Mouse CD94/NKG2A is a natural killer cell receptor for the nonclassical major histocompatibility complex (MHC) class I molecule Qa-1(b)

Long-term donor chimerism after MHC (RT1) mismatched bone marrow transplantation in the rat: the role of host alloreactive NK cells

We have investigated the role of major histocompatibility complex (MHC) (RT1) disparities in the engraftment of bone marrow (BM) cells after whole body irradiation of rats. Mononuclear BM cells from PVG.RT7.2 (RT1c) rats were injected i.v. into sublethally (10Gy) whole body irradiated PVG (RT1c) rats and RT1 congenic and recombinant PVG rats. Repopulation of the BM, spleen, and blood with donor cells was assessed by FACS analysis of cells labelled with the fluorescein isothiocyanate (FITC)-labelled HIS41 monoclonal antibody (MoAb) against the RT7.2 marker. In RT1 matched (PVG.RT7.2 --> PVG) and RT1-mismatched combinations (PVG.RT7.2 --> PVG.1AV1), where radioresistant host natural killer (NK) cells could not recognize the BM inoculum as foreign, a donor chimerism close to 100% was observed after 6-8 weeks. However, in rat strain combinations where host NK cells could recognize an RT1 mismatch, almost no donor cells survived, and the rats were repopulated with leukocytes of host origin. In intra-MHC recombinant rat strains the element determining rejection or acceptance of the allograft mapped to the RT1-B/D-C/E/M region in PVG.R8 and PVG.R23 rats, in accordance with the patterns of NK alloreactivity in these strain combinations. NK cells may therefore be a primary obstacle to successful allogeneic BM engraftment in this model.

MHC class I recognition by NK receptors in the Ly49 family is strongly influenced by the beta 2-microglobulin subunit

NK cell recognition of targets is strongly affected by MHC class I specific receptors. The recently published structure of the inhibitory receptor Ly49A in complex with H-2Dd revealed two distinct sites of interaction in the crystal. One of these involves the alpha1, alpha2, alpha3, and beta2-microglobulin (beta2m) domains of the MHC class I complex. The data from the structure, together with discrepancies in earlier studies using MHC class I tetramers, prompted us to study the role of the beta2m subunit in MHC class I-Ly49 interactions. Here we provide, to our knowledge, the first direct evidence that residues in the beta2m subunit affect binding of MHC class I molecules to Ly49 receptors. A change from murine beta2m to human beta2m in three different MHC class I molecules, H-2Db, H-2Kb, and H-2Dd, resulted in a loss of binding to the receptors Ly49A and Ly49C. Analysis of the amino acids involved in the binding of Ly49A to H-2Dd in the published crystal structure, and differing between the mouse and the human beta2m, suggests the cluster formed by residues Lys3, Thr4, Thr28, and Gln29, as a potentially important domain for the Ly49A-H-2Dd interaction. Another possibility is that the change of beta2m indirectly affects the conformation of distal parts of the MHC class I molecule, including the alpha1 and alpha2 domains of the heavy chain.

MHC Class I-Ly49 Interactions Shape the Ly49 Repertoire on Murine NK Cells

This study aims to determine how the interaction of Ly49 receptors with MHC class I molecules shapes the development of the Ly49 repertoire. We have examined the percentage of NK cells that expressed Ly49A, Ly49G2, and Ly49D in single and double Ly49A/C-transgenic mice on four different MHC backgrounds, H-2(b), H-2(d), H-2(b/d), and beta(2)-microglobulin(-/-). The results show that the total numbers of NK cells were not different among the strains. The prior expression of a Ly49 receptor capable of binding to self MHC class I altered the percentage of NK cells expressing endogenous Ly49A, Ly49G2, and Ly49D even in mice in which no MHC ligand was present for the latter receptors. The NK cells in the Ly49-transgenic mice expressed the same level of endogenous Ly49 receptors as wild-type mice of a similar MHC background. In contrast, the number of NK T cells was reduced in mice in which the Ly49 transgene could bind to a MHC class I molecule. The onset of Ly49 receptor expression on NK cells during ontogeny was not altered in the presence of transgenic Ly49 receptors. These data support a sequential model and argue against a selection model for Ly49 repertoire development on NK cells.

NK and T cells constitute two major, functionally distinct intestinal epithelial lymphocyte subsets in the chicken

Non-mammalian NK cells have not been characterized in detail; however, their analysis is essential for the understanding of the NK cell receptor phylogeny. As a first step towards defining chicken NK cells, several tissues were screened for the presence of NK cells, phenotypically defined as CD8(+) cells lacking T- or B-lineage specific markers. By this criteria, approximately 30% of CD8(+) intestinal intraepithelial lymphocytes (IEL), but <1% of splenocytes or peripheral blood lymphocytes were defined as NK cells. These CD8(+)CD3(-) IEL were used for the generation of the 28-4 mAb, immunoprecipitating a 35-kDa glycoprotein with a 28-kDa protein core. The CD3 and 28-4 mAb were used to separate IEL into CD3(+) IEL T cells and 28-4(+) cells, both co-expressing the CD8 antigen. During ontogeny, 28-4(+) cells were abundant in the IEL and in the embryonic spleen, where two subsets could be distinguished according to their CD8 and c-kit expression. Most importantly, 28-4(+) IEL lysed NK-sensitive targets, whereas intestinal T cells did not have any spontaneous cytolytic activity. These results define two major, phenotypically and functionally distinct IEL subpopulations, and imply an important role of NK cells in the mucosal immune system.

Recognition of tumor cells by the innate immune system

There has been a rapid increase in our understanding of the cellular components of the innate immune system, the receptors used to distinguish changes in homeostasis, and how these components integrate into an anti-tumor effector response. Recently, significant progress has been made in the identification of ligands for receptors that activate NK cells, and the results have implications for the recognition of tumor cells.

Expression of Ly49E and CD94/NKG2 on fetal and adult NK cells

Murine NK cells express inhibitory receptors belonging to the Ly49 and CD94/NKG2 family. Ly49E and CD94 are the only NK cell receptor transcripts detectable in fetal NK cells. Still unproved is the surface expression of Ly49E on NK cells. Here we generated two novel mAbs, a mAb recognizing Ly49E with cross-reactivity to Ly49C, and a mAb against NKG2A/C/E. Ly49E was immunoprecipitated as a disulfide-linked homodimer with 46-kDa subunits. Removal of N-linked carbohydrates revealed a 31-kDa protein backbone. NKG2A was immunoprecipitated as a 38-kDa protein. Although the frequency of fetal NK cells expressing Ly49E was higher than 25%, it decreased drastically from 2 wk after birth. Phenotypic analysis showed that approximately 90% of fetal NK cells and approximately 50% of adult NK cells express high levels of CD94/NKG2. The remaining 50% of adult NK cells expressed low surface levels of CD94/NKG2. Expression of Ly49E and CD94/NKG2 was not restricted to NK cells, but was also observed on NK T and memory T cells. Functional analysis showed that sorted Ly49E(+) and CD94/NKG2(+) fetal NK cells could discriminate between MHC class I-positive and MHC class I-negative tumor cells. We also demonstrated that Ly49E becomes phosphorylated following pervanadate stimulation of fetal NK cells. The expression levels of Ly49E and CD94/NKG2 were similar in wild-type compared with beta(2)-microglobulin(-/-) mice. In conclusion, generation of mAbs against Ly49E and NKG2 extended the phenotypic and functional characterization of NK cells.

Distinguishing self from nonself: immunogenicity of the murine H47 locus is determined by a single amino acid substitution in an unusual peptide

Histocompatibility (H) Ags are responsible for chronic graft rejection and graft vs host disease in solid tissue and bone marrow transplantation among MHC-matched individuals. Here we defined the molecular basis of self-nonself discrimination for the murine chromosome 7 encoded H47 histocompatibility locus, known by its trait of graft-rejection for over 40 years. H47 encodes a novel, highly conserved cell surface protein containing the SCILLYIVI (SII9) nonapeptide in its transmembrane region. The p7 isoleucine-to-phenylalanine substitution in SII9 defined the antigenic polymorphism and T cell specificity. Despite absence of the canonical consensus motif and weak binding to D(b) MHC I, both H47 peptides were presented to CTLs. However, unlike all the other known H loci, the relative immunogenicity of both H47 alleles varied dramatically and was profoundly influenced by neighboring H loci. The results provide insights into the peptide universe that defines nonself and the basis of histoincompatibility.

Mouse CD94 participates in Qa-1-mediated self recognition by NK cells and delivers inhibitory signals independent of Ly-49

Inhibitory receptors expressed on NK cells recognize MHC class I molecules and transduce negative signals to prevent the lysis of healthy autologous cells. The lectin-like CD94/NKG2 heterodimer has been studied extensively as a human inhibitory receptor. In contrast, in mice, another lectin-like receptor, Ly-49, was the only known inhibitory receptor until the recent discovery of CD94/NKG2 homologues in mice. Here we describe the expression and function of mouse CD94 analyzed by a newly established mAb. CD94 was detected on essentially all NK and NK T cells as well as small fractions of T cells in all mouse strains tested. Two distinct populations were identified among NK and NK T cells, CD94(bright) and CD94(dull) cells, independent of Ly-49 expression. The anti-CD94 mAb completely abrogated the inhibition of target killing mediated by NK recognition of Qa-1/Qdm peptide on target cells. Importantly, CD94(bright) but not CD94(dull) cells were found to be functional in the Qa-1/Qdm-mediated inhibition. In the presence of the mAb, activated NK cells showed substantial cytotoxicity against autologous target cells as well as enhanced cytotoxicity against allogeneic and "missing self" target cells. These results suggest that mouse CD94 participates in the protection of self cells from NK cytotoxicity through the Qa-1 recognition, independent of inhibitory receptors for classical MHC class I such as Ly-49.

The non-classical MHC class I molecule Qa-1(b) inhibits classical MHC class I-restricted cytotoxicity of cytotoxic T lymphocytes

The CD94/NKG2A heterodimer is an inhibitory receptor expressed on a subset of mouse NK cells. CD94/NKG2A recognizes the non-classical MHC class I (class Ib) molecule Qa-1(b) and inhibits NK cytotoxicity. Qa-1(b) presents a peptide derived from the leader sequence of classical MHC class I molecules. Here, we examined the role of CD94/NKG2A in T cell-mediated cytotoxicity. Soluble tetrameric Qa-1(b) bound to almost all CD8(+), but not CD4(+), T cells. This binding seems to be mediated by CD8, because COS cells transfected with CD8 also bound Qa-1(b) tetramer. Therefore, the expression of CD94/NKG2 in T cells was further examined by single-cell RT-PCR. Most murine CD8(+) T cells constitutively expressed CD94 and NKG2A transcripts, whereas they were not detected in CD4(+) T cells. Co-expression of Qa-1(b) and D(k) on target cells significantly inhibited cytotoxicity of D(k)-specific cytotoxic T lymphocytes generated by mixed lymphocyte reaction, indicating that Qa-1(b) on antigen-presenting cells interacts with CD94/NKG2A on CD8 T cells and regulates classical MHC class I-restricted cytotoxic T cells. These results suggest a significant role of CD94/NKG2A as an inhibitory receptor on CD8(+) T cells.

Expression of natural killer receptor alleles at different Ly49 loci occurs independently and is regulated by major histocompatibility complex class I molecules

Ly49 receptor genes are expressed by subsets of natural killer (NK) cells in an overlapping fashion, accounting for the capacity of NK subsets to attack host cells that have selectively downregulated self-major histocompatibility complex (MHC) class I molecules. It was shown previously that most NK cells express only one or the other allele of a given Ly49 gene, while a smaller population expresses both alleles. However, the methods used to detect monoallelic and biallelic cells were nonquantitative. Here, new allele-specific antibodies were used to provide the first quantitative examination of biallelic and monoallelic expression of Ly49A and Ly49G2. The results demonstrate conclusively that most Ly49A(+) and Ly49G2(+) NK cells express the corresponding gene in a monoallelic fashion, with a smaller subset expressing both alleles. Unexpectedly, biallelic Ly49A(+) NK cells were more numerous than predicted by completely independent allelic expression, suggesting some heterogeneity among NK progenitors in the potential to express a given Ly49 gene. The data also show that cells expressing one allele of Ly49G2 may express Ly49A from the same or opposite chromosome with equal likelihood, indicating that the expressed allele is chosen independently for different Ly49 genes. Finally, the data demonstrate that biallelic expression of Ly49A or Ly49G2 occurs least frequently in mice that express ligands for these receptors (H-2(d) mice), and most frequently in class I-deficient mice. Thus, biallelic expression of Ly49 genes is regulated by interactions of NK cell progenitors with MHC class I molecules.

Functional analysis of the molecular factors controlling Qa1-mediated protection of target cells from NK lysis

CD94/NKG2 receptors on mouse NK cells recognize the nonclassical class I molecule Qa1 and can deliver inhibitory signals that prevent NK cells from lysing Qa1-expressing cells. However, the exact circumstances under which Qa1 protects cells from NK lysis and, in particular, the role of the dominant Qa1-associated peptide, Qdm, are unclear. In this study, we examined in detail the lysis of Qa1-expressing cells by fetal NK cells that express CD94/NKG2 receptors for Qa1 but that lack receptors for classical class I molecules. Whereas mouse L cells and human C1R cells transfected with Qa1 were resistant to lysis by these effectors, Qa1-transfected TAP-deficient human T2 cells showed no resistance despite expressing high levels of surface Qa1. However, these cells could be efficiently protected by exposure to low concentrations of Qdm peptide or certain Qdm-related peptides. By contrast, even prolonged exposure of TAP-deficient RMA/S cells to high doses of Qdm peptide failed to induce levels of surface Qa1 detectable with a Qa1-specific mAb or to protect them from NK lysis, although such treatment induced sensitivity to lysis by Qa1-specific CTL. Collectively, these findings indicate that high surface expression of Qa1 is necessary but not sufficient for protection, and that effective protection requires the expression of sufficient levels of suitable Qa1-peptide complexes to overcome activatory signals. Results obtained with a series of substituted Qdm peptides suggest that residues at positions 3, 4, 5, and 8 of the Qdm sequence, AMAPRTLLL, are important for recognition of Qa1-Qdm complexes by inhibitory CD94/NKG2 receptors.

Definition of a novel binding site on CD8 cells for a conserved region of the MHC class Ib molecule Qa-1 that regulates IFN-gamma expression

Natural killer (NK) cells and activated CD8 cells both express cytotoxic activity and produce substantial levels of IFN-gamma in response to viral and bacterial infections. In the case of NK cells, cellular activation and IFN-gamma expression are regulated by an interaction between NK receptors and MHC class Ib molecules, including HLA-E/Qa-1. We have used soluble tetrameric complexes of the murine class Ib molecule Qa-1 to define the significance of this interaction for CD8 cells. We find that all CD8 cells express a receptor for Qa-1 and that ligation of this receptor by Qa-1 results in up-regulation of IFN-gamma production.

Divergent and convergent evolution of NK-cell receptors

Natural killer (NK)-cell receptors specific for major histocompatibility complex (MHC) class I molecules have been identified in humans and mice. Some of the most important receptors are structurally unrelated in the two species: the murine Ly-49 receptors are C-type lectins, while human killer-cell inhibitory receptors (KIRs) belong to the immunoglobulin superfamily. Here, Roland Barten and colleagues describe the divergent and convergent evolution of NK-cell receptors.

Factors controlling the trafficking and processing of a leader-derived peptide presented by Qa-1

Many leader-derived peptides require TAP for presentation by class I molecules. This TAP dependence can either be ascribed to the inability of proteases resident in the endoplasmic reticulum (ER) to trim leader peptide precursors into the appropriate epitope or the failure of a portion of the leader segment to gain access to the lumen of the ER. Using the Qa-1 binding epitope, Qdm derived from a class Ia leader as a model, we show that many cell types lack ER protease activity to trim this peptide at its C terminus. However, both T1 and T2 cells contain appropriate protease activity to process the full length D(d) leader (DL) when introduced into the ER lumen. Nevertheless, both T1 cells treated with the TAP inhibitor ICP47 and TAP(-) T2 cells fail to present this epitope from either the intact D(d) molecule or a minigene encoding the DL. This indicates that the portion of the leader containing Qdm does not gain access to the ER. However, changing the Arg at P7 of the DL to a Cys can alter its trafficking and allows for TAP-independent presentation of the Qdm epitope.

Comparative ability of Qdm/Qa-1b, kb, and Db to protect class Ilow cells from NK-mediated lysis in vivo

The class Ib molecule Qa-1(b) binds the class Ia leader peptide, Qdm, which reacts with CD94/NKG2R on NK cells. We have generated a gene that encodes the Qdm peptide covalently attached to ss(2)-microglobulin (ss(2)M) by a flexible linker (Qa-1 determinant modifier (Qdm)-ss(2)M). When this construct is expressed in TAP-2(-) or ss(2)M(-) cells, it allows for the expression of a Qdm-ss(2)M protein that associates with Qa-1(b) to generate the Qdm epitope, as detected by Qdm/Qa-1(b)-specific CTL. To test the biological significance of expression of this engineered molecule, we injected TAP-2(-) RMAS-Qdm-ss(2)M cells into C57BL/6 mice and measured their NK cell-mediated clearance from the lungs at 2 h. RMAS cells transfected with Qdm-ss(2)M were resistant to lung clearance, similar to RMA cells or RMAS cells in anti-asialo-GM(1)-treated mice, while untransfected or ss(2)M-transfected RMAS cells were rapidly cleared. Further, pulsing RMAS cells with either Qdm, a K(b)-, or D(b)-binding peptide showed equivalent protection from clearance, indicating that a single class Ia or Ib molecule can afford complete protection from NK cells in this system. In contrast, injection of RMAS cells into DBA/2 animals, which express low levels of receptors for Qdm/Qa-1(b), resulted in protection from lung clearance if pulsed with a K(b)- or D(b)-binding peptide, but not the Qa-1(b)-binding peptide, Qdm.

MHC class Ib-restricted CTL provide protection against primary and secondary Listeria monocytogenes infection

Infection of B6 mice with the intracellular pathogen Listeria monocytogenes (LM) results in the activation of CD8(+) T cells that respond to Ag presented by both MHC class Ia and class Ib molecules. Enzyme-linked immunospot analysis reveals that these CTL populations expand and contract at different times following a primary sublethal LM infection. Between days 4 and 6 postinfection, class Ib-restricted CTL exhibit a rapid proliferative response that is primarily H2-M3 restricted. The peak response of class Ia-restricted CD8(+) T cells occurs a few days later, after the majority of bacteria have been cleared. Although class Ia-restricted CTL exhibit a vigorous recall response to secondary LM infection, we observe limited expansion of class Ib-restricted memory CTL, even in MHC class Ia-deficient mice (B6.K(b-/-)D(b-/-)). Despite this lack of enhanced expansion in vivo, class Ib-restricted memory CTL retain the ability to proliferate and expand when provided with Ag in vitro. Furthermore, we demonstrate that in vivo depletion of CD8(+) T cells in LM-immune B6.K(b-/-)D(b-/-) mice severely impairs memory protection. Together, these data demonstrate that class Ib-restricted CTL play an important role in clearing a primary LM infection and generate a memory population capable of providing significant protection against subsequent infection.

Tracking the response of natural killer T cells to a glycolipid antigen using CD1d tetramers

A major group of natural killer (NK) T cells express an invariant Valpha14(+) T cell receptor (TCR) specific for the lipoglycan alpha-galactosylceramide (alpha-GalCer), which is presented by CD1d. These cells may have an important immune regulatory function, but an understanding of their biology has been hampered by the lack of suitable reagents for tracking them in vivo. Here we show that tetramers of mouse CD1d loaded with alpha-GalCer are a sensitive and highly specific reagent for identifying Valpha14(+) NK T cells. Using these tetramers, we find that alpha-GalCer-specific T lymphocytes are more widely distributed than was previously appreciated, with populations of largely NK1.1(-) but tetramer-binding T cells present in the lymph nodes and the intestine. Injection of alpha-GalCer leads to the production of both interferon gamma and interleukin 4 by nearly all NK T cells in the liver and the majority of the spleen within 2 h. These cells mostly disappear by 5 h, and they do not reappear after 1 wk. Curiously, tetramer-positive thymocytes do not rapidly synthesize cytokines, nor do they undergo decreases in cell number after lipid antigen stimulation, although they express equivalent TCR levels. In summary, the data presented here demonstrate that alpha-GalCer-specific NK T cells undergo a unique and highly compartmentalized response to antigenic stimulation.

Analysis of Qa-1(b) peptide binding specificity and the capacity of CD94/NKG2A to discriminate between Qa-1-peptide complexes

The major histocompatibility complex class Ib protein, Qa-1(b), serves as a ligand for murine CD94/NKG2A natural killer (NK) cell inhibitory receptors. The Qa-1(b) peptide-binding site is predominantly occupied by a single nonameric peptide, Qa-1 determinant modifier (Qdm), derived from the leader sequence of H-2D and L molecules. Five anchor residues were identified in this study by measuring the peptide-binding affinities of substituted Qdm peptides in experiments with purified recombinant Qa-1(b). A candidate peptide-binding motif was determined by sequence analysis of peptides eluted from Qa-1 that had been folded in the presence of random peptide libraries or pools of Qdm derivatives randomized at specific anchor positions. The results indicate that Qa-1(b) can bind a diverse repertoire of peptides but that Qdm has an optimal primary structure for binding Qa-1(b). Flow cytometry experiments with Qa-1(b) tetramers and NK target cell lysis assays demonstrated that CD94/NKG2A discriminates between Qa-1(b) complexes containing peptides with substitutions at nonanchor positions P4, P5, or P8. Our findings suggest that it may be difficult for viruses to generate decoy peptides that mimic Qdm and raise the possibility that competitive replacement of Qdm with other peptides may provide a novel mechanism for activation of NK cells.

DAP12-deficient mice fail to develop autoimmunity due to impaired antigen priming

DAP12 is an ITAM-bearing membrane adaptor molecule implicated in the activation of NK and myeloid cells. In mice rendered DAP12 deficient by targeted gene disruption, lymphoid and myeloid development was apparently normal, although the activating Ly49 receptors on NK cells were downregulated and nonfunctional. To analyze the consequences of DAP12 deficiency in vivo, we examined the susceptibility of DAP12-/- mice to experimental autoimmune encephalomyelitis (EAE). DAP12-/- mice were resistant to EAE induced by immunization with myelin oligodendrocyte glycoprotein (MOG) peptide. Resistance was associated with a strongly diminished production of IFNgamma by myelin-reactive CD4+ T cells due to inadequate T cell priming in vivo. These data suggest that DAP12 signaling may be required for optimal antigen-presenting cell (APC) function or inflammation.

Host immune response to intracellular bacteria: A role for MHC-linked class-Ib antigen-presenting molecules

MHC-linked class-Ib molecules are a subfamily of class-I molecules that display limited genetic polymorphism. At one time these molecules were considered to have an enigmatic function. However, recent studies have shown that MHC-linked class-Ib molecules can function as antigen presentation structures that bind bacteria-derived epitopes for recognition by CD8+ effector T cells. This role for class-Ib molecules has been demonstrated across broad classes of intracellular bacteria including Listeria moncytogenes, Salmonella typhimurium, and Mycobacterium tuberculosis. Additionally, evidence is emerging that MHC-linked class-Ib molecules also serve an integral role as recognition elements for NK cells as well as several TCR alpha/beta and TCR gamma/delta T-cell subsets. Thus, MHC-linked class-Ib molecules contribute to the host immune response by serving as antigen presentation molecules and recognition ligands in both the innate and adaptive immune response to infection. In this review, we will attempt to summarize the work that supports a role for MHC-linked class-Ib molecules in the host response to infection with intracellular bacteria.

Transgenic expression of Ly49A on T cells impairs a specific antitumor response

Inhibitory MHC receptors determine the reactivity and specificity of NK cells. These receptors can also regulate T cells by modulating TCR-induced effector functions such as cytotoxicity, cytokine production, and proliferation. Here we have assessed the capacity of mouse T cells expressing the inhibitory MHC class I receptor Ly49A to respond to a well-defined tumor Ag in vivo using Ly49A transgenic mice. We find that the presence of Ly49A on the vast majority of lymphocytes prevents the development of a significant Ag-specific CD8+ T cell response and, consequently, the rejection of the tumor. Despite minor alterations in the TCR repertoire of CD8+ T cells in the transgenic lines, precursors of functional tumor-specific CD8+ T cells exist but could not be activated most likely due to a lack of appropriate CD4+ T cell help. Surprisingly, all of these effects are observed in the absence of a known ligand for the Ly49A receptor as defined by its ability to regulate NK cell function. Indeed, we found that the above effects on T cells may be based on a weak interaction of Ly49A with Kb or Db class I molecules. Thus, our data demonstrate that enforced expression of a Ly49A receptor on conventional T cells prevents a specific immune response in vivo and suggest that the functions of T and NK cells are differentially sensitive to the presence of inhibitory MHC class I receptors.

Ligands for the murine NKG2D receptor: expression by tumor cells and activation of NK cells and macrophages

Natural killer (NK) cells attack tumor and infected cells, but the receptors and ligands that stimulate them are poorly understood. Here we report the expression cloning of two murine ligands for the lectin-like receptor NKG2D. The two ligands, H-60 and Rae1 beta, are distant relatives of major histocompatibility complex class I molecules. NKG2D ligands are not expressed by most normal cells but are up-regulated on numerous tumor cells. We show that mouse NKG2D is expressed by NK cells, activated CD8+ T cells and activated macrophages. Expression of either NKG2D ligand by target cells triggers NK cell cytotoxicity and interferon-gamma secretion by NK cells, as well as nitric oxide release and tumor necrosis factor alpha transcription by macrophages. Thus, through their interaction with NKG2D, H-60 and Rae1 beta are newly identified potent stimulators of innate immunity.

Impaired natural killing of MHC class I-deficient targets by NK cells expressing a catalytically inactive form of SHP-1

NK cell function is negatively regulated by MHC class I-specific inhibitory receptors. Transduction of the inhibitory signal involves protein tyrosine phosphatases such as SHP-1 (SH2-containing protein tyrosine phosphatase-1). To investigate the role of SHP-1 for NK cell development and function, we generated mice expressing a catalytically inactive, dominant-negative mutant of SHP-1 (dnSHP-1). In this paper we show that expression of dnSHP-1 does not affect the generation of NK cells even though MHC receptor-mediated inhibition is partially impaired. Despite this defect, these NK cells do not kill syngeneic, normal target cells. In fact dnSHP-1-expressing NK cells are hyporesponsive toward MHC-deficient target cells, suggesting that non-MHC-specific NK cell activation is significantly reduced. In contrast, these NK cells mediate Ab-dependent cell-mediated cytotoxicity and prevent the engraftment with beta2-microglobulin-deficient bone marrow cells. A similar NK cell phenotype is observed in viable motheaten (mev) mice, which show reduced SHP-1 activity due to a mutation in the Shp-1 gene. In addition, NK cells in both mouse strains show a tendency to express more inhibitory MHC-specific Ly49 receptors. Our results demonstrate the importance of SHP-1 for the generation of functional NK cells, which are able to react efficiently to the absence of MHC class I molecules from normal target cells. Therefore, SHP-1 may play an as-yet-unrecognized role in some NK cell activation pathways. Alternatively, a reduced capacity to transduce SHP-1-dependent inhibitory signals during NK cell development may be compensated by the down-modulation of NK cell triggering pathways.

The specific regulation of immune responses by CD8+ T cells restricted by the MHC class Ib molecule, Qa-1

Over the last three decades considerable evidence has accumulated that CD8(+) T cells regulate peripheral immune responses, in part, by specifically controlling the outgrowth of antigen-triggered CD4(+) T cells. This regulatory function of CD8(+) T cells has been shown, in vivo, to control the emergence of autoreactive CD4(+) T cells as well as CD4(+) T cells reactive to conventional antigens, including alloantigens. In this review, we summarize the evidence that this immune suppression mediated by CD8(+) T cells is dependent, in part, on specific cognate interactions between MHC class I-restricted regulatory CD8(+) cells and antigen-activated CD4(+) T cells. Moreover, we review the evidence that regulatory CD8(+) T cells recognize antigen-activated CD4(+) T cells in a TCR specific manner restricted by the MHC class Ib molecule, Qa-1. The Qa-1 molecule may be uniquely qualified to serve this MHC restrictive function because, unlike conventional MHC molecules, it is preferentially and transiently expressed on activated and not resting CD4(+) T cells. This may assure that only recently antigen-activated CD4(+) T cells expressing Qa-1/TCR peptide complexes will induce regulatory CD8(+) T cells and subsequently become susceptible to regulation. Because Qa-1 also binds to self Qdm peptides that trigger NK (CD94/ NKG2) receptors on CD8(+) T cells, the machinery for homeostatic regulation of regulatory CD8(+) T cells can be envisioned. Finally, we propose a model by which these TCR specific, Qa-1-restricted regulatory CD8(+) T cells selectively downregulate antigen-activated T cells expressing TCRs of certain affinities. Ultimately these regulatory CD8(+) T cells control the peripheral TCR repertoire during the course of immune responses to both self and foreign antigens.

Viral subversion of the immune system

This review describes the diverse array of pathways and molecular targets that are used by viruses to elude immune detection and destruction. These include targeting of pathways for major histocompatibility complex-restricted antigen presentation, apoptosis, cytokine-mediated signaling, and humoral immune responses. The continuous interactions between host and pathogens during their coevolution have shaped the immune system, but also the counter measures used by pathogens. Further study of their interactions should improve our ability to manipulate and exploit the various pathogens.

The mouse as a model for the effects of MHC genes on human disease

As mice are often used to model human major histocompatibility complex (MHC)-associated diseases, it is important to understand how their MHC regions differ at the DNA level. The sequencing of the mouse MHC (H2 region) has enabled a detailed map of this region to be assembled for comparison with the human MHC. Here, Richard Allcock and colleagues outline the similarities between the human and mouse MHC regions and discuss notable differences that might affect disease models.

Inhibition of human NK cell-mediated killing by CD1 molecules

It is now well established that NK cells recognize classical and nonclassical MHC class I molecules and that such recognition typically results in the inhibition of target cell lysis. Given the known structural similarities between MHC class I and non-MHC-encoded CD1 molecules, we investigated the possibility that human CD1a, -b, and -c proteins might also function as specific target structures for NK cell receptors. Here we report that expression of CD1a, -b, or -c can partially inhibits target cell lysis by freshly isolated human NK cells and cultured NK lines. The inhibitory effects of CD1 molecules on NK cell could be shown upon expression of individual CD1 proteins in transfected NK-sensitive target cells, and these effects could be reversed by incubation of the target cells with mAbs specific for the expressed form of CD1. Inhibitory effects of CD1 expression on NK-mediated lysis could also be shown for cultured human dendritic cells, which represent a cell type that prominently expresses the various CD1 proteins in vivo. In addition, the bacterial glycolipid Ags known to be bound and presented by CD1 proteins could significantly augment the observed inhibitory effects on target cell lysis by NK cells.

In vivo identification of glycolipid antigen-specific T cells using fluorescent CD1d tetramers

The CD1 family of major histocompatibility complex (MHC)-like molecules specializes in presenting lipid and glycolipid antigens to alpha/beta T lymphocytes, but little is known about the size of the CD1-restricted T cell population or the frequency of T lymphocytes specific for a given glycolipid antigen. Here, we report the generation and use of mouse CD1d1-glycolipid tetramers to visualize CD1d-restricted T cells. In contrast with previous BIAcore-based estimates of very short half-lives for CD1d-glycolipid complexes, we found that the dissociation rate of several different CD1d-glycolipid complexes was very slow. Fluorescent tetramers of mouse CD1d1 complexed with alpha-galactosylceramide (alphaGalCer), the antigen recognized by mouse Valpha14-Jalpha281/Vbeta8 and human Valpha24-JalphaQ/Vbeta11 natural killer T (NKT) cell T cell receptors (TCRs), allowed us for the first time to accurately describe, based on TCR specificity, the entire population of NKT cells in vivo and to identify a previously unrecognized population of NK1.1-negative "NKT" cells, which expressed a different pattern of integrins. In contrast, natural killer (NK) cells failed to bind the tetramers either empty or loaded with alphaGalCer, suggesting the absence of a CD1d-specific, antigen-nonspecific NK receptor. Mouse CD1d1-alphaGalCer tetramers also stained human NKT cells, indicating that they will be useful for probing a range of mouse and human conditions such as insulin-dependent diabetes mellitus, tumor rejection, and infectious diseases where NKT cells play an important role.

Rapid evolution of NK cell receptor systems demonstrated by comparison of chimpanzees and humans

That NK cell receptors engage fast-evolving MHC class I ligands suggests that they, too, evolve rapidly. To test this hypothesis, the structure and class I specificity of chimpanzee KIR and CD94:NKG2 receptors were determined and compared to their human counterparts. The KIR families are divergent, with only three KIR conserved between chimpanzees and humans. By contrast, CD94:NKG2 receptors are conserved. Whereas receptors for polymorphic class I are divergent, those for nonpolymorphic class I are conserved. Although chimpanzee and human NK cells exhibit identical receptor specificities for MHC-C, they are mediated by nonorthologous KIR. These results demonstrate the rapid evolution of NK cell receptor systems and imply that "catching up" with class I is not the only force driving this evolution.

The genomic organization of the mouse CD94 C-type lectin gene

The mouse natural killer (NK) gene complex is located on chromosome 6 and contains a number of genes encoding C-type lectin receptors which have been found to regulate NK cell function. Among these are CD94 and four NKG2 genes. Like its human counterpart, the mouse CD94 protein associates with different NKG2 isoforms and recognizes the atypical MHC class I molecule Qa-1b. Here, the genomic organization of the mouse CD94 gene was determined by analysing a BAC clone containing the CD94 gene. The mouse CD94 gene contains six exons separated by five introns. Exons I and II encode the 5' untranslated region (UTR) and the transmembrane domain. Exon III encodes the stalk region and exons IV-VI encode the carbohydrate recognition domain (CRD). Furthermore, we cloned and sequenced the CD94 promoter region, and putative regulatory DNA elements were identified. Further studies on the CD94 promoter region may help to elucidate the restricted expression pattern of CD94 in NK cells and a subpopulation of T cells.

Retinoic acid early inducible genes define a ligand family for the activating NKG2D receptor in mice

Here we describe a family of GPI-anchored cell surface proteins that function as ligands for the mouse activating NKG2D receptor. These molecules are encoded by the retinoic acid early inducible (RAE-1) and H60 minor histocompatibility antigen genes on mouse chromosome 10 and show weak homology with MHC class I. Expression of the NKG2D ligands is low or absent on normal, adult tissues; however, they are constitutively expressed on some tumors and upregulated by retinoic acid. Ectopic expression of RAE-1 and H60 confers target susceptibility to NK cell attack. These studies identify a family of ligands for the activating NKG2D receptor on NK and T cells, which may play an important role in innate and adaptive immunity.

Inducible expression of the gp49B inhibitory receptor on NK cells

Murine NK cells express inhibitory receptors belonging to the C-type lectin-like (Ly-49, CD94/NKG2) and Ig superfamily-related (gp49) receptors. The murine gp49B receptor displays structural homology with human killer inhibitory receptors, and was previously identified to be a receptor on mast cells and activated NK cells. The gp49B receptor is highly related to gp49A, a receptor with unknown function. In this study, using a novel mAb produced against soluble gp49B molecules that cross-reacts with gp49A, we examined the cellular distribution and function of these receptors. gp49 is constitutively expressed on cells of the myeloid lineage throughout development, as well as on mature cells. Importantly, gp49 is not expressed on spleen- and liver-derived lymphocytes, including NK cells, but its expression is induced in vitro on NK cells following IL-2 stimulation, or in vivo by infection with murine CMV. Molecular studies revealed that both the immunoreceptor tyrosine-based inhibitory motif-containing gp49B as well as immunoreceptor tyrosine-based inhibitory motif-less gp49A receptors are up-regulated on NK cells following murine CMV infection. When co-cross-linked with NK1.1, gp49B can inhibit NK1.1-mediated cytokine release by NK cells. Taken together, these studies demonstrate that the expression of gp49B on NK cells is regulated, providing the first example of an in vivo activation-induced NK cell inhibitory receptor, in contrast to the constitutively expressed Ly49 family.

The mouse tumor cell lines EL4 and RMA display mosaic expression of NK-related and certain other surface molecules and appear to have a common origin

As a potential means for facilitating studies of NK cell-related molecules, we examined the expression of these molecules on a range of mouse tumor cell lines. Of the lines we initially examined, only EL4 and RMA expressed such molecules, both lines expressing several members of the Ly49 and NKRP1 families. Unexpectedly, several of the NK-related molecules, together with certain other molecules including CD2, CD3, CD4, CD32, and CD44, were often expressed in a mosaic manner, even on freshly derived clones, indicating frequent switching in expression. In each case examined, switching was controlled at the mRNA level, with expression of CD3zeta determining expression of the entire CD3-TCR complex. Each of the variable molecules was expressed independently, with the exception that CD3 was restricted to cells that also expressed CD2. Treatment with drugs that affect DNA methylation and histone acetylation could augment the expression of at least some of the variable molecules. The striking phenotypic similarity between EL4 and RMA led us to examine the state of their TCRbeta genes. Both lines had identical rearrangements on both chromosomes, indicating that RMA is in fact a subline of EL4. Overall, these findings suggest that EL4 is an NK-T cell tumor that may have retained a genetic mechanism that permits the variable expression of a restricted group of molecules involved in recognition and signaling.

Nonstochastic coexpression of activation receptors on murine natural killer cells

Murine natural killer cells (NK) express lectin-like activation and inhibitory receptors, including the CD94/NKG2 family of receptors that bind Qa-1, and the Ly-49 family that recognizes major histocompatibility complex class I molecules. Here, we demonstrate that cross-linking of NK cells with a new specific anti-Ly-49H mAb induced NK cell cytotoxicity and cytokine production. Ly-49H is expressed on a subset of NK cells and can be coexpressed with Ly-49 inhibitory receptors. However, unlike Ly-49 inhibitory receptors, Ly-49H is not detectable on naive splenic CD3(+) T cells, indicating that Ly-49H may be an NK cell-specific activation receptor. In further contrast to the stochastically expressed Ly-49 inhibitory receptors, Ly-49H is preferentially expressed with the Ly-49D activation receptor, and expression of both Ly-49H and Ly-49D is augmented on NK cells that lack receptors for Qa-1 tetramers. On developing splenic NK1.1(+) cells, Ly-49D and Ly-49H are expressed later than the inhibitory receptors. These results directly demonstrate that Ly-49H activates primary NK cells, and suggest that expression of Ly-49 activation receptors by NK cells may be specifically regulated on NK cell subsets. The simultaneous expression of multiple activation receptors by individual NK cells contrasts with that of T cell antigen receptors and is relevant to the role of NK cells in innate immunity.

A sequence-ready physical map of the region containing the human natural killer gene complex on chromosome 12p12.3-p13.2

We developed a sequence-ready physical map of a part of human chromosome 12p12.3-p13.2 where the natural killer gene complex (NKC) is located. The NKC includes a cluster of genes with structure similar to that of the Ca(2+)-dependent lectin superfamily of glycoproteins that are expressed on the surface of most natural killer (NK) cells and a subset of T cells. These killer cell lectin-like receptors (KLR) are involved in NK target cell recognition, leading to activation or inhibition of NK cell function. We used a number of sequence-tagged site (STS) markers from this region to screen two large insert bacterial artificial chromosome (BAC) libraries and a bacteriophage P1-derived (PAC) chromosome library. The clones were assembled into contiguous sets by STS content analysis. The 72-BAC and 11-PAC contig covers nearly 2 Mb of DNA and provides an average marker resolution of 26 kb. We have precisely localized 17 genes, 5 expressed sequence tags, and 49 STSs within this contig. Of this total number of STS, 30 are newly developed by clone-end sequencing. We established the order of the genes as tel-M6PR-MAFAL (HGMW-approved symbol KLRG1)-A2M-PZP-A2MP-NKRP1A (HGMW-approved symbol KLRB1)-CD69-AICL (HGMW-approved symbol CLECSF2)-KLRF1-OLR1-CD94 (HGMW-approved symbol KLRD1)-NKG2D (HGMW-approved symbol D12S2489E)-PGFL-NKG2F (HGMW-approved symbol KLRC4)-NKG2E (HGMW-approved symbol KLRC3)-NKG2A (HGMW-approved symbol KLRC1)-LY49L (HGMW-approved symbol KLRA1)-cen. This map would facilitate the cloning of new KLR genes and the complete sequencing of this region.

Cytomegalovirus MHC class I homologues and natural killer cells: an overview

Viruses that establish a persistent infection with their host have evolved numerous strategies to evade the immune system. Consequently, they are useful tools to dissect the complex cellular processes that comprise the immune response. Rapid progress has been made in recent years in defining the role of cellular MHC class I molecules in regulating the response of natural killer (NK) cells. Concomitantly, the roles of the MHC class I homologues encoded by human and mouse cytomegaloviruses in evading or subverting NK cell responses has received considerable interest. This review discusses the results from a number of studies that have pursued the biological function of the viral MHC class I homologues. Based on the evidence from these studies, hypotheses for the possible role of these intriguing molecules are presented.

NK cell recognition of non-classical HLA class I molecules

NK cells recognize several HLA class Ib molecules employing both immunoglobulin-like (Ig-like) and C-type lectin receptors. The CD94/NKG2 and NKG2D lectin-like molecules, respectively, interact with HLA-E and MICA; CD94/NKG2A functions as an inhibitory receptor, while CD94/NKG2C and NKG2D trigger NK cell activity. HLA-E predominantly presents nonamers from the leader sequences of other class I molecules; a peptide derived from HLA-G1 constitutes the highest affinity ligand for both CD94/NKG2 receptors. Members of the Ig-like transcript (ILT) or leucocyte Ig-like receptor (LIR) family (ILT2 or LIR-1 and ILT4 or LIR-2), expressed by other leucocyte lineages, interact with a broad spectrum of HLA class Ia molecules and HLA-G1. Among Ig-like KIRs, the KIR2DL4 (p49) receptor has been shown to specifically recognize HLA-G1; this molecule displays an unusual hybrid structure, sharing features with inhibitory and triggering KIRs.

Natural killer cell surveillance of intracellular antigen processing pathways mediated by recognition of HLA-E and Qa-1b by CD94/NKG2 receptors

HLA-E binds specifically to MHC class Ia leader peptides in a TAP (transporter associated with antigen processing)-dependent manner. It interacts with CD94/NKG2A receptors on natural killer cells and this inhibits natural killer cell lysis of the cell displaying HLA-E. The crystal structure of HLA-E demonstrates that the specificity of leader peptide binding is a structurally defined intrinsic property of HLA-E.

Reconstitution of bacterial expressed human CD94: the importance of the stem region for dimer formation

Human CD94 is a subunit of the disulfide-linked, heterodimeric natural killer (NK) cell surface receptor CD94/NKG2. This receptor, a member of the C-type lectin superfamily, participates in regulating NK cell directed lysis through interaction with the major histocompatibility antigen HLA-E. Two forms of CD94 were expressed using a bacterial expression system and refolded in vitro. One form, residues 34-179, designated S34, corresponds to the entire extracellular region of the receptor, including a 23-residue stem region, and the other, residues 51-179, designated E51, corresponds only to the putative carbohydrate recognition domain of the receptor. The refolded full-length S34 protein existed as a noncovalent dimer initially but formed an interchain disulfide bond upon storage for several months. In contrast, the stemless construct, E51, existed largely as a monomeric form. The stem region of S34, residues 34-56, is sensitive to proteolysis and its absence results in dissociation of the dimer. This suggests that the residues in the stem region of CD94 help to stabilize the dimeric conformation.

The murine liver-specific nonclassical MHC class I molecule Q10 binds a classical peptide repertoire

The biological properties of the nonclassical class I MHC molecules secreted into blood and tissue fluids are not currently understood. To address this issue, we studied the murine Q10 molecule, one of the most abundant, soluble class Ib molecules. Mass spectrometry analyses of hybrid Q10 polypeptides revealed that alpha1alpha2 domains of Q10 associate with 8-9 long peptides similar to the classical class I MHC ligands. Several of the sequenced peptides matched intracellularly synthesized murine proteins. This finding and the observation that the Q10 hybrid assembly is TAP2-dependent supports the notion that Q10 groove is loaded by the classical class I Ag presentation pathway. Peptides eluted from Q10 displayed a binding motif typical of H-2K, D, and L ligands. They carried conserved residues at P2 (Gly), P6 (Leu), and Pomega (Phe/Leu). The role of these residues as anchors/auxiliary anchors was confirmed by Ala substitution experiments. The Q10 peptide repertoire was heterogeneous, with 75% of the groove occupied by a multitude of diverse peptides; however, 25% of the molecules bound a single peptide identical to a region of a TCR V beta-chain. Since this peptide did not display enhanced binding affinity for Q10 nor does its origin and sequence suggest that it is functionally significant, we propose that the nonclassical class I groove of Q10 resembles H-2K, D, and L grooves more than the highly specialized clefts of nonclassical class I Ags such as Qa-1, HLA-E, and M3.

NK cell activation: distinct stimulatory pathways counterbalancing inhibitory signals

A delicate balance between positive and negative signals regulates NK cell effector function. Activation of NK cells may be initiated by the triggering of multiple adhesion or costimulatory molecules, and can be counterbalanced by inhibitory signals induced by receptors for MHC class I. A common pathway of inhibitory signaling is provided by immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the cytoplasmic domains of these receptors which mediate the recruitment of SH2 domain-bearing tyrosine phosphate-1 (SHP-1). In contrast to the extensive progress that has been made regarding the negative regulation of NK cell function, our knowledge of the signals that activate NK cells is still poor. Recent studies of the activating receptor complexes have shed new light on the induction of NK cell effector function. Several NK receptors using novel adaptors with immunoreceptor tyrosine-based activation motifs (ITAMs) and with PI 3-kinase recruiting motifs have been implicated in NK cell stimulation.

Paired inhibitory and triggering NK cell receptors for HLA class I molecules

Human natural killer (NK) cells specifically interact with major histocompatibility complex (MHC) class I molecules employing different receptor systems, shared with subsets of alphabeta and gammadelta T lymphocytes. Killer cell immunoglobulin-like receptors (KIRs) recognize groups of human leukocyte antigen (HLA) class Ia proteins displaying common structural features at the alpha-1 domain; among them, KIR2DL4 has been proposed to specifically interact with the class Ib molecule HLA-G1. Members of a related family of immunoglobulin (Ig)-like receptors (ILT2 or LIR-1 and ILT4 or LIR-2), expressed by other leukocyte lineages, interact with a broad spectrum of class Ia molecules and HLA-G1. On the other hand, CD94/NKG2-A(-C) and NKG2D lectin-like receptors, respectively, recognize the class Ib molecules HLA-E and MICA. A recurrent finding within the different receptor families is the existence of pairs of homologous molecules that often share the same ligands but display divergent functions. Inhibitory receptors tend to exhibit an affinity for HLA molecules higher than their activating counterparts. Recruitment of SH2 domain-bearing tyrosine phosphatases (SHP) by cytoplasmic phosphorylated immunoreceptor tyrosine-based inhibition motifs (ITIMs) is a crucial event for the inhibitory signalling pathway. By contrast, triggering receptors assemble with homodimers of immune tyrosine-based activation motif (ITAM)-bearing adaptor molecules (i.e., DAP12, CD3 xi) that engage tyrosine kinases (ZAP70 and syk).

Recognition of the class Ib molecule Qa-1(b) by putative activating receptors CD94/NKG2C and CD94/NKG2E on mouse natural killer cells

The heterodimeric CD94/NKG2A receptor, expressed by mouse natural killer (NK) cells, transduces inhibitory signals upon recognition of its ligand, Qa-1(b), a nonclassical major histocompatibility complex class Ib molecule. Here we clone and express two additional receptors, CD94/NKG2C and CD94/NKG2E, which we show also bind to Qa-1(b). Within their extracellular carbohydrate recognition domains, NKG2C and NKG2E share extensive homology with NKG2A (93-95% amino acid similarity); however, NKG2C/E receptors differ from NKG2A in their cytoplasmic domains (only 33% similarity) and contain features that suggest that CD94/NKG2C and CD94/NKG2E may be activating receptors. We employ a novel blocking anti-NKG2 monoclonal antibody to provide the first direct evidence that CD94/NKG2 molecules are the only Qa-1(b) receptors on NK cells. Molecular analysis reveals that NKG2C and NKG2E messages are extensively alternatively spliced and approximately 20-fold less abundant than NKG2A message in NK cells. The organization of the mouse Cd94/Nkg2 gene cluster, presented here, shows striking similarity with that of the human, arguing that the entire CD94/NKG2 receptor system is relatively primitive in origin. Analysis of synonymous substitution frequencies suggests that within a species, NKG2 genes may maintain similarities with each other by concerted evolution, possibly involving gene conversion-like events. These findings have implications for understanding NK cells and also raise new possibilities for the role of Qa-1 in immune responses.

IFN-γ Production and Cytotoxicity of IL-2-Activated Murine NK Cells Are Differentially Regulated by MHC Class I Molecules

Activation of NK cells by target cells leads to cytotoxicity as well as production of various cytokines including IFN-γ. MHC class I molecules on target cells regulate NK cytotoxicity. However, little is known about the regulation of IFN-γ production by NK cells. We examined the production of IFN-γ in individual murine NK cells stimulated with tumor cell lines by flow cytometric analysis of intracellular IFN-γ. Among several tumor lines tested, the rat basophilic leukemia line RBL-1 induced particularly high level of IFN-γ production in IL-2-activated NK cells, whereas other lines, including the prototypic NK target YAC-1, induced very low or no IFN-γ production. Transfection of murine classical MHC class I molecules into RBL-1 cells substantially inhibited IFN-γ production. This inhibition of IFN-γ production by MHC class I was independent of Ly-49 or CD94/NKG2A expression on NK cells. These results indicate that some target cells directly stimulate IL-2-activated NK cells and induce IFN-γ production, but the requirements for the induction of IFN-γ production seem different from those for NK cytotoxicity. Furthermore, similar to NK cytotoxicity, induction of IFN-γ production is inhibited by MHC class I on stimulating cells. However, the MHC class I-specific receptors inhibiting IFN-γ production are different from those for NK cytotoxicity.

Expression of a nonclassical MHC class Ib molecule in the eye

BACKGROUND

MHC class Ia molecules are absent, or expressed at low levels, on cells lining the anterior chamber of the eye, an immune-privileged site. Although the scarcity of class Ia MHC antigens may protect cells from T cell-mediated tissue injury, it may also render them vulnerable to natural killer cell-mediated cytolysis. There is growing evidence that MHC class Ib molecules share similar functions to class Ia. In this study, we examine the expression and distribution of Qa-2, one of the best-characterized murine MHC class Ib molecules in the eye.

METHODS

The transcription of Qa-2 mRNA in whole eye and eye-derived cells was analyzed by sensitive and specific RNase protection and reverse transcription-polymerase chain reaction assays. Immunohistochemistry, flow cytometry, and ELISA were used to determine whether Qa-2 was expressed as cell surface proteins. Expression levels of Qa-2 were monitored in resting cells and cells stimulated with interferon-gamma.

RESULTS

Expression of membrane-bound and soluble Qa-2 isoforms was detected in various tissues of the eye, including cell subsets lining the anterior chamber. Immunohistological staining revealed Qa-2 expression on corneal epithelium as well as endothelium, iris ciliary bodies, and retina. These observations were confirmed by analysis of cultured, eye-derived cells. Qa-2 expression was inducible by interferon-gamma. Qa-2 was not detected in lens cells.

CONCLUSIONS

The results demonstrate that membrane-bound and soluble MHC class Ib molecules are expressed by eye cells. Expression of Qa-2 in the corneal endothelium and other substructures lining the anterior chamber suggests that this class Ib protein may contribute to the immune-privileged status of the anterior chamber.

Impaired Anti-Viral T Cell Responses Due to Expression of the LY49A Inhibitory Receptor

Inhibitory receptors specific for alleles of MHC class I proteins play an important role in determining the reactivity and specificity of NK cells. To determine whether these receptors are also able to regulate T cell functions, we have studied anti-viral immune responses in mice transgenic for a class I-specific inhibitory receptor, Ly49A. Although nontransgenic mice express Ly49A primarily on NK cells and some T cells, the Ly49A transgenic mice express Ly49A on all lymphocytes, including T cells. We have assessed the activation, expansion, cytokine production, and cytotoxic activity of CD8 T cells in both transgenic and nontransgenic mice following infection with lymphocytic choriomeningitis virus. As expected, nontransgenic mice made a potent virus-specific CD8 T cell response following virus infection. However, as measured in cytolysis assays and by cytokine production, virus-specific CD8 T cell activity was reduced in Ly49A transgenic mice. This inhibition was largely, but not always exclusively, dependent upon the presence, either in vivo or in vitro, of the Ly49A ligand, H-2Dd. Strikingly Ly49A transgenic mice have reduced capacity to control infection with the virulent lymphocytic choriomeningitis virus variant clone 13. Overall, these studies demonstrate that expression of killer inhibitory receptors can modulate anti-viral T cell responses in vivo and in vitro.