T cell repertoire diversity is required for relapses in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis

T Cell Repertoire During Ontogeny and Characteristics in Inflammatory Disorders in Adults and Childhood

Since the first day of life, a newborn has to deal with various pathogens from the environment. While passive immune protection is provided by diaplacental maternal antibodies, the development of cellular immunity is ongoing. A mature immune system should be able not only to defend against pathogens, but should also be able to differentiate between self- and non-self-antigens. Dysregulation in the development of cellular immunity can lead to severe disorders like immunodeficiency, autoimmunity and chronic inflammation. In this review, we explain the role of T cell immunity in antigen detection and summarize the characteristics of a mature TCR repertoire as well as the current state of knowledge about the development of the TCR repertoire in ontogenesis. In addition, methods of assessments are outlined, with a focus on the advantages and disadvantages of advanced methods such as next generation sequencing. Subsequently, we provide an overview of various disorders occuring in early childhood like immunodeficiencies, autoimmunity, allergic diseases and chronic infections and outline known changes in the TCR repertoire. Finally, we summarize the latest findings and discuss current research gaps as well as potential future developments.

T-cell repertoire profiling by next-generation sequencing reveals tissue migration dynamics of TRBV13-family clonotypes in a common experimental autoimmune encephalomyelitis mouse model

The experimental autoimmune encephalomyelitis (EAE) model is indispensable for autoimmunity research, but model-specific T cell dynamics are sparsely studied. We used next-generation immunosequencing across lymphoid organs, blood and spinal cord in response to immunization with myelin basic protein (MBP) to study T cell repertoires and migration patterns. Surprisingly, most spinal cord T cells were unique to the individual animal despite the existence of shared MBP-specific clones, suggesting a previously underestimated T cell diversity. Almost complete emigration of pathogenic clones from blood to spinal cord indicates that blood is not a suitable compartment to study EAE-mediating T cells.

Distinct inflammatory phenotypes of microglia and monocyte-derived macrophages in Alzheimer's disease models: effects of aging and amyloid pathology

Alzheimer's disease (AD) is a neurodegenerative disease characterized by formation of amyloid‐β (Aβ) plaques, activated microglia, and neuronal cell death leading to progressive dementia. Recent data indicate that microglia and monocyte‐derived macrophages (MDM) are key players in the initiation and progression of AD, yet their respective roles remain to be clarified. As AD occurs mostly in the elderly and aging impairs myeloid functions, we addressed the inflammatory profile of microglia and MDM during aging in TgAPP/PS1 and TgAPP/PS1dE9, two transgenic AD mouse models, compared to WT littermates. We only found MDM infiltration in very aged mice. We determined that MDM highly expressed activation markers at basal state. In contrast, microglia exhibited an activated phenotype only with normal aging and Aβ pathology. Our study showed that CD14 and CD36, two receptors involved in phagocytosis, were upregulated during Aβ pathogenesis. Moreover, we observed, at the protein levels in AD models, higher production of pro‐inflammatory mediators: IL‐1β, p40, iNOS, CCL‐3, CCL‐4, and CXCL‐1. Taken together, our data indicate that microglia and MDM display distinct phenotypes in AD models and highlight the specific effects of normal aging vs Aβ peptides on inflammatory processes that occur during the disease progression. These precise phenotypes of different subpopulations of myeloid cells in normal and pathologic conditions may allow the design of pertinent therapeutic strategy for AD.

Hippocampal T cell infiltration promotes neuroinflammation and cognitive decline in a mouse model of tauopathy

Alzheimer's disease is characterized by the combined presence of amyloid plaques and tau pathology, the latter being correlated with the progression of clinical symptoms. Neuroinflammatory changes are thought to be major contributors to Alzheimer's disease pathophysiology, even if their precise role still remains largely debated. Notably, to what extent immune responses contribute to cognitive impairments promoted by tau pathology remains poorly understood. To address this question, we took advantage of the THY-Tau22 mouse model that progressively develops hippocampal tau pathology paralleling cognitive deficits and reappraised the interrelationship between tau pathology and brain immune responses. In addition to conventional astroglial and microglial responses, we identified a CD8-positive T cell infiltration in the hippocampus of tau transgenic mice associated with an early chemokine response, notably involving CCL3. Interestingly, CD8-positive lymphocyte infiltration was also observed in the cortex of patients exhibiting frontemporal dementia with P301L tau mutation. To gain insights into the functional involvement of T cell infiltration in the pathophysiological development of tauopathy in THY-Tau22 mice, we chronically depleted T cells using anti-CD3 antibody. Such anti-CD3 treatment prevented hippocampal T cell infiltration in tau transgenic animals and reverted spatial memory deficits, in absence of tau pathology modulation. Altogether, these data support an instrumental role of hippocampal T cell infiltration in tau-driven pathophysiology and cognitive impairments in Alzheimer's disease and other tauopathies.

Bispecificity for myelin and neuronal self-antigens is a common feature of CD4 T cells in C57BL/6 mice

The recognition of multiple ligands by a single TCR is an intrinsic feature of T cell biology, with important consequences for physiological and pathological processes. Polyspecific T cells targeting distinct self-antigens have been identified in healthy individuals as well as in the context of autoimmunity. We have previously shown that the 2D2 TCR recognizes the myelin oligodendrocyte glycoprotein epitope (MOG)35-55 as well as an epitope within the axonal protein neurofilament medium (NF-M15-35) in H-2(b) mice. In this study, we assess whether this cross-reactivity is a common feature of the MOG35-55-specific T cell response. To this end, we analyzed the CD4 T cell response of MOG35-55-immunized C57BL/6 mice for cross-reactivity with NF-M15-35. Using Ag recall responses, we established that an important proportion of MOG35-55-specific CD4 T cells also responded to NF-M15-35 in all mice tested. To study the clonality of this response, we analyzed 22 MOG35-55-specific T cell hybridomas expressing distinct TCR. Seven hybridomas were found to cross-react with NF-M15-35. Using an alanine scan of NF-M18-30 and an in silico predictive model, we dissected the molecular basis of cross-reactivity between MOG35-55 and NF-M15-35. We established that NF-M F24, R26, and V27 proved important TCR contacts. Strikingly, the identified TCR contacts are conserved within MOG38-50. Our data indicate that due to linear sequence homology, part of the MOG35-55-specific T cell repertoire of all C57BL/6 mice also recognizes NF-M15-35, with potential implications for CNS autoimmunity.

Targeting T cells responsive to the priming epitope prevent the relapsing phase of experimental autoimmune encephalomyelitis

Upon recovery from the initial episode of experimental autoimmune encephalomyelitis (EAE), virtually all SJL mice develop relapsing/remitting episodes of disease. These relapses may occur due to the reactivation of memory T cells initially stimulated as part of the disease-inducing protocol or naïve T-cell populations stimulated by distinct encephalitogens derived from the inflammatory disease process (epitope spread). We have used encephalitogen-specific non-linear peptide octamers to modify the course of relapsing EAE (rEAE) in SJL mice immunized with an oliogodendrocyte-specific protein peptide (OSP 55-71). Our studies show that the peptide-octamers, which target the T cells stimulated by the priming encephalitogen, but not other candidate encephalitogens, prevent rEAE.

CD24 on thymic APCs regulates negative selection of myelin antigen-specific T lymphocytes

Negative selection plays a key role in the clonal deletion of autoreactive T cells in the thymus. However, negative selection is incomplete; as high numbers of autoreactive T cells can be detected in normal individuals, mechanisms that regulate negative selection must exist. In this regard, we previously reported that CD24, a GPI-anchored glycoprotein, is required for thymic generation of autoreactive T lymphocytes. The CD24-deficient 2D2 TCR transgenic mice (2D2(+) CD24(-/-) ), whose TCR recognizes myelin oligodendrocyte glycoprotein (MOG), fail to generate functional 2D2 T cells. However, it was unclear if CD24 regulated negative selection, and if so, what cellular mechanisms were involved. Here, we show that elimination of MOG or Aire gene expression in 2D2(+) CD24(-/-) mice - through the creation of 2D2(+) CD24(-/-) MOG(-/-) or 2D2(+) CD24(/) ∼Aire(-/-) mice - completely restores thymic cellularity and function of 2D2 T cells. Restoration of CD24 expression on DCs, but not on thymocytes also partially restores 2D2 T-cell generation in 2D2(+) CD24(-/-) mice. Taken together, we propose that CD24 expression on thymic antigen-presenting cells (mTECs, DCs) down-regulates autoantigen-mediated clonal deletion of autoreactive thymocytes.

Autoimmune regulator (AIRE)-deficient CD8+CD28low regulatory T lymphocytes fail to control experimental colitis

Mutations in the gene encoding the transcription factor autoimmune regulator (AIRE) are responsible for autoimmune polyendocrinopathy candidiasis ectodermal dystrophy syndrome. AIRE directs expression of tissue-restricted antigens in the thymic medulla and in lymph node stromal cells and thereby substantially contributes to induction of immunological tolerance to self-antigens. Data from experimental mouse models showed that AIRE deficiency leads to impaired deletion of autospecific T-cell precursors. However, a potential role for AIRE in the function of regulatory T-cell populations, which are known to play a central role in prevention of immunopathology, has remained elusive. Regulatory T cells of CD8(+)CD28(low) phenotype efficiently control immune responses in experimental autoimmune and colitis models in mice. Here we show that CD8(+)CD28(low) regulatory T lymphocytes from AIRE-deficient mice are transcriptionally and phenotypically normal and exert efficient suppression of in vitro immune responses, but completely fail to prevent experimental colitis in vivo. Our data therefore demonstrate that AIRE plays an important role in the in vivo function of a naturally occurring regulatory T-cell population.

Accelerated course of experimental autoimmune encephalomyelitis in PD-1-deficient central nervous system myelin mutants

It is assumed that the onset and course of autoimmune inflammatory central nervous system (CNS) disorders (eg, multiple sclerosis) are influenced by factors that afflict immune regulation as well as CNS vulnerability. We challenged this concept experimentally by investigating how genetic alterations that affect myelin (primary oligodendrocyte damage in PLPtg mice) and/or T-cell regulation (deficiency of PD-1) influence both the onset and course of an experimental autoimmune CNS inflammatory disease [MOG(35-55)-induced experimental autoimmune encephalomyelitis (EAE)]. We observed that double pathology was associated with a significantly earlier onset of disease, a slight increase in the neurological score, an increase in the number of infiltrating cells, and enhanced axonal degeneration compared with wild-type mice and the respective, single mutant controls. Double-mutant PLPtg/PD-1(-/-) mice showed an increased production of interferon-gamma by CNS immune cells at the peak of disease. Neither PD-1 deficiency nor oligodendropathy led to detectable spread of antigenic MHC class I- or class II-restricted epitopes during EAE. However, absence of PD-1 clearly increased the propensity of T lymphocytes to expand, and the number of clonal expansions reliably reflected the severity of the EAE disease course. Our data show that the interplay between immune dysregulation and myelinopathy results in a stable exacerbation of actively induced autoimmune CNS inflammation, suggesting that the combination of several pathological issues contributes significantly to disease susceptibility or relapses in human disease.

Clinical, pathological, and immunologic aspects of the multiple sclerosis model in common marmosets (Callithrix jacchus)

The efficacy of many new immunomodulatory therapies for multiple sclerosis (MS) patients has often been disappointing, reflecting our incomplete understanding of this enigmatic disease. There is a growing awareness that, at least in part, there may be limited applicability to the human disease of results obtained in the widely studied MS model experimental autoimmune encephalomyelitis in rodents. This review describes the experimental autoimmune encephalomyelitis model developed in a small neotropical primate, the common marmoset (Callithrix jacchus). The model has features including clinicopathologic correlation patterns, lesion heterogeneity, immunologic mechanisms, and disease markers that more closely mimic the human disease. Several unique features of experimental autoimmune encephalomyelitis in marmosets, together with their outbred nature and close genetic and immunologic similarities to humans, create an attractive experimental model for translational research into MS, particularly for the preclinical evaluation of new biologic therapeutic molecules that cannot be investigated in rodents because of their species specificity. Moreover, this model provides new insights into possible pathogenetic mechanisms in MS.

Follicular helper T cells: lineage and location

Follicular helper T (Tfh) cells are the class of effector T helper cells that regulates the step-wise development of antigen-specific B cell immunity in vivo. Deployment of CXCR5+ Tfh cells to B cell zones of lymphoid tissues and stable cognate interactions with B cells are central to the delivery of antigen-specific Tfh cell function. Here, we review recent advances that have helped to unravel distinctive elements of developmental programming for Tfh cells and unique effector Tfh cell functions focused on antigen-primed B cells. Understanding the regulatory functions of Tfh cells in the germinal center and the subsequent regulation of memory B cell responses to antigen recall represent the frontiers of this research area with the potential to alter fundamentally the design of future vaccines.

The role of production frequency in the sharing of simian immunodeficiency virus-specific CD8+ TCRs between macaques

In some epitope-specific responses, T cells bearing identical TCRs occur in many MHC-matched individuals. The sharing of public TCRs is unexpected, given the enormous potential diversity of the TCR repertoire. We have previously studied the sharing of TCR beta-chains in the CD8(+) T cell responses to two influenza epitopes in mice. Analysis of these TCRbeta repertoires suggests that, even with unbiased V(D)J recombination mechanisms, some TCRbetas can be produced more frequently than others, by a process of convergent recombination. The TCRbeta production frequency was shown to be a good predictor of the observed sharing of epitope-specific TCRbetas between mice. However, this study was limited to immune responses in an inbred population. In this study, we investigated TCRbeta sharing in CD8(+) T cell responses specific for the immunodominant Mamu-A*01-restricted Tat-SL8/TL8 and Gag-CM9 epitopes of SIV in rhesus macaques. Multiple data sets were used, comprising a total of approximately 6000 TCRbetas sampled from 20 macaques. We observed a spectrum in the number of macaques sharing epitope-specific TCRbetas in this outbred population. This spectrum of TCRbeta sharing was negatively correlated with the minimum number of nucleotide additions required to produce the sequences and strongly positively correlated with the number of observed nucleotide sequences encoding the amino acid sequences. We also found that TCRbeta sharing was correlated with the number of times, and the variety of different ways, the sequences were produced in silico via random gene recombination. Thus, convergent recombination is a major determinant of the extent of TCRbeta sharing.

The molecular basis for public T-cell responses?

Public T-cell responses, in which T cells bearing identical T-cell receptors (TCRs) are observed to dominate the response to the same antigenic epitope in multiple individuals, have long been a focus of immune T-cell repertoire studies. However, the mechanism that enables the survival of a specific TCR from the diverse repertoire produced in the thymus through to its involvement in a public immune response remains unclear. In this Opinion article, we propose that the frequency of production of T cells bearing different TCRs during recombination has an important role in the sharing of TCRs in an immune response, with variable levels of 'convergent recombination' driving production frequencies.