Murine class I major histocompatibility complex H-2Dd: expression, refolding and crystallization

l- to d-Amino Acid Substitution in the Immunodominant LCMV-Derived Epitope gp33 Highlights the Sensitivity of the TCR Recognition Mechanism for the MHC/Peptide Structure and Dynamics

Presentation of pathogen-derived epitopes by major histocompatibility complex I (MHC-I) can lead to the activation and expansion of specific CD8⁺ T cell clones, eventually resulting in the destruction of infected target cells. Altered peptide ligands (APLs), designed to elicit immunogenicity toward a wild-type peptide, may affect the overall stability of MHC-I/peptide (pMHC) complexes and modulate the recognition by T cell receptors (TCR). Previous works have demonstrated that proline substitution at position 3 (p3P) of different MHC-restricted epitopes, including the immunodominant LCMV-derived epitope gp33 and escape variants, may be an effective design strategy to increase epitope immunogenicity. These studies hypothesized that the p3P substitution increases peptide rigidity, facilitating TCR binding. Here, molecular dynamics simulations indicate that the p3P modification rigidifies the APLs in solution predisposing them for the MHC-I loading as well as once bound to H-2D^b, predisposing them for TCR binding. Our results also indicate that peptide position 6, key for interaction of H-2D^b/gp33 with the TCR P14, takes a suboptimal conformation before as well as after binding to the TCR. Analyses of H-2D^b in complex with APLs, in which position 6 was subjected to an l- to d-amino acid modification, revealed small conformational changes and comparable pMHC thermal stability. However, the l- to d-modification reduced significantly the binding to P14 even in the presence of the p3P modification. Our combined data highlight the sensitivity of the TCR for the conformational dynamics of pMHC and provide further tools to dissect and modulate TCR binding and immunogenicity via APLs.

The discriminatory power of the T cell receptor

T cells use their T cell receptors (TCRs) to discriminate between lower-affinity self and higher-affinity non-self peptides presented on major histocompatibility complex (pMHC) antigens. Although the discriminatory power of the TCR is widely believed to be near-perfect, technical difficulties have hampered efforts to precisely quantify it. Here, we describe a method for measuring very low TCR/pMHC affinities and use it to measure the discriminatory power of the TCR and the factors affecting it. We find that TCR discrimination, although enhanced compared with conventional cell-surface receptors, is imperfect: primary human T cells can respond to pMHC with affinities as low as K_D ∼ 1 mM. The kinetic proofreading mechanism fit our data, providing the first estimates of both the time delay (2.8 s) and number of biochemical steps (2.67) that are consistent with the extraordinary sensitivity of antigen recognition. Our findings explain why self pMHC frequently induce autoimmune diseases and anti-tumour responses, and suggest ways to modify TCR discrimination.

Tuning antiviral CD8 T-cell response via proline-altered peptide ligand vaccination

Viral escape from CD8+ cytotoxic T lymphocyte responses correlates with disease progression and represents a significant challenge for vaccination. Here, we demonstrate that CD8+ T cell recognition of the naturally occurring MHC-I-restricted LCMV-associated immune escape variant Y4F is restored following vaccination with a proline-altered peptide ligand (APL). The APL increases MHC/peptide (pMHC) complex stability, rigidifies the peptide and facilitates T cell receptor (TCR) recognition through reduced entropy costs. Structural analyses of pMHC complexes before and after TCR binding, combined with biophysical analyses, revealed that although the TCR binds similarly to all complexes, the p3P modification alters the conformations of a very limited amount of specific MHC and peptide residues, facilitating efficient TCR recognition. This approach can be easily introduced in peptides restricted to other MHC alleles, and can be combined with currently available and future vaccination protocols in order to prevent viral immune escape.

Biochemical and biophysical comparison of human and mouse beta-2 microglobulin reveals the molecular determinants of low amyloid propensity

The molecular bases of amyloid aggregation propensity are still poorly understood, especially for proteins that display a stable folded native structure. A prototypic example is human beta-2 microglobulin (β2m), which, when accumulated in patients, gives rise to dialysis-related amyloidosis. Interestingly, although the physiologic concentration of β2m in mice is five times higher than that found in human patients, no amyloid deposits are observed in mice. Moreover, murine β2m (mβ2m) not only displays a lower amyloid propensity both in vivo and in vitro but also inhibits the aggregation of human β2m in vitro. Here, we compared human and mβ2m for their aggregation propensity, ability to form soluble oligomers, stability, three-dimensional structure and dynamics. Our results indicate that mβ2m low-aggregation propensity is due to two concomitant aspects: the low-aggregation propensity of its primary sequence combined with the absence of high-energy amyloid-competent conformations under native conditions. The identification of the specific properties determining the low-aggregation propensity of mouse β2m will help delineate the molecular risk factors which cause a folded protein to aggregate.

Successive crystal structure snapshots suggest the basis for MHC class I peptide loading and editing by tapasin

MHC-I epitope presentation to CD8⁺ T cells is directly dependent on peptide loading and selection during antigen processing. However, the exact molecular bases underlying peptide selection and binding by MHC-I remain largely unknown. Within the peptide-loading complex, the peptide editor tapasin is key to the selection of MHC-I-bound peptides. Here, we have determined an ensemble of crystal structures of MHC-I in complex with the peptide exchange-associated dipeptide GL, as well as the tapasin-associated scoop loop, alone or in combination with candidate epitopes. These results combined with mutation analyses allow us to propose a molecular model underlying MHC-I peptide selection by tapasin. The N termini of bound peptides most probably bind first in the N-terminal and middle region of the MHC-I peptide binding cleft, upon which the peptide C termini are tested for their capacity to dislodge the tapasin scoop loop from the F pocket of the MHC-I cleft. Our results also indicate important differences in peptide selection between different MHC-I alleles.

The Immunogenicity of a Proline-Substituted Altered Peptide Ligand toward the Cancer-Associated TEIPP Neoepitope Trh4 Is Unrelated to Complex Stability

Human cancers frequently display defects in Ag processing and presentation allowing for immune evasion, and they therefore constitute a significant challenge for T cell-based immunotherapy. We have previously demonstrated that the antigenicity of tumor-associated Ags can be significantly enhanced through unconventional residue modifications as a novel tool for MHC class I (MHC-I)-based immunotherapy approaches. We have also previously identified a novel category of cancer neo-epitopes, that is, T cell epitopes associated with impaired peptide processing (TEIPP), that are selectively presented by MHC-I on cells lacking the peptide transporter TAP. In this study, we demonstrate that substitution of the nonanchoring position 3 into a proline residue of the first identified TEIPP peptide, the murine Trh4, results in significantly enhanced recognition by antitumor CTLs toward the wild-type epitope. Although higher immunogenicity has in most cases been associated with increased MHC/peptide complex stability, our results demonstrate that the overall stability of H-2D^b in complex with the highly immunogenic altered peptide ligand Trh4-p3P is significantly reduced compared with wild-type H-2D^b/Trh4. Comparison of the crystal structures of the H-2D^b/Trh4-p3P and H-2D^b/Trh4 complexes revealed that the conformation of the nonconventional methionine anchor residue p5M is altered, deleting its capacity to form adequate sulfur-π interactions with H-2D^b residues, thus reducing the overall longevity of the complex. Collectively, our results indicate that vaccination with Thr4-p3P significantly enhances T cell recognition of targets presenting the wild-type TEIPP epitope and that higher immunogenicity is not necessarily directly related to MHC/peptide complex stability, opening for the possibility to design novel peptide vaccines with reduced MHC/peptide complex stability.

Crystal structures of H-2Db in complex with the LCMV-derived peptides GP92 and GP392 explain pleiotropic effects of glycosylation on antigen presentation and immunogenicity

Post-translational modifications significantly broaden the epitope repertoire for major histocompatibility class I complexes (MHC-I) and may allow viruses to escape immune recognition. Lymphocytic choriomeningitis virus (LCMV) infection of H-2b mice generates CD8+ CTL responses directed towards several MHC-I-restricted epitopes including the peptides GP92 (CSANNSHHYI) and GP392 (WLVTNGSYL), both with a N-glycosylation site. Interestingly, glycosylation has different effects on the immunogenicity and association capacity of these two epitopes to H-2Db. To assess the structural bases underlying these functional results, we determined the crystal structures of H-2Db in complex with GP92 (CSANNSHHYI) and GP392 (WLVTNGSYL) to 2.4 and 2.5 Å resolution, respectively. The structures reveal that while glycosylation of GP392 most probably impairs binding, the glycosylation of the asparagine residue in GP92, which protrudes towards the solvent, possibly allows for immune escape and/or forms a neo-epitope that may select for a different set of CD8 T cells. Altogether, the presented results provide a structural platform underlying the effects of post-translational modifications on epitope binding and/or immunogenicity, resulting in viral immune escape.

The MHC Class I Cancer-Associated Neoepitope Trh4 Linked with Impaired Peptide Processing Induces a Unique Noncanonical TCR Conformer

MHC class I downregulation represents a significant challenge for successful T cell-based immunotherapy. T cell epitopes associated with impaired peptide processing (TEIPP) constitute a novel category of immunogenic Ags that are selectively presented on transporter associated with Ag processing-deficient cells. The TEIPP neoepitopes are CD8 T cell targets, derived from nonmutated self-proteins that might be exploited to prevent immune escape. In this study, the crystal structure of H-2D(b) in complex with the first identified TEIPP Ag (MCLRMTAVM) derived from the Trh4 protein has been determined to 2.25 Å resolution. In contrast to prototypic H-2D(b) peptides, Trh4 takes a noncanonical peptide-binding pattern with extensive sulfur-π interactions that contribute to the overall complex stability. Importantly, the noncanonical methionine at peptide position 5 acts as a main anchor, altering only the conformation of the H-2D(b) residues Y156 and H155 and thereby forming a unique MHC/peptide conformer that is essential for recognition by TEIPP-specific T cells. Substitution of peptide residues p2C and p5M to the conservative α-aminobutyric acid and norleucine, respectively, significantly reduced complex stability, without altering peptide conformation or T cell recognition. In contrast, substitution of p5M to a conventional asparagine abolished recognition by the H-2D(b)/Trh4-specific T cell clone LnB5. We anticipate that the H-2D(b)/Trh4 complex represents the first example, to our knowledge, of a broader repertoire of alternative MHC class I binders.

Peptide-independent stabilization of MHC class I molecules breaches cellular quality control

The intracellular trafficking of major histocompatibility complex class I (MHC-I) proteins is directed by three quality control mechanisms that test for their structural integrity, which is correlated to the binding of high-affinity antigenic peptide ligands. To investigate which molecular features of MHC-I these quality control mechanisms detect, we have followed the hypothesis that suboptimally loaded MHC-I molecules are characterized by their conformational mobility in the F-pocket region of the peptide-binding site. We have created a novel variant of an MHC-I protein, K(b)-Y84C, in which two α-helices in this region are linked by a disulfide bond that mimics the conformational and dynamic effects of bound high-affinity peptide. K(b)-Y84C shows a remarkable increase in the binding affinity to its light chain, beta-2 microglobulin (β2m), and bypasses all three cellular quality control steps. Our data demonstrate (1) that coupling between peptide and β2m binding to the MHC-I heavy chain is mediated by conformational dynamics; (2) that the folded conformation of MHC-I, supported by β2m, plays a decisive role in passing the ER-to-cell-surface transport quality controls; and (3) that β2m association is also tested by the cell surface quality control that leads to MHC-I endocytosis.

Class I major histocompatibility complex, the trojan horse for secretion of amyloidogenic β2-microglobulin

To form extracellular aggregates, amyloidogenic proteins bypass the intracellular quality control, which normally targets unfolded/aggregated polypeptides. Human D76N β₂-microglobulin (β₂m) variant is the prototype of unstable and amyloidogenic protein that forms abundant extracellular fibrillar deposits. Here we focus on the role of the class I major histocompatibility complex (MHCI) in the intracellular stabilization of D76N β₂m. Using biophysical and structural approaches, we show that the MHCI containing D76N β₂m (MHCI₇₆) displays stability, dissociation patterns, and crystal structure comparable with those of the MHCI with wild type β₂m. Conversely, limited proteolysis experiments show a reduced protease susceptibility for D76N β₂m within the MHCI₇₆ as compared with the free variant, suggesting that the MHCI has a chaperone-like activity in preventing D76N β₂m degradation within the cell. Accordingly, D76N β₂m is normally assembled in the MHCI and circulates as free plasma species in a transgenic mouse model.

Proline substitution independently enhances H-2D(b) complex stabilization and TCR recognition of melanoma-associated peptides

The immunogenicity of H-2D(b) (D(b)) restricted epitopes can be significantly increased by substituting peptide position 3 to a proline (p3P). The p3P modification enhances MHC stability without altering the conformation of the modified epitope allowing for T-cell cross-reactivity with the native peptide. The present study reveals how specific interactions between p3P and the highly conserved MHC heavy chain residue Y159 increase the stability of D(b) in complex with an optimized version of the melanoma-associated epitope gp10025-33 . Furthermore, the p3P modification directly increased the affinity of the D(b)/gp10025-33 -specific T-cell receptor (TCR) pMel. Surprisingly, the enhanced TCR binding was independent from the observed increased stability of the optimized D(b)/gp10025-33 complex and from the interactions formed between p3P and Y159, indicating a direct effect of the p3P modification on TCR recognition.

Unexpected T-cell recognition of an altered peptide ligand is driven by reversed thermodynamics

The molecular basis underlying T-cell recognition of MHC molecules presenting altered peptide ligands is still not well-established. A hierarchy of T-cell activation by MHC class I-restricted altered peptide ligands has been defined using the T-cell receptor P14 specific for H-2D(b) in complex with the immunodominant lymphocytic choriomeningitis virus peptide gp33 (KAVYNFATM). While substitution of tyrosine to phenylalanine (Y4F) or serine (Y4S) abolished recognition by P14, the TCR unexpectedly recognized H-2D(b) in complex with the alanine-substituted semiagonist Y4A, which displayed the most significant structural modification. The observed functional hierarchy gp33 > Y4A > Y4S = Y4F was neither due to higher stabilization capacity nor to differences in structural conformation. However, thermodynamic analysis demonstrated that while recognition of the full agonist H-2D(b) /gp33 was strictly enthalpy driven, recognition of the weak agonist H-2D(b) /Y4A was instead entropy driven with a large reduction in the favorable enthalpy term. The fourfold larger negative heat capacity derived for the interaction of P14 with H-2D(b) /gp33 compared with H-2D(b) /Y4A can possibly be explained by higher water entrapment at the TCR/MHC interface, which is also consistent with the measured opposite entropy contributions for the interactions of P14 with both MHCs. In conclusion, this study demonstrates that P14 makes use of different strategies to adapt to structural modifications in the MHC/peptide complex.

Inflammation-associated nitrotyrosination affects TCR recognition through reduced stability and alteration of the molecular surface of the MHC complex

Nitrotyrosination of proteins, a hallmark of inflammation, may result in the production of MHC-restricted neoantigens that can be recognized by T cells and bypass the constraints of immunological self-tolerance. Here we biochemically and structurally assessed how nitrotyrosination of the lymphocytic choriomeningitis virus (LCMV)-associated immunodominant MHC class I-restricted epitopes gp33 and gp34 alters T cell recognition in the context of both H-2D^b and H-2K^b. Comparative analysis of the crystal structures of H-2K^b/gp34 and H-2K^b/NY-gp34 demonstrated that nitrotyrosination of p3Y in gp34 abrogates a hydrogen bond interaction formed with the H-2K^b residue E152. As a consequence the conformation of the TCR-interacting E152 was profoundly altered in H-2K^b/NY-gp34 when compared to H-2K^b/gp34, thereby modifying the surface of the nitrotyrosinated MHC complex. Furthermore, nitrotyrosination of gp34 resulted in structural over-packing, straining the overall conformation and considerably reducing the stability of the H-2K^b/NY-gp34 MHC complex when compared to H-2K^b/gp34. Our structural analysis also indicates that nitrotyrosination of the main TCR-interacting residue p4Y in gp33 abrogates recognition of H-2D^b/gp33-NY complexes by H-2D^b/gp33-specific T cells through sterical hindrance. In conclusion, this study provides the first structural and biochemical evidence for how MHC class I-restricted nitrotyrosinated neoantigens may enable viral escape and break immune tolerance.

Expression, refolding and crystallization of murine MHC class I H-2Db in complex with human beta2-microglobulin

Beta2-microglobulin (beta2m) is non-covalently linked to the major histocompatibility (MHC) class I heavy chain and interacts with CD8 and Ly49 receptors. Murine MHC class I can bind human beta2m (hbeta2m) and such hybrid molecules are often used in structural and functional studies. The replacement of mouse beta2m (mbeta2m) by hbeta2m has important functional consequences for MHC class I complex stability and specificity, but the structural basis for this is unknown. To investigate the impact of species-specific beta2m subunits on MHC class I conformation, murine MHC class I H-2Db in complex with hbeta2m and the peptide gp33 derived from lymphocytic choriomeningitis virus (LCMV) has been expressed, refolded in vitro and crystallized. Crystals containing two complexes per asymmetric unit and belonging to the space group P2(1), with unit-cell parameters a = 68.1, b = 65.2, c = 101.9 A, beta = 102.4 degrees, were obtained.

Expression of human immunodeficiency virus type 1 gp120 from herpes simplex virus type 1-derived amplicons results in potent, specific, and durable cellular and humoral immune responses

Herpes simplex virus type 1 (HSV-1) infects a wide range of cells, including dendritic cells. Consequently, HSV-1 vectors may be capable of eliciting strong immune responses to vectored antigens. To test this hypothesis, an HSV-1 amplicon plasmid encoding human immunodeficiency virus type 1 gp120 was constructed, and murine immune responses to helper virus-free amplicon preparations derived from this construct were evaluated. Initial studies revealed that a single intramuscular (i.m.) injection of 10(6) infectious units (i.u.) of HSV:gp120 amplicon particles (HSV:gp120) elicited Env-specific cellular and humoral immune responses. A potent, CD8(+)-T-cell-mediated response to an H-2D(d)-restricted peptide from gp120 (RGPGRAFVTI) was measured by a gamma interferon ELISPOT and was confirmed by standard cytotoxic-T-lymphocyte assays. Immunoglobulin G enzyme-linked immunosorbent assay analysis showed the induction of a strong, Env-specific antibody response. An i.m. or an intradermal administration of HSV:gp120 at the tail base elicited a more potent cellular immune response than did an intraperitoneal (i.p.) inoculation, although an i.p. introduction generated a stronger humoral response. The immune response to HSV:gp120 was durable, with robust cellular and humoral responses persisting at 171 days after a single 10(6)-i.u. inoculation. The immune response to HSV:gp120 was also found to be dose dependent: as few as 10(4) i.u. elicited a strong T-cell response. Finally, HSV:gp120 elicited significant Env-specific cellular immune responses even in animals that had been previously infected with wild-type HSV-1. Taken together, these data strongly support the use of helper-free HSV-1 amplicon particles as vaccine delivery vectors.

The role of an exposed loop in the alpha2 domain in the mouse MHC class I H-2Dd molecule for recognition by the monoclonal antibody 34-5-8S and the NK-cell receptor Ly49A

Natural killer (NK) cells express major histocompatibility complex (MHC) class I-specific inhibitory receptors. The region mediating the protective effect of the MHC class I molecule H-2Dd (Dd), recognized by the inhibitory receptor Ly49A, has been mapped to the alpha1/alpha2 domains. Here we have focused on an exposed loop in the N-terminal part of the alpha2 domain, which constitutes a major structural motif that differs between Dd (strong binding to Ly49A) and Db (weak binding to Ly49A at best). We mutated the residues 103, 104 and 107 in Dd to the corresponding amino acids in Db. The Dd mutant molecule retained the ability to be stabilized by a Dd-binding peptide. However, the mutation totally abolished the recognition by the conformational dependent monoclonal antibody (MoAb) 34-5-8S, known to inhibit the interaction between Dd and Ly49A. In addition, there was a marked impairment of the binding to Ly49A as evaluated by the ability of tetramers of the Dd mutant molecule to bind to Ly49A-transfected reporter cells and spleen cells. These results demonstrate that the introduced changes at positions 103, 104 and 107 directly or indirectly affect the epitopes for the MoAb 34-5-8S and the Ly49A receptor.

NK cell inhibitory receptor Ly-49C residues involved in MHC class I binding

Mouse NK cells express Ly-49 receptors specific for classical MHC class I molecules. Several of the Ly-49 receptors have been characterized in terms of function and ligand specificity. However, the only Ly-49 receptor-ligand interaction previously described in detail is that between Ly-49A and H-2D(d), as studied by point mutations in the ligand and the crystal structure of the co-complex of these molecules. It is not known whether other Ly-49 receptors bind MHC class I in a similar manner as Ly-49A. Here we have studied the effect of mutations in Ly-49C on binding to the MHC class I molecules H-2K(b), H-2D(b), and H-2D(d). The MHC class I molecules were used as soluble tetramers to stain transiently transfected 293T cells expressing the mutated Ly-49C receptors. Three of nine mutations in Ly-49C led to loss of MHC class I binding. The three Ly-49C mutations that affected MHC binding correspond to Ly-49A residues that are in contact or close to H-2D(d) in the co-crystal, demonstrating that MHC class I binding by Ly-49C is dependent on residues in the same area as that used by Ly-49A for ligand contacts.

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

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

Emergence of CD8+ T cells expressing NK cell receptors in influenza A virus-infected mice

Both innate and adaptive immune responses play an important role in the recovery of the host from viral infections. In the present report, a subset of cells coexpressing CD8 and NKR-P1C (NK1.1) was found in the lungs of mice infected with influenza A virus. These cells were detected at low numbers in the lungs of uninfected mice, but represented up to 10% of the total CD8(+) T cell population at day 10 postinfection. Almost all of the CD8(+)NK1.1(+) cells were CD8alphabeta(+)CD3(+)TCRalphabeta(+) and a proportion of these cells also expressed the NK cell-associated Ly49 receptors. Interestingly, up to 30% of these cells were virus-specific T cells as determined by MHC class I tetramer staining and by intracellular staining of IFN-gamma after viral peptide stimulation. Moreover, these cells were distinct from conventional NKT cells as they were also found at increased numbers in influenza-infected CD1(-/-) mice. These results demonstrate that a significant proportion of CD8(+) T cells acquire NK1.1 and other NK cell-associated molecules, and suggests that these receptors may possibly regulate CD8(+) T cell effector functions during viral infection.

T cell tolerance based on avidity thresholds rather than complete deletion allows maintenance of maximal repertoire diversity

Given the flexible nature of TCR specificity, deletion or permanent disabling of all T cells with the capacity to recognize self peptides would severely limit the diversity of the repertoire and the capacity to recognize foreign Ags. To address this, we have investigated the patterns of CD8+ CTL reactivity to a naturally H-2Kb-presented self peptide derived from the elongation factor 1alpha (EF1alpha). EF1alpha occurs as two differentially expressed isoforms differing at one position of the relevant peptide. Low avidity CTLs could be raised against both variants of the EF1alpha peptide. These CTLs required 100-fold more peptide-H-2Kb complexes on the target cell compared with CTLs against a viral peptide, and did not recognize the naturally expressed levels of EF1alpha peptides. Thus, low avidity T cells specific for these self peptides escape tolerance by deletion, despite expression of both EF1alpha isoforms in dendritic cells known to mediate negative selection in the thymus. The low avidity in CTL recognition of these peptides correlated with low TCR affinity. However, self peptide-specific CTLs expressed elevated levels of CD8. Furthermore, CTLs generated against altered self peptide variants displayed intermediate avidity, indicating cross-reactivity in induction of tolerance. We interpret these data, together with results previously published by others, in an avidity pit model based on avidity thresholds for maintenance of both maximal diversity and optimal self tolerance in the CD8+ T cell repertoire.