Cutting edge: CD40-induced expression of recombination activating gene (RAG) 1 and RAG2: a mechanism for the generation of autoaggressive T cells in the periphery

Canine diabetes mellitus demonstrates multiple markers of chronic inflammation including Th40 cell increases and elevated systemic-immune inflammation index, consistent with autoimmune dysregulation

Introduction

Canine diabetes mellitus (CDM) is a relatively common endocrine disease in dogs. Many CDM clinical features resemble human type 1 diabetes mellitus (T1DM), but lack of autoimmune biomarkers makes calling the disease autoimmune controversial. Autoimmune biomarkers linking CDM and T1DM would create an alternative model for drug development impacting both human and canine disease.

Methods

We examined peripheral blood of diagnosed CDM dog patients comparing it to healthy control (HC) dogs. Dogs were recruited to a study at the Colorado State University Veterinary Teaching Hospital and blood samples collected for blood chemistry panels, complete blood counts (CBC), and immunologic analysis. Markers of disease progression such as glycated albumin (fructosamine, the canine equivalent of human HbA1c) and c-peptide were addressed.

Results

Significant differences in adaptive immune lymphocytes, innate immune macrophages/monocytes and neutrophils and differences in platelets were detected between CDM and HC based on CBC. Significant differences in serum glucose, cholesterol and the liver function enzyme alkaline phosphatase were also detected. A systemic immune inflammation index (SII) and chronic inflammation index (CII) as measures of dynamic changes in adaptive and innate cells between inflammatory and non-inflammatory conditions were created with highly significant differences between CDM and HC. Th40 cells (CD4+CD40+ T cells) that are demonstrably pathogenic in mouse T1DM and able to differentiate diabetic from non-diabetic subjects in human T1DM were significantly expanded in peripheral blood mononuclear cells.

Conclusions

Based on each clinical finding, CDM can be categorized as an autoimmune condition. The association of significantly elevated Th40 cells in CDM when compared to HC or to osteoarthritis, a chronic but non-autoimmune disease, suggests peripheral blood Th40 cell numbers as a biomarker that reflects CDM chronic inflammation. The differences in SII and CII further underscore those findings.

Modulating CD40 and integrin signaling in the proinflammatory nexus using a 15-amino-acid peptide, KGYY15

CD40 signaling has long been a target in autoimmunity. Attempts to block signaling between CD40 and CD154 during clinical trials using monoclonal antibodies suffered severe adverse events. Previously, we developed a peptide, KGYY15, that targets CD40 and, in preclinical trials, prevents type 1 diabetes in >90% of cases and reverses new-onset hyperglycemia in 56% of cases. It did so by establishing normal effector T-cell levels rather than ablating the cells and causing immunosuppression. However, the relationship between KGYY15 and other elements of the complex signaling network of CD40 is not clear. Studying interactions between proteins from autoimmune and nonautoimmune mice, we demonstrate interactions between CD40 and integrin CD11a/CD18, which complicates the understanding of the inflammatory nexus and how to prevent autoinflammation. In addition to interacting with CD40, KGYY15 interacts with the integrins CD11a/CD18 and CD11b/CD18. We argue that modulation of CD40-CD154 signaling may be more advantageous than complete inhibition because it may preserve normal immunity to pathogens.

Ocrevus reduces TH40 cells, a biomarker of systemic inflammation, in relapsing multiple sclerosis (RMS) and in progressive multiple sclerosis (PMS)

Treating MS has been difficult. One successful drug is Ocrelizumab (anti-CD20), used for the chronic relapsing MS (RMS) and the progressive MS (PMS) forms. TH40 cells are pathogenic effector T cells that increase in percentage and numbers during chronic inflammation. Here we show that in the earliest MS course, clinically isolated syndrome (CIS), TH40 cells expand in number. In PMS TH40 cell numbers remain expanded demonstrating sustained chronic inflammation. In RMS TH40 cells were found in CSF and express CD20. Ocrelizumab reduced TH40 cells to healthy control levels in patients. During treatment inflammatory cytokine producing TH40 cells were decreased.

T cell receptor revision and immune repertoire changes in autoimmune diseases

Autoimmune disease (AID) is a condition in which the immune system breaks down and starts to attack the body. Some common AIDs include systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes mellitus and so forth. The changes in T-cell receptor (TCR) repertoire have been found in several autoimmune diseases, and may be responsible for the breakdown of peripheral immune tolerance. In this review, we discussed the processes of TCR revision in peripheral immune environment, the changes in TCR repertoire that occurred in various AIDs, and the specifically expanded T cell clones. We hope our discussion can provide insights for the future studies, helping with the discovery of disease biomarkers and expanding the strategies of immune-targeted therapy. HighlightsRestricted TCR repertoire and biased TCR-usage are found in a variety of AIDs.TCR repertoire shows tissue specificity in a variety of AID diseases.The relationship between TCR repertoire diversity and disease activity is still controversial in AIDs.Dominant TCR clonotypes may help to discover new disease biomarkers and expand the strategies of immune-targeted therapy.

T Cells Expressing Receptor Recombination/Revision Machinery Are Detected in the Tumor Microenvironment and Expanded in Genomically Over-unstable Models

Tumors undergo dynamic immunoediting as part of a process that balances immunologic sensing of emerging neoantigens and evasion from immune responses. Tumor-infiltrating lymphocytes (TIL) comprise heterogeneous subsets of peripheral T cells characterized by diverse functional differentiation states and dependence on T-cell receptor (TCR) specificity gained through recombination events during their development. We hypothesized that within the tumor microenvironment (TME), an antigenic milieu and immunologic interface, tumor-infiltrating peripheral T cells could reexpress key elements of the TCR recombination machinery, namely, Rag1 and Rag2 recombinases and Tdt polymerase, as a potential mechanism involved in the revision of TCR specificity. Using two syngeneic invasive breast cancer transplantable models, 4T1 and TS/A, we observed that Rag1, Rag2, and Dntt in situ mRNA expression characterized rare tumor-infiltrating T cells. In situ expression of the transcripts was increased in coisogenic Mlh1-deficient tumors, characterized by genomic overinstability, and was also modulated by PD-1 immune-checkpoint blockade. Through immunolocalization and mRNA hybridization analyses, we detected the presence of rare TDT+RAG1/2+ cells populating primary tumors and draining lymph nodes in human invasive breast cancer. Analysis of harmonized single-cell RNA-sequencing data sets of human cancers identified a very small fraction of tumor-associated T cells, characterized by the expression of recombination/revision machinery transcripts, which on pseudotemporal ordering corresponded to differentiated effector T cells. We offer thought-provoking evidence of a TIL microniche marked by rare transcripts involved in TCR shaping.

Biomarker discovery in pre-Type 1 Diabetes; Th40 cells as a predictive risk factor

CONTEXT

The incidence of Type 1 Diabetes (T1D) is increasing worldwide. The quest to understand T1D etiology as well as how to predict diabetes is ongoing and, in many ways, those goals intertwine. While genetic components associate with T1D, not all T1D individuals have those components and not all subjects with those components develop disease.

OBJECTIVE

More robust methods for prediction of T1D are needed. Can high CD4+CD40+ T cell (Th40) levels be used as a biomarker in addition to other markers?

METHODS

Th40 levels were assessed along with other parameters in blood collected from prediabetic TrialNet subjects.

RESULTS

Pre-diabetic subjects, stratified according to their Th40 cell levels, demonstrate patterns that parallel those seen between control and T1D subjects. Cytokine patterns are significantly different between Th40-high and -low subjects and a CD4/CD8 double-positive population is more represented in Th40-high groups. Subjects experiencing impaired glucose tolerance present a significantly higher Th40 level than control subjects do. HLA DR4/DR4 and DQ8/DQ8, HLAs associated with T1D, are more likely found among Th40-high subjects. Interestingly, HLA DR4/DR4 subjects were significantly older compared with all other subjects, suggesting that this haplotype together with a high Th40 level may represent someone who will onset after age 30, which is reported for 42% of T1D cases.

CONCLUSION

Considering the differences found in relation to prediabetic Th40 cell level, it may be possible to devise methods that more accurately predicts who will proceed toward diabetes and, possibly, at what stage of prediabetes a subject is.

WITHDRAWN: T cell receptor revision and immune repertoire changes in autoimmune diseases

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CD40-mediated signalling influences trafficking, T-cell receptor expression, and T-cell pathogenesis, in the NOD model of type 1 diabetes

CD40 plays a critical role in the pathogenesis of type 1 diabetes (T1D). The mechanism of action, however, is undetermined, probably because CD40 expression has been grossly underestimated. CD40 is expressed on numerous cell types that now include T cells and pancreatic β cells. CD40⁺ CD4⁺ cells [T helper type 40 (TH40)] prove highly pathogenic in NOD mice and in translational human T1D studies. We generated BDC2.5.CD40^-/- and re-derived NOD.CD154^-/- mice to better understand the CD40 mechanism of action. Fully functional CD40 expression is required not only for T1D development but also for insulitis. In NOD mice, TH40 cell expansion in pancreatic lymph nodes occurs before insulitis and demonstrates an activated phenotype compared with conventional CD4⁺ cells, apparently regardless of antigen specificity. TH40 T-cell receptor (TCR) usage demonstrates increases in several Vα and Vβ species, particularly Vα3.2⁺ that arise early and are sustained throughout disease development. TH40 cells isolated from diabetic pancreas demonstrate a relatively broad TCR repertoire rather than restricted clonal expansions. The expansion of the Vα/Vβ species associated with diabetes depends upon CD40 signalling; NOD.CD154^-/- mice do not expand the same TCR species. Finally, CD40-mediated signals significantly increase pro-inflammatory Th1- and Th17-associated cytokines whereas CD28 co-stimulus alternatively promotes regulatory cytokines.

Th40 cells (CD4+CD40+ Tcells) drive a more severe form of Experimental Autoimmune Encephalomyelitis than conventional CD4 T cells

CD40-CD154 interaction is critically involved in autoimmune diseases, and CD4 T cells play a dominant role in the Experimental Autoimmune Encephalomyelitis (EAE) model of Multiple Sclerosis (MS). CD4 T cells expressing CD40 (Th40) are pathogenic in type I diabetes but have not been evaluated in EAE. We demonstrate here that Th40 cells drive a rapid, more severe EAE disease course than conventional CD4 T cells. Adoptively transferred Th40 cells are present in lesions in the CNS and are associated with wide spread demyelination. Primary Th40 cells from EAE-induced donors adoptively transfer EAE without further in-vitro expansion and without requiring the administration of the EAE induction regimen to the recipient animals. This has not been accomplished with primary, non-TCR-transgenic donor cells previously. If co-injection of Th40 donor cells with Freund's adjuvant (CFA) in the recipient animals is done, the disease course is more severe. The CFA component of the EAE induction regimen causes generalized inflammation, promoting expansion of Th40 cells and infiltration of the CNS, while MOG-antigen shapes the antigen-specific TCR repertoire. Those events are both necessary to precipitate disease. In MS, viral infections or trauma may induce generalized inflammation in susceptible individuals with subsequent disease onset. It will be important to further understand the events leading up to disease onset and to elucidate the contributions of the Th40 T cell subset. Also, evaluating Th40 levels as predictors of disease onset would be highly useful because if either the generalized inflammation event or the TCR-honing can be interrupted, disease onset may be prevented.

Overlooked Mechanisms in Type 1 Diabetes Etiology: How Unique Costimulatory Molecules Contribute to Diabetogenesis

Type 1 Diabetes (T1D) develops when immune cells invade the pancreatic islets resulting in loss of insulin production in beta cells. T cells have been proven to be central players in that process. What is surprising, however, is that classic mechanisms of tolerance cannot explain diabetogenesis; alternate mechanisms must now be considered. T cell receptor (TCR) revision is the process whereby T cells in the periphery alter TCR expression, outside the safety-net of thymic selection pressures. This process results in an expanded T cell repertoire, capable of responding to a universe of pathogens, but limitations are that increased risk for autoimmune disease development occurs. Classic T cell costimulators including the CD28 family have long been thought to be the major drivers for full T cell activation. In actuality, CD28 and its family member counterparts, ICOS and CTLA-4, all drive regulatory responses. Inflammation is driven by CD40, not CD28. CD40 as a costimulus has been largely overlooked. When naïve T cells interact with antigen presenting cell CD154, the major ligand for CD40, is induced. This creates a milieu for T cell (CD40)-T cell (CD154) interaction, leading to inflammation. Finally, defined pathogenic effector cells including TH40 (CD4⁺CD40⁺) cells can express FOXP3 but are not Tregs. The cells loose FOXP3 to become pathogenic effector cells. Each of these mechanisms creates novel options to better understand diabetogenesis and create new therapeutic targets for T1D.

Of the multiple mechanisms leading to type 1 diabetes, T cell receptor revision may play a prominent role (is type 1 diabetes more than a single disease?)

A single determinant factor for autoimmunity does not exist; disease development probably involves contributions from genetics, the environment and immune dysfunction. Type 1 diabetes is no exception. Genomewide-associated studies (GWAS) analysis in T1D has proved disappointing in revealing contributors to disease prediction; the only reliable marker has been human leucocyte antigen (HLA). Specific HLAs include DR3/DR4/DQ2/DQ8, for example. Because HLA molecules present antigen to T cells, it is reasonable that certain HLA molecules have a higher affinity to present self-antigen. Recent studies have shown that additional polymorphisms in HLA that are restricted to autoimmune conditions are further contributory. A caveat is that not all individuals with the appropriate 'pro-autoimmune' HLA develop an autoimmune disease. Another crucial component is autoaggressive T cells. Finding a biomarker to discriminate autoaggressive T cells has been elusive. However, a subset of CD4 helper cells that express the CD40 receptor have been described as becoming pathogenic. An interesting function of CD40 on T cells is to induce the recombination-activating gene (RAG)1/RAG2 T cell receptor recombination machinery. This observation is contrary to immunology paradigms that changes in TCR molecules cannot take place outside the thymic microenvironment. Alteration in TCR, called TCR revision, not only occurs, but may help to account for the development of autoaggressive T cells. Another interesting facet is that type 1 diabetes (T1D) may be more than a single disease; that is, multiple cellular components contribute uniquely, but result ultimately in the same clinical outcome, T1D. This review considers the process of T cell maturation and how that could favor auto-aggressive T cell development in T1D. The potential contribution of TCR revision to autoimmunity is also considered.

A CD40-targeted peptide controls and reverses type 1 diabetes in NOD mice

AIMS/HYPOTHESIS

The CD40-CD154 interaction directs autoimmune inflammation. Therefore, a long-standing goal in the treatment of autoimmune disease has been to control the formation of that interaction and thereby prevent destructive inflammation. Antibodies blocking CD154 are successful in mouse models of autoimmune disease but, while promising when used in humans, unfortunate thrombotic events have occurred, forcing the termination of those studies.

METHODS

To address the clinical problem of thrombotic events caused by anti-CD154 antibody treatment, we created a series of small peptides based on the CD154 domain that interacts with CD40 and tested the ability of these peptides to target CD40 and prevent type 1 diabetes in NOD mice.

RESULTS

We identified a lead candidate, the 15-mer KGYY15 peptide, which specifically targets CD40-positive cells in a size- and sequence-dependent manner. It is highly efficient in preventing hyperglycaemia in NOD mice that spontaneously develop type 1 diabetes. Importantly, KGYY15 can also reverse new-onset hyperglycaemia. KGYY15 is well tolerated and functions to control the cytokine profile of culprit Th40 effector T cells. The KGYY15 peptide is 87% homologous to the human sequence, suggesting that it is an important candidate for translational studies.

CONCLUSIONS/INTERPRETATION

Peptide KGYY15 constitutes a viable therapeutic option to antibody therapy in targeting the CD40-CD154 interaction in type 1 diabetes. Given the involvement of CD40 in autoimmunity in general, it will also be important to evaluate KGYY15 in the treatment of other autoimmune diseases. This alternative therapeutic approach opens new avenues of exploration in targeting receptor-ligand interactions.

Pro-inflammatory T-lymphocytes rapidly infiltrate into the brain and contribute to neuronal injury following cardiac arrest and cardiopulmonary resuscitation

Although inflammatory mechanisms have been linked to neuronal injury following global cerebral ischemia, the presence of infiltrating peripheral immune cells remains understudied. We performed flow cytometry of single cell suspensions obtained from the brains of mice at varying time points after global cerebral ischemia induced by cardiac arrest and cardiopulmonary resuscitation (CA/CPR) to characterize the influx of lymphocytes into the injured brain. We observed that CA/CPR caused a large influx of lymphocytes within 3h of resuscitation that was maintained for the 3day duration of our experiments. Using cell staining flow cytometry we observed that the large majority of infiltrating lymphocytes were CD4(+) T cells. Intracellular stains revealed a large proportion of pro-inflammatory T cells expressing either TNFα or INFγ. Importantly, the lack of functional T cells in TCRα knockout mice reduced neuronal injury following CA/CPR, implicating pro-inflammatory T cells in the progression of ischemic neuronal injury. Finally, we made the remarkable observation that the novel CD4(+)CD40(+) (Th40) population of pro-inflammatory T cells that are strongly associated with autoimmunity are present in large numbers in the injured brain. These data indicate that studies investigating the neuro-immune response after global cerebral ischemia should consider the role of infiltrating T cells in orchestrating the acute and sustained immune response.

Receptor revision in CD4 T cells is influenced by follicular helper T cell formation and germinal-center interactions

Peripheral CD4 T cells in Vβ5 transgenic (Tg) C57BL/6J mice undergo tolerance to an endogenous superantigen encoded by mouse mammary tumor virus 8 (Mtv-8) by either deletion or T-cell receptor (TCR) revision. Revision is a process by which surface expression of the Vβ5(+) TCR is down-regulated in response to Mtv-8 and recombination activating genes are expressed to drive rearrangement of the endogenous TCRβ locus, effecting cell rescue through the expression of a newly generated, non-self-reactive TCR. In an effort to identify the microenvironment in which revision takes place, we show here that the proportion of T follicular helper cells (Tfh) and production of high-affinity antibody during a primary response are increased in Vβ5 Tg mice in an Mtv-8-dependent manner. Revising T cells have a Tfh-like surface phenotype and transcription factor profile, with elevated expression of B-cell leukemia/lymphoma 6 (Bcl-6), CXC chemokine receptor 5, programmed death-1, and other Tfh-associated markers. Efficient revision requires Bcl-6 and is inhibited by B lymphocyte-induced maturation protein-1. Revision completes less efficiently in the absence of signaling lymphocytic activation molecule-associated protein although initiation proceeds normally. These data indicate that Tfh formation is required for the initiation of revision and germinal-center interactions for its completion. The germinal center is known to provide a confined space in which B-cell antigen receptors undergo selection. Our data extend the impact of this selective microenvironment into the arena of T cells, suggesting that this fluid structure also provides a regulatory environment in which TCR revision can safely take place.

Defining a new biomarker for the autoimmune component of Multiple Sclerosis: Th40 cells

Multiple Sclerosis (MS) is a chronic inflammatory, neurodegenerative disease. Diagnosis is very difficult requiring defined symptoms and multiple CNS imaging. A complicating issue is that almost all symptoms are not disease specific for MS. Autoimmunity is evident, yet the only immune-related diagnostic tool is cerebral-spinal fluid examination for oligoclonal bands. This study addresses the impact of Th40 cells, a pathogenic effector subset of helper T cells, in MS. MS patients including relapsing/remitting MS, secondary progressive MS and primary progressive MS were examined for Th40 cell levels in peripheral blood and, similar to our findings in autoimmune type 1 diabetes, the levels were significantly (p<0.0001) elevated compared to controls including healthy non-autoimmune subjects and another non-autoimmune chronic disease. Classically identified Tregs were at levels equivalent to non-autoimmune controls but the Th40/Treg ratio still predicted autoimmunity. The cohort displayed a wide range of HLA haplotypes including the GWAS identified predictive HLA-DRB1*1501 (DR2). However half the subjects did not carry DR2 and regardless of HLA haplotype, Th40 cells were expanded during disease. In RRMS Th40 cells demonstrated a limited TCR clonality. Mechanistically, Th40 cells demonstrated a wide array of response to CNS associated self-antigens that was dependent upon HLA haplotype. Th40 cells were predominantly memory phenotype producing IL-17 and IFNγ with a significant portion producing both inflammatory cytokines simultaneously suggesting an intermediary between Th1 and Th17 phenotypes.

CD40 interacts directly with RAG1 and RAG2 in autoaggressive T cells and Fas prevents CD40-induced RAG expression

CD4(+) T cells expressing CD40 (Th40 cells) constitute a pathogenic T-cell subset that is necessary and sufficient to transfer autoimmune disease. We have previously demonstrated that CD40 signals peripheral Th40 cells to induce RAG1 and RAG2 expression, proteins necessary for the expression of T-cell receptor (TCR), leading to TCR revision. The dependency of TCR expression in the thymus on RAG proteins has long been known. However, despite numerous publications, there is controversy as to whether TCR expression can be altered in the periphery, post-thymic selective pressures. Therefore, a better understanding of TCR expression in primary peripheral cells is needed. We now show that the CD40 protein itself interacts with RAG1 and RAG2 as well as with Ku70 and translocates to the nucleus in Th40 cells. This indicates that the CD40 molecule is closely involved in the mechanism of TCR expression in the periphery. In addition, Fas signals act as a silencing mechanism for CD40-induced RAGs and prevent CD40 translocation to the nucleus. It will be important to further understand the involvement of CD40 in peripheral TCR expression and how TCR revision impacts auto-antigen recognition in order to effectively target and tolerize autoaggressive T cells in autoimmune disease.

An alternative role for Foxp3 as an effector T cell regulator controlled through CD40

The BDC2.5 T cell clone is highly diabetogenic, but the transgenic mouse generated from that clone is surprisingly slow in diabetes development. Although defining pathogenic effector T cells in autoimmunity has been inconsistent, CD4(+) cells expressing the CD40 receptor (Th40 cells) are highly diabetogenic in NOD mice, and NOD.BDC2.5.TCR.Tg mice possess large numbers of these cells. Given the importance of CD40 for pathogenic T cell development, BDC2.5.CD40(-/-) mice were created. Regulatory T cells, CD4(+)CD25(hi)Foxp3(+), develop normally, but pathogenic effector cells are severely reduced in number. Th40 cells from diabetic BDC2.5 mice rapidly induce diabetes in NOD.scid recipients, but Th40 cells from prediabetic mice transfer diabetes very slowly. Demonstrating an important paradigm shift, effector Th40 cells from prediabetic mice are Foxp3(+). As mice age, moving to type 1 diabetes development, Th40 cells lose Foxp3. When Th40 cells that are Foxp3(+) are transferred to NOD.scid recipients, disease is delayed. Th40 cells that are Foxp3(-) rapidly transfer disease. Th40 cells from BDC2.5.CD40(-/-) mice do not transfer disease nor do they lose Foxp3 expression. Mechanistically, Foxp3(+) cells produce IL-17 but do not produce IFN-γ, whereas Foxp3(-) Th40 cells produce IFN-γ and IL-2. This poses a new consideration for the function of Foxp3, as directly impacting effector T cell function.

The dynamic lives of T cells: new approaches and themes

Activated T cells have classically been thought to progress unidirectionally through discrete phenotypic states and differentiate into static lineages. It is increasingly evident, however, that T cells exhibit much more complex and flexible dynamic behaviors than initially appreciated, and that these behaviors influence the efficacy of T cell responses to immunological challenges. In this review, we discuss how new technologies for monitoring the dynamics of T cells are enhancing the resolution of the fine phenotypic and functional heterogeneity within populations of T cells and revealing how individual T cells transition among a continuum of states. Such insights into the dynamic properties of T cells should improve immune monitoring and inform strategies for therapeutic interventions.

Antigenic stimulation induces recombination activating gene 1 and terminal deoxynucleotidyl transferase expression in a murine T-cell hybridoma

Secondary rearrangements of the T cell receptor (TCR) represent a genetic correction mechanism which changes T cell specificity by re-activating V(D)J recombination in peripheral T cells. Murine T-cell hybridoma A1.1 was employed to investigate whether antigenic stimulation induced re-expression of recombinase genes and altered TCR Vβ expression. Following repeated antigenic stimulation, A1.1 cells were induced to re-express recombination activating gene (RAG)1 and terminal deoxynucleotidyl transferase (TdT) which are generally considered prerequisite to TCR gene rearrangement. Accompanied with the significant changes in TCR mRNA levels over time, it is suggested that secondary rearrangements may be induced in A1.1 cells, which represent a mature T cell clone capable of re-expressing RAG genes and possesses the prerequisite for secondary V(D)J rearrangement.

CD40 engagement of CD4+ CD40+ T cells in a neo-self antigen disease model ablates CTLA-4 expression and indirectly impacts tolerance

Biomarkers defining pathogenic effector T (Teff) cells slowly have been forthcoming and towards this we identified CD4(+) T cells that express CD40 (CD4(+) CD40(+) ) as pathogenic in the NOD type 1 diabetes (T1D) model. CD4(+) CD40(+) T cells rapidly and efficiently transfer T1D to NOD.scid recipients. To study the origin of CD4(+) CD40(+) T cells and disease pathogenesis, we employed a dual transgenic model expressing OVA(323-339) peptide as a neo-self antigen on islet β cells and medullary thymic epithelial cells (mTECs) and a transgenic TCR recognizing the OVA(323-339) peptide. CD4(+) CD40(+) T cells and Treg cells each recognizing the cognate neo-antigen, rather than being deleted through central tolerance, drastically expanded in the thymus. In pancreatic lymph nodes of DO11.RIPmOVA mice, CD4(+) CD40(+) T cells and Treg cells are expanded in number compared with DO11 mice and importantly, Treg cells remain functional throughout the disease process. When exposed to neo-self antigen, CD4(+) CD40(+) T cells do not express the auto-regulatory CTLA-4 molecule while naïve CD4(+) CD40(+) T cells do. DO11.RIPmOVA mice develop autoimmune-type diabetes. CD40 engagement has been shown to prevent CTLA-4 expression and injecting anti-CD40 in DO11.RIPmOVA mice significantly exacerbates disease. These data suggest a unique means by which CD4(+) CD40(+) T cells thwart tolerance.

Modulation of TCRβ surface expression during TCR revision

TCR revision is a tolerance mechanism by which self-reactive TCRs expressed by mature CD4(+) peripheral T cells are replaced by receptors encoded by genes generated by post-thymic DNA rearrangement. The downmodulation of surface TCR expression initiates TCR revision, and serves as a likely trigger for the induction of the recombinase machinery. We show here in a Vβ5 transgenic mouse model system that downregulation of the self-reactive transgene-encoded TCR is not maintained by transgene loss or diminished transcription or translation. The downregulation of surface TCR expression likely occurs in two stages, only one of which requires tolerogen expression.

TCR transfer induces TCR-mediated tonic inhibition of RAG genes in human T cells

Induction of the TCR signaling pathway terminates the expression of RAG genes, and a link between this pathway and their transcriptional control is evident from the recent demonstration of their re-expression if the TCR is subsequently lost or down-regulated. Since unstimulated T cells display a steady-state level of "tonic" TCR signaling, i.e. in the absence of any antigenic stimulus, it was uncertain whether this control was exerted through ligand-dependent or ligand-independent TCR signaling. Here we demonstrate for the first time that exogenous TCR α and β chains transferred into the human immature RAG(+) T cell line Sup-T1 by lentiviral transduction inhibit RAG expression through tonic signaling, and that this inhibition could itself be reverted by pharmacological tonic pathway inhibitors. We also suggest that mature T cells already expressing an endogenous TCR on their surface maintain some levels of plasticity at the RAG locus when their basal TCR signaling is interfered with. Lastly, we show that the TCR constructs employed in TCR gene therapy do not possess the same basal signaling transduction capability, a feature that may have therapeutic implications.

TCR revision generates functional CD4+ T cells

CD4(+)Vβ5(+) peripheral T cells in C57BL/6 mice respond to encounter with a peripherally expressed endogenous superantigen by undergoing either deletion or TCR revision. In this latter process, cells lose surface Vβ5 expression and undergo RAG-dependent rearrangement of endogenous TCRβ genes, driving surface expression of novel TCRs. Although postrevision CD4(+)Vβ5(-)TCRβ(+) T cells accumulate with age in Vβ5 transgenic mice and bear a diverse TCR Vβ repertoire, it is unknown whether they respond to homeostatic and antigenic stimuli and thus may benefit the host. We demonstrate in this study that postrevision cells are functional. These cells have a high rate of steady-state homeostatic proliferation in situ, and they undergo extensive MHC class II-dependent lymphopenia-induced proliferation. Importantly, postrevision cells do not proliferate in response to the tolerizing superantigen, implicating TCR revision as a mechanism of tolerance induction and demonstrating that TCR-dependent activation of postrevision cells is not driven by the transgene-encoded receptor. Postrevision cells proliferate extensively to commensal bacterial Ags and can generate I-A(b)-restricted responses to Ag by producing IFN-γ following Listeria monocytogenes challenge. These data show that rescued postrevision T cells are responsive to homeostatic signals and recognize self- and foreign peptides in the context of self-MHC and are thus useful to the host.

CD40 glycoforms and TNF-receptors 1 and 2 in the formation of CD40 receptor(s) in autoimmunity

The CD40-CD154 dyad is an intensely studied field as is glycosylation status and both impact immunological functions and autoimmune conditions. CD40 has several isoforms, is modified by glycosylation, and trimerizes to form the functional receptor. We described a CD4(+)CD40(+) T cell (Th40) subset which is expanded in autoimmunity and is necessary and sufficient in transferring type 1 diabetes. Glycosylation impacts immunological events and T cells from autoimmune mouse strains express 30-40% less GlcNAc-branched N-glycans than T cells from non-autoimmune strains, a decrease known to activate T cells. Here we demonstrate that several CD40 receptor constellations exist on CD4 T cells. However, rather than containing different isoforms of CD40 they contain different glycoforms of isoform I. The glycoform profile is dependent on availability of CD154 and autoimmune NOD mice express a high level of a less glycosylated form. Interestingly, CD40 stimulation induces some CD40 receptor constellations that contain TNF-receptors 1 and 2 and targeting of those alters CD40 signaling outcomes in NOD Th40 cells. CD40-stimulation in vivo of non-autoimmune BALB/c mice expands the Th40 population and alters the CD40 glycoform profile of those cells to appear more like that of autoimmune prone NOD mice. Further understanding the dynamics and composition of the different CD40 receptor constellations will provide important insights into treatment options in autoimmunity.

T-cell receptor revision: friend or foe?

T-cell receptor (TCR) revision is a process of tolerance induction by which peripheral T cells lose surface expression of an autoreactive TCR, reinduce expression of the recombinase machinery, rearrange genes encoding extrathymically generated TCRs for antigen, and express these new receptors on the cell surface. We discuss the evidence for this controversial tolerance mechanism below. Despite the apparent heresy of post-thymic gene rearrangement, we argue here that TCR revision follows the rules obeyed by maturing thymocytes undergoing gene recombination. Expression of the recombinase is carefully controlled both spatially and temporally, and may be initiated by loss of signals through surface TCRs. The resulting TCR repertoire is characterized by its diversity, self major histocompatibility complex restriction, self tolerance, and ability to mount productive immune responses specific for foreign antigens. Hence, TCR revision is a carefully regulated process of tolerance induction that can contribute to the protection of the individual against invading pathogens while preserving the integrity of self tissue.

Functional roles for T cell CD40 in infection and autoimmune disease: the role of CD40 in lymphocyte homeostasis

CD40 stimulation on monocytes/macrophages, dendritic cells, and B-lymphocytes has been the subject of much study. It is well recognized that activation of CD40 on antigen presenting cells by its ligand, CD154, expressed on T-lymphocytes, contributes to the pro-inflammatory response necessary for eradication of infection, yet pathological in autoimmunity. However, there is evidence that CD40 is also expressed on T-lymphocytes and can act as a costimulatory molecule. While the exact role of CD40 on CD8 T cells remains controversial, it does appear to contribute to the adaptive immune response against infection. CD40 on CD4 T cells, on the other hand, plays a functional role in the autoimmune disease process. Further dissection of the exact nature and role of CD40 in T cell activation could lead the way to more effective vaccines and novel therapeutics for autoimmune diseases.

CD40 on NOD CD4 T cells contributes to their activation and pathogenicity

Our goals in this study were to investigate conditions under which T cells from NOD mice express CD40 and to determine how CD40 on autoreactive CD4 T cells contributes to their pathogenicity in T1D. Using CD40-positive diabetogenic T cell clones and CD4 T cells from NOD mice, we examined expression of CD40 upon activation through the TCR and costimulation through either CD28 or CD40. Our results indicate that CD40 expression is increased upon activation with antigen/MHC and that activation of NOD CD4 T cells through TCR/CD40 rapidly induced CD40 expression. Furthermore, CD40 costimulation promoted T cell proliferation to the same extent as costimulation through TCR/CD28. Importantly, costimulation of CD4 T cells through CD40 also interfered with T cell homeostasis by altering regulation of CTLA-4 expression. Through CD40-CD154 blocking studies, we demonstrated that signaling between T cells through CD40 and its ligand contributes to activation of pathogenic T cells and that blocking CD40 on T cells abrogates their ability to transfer diabetes. Thus, costimulation through CD40 on NOD T cells contributes to their pathogenicity by providing additional pathways for activation and by inhibiting upregulation of CTLA-4 during T cell activation.

Disruption of the homeostatic balance between autoaggressive (CD4+CD40+) and regulatory (CD4+CD25+FoxP3+) T cells promotes diabetes

Although regulatory T cells (Tregs) are well described, identifying autoaggressive effector T cells has proven more difficult. However, we identified CD4loCD40+ (Th40) cells as being necessary and sufficient for diabetes in the NOD mouse model. Importantly, these cells are present in pancreata of prediabetic and diabetic NOD mice, and Th40 cells but not CD4+CD40(-) T cells transfer progressive insulitis and diabetes to NOD.scid recipients. Nonobese-resistant (NOR) mice have the identical T cell developmental background as NOD mice, yet they are diabetes-resistant. The seminal issue is how NOR mice remain tolerant to diabetogenic self-antigens. We show here that autoaggressive T cells develop in NOR mice and are confined to the Th40 subset. However, NOR mice maintain Treg numbers equivalent to their Th40 numbers. NOD mice have statistically equal numbers of CD4+CD25+forkhead box P3+intrinsic Tregs compared with NOR or nonautoimmune BALB/c mice, and NOD Tregs are equally as suppressive as NOR Tregs. A critical difference is that NOD mice develop expanded numbers of Th40 cells. We suggest that a determinant factor for autoimmunity includes the Th40:Treg ratio. Mechanistically, NOD Th40 cells have low susceptibility to Fas-induced cell death and unlike cells from NOR and BALB/c mice, have predominantly low Fas expression. CD40 engagement of Th40 cells induces Fas expression but further confers resistance to Fas-mediated cell death in NOD mice. A second fundamental difference is that NOD Th40 cells undergo much more rapid homeostatic expansion than Th40 cells from NOR mice.

High distribution of CD40 and TRAF2 in Th40 T cell rafts leads to preferential survival of this auto-aggressive population in autoimmunity

Background

CD40–CD154 interactions have proven critical in autoimmunity, with the identification of CD4^loCD40⁺ T cells (Th40 cells) as harboring an autoaggressive T cell population shedding new insights into those disease processes. Th40 cells are present at contained levels in non-autoimmune individuals but are significantly expanded in autoimmunity. Th40 cells are necessary and sufficient in transferring type 1 diabetes in mouse models. However, little is known about CD40 signaling in T cells and whether there are differences in that signaling and subsequent outcome depending on disease conditions. When CD40 is engaged, CD40 and TNF-receptor associated factors, TRAFs, become associated with lipid raft microdomains. Dysregulation of T cell homeostasis is emerging as a major contributor to autoimmune disease and thwarted apoptosis is key in breaking homeostasis.

Methodology/Principal Findings

Cells were sorted into CD4^hi and CD4^lo (Th40 cells) then treated and assayed either as whole or fractionated cell lysates. Protein expression was assayed by western blot and Nf-κB DNA-binding activity by electrophoretic mobility shifts. We demonstrate here that autoimmune NOD Th40 cells have drastically exaggerated expression of CD40 on a per-cell-basis compared to non-autoimmune BALB/c. Immediately ex-vivo, untreated Th40 cells from NOD mice have high levels of CD40 and TRAF2 associated with the raft microdomain while Th40 cells from NOR and BALB/c mice do not. CD40 engagement of Th40 cells induces Nf-κB DNA-binding activity and anti-apoptotic Bcl-X_L expression in all three mouse strains. However, only in NOD Th40 cells is anti-apoptotic cFLIP_p43 induced which leads to preferential survival and proliferation. Importantly, CD40 engagement rescues NOD Th40 cells from Fas-induced death.

Conclusions/Significance

CD40 may act as a switch between life and death promoting signals and NOD Th40 cells are poised for survival via this switch. This may explain how they expand in autoimmunity to thwart T cell homeostasis.

Cutting edge: TCR revision affects predominantly Foxp3 cells and skews them toward the Th17 lineage

CD4(+) T cells respond to peripheral endogenous superantigen stimulation by undergoing deletion or TCR revision. The latter involves RAG re-expression, TCR gene rearrangement, and expression of a novel TCR. TCR-revised T cells are functional and express a diverse TCR repertoire. Because TCR revision harbors the potential to create self-reactivity, it is important to explore whether T cells known to be self-reactive (regulatory T cells) or those involved in autoimmunity (Th17 cells) arise from TCR revision. Interestingly, we observed that Foxp3(+) cells are excluded from revising their TCR and that only a small fraction of postrevision cells expresses Foxp3. In contrast, Th17 cells are 20 times more frequent among revised than among C57BL/6 CD4(+) T cells, indicating that postrevision cells are biased toward the Th17 lineage. The link between Th17 differentiation and TCR revision might be highly relevant to the role of Th17 cells in promoting autoimmunity.

Extrathymic rearrangement of alphabetaT-lymphocyte antigen receptor genes during pregnancy

The existence of alphabetaT-lymphocyte differentiation processes have been demonstrated in mouse peripheral lymphoid organs during pregnancy. Study of pregnant Swiss mice has shown that the development of the second half of gestation is accompanied by expression of RAG-1 recombinase mRNA and unrearranged TCR alpha-chain (pre-TCRalpha) preferentially in T-lymphocytes of lymph nodes involved in uterine drainage (para-aortal lymph nodes), and to a lesser extent in other lymph nodes (mainly from axillary lymph nodes). The data suggest that during pregnancy the differentiation of alphabetaT lymphocytes may occur not only in central (thymus) but also in peripheral lymphoid organs.

Age-dependent TCR revision mediated by interaction between alphabeta TCR and self-antigens

Interactions between TCR and self-peptide/MHC complex play an important role in homeostasis and Ag reactivity of mature peripheral T cells. In this report, we demonstrate that the interactions between mature peripheral T cells and endogenous Ags have a negative impact on the maintenance of foreign Ag-specific T cells in an age-dependent manner. This is mediated by RAG-dependent secondary rearrangement of the TCR alpha-chain (receptor revision). The TCR revision in mature T cells is readily observed in mouse expressing transgenic TCR alpha-chain inserted into the physiological locus (knockin mouse) but not in conventional transgenic mouse with an identical TCR alpha-chain. Thus, our results suggest that under physiological conditions in which all TCR alpha-chains are susceptible to deletion by secondary rearrangement, TCR revision in mature peripheral T cells is an ongoing process in adult animals and contributes to age-dependent changes in T cell function and repertoire.

An in vitro model of T cell receptor revision in mature human CD8+ T cells

V(D)J recombination is a mechanism peculiar to the somatic rearrangement of antigen receptor genes. It requires both expression of the RAG-1 and RAG-2 recombinases and accessibility of the substrate to its recombinase and post-cleavage/DNA repair stage. TCR revision is a genetic correction mechanism that changes T cell specificity by re-activating V(D)J recombination in peripheral T cells. This process is now well described in both normal or pathological murine and human settings. Many of its features, such as the question of whether it occurs in truly mature T cells, remain to be elucidated. Its occurrence in human CD8+ T cells is also an open question. We have therefore established an in vitro model of TCR revision in mature human CD8+ T cells to determine whether down-regulation of the TCR/CD3 complex from the cell surface in the presence of IL7 as a factor favouring chromatin remodelling initiates a TCR revision pathway. Only mature CD8+ T cells carrying already-formed antigen receptors were used. CD8+ T cells treated with anti-CD3 and IL7 showed rearrangement intermediates and expressed new Vbeta-chains on their surface. Investigation of the molecular pathway thus induced disclosed up-regulation of the RAG-2 transcript, but absence of the 'canonical' RAG-1 mRNA. A surprising finding was the demonstration of alternative splice forms of this mRNA, already expressed in untreated CD8+ T cells, encoding for the full-length RAG-1 protein, which was increased three-fold in the treated cells. All the V(D)J requirements were thus fulfilled when mature human CD8+ T cells were stimulated with anti-CD3 and IL7. Induction of TCR revision in vitro in mature T cells is an easily controllable system that could be employed in further studies to elucidate the molecular pathways involved in secondary V(D)J rearrangements in peripheral cells.

A unique T cell subset described as CD4loCD40+ T cells (TCD40) in human type 1 diabetes

Human T1D pancreatic lymph nodes contain diabetes-autoantigen responsive T cells but identification of such T cells in the periphery has proven difficult. Here we describe a unique T cell subset defined by CD4(lo) and CD40 expression (T(CD40)) that is significantly expanded in peripheral blood of T1D but not control or T2D subjects. The HLA-DR3 and DR4 alleles are considered high risk factors for T1D and T(CD40) expansion occurs in T1D subjects carrying HLA DR3 or DR4 haplotypes but, T1D subjects who do not carry either DR3 or DR4 haplotypes still have an expanded percentage of T(CD40) cells. Non-autoimmune subjects, even DR3(+) and DR4(+), do not have elevated percentages of T(CD40) cells. The majority of T(CD40) cells in T1D carry a memory phenotype and a portion of those proliferates when exposed to diabetes-associated self-antigens. A greater number of memory T(CD40) cells express CXCR3 when compared to CD40(-) memory cells and that number is significantly expanded in T1D compared to control subjects. If only total CD4(+) T cells are compared no difference in CXCR3 is seen. Furthermore, T(CD40) cells produce a Th1, pro-inflammatory cytokine profile. In healthy controls, T(CD40) cells have equally Th1 and Th2 profiles.

A costimulatory function for T cell CD40

CD40 plays a significant role in the pathogenesis of inflammation and autoimmunity. B cell CD40 directly activates cells, which can result in autoantibody production. T cells can also express CD40, with an increased frequency and amount of expression seen in CD4(+) T lymphocytes of autoimmune mice, including T cells from mice with collagen-induced arthritis. However, the mechanisms of T cell CD40 function have not been clearly defined. To test the hypothesis that CD40 can serve as a costimulatory molecule on T lymphocytes, CD40(+) T cells from collagen-induced arthritis mice were examined in parallel with mouse and human T cell lines transfected with CD40. CD40 served as effectively as CD28 in costimulating TCR-mediated activation, including induction of kinase and transcription factor activities and production of cytokines. An additional enhancement was seen when both CD40 and CD28 signals were combined with AgR stimulation. These findings reveal potent biologic functions for T cell CD40 and suggest an additional means for amplification of autoimmune responses.

The DDE recombinases: diverse roles in acquired and innate immunity

BACKGROUND

The RAG proteins required for V(D)J recombination of immunoglobulin and T-cell receptor genes in the acquired immune response contain a magnesium ion-binding site termed a DDE site, composed of D (aspartic acid) and E (glutamic acid) amino acids. A similar DDE-like magnesium binding site also is present in transposases, retroviral integrases, and the innate antiviral response enzymes RNAse H and RNA-induced silencing complex (RISC).

OBJECTIVE

To help clinicians understand immunodeficiency that results from deficiencies of RAG protein functions, such as severe combined immunodeficiency disorders, Omenn syndrome, and ataxia telangiectasia, and to be familiar with the diverse roles of other DDE enzymes.

METHODS

Literature published in peer-reviewed journals during the past 2 decades that identified and characterized DDE enzymes, including RAG proteins, RISC and RNA silencing, RNAse H, retroviral integrases, transposases, and a putative DDE recombinase required for herpes virus replication, was selectively reviewed and summarized by the author.

RESULTS

DDE enzymes play a critical role in acquired immunity through RAG-mediated immunoglobulin and T-cell receptor V(D)J recombination in innate immunity through RISC and RNAse H. Paradoxically, DDE enzymes are critical components of pathogen-specific enzymes such as retroviral integrase and other pathogen-encoded proteins.

CONCLUSION

Because of their critical role in acquired and innate immunity, the DDE recombinases are attractive targets for novel pharmacologic therapies. Currently, retroviral integrase inhibitors in clinical trial for human immunodeficiency virus infection appear to be safe and effective and could provide a paradigm for inactivating DDE sites in other viral pathogens, as well as RAG and RISC.

Revision of the antigen receptor of T-lymphocytes

This review considers a crucially new mechanism of T-cell antigen-recognizing repertoire formation. It includes the revision of T-cell antigen receptor (TCR), which implies the secondary rearrangement of TCR genes in peripheral T-lymphocytes and surface expression of a new antigen receptor with altered specificity. Factors and mechanisms involved in the induction of this process have been analyzed. Certain attention is paid to a possible role of TCR revision in the formation of peripheral tolerance in the processes of "avidity maturation" of T-lymphocytes during immune response and also negative consequences related to appearance of potentially autoreactive clones in the periphery.

Re-shaping the T cell repertoire: TCR editing and TCR revision for good and for bad

Protection against the universe of pathogens requires a functional, diverse T cell repertoire. However, the price that is paid for an evolved, effective immune system includes the potential danger of generating autoaggressive T cells. Autoimmune diseases result from inherent breach of tolerance to self-antigens leading to disruption of the regulatory to autoaggressive T cell homeostatic balance. The immune system has evolved mechanisms to control those processes. For T cells, positive and negative selection in the thymus assures that only fully functional, non-self-reactive T cells will populate the periphery. Failure of this central tolerance would result in autoaggressive T cells escaping into the periphery. However, other means of escaping negative selection can occur in the periphery, i.e., TCR revision, or the altering of TCR expression after thymic egress. Here the potential benefits, i.e., expansion and re-shaping of the T cell repertoire as potentiated by TCR editing and revision are considered. Furthermore, the potential to develop autoaggressive TCR and thus enhance autoimmunity is considered.

PKB Rescues Calcineurin/NFAT-Induced Arrest of Rag Expression and Pre-T Cell Differentiation

Protein kinase B (PKB), an Ag receptor activated serine-threonine kinase, controls various cellular processes including proliferation and survival. However, PKB function in thymocyte development is still unclear. We report PKB as an important negative regulator of the calcineurin (CN)-regulated transcription factor NFAT in early T cell differentiation. Expression of a hyperactive version of CN induces a profound block at the CD25+CD44- double-negative (DN) 3 stage of T cell development. We correlate this arrest with up-regulation of Bcl-2, CD2, CD5, and CD27 proteins and constitutive activation of NFAT but a severe impairment of Rag1, Rag2, and intracellular TCR-beta as well as intracellular TCR-gammadelta protein expression. Intriguingly, simultaneous expression of active myristoylated PKB inhibits nuclear NFAT activity, restores Rag activity, and enables DN3 cells to undergo normal differentiation and expansion. A correlation between the loss of NFAT activity and Rag1 and Rag2 expression is also found in myristoylated PKB-induced CD4+ lymphoma cells. Furthermore, ectopic expression of NFAT inhibits Rag2 promoter activity in EL4 cells, and in vivo binding of NFATc1 to the Rag1 and Rag2 promoter and cis-acting transcription regulatory elements is verified by chromatin immunoprecipitation analysis. The regulation of CN/NFAT signaling by PKB may thus control receptor regulated changes in Rag expression and constitute a signaling pathway important for differentiation processes in the thymus and periphery.

A role for STAT5A/B in protection of peripheral T-lymphocytes from postactivation apoptosis: Insights from gene expression profiling

Activation of the transcription factors STAT5A and STAT5B by JAK1 and JAK3 tyrosine kinases is a key event in downstream signaling of gammac (common gamma chain)-family cytokine receptors in lymphoid cells. STAT5A/B-deficiency in mice causes, among other consequences, a reduced size and altered composition of the peripheral T-cell pool with predominance of an activated or memory-like population (CD4(+)/CD44(high)/CD62L(low)) and a proliferative deficiency following antigenic stimulation and subsequent IL-2 treatment. To further elucidate the critical function of STAT5A/B in homeostasis and activation of murine naïve peripheral T-lymphocytes, we have analyzed global gene expression of STAT5A/B-deficient versus wild-type splenic T-cells by profiling with high-density oligonucleotide arrays (Affymetrix). Comparison of (1) basal gene expression of untreated peripheral STAT5A/B-deficient and control T-cells and (2) immediate early gene induction upon in vitro stimulation of either population with IL-2 has revealed differential expression of a broad range of genes potentially contributing to the defects of STAT5A/B deficient T-cells. In the context of enhanced apoptotic rates of STAT5A/B(-/-)-T-cells in vivo and upon TCR-stimulation in culture our data suggest a role for STAT5 in post-activation survival beyond regulation of antiapoptotic Bcl-2 proteins and hence provide new insights into the nature of the late proliferative block in the T-cell compartment caused by STAT5-deficiency.

Different Diabetogenic Potential of Autoaggressive CD8+ Clones Associated with IFN-γ-Inducible Protein 10 (CXC Chemokine Ligand 10) Production but Not Cytokine Expression, Cytolytic Activity, or Homing Characteristics

Type 1 diabetes mellitus is an autoimmune disease characterized by T cell-mediated destruction of the insulin-producing beta cells in the islets of Langerhans. From studies in animal models, CD8(+) T cells recognizing autoantigens such as islet-specific glucose-6-phosphatase catalytic subunit-related protein, insulin, or glutamic acid decarboxylase (GAD) are believed to play important roles in both the early and late phases of beta cell destruction. In this study, we investigated the factors governing the diabetogenic potential of autoreactive CD8(+) clones isolated from spleens of NOD mice that had been immunized with GAD65(515-524) or insulin B-chain(15-23) peptides. Although these two clones were identical in most phenotypic and functional aspects, for example cytokine production and killing of autologous beta cells, they differed in the expression of IFN-gamma-inducible protein-10, which was only produced at high levels by the insulin-specific clone, but not by the GAD65-specific clone, and other autoantigen-specific nonpathogenic CD8 T cell clones. Interestingly, upon i.p. injection into neonatal mice, only the insulin B-chain(15-23)-reactive CD8(+) T clone accelerated diabetes in all recipients after 4 wk, although both insulin- and GAD-reactive clones homed to pancreas and pancreatic lymph nodes with similar kinetics. Diabetes was associated with increased pancreatic T cell infiltration and, in particular, recruitment of macrophages. Thus, secretion of IFN-gamma-inducible protein-10 by autoaggressive CD8(+) lymphocytes might determine their diabetogenic capacity by affecting recruitment of cells to the insulitic lesion.

Cutting edge: TCR revision occurs in germinal centers

Mouse CD4(+)Vbeta5(+) T cells recognize a peripherally expressed superantigen encoded by an endogenous retrovirus. Ag encounter tolerizes the mature CD4 T cell compartment, either by deletion of autoreactive cells or by TCR revision. This latter process is driven by TCRbeta rearrangement through RAG activity and results in the rescue of cells expressing novel TCRs that no longer recognize the tolerogen. Consistent with the notion that revising T cells represent a distinct peripheral T cell population, we now show that these lymphocyte blasts express a hybrid effector/memory phenotype and are not undergoing cell division. A population of revising T cells is CD40(+), expresses the germinal center (GC) marker CXCR5, and is Vbeta5(low)Thy-1(low). Histology reveals that, consistent with their surface Ag phenotype, T cells undergoing TCR revision are enriched in splenic GCs. These data demonstrate that TCR revision is a multistep tolerance pathway supported by the unique microenvironment provided by GCs.

Multiple T-cell clones specific for the same foreign pMHC ligand can be generated from a single, ancestral TCR-VDJbeta precursor

Owing to ordered, stage-specific T-cell receptor (TCR) gene rearrangements and cell division during T-cell development, small cohorts of "half-sibling" T cells sharing an ancestral TCR VDJbeta rearrangement but expressing different TCR alpha-locus rearrangements may be selected into the mature T-cell repertoire. We wondered whether different alphabetaTCRs expressed by T cells from the same ancestral VDJbeta cohort might be capable of recognizing the same foreign peptide-major histocompatibility complex complex (pMHC). By a combined flow cytometric and single-cell polymerase chain reaction (PCR) approach to analyze TCRs selected by the previously defined foreign antigen, pCW3170-179/H-2Kd, we were able to identify cohorts of half-sibling antigen-specific CD8 T cells after their expansion in immunized mice. We amplified residual DJbeta rearrangements as clonal markers to confirm that the shared VDJbeta sequences represent ancestral rearrangements rather than identical but independent ones. An intriguing explanation of our findings would be that only a very limited repertoire of TCR alpha-chains is selected to pair with a given TCR beta-chain during T-cell development.

Receptor revision in T cells: an open question?

During lymphocyte development, both B and T cells assemble antigen receptor variable region genes from germline gene segments, allowing the expression of unique receptors in each clonally derived lymphocyte. Previously, it was shown that in certain cases, progenitor and immature B cells are capable of editing their receptors to a new specificity on encounter with self-antigens. Although the existence of such a process in T cells remains controversial, recent studies suggest that mature T cells are able to similarly revise their receptors in the periphery.