Positive selection of self- and alloreactive CD8+ T cells in Tap-1 mutant mice

Different routes of MHC-I delivery to phagosomes and their consequences to CD8 T cell immunity

Dendritic cells (DCs) present internalized antigens to CD8 T cells through cross-presentation by major histocompatibility complex class I (MHC-I) molecules. While conventional cDC1 excel at cross-presentation, cDC2 can be licensed to cross-present during infection by signals from inflammatory receptors, most prominently Toll-like receptors (TLRs). At the core of the regulation of cross-presentation by TLRs is the control of subcellular MHC-I traffic. Within DCs, MHC-I are enriched within endosomal recycling compartments (ERC) and traffic to microbe-carrying phagosomes under the control of phagosome-compartmentalized TLR signals to favor CD8 T cell cross-priming to microbial antigens. Viral blockade of the transporter associated with antigen processing (TAP), known to inhibit the classic MHC-I presentation of cytoplasmic protein-derived peptides, depletes the ERC stores of MHC-I to simultaneously also block TLR-regulated cross-presentation. DCs counter this impairment in the two major pathways of MHC-I presentation to CD8 T cells by mobilizing noncanonical cross-presentation, which delivers MHC-I to phagosomes from a new location in the ER-Golgi intermediate compartment (ERGIC) where MHC-I abnormally accumulate upon TAP blockade. Noncanonical cross-presentation thus rescues MHC-I presentation and cross-primes TAP-independent CD8 T cells best-matched against target cells infected with immune evasive viruses. Because noncanonical cross-presentation relies on a phagosome delivery route of MHC-I that is not under TLR control, it risks potential cross-presentation of self-antigens during infection. Here I review these findings to illustrate how the subcellular route of MHC-I to phagosomes critically impacts the regulation of cross-presentation and the nature of the CD8 T cell response to infection and cancer. I highlight important and novel implications to CD8 T cell vaccines and immunotherapy.

TAP dysfunction in dendritic cells enables noncanonical cross-presentation for T cell priming

Classic major histocompatibility complex class I (MHC-I) presentation relies on shuttling cytosolic peptides into the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP). Viruses disable TAP to block MHC-I presentation and evade cytotoxic CD8+ T cells. Priming CD8+ T cells against these viruses is thought to rely solely on cross-presentation by uninfected TAP-functional dendritic cells. We found that protective CD8+ T cells could be mobilized during viral infection even when TAP was absent in all hematopoietic cells. TAP blockade depleted the endosomal recycling compartment of MHC-I molecules and, as such, impaired Toll-like receptor-regulated cross-presentation. Instead, MHC-I molecules accumulated in the ER-Golgi intermediate compartment (ERGIC), sequestered away from Toll-like receptor control, and coopted ER-SNARE Sec22b-mediated vesicular traffic to intersect with internalized antigen and rescue cross-presentation. Thus, when classic MHC-I presentation and endosomal recycling compartment-dependent cross-presentation are impaired in dendritic cells, cell-autonomous noncanonical cross-presentation relying on ERGIC-derived MHC-I counters TAP dysfunction to nevertheless mediate CD8+ T cell priming.

A novel spontaneous mutation in the TAP2 gene unravels its role in macrophage survival

We report a new mouse strain with a single point mutation in the type 2 transporter associated with antigen processing (TAP2). This strain randomly arose in one of our C57BL/6J mouse colonies and was initially discovered because of the lack of CD8⁺ T cells in the periphery. Following our observation, we subsequently revealed a lack of cell surface MHC-I expression, derived from TAP2 protein deficiency. Our strain, named eightless, has a C to T substitution in exon 5 resulting in a glutamine to stop codon substitution at position 285 in the TAP2 protein. Interestingly, in addition to the expected lack of CD8⁺ T cell phenotype, eightless mice have a diminished number of macrophages in their peritoneum. Moreover, following peritoneal inflammation, elicited eightless macrophages showed impaired survival both in vivo and ex vivo. Our study describes the first ever TAP2 complete knockout mouse strain and provides a possible explanation for why patients with TAP2 deficiency syndrome present clinical manifestations that would suggest a phagocyte defect rather than a lack of CD8⁺ T cells.

Alternative Antigen Processing for MHC Class I: Multiple Roads Lead to Rome

The well described conventional antigen-processing pathway is accountable for most peptides that end up in MHC class I molecules at the cell surface. These peptides experienced liberation by the proteasome and transport by the peptide transporter TAP. However, there are multiple roads that lead to Rome, illustrated by the increasing number of alternative processing pathways that have been reported during last years. Interestingly, TAP-deficient individuals do not succumb to viral infections, suggesting that CD8 T cell immunity is sufficiently supported by alternative TAP-independent processing pathways. To date, a diversity of viral and endogenous TAP-independent peptides have been identified in the grooves of different MHC class I alleles. Some of these peptides are not displayed by normal TAP-positive cells and we therefore called them TEIPP, for “T-cell epitopes associated with impaired peptide processing.” TEIPPs are hidden self-antigens, are derived from normal housekeeping proteins, and are processed via unconventional processing pathways. Per definition, TEIPPs are presented via TAP-independent pathways, but recent data suggest that part of this repertoire still depend on proteasome and metalloprotease activity. An exception is the C-terminal peptide of the endoplasmic reticulum (ER)-membrane-spanning ceramide synthase Trh4 that is surprisingly liberated by the signal peptide peptidase (SPP), the proteolytic enzyme involved in cleaving leader sequences. The intramembrane cleaving SPP is thereby an important contributor of TAP-independent peptides. Its family members, like the Alzheimer’s related presenilins, might contribute as well, according to our preliminary data. Finally, alternative peptide routing is an emerging field and includes processes like the unfolded protein response, the ER-associated degradation, and autophagy-associated vesicular pathways. These data convince us that there is a world to be discovered in the field of unconventional antigen processing.

Latent tuberculosis infection: myths, models, and molecular mechanisms

The aim of this review is to present the current state of knowledge on human latent tuberculosis infection (LTBI) based on clinical studies and observations, as well as experimental in vitro and animal models. Several key terms are defined, including "latency," "persistence," "dormancy," and "antibiotic tolerance." Dogmas prevalent in the field are critically examined based on available clinical and experimental data, including the long-held beliefs that infection is either latent or active, that LTBI represents a small population of nonreplicating, "dormant" bacilli, and that caseous granulomas are the haven for LTBI. The role of host factors, such as CD4(+) and CD8(+) T cells, T regulatory cells, tumor necrosis factor alpha (TNF-α), and gamma interferon (IFN-γ), in controlling TB infection is discussed. We also highlight microbial regulatory and metabolic pathways implicated in bacillary growth restriction and antibiotic tolerance under various physiologically relevant conditions. Finally, we pose several clinically important questions, which remain unanswered and will serve to stimulate future research on LTBI.

New role of signal peptide peptidase to liberate C-terminal peptides for MHC class I presentation

The signal peptide peptidase (SPP) is an intramembrane cleaving aspartyl protease involved in release of leader peptide remnants from the endoplasmic reticulum membrane, hence its name. We now found a new activity of SPP that mediates liberation of C-terminal peptides. In our search for novel proteolytic enzymes involved in MHC class I (MHC-I) presentation, we found that SPP generates the C-terminal peptide-epitope of a ceramide synthase. The display of this immunogenic peptide-MHC-I complex at the cell surface was independent of conventional processing components like proteasome and peptide transporter TAP. Absence of TAP activity even increased the MHC-I presentation of this Ag. Mutagenesis studies revealed the crucial role of the C-terminal location of the epitope and "helix-breaking" residues in the transmembrane region just upstream of the peptide, indicating that SPP directly liberated the minimal 9-mer peptide. Moreover, silencing of SPP and its family member SPPL2a led to a general reduction of surface peptide-MHC-I complexes, underlining the involvement of these enzymes in Ag processing and presentation.

TAP1-deficiency does not alter atherosclerosis development in Apoe-/- mice

Antigen presenting cells (APC) have the ability to present both extra-cellular and intra-cellular antigens via MHC class I molecules to CD8(+) T cells. The cross presentation of extra-cellular antigens is reduced in mice with deficient Antigen Peptide Transporter 1 (TAP1)-dependent MHC class I antigen presentation, and these mice are characterized by a diminished CD8(+) T cell population. We have recently reported an increased activation of CD8(+) T cells in hypercholesterolemic Apoe(-/-) mice. Therefore, this study included TAP1-deficient Apoe(-/-) mice (Apoe(-/-)Tap1(-/-)) to test the atherogenicity of CD8(+) T cells and TAP1-dependent cross presentation in a hypercholesterolemic environment. As expected the CD8(+) T cell numbers were low in Apoe(-/-)Tap1(-/-) mice in comparison to Apoe(-/-) mice, constituting ~1% of the lymphocyte population. In spite of this there were no differences in the extent of atherosclerosis as assessed by en face Oil Red O staining of the aorta and cross-sections of the aortic root between Apoe(-/-)Tap1(-/-) and Apoe(-/-) mice. Moreover, no differences were detected in lesion infiltration of macrophages or CD3(+) T cells in Apoe(-/-)Tap1(-/-) compared to Apoe(-/-) mice. The CD3(+)CD4(+) T cell fraction was increased in Apoe(-/-)Tap1(-/-) mice, suggesting a compensation for the decreased CD8(+) T cell population. Interestingly, the fraction of CD8(+) effector memory T cells was increased but this appeared to have little impact on the atherosclerosis development.In conclusion, Apoe(-/-)Tap1(-/-) mice develop atherosclerosis equal to Apoe(-/-) mice, indicating a minor role for CD8(+) T cells and TAP1-dependent antigen presentation in the disease process.

Similar intracellular peptide profile of TAP1/β2 microglobulin double-knockout mice and C57BL/6 wild-type mice as revealed by peptidomic analysis

Cells produce and use peptides in distinctive ways. In the present report, using isotope labeling plus semi-quantitative mass spectrometry, we evaluated the intracellular peptide profile of TAP1/β2m⁻(/)⁻ (transporter associated with antigen-processing 1/ß2 microglobulin) double-knockout mice and compared it with that of C57BL/6 wild-type animals. Overall, 92 distinctive peptides were identified, and most were shown to have a similar concentration in both mouse strains. However, some peptides showed a modest increase or decrease (~2-fold), whereas a glycine-rich peptide derived from the C-terminal of neurogranin (KGPGPGGPGGAGGARGGAGGGPSGD) showed a substantial increase (6-fold) in TAP1/β2m⁻(/)⁻ mice. Thus, TAP1 and β2microglobulin have a small influence on the peptide profile of neuronal tissue, suggesting that the presence of peptides derived from intracellular proteins in neuronal tissue is not associated with antigens of the class I major histocompatibility complex. Therefore, it is possible that these intracellular peptides play a physiological role.

On the issue of peptide recognition in T cell development

CD4-CD8 double positive (DP) thymocytes undergo a differentiation process in the thymus where they are selected based on their ability to recognize peptide antigens presented on self major histocompatibility complex (MHC) molecules. The first stage of this process is positive selection, a quality-control mechanism which ensures that the T cell receptors (TCR) presented on developing thymocytes can transmit signals via peptides presented on either MHC class I (MHC1) or MHC class II (MHC2) molecules. Work over the past decade has revealed that the peptides that drive positive selection of both CD4 and CD8 lineage cells deliver only weak TCR signals. In line with these observations, specialized protein degradation machineries have been discovered in the thymic cortex that presumably generate specialized low-affinity peptide repertoires for presentation on MHC1 and MHC2 molecules. TCR signals transduced through these weak-affinity ligands in the early stages of positive selection alter the kinetics of expression of CD4 and CD8 molecules and play a crucial role in commitment of thymocytes to either the CD4 or CD8 lineages. In this work, we review the experiments that explore the peptide repertoires that are presented to developing thymocytes during positive selection, the observed signaling patterns that lead to CD4 versus CD8 lineage commitment, and speculate about how specialized organization of the signaling machinery in DP thymocytes may allow for efficient transduction of weak signals during the course of positive selection.

Systemic control of plasmacytoid dendritic cells by CD8+ T cells and commensal microbiota

The composition of the intestinal microbial community is a distinctive individual trait that may divergently influence host biology. Because dendritic cells (DC) regulate the quality of the host response to microbiota, we evaluated DC in mice bearing distinct enteric microbial communities divergent for colitis susceptibility. Surprisingly, a selective, systemic reduction of plasmacytoid dendritic cells (pDC) was observed in isogenic mice with different microbiota: restricted flora (RF) vs specific pathogen free (SPF). This reduction was not observed in germfree mice, suggesting that the pDC deficiency was not simply due to a lack of intestinal microbial products. The microbial action was linked to cytotoxic CD8(+) T cells, since pDC in RF mice were preserved in the CD8(-/-) and perforin(-/-) genotypes, partially restored by anti-CD8beta Ab, and augmented in SPF mice bearing the TAP(-/-) genotype. Direct evidence for pDC cytolysis was obtained by rapid and selective pDC depletion in SPF mice transferred with RF CD8(+) T cells. These data indicate that commensal microbiota, via CTL activation, functionally shape systemic immune regulation that may modify risk of inflammatory disease.

Systemic CD8 T-cell memory response to a Salmonella pathogenicity island 2 effector is restricted to Salmonella enterica encountered in the gastrointestinal mucosa

To better understand the evolution of a systemic memory response to a mucosal pathogen, we monitored antigen-specific OT1 CD8 T-cell responses to a fusion of the SspH2 protein and the peptide SIINFEKL stably expressed from the chromosome of Salmonella enterica and loaded into the class I pathway of antigen presentation of professional phagocytes through the Salmonella pathogenicity island 2 type III secretion system (TTSS). This strategy has revealed that effector memory CD8 T cells with low levels of CD62L expression (CD62L(low)) are maintained in systemic sites months after vaccination in response to low-grade infections with Salmonella. However, the CD8 T-cell pool eventually declines. Low numbers of central memory cells surviving after prolonged resting from an antigen encounter can nevertheless reconstitute the systemic effector memory pool in a route-specific recall response to cognate antigens encountered in the gut. Accordingly, populations of CD62L(high) interleukin-7 receptor-positive progenitor central memory cells grafted into naïve mice expand in response to orally administered Salmonella expressing the chromosomal translational fusion of sspH2 and the sequence encoding the SIINFEKL peptide but fail to proliferate following systemic stimulation. Moreover, populations of systemic memory CD8 T cells restricted to Salmonella in oral vaccines selectively expand in response to cognate antigens presented by cells isolated from mesenteric lymph nodes (MLN). Together, these findings have revealed the imprinting of systemic CD8 central memory T-cell recall responses against enteropathogens by MLN. MLN restriction represents a novel mechanism by which systemic CD8 T-cell immunity is confined to periods of high risk for extraintestinal dissemination.

TAP1-/- mice present oligoclonal BV-BJ expansions following the rejection of grafts bearing self antigens

Our previous work showed that transporter associated with antigen processing 1 (TAP1)-/- (H-2b) mice rejected grafts from H-2b mice which display a normal density of class I major histocompatibility complex (MHC) molecules at the cell surface. Our results indicated that H-2b molecules themselves may be a target in this kind of rejection and that CD4+ T cells play a major role in this autoreactive process. Our data also suggested that TAP1-/- mice, in addition to the well-recognized phenotype of class I and CD8+ T-cell deficiency, present a functional alteration in their autoreactive CD4+ T-cell repertoires. In this model of inflammatory autoreactivity to modified self, we have analysed T-cell receptor (TCR) V-beta-J-beta (BV-BJ) usage by complementarity determining region 3 (CDR3) length spectratyping in splenocytes from naïve TAP1-/- mice and transplanted TAP1-/- mice that rejected B6 heart grafts or responded to synthetic self H-2Kb peptides. Importantly, oligoclonal T-cell expansions shared by different animals were detected in the peripheral T-cell repertoire of transplanted TAP1-/- mice. Such public expansions were also induced in vitro by H-2Kb peptides, suggesting that dominant class I peptides can induce preferential expansions of restricted T-cell populations during rejection. Some of these public T-cell expansions were also detected in transplanted mice even before in vitro stimulation with peptides, indicating that post-transplantation expansion of these populations had occurred in vivo. The functional activity of these T-cell populations awaits elucidation, as do the underlying mechanisms involved in the inflammatory autoreactive process, in TAP1-/- mice.

SOCS-1 regulates IL-15-driven homeostatic proliferation of antigen-naive CD8 T cells, limiting their autoimmune potential

Mice that are deficient in suppressor of cytokine signaling–1 (SOCS-1) succumb to neonatal mortality that is associated with extensive cellular infiltration of many tissues. T cells seem to be necessary for disease, which can be alleviated largely by neutralizing interferon-γ. Examining T cell receptor (TCR) specificity shows that even monospecific T cells can mediate disease in SOCS-1–deficient mice, although disease onset is substantially faster with a polyclonal T cell repertoire. A major phenotype of SOCS-1^−/− mice is the accumulation of CD44^highCD8⁺ peripheral T cells. We show that SOCS-1–deficient CD8, but not CD4, T cells proliferate when transferred into normal (T cell–sufficient) mice, and that this is dependent on two signals: interleukin (IL)-15 and self-ligands that are usually only capable of stimulating homeostatic expansion in T cell–deficient mice. Our findings reveal that SOCS-1 normally down-regulates the capacity of IL-15 to drive activation and proliferation of naive CD8 T cells receiving TCR survival signals from self-ligands. We show that such dysregulated proliferation impairs the deletion of a highly autoreactive subset of CD8 T cells, and increases their potential for autoimmunity. Therefore, impaired deletion of highly autoreactive CD8 T cells, together with uncontrolled activation of naive CD8 T cells by homeostatic survival ligands, may provide a basis for the T cell–mediated disease of SOCS-1^−/− mice.

Autoreactivity to self H-2Kb peptides in TAP1 mice. Intravenous administration of H-2Kb class I-derived peptides induces long-term survival of grafts from C57BL/6 donors

We and others have previously shown that TAP1-/- mice (H-2b) reject grafts from donors without major histocompatibility complex (MHC) disparity that express wild-type levels of H-2b class I molecules (C57BL/6, TAP1+/+ mice). In this same model, we also showed that subcutaneous priming of TAP1-/- mice with synthetic peptides derived from the H-2Kb molecule accelerated graft rejection and that in vivo depletion of CD4+ T cells induced a significant prolongation of graft survival, suggesting an important role for CD4 T cells. We hypothesize that, in this model, rejection is triggered by the recognition of class I molecules or derived peptides, in an inflammatory microenvironment, by a functionally altered autoreactive T-cell repertoire that escapes the control of peripheral regulatory mechanisms. In the present study, we analysed the cellular autoreactivity induced by synthetic peptides derived from the H-2Kb sequence in naive and TAP1-/- mice transplanted with C57BL/6 grafts, and investigated whether intravenous modulation of autoreactivity to these peptides induced transplantation tolerance. We showed that TAP1-/- mice have peripheral autoreactive T cells that recognize H-2Kb peptides. A significant amplification of proliferation against these peptides was detected in TAP1-/- mice that rejected grafts, indicating that the inflammatory context of transplantation induced peripheral expansion of these autoreactive T cells. Furthermore, intravenous injection of H-2Kb-derived peptides significantly prolonged graft survival in some animals. In these mice (> 100 days graft survival), we observed intragraft inhibition of interferon-gamma and interleukin-10 expression, suggesting that these cytokines have an active role during the rejection. In conclusion, our present data indicate that inflammatory autoreactive T cells directed against H-2Kb peptides can be inhibited in the periphery to prolong graft survival in TAP1-/- mice.

Genetic susceptibility to type 1 diabetes in the intracellular pathway of antigen processing - a subject review and cross-study comparison

Ligand binding grooves of MHC class I molecules are able to load a panel of endogenous peptides of varying length and sequence derived from self or foreign origin to activate or deactivate cytotoxic CD8(+) T cells. Peptides are assembled with class I molecules by pathways that are either dependent or independent of transport by ABC proteins (TAP) and degradation in the immunoproteasome by its subunits LMP2 and LMP7. Those peptides that require TAP and LMP treatment appear to be subject to control and optimization by TAP for proper customizing and efficient presentation. Therefore, allelic variations in the coding sequences of TAP and LMP were suspected for a long time to be responsible for improper antigen processing, interruption of self-peptide presentation and reduced cell surface expression of MHC class I molecules resulting in the activation of autoreactive CD8(+) T cells. In this article we reviewed the controversial findings regarding the role of TAP and LMP genes in autoimmune diabetes and reevaluated data of eleven separate studies in a cross-study analysis by genotype and HLA haplotype matching. We could confirm previous results by showing that TAP2*651-A/F and TAP2*687-A/A are significantly associated with disease, independently of linkage disequilibrium (LD). LMP2-R/H surprisingly seems to be primarily disease-conferring although a weak association with DR4 serotypes can be observed. Our analysis also suggests that LMP7-B/B, TAP1-A/A and TAP2*687-A/B are the protective genotypes and that these associations are not secondary to LD with DRB1. Consequently, intracellular antigen processing associated with TAP- and proteasome-dependent pathways seems to be a critical element in T cell selection for the retention of a balanced immunity.

Cutting Edge: Antibody Production to Pneumococcal Polysaccharides Requires CD1 Molecules and CD8+T Cells

T cell involvement in Ab responses to thymus-independent type 2 Ags is an immunologic enigma. The identity of these cells and the mechanisms of their TCR engagement to carbohydrate molecules remain unknown. We measured IgG Ab production after immunization with pneumococcal polysaccharides in mice with disruptions in selected genes of the T cell pathway. Nonclassical MHC class I-like CD1 molecules and MHC class I-dependent CD8+ cells were found to be essential. Our findings set forth a new paradigm for humoral responses in which CD1 expression as well as a subset of CD8+ cells are required to provide helper function for Ab production against thymus-independent type 2 polysaccharides, similar to MHC class II-restricted CD4+ cells for protein Ags.

Involvement of natural killer T cells and granulocytes in the inflammation induced by partial hepatectomy

BACKGROUND/AIMS

Natural killer T (NKT) cells are present in the liver of mice. We examined whether NKT cells and other leukocytes were associated with hepatic inflammation after partial hepatectomy.

METHODS

Approximately 70% of the liver was removed from mice using the method described by Higgins and Anderson.

RESULTS

Partial hepatectomy induced the expansion of NKT cells in the liver and the elevation of transaminase. These responses were completely suppressed by the administration of tacrolimus. NKT cell-deficient mice showed a decreased level of transaminase after partial hepatectomy. Perforin (-/-) mice showed an elevation of transaminase while B6-gld/gld mice (Fas ligand-) showed a decreased elevation of transaminase. In TAP-1(-/-) mice which lacked CD8+ cytotoxic T cells, inflammation remained at a normal level after partial hepatectomy. Since NKT cell-deficient mice showed up to 50% decrease in the level of inflammation, we examined the association of other leukocytes with the remaining inflammation. The number and proportion of granulocytes were increased by partial hepatectomy.

CONCLUSIONS

Both NKT cells and granulocytes participated in the hepatic inflammation after partial hepatectomy. The function of NKT cells, but not of granulocytes, was found to be sensitive to the immunosuppressive effect of tacrolimus.

Characterization of extrathymic CD8 alpha beta T cells in the liver and intestine in TAP-1 deficient mice

TAP-1 deficient (-/-) mice cannot transport MHC class I antigens onto the cell surface, which results in failure of the generation of CD8+ T cells in the thymus. In a series of recent studies, it has been proposed that extrathymic T cells are generated in the liver and at other extrathymic sites (e.g. the intestine). It was therefore investigated whether CD8+ extrathymic T cells require an interaction with MHC class I antigens for their differentiation in TAP-1(-/-) mice. Although CD8+ thymically derived T cells were confirmed to be absent in the spleen as well as in the thymus, CD8 alpha beta+ T cells were abundant in the livers and intestines of TAP-1(-/-) mice. These CD8+ T cells expanded in the liver as a function of age and were mainly confined to a NK1.1-CD3int population which is known to be truly of extrathymic origin. Hepatic lymphocytes, which contained CD8+ T cells and which were isolated from TAP-1(-/-) mice (H-2b), responded to neither mutated MHC class I antigens (bm1) nor allogeneic MHC class I antigens (H-2d) in in vitro mixed lymphocyte cultures. However, the results from repeated in vivo stimulations with alloantigens (H-2d) were interesting. Allogeneic cytotoxicity was induced in liver lymphocytes in TAP-1(-/-) mice, although the magnitude of cytotoxicity was lower than that of liver lymphocytes in immunized B6 mice. All allogeneic cytotoxicity disappeared with the elimination of CD8+ cells in TAP-1(-/-) mice. These results suggest that the generation and function of CD8+ extrathymic T cells are independent of the existence of the MHC class I antigens of the mouse but have a limited allorecognition ability.

Memory phenotype of CD8+ T cells in MHC class Ia-deficient mice

B6.K(b-)D(b-) mice are devoid of class Ia but express normal levels of class Ib molecules. They have low levels of CD8 T cells in both the thymus as well as peripheral T cell compartments. Although the percentage of splenic CD8 alpha alpha T cells is increased in these animals, approximately 90% of CD8 T cells are CD8 alpha beta. In contrast to B6 animals, most of the CD8 T cells from these mice have a memory phenotype (CD44(high)CD122(high) CD62L(low)) including both CD8 alpha beta and CD8 alpha alpha subsets. In the thymus of B6.K(b-)D(b-) animals, there is a decrease in the percentage of SP CD8 T cells, although most are CD44(low), similar to that seen in B6 mice. The spleens from day 1-old B6 and B6.K(b-)D(b-) mice have a relatively high proportion of CD44(high)CD62L(low) CD8 T cells. However, by day 28 most CD8 T cells in B6 mice have a naive phenotype while in B6.K(b-)D(b-) mice the memory phenotype remains. Unlike CD44(high) cells that are found in B6 animals, most CD44(high) cells from B6.K(b-)D(b-) mice do not secrete IFN-gamma rapidly upon activation. The paucity of CD8 T cells in B6.K(b-)D(b-) mice might be due in part to their inability to undergo homeostatic expansion. Consistent with this, we found that CD8 T cells from these animals expand poorly in X-irradiated syngeneic hosts compared with B6 CD8 T cells that respond to class Ia Ags. We examined homeostatic expansion of B6 CD8 T cells in single as well as double class Ia knockout mice and were able to estimate the fraction of cells reactive against class Ia vs class Ib molecules.

Polymorphisms of human TAP2 detected by denaturing gradient gel electrophoresis

The human transporter associated with antigen processing (TAP1 and TAP2) genes are located in the human leukocyte antigen (HLA) class II region of the genome and encode proteins that form a heterodimer essential for the transport of endogenous peptides into the endoplasmic reticulum for assembly with HLA class I molecules. Type 1 diabetes is an autoimmune disease that is associated with the HLA region of the genome, with HLA class II genes conferring the greatest statistical risk. The presentation of self-peptides by HLA class I molecules is defective in individuals with this disease, and both TAP1 and TAP2 are potential contributors to this defect. Denaturing gradient gel electrophoresis (DGGE) was applied to screen all 11 exons and the 3' flanking region of TAP2 for polymorphisms in individuals with type 1 diabetes patients and controls. Seventy polymorphisms, including 51 in introns, 4 in the 3' flanking region, and 15 in exons, were identified. Sequencing of polymorphic DNA fragments revealed several new polymorphisms, including a Gln --> Arg substitution at codon 611 and a GT --> GC polymorphism affecting the donor splice site of intron 4, that might be of functional significance. None of the polymorphisms examined differed in frequency between individuals with type 1 diabetes and controls.

Antigen degradation or presentation by MHC class I molecules via classical and non-classical pathways

Major histocompatibility complex (MHC) class I molecules usually present endogenous peptides at the cell surface. This is the result of a cascade of events involving various dedicated proteins like the peptide transporter associated with antigen processing (TAP) and the ER chaperone tapasin. However, alternative ways for class I peptide loading exist which may be highly relevant in a process called cross-priming. Both pathways are described here in detail. One major difference between these pathways is that the proteases involved in the generation of peptides are different. How proteases and peptidases influence peptide generation and degradation will be discussed. These processes determine the amount of peptides available for TAP translocation and class I binding and ultimately the immune response.

Rejection of grafts with no H-2 disparity in TAP1 mutant mice: CD4 T cells are important effector cells and self H-2b class I molecules are target

Our previous results showed that TAP1 mutant mice rejected heart and skin grafts from donors with no H-2 disparity that express normal density of MHC class I molecules at the cell surface. During rejection, CD4 cells were predominant and essentially, no CD8 cells were found infiltrating the grafts. We hypothesized that TAP1 mutant mice, which developed and matured in an MHC class I-deficient environment, may have selected a repertoire of T cells with distinct reactivity to self class I molecules. The rejection of grafts with no H-2 disparity could be mediated by CD4+ T cells reactive to wild type H-2b class I molecules, or derived peptides, in the context of self-APC. Accordingly, we observed that transplanted TAP1 mutant mice presented a significant amplification of the proliferative T cell response to H-2Kb peptides, indicating that the stimulus with the graft was sufficient to induce peripheral expansion of these T cell repertoires. Therefore, the response to H-2Kb molecules could be a relevant pathway of activating T cells and triggering rejection of grafts expressing normal levels of these class I molecules. To test our hypothesis, we investigate the effect of pre-transplantation H-2Kb peptide-immunization on TAP1 mutant, which were then transplanted with C57BL/6 skin grafts (H-2b). Mice were immunized with a pool of five peptides derived from the polymorphic region of Kb alpha chain, before tail skin grafting. To study the role of CD4+ T cells in the rejection of C57BL/6 skin grafts, mice were in vivo depleted with an anti-CD4 monoclonal antibody GK1.5, and transplant evolution was observed. Sensitization of TAP1 mutant mice with H-2Kb peptides accelerated the rejection of skin grafts. Immunized mice rejected grafts with a MST of 13 days, compared to 16 days for the non-immunized mice (P=0.0089). The significant acceleration of graft rejection, induced by immunization with H-2Kb peptides, indicates that these peptides are capable of mobilizing effector T-cells that participate in rejection. These results support our hypothesis that class I molecules may be a target in the rejection of grafts with no MHC disparity. Depletion of CD4 T-cells resulted in a significant delay in rejection compared with the untreated control group. The MST of skin grafts in the controls was 16 days, whereas CD4-depleted recipients rejected skin grafts with a MST of 41 days (P=0.025). Moreover, some animals did not show macroscopic signs of rejection up to > 100 days posttransplantation. The contribution of CD4+ T cells to skin graft rejection, in our model, may reflect the occurrence of the presentation of H-2b peptides during graft rejection, in the context of self-APC. In conclusion, our results demonstrate an important role for H-2b molecules and CD4 T cells in the rejection of C57BL/6 grafts by TAP1 mutant mice. The low expression of MHC-I molecules on TAP1-/- mice may be determinant in the selection of a T cell repertoire strongly reactive to self MHC class I molecules which probably escapes the control of peripheral regulatory mechanisms.

Predisposing factors in the spondyloarthropathies: new insights into the role of HLA-B27

Spondyloarthropathies represent complex genetic diseases whose development is influenced by environmental factors. Estimates suggest that three to nine loci may be responsible for the majority of the genetic susceptibility to ankylosing spondylitis. The only susceptibility locus identified to date in multiple populations is HLA-B, where several HLA-B27 alleles (subtypes) are strongly associated with disease. Recent evidence implicates cytochrome P450 2D6 as a second locus, although its influence on overall risk appears small. Despite considerable efforts to define how HLA-B27 contributes to disease, its role remains enigmatic. Increasing evidence suggests it has effects that are unrelated to its physiologic function. The basis for this is unknown but may be a consequence of the unusual tendency of this allele to misfold.

Development of spontaneous arthritis in beta2-microglobulin-deficient mice without expression of HLA-B27: association with deficiency of endogenous major histocompatibility complex class I expression

OBJECTIVE

Mice deficient in beta2-microglobulin (beta2m), but expressing the human major histocompatibility complex (MHC) class I molecule HLA-B27, have been reported to develop spontaneous inflammatory arthritis (SA). We sought to determine whether, under certain conditions, beta2m deficiency alone was sufficient to cause SA, and if this might be a result of class I deficiency.

METHODS

The following types of mice were produced: mice of the MHC b haplotype genetically deficient in beta2m (beta2m(0)) on several genetic backgrounds (C57BL/6J [B6], BALB/cJ, SJL/J, MRL/MpJ, and B6,129), mice deficient in the transporter associated with antigen processing (TAP1(0)) on a B6,129 background, and HLA-B27-transgenic beta2m(0) mice on a B6 background. Cohorts were transferred from specific pathogen-free (SPF) to conventional (non-SPF) animal rooms, and evaluated clinically and histologically for the development of SA.

RESULTS

SA occurred in TAP1(0) and beta2m(0)/class I-deficient mice with a mixed B6,129 genome at a frequency of 30-50%, while 10-15% of B6, SJL/J, and BALB/cJ beta2m(0) mice developed this arthropathy. MRL/ MpJ beta2m(0) mice were unaffected. Expression of B27 did not increase the frequency of SA in B27-transgenic B2m(0) B6 mice compared with that in beta2m(0) B6 controls.

CONCLUSION

Class I deficiency is sufficient to cause SA in mice. The frequency of disease, as well as B27-specific SA, is markedly dependent on a non-MHC genetic background. These results suggest that class I deficiency in a genetically susceptible mouse can mimic B27-associated arthropathy.

Impaired immune responses and altered peptide repertoire in tapasin-deficient mice

Tapasin is a component of the major histocompatibility complex (MHC) class I antigen-loading complex. Here we show that mice with a disrupted tapasin gene display reduced MHC class I expression. Cytotoxic T cell (CTL) responses to viruses are impaired, and dendritic cells of tapasin-deficient mice do not cross-present protein antigen via the MHC class I pathway, indicating a defect in antigen processing. Natural killer (NK) cells from tapasin-deficient mice have an altered repertoire and are self-tolerant. In addition, the repertoire of class I-bound peptides is altered towards less stably binding ones. Thus tapasin plays a role in CTL and NK immune responses and in optimal peptide selection.

The role of the proteasome in autoimmunity

Type 1 diabetes is believed to be caused by T cell-mediated autoimmunity, with a prediabetic state characterized by the production of autoantibodies specific for proteins expressed by pancreatic beta cells. The non-obese diabetic (NOD) mouse is a spontaneous model of Type 1 diabetes with a strong genetic component that maps to the major histocompatibility complex (MHC) region of the genome. A specific proteasome defect has now been identified in NOD mouse lymphocytes that results from down-regulation of expression of the proteasome subunit LMP2, which is encoded by a gene in the MHC genomic region. This defect both prevents the proteolytic processing required for the production and activation of the transcription factor nuclear factor-kappaB (NF-kappaB), which plays an important role in immune and inflammatory responses, in addition to increasing the susceptibility of the affected cells to apoptosis induced by tumor necrosis factor-alpha (TNF-alpha). The proteasome dysfunction is both tissue- and developmental stage-specific and likely contributes to disease pathogenesis and tissue targeting.

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.

Beta 2-microglobulin/CD8 -/- mice reveal significant role for CD8+ T cells in graft rejection responses in beta 2-microglobulin -/- mice

Beta 2-microglobulin (beta 2m) -/- mice have often been used as a model to investigate host resistance to grafted tissues in the absence of CD8+ T cells. However, the realization that beta 2m -/- mice have a small pool of CD8+ T cells imply that these cells may take part in immune responses in vivo. To directly address the role of CD8+ T cell responses in beta 2m -/- mice, we introduced a CD8 null mutation into these mice. The beta 2m/CD8 -/- mice and the corresponding control mice were primed, and challenged with syngeneic tumour grafts. While beta 2m -/- mice readily cleared such tumour grafts, similar tumour grafts grew progressively in a dose dependent manner in the beta 2m/CD8 -/- mice. The present results imply that residual CD8+ T cells in beta 2m -/- mice may carry out significant biological functions, and suggest that studies using beta 2m -/- mice as a model for CD8+ T cell deficiency must be regarded with some caution.

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.

Defective function of the proteasome in autoimmunity: involvement of impaired NF-kappaB activation

Type 1 diabetes (also known as insulin-dependent diabetes mellitus or juvenile-onset diabetes) is usually caused by T cell-mediated autoimmunity, with a prediabetic state characterized by the production of autoantibodies specific for proteins expressed by pancreatic beta cells. The nonobese patient with diabetes (NOD) mouse is a spontaneous model of type 1 diabetes with a strong genetic component that maps to the major histocompatibility complex (MHC) region of the genome. A specific proteasome defect has been identified in NOD mouse in select lymphocytic and monocytic lineages that results from down-regulation of expression of the proteasome subunit LMP2, which is encoded by a gene in the MHC genomic region. This defect prevents the proteolytic processing required for the production and activation of the transcription factor nuclear factor-kappaB (NF-kappaB), which plays important roles in immune and inflammatory responses, as well as increases the susceptibility of the affected cells to apoptosis induced by tumor necrosis factor-alpha (TNF-alpha). The novel role of the proteasome in dysfunction in autoimmunity is presented and documented to be both tissue and developmental stage specific. We propose a role of the proteasome as a step in disease pathogenesis and tissue targeting.

Homeostatic expansion and phenotypic conversion of naïve T cells in response to self peptide/MHC ligands

Recent data suggest that survival of resting, naïve T cells requires an interaction with self MHC molecules. From analysis of the class I MHC-restricted T cell receptor transgenic strain OT-I, we report a different response. Rather than merely surviving, these T cells proliferated slowly after transfer into T-depleted syngeneic hosts. This expansion required both T cell "space" and expression of normal levels of self class I MHC molecules. Furthermore, we demonstrate that during homeostatic expansion in a suitable environment, naïve phenotype (CD44(low)) OT-I T cells converted to memory phenotype (CD44(med/high)), despite the absence of foreign antigenic stimulation. On the other hand, cells undergoing homeostatic expansion did not acquire cytolytic effector function. The significance of these data for reactivity of T cells with self peptide/MHC ligands and the implications for normal and abnormal T cell homeostasis are discussed.

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.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

Recognition of the major histocompatibility complex restriction element modulates CD8(+) T cell specificity and compensates for loss of T cell receptor contacts with the specific peptide

Triggering of a T cell requires interaction between its specific receptor (TCR) and a peptide antigen presented by a self-major histocompatibility complex (MHC) molecule. TCR recognition of self-MHC by itself falls below the threshold of detection in most systems due to low affinity. To study this interaction, we have used a read-out system in which antigen-specific effector T cells are confronted with targets expressing high levels of MHC compared with the selecting and priming environment. More specifically, the system is based on CD8(+) T cells selected in an environment with subnormal levels of MHC class I in the absence of beta2-microglobulin. We observe that the MHC restriction element can trigger viral peptide-specific T cells independently of the peptide ligand, provided there is an increase in self-MHC density. Peptide-independent triggering required at least four times the natural in vivo level of MHC expression. Furthermore, recognition of the restriction element at expression levels below this threshold was still enough to compensate for lack of affinity to peptides carrying alanine substitutions in major TCR contact residues. Thus, the specificity in TCR recognition and T cell activation is fine tuned by the avidity for self-MHC, and TCR avidities for peptide and MHC may substitute for each other. These results demonstrate a functional role for TCR avidity for self-MHC in tuning of T cell specificity, and support a role for cross-reactivity on "self" during T cell selection and activation.

Major histocompatibility complex (MHC) class I KbDb -/- deficient mice possess functional CD8+ T cells and natural killer cells

We obtained mice deficient for major histocompatibility complex (MHC) molecules encoded by the H-2K and H-2D genes. H-2 KbDb -/- mice express no detectable classical MHC class I-region associated (Ia) heavy chains, although beta2-microglobulin and the nonclassical class Ib proteins examined are expressed normally. KbDb -/- mice have greatly reduced numbers of mature CD8+ T cells, indicating that selection of the vast majority (>90%) of CD8+ T cells cannot be compensated for by beta2-microglobulin-associated molecules other than classical H-2K and D locus products. In accord with the greatly reduced number of CD8+ T cells, spleen cells from KbDb -/- mice do not generate cytotoxic responses in primary mixed-lymphocyte cultures against MHC-disparate (allogeneic) cells. However, in vivo priming of KbDb -/- mice with allogeneic cells resulted in strong CD8+ MHC class Ia-specific allogeneic responses. Thus, a minor population of functionally competent peripheral CD8+ T cells capable of strong cytotoxic activity arises in the complete absence of classical MHC class Ia molecules. KbDb -/- animals also have natural killer cells that retain their cytotoxic potential.

Antigen processing and autoimmunity. Evaluation of mRNA abundance and function of HLA-linked genes

Quantitative defects in the density of conformationally correct human lymphocyte antigen (HLA) class I complexes on the surface of lymphocytes are apparent in patients with diverse HLA-linked autoimmune diseases, including Type I diabetes and Sjögren's syndrome. First, HLA class I expression was investigated in individuals with two rare and genetically divergent polyglandular autoimmune diseases. Polyglandular failure patients whose disease showed HLA linkage, but not those whose disease was not HLA linked, exhibited decreased HLA class I expression on the surface of their lymphocytes as well as a reduced abundance of transcripts of the HLA-linked genes Tap1 and Tap2, both of which encode proteins that contribute to HLA class I processing. Second, lymphocytes from patients with insulin-dependent diabetes mellitus (IDDM), Sjögren's syndrome, Graves' disease, and Hashimoto's disease showed varying degrees of decreased abundance of mRNAs that encode Tap1, Tap2, Lmp2, or Lmp7 (the latter two proteins also contribute to HLA class I processing). Third, in twins discordant for IDDM, reduced transcript abundance was preferential to diabetic subjects. Fourth, functional assays of isolated diabetic proteasomes, the peptide cutting complex containing LMP2 and LMP7 proteins, revealed altered peptidase activity. These data suggest that defective transcription of HLA class I-processing genes could contribute to the quantitative defect in cell-surface expression in autoimmune lymphocytes of HLA-controlled disease.

Cutting Edge: A Critical, Invariant Chain-Independent Role for H2-M in Antigen Presentation

Antigen presentation by MHC class II (class II) is facilitated by the accessory molecules, invariant chain (Ii) and H2-M. Ii associates with class II during biosynthesis and promotes transport of class II to Ag-loading compartments. One function of H2-M is the removal of Ii fragments from MHC class II. We have previously demonstrated that Ii-deficient mice, unlike class II-deficient mice, are resistant to L. major infection. In the present study, we found that H2-M-deficient (H2-M0) mice were susceptible to progressive infection with L. major. The dispensability of Ii for control of L. major allowed genetic analysis of whether H2-M functions by association with or independently of Ii. In contrast to Ii-deficient (Ii0) mice, Ii0H2-M0 mice were as susceptible to L. major as H2-M0 mice. Thus, H2-M has an essential, Ii-independent function during presentation of microbial pathogens.

Rejection of Allogeneic and Syngeneic But Not MHC Class I-Deficient Tumor Grafts by MHC Class I-Deficient Mice

The ability of TAP1−/−, β2m−/−, and TAP1/β2m−/− mice to mount rejection responses against allogeneic, syngeneic, and MHC class I-deficient tumor grafts was examined. The results demonstrate a potent ability of TAP1−/− and β2m−/− as well as TAP1/β2m−/− mice to reject allogeneic tumors. In contrast to published data, rejection of syngeneic MHC class I-expressing tumors was also observed. This response was specific for the MHC class I-deficient mice, since wild-type mice did not reject syngeneic MHC class I-positive tumors under identical experimental conditions. The rejection response of syngeneic tumors required preimmunization of the mice and was MHC class I specific at the level of priming as well as at the level of the tumor target. Finally, MHC class I-deficient tumor grafts were accepted in MHC class I-deficient mice while similar grafts were rejected in wild-type mice. In summary, while MHC class I-deficient mice have retained a capacity to reject allogeneic tumors, they have gained an ability to reject syngeneic MHC class I-positive tumors and lost the ability to reject MHC class I-negative tumors. The present results are discussed in relation to the role of MHC class I molecules in selecting functional CD8+ T and NK cell repertoires, and the development of cell-mediated immunity.

Allospecific cytotoxic T cells generated from beta 2m-/- mice in primary MLC: analysis of activation requirements, specificity, and phenotype

It has been demonstrated by several investigators that beta 2m-/- knockout mice are deficient in the expression of MHC Class I molecules but can nevertheless generate CD8(+) allospecific cytotoxic T cells following vigorous in vivo priming. We demonstrate here that in vivo priming is not necessary to generate MHC Class I allospecific CTL from beta 2m-/- mice. When splenocytes from naive unprimed beta 2m-/- mice were provided exogenous cytokines in MHC Class I disparate primary MLC, allospecific cytolytic effectors were generated. beta 2m-/- MHC Class I allospecific CTL that were CD3+ and Thy1.2+ were otherwise heterogeneous in phenotype, including CD8+, CD4+, CD8-CD4-, TCR alpha beta+, and TCR gamma delta+ T cells. This phenotypic variability of beta 2m-/- CTL generated in primary MLC reveals the diversity of CTL precursors that develop in vivo in the absence of MHC Class I.

H2-M mutant mice are defective in the peptide loading of class II molecules, antigen presentation, and T cell repertoire selection

H2-M is a nonconventional major histocompatibility complex (MHC) class II molecule that has been implicated in the loading of peptides onto conventional class II molecules. We generated mice with a targeted mutation in the H2-Ma gene, which encodes a subunit for H2-M. Although the mutant mice express normal class II cell surface levels, these are structurally distinct from the compact SDS-resistant complexes expressed by wild-type cells and are predominantly bound by class II-associated invariant chain peptides (CLIPs). Cells from these animals are unable to present intact protein antigens to class II-restricted T cells and show reduced capacity to present exogenous peptides. Numbers of mature CD4+ T lymphocytes in mutant mice are reduced 3- to 4-fold and exhibit altered reactivities. Overall, this phenotype establishes an important role for H2-M in regulating MHC class II function in vivo and supports the notion that self-peptides contribute to the specificity of T cell positive selection.

TAP1-deficient mice select a CD8+ T cell repertoire that displays both diversity and peptide specificity

Mice deficient in the gene encoding the transporter associated with antigen processing 1 (TAP1) are defective in providing major histocompatibility complex (MHC) class I molecules with cytosolic peptides. Consequently, these mice express reduced levels of MHC class I glycoproteins on the cell surface, and have reduced numbers of CD8+ T cells in the periphery. In the present study, we have addressed the diversity and specificity of the peripheral CD8+ T cell population in TAP1 -/- mice. CD8+ T cells were polyclonal with regard to T cell receptor (TCR) V beta expression. Overall, V beta usage in TAP1 -/- mice appear to be very similar to that in wild-type mice, with significantly reduced levels of V beta 5.1/5.2-expressing CD8+ T cells as the only clear exception. This polyclonal population of CD8+ T cells readily mounted epitope-specific CTL responses against four out of five well-defined MHC class I-restricted peptides. In contrast to allospecific CTL, peptide-specific CTL from TAP1 -/- mice did not cross-react on cells expressing normal levels of H-2b class I. The present results demonstrate that a polyclonal CD8+ T cell repertoire, displaying both diversity and peptide specificity, is positively selected in mice devoid of a functional peptide transporter. These observations imply that TAP-dependent peptides are not absolutely required for positive selection of a functionally diverse repertoire of CD8+ T cells.

Tap-1 and Tap-2 gene therapy selectively restores conformationally dependent HLA Class I expression in type I diabetic cells

Genetic susceptibility to many autoimmune diseases, including insulin-dependent diabetes mellitus (IDDM) is statistically linked to the HLA class II region of chromosome 6. However, a distinguishing feature of patients with HLA class II-linked autoimmune disease is an abnormally low density of conformationally correct, self-peptide filled HLA class I molecules on the lymphocyte cell surface. The transporters associated with antigen processing (Tap-1 and Tap-2) are essential for normal class I expression and presentation of intracellular peptides, and these genes are located within the HLA class II region. The aims of this project were to determine if Tap genes could be implicated in the defective class I expression associated with IDDM by using a novel Epstein-Barr virus (EBV)-mediated gene transfer system to introduce a cloned, normal Tap-2 or Tap-1 gene into B cell lines from normal and IDDM patients and analyzing the effect on conformationally dependent class I expression. The results show that Tap-2 gene transfer in B cells from 40% of randomly selected IDDM patients increased expression of conformationally correct, cell-surface class I molecules to levels comparable with similarly treated B cells from normal control individuals. B cells from another 40% of IDDM patients responded to Tap-1 gene transfer. These effects were specific because B cells from normal individuals did not respond to Tap-1 or Tap-2 gene transfer with increased class I expression, and B cells from IDDM patients responding to Tap-2 gene transfer did not respond to Tap-1 gene transfer and vice versa. Thus, these complementation studies identify distinct, non-overlapping subsets of IDDM patients whose class I defect in B cells can be reversed by Tap-1 or Tap-2 gene transfer. The increase in class I expression induced by Tap gene transfer is associated with a reduction in the number of peptide-empty class I molecules as demonstrated by the response to exogenous peptide loading. Furthermore, the increase in self-peptide filled class I molecules induced by Tap gene transfer into B cells from IDDM patients is associated with restored antigen presentation to autologous T cells. These studies conclude that Tap gene dysfunctions may contribute to the defect in class I phenotype and antigen presentation demonstrated by IDDM patients. Defective presentation of self-peptides by antigen presenting cells can lead to the failed T cell education and tolerance to self antigens evident in IDDM. These studies functionally identify HLA class II region genes that contribute to an immunologic defect in IDDM.

Reduced expression of peptide-loaded HLA class I molecules on multiple sclerosis lymphocytes

Lymphocytes from patients with HLA class II-linked autoimmune diseases such as type I diabetes, systemic lupus erythematosus, rheumatoid arthritis, and Graves' have recently been shown to have a decrease in the expression of self-peptide-filled HLA class I antigens on the surface of peripheral lymphocytes. The human demyelinating diseases of multiple sclerosis in some cases are also associated with the presence of certain HLA class II genes, which may in turn be linked to genes in the class II region that control class I expression. Hence, we studied fresh peripheral blood mononuclear cells (PBMCs) and newly produced Epstein-Barr virus (EBV)-transformed cell lines from multiple sclerosis patients for the class I defect. Unseparated PBMCs, as well as T cells, B cells, and macrophages from multiple sclerosis patients had a decrease in the amount of conformationally correct peptide-filled HLA class I molecules on the cell surface compared with matched controls detectable by flow cytometry. To demonstrate the independence of this defect from exogenous serum factors, newly produced EBV-transformed cell lines from B cells of patients with multiple sclerosis maintained the defect. In addition, DR2 +/+, +/-, and -/- EBV-transformed B cells from these patients similarly demonstrated the self-antigen presentation defect. Analysis of a set of discordant multiple sclerosis twins revealed the class I defect was exclusively found on the affected twin lymphocytes, suggesting a role of this class I complex in disease expression. These data indicate that multiple sclerosis patients have abnormal presentation of self-antigens.(ABSTRACT TRUNCATED AT 250 WORDS)