Increase in positive selection of CD8+ T cells in TAP1-mutant mice by human beta 2-microglobulin transgene

Dual role of the peptide-loading complex as proofreader and limiter of MHC-I presentation

Antigen presentation via major histocompatibility complex class I (MHC-I) molecules is essential for surveillance by the adaptive immune system. Central to this process is the peptide-loading complex (PLC), which translocates peptides from the cytosol to the endoplasmic reticulum and catalyzes peptide loading and proofreading of peptide-MHC-I (pMHC-I) complexes. Despite its importance, the impact of individual PLC components on the presented pMHC-I complexes is still insufficiently understood. Here, we used stoichiometrically defined antibody-nanobody complexes and engineered soluble T cell receptors (sTCRs) to quantify different MHC-I allomorphs and defined pMHC-I complexes, respectively. Thereby, we uncovered distinct effects of individual PLC components on the pMHC-I surface pool. Knockouts of components of the PLC editing modules, namely tapasin, ERp57, or calreticulin, changed the MHC-I surface composition to a reduced proportion of HLA-A*02:01 presentation compensated by a higher ratio of HLA-B*40:01 molecules. Intriguingly, these knockouts not only increased the presentation of suboptimally loaded HLA-A*02:01 complexes but also elevated the presentation of high-affinity peptides overexpressed in the cytosol. Our findings suggest that the components of the PLC editing module serve a dual role, acting not only as peptide proofreaders but also as limiters for abundant peptides. This dual function ensures the presentation of a broad spectrum of antigenic peptides.

Conformational sensing of major histocompatibility complex (MHC) class I molecules by immune receptors and intracellular assembly factors

Major histocompatibility complex class I (MHC-I) molecules play a critical role in both innate and adaptive immune responses. The heterodimeric complex of a polymorphic MHC-I heavy chain and a conserved light chain binds to a diverse set of peptides which are presented at the cell surface. Peptide-free (empty) versions of MHC-I molecules are typically retained intracellularly due to their low stability and bound by endoplasmic reticulum chaperones and assembly factors. However, emerging evidence suggests that at least some MHC-I allotypes are relatively stable and detectable at the cell-surface as peptide-deficient conformers, under some conditions. Such MHC-I conformers interact with multiple immune receptors to mediate various immunological functions. Furthermore, conformational sensing of MHC-I molecules by intracellular assembly factors and endoplasmic reticulum chaperones influences the peptide repertoire, with profound consequences for immunity. In this review, we discuss recent advances relating to MHC-I conformational variations and their pathophysiological implications.

MHC class I family proteins retard systemic lupus erythematosus autoimmunity and B cell lymphomagenesis

Dysregulation of the T cell-dependent Ab response can lead to numerous immunological disorders, ranging from systemic lupus erythematosus to B cell lymphomas. Cellular processes governed by MHC class II proteins play a major role in this response and its dysregulation. The extent to which processes controlled by the diverse family of MHC class I proteins impact such autoimmune and neoplastic disorders, however, is less clear. In this study, we genetically dissect the contributions of individual MHC class I family members and the pathological processes under their control in the systemic lupus erythematosus-like disease of BXSB.Yaa mice and B cell lymphomagenesis of SJL mice. This study reveals a powerful repressive regulatory axis comprised of MHC class I-dependent CD8(+) T cells and NK cells. These results indicate that the predominant role of the MHC class I protein family in such immunological disorders is to protect from more aggressive diseases.

Defective MHC class I antigen surface expression promotes cellular survival through elevated ER stress and modulation of p53 function

Defects in Major Histocompatibility class I cell surface expression is thought to allow escape of tumor cells from immune surveillance. Hitherto, it is unclear whether this deficiency confers immune-independent survival advantage. We show here that class I cell surface expression deficiency due to defects in beta2 microglobulin or the transporter-associated with antigen processing (TAP) results in resistance to apoptosis in response to various cytotoxic signals. Reduced apoptosis correlated with altered p53 activation, which was due to compromised nuclear translocation of p53. Binding of p53 to glycogen synthase kinase-3beta (GSK3beta), which is known to phosphorylate and lead to cytoplasmic sequestration of p53, was enhanced in these cells. Consistently, endoplasmic reticulum (ER) stress, which promotes binding of p53 to GSK3beta was constitutively elevated in the absence of class I cell surface expression. Taken together, the results suggest a non-immunological causal role for defective class I cell surface expression in regulating cellular survival in a p53-dependent manner, through the upregulation of ER stress, which could be another mechanism leading to carcinogenesis.

HLA class I transgenic mice: development, utilisation and improvement

Classical major histocompatibility complex (MHC) class I antigens are trimeric molecules found on the surface of nucleated cells in all jawed vertebrates. MHC I are recognised by two families of receptors: clonotypic T cell receptors expressed on the surface of CD8+ cytotoxic T lymphocytes (CTLs), and monomorphic receptors expressed by both natural killer cells and CTLs. The production of MHC I molecules within the cells is a sequential process performed with the help of interacting proteins: proteases, chaperones, transporters and so on. Although largely homologous in their structure, organisation and function, the human and mouse MHC I antigen processing and presentation machineries show fine differences. Transgenesis and 'knockout' or 'knock-in' technologies permit the addition of relevant human genes or the replacement of mouse genes by their human orthologues in order to produce immunologically humanised mice. Such experimental animals are especially relevant for the comparative evaluation of immunotherapies and for the characterisation of MHC I peptide epitopes. This review presents the similarities and differences between mouse and human MHC I antigen processing machinery, and describes the development and utilisation of improving mouse models of human cytotoxic T cell immunity.

Coevolution of TCR-MHC interactions: conserved MHC tertiary structure is not sufficient for interactions with the TCR

The specificity for self-MHC that is necessary for T cell function is a consequence of intrathymic selection during which T cell antigen receptors (TCRs) expressed by immature thymocytes are tested for their affinity for self-peptide:self-MHC. The germ-line-encoded segments of the TCR, however, are believed to have an innate specificity for structural features of MHC molecules. We directly tested this hypothesis by generating a transgenic mouse system in which the protein HLA-DM is expressed at the surface of thymic cortical epithelial cells in the absence of classical MHC molecules. The specialized intracellular function of HLA-DM has removed this MHC class II-like protein from the evolutionary forces that have been hypothesized to shape TCR-MHC interactions. Our study shows that a structural mimic of MHC class II is not sufficient to appropriately interact with the TCRs expressed by developing thymocytes. This result emphasizes the unique complementarity of TCR-MHC interactions that are maintained by the evolutionary pressures dictated by positive selection.

Tapasin-/- and TAP1-/- macrophages are deficient in vacuolar alternate class I MHC (MHC-I) processing due to decreased MHC-I stability at phagolysosomal pH

Alternate class I MHC (MHC-I) Ag processing via cytosolic or vacuolar pathways leads to cross-presentation of exogenous Ag to CD8 T cells. Vacuolar alternate MHC-I processing involves phagolysosomal Ag proteolysis and peptide binding to MHC-I in post-Golgi compartments. We report the first study of alternate MHC-I Ag processing in tapasin(-/-) cells and experiments with tapasin(-/-) and TAP1(-/-) macrophages that characterize alternate MHC-I processing. Tapasin promotes retention of MHC-I in the endoplasmic reticulum (ER) for loading with high affinity peptides, whereas tapasin(-/-) cells allow poorly loaded MHC-I molecules to exit the ER. Hypothetically, we considered that a large proportion of post-Golgi MHC-I on tapasin(-/-) cells might be peptide-receptive, enhancing alternate MHC-I processing. In contrast, alternate MHC-I processing was diminished in both tapasin(-/-) and TAP1(-/-) macrophages. Nonetheless, these cells efficiently presented exogenous peptide, suggesting a loss of MHC-I stability or function specific to vacuolar processing compartments. Tapasin(-/-) and TAP1(-/-) macrophages had decreased MHC-I stability and increased susceptibility of MHC-I to inactivation by acidic conditions (correlating with vacuolar pH). Incubation of tapasin(-/-) or TAP1(-/-) cells at 26 degrees C decreased susceptibility of MHC-I to acid pH and reversed the deficiency in alternate MHC-I processing. Thus, tapasin and TAP are required for MHC-I to bind ER-derived stabilizing peptides to achieve the stability needed for alternate MHC-I processing via peptide exchange in acidic vacuolar processing compartments. Acidic pH destabilizes MHC-I, but also promotes peptide exchange, thereby enhancing alternate MHC-I Ag processing. These results are consistent with alternate MHC-I Ag processing mechanisms that involve binding of peptides to MHC-I within acidic vacuolar compartments.

A low affinity TCR ligand restores positive selection of CD8+ T cells in vivo

The T cell repertoire is shaped by the processes of positive and negative selection. During development, the TCR binds self peptide-MHC complexes in the thymus, and the kinetics of this interaction are thought to determine the thymocyte's fate. For development of CD8(+) T cells, the data supporting such a model have been obtained using fetal thymic organ culture. To confirm the fidelity of this model in vivo, we studied development of OT-I TCR-transgenic mice that expressed different individual K(b) binding peptides in thymic epithelial cells under the control of the human keratin 14 promoter. We used a system that allowed TAP-independent expression of the peptide-MHC complex, such that the ability of given peptides to restore positive selection in TAP(o) mice could be assessed. We found that transgenic expression of a TCR antagonist peptide (E1) in vivo efficiently restored positive selection of OT-I T cells in TAP(o) mice. An unrelated transgenic peptide (SIY) did not restore selection of OT-I T cells, nor did the E1-transgenic peptide restore selection of an unrelated receptor (2C), showing that positive selection is peptide specific in vivo, as observed in organ cultures. Neither E1 nor SIY transgenes increased the polyclonal CD8 T cell repertoire size in non-TCR-transgenic animals, arguing that single class I binding peptides do not detectably affect the size of the CD8 T cell repertoire when expressed at low levels. We also observed that OT-I T cells selected in TAP(o)-E1 mice were functional in their response to Ag; however, there was a lag in this response, suggesting that the affinity of the TCR interaction with MHC-self peptide can result in fine-tuning of the T cell response.

Positive selection of an MHC class-I restricted TCR in the absence of classical MHC class I molecules

The H-2Ld alloreactive 2C T cell receptor (TCR) is commonly considered as being positively selected on the H-2Kb molecule. Surprisingly, 2C TCR+ CD8+ single-positive T cells emerge in massive numbers in fetal thymic organ culture originating from 2C transgenic, H-2KbD(b-/-) (2C+KbD(b-/-)) but not in fetal thymic organ culture from beta2-microglobulin(-/-) 2C transgenic animals. Mature CD8+ T cells are observed in newborn but not in adult 2C+KbD(b-/-) mice. These CD8+ T cells express the alpha4beta7 integrin, which allows them to populate the intestine, a pattern of migration visualized by intrathymic injection of FITC and subsequent accrual of FITC-labeled lymphocytes in the gut. We conclude that the 2C TCR is reactive not only with H-2Ld and H-2Kb, but also with nonclassical MHC class I products to enable positive selection of 2C+ T cells in the fetal and newborn thymus and to support their maintenance in the intestine.

Population biology of lymphocytes: the flight for survival

In this essay we suggest that the primary goal of the cells of the immune system is to ensure their own growth and survival. In adults, in steady-state conditions, the number and distribution of lymphocyte populations is under homeostatic control. New lymphocytes that are continuously produced in primary and secondary lymphoid organs must compete with resident cells for survival. We discuss recent findings supporting lymphocyte survival as a continuous active process and implicating cognate receptor engagement as fundamental survival signals for both T and B lymphocytes. The conflict of survival interests between different cell types gives rise to a pattern of interactions that mimics the behavior of complex ecological systems. In their flight for survival and in response to competition, lymphocytes use different survival signals within different ecological niches during cell differentiation. This is the case for T and B lymphocytes and also for naive and memory/activated T and B cells. We discuss how niche differentiation allows the co-existence of different cell types and guarantees both repertoire diversity and efficient immune responses.

Naturally occurring TAP-dependent specific T-cell tolerance for a variant of an immunodominant retroviral cytotoxic T-lymphocyte epitope

Upon immunization and restimulation with tumors induced by the endogenous AKR/Gross murine leukemia virus (MuLV), C57BL/6 mice generate vigorous H-2K(b)-restricted cytotoxic T-lymphocyte (CTL) responses to a determinant (KSPWFTTL) derived from the p15E transmembrane portion of the viral envelope glycoprotein. By contrast, the highly homologous determinant RSPWFTTL, expressed by tumor cells induced by Friend/Moloney/Rauscher (FMR) MuLV, is not immunogenic, even when presented to the immune system as vaccinia virus-encoded cytosolic or endoplasmic reticulum (ER)-targeted minigene products. Such minigene products are usually highly immunogenic since they bypass the need for cells to liberate the peptide or transport the peptide into the ER by the transporter associated with antigen processing (TAP). Using KSPWFTTL-specific CTLs that cross-react with RSPWFTTL, we previously demonstrated that presentation of RSPWFTTL from its natural viral gene product is TAP dependent. Here, we show first that C57BL/6 mice express mRNA encoding RSPWFTTL but not KSPWFTTL and second that the ER-targeted RSPWFTTL minigene product is highly immunogenic in C57BL/6 mice with a targeted deletion in TAP1. These findings provide the initial demonstration of TAP-dependent tolerance induction to a specific self peptide and demonstrate that this contributes to the differential recognition of RSPWFTTL and KSPWFTTL by C57BL/6 mice.

Positive selection is limited by available peptide-dependent MHC conformations

Recent data suggest that the diversity of self peptides presented in the thymus during development contributes to positive selection of a diverse T cell repertoire. We sought to determine whether a previously defined "hole in the immunological repertoire" could be explained by the absence of an appropriate selecting self peptide. The repertoire defect in question is the inability of bm8 mice to make an H-2K-restricted response to OVA. Like other OVA-specific, H-2K-restricted receptors, OT-I-transgenic T cells are not positively selected in bm8 mice. Using criteria we had previously established for identifying positive selection ligands, we found peptides that could restore positive selection of OT-I thymocytes in bm8 mice. Thus, the T cell repertoire can be limited by a requirement for specific self peptides during development. Data with MHC-specific Abs suggested that peptides might be able to force MHC residues to adopt different conformations in Kb vs Kbm8. This shows that peptides can potentially contribute to ligand diversity both directly (via variability in the solvent-exposed side chains) and indirectly (through their effect on the MHC conformation). Our data support a model where self peptide diversity allows selection of T cells specific for a broad range of MHC conformations.

T cell receptor interactions with class I heavy-chain influence T cell selection

The interaction of the T cell receptor (TCR) with peptide in the binding site of the major histocompatibility complex molecule provides the basis for T cell recognition during immune surveillance, repertoire development, and tolerance. Little is known about the extent to which repertoire selection is influenced directly by variation of the structure of the class I heavy chain. We find that the 2C TCR, normally positively selected in the context of the K(b) molecule, is minimally selected into the CD8 lineage in the absence of antigen-processing genes. This finding underscores the importance of peptides in determining the positive-selecting class I ligands in the thymus. In contrast, K(bm3), a variant class I molecule that normally exerts a negative selection pressure on 2C-bearing T cells, positively selects 2C transgenic T cells into the CD8 lineage in an antigen-processing gene-deficient environment. These findings indicate that structural changes in the heavy chain can have direct influence in T cell recognition, from which we conclude that the nature of TCR interaction with class I heavy chain influences the array of TCRs selected during development of the functional adult repertoire.

Peripheral T cell survival

T cell survival in the periphery is an active process, depending on continuous TCR engagement by peptide-MHC complexes and/or response to environmental cytokines. Naive T cells require interactions with the MHC restricting element. The survival requirements of memory T cells are as yet insufficiently characterized, but MHC-restricted interactions are not necessary.

A new MHC locus that influences class I peptide presentation

We have investigated the HLA-B27-restricted CTL response to HY minor histocompatibility antigens in rats and mice transgenic for HLA-B27 and human beta2-microglobulin. A polymorphism was found at a locus within the H2 complex, producing two distinct but overlapping sets of B27-presented HY peptides. The locus, named Cim2, mapped between the K and Pb loci, and its product is therefore distinct from TAP, LMP, and tapasin. Identical findings in rats and mice, including identical HY peptide sequences and the failure of a rat Tap2A transgene to alter CTL recognition, suggest that a homologous locus with similar polymorphism exists in the rat. Cim2, or a closely linked locus, was found to exert a broad effect on peptide loading of both HLA-B27 and mouse class I alleles. The data thus establish a strong, previously unrecognized MHC-encoded influence on the class I antigen pathway.

Differential requirements for survival and proliferation of CD8 naïve or memory T cells

The requisite molecular interactions for CD8 T cell memory were determined by comparison of monoclonal naïve and memory CD8(+) T cells bearing the T cell receptor (TCR) for the HY antigen. Naïve T cells required only the right major histocompatibility complex (MHC) class I-restricting molecule to survive; to expand, they also needed antigen. In contrast, for survival, memory cells did not require the restricting MHC allele, but needed only a nonspecific class I; for expansion the correct class I, but not antigen, was required. Thus, maintenance of CD8 T cell memory still required TCR-MHC class I interactions, but memory T cells may have a lower functional activation threshold that facilitates secondary responses.

Identification of a naturally occurring ligand for thymic positive selection

In the thymus, positive and negative selection shape the T cell repertoire. It has previously been shown that positive selection, like negative selection, is the result of the interaction of the TCR with self-peptides bound to MHC. However, little is known about the number or nature of the self-peptide ligands that mediate positive selection in vivo. We devised a novel assay with enhanced sensitivity for low affinity TCR ligands to identify self-peptides that may be biologically relevant. At least eight K(b)-bound self-peptides were detected by this assay using thymocytes bearing the OT-I TCR (specific for OVAp/K(b)). The sequence of one of these peptides was determined using the recently developed technique of membrane preconcentration-capillary electrophoresis-tandem mass spectrometry. This peptide, CP alpha1, has limited sequence similarity to OVAp, yet was found to induce positive selection of OT-I thymocytes in fetal thymic organ culture.

Positive selection of T cells: fastidious or promiscuous?

Several studies reported during the past year, most of which exploited novel in vivo positive selection systems, have addressed the basis of peptide involvement in positive selection of T cells. The very flexible, yet specific, requirements the studies demonstrate differ somewhat from the very specific requirements reported in earlier experiments relying on in vitro selection systems.

A mouse cytomegalovirus glycoprotein, gp34, forms a complex with folded class I MHC molecules in the ER which is not retained but is transported to the cell surface

Murine cytomegalovirus (MCMV) interferes with antigen presentation by means of retaining major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum (ER). Here we identify and characterize an MCMV-encoded glycoprotein, gp34, which tightly associates with properly conformed MHC class I molecules in the ER. Gp34 is synthesized in large quantities during MCMV infection and it leaves the ER only in association with MHC class I complexes. Many but not all class I molecules are retained in the ER during the early phase of MCMV infection, and we observe an inverse correlation between amounts of gp34 synthesized during the course of infection and class I retention. An MCMV deletion mutant lacking several genes, including the gene encoding gp34, shows increased class I retention. Thus, MCMV gp34 may counteract class I retention, perhaps to decrease susceptibility of infected cells to recognition by natural killer cells.

Positive selection of T cells induced by viral delivery of neopeptides to the thymus

The relation between an antigenic peptide that can stimulate a mature T cell and the natural peptide that promoted selection of this cell in the thymus is still unknown. An experimental system was devised to address this issue in vivo-mice expressing neopeptides in thymic stromal cells after adenovirus-mediated delivery of invariant chain-peptide fusion proteins. In this system, selection of T cells capable of responding to a given antigenic peptide could be promoted by the peptide itself, by closely related analogs lacking agonist and antagonist activity, or by ostensibly unrelated peptides. However, the precise repertoire of T cells selected was dictated by the particular neopeptide expressed.

Positive selection of CD4+ and CD8+ T cells

Significant progress has been made in characterizing intermediates and defining individual steps of positive selection, providing important insights into mechanisms of CD4/CD8 lineage commitment. New evidence suggests that specific recognition of peptides may be important for positive selection of CD4+ T cells. Several studies have defined signal-transduction pathways important for positive selection and have provided evidence that distinct signaling pathways may regulate positive versus negative selection.

Mature T cell reactivity altered by peptide agonist that induces positive selection

Recent studies have investigated how defined peptides influence T cell development. Using a T cell receptor-transgenic beta2-microglobulin-deficient model, we have examined T cell maturation in fetal thymic organ cultures in the presence of various peptides containing single-alanine substitutions of the strong peptide agonist, p33. Cocultivation with the peptide A4Y, which contains an altered T cell contact residue, resulted in efficient positive selection. Several in vitro assays demonstrated that A4Y was a moderate agonist relative to p33. Although A4Y promoted positive selection over a wide concentration range, high doses of this peptide could not induce clonal deletion. Thymocytes maturing in the presence of A4Y were no longer able to respond to A4Y, but could proliferate against p33. These studies demonstrate that (a) peptides that induce efficient positive selection at high concentrations are not exclusively antagonists; (b) some agonists do not promote clonal deletion; (c) positive selection requires a unique T cell receptor-peptide-major histocompatibility complex interaction; and (d) interactions with selecting peptides during T cell ontogeny may define the functional reactivity of mature T cells.

Biosynthesis of major histocompatibility complex molecules and generation of T cells in Ii TAP1 double-mutant mice

Major histocompatibility complex (MHC) class I and II molecules are loaded with peptides in distinct subcellular compartments. The transporter associated with antigen processing (TAP) is responsible for delivering peptides derived from cytosolic proteins to the endoplasmic reticulum, where they bind to class I molecules, while the invariant chain (Ii) directs class II molecules to endosomal compartments, where they bind peptides originating mostly from exogenous sources. Mice carrying null mutations of the TAP1 or Ii genes (TAP10) or Ii0, respectively) have been useful tools for elucidating the two MHC/peptide loading pathways. To evaluate to what extent these pathways functionally intersect, we have studied the biosynthesis of MHC molecules and the generation of T cells in Ii0TAP10 double-mutant mice. We find that the assembly and expression of class II molecules in Ii0 and Ii0TAP10 animals are indistinguishable and that formation and display of class I molecules is the same in TAP10 and Ii0TAP10 animals. Thymic selection in the double mutants is as expected, with reduced numbers of both CD4+ CD8- and CD4- CD8+ thymocyte compartments. Surprisingly, lymph node T-cell populations look almost normal; we propose that population expansion of peripheral T cells normalizes the numbers of CD4+ and CD8+ cells in Ii0TAP10 mice.

Misfolded major histocompatibility complex class I molecules accumulate in an expanded ER-Golgi intermediate compartment

Misfolded membrane proteins are rapidly degraded, often shortly after their synthesis and insertion in the endoplasmic reticulum (ER), but the exact location and mechanisms of breakdown remain unclear. We have exploited the requirement of MHC class I molecules for peptide to achieve their correct conformation: peptide can be withheld by introducing a null mutation for the MHC-encoded peptide transporter, TAP. By withholding TAP-dependent peptides, the vast majority of newly synthesized class I molecules fails to leave the endoplasmic reticulum and is degraded. We used mice transgenic for HLA-B27 on a TAP1-deficient background to allow visualization by immunoelectron microscopy of misfolded HLA-B27 molecules in thymic epithelial cells. In such HLA transgenic animals, the TAP mutation can be considered a genetic switch that allows control over the extent of folding of the protein of interest, HLA-B27, while the rate of synthesis of the constituent subunits remains unaltered. In TAP1-deficient, HLA-B27 transgenic animals, HLA-B27 molecules fail to assemble correctly, and do not undergo carbohydrate modifications associated with the Golgi apparatus, such as conversion to Endoglycosidase H resistance, and acquisition of sialic acids. We show that such molecules accumulate in an expanded network of tubular and fenestrated membranes. This compartment has its counterpart in normal thymic epithelial cells, and is identified as an ER-Golgi intermediate. We detect the presence of ubiquitin and ubiquitin-conjugating enzymes in association with this compartment, suggesting a nonlysosomal mode of degradation of its contents.