1 mice were determined by fluorescence-activated cell analysis using anti-TCR Ab and CD1/α-galactosylceramide tetramer. Tail DNA of 138 female BC1 mice was analyzed for PCR product length polymorphisms at 181 simple sequence repeats, providing greater than 90% coverage of the autosomal genome with an average marker separation of 8 cM. Two loci exhibiting significant linkage to NKT cell numbers were identified; the most significant (Nkt1) was on distal chromosome 1, in the same region as the NOD mouse lupus susceptibility gene Babs2/Bana3. The second most significant locus (Nkt2) mapped to the same region as Idd13, a NOD-derived diabetes susceptibility gene on chromosome 2.