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Fibroblasts have recently attracted attention as a key stromal component that controls the immune responses in lymphoid tissues. The thymus has a unique microenvironment comprised of a variety of stromal cells, including fibroblasts and thymic epithelial cells (TECs), the latter of which is known to be important for T cell development because of their ability to express self‐antigens. Thymic fibroblasts contribute to thymus organogenesis during embryogenesis and form the capsule and medullary reticular network in the adult thymus. However, the immunological significance of thymic fibroblasts has thus far only been poorly elucidated. In this review, we will summarize the current views on the development and functions of thymic fibroblasts as revealed by new technologies such as multicolor flow cytometry and single cell–based transcriptome profiling. Furthermore, the recently discovered role of medullary fibroblasts in the establishment of T cell tolerance by producing a unique set of self‐antigens will be highlighted.

T cells undergo rigorous selection in the thymus to ensure self-tolerance and prevent autoimmunity, with this process requiring innocuous self-antigens (Ags) to be presented to thymocytes. Self-Ags are either expressed by thymic stroma cells or transported to the thymus from the periphery by migratory dendritic cells (DCs); meanwhile, small blood-borne peptides can access the thymic parenchyma by diffusing across the vascular lining. Here we describe an additional pathway of thymic Ag acquisition that enables circulating antigenic macromolecules to access both murine and human thymi. This pathway depends on a subset of thymus-resident DCs, distinct from both parenchymal and circulating migratory DCs, that are positioned in immediate proximity to thymic microvessels where they extend cellular processes across the endothelial barrier into the blood stream. Transendothelial positioning of DCs depends on DC-expressed CX3CR1 and its endothelial ligand, CX3CL1, and disrupting this chemokine pathway prevents thymic acquisition of circulating proteins and compromises negative selection of Ag-reactive thymocytes. Thus, transendothelial DCs represent a mechanism by which the thymus can actively acquire blood-borne Ags to induce and maintain central tolerance., T cells are selected in the thymus, through interaction with self-antigens, to remove autoreactive cells. Here the authors show that a specialized thymic dendritic cell subset juxtaposes to microvessels, requires CX3CR1/CX3CL1 for this positioning, and has processes extruding into the blood stream to sample soluble macromolecules and assist in T cell selection.

The CD8αα homodimer is crucial to both thymic T cell selection and the antigen recognition of cytotoxic T cells. The CD8-pMHC-I interaction can enhance CTL immunity via stabilizing the TCR-pMHC-I interaction and optimizing the cross-reactivity and Ag sensitivity of CD8+ T cells at various stages of development. To date, only human and mouse CD8-pMHC-I complexes have been determined. Here, we resolved the pBF2*1501 complex and the cCD8αα/pBF2*1501 and cCD8αα/pBF2*0401 complexes in nonmammals for the first time. Remarkably, cCD8αα/pBF2*1501 and the cCD8αα/pBF2*0401 complex both exhibited two binding modes, including an “antibody-like” mode similar to that of the known mammal CD8/pMHC-I complexes and a “face-to-face” mode that has been observed only in chickens to date. Compared to the “antibody-like” mode, the “face-to-face” binding mode changes the binding orientation of the cCD8αα homodimer to pMHC-I, which might facilitate abundant γδT cells to bind diverse peptides presented by limited BF2 alleles in chicken. Moreover, the forces involving in the interaction of cCD8αα/pBF2*1501 and the cCD8αα/pBF2*0401 are different in this two binding model, which might change the strength of the CD8-pMHC-I interaction, amplifying T cell cross-reactivity in chickens. The coreceptor CD8αα of TCR has evolved two peptide-MHC-I binding patterns in chickens, which might enhance the T cell response to major or emerging pathogens, including chicken-derived pathogens that are relevant to human health, such as high-pathogenicity influenza viruses.

The costimulatory molecule CD226 is highly expressed on effector/memory T cells and natural killer cells. Costimulatory signals received by T cells can impact both central and peripheral tolerance mechanisms. Genetic polymorphisms in CD226 have been associated with susceptibility to type 1 diabetes and other autoimmune diseases. We hypothesized that genetic deletion of Cd226 in the non-obese diabetic (NOD) mouse would impact type 1 diabetes incidence by altering T cell activation. CD226 knockout (KO) NOD mice displayed decreased disease incidence and insulitis in comparison to wild-type (WT) controls. Although female CD226 KO mice had similar levels of sialoadenitis as WT controls, male CD226 KO mice showed protection from dacryoadenitis. Moreover, CD226 KO T cells were less capable of adoptively transferring disease compared to WT NOD T cells. Of note, CD226 KO mice demonstrated increased CD8+ single positive (SP) thymocytes, leading to increased numbers of CD8+ T cells in the spleen. Decreased percentages of memory CD8+CD44+CD62L– T cells were observed in the pancreatic lymph nodes of CD226 KO mice. Intriguingly, CD8+ T cells in CD226 KO mice showed decreased islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP)-tetramer and CD5 staining, suggesting reduced T cell receptor affinity for this immunodominant antigen. These data support an important role for CD226 in type 1 diabetes development by modulating thymic T cell selection as well as impacting peripheral memory/effector CD8+ T cell activation and function.

Early thymic progenitors (ETPs) are bone marrow–derived hematopoietic stem cells that remain multipotent and give rise to a variety of lineage-specific cells. Recently, we discovered a subset of murine ETPs that expresses the IL-4Rα/IL-13Rα1 heteroreceptor (HR) and commits only to the myeloid lineage. This is because IL-4/IL-13 signaling through the HR inhibits their T cell potential and enacts commitment of HR+ETPs to thymic resident CD11c+CD8α+ dendritic cells (DCs). In this study, we discovered that HR+-ETP–derived DCs function as APCs in the thymus and promote deletion of myelin-reactive T cells. Furthermore, this negative T cell selection function of HR+-ETP–derived DCs sustains protection against experimental allergic encephalomyelitis, a mouse model for human multiple sclerosis. These findings, while shedding light on the intricacies underlying ETP lineage commitment, reveal a novel, to our knowledge, function by which IL-4 and IL-13 cytokines condition thymic microenvironment to rheostat T cell selection and fine-tune central tolerance.

The contributions of the peripheral adaptive and innate immune systems to CNS autoimmunity have been extensively studied. However, the role of thymic selection in these conditions is much less well understood. The thymus is the primary lymphoid organ for the generation of T cells; thymic mechanisms ensure that cells with an overt autoreactive specificity are eliminated before they emigrate to the periphery and control the generation of thymic regulatory T cells. Evidence from animal studies demonstrates that thymic T cell selection is important for establishing tolerance to autoantigens. However, there is a considerable knowledge gap regarding the role of thymic selection in autoimmune conditions of the human CNS. In this Review , we critically examine the current body of experimental evidence for the contribution of thymic tolerance to CNS autoimmune diseases. An understanding of why dysfunction of either thymic or peripheral tolerance mechanisms rarely leads to CNS inflammation is currently lacking. We examine the potential of de novo T cell formation and thymic selection as novel therapeutic avenues and highlight areas for future study that are likely to make these targets the focus of future treatments.

Cells of hematopoietic origin express high levels of the immunoproteasome, a cytokine-inducible variant of the proteasome which has been implicated in regulating inflammatory responses and antigen presentation. In the thymus, medullary thymic epithelial cells (mTECs) and cortical thymic epithelial cells (cTECs) do express different proteasome subunits exerting chymotrypsin-like activities suggesting distinct functions in thymic T cell selection. Employing the lymphocytic choriomeningitis virus (LCMV) infection model, we could show that the immunoproteasome subunit LMP7 was absolutely required for the generation of LCMV GP118-125-specific T cells although the class I mediated presentation of GP118-125 was not dependent on LMP7. Using bone marrow chimeras and adoptive transfer of LMP7-deficient CD8+ T cells into RAG1-deficient mice we show that LMP7-deficient mice lacked GP118-125-specific T cell precursors and that LMP7 was required in radioresistant cells – most likely thymic epithelial cells - to enable their selection. Since LMP7 is strongly expressed in negatively selecting mTECs but barely in positively selecting cTECs our data suggest that LMP7 was required to avoid excessive negative selection of GP118-125-specific T cell precursors. Taken together, this study demonstrates that the immunoproteasome is a crucial factor for filling up holes within the cytotoxic T cell repertoire. This article is protected by copyright. All rights reserved

Lately, it has become clear that regulatory T cells (Tregs) play a major role in the maintenance of peripheral tolerance and control of autoimmunity. Despite these critical functions, the process underlying the development of Tregs remains largely undefined. Herein, altered peptide ligand (APL) variants derived from the proteolipid protein-1 (PLP1) epitope were expressed on immunoglobulins (Igs) and the resulting Ig-APLs were used to deliver the APLs from mother to fetus through the maternal placenta to influence thymic T cell selection. This delivery system was then adapted to the SJL/J mouse, a strain that expresses only the DM20 form of PLP, which lacks the dominant PLP1 epitope in the thymus during fetal and neonatal development. This model, which restores thymic T cell selection for PLP1, was then used to determine whether affinity plays a role in the development of Tregs. The findings show that fetal exposure to low-affinity peptide ligand was unable to drive development of Tregs while variants with higher affinity to the TCR resulted in significant seeding of the periphery with mature, naive Tregs. Thus, contrary to pathogenic T cells, Tregs require avid TCR-ligand interaction to undergo thymic development and maturation.

Classical CD4+ and CD8+ T cells recognize Ag presented by MHC class II (MHCII) and MHC class I (MHCI), respectively. However, our results show that CD4−/− mice mount a strong, readily detectable CD8+ T cell response to MHCII-restricted epitopes after a primary bacterial or viral infection. These MHCII-restricted CD8+CD4− T cells are more similar to classical CD8+ T cells than to CD4+ T cells in their expression of effector functions during a primary infection, yet they also differ from MHCI-restricted CD8+ T cells by their inability to produce high levels of the cytolytic molecule granzyme B. After resolution of a primary infection, epitope-specific MHCII-restricted T cells in CD4−/− mice persist for a long period of time as memory T cells. Surprisingly, upon reinfection the secondary MHCII-restricted response in CD4−/− mice consists mainly of CD8−CD4− T cells. In contrast to CD8+ T cells, MHCII-restricted CD8−CD4− T cells are capable of producing IL-2 in addition to IFN-γ and thus appear to have attributes characteristic of CD4+ T cells rather than CD8+ T cells. Therefore, MHCII-restricted T cells in CD4−/− mice do not share all phenotypic and functional characteristics with MHCI-restricted CD8+ T cells or with MHCII-restricted CD4+ T cells, but, rather, adopt attributes from each of these subsets. These results have implications for understanding thymic T cell selection and for elucidating the mechanisms regulating the peripheral immune response and memory differentiation.

Rheumatoid arthritis (RA), which afflicts about 1% of the world population, is a chronic systemic inflammatory disease of unknown aetiology that primarily affects the synovial membranes of multiple joints. Although CD4(+) T cells seem to be the prime mediators of RA, it remains unclear how arthritogenic CD4(+) T cells are generated and activated. Given that highly self-reactive T-cell clones are deleted during normal T-cell development in the thymus, abnormality in T-cell selection has been suspected as one cause of autoimmune disease. Here we show that a spontaneous point mutation of the gene encoding an SH2 domain of ZAP-70, a key signal transduction molecule in T cells, causes chronic autoimmune arthritis in mice that resembles human RA in many aspects. Altered signal transduction from T-cell antigen receptor through the aberrant ZAP-70 changes the thresholds of T cells to thymic selection, leading to the positive selection of otherwise negatively selected autoimmune T cells. Thymic production of arthritogenic T cells due to a genetically determined selection shift of the T-cell repertoire towards high self-reactivity might also be crucial to the development of disease in a subset of patients with RA.

Epithelial cells have central roles in the development of the thymus and in the selection of T cells in the thymus. Several recent reports have demonstrated active autophagy in thymic epithelial cells. The lipidated form of the autophagy-associated protein LC3 is present in the thymus, and autophagic vesicles are abundant in thymic epithelium. Recombinant fusion of green fluorescent protein to LC3 in a transgenic mouse model has confirmed the high proportion of autophagy-positive thymic epithelial cells. Suppression of autophagy by targeted gene deletion and grafting of thymi from autophagy-deficient embryos into athymic mice led to the development of severe autoimmunity in multiple organs, indicating a failure of autophagy-deficient thymic epithelium to appropriately support the negative selection of autoreactive T cells. Depending on expression levels and intracellular localization of antigens, autophagy appears to be essential for the loading of some but not all self-peptides onto MHC class II molecules within medullary thymic epithelial cells. In spite of these data arguing for a critical direct role of epithelial cells in antigen presentation within the thymus, signs of autoimmune disease do not develop in mouse models that lack either one of the autophagy-related genes Atg5 or Atg7 in the endogenous thymic epithelium. Thus, autophagy is highly active in normal thymic epithelium and involved in distinct aspects of T cell selection; however, its suppression does not lead to severe autoimmunity.

Affinity of a T cell receptor (TCR) refers to the strength of binding between a single TCR and a peptide/MHC (pMHC) complex while avidity of a TCR refers to the overall strength of binding between multiple cell-bound receptors and their ligands. Affinity of a TCR plays a role in thymic T cell selection and the generation of the TCR repertoire. In the absence of sensitivity to strong or weak antigen receptor signals, the homeostasis of the immune system is compromised and the risk of autoimmunity and/or infection ensues. Over the past few decades, T cells which have been genetically modified to target tumor antigens have been used to treat cancer patients. Antibody-based chimeric antigen receptors (CAR) were the first molecules used to redirect the specificity of normal T cells. CAR gene modified T cells can direct tumor rejection in mice and humans. Another class of receptors used to redirect the specificity of T cells is the T cell receptor (TCR). TCR gene modified T cells can also direct tumor rejection in mice and humans. CAR and TCR engineered T cells reactive against tumors have emerged as a promising advance in tumor immunotherapy. The rationale of this chapter is to study how CAR and TCR gene modified T cells modulate tumor immunity.

This study assessed changes in gene expression and their association with fatigue in head and neck cancer (HNC) patients receiving Intensity-Modulated Radiotherapy (IMRT). Consented patients were investigated pre- and 1-month post-IMRT. Fatigue was assessed by Multidimensional Fatigue Inventory-20. RNA was isolated from peripheral blood mononuclear cells and analyzed on Illumina Whole Genome BeadChips microarray. Forty non-metastatic HNC patients were enrolled; 70% received chemoIMRT. 1076 genes demonstrated expression changes from pre- to post-IMRT that were significantly correlated with changes in fatigue; 323 (30%) were up-regulated and 753 (70%) were down-regulated from pre- to post-IMRT. Gene enrichment analysis was performed using MetaCore/Cytoscape (FDR 0.01). Among top 200 genes, the top two biological pathways in up-regulated genes were related to IL6 regulation and Legionellosis in innate immune response. The top two biological pathways in down-regulated genes were related to thymic T cell selection and IL12 regulation in acquired immune response. Among top 200 up-regulated genes, the top network of transcription factors/receptors, such as RAGE, AP-1, and CREB1, was significantly over-represented for genes involved in body’s response to treatment-related stimuli. The second most significant network, including TLR, NFkB and its family, was significantly over-represented for genes involved in body’s immune and defense system. These findings suggest that fatigue is associated with expression changes in genes related to inflammation and immune response. Future large studies are needed to verify these findings.

T cell receptors (TCRs) recognize peptides presented by MHC molecules (pMHC) on an antigen-presenting cell (APC) to discriminate foreign from self antigens and initiate adaptive immune responses. In addition, T cell activation generally requires binding of this same pMHC to a CD4 or CD8 co-receptor, resulting in assembly of a TCR–pMHC–CD4 or TCR–pMHC–CD8 complex and recruitment of Lck via its association with the co-receptor. Here we review structural and biophysical studies of CD4 and CD8 interactions with MHC molecules and TCR–pMHC complexes. Crystal structures have been determined of CD8alpha/alpha and CD8alpha/beta in complex with MHC class I, of CD4 bound to MHC class II, and of a complete TCR–pMHC–CD4 ternary complex. Additionally, the binding of these co-receptors to pMHC and TCR–pMHC ligands has been investigated both in solution and in situ at the T cell–APC interface. Together, these studies have provided key insights into the role of CD4 and CD8 in T cell activation, and into how these co-receptors focus TCR on MHC to guide TCR docking on pMHC during thymic T cell selection.

SKG mouse is a murine model of autoimmune arthritis. A spontaneous point mutation of the gene encoding an SH2 domain of the ζ-associated protein of 70 kDa gene (ZAP-70), a key signal transduction molecule in T cells, causes chronic autoimmune arthritis in SKG mice that resembles human RA in many aspects. Altered signal transduction from T-cell antigen receptor through the aberrant ZAP-70 changes the thresholds of T cells to thymic selection, leading to the positive selection of otherwise negatively selected autoimmune T cells. Based on the finding that the skg-mutation of ZAP-70 causes autoimmune arthritis, we then examined how attenuated TCR signaling affects the spectrum of autoimmune diseases. In a set of mice with the mutation, the amount of ZAP-70 protein as well as its tyrosine phosphorylation upon TCR stimulation decreased from +/+, skg/+, skg/skg, to skg/− mice in a stepwise manner. The reduction resulted in graded alterations of thymic positive and negative selection of self-reactive T cells and Foxp3+ natural regulatory T cells (Tregs) and their respective functions. Consequently, skg/− mice spontaneously developed autoimmune arthritis even in a microbially clean environment, whereas skg/skg mice required stimulation through innate immunity for disease manifestation. After Treg depletion, organ-specific autoimmune diseases, especially autoimmune gastritis, predominantly developed in +/+, at a lesser incidence in skg/+, but not in skg/skg BALB/c mice, which suffered from other autoimmune diseases, especially autoimmune arthritis. In correlation with this change, gastritis-mediating TCR transgenic T cells were positively selected in +/+, less in skg/+, but not in skg/skg BALB/c mice. Similarly, on the genetic background of diabetes-prone NOD mice, diabetes spontaneously developed in +/+, at a lesser incidence in skg/+, but not in skg/skg mice, which instead succumbed to arthritis. Thus, the graded attenuation of TCR signaling alters the repertoire and the function of autoimmune T cells and natural Tregs in a progressive manner. It also changes the dependency of disease development on environmental stimuli. These findings collectively provide a model of how genetic anomaly of T cell signaling contributes to the development of autoimmune disease.

The mechanisms underlying tolerance to noninherited maternal Ags (NIMA) are not fully understood. In this study, we designed a double-transgenic model in which all the offspring’s CD8+ T cells corresponded to a single clone recognizing the Kb MHC class I protein. In contrast, the mother and the father of the offspring differed by the expression of a single Ag, Kb, that served as NIMA. We investigated the influence of NIMA exposure on the offspring thymic T cell selection during ontogeny and on its peripheral T cell response during adulthood. We observed that anti-Kb thymocytes were exposed to NIMA and became activated during fetal life but were not deleted. Strikingly, adult mice exposed to NIMA accepted permanently Kb+ heart allografts despite the presence of normal levels of anti-Kb TCR transgenic T cells. Transplant tolerance was associated with a lack of a proinflammatory alloreactive T cell response and an activation/expansion of T cells producing IL-4 and IL-10. In addition, we observed that tolerance to NIMA Kb was abrogated via depletion of CD4+ but not CD8+ T cells and could be transferred to naive nonexposed mice via adoptive transfer of CD4+CD25high T cell expressing Foxp3 isolated from NIMA mice.

The design of a specific blocking peptide for the immunosuppressive therapy of an autoimmune disease requires the identification of peptides of an autoantigen that are physiologically processed in vivo and bind to MHC-encoded membrane glycoproteins. However, knowledge of how an antigen is physiologically processed by antigen-presenting cells (APC) in vivo , particularly in the thymus, is lacking. It is also unknown whether the processing of an antigen by different APC in the thymus can influence thymic T-cell selection. This is an important consideration for attempts to delete or inactive autoreactive T cells that elicit autoimmune disease. To address these issues, we investigated the processing of biosynthetically labelled recombinant human insulin (rHl), a model autoantigen, injected into mice and characterized the insulin peptides associated with MHC class II molecules on thymic epithelial cells and dendritic cells. These APC were found to differ in the way they process insulin. The detection of MHC-class-II-bound insulin peptides on the surface of the epithelial cells but not the dendritic cells correlated with their capacity to either present or not present insulin to T cells, respectively. Thus, antigen processing may control the appearance of different peptide-MHC class II complexes on thymic APC that mediate positive and negative selection, and thereby influence the development of the T-cell repertoire. Our findings could have important bearing on the future design of synthetic blocking peptides that reduce or eliminate the onset of autoimmune disease.

IL-12 has been reported to affect thymic T cell selection, but the role of IL-12 in thymic involution has not been studied. We found that in vivo, IL-12b knockout (IL-12b−/−) mice exhibited accelerated thymic involution compared with wild-type (WT) B6 mice. This is characterized by an increase in thymocytes with the early development stage phenotype of CD25−CD44+CD4−CD8− in aged IL-12b−/− mice. Histologically, there were accelerated degeneration of thymic extracellular matrix and blood vessels, a significantly decreased thymic cortex/medulla ratio, and increased apoptotic cells in aged IL-12b−/− mice compared with WT mice. There was, however, no apparent defect in thymic structure and thymocyte development in young IL-12−/− mice. These results suggest the importance of IL-12 in maintaining thymic integrity and function during the aging process. Surprisingly, in WT B6 mice, there was no age-related decrease in the levels of IL-12 produced from thymic dendritic cells. Stimulation of thymocytes with IL-12 alone also did not enhance the thymocyte proliferative response in vitro. IL-12, however, provided a strong synergistic effect to augment the IL-7 or IL-2 induced thymocyte proliferative response, especially in aged WT and IL-12b−/− mice. Our data strongly support the role of IL-12 as an enhancement cytokine, which acts through its interactions with other cytokines to maintain thymic T cell function and development during aging.

Intrathymic T cell selection and peripheral activation of mature T cells are crucial for self-recognition and the general immune response to viral, bacterial, and tumor antigens. The T cell coreceptors, CD4 and CD8, contribute to the regulation of these processes. The importance of interactions between CD4 and molecules encoded by the class II major histocompatibility complex (MHC) for thymic T cell selection has been clearly established, however, the role of CD4-MHC class II interactions in T helper (TH) cell differentiation, in the maintenance of homeostasis in the peripheral immune system, and in the generation of memory TH cells is largely unclear. Here, we present evidence for a role of CD4 in controlling homeostasis in the peripheral immune system. We also demonstrate the importance of CD4-MHC class II interactions in inducing these previously not recognized functions of CD4.

Glucocorticoid (GC) hormones play an important role in thymic T cell selection and in the development of autoimmune diseases. Previous studies have shown that the mammalian thymus itself is able to produce GC. In order to assess the importance of these findings in terms of the evolutionary development of the immune system, we investigated the functional presence of steroidogenic enzymes in primary lymphoid organs of chickens, which represent one of the best studied non-mammalian species. To this end, we attempted to demonstrate enzyme activities of the whole set of steroidogenic enzymes for the synthesis of GC in the bursa of Fabricius and the thymus. We isolated steroidogenic organelles from primary lymphoid tissues, incubated these with radioactive (precursor) steroids in vitro and visualized the resulting products by thin-layer chromatography. Our results show that the chicken bursa as well as the chicken thymus possesses all enzymes and cofactors required for GC production. The observation of GC production in an organ responsible for B cell selection and maturation is a further step in uncovering the yet ill-defined mechanism of B cell selection. These results provide the biochemical basis for the in situ hormonal effects, and underline the general importance of GC hormones on T and B lymphocyte development and selection.

Intraepithelial lymphocytes (IEL) are associated with the intestinal tract, respiratory tract, genitourinary tract epithelium, and the skin and are the first immune system cells to encounter pathogens that have invaded an epithelial surface. IEL are predominantly T cells (CD3+) with CD8+ cells predominating at most, but not all, sites. Both TCR alphabeta+ and TCR gammadelta+ cells are found within IEL populations and an increasing body of evidence suggests that some IEL may arise extrathymically. The presence within intestinal IEL of cells expressing potentially self-reactive TCR suggests that T cell selection within epithelia may differ from thymic T cell selection although recent evidence suggests that these cells may in fact be nonresponsive. IEL exhibit various cytotoxic activities including alloreactive and virus-specific CTL activity, NK activity and spontaneous cytotoxicity, activities consistent with an immune surveillance or first line of defence role. IEL also appear activated in vivo and secrete a variety of cytokines. Subsets of IEL have been shown to provide B cell help, to play a role in the maintenance of oral tolerance and to regulate epithelial cell function. In this review the morphology, distribution and phenotype of IEL, the potential for extrathymic development and possible functions of this unique lymphoid population are discussed.

The adult T cell repertoire is the result of positive and negative intrathymic selective processes. Since non-self antigens are not present in the thymus, self peptides are believed to play a role in the thymic T cell selection. T cell lines raised against a peptide from the second complementarity-determining region (CDR2) of the S107 germ-line immunoglobulin (Ig) do not respond to the intact Ig, indicating that in adult animals this CDR2 epitope is cryptic in the context of intact S107 Ig. To show that the response to self Ig peptide is the result of positive selection, the fetal expression of the S107 germ-line Ig was suppressed by maternal anti-idiotype (Id) injection. Id S107-suppressed mice responded poorly to the CDR2 peptide, indicating that S107 Ig selects positively T cells responding to a polymorphic self epitope. Cross-reactivity of cryptic self Ig with non-self epitopes could be a mechanism to increase the T cell repertoire for foreign antigens.

The etiology of autoimmunity in humans remains poorly defined, and animal models provide a unique opportunity to study potential autoimmune mechanisms. A novel model of autoimmune inflammatory arthritis results from a point mutation in the zeta-associated-protein of 70 kDa (ZAP-70), which causes abnormal thymic T cell selection and survival of autoreactive clones. Although the resulting clinical and pathologic abnormalities are clearly T cell-dependent, macrophage and fibroblast cytokines such as IL-1 and TNF-alpha are required for full expression of the disease. The studies of Hata et al. raise the intriguing possibility that traditional proinflammatory cytokine networks represent common effector mechanisms in inflammatory joint diseases such as rheumatoid arthritis. Hence, effective therapeutic interventions can target either unique etiologic pathways related to adaptive immune responses or shared terminal mechanisms.

The development of autoimmune disorders is incompletely understood. Inefficient thymic T cell selection against self-peptides presented by major histocompatibility antigens (HLA in humans) may contribute to the emergence of auto-reactive effector cells, and molecular mimicry between foreign and self-peptides could promote T cell cross-reactivity. A pair of class I subtypes, HLA-B2705 and HLA-B2709, have previously been intensely studied, because they are distinguished from each other only by a single amino acid exchange at the floor of the peptide-binding groove, yet are differentially associated with the autoinflammatory disorder ankylosing spondylitis. Using X-ray crystallography in combination with ensemble refinement, we find that the non-disease-associated subtype HLA-B2709, when presenting the self-peptide pGR (RRRWHRWRL), exhibits elevated conformational dynamics, and the complex can also be recognized by T cells. Both features are not observed in case of the sequence-related self-peptide pVIPR (RRKWRRWHL) in complex with this subtype, and T cell cross-reactivity between pGR, pVIPR, and the viral peptide pLMP2 (RRRWRRLTV) is only rarely observed. The disease-associated subtype HLA-B2705, however, exhibits extensive conformational flexibility in case of the three complexes, all of which are also recognized by frequently occurring cross-reactive T cells. A comparison of the structural and dynamic properties of the six HLA-B27 complexes, together with their individual ability to interact with T cells, permits us to correlate the flexibility of HLA-B27 complexes with effector cell reactivity. The results suggest the existence of an inverse relationship between conformational plasticity of peptide-HLA-B27 complexes and the efficiency of negative selection of self-reactive cells within the thymus.