Transporters associated with antigen processing (TAP)-independent presentation of soluble insulin to alpha/beta T cells by the class Ib gene product, Qa-1(b)

Imprint of Initial Education and Loss of Ly49C/I in Activated Natural Killer Cells of TAP1-KO and C57BL/6 Wildtype Mice

Natural killer (NK) cells are important effectors of the innate immune system and participate in the first line of defense against infections and tumors. Prior to being functional, these lymphocytes must be educated or licensed through interactions of their major histocompatibility complex class I molecules with self-specific inhibitory receptors that recognize them. In the absence of such contacts, caused by either the lack of expression of the inhibitory receptors or a very low level of major histocompatibility complex class I (MHC class I) proteins, NK cells are hypo-reactive at baseline (ex vivo). After stimulation (assessed through plate-bound antibodies against activating receptors or culture in the presence of cytokines such as interleukin (IL)-2 or IL-15) however, they can become cytotoxic and produce cytokines. This is particularly the case in transporter associated with antigen processing (TAP)-deficient mice, which we investigated in the present study. Transporter associated with antigen processing transports endogenous peptides from the cytosol to the endoplasmic reticulum, where they are loaded on nascent MHC class I molecules, which then become stable and expressed at the cell surface. Consequently, TAP-KO mice have very low levels of MHC class I expression. We present a study about phenotypic and functional aspects of NK cells in two mouse strains, C57BL/6 wildtype and TAP1-KO in spleen and lung. We observed that in both types of mice, on the same genetic background, the initial pattern of education, conferred to the cells via the inhibitory receptors Ly49C/I and NKG2A, was maintained even after a strong stimulation by the cytokines interleukin-2, interleukin-12, interleukin-15 and interleukin-18. Furthermore, the percentages of activated NK cells expressing Ly49C/I and Ly49I were strongly down-modulated under these conditions. We completed our investigations with phenotypic studies of NK cells from these mice.

The nonclassical immune surveillance for ERAAP function

The peptide repertoire presented by MHC class I molecules on the cell surface is essential for the immune surveillance of intracellular pathogens and transformed cells. The generation of this peptide repertoire is critically dependent on the endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP). Loss of ERAAP function leads to the generation of a profoundly disrupted peptide repertoire including many novel and immunogenic peptides. Strikingly, a large fraction of these novel peptides on ERAAP-KO cells are presented by the nonclassical MHC Ib molecule, Qa-1^b. One immunodominant Qa-1^b-restricted novel peptide is recognized by a unique CD8⁺ T cell population showing features of both conventional cytotoxic T cells and unconventional innate-like T cells. While much remains to be uncovered, here we summarize the latest discoveries of our lab on the important immune surveillance of ERAAP function mediated by nonclassical MHC Ib molecules and their unusual cognate T cells.

T Cells Engaging the Conserved MHC Class Ib Molecule Qa-1b with TAP-Independent Peptides Are Semi-Invariant Lymphocytes

The HLA-E homolog in the mouse (Qa-1^b) is a conserved MHC class Ib molecule presenting monomorphic peptides to germline-encoded natural killer receptor CD94/NKG2A. Previously, we demonstrated the replacement of this canonical peptide by a diverse peptidome upon deficiency of the TAP peptide transporter. Analysis of this Qa-1^b-restricted T cell repertoire against these non-mutated neoantigens revealed characteristics of conventional hypervariable CD8⁺ T cells, but also of invariant T cell receptor (TCR)αβ T cells. A shared TCR Vα chain was used by this subset in combination with a variety of Vβ chains. The TCRs target peptide ligands that are conserved between mouse and man, like the identified peptide derived from the transcriptional cofactor Med15. The thymus selection was studied in a TCR-transgenic mouse and emerging naïve CD8⁺ T cells displayed a slightly activated phenotype, as witnessed by higher CD122 and Ly6C expression. Moreover, the Qa-1^b protein was dispensable for thymus selection. Importantly, no self-reactivity was observed as reported for other MHC class Ib-restricted subsets. Naïve Qa-1^b restricted T cells expanded, contracted, and formed memory cells in vivo upon peptide vaccination in a similar manner as conventional CD8⁺ T cells. Based on these data, the Qa-1^b restricted T cell subset might be positioned closest to conventional CD8⁺ T cells of all MHC class Ib populations.

Expression of the mouse MHC class Ib H2-T11 gene product, a paralog of H2-T23 (Qa-1) with shared peptide-binding specificity

The mouse MHC class Ib gene H2-T11 is 95% identical at the DNA level to H2-T23, which encodes Qa-1, one of the most studied MHC class Ib molecules. H2-T11 mRNA was observed to be expressed widely in tissues of C57BL/6 mice, with the highest levels in thymus. To circumvent the availability of a specific mAb, cells were transduced with cDNA encoding T11 with a substituted α3 domain. Hybrid T11D3 protein was expressed at high levels similar to control T23D3 molecules on the surface of both TAP(+) and TAP(-) cells. Soluble T11D3 was generated by folding in vitro with Qa-1 determinant modifier, the dominant peptide presented by Qa-1. The circular dichroism spectrum of this protein was similar to that of other MHC class I molecules, and it was observed to bind labeled Qa-1 determinant modifier peptide with rapid kinetics. By contrast to the Qa-1 control, T11 tetramers did not react with cells expressing CD94/NKG2A, supporting the conclusion that T11 cannot replace Qa-1 as a ligand for NK cell inhibitory receptors. T11 also failed to substitute for Qa-1 in the presentation of insulin to a Qa-1-restricted T cell hybridoma. Despite divergent function, T11 was observed to share peptide-loading specificity with Qa-1. Direct analysis by tandem mass spectrometry of peptides eluted from T11D3 and T23D3 isolated from Hela cells demonstrated a diversity of peptides with a clear motif that was shared between the two molecules. Thus, T11 is a paralog of T23 encoding an MHC class Ib molecule that shares peptide-binding specificity with Qa-1 but differs in function.

Beyond the hormone: insulin as an autoimmune target in type 1 diabetes

Insulin is not only the hormone produced by pancreatic β-cells but also a key target antigen of the autoimmune islet destruction leading to type 1 diabetes. Despite cultural biases between the fields of endocrinology and immunology, these two facets should not be regarded separately, but rather harmonized in a unifying picture of diabetes pathogenesis. There is increasing evidence suggesting that metabolic factors (β-cell dysfunction, insulin resistance) and immunological components (inflammation and β-cell-directed adaptive immune responses) may synergize toward islet destruction, with insulin standing at the crossroad of these pathways. This concept further calls for a revision of the classical dichotomy between type 1 and type 2 diabetes because metabolic and immune mechanisms may both contribute to different extents to the development of different forms of diabetes. After providing a background on the mechanisms of β-cell autoimmunity, we will explain the role of insulin and its precursors as target antigens expressed not only by β-cells but also in the thymus. Available knowledge on the autoimmune antibody and T-cell responses against insulin will be summarized. A unifying scheme will be proposed to show how different aspects of insulin biology may lead to β-cell destruction and may be therapeutically exploited. We will argue about possible reasons why insulin remains the mainstay of metabolic control in type 1 diabetes but has so far failed to prevent or halt β-cell autoimmunity as an immune modulatory reagent.

Control of T cell reactivation by regulatory Qa-1-restricted CD8+ T cells

Administration of attenuated pathogenic T cell clones, a procedure known as T cell vaccination, induces CD8+ T cells specific for peptides derived from the Vbeta-chain of the TCR presented by the MHC class Ib molecule, Qa-1 expressed on the vaccine cells. These regulatory CD8+ T cells have the capacity to control the activation of endogenous T cells expressing the same TCR Vbeta-chain as the vaccinating cells. We hypothesized that vaccination with NKT cells could also induce Qa-1-restricted CD8+ T cells that would control NKT cell activation. We tested this hypothesis in a murine model of Con A-induced hepatitis that is induced by NKT cells. Vaccination with NKT cells effectively induced protective Qa-1-restricted CD8+ T cells that prevented hepatitis. Surprisingly, upon vaccination with T cells expressing Vbeta-chains irrelevant to NKT cells, we discovered that the specificity of vaccine-induced Qa-1-restricted CD8+ T cells was not limited to the Vbeta-chain of the vaccinating cells. We further show that these regulatory Qa-1-restricted CD8+ T cells arise spontaneously upon polyclonal activation of T cells in the absence of deliberate T cell vaccination. These experiments provide new insight into a CD8+ T cell compartment that regulates the immediate reactivation of conventional T cells and NKT cells.

Polyclonal MHC Ib-restricted CD8+ T cells undergo homeostatic expansion in the absence of conventional MHC-restricted T cells

Naive T cells have the capacity to expand in a lymphopenic environment in a process called homeostatic expansion, where they gain a memory-like phenotype. Homeostatic expansion is dependent on competition for a number of factors, including growth factors and interactions with their selecting self-MHC molecules. In contrast to conventional T cells, it is unclear whether class Ib-restricted CD8+ T cells have a capacity to undergo homeostatic expansion. In this study, we demonstrate that polyclonal MHC Ib-restricted CD8+ T cells can undergo homeostatic expansion and that their peripheral expansion is suppressed by conventional MHC-restricted T cells. The acute depletion of CD4+ T cells in MHC class Ia-deficient Kb-/-Db-/- mice led to the substantial expansion of class Ib-restricted CD8+ T cells. Adoptive transfer of class Ib-restricted CD8+ T cells to congenic lymphopenic recipients revealed their ability to undergo homeostatic expansion in a MHC Ib-dependent manner. To further study the homeostatic expansion of MHC Ib-restricted T cells in the absence of all conventional MHC-restricted T cells, we generated mice that express only MHC Ib molecules by crossing H-2Kb-/-Db-/- with CIITA-/- mice. CD8+ T cells in these mice exhibit all of the hallmarks of naive T cells actively undergoing homeostatic expansion with constitutive memory-like surface and functional phenotype. These findings provide direct evidence that MHC Ib-restricted CD8+ T cells have the capacity to undergo homeostatic expansion. Their peripheral expansion is suppressed under normal conditions by a numerical excess of conventional MHC class Ia- and class II-restricted T cells.

The immunoregulatory effects of Qa-1

The immune system is not only well equipped to control infections but also tightly controlled to prevent autoimmune disease. Despite the negative selection of T-cell clones in the thymus, mature T cells capable of recognizing self-antigens are present in every individual. Several types of specialized regulatory cells maintain homeostasis and prevent expansion of autoreactive T cells. In this issue of Immunological Reviews, the role of CD4+ regulatory T cells is extensively discussed. Suppression of T-cell responses by CD8+ T cells has received less attention. Here, we review research on Qa-1-restricted CD8+ regulatory T cells. We focus on the role of this class Ib major histocompatibility complex (MHC) molecule in both CD8+ regulatory T-cell activity and protection of activated T cells.

Non-classical MHC class I molecules on intestinal epithelial cells: mediators of mucosal crosstalk

The mucosal immune environment consists of a complex combination of lymphoid cells, non-lymphoid cells, and lumenal bacteria. Signals from lumenal bacteria are constantly transmitted to the underlying tissues across the intestinal epithelial barrier. Intestinal epithelial cells (IECs) can sense these signals, integrate them, and interpret them for lamina propria lymphoid populations. One mechanism by which these signals are communicated is by the expression of non-classical major histocompatibility complex (MHC) class I molecules by IECs. Epithelial cells can express a surprising variety of non-classical MHC class I molecules. In some cases, IECs can act as non-professional antigen-presenting cells utilizing the expression of such non-classical MHC class I molecules to directly present bacterial antigens. In other cases, the expression of non-classical MHC class I molecules may act as a co-stimulatory molecule or adhesion molecule that can modify the mucosal immune response. Finally, the expression of these molecules on IECs can lead to a broad array of responses ranging from tolerance to inflammation. Overall, the IEC, via the expression of non-classical MHC class I molecules, is a central mediator of the constant crosstalk between the intestinal lumen and the mucosal immune system.

An MHC Class Ib-Restricted TCR That Cross-Reacts with an MHC Class Ia Molecule

TCR transgenic 6C5 T cells recognize an insulin B chain epitope presented by the nonclassical class I MHC molecule, Qa-1(b). Positive selection of these T cells was shown previously to require Qa-1(b). Despite dedicated specificity for Qa-1(b), evidence presented in the current study indicates that 6C5 T cells can cross-recognize a classical class I molecule. Clonal deletion was observed unexpectedly in 6C5.H-2(bxq) mice, which do not express I-E MHC class II molecules and thus should not be subject to superantigen-mediated negative selection. 6C5 T cells were observed to respond in vivo and in vitro to spleen cells from allogeneic H-2(q) mice, and specificity was mapped to D(q). Evidence was obtained for direct recognition of D(q), rather than indirect presentation of a D(q)-derived peptide presented by Qa-1(b). Polyclonal CD8(+) T cells from class Ia-deficient K(b)D(b-/-) mice reacted in vitro to allogeneic spleen cells with an apparent frequency comparable to conventional class Ia-restricted T cells. Our results provide a clear example of a Qa-1-specific TCR that can cross-react with a class Ia molecule and evidence supporting the idea that this may be a common property of T cells selected by class Ib molecules.

MHC class Ib molecules bridge innate and acquired immunity

Our understanding of the classical MHC class I molecules (MHC class Ia molecules) has long focused on their extreme polymorphism. These molecules present peptides to T cells and are central to discrimination between self and non-self. By contrast, the functions of the non-polymorphic MHC class I molecules (MHC class Ib molecules) have been elusive, but emerging evidence reveals that, in addition to antigen presentation, MHC class Ib molecules are involved in immunoregulation. As we discuss here, the subset of MHC class Ib molecules that presents peptides to T cells bridges innate and acquired immunity, and this provides insights into the origins of acquired immunity.

Homeostatic proliferation of a Qa-1b-restricted T cell: a distinction between the ligands required for positive selection and for proliferation in lymphopenic hosts

Naive T cells proliferate in response to self MHC molecules after transfer into lymphopenic hosts, a process that has been termed homeostatic proliferation (HP). Previous studies have demonstrated that HP is driven by low level signaling induced by interactions with the same MHC molecules responsible for positive selection in the thymus. Little is known about the homeostatic regulation of T cells specific for class Ib molecules, including Qa-1 and H2-M3, though it has been suggested that their capacity to undergo homeostatic expansion may be inherently limited. In this study, we demonstrate that naive 6C5 TCR transgenic T cells with specificity for Qa-1(b) have a capacity similar to conventional T cells to undergo HP after transfer into sublethally irradiated mice. Proliferation was largely dependent on the expression of beta(2)-microglobulin, and experiments with congenic recipients expressing Qa-1(a) instead of Qa-1(b) demonstrated that HP is specifically driven by Qa-1(b) and not through cross-recognition of classical class I molecules. Thus, the same MHC molecule that mediates positive selection of 6C5 T cells is also required for HP. Homeostatic expansion, like positive selection, occurs in the absence of a Qa-1 determinant modifier, the dominant self-peptide bound to Qa-1 molecules. However, experiments with TAP(-/-) recipients demonstrate a clear distinction between the ligand requirements for thymic selection and HP. Positive selection of 6C5 T cells is dependent on TAP function, thus selection is presumably mediated by TAP-dependent peptides. By contrast, HP occurs in TAP(-/-) recipients, providing an example where the ligand requirements for HP are less stringent than for thymic selection.

Qa-1 restriction of CD8+ suppressor T cells

There is increasing evidence that the immune response can be inhibited by several T cell subsets, including NK T cells, CD25+CD4+ T cells, and a subpopulation of CD8+ T cells. Animal model studies of multiple sclerosis have suggested an important role for suppressor CD8+ T cells in protection against disease recurrence and exacerbation. The molecular lynchpin of CD8+ suppressive activity is the murine MHC molecule Qa-1, termed HLA-E in humans. Here we summarize findings from work on Qa-1 that have begun to delineate suppressor CD8+ T cells and their mechanisms of action in the context of self tolerance and autoimmune disease.

Polymorphism and conservation of the genes encoding Qa1 molecules

To evaluate the polymorphism and conservation of the major histocompatibility complex class Ib molecule Qa1 in wild mouse populations, we determined the nucleotide sequence of exons 1-3 of Qa1 of eight mouse haplotypes derived from wild mice, including Mus musculus domesticus, M. m. castaneus, M. m. bactrianus, and M. spretus, as well as two t haplotypes. Our data identify eight new alleles of Qa1. Taken together with previously published data on Qa1 among the common laboratory inbred strains, and in agreement with cytotoxic T-lymphocyte, serological, and biochemical data, these results further confirm the existence of two families of Qa1 molecules, Qa1(a)-like and Qa1(b)-like, and illuminate the extreme conservation of the peptide-binding region of these molecules, even across species.

Differential Requirement for Tapasin in the Presentation of Leader- and Insulin-Derived Peptide Antigens to Qa-1b-Restricted CTLs

The loading of MHC class I molecules with peptides involves a variety of accessory proteins, including TAP-associated glycoprotein (tapasin), which tethers empty MHC class I molecules to the TAP peptide transporter. We have evaluated the role of tapasin for the assembly of peptides with the class Ib molecule Qa-1b. In normal cells, Qa-1b is predominantly bound by a peptide, the Qa-1 determinant modifier (Qdm), derived from the signal sequence of class Ia molecules. Our results show that tapasin links Qa-1b to the TAP peptide transporter, and that tapasin facilitates the delivery of Qa-1b molecules to the cell surface. Tapasin was also required for the presentation of endogenous Qdm peptides to Qdm-specific, Qa-1b-restricted CTLs. In sharp contrast, tapasin expression was dispensable for the presentation of an insulin peptide to insulin-specific, Qa-1b-restricted CTL isolated from TCR transgenic mice. However, tapasin deficiency significantly impaired the positive selection of these insulin-specific, Qa-1b-restricted transgenic CD8+ T cells. These findings reveal that tapasin plays a differential role in the loading of Qdm and insulin peptides onto Qa-1b molecules, and that tapasin is dispensable for retention of empty Qa-1b molecules in the endoplasmic reticulum, and are consistent with the proposed peptide-editing function of tapasin.

Infection-induced expansion of a MHC Class Ib-dependent intestinal intraepithelial gammadelta T cell subset

Salmonella species invade the host via the intestinal epithelium. Hence, intestinal intraepithelial lymphocytes (iIELs) are potentially the first element of the immune system to encounter Salmonella during infection. In this study, we demonstrate, in a mouse model, the expansion of a CD8alphabeta(+)CD94(-)TCRgammadelta(+) T cell subset within the iIEL population in response to oral infection with virulent or avirulent Salmonella. This population can be detected 3 days following infection, represents up to 15% of the TCRgammadelta(+) iIELs, and is dependent on the MHC class Ib molecule T23 (Qa-1). Qa-1 is expressed by intestinal epithelial cells and thus accessible for iIEL recognition. Such cells may play a role in the early immune response to Salmonella.

Suppression of autoimmune disease after vaccination with autoreactive T cells that express Qa-1 peptide complexes

The ability of autoreactive T cells to provoke autoimmune disease is well documented. The finding that immunization with attenuated autoreactive T cells (T cell vaccination, or TCV) can induce T cell-dependent inhibition of autoimmune responses has opened the possibility that regulatory T cells may be harnessed to inhibit autoimmune disease. Progress in the clinical application of TCV, however, has been slow, in part because the underlying mechanism has remained clouded in uncertainty. We have investigated the molecular basis of TCV-induced disease resistance in two murine models of autoimmunity: herpes simplex virus-1 (KOS strain)-induced herpes stromal keratitis and murine autoimmune diabetes in non-obese diabetic (NOD) mice. We find that the therapeutic effects of TCV depend on activation of suppressive CD8 cells that specifically recognize Qa-1-bound peptides expressed by autoreactive CD4 cells. We clarify the molecular interaction between Qa-1 and self peptides that generates biologically active ligands capable of both inducing suppressive CD8 cells and targeting them to autoreactive CD4 cells. These studies suggest that vaccination with peptide-pulsed cells bearing the human equivalent of murine Qa-1 (HLA-E) may represent a convenient and effective clinical approach to cellular therapy of autoimmune disease.

CD8 T cells can reject major histocompatibility complex class I-deficient skin allografts

Following transplantation, recipient T cells can recognize and respond to donor antigens expressed directly on donor cells, and can respond to donor-derived peptides that have been processed and presented in the context of recipient MHC through the indirect pathway. Indirectly primed CD4(+) T cells have been well studied in transplantation, but little information is available regarding whether indirectly primed CD8(+) T cells participate in rejection. To address this, we placed MHC class I-deficient D(b)K(b) knockout skin grafts onto allogeneic H-2 (k) SCID recipients followed by adoptive transfer of purified H-2 (k) CD8(+) T cells. The MHC class I-deficient grafts were rejected and only CD8(+) T cells were detectable in the recipient lymphoid organs and in the skin grafts. Immunohistochemical analysis showed that CD8(+) T cells were found in close proximity to vascular endothelial cells and to recipient infiltrating macrophages, suggesting specific interactions. The data demonstrate that cross-primed polyclonal CD8(+) T cells can function as active participants in the effector phase of rejection. The findings confirm and extend previous studies using a monoclonal TCR transgenic T cell and shed light on mechanisms of acute and chronic graft injury that are potentially relevant to human transplant recipients.

Analysis of regulatory CD8 T cells in Qa-1-deficient mice

The mouse protein Qa-1 (HLA-E in humans) is essential for immunological protection and immune regulation. Although Qa-1 has been linked to CD8 T cell-dependent suppression, the physiological relevance of this observation is unclear. We generated mice deficient in Qa-1 to develop an understanding of this process. Qa-1-deficient mice develop exaggerated secondary CD4 responses to foreign and self peptides. Enhanced responses to proteolipid protein self peptide were associated with resistance of Qa-1-deficient CD4 T cells to Qa-1-restricted CD8 T suppressor activity and increased susceptibility to experimental autoimmune encephalomyelitis. These findings delineate a Qa-1-dependent T cell-T cell inhibitory interaction that prevents the pathogenic expansion of autoreactive CD4 T cell populations and consequent autoimmune disease.

The nonclassical MHC class I molecule Qa-1 forms unstable peptide complexes

The MHC class Ib molecule Qa-1 is the primary ligand for mouse CD94/NKG2A inhibitory receptors expressed on NK cells, in addition to presenting Ags to a subpopulation of T cells. CD94/NKG2A receptors specifically recognize Qa-1 bound to the MHC class Ia leader sequence-derived peptide Qdm. Qdm is the dominant peptide loaded onto Qa-1 under physiological conditions and this peptide has an optimal sequence for binding to Qa-1. Peptide dissociation experiments demonstrated that Qdm dissociates from soluble or cell surface Qa-1(b) molecules with a t(1/2) of approximately 1.5 h at 37 degrees C. In comparison, complexes of an optimal peptide (SIINFEKL) bound to the MHC class Ia molecule H-2K(b) dissociated with a t(1/2) in the range from 11 to 31 h. In contrast to K(b), the stability of cell surface Qa-1(b) molecules was independent of bound peptides, and several observations suggested that empty cell surface Qa-1(b) molecules might be unusually stable. Consistent with the rapid dissociation rate of Qdm from Qa-1(b), cells become susceptible to lysis by CD94/NKG2A(+) NK cells under conditions in which new Qa-1(b)/Qdm complexes cannot be continuously generated at the cell surface. These results support the hypothesis that Qa-1 has been selected as a specialized MHC molecule that is unable to form highly stable peptide complexes. We propose that the CD94/NKG2A-Qa-1/Qdm recognition system has evolved as a rapid sensor of the integrity of the MHC class I biosynthesis and Ag presentation pathway.

A peptide from heat shock protein 60 is the dominant peptide bound to Qa-1 in the absence of the MHC class Ia leader sequence peptide Qdm

The MHC class Ib molecule Qa-1 binds specifically and predominantly to a single 9-aa peptide (AMAPRTLLL) derived from the leader sequence of many MHC class Ia proteins. This peptide is referred to as Qdm. In this study, we report the isolation and sequencing of a heat shock protein 60-derived peptide (GMKFDRGYI) from Qa-1. This peptide is the dominant peptide bound to Qa-1 in the absence of Qdm. A Qa-1-restricted CTL clone recognizes this heat shock protein 60 peptide, further verifying that it binds to Qa-1 and a peptide from the homologous Salmonella typhimurium protein GroEL (GMQFDRGYL). These observations have implications for how Qa-1 can influence NK cell and T cell effector function via the TCR and CD94/NKG2 family members, and how this effect can change under conditions that cause the peptides bound to Qa-1 to change.

A novel single chain I-A(b) molecule can stimulate and stain antigen-specific T cells

Multimers of soluble major histocompatibility complex class I and II molecules have proven to be useful reagents in quantifying and following specific T cell populations. This study describes the design, generation, and characterization of a novel, single chain I-A(b) molecule which utilizes a unique linker derived from the murine invariant chain. A fragment of the invariant chain, residues 58-85, binds to a region proximal to the class II peptide binding groove and stabilizes occupancy of the class II invariant chain-associated peptide. We have utilized this fragment, replacing CLIP with the Ealpha peptide sequence, to lock the attached peptide into the class II binding groove. The single chain I-A(b) molecule was recognized by a panel of conformation-sensitive, I-A(b)-specific, monoclonal antibodies. Membrane-bound and soluble forms of the single chain I-A(b) stimulated an antigen-specific T cell hybridoma, and tetramers made from soluble monomers stained these cells. The unique features of this molecule may be useful in the design of recombinant T cell receptor ligands containing peptides with low affinity for MHC.

MHC specificity of iIELs

Intestinal intra-epithelial lymphocytes (iIELs) are a major lymphocyte population, reside in close proximity to the intestinal lumen and are conserved throughout vertebrate evolution. iIELs consist of several unique T-cell phenotypes and express both non-rearranged innate immune receptors and rearranged adaptive immune receptors. The ligands for the innate immune receptors on iIELs, such as NKG2D (natural killer-cell receptor), often bind to non-classical MHC class I molecules, such as the human MHC class I-related molecules MICA or MICB. These ligands costimulate T-cell receptor (TCR)-mediated signaling. In most cases, the MHC molecules that bind to the TCR are still unknown. However, recent efforts to understand the MHC molecules that are involved in the development of and antigen recognition by iIELs have revealed several important results. Here, we focus systematically on recent developments in innate immunity and in TCR recognition of different subtypes of iIELs by various MHC molecules.

Peptide-independent folding and CD8 alpha alpha binding by the nonclassical class I molecule, thymic leukemia antigen

The nonclassical class I molecule, thymic leukemia (TL), has been shown to be expressed on intestinal epithelial cells and to interact with CD8(+) intraepithelial T lymphocytes. We generated recombinant soluble TL (T18(d)) H chains in bacteria as inclusion bodies and refolded them with beta(2)-microglobulin in the presence or absence of a random peptide library. Using a mAb, HD168, that recognizes a conformational epitope on native TL molecules, we observed that protein folds efficiently in the absence of peptide. Circular dichroism analysis demonstrated that TL molecules have structural features similar to classical class I molecules. Moreover, thermal denaturation experiments indicated that the melting temperature for peptide-free TL is similar to values reported previously for conventional class I-peptide complexes. Our results also show that CD8alphaalpha binding is not dependent on either TL-associated peptide or TL glycosylation.

CD94/NKG2 expression does not inhibit cytotoxic function of lymphocytic choriomeningitis virus-specific CD8+ T cells

Murine Ag-specific CD8(+) T cells express various NK markers and NK inhibitory receptors that have been proposed to modulate immune responses. Following acute infection of C57BL/6 and BALB/cJ mice with lymphocytic choriomeningitis virus (LCMV), we observed that Ag-specific CD8(+) T cells expressed CD94/NKG2. Only slight expression of Ly49A and Ly49C receptors was observed on NP396-specific T cells, while all NP396-specific T cells expressed the NKT cell marker U5A2-13 Ag. Expression of CD94/NKG2 was maintained for at least 1 year following LCMV infection, as was the NKT cell marker. By means of cell sorting and quantitative PCR, we found that NP118-specific CD8(+) T cells primarily express transcripts for inhibitory NKG2 receptor isoforms. CD94/NKG2 expression was also observed on Ag-specific CD8(+) T cells following infection with polyoma virus, influenza virus, and Listeria monocytogenes, suggesting that it may be a common characteristic of Ag-specific CD8(+) T cells following infection with viral or bacterial pathogens. Expression of CD94/NKG2 on memory-specific CD8(+) T cells did not change following secondary challenge with LCMV clone 13 and did not inhibit viral clearance. Furthermore, we found no evidence that CD94/NKG2 inhibits either the lytic function of LCMV-specific T cells or their capacity to produce effector cytokines upon peptide stimulation. Finally, down-regulation of CD94/NKG2 was found to occur only during chronic LCMV infection. Altogether, this study suggests that CD94/NKG2 expression is not necessarily correlated with inhibition of T cell function.

Positive selection of a Qa-1-restricted T cell receptor with specificity for insulin

The phenotype and development of T cells from transgenic mice expressing a T cell receptor with specificity for insulin presented by the MHC class Ib molecule Qa-1(b) was investigated. Peripheral T cells from the transgenic mice express CD8 and, after activation, kill Qa-1(b)-positive lymphoid target cells in the presence of soluble insulin. Thymic selection requires expression of Qa-1(b) but not the dominant Qa-1-associated peptide, Qdm. In contrast to conventional T cells, selection is at least as efficient when the selecting ligand is expressed only on hematopoietic lineage cells as compared to expression on epithelial cells in the thymus. Our findings suggest that there is a dedicated population of Qa-1-restricted T cells that are selected by interaction with Qa-1 and that the cellular requirements for selection may differ from conventional T cells.

MHC class I pathway is not required for the development of crescentic glomerulonephritis in mice

MHC II and CD4+ T cells are required for anti-glomerular basement membrane (GBM) globulin-initiated crescentic glomerulonephritis (GN) in mice, but the role of MHC I and CD8+ T cells is unclear. The cytolytic function of CD8+ T cells requires recognition of peptide antigens presented on MHC I. CD8+ T cells can also perform helper functions via cytokine production. The contribution of MHC I to crescentic GN was investigated using TAP-1 gene knock out (TAP-1-/-) mice, which have deficient MHC I antigen presentation. Heterozygous TAP-1 mice have normal MHC I expression and developed GN with crescents in 42 +/- 4% of glomeruli (normal 0%), proteinuria (9.1 +/- 1.6 mg/20 h, normal 1.5 +/- 0.3 mg/20 h) and impaired renal function (creatinine clearance 110 +/- 8 microl/min, normal 193 +/- 10 microl/min) following administration of sheep anti-mouse GBM globulin. TAP-1-/- mice, which have extremely low MHC I expression and reduced CD8+ T cells, developed similar GN with 39 +/- 3% crescents, proteinuria (12.7 +/- 4.3 mg/20 h) and impaired renal function (creatinine clearance 123 +/- 20 microl/min). In vivo antibody-induced CD8 depletion did not attenuate crescent formation or protect renal function in C57Bl/6 mice developing GN, although significant reduction in proteinuria (5.3 +/- 1.2 mg/20 h, P = 0. 012) and glomerular recruitment of CD4+ T cells and macrophages were observed compared with control treated mice with GN. These data demonstrate that MHC I is not required for development of crescentic GN in mice. The MHC I-independent contribution of CD8+ T cells to proteinuria and inflammatory cell recruitment suggests that they may serve a 'helper' rather than cytolytic role in this disease.

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.

The epitope detected by cytotoxic T lymphocytes against thymus leukemia (TL) antigen is TAP independent

Thymus leukemia (TL) antigens belong to the family of MHC class Ib antigens. We have shown in our previous studies that they serve as transplantation antigens, and can be recognized by both TCR alpha beta and TCR gamma delta cytotoxic T lymphocytes (CTL) with TL but not H-2 restriction. Although TL are known to be expressed TAP independently, it is unclear whether peptide loading on TL molecules is necessary for the formation of CTL epitopes. In the present study, we first showed that TL expression is beta(2)-microglobulin (beta(2)m)-dependent but TAP1 independent by flow cytometric analysis of thymocytes from beta(2)m- or TAP1-deficient mice crossed with TL transgenic mice expressing Tla(a)-3-TL on their thymocytes. Subsequently, we investigated the epitope recognized by CTL derived from C3H mice immunized with skin from a transgenic mouse expressing T3(b)-TL ubiquitously. Bulk CTL lines against TL from primary mixed lymphocyte cultures showed comparable cytotoxicity against T3(b)-TL transfectants of TAP2-deficient murine RMA-S grown at 37 degrees C to that against those grown at 25 degrees C. Furthermore, TCR alpha beta and TCR gamma delta CTL clones against TL recognized TL expressed on T3(b)-TL transfectants of RMA-S and Drosophila melanogaster cells having broad defects in peptide loading of MHC, and lysed these target cells. These results together indicate that TL-specific CTL populations primarily recognize epitopes expressed TAP independently.

Relative contributions of distinct MHC class I-dependent cell populations in protection to tuberculosis infection in mice

A necessary role for cytotoxic T lymphocytes in protection against Mycobacterium tuberculosis (MTB) has been suggested by studies of the beta2-microglobulin-deficient mouse, which is unable to present antigens through MHC class I and class I-like molecules and invariably succumbs early after infection. To identify the relative contributions of distinct putative MHC class I-dependent cell populations in protection against tuberculosis, we compared a variety of gene-disrupted mouse strains for susceptibility to MTB infection. Among the strains tested, the most susceptible mice, as measured by survival time and bacterial loads, were the beta2-microglobulin(-/-), followed by transporter associated with antigen processing deficient (TAP1(-/-)), CD8alpha(-/-), perforin(-/-), and CD1d(-/-) mice. These findings indicated that (i) CD8(+) T cells contribute to protection against MTB, and their protective activity is only partially dependent on perforin; (ii) beta2-microglobulin-dependent T cell populations distinct from CD8(+) T cells also contribute to anti-MTB immunity; and (iii) protective immune mechanisms are predominantly TAP-dependent, although TAP-independent mechanisms also contribute to protection. Because CD1d-deficient animals were fully resistant to MTB, other TAP-independent mechanisms must contribute to protection. We suggest here that both classical and nonclassical MHC class I-restricted T cells, distinct from CD1d-restricted cells, may be involved in protective immune responses against tuberculosis.

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.

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.

Cutting Edge: TCRαβ+ CD8αα+ T Cells Are Found in Intestinal Intraepithelial Lymphocytes of Mice That Lack Classical MHC Class I Molecules

TCRαβ+ intestinal intraepithelial lymphocytes (IEL) can express either the typical CD8αβ heterodimer or an unusual CD8αα homodimer. Both types of CD8+ IEL require class I molecules for their differentiation, since they are absent in β2m−/− mice. To gain insight into the role of class I molecules in forming TCRαβ+ CD8+ IEL populations, we have analyzed the IEL in mice deficient for either TAP, β2m, CD1, or K and D. We find that K−/−D−/− mice have TCRαβ+ CD8αα+ IEL, although they are deficient for TCRαβ+ CD8αβ+ cells. This indicates that at least some TCRαβ+ CD8αα+ IEL require only nonclassical class I molecules for their development. Surprisingly, the TCRαβ+ CD8αα+ IEL are significantly increased in K−/−D−/− mice, suggesting a complex interaction between CD8+ IEL and class I molecules that might include direct or indirect negative regulation by K and D, as well as positive effects mediated by nonclassical class I molecules.

Selection and expansion of CD8alpha/alpha(1) T cell receptor alpha/beta(1) intestinal intraepithelial lymphocytes in the absence of both classical major histocompatibility complex class I and nonclassical CD1 molecules

Intestinal intraepithelial lymphocytes (IELs) in mice include two main subsets of TCR-alpha/beta(1) cells which differ functionally and ontogenically from each other. One expresses the CD8alpha/alpha homodimer, whereas the other expresses the CD8alpha/beta heterodimer. Although the presence of all CD8(+)TCR-alpha/beta(1) IELs is dependent on beta2-microglobulin molecules, the nature of the major histocompatibility complex (MHC) class I molecules recognized by the CD8alpha/alpha and the CD8alpha/beta(1) subsets has remained elusive. Using mutant mice lacking the expression of both H2-K(b) and H2-D(b), we show that the CD8alpha/beta(1)TCR-alpha/beta(1) subset is dependent on K or D molecules, whereas the CD8alpha/alpha(1)TCR-alpha/beta(1) subset is independent of classical MHC class I molecules. Furthermore, the CD8alpha/alpha(1) cells are conserved in mice lacking expression of CD1, a nonclassical MHC class I-like molecule previously proposed to be a potential ligand for IELs. Using transporter associated with antigen processing (TAP)-deficient mice, this cell population can be further separated into a TAP-dependent and a TAP-independent subset, suggesting either the recognition of two nonclassical MHC-like molecules, only one of which is TAP dependent, or the involvement of a single nonclassical MHC-like molecule that is only partially TAP dependent. These findings demonstrate that CD8alpha/beta(1)TCR-alpha/beta(1) IELs are restricted by H-2K and H-2D molecules, whereas the unusual subset of CD8alpha/alpha(1)TCR-alpha/beta(1) resident IELs recognize nonclassical MHC class I-like molecules that are distinct from CD1.

Assessment of the Role of Determinant Selection in Genetic Control of the Immune Response to Insulin in H-2b Mice

The immune response to insulin is regulated by MHC class II genes. Immune response (Ir) gene-linked low responsiveness to protein Ags can be mediated by the low affinity of potential antigenic determinants for MHC molecules (determinant selection) or by the influence of MHC on the functional T cell repertoire. Strong evidence exists that determinant selection plays a key role in epitope immunodominance and Ir gene-linked unresponsiveness. However, the actual measurement of relative MHC-binding affinities of all potential peptides derived from well-characterized model Ags under Ir gene regulation has been very limited. We chose to take advantage of the simplicity of the structure of insulin to study the mechanism of Ir gene control in H-2b mice, which respond to beef insulin (BINS) but not pork insulin (PINS). Peptides from these proteins, including the immunodominant A(1–14) determinant, were observed to have similar affinities for purified IAb in binding experiments. Functional and biochemical experiments suggested that PINS and BINS are processed with similar efficiency. The T cell response to synthetic pork A(1–14) was considerably weaker than the response to the BINS peptide. We conclude that the poor immunogenicity of PINS in H-2b mice is a consequence of the T cell repertoire rather than differences in processing and presentation.

Susceptibility of mice deficient in CD1D or TAP1 to infection with Mycobacterium tuberculosis

Cellular immunity against Mycobacterium tuberculosis controls infection in the majority of infected humans. Studies in mice have delineated an important role for CD4(+) T cells and cytokines including interferon gamma and tumor necrosis factor alpha in the response to infection with mycobacteria. Recently, the identification of CD8(+) CD1-restricted T cells that kill M. tuberculosis organisms via granulysin and the rapid death after infection of beta2 microglobulin deficient mice in humans has drawn attention to a critical role for CD8(+) T cells. The nature of mycobacterial-specific CD8(+) T cells has been an enigma because few have been identified in any species. Here, we delineate the contribution of class I MHC-restricted T cells in the defense against tuberculosis as transporter associated with antigen processing (TAP)1-deficient mice died rapidly, bore a greater bacterial burden, and had more severe tissue pathology than control mice. In contrast, CD1D-/- mice were not significantly different in their susceptibility to infection than control mice. This data demonstrates a critical role for TAP-dependent peptide antigen presentation and provides further evidence that class I MHC-restricted CD8(+) T cells, the major T cell subset activated by this antigen processing pathway, play an essential role in immunity to tuberculosis.

Cell surface expression of HLA-E molecules on PBMC from a TAP1-deficient patient

It was recently revealed from studies on TAP-deficient cell lines that HLA-E molecules are associated with nonamer peptides derived from certain HLA class I leader sequences and are expressed on the cell surface in a TAP-dependent manner. We have previously reported a homozygous TAP1 gene mutation in a HLA class I-deficient patient. In the present report, we demonstrate HLA-E molecule expression on the surface of the peripheral blood mononuclear cells (PBMC) of the TAP1-deficient patient. The HLA-E expression level on the monocytes of the patient was as high as that in healthy donors, whereas the HLA-E expression level on the lymphocytes of the patient was slightly lower. On the other hand, HLA-E expression was not detected on KMW-B2 cells, an Epstein-Barr virus (EBV)-transformed B-cell line derived from the lymphocytes of the TAP1-deficient patient. These data suggest the existence of TAP-dependent and -independent pathways for the surface expression of HLA-E molecules.

Mechanisms of antigen presentation

T-lymphocytes recognize short peptide antigens bound stably to polymorphic major histocompatibility complex (MHC)-encoded glycoproteins expressed on the surface of antigen-presenting cells (APC). Two general pathways have evolved to generate peptide-MHC complexes. The MHC class II antigen processing pathway provides a mechanism for sampling proteins present in endosomal compartments. CD4+ regulatory T-cells recognize peptides bound to MHC class II molecules, which are selectively expressed in specialized APC that have efficient mechanisms for uptake of microbial antigens, and express costimulatory molecules required for activating naive T-cells. CD8+ T-cells recognize peptides bound to MHC class I molecules. Class I molecules are widely expressed and bind peptides derived from the normal turnover of cellular proteins, providing a mechanism to display a sampling of cellular components to be monitored for abnormalities by cytotoxic T-cells. Specialized accessory proteins influence the efficiency of antigen presentation and the specificity of immune responses through their roles in generating peptides, targeting antigen and MHC glycoproteins to selected intracellular compartments, and by direct participation in the peptide-loading mechanism. It has recently been discovered that some viruses have evolved ways to inhibit or subvert discrete steps in antigen processing, providing a mechanism to evade immune recognition.

Functions of nonclassical MHC and non-MHC-encoded class I molecules

Fascinating recent discoveries have focused attention on the nonclassical class I molecules. They can exert their function at most levels of the immune response, being part of both innate and adaptive immune systems. They not only have specialized antigen-presentation functions but also play important immunoregulatory roles: HLA-E regulates natural killer cells by interacting with CD94/NKG2 receptors; the MIC (MHC class I chain related) glycoproteins appear crucial to the activation of gammadelta T cells in the gastrointestinal epithelium; HLA-G may play a role in controlling the immune response to the fetus; and CD1 molecules are important in defense against bacterial infections, as well as in the development and regulation of a subset of NKT cells expressing a highly restricted TCR repertoire; however not all nonclassical class I molecules have an immunological function, as demonstrated by HFE which is implicated in iron metabolism.

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

Natural killer (NK) cells preferentially lyse targets that express reduced levels of major histocompatibility complex (MHC) class I proteins. To date, the only known mouse NK receptors for MHC class I belong to the Ly49 family of C-type lectin homodimers. Here, we report the cloning of mouse NKG2A, and demonstrate it forms an additional and distinct class I receptor, a CD94/NKG2A heterodimer. Using soluble tetramers of the nonclassical class I molecule Qa-1(b), we provide direct evidence that CD94/NKG2A recognizes Qa-1(b). We further demonstrate that NK recognition of Qa-1(b) results in the inhibition of target cell lysis. Inhibition appears to depend on the presence of Qdm, a Qa-1(b)-binding peptide derived from the signal sequences of some classical class I molecules. Mouse NKG2A maps adjacent to CD94 in the heart of the NK complex on mouse chromosome six, one of a small cluster of NKG2-like genes. Our findings suggest that mouse NK cells, like their human counterparts, use multiple mechanisms to survey class I expression on target cells.