Age-Related Changes in Thymic Central Tolerance

Macroprolactin in mothers and their babies: what is its origin?

OBJECTIVES

Macroprolactinemia is one of the major causes of hyperprolactinemia. The aim of this study was to clarify the origin of macroprolactin (macro-PRL).

METHODS

We examined macro-PRL in the sera of 826 pregnant women and in those of their babies' umbilical cords at delivery. Macro-PRL was evaluated by precipitation with polyethylene glycol (PEG), gel filtration chromatography (GFC), and absorption with protein G (PG).

RESULTS

We detected macro-PRL in 16 out of the 826 pregnant women (1.94 %) and in 14 of their babies, which may indicate the possibility of hereditary origin of macro-PRL. However, the macro-PRL ratios of the babies correlated positively with those of their mothers (r=0.72 for GFC, p<0.001 and r=0.77 for PG, p<0.001), suggesting that the immunoglobulin (Ig)G-type anti-PRL autoantibodies might be actively transferred to babies via the placenta and form macro-PRL by binding to their babies' PRL or PRL-IgG complexes may possibly pass through the placenta. There were two cases in which only mothers had macro-PRL, indicating that the mothers had autoantibodies that did not pass through the placenta, such as IgA, PRL bound to the other proteins or PRL aggregates. No cases were found in which only the babies had macro-PRL and their mothers did not, suggesting that macro-PRL might not arise by non-hereditary congenital causes.

CONCLUSIONS

Macro-PRL in women of reproductive age might be mostly IgG-type anti-PRL autoantibody-bound PRL. The likely origin of macro-PRL in babies is the transplacental transfer of IgG-type anti-PRL autoantibodies or PRL-IgG complexes from the mothers to their babies.

Generation and repair of thymic epithelial cells

In the vertebrate immune system, thymus stromal microenvironments support the generation of αβT cells from immature thymocytes. Thymic epithelial cells are of particular importance, and the generation of cortical and medullary epithelial lineages from progenitor stages controls the initiation and maintenance of thymus function. Here, we discuss the developmental pathways that regulate thymic epithelial cell diversity during both the embryonic and postnatal periods. We also examine how thymus microenvironments respond to injury, with particular focus on mechanisms that ensure regeneration of thymic epithelial cells for the restoration of thymus function.

A Chemical Approach to Assess the Impact of Post-translational Modification on MHC Peptide Binding and Effector Cell Engagement

The human major histocompatibility complex (MHC) plays a pivotal role in the presentation of peptidic fragments from proteins, which can originate from self-proteins or from nonhuman antigens, such as those produced by viruses or bacteria. To prevent cytotoxicity against healthy cells, thymocytes expressing T cell receptors (TCRs) that recognize self-peptides are removed from circulation (negative selection), thus leaving T cells that recognize nonself-peptides. Current understanding suggests that post-translationally modified (PTM) proteins and the resulting peptide fragments they generate following proteolysis are largely excluded from negative selection; this feature means that PTMs can generate nonself-peptides that potentially contribute to the development of autoreactive T cells and subsequent autoimmune diseases. Although it is well-established that PTMs are prevalent in peptides present on MHCs, the precise mechanisms by which PTMs influence the antigen presentation machinery remain poorly understood. In the present work, we introduce chemical modifications mimicking PTMs on synthetic peptides. This is the first systematic study isolating the impact of PTMs on MHC binding and also their impact on TCR recognition. Our findings reveal various ways PTMs alter antigen presentation, which could have implications for tumor neoantigen presentation.

Antigen presentation for central tolerance induction

The extent of central T cell tolerance is determined by the diversity of self-antigens that developing thymocytes 'see' on thymic antigen-presenting cells (APCs). Here, focusing on insights from the past decade, we review the functional adaptations of medullary thymic epithelial cells, thymic dendritic cells and thymic B cells for the purpose of tolerance induction. Their distinct cellular characteristics range from unconventional phenomena, such as promiscuous gene expression or mimicry of peripheral cell types, to strategic positioning in distinct microenvironments and divergent propensities to preferentially access endogenous or exogenous antigen pools. We also discuss how 'tonic' inflammatory signals in the thymic microenvironment may extend the intrathymically visible 'self' to include autoantigens that are otherwise associated with highly immunogenic peripheral environments.

Comprehensive Flow Cytometric, Immunohistologic, and Molecular Assessment of Thymus Function in Rhesus Macaques

The critical importance of the thymus for generating new naive T cells that protect against novel infections and are tolerant to self-antigens has led to a recent revival of interest in monitoring thymic function in species other than humans and mice. Nonhuman primates such as rhesus macaques (Macaca mulatta) provide particularly useful animal models for translational research in immunology. In this study, we tested the performance of a 15-marker multicolor Ab panel for flow cytometric phenotyping of lymphocyte subsets directly from rhesus whole blood, with validation by thymectomy and T cell depletion. Immunohistochemical and multiplex RNA expression analysis of thymus tissue biopsies and molecular assays on PBMCs were used to further validate thymus function. Results identify Ab panels that can accurately classify rhesus naive T cells (CD3+CD45RA+CD197+ or CD3+CD28+CD95-) and recent thymic emigrants (CD8+CD28+CD95-CD103+CD197+) using just 100 µl of whole blood and commercially available fluorescent Abs. An immunohistochemical panel reactive with pan-cytokeratin (CK), CK14, CD3, Ki-67, CCL21, and TdT provides histologic evidence of thymopoiesis from formalin-fixed, paraffin-embedded thymus tissues. Identification of mRNAs characteristic of both functioning thymic epithelial cells and developing thymocytes and/or molecular detection of products of TCR gene rearrangement provide additional complementary methods to evaluate thymopoiesis, without requiring specific Abs. Combinations of multiparameter flow cytometry, immunohistochemistry, multiplex gene expression, and TCR excision circle assays can comprehensively evaluate thymus function in rhesus macaques while requiring only minimal amounts of peripheral blood or biopsied thymus tissue.

Gene Regulatory Programs that Specify Age-Related Differences during Thymocyte Development

T cell development is fundamental to immune system establishment, yet how this development changes with age remains poorly understood. Here, we construct a transcriptional and epigenetic atlas of T cell developmental programs in neonatal and adult mice, revealing the ontogeny of divergent gene regulatory programs and their link to age-related differences in phenotype and function. Specifically, we identify a gene module that diverges with age from the earliest stages of genesis and includes programs that govern effector response and cell cycle regulation. Moreover, we reveal that neonates possess more accessible chromatin during early thymocyte development, likely establishing poised gene expression programs that manifest later in thymocyte development. Finally, we leverage this atlas, employing a CRISPR-based perturbation approach coupled with single-cell RNA sequencing as a readout to uncover a conserved transcriptional regulator, Zbtb20, that contributes to age-dependent differences in T cell development. Altogether, our study defines transcriptional and epigenetic programs that regulate age-specific differences in T cell development.

The aged microenvironment impairs BCL6 and CD40L induction in CD4+ T follicular helper cell differentiation

Weakened germinal center responses by the aged immune system result in diminished immunity against pathogens and reduced efficacy of vaccines. Prolonged contacts between activated B cells and CD4+ T cells are crucial to germinal center formation and T follicular helper cell (Tfh) differentiation, but it is unclear how aging impacts the quality of this interaction. Peptide immunization confirmed that aged mice have decreased expansion of antigen-specific germinal center B cells and reduced antibody titers. Furthermore, aging was associated with accumulated Tfh cells, even in naïve mice. Despite increased numbers, aged Tfh had reduced expression of master transcription factor BCL6 and increased expression of the ectonucleotidase CD39. In vitro activation revealed that proliferative capacity was maintained in aged CD4+ T cells, but not the costimulatory molecule CD40L. When activated in vitro by aged antigen-presenting cells, young CD4+ naïve T cells generated reduced numbers of activated cells with upregulated CD40L. To determine the contribution of cell-extrinsic influences on antigen-specific Tfh induction, young, antigen-specific B and CD4+ T cells were adoptively transferred into aged hosts prior to peptide immunization. Transferred cells had reduced expansion and differentiation into germinal center B cell and Tfh and reduced antigen-specific antibody titers when compared to young hosts. Young CD4+ T cells transferred aged hosts differentiated into Tfh cells with reduced PD-1 and BCL6 expression, and increased CD39 expression, though they maintained their mitochondrial capacity. These results highlight the role of the lymphoid microenvironment in modulating CD4+ T cell differentiation, which contributes to impaired establishment and maintenance of germinal centers.

Transposable elements regulate thymus development and function

Transposable elements (TEs) are repetitive sequences representing ~45% of the human and mouse genomes and are highly expressed by medullary thymic epithelial cells (mTECs). In this study, we investigated the role of TEs on T-cell development in the thymus. We performed multiomic analyses of TEs in human and mouse thymic cells to elucidate their role in T-cell development. We report that TE expression in the human thymus is high and shows extensive age- and cell lineage-related variations. TE expression correlates with multiple transcription factors in all cell types of the human thymus. Two cell types express particularly broad TE repertoires: mTECs and plasmacytoid dendritic cells (pDCs). In mTECs, transcriptomic data suggest that TEs interact with transcription factors essential for mTEC development and function (e.g., PAX1 and REL), and immunopeptidomic data showed that TEs generate MHC-I-associated peptides implicated in thymocyte education. Notably, AIRE, FEZF2, and CHD4 regulate small yet non-redundant sets of TEs in murine mTECs. Human thymic pDCs homogenously express large numbers of TEs that likely form dsRNA, which can activate innate immune receptors, potentially explaining why thymic pDCs constitutively secrete IFN ɑ/β. This study highlights the diversity of interactions between TEs and the adaptive immune system. TEs are genetic parasites, and the two thymic cell types most affected by TEs (mTEcs and pDCs) are essential to establishing central T-cell tolerance. Therefore, we propose that orchestrating TE expression in thymic cells is critical to prevent autoimmunity in vertebrates.

Overcoming Xenoantigen Immunity to Enable Cellular Tracking and Gene Regulation with Immune-competent "NoGlow" Mice

The ability to temporally regulate gene expression and track labeled cells makes animal models powerful biomedical tools. However, sudden expression of xenobiotic genes [e.g., GFP, luciferase (Luc), or rtTA3] can trigger inadvertent immunity that suppresses foreign protein expression or results in complete rejection of transplanted cells. Germline exposure to foreign antigens somewhat addresses these challenges; however, native fluorescence and bioluminescence abrogates the utility of reporter proteins and highly spatiotemporally restricted expression can lead to suboptimal xenoantigen tolerance. To overcome these unwanted immune responses and enable reliable cell tracking/gene regulation, we developed a novel mouse model that selectively expresses antigen-intact but nonfunctional forms of GFP and Luc, as well as rtTA3, after CRE-mediated recombination. Using tissue-specific CREs, we observed model and sex-based differences in immune tolerance to the encoded xenoantigens, illustrating the obstacles of tolerizing animals to foreign genes and validating the utility of these "NoGlow" mice to dissect mechanisms of central and peripheral tolerance. Critically, tissue unrestricted NoGlow mice possess no detectable background fluorescence or luminescence and exhibit limited adaptive immunity against encoded transgenic xenoantigens after vaccination. Moreover, we demonstrate that NoGlow mice allow tracking and tetracycline-inducible gene regulation of triple-transgenic cells expressing GFP/Luc/rtTA3, in contrast to transgene-negative immune-competent mice that eliminate these cells or prohibit metastatic seeding. Notably, this model enables de novo metastasis from orthotopically implanted, triple-transgenic tumor cells, despite high xenoantigen expression. Altogether, the NoGlow model provides a critical resource for in vivo studies across disciplines, including oncology, developmental biology, infectious disease, autoimmunity, and transplantation.

SIGNIFICANCE

Multitolerant NoGlow mice enable tracking and gene manipulation of transplanted tumor cells without immune-mediated rejection, thus providing a platform to investigate novel mechanisms of adaptive immunity related to metastasis, immunotherapy, and tolerance.

Investigating the causal relationship between immune cell and Alzheimer's disease: a mendelian randomization analysis

BACKGROUND

Complex interactions between the immune system and the brain may affect neural development, survival, and function, with etiological and therapeutic implications for neurodegenerative diseases. However, previous studies investigating the association between immune inflammation and Alzheimer's disease (AD) have yielded inconsistent results.

METHODS

We applied Mendelian randomization (MR) to examine the causal relationship between immune cell traits and AD risk using genetic variants as instrumental variables. MR is an epidemiological study design based on genetic information that reduces the effects of confounding and reverse causation. We analyzed the causal associations between 731 immune cell traits and AD risk based on publicly available genetic data.

RESULTS

We observed that 5 immune cell traits conferred protection against AD, while 7 immune cell traits increased the risk of AD. These immune cell traits mainly involved T cell regulation, monocyte activation and B cell differentiation. Our findings suggest that immune regulation may influence the development of AD and provide new insights into potential targets for AD prevention and treatment. We also conducted various sensitivity analyses to test the validity and robustness of our results, which revealed no evidence of pleiotropy or heterogeneity.

CONCLUSION

Our research shows that immune regulation is important for AD and provides new information on potential targets for AD prevention and treatment. However, this study has limitations, including the possibility of reverse causality, lack of validation in independent cohorts, and potential confounding by population stratification. Further research is needed to validate and amplify these results and to elucidate the potential mechanisms of the immune cell-AD association.

Morphometric Analysis of the Thymic Epithelial Cell (TEC) Network Using Integrated and Orthogonal Digital Pathology Approaches

The thymus, a central primary lymphoid organ of the immune system, plays a key role in T cell development. Surprisingly, the thymus is quite neglected with regards to standardized pathology approaches and practices for assessing structure and function. Most studies use multispectral flow cytometry to define the dynamic composition of the thymus at the cell population level, but they are limited by lack of contextual insight. This knowledge gap hinders our understanding of various thymic conditions and pathologies, particularly how they affect thymic architecture, and subsequently, immune competence. Here, we introduce a digital pathology pipeline to address these challenges. Our approach can be coupled to analytical algorithms and utilizes rationalized morphometric assessments of thymic tissue, ranging from tissue-wide down to microanatomical and ultrastructural levels. This pipeline enables the quantitative assessment of putative changes and adaptations of thymic structure to stimuli, offering valuable insights into the pathophysiology of thymic disorders. This versatile pipeline can be applied to a wide range of conditions that may directly or indirectly affect thymic structure, ranging from various cytotoxic stimuli inducing acute thymic involution to autoimmune diseases, such as myasthenia gravis. Here, we demonstrate applicability of the method in a mouse model of age-dependent thymic involution, both by confirming established knowledge, and by providing novel insights on intrathymic remodeling in the aged thymus. Our orthogonal pipeline, with its high versatility and depth of analysis, promises to be a valuable and practical toolset for both basic and translational immunology laboratories investigating thymic function and disease.

A guide to thymic selection of T cells

The thymus is an evolutionarily conserved organ that supports the development of T cells. Not only does the thymic environment support the rearrangement and expression of diverse T cell receptors but also provides a unique niche for the selection of appropriate T cell clones. Thymic selection ensures that the repertoire of available T cells is both useful (being MHC-restricted) and safe (being self-tolerant). The unique antigen-presentation features of the thymus ensure that the display of self-antigens is optimal to induce tolerance to all types of self-tissue. MHC class-specific functions of CD4+ T helper cells, CD8+ killer T cells and CD4+ regulatory T cells are also established in the thymus. Finally, the thymus provides signals for the development of several minor T cell subsets that promote immune and tissue homeostasis. This Review provides an introductory-level overview of our current understanding of the sophisticated thymic selection mechanisms that ensure T cells are useful and safe.

Thymosin β4 Regulates the Differentiation of Thymocytes by Controlling the Cytoskeletal Rearrangement and Mitochondrial Transfer of Thymus Epithelial Cells

The thymus is one of the most crucial immunological organs, undergoing visible age-related shrinkage. Thymic epithelial cells (TECs) play a vital role in maintaining the normal function of the thymus, and their degeneration is the primary cause of age-induced thymic devolution. Thymosin β4 (Tβ4) serves as a significant important G-actin sequestering peptide. The objective of this study was to explore whether Tβ4 influences thymocyte differentiation by regulating the cytoskeletal rearrangement and mitochondrial transfer of TECs. A combination of H&E staining, immunofluorescence, transmission electron microscopy, RT-qPCR, flow cytometry, cytoskeletal immunolabeling, and mitochondrial immunolabeling were employed to observe the effects of Tβ4 on TECs' skeleton rearrangement, mitochondrial transfer, and thymocyte differentiation. The study revealed that the Tβ4 primarily regulates the formation of microfilaments and the mitochondrial transfer of TECs, along with the formation and maturation of double-negative cells (CD4-CD8-) and CD4 single-positive cells (CD3+TCRβ+CD4+CD8-) thymocytes. This study suggests that Tβ4 plays a crucial role in thymocyte differentiation by influencing the cytoskeletal rearrangement and mitochondrial transfer of TECs. These effects may be associated with Tβ4's impact on the aggregation of F-actin. This finding opens up new avenues for research in the field of immune aging.

Chest Computed Tomography Characteristics of Critically Ill COVID-19 Patients with Auto-antibodies Against Type I Interferons

PURPOSE

Patients with auto-antibodies neutralizing type I interferons (anti-IFN auto-Abs) are at risk of severe forms of coronavirus disease 19 (COVID-19). The chest computed tomography (CT) scan characteristics of critically ill COVID-19 patients harboring these auto-Abs have never been reported.

METHODS

Bicentric ancillary study of the ANTICOV study (observational prospective cohort of severe COVID-19 patients admitted to the intensive care unit (ICU) for hypoxemic acute respiratory failure between March 2020 and May 2021) on chest CT scan characteristics (severity score, parenchymal, pleural, vascular patterns). Anti-IFN auto-Abs were detected using a luciferase neutralization reporting assay. Imaging data were collected through independent blinded reading of two thoracic radiologists of chest CT studies performed at ICU admission (± 72 h). The primary outcome measure was the evaluation of severity by the total severity score (TSS) and the CT severity score (CTSS) according to the presence or absence of anti-IFN auto-Abs.

RESULTS

Two hundred thirty-one critically ill COVID-19 patients were included in the study (mean age 59.5 ± 12.7 years; males 74.6%). Day 90 mortality was 29.5% (n = 72/244). There was a trend towards more severe radiological lesions in patients with anti-IFN auto-Abs than in others, not reaching statistical significance (median CTSS 27.5 (21.0-34.8) versus 24.0 (19.0-30.0), p = 0.052; median TSS 14.5 (10.2-17.0) versus 12.0 (9.0-15.0), p = 0.070). The extra-parenchymal evaluation found no difference in the proportion of patients with pleural effusion, mediastinal lymphadenopathy, or thymal abnormalities in the two populations. The prevalence of pulmonary embolism was not significantly different between groups (8.7% versus 5.3%, p = 0.623, n = 175).

CONCLUSION

There was no significant difference in disease severity as evaluated by chest CT in severe COVID-19 patients admitted to the ICU for hypoxemic acute respiratory failure with or without anti-IFN auto-Abs.

Multi-variate model of T cell clonotype competition and homeostasis

Diversity of the naive T cell repertoire is maintained by competition for stimuli provided by self-peptides bound to major histocompatibility complexes (self-pMHCs). We extend an existing bi-variate competition model to a multi-variate model of the dynamics of multiple T cell clonotypes which share stimuli. In order to understand the late-time behaviour of the system, we analyse: (i) the dynamics until the extinction of the first clonotype, (ii) the time to the first extinction event, (iii) the probability of extinction of each clonotype, and (iv) the size of the surviving clonotypes when the first extinction event takes place. We also find the probability distribution of the number of cell divisions per clonotype before its extinction. The mean size of a new clonotype at quasi-steady state is an increasing function of the stimulus available to it, and a decreasing function of the fraction of stimuli it shares with other clonotypes. Thus, the probability of, and time to, extinction of a new clonotype entering the pool of T cell clonotypes is determined by the extent of competition for stimuli it experiences and by its initial number of cells.

The Role of Immune Dysfunction in Parkinson's Disease Development

Recent research has unveiled intriguing insights suggesting that the body's immune system may be implicated in Parkinson's disease (PD) development. Studies have observed disparities in pro-inflammatory and anti-inflammatory markers between PD patients and healthy individuals. This finding underscores the potential influence of immune system dysfunction in the genesis of this condition. A dysfunctional immune system can serve as a primary catalyst for systemic inflammation in the body, which may contribute to the emergence of various brain disorders. The identification of several genes associated with PD, as well as their connection to neuroinflammation, raises the likelihood of disease susceptibility. Moreover, advancing age and mitochondrial dysfunction can weaken the immune system, potentially implicating them in the onset of the disease, particularly among older individuals. Compromised integrity of the blood-brain barrier could facilitate the immune system's access to brain tissue. This exposure may lead to encounters with native antigens or infections, potentially triggering an autoimmune response. Furthermore, there is mounting evidence supporting the notion that gut dysbiosis might represent an initial trigger for brain inflammation, ultimately promoting neurodegeneration. In this comprehensive review, we will delve into the numerous hypotheses surrounding the role of both innate and adaptive immunity in PD.

Aging of lymphoid stromal architecture impacts immune responses

The secondary lymphoid organs (SLOs) undergo structural changes with age, which correlates with diminishing immune responses against infectious disease. A growing body of research suggests that the aged tissue microenvironment can contribute to decreased immune function, independent of intrinsic changes to hematopoietic cells with age. Stromal cells impart structural integrity, facilitate fluid transport, and provide chemokine and cytokine signals that are essential for immune homeostasis. Mechanisms that drive SLO development have been described, but their roles in SLO maintenance with advanced age are unknown. Disorganization of the fibroblasts of the T cell and B cell zones may reduce the maintenance of naïve lymphocytes and delay immune activation. Reduced lymphatic transport efficiency with age can also delay the onset of the adaptive immune response. This review focuses on recent studies that describe age-associated changes to the stroma of the lymph nodes and spleen. We also review recent investigations into stromal cell biology, which include high-dimensional analysis of the stromal cell transcriptome and viscoelastic testing of lymph node mechanical properties, as they constitute an important framework for understanding aging of the lymphoid tissues.

Chest computed tomography characteristics of critically ill COVID-19 patients with auto-antibodies against type I interferons

Purpose

patients with auto-antibodies neutralizing type I interferons (anti-IFN auto-Abs) are at risk of severe forms of coronavirus disease 19 (COVID-19). The chest computed tomography (CT) scan characteristics of critically ill COVID-19 patients harboring these auto-Abs have never been reported.

Methods

Bicentric ancillary study of the ANTICOV study (observational prospective cohort of severe COVID-19 patients admitted to the intensive care unit (ICU) for hypoxemic acute respiratory failure) on chest CT scan characteristics (severity score, parenchymal, pleural, vascular patterns). Anti-IFN auto-Abs were detected using a luciferase neutralization reporting assay. Imaging data were collected through independent blinded reading of two thoracic radiologists of chest CT studies performed at ICU admission (±72h). The primary outcome measure was the evaluation of severity by the total severity score (TSS) and the CT severity score (CTSS) according to the presence or absence of anti-IFN auto-Abs.

Results

231 critically ill COVID-19 patients were included in the study (mean age 59.5±12.7 years; males 74.6%). Day 90 mortality was 29.5% (n=72/244). There was a trend towards more severe radiological lesions in patients with auto-IFN anti-Abs than in others, not reaching statistical significance (median CTSS 27.5 (21.0-34.8] versus 24.0 (19.0-30.0), p=0.052; median TSS 14.5 (10.2-17.0) versus 12.0 (9.0-15.0), p=0.070). The extra-parenchymal evaluation found no difference in the proportion of patients with pleural effusion, mediastinal lymphadenopathy or thymal abnormalities in the two populations. The prevalence of pulmonary embolism was not significantly different between groups (8.7% versus 5.3%, p=0.623, n=175).

Conclusion

There was no significant difference in disease severity as evaluated by chest CT in severe COVID-19 patients admitted to the ICU for hypoxemic acute respiratory failure with or without anti-IFN auto-Abs.

Shaping immunity for life: Layered development of CD8+ T cells

Historically, the immune system was believed to develop along a linear axis of maturity from fetal life to adulthood. Now, it is clear that distinct layers of immune cells are generated from unique waves of hematopoietic progenitors during different windows of development. This model, known as the layered immune model, has provided a useful framework for understanding why distinct lineages of B cells and γδ T cells arise in succession and display unique functions in adulthood. However, the layered immune model has not been applied to CD8+ T cells, which are still often viewed as a uniform population of cells belonging to the same lineage, with functional differences between cells arising from environmental factors encountered during infection. Recent studies have challenged this idea, demonstrating that not all CD8+ T cells are created equally and that the functions of individual CD8+ T cells in adults are linked to when they were created in the host. In this review, we discuss the accumulating evidence suggesting there are distinct ontogenetic subpopulations of CD8+ T cells and propose that the layered immune model be extended to the CD8+ T cell compartment.

Deciphering Differential Behavior of Immune Responses as the Foundation for Precision Dosing in Allergen Immunotherapy

Like in many fields of medicine, the concept of precision dosing has re-emerged in routine practice in allergology. Only one retrospective study on French physicians' practice has addressed this topic so far and generated preliminary data supporting dose adaptation, mainly based on experience, patient profile understanding and response to treatment. Both intrinsic and extrinsic factors shape the individual immune system response to allergen immunotherapy (AIT). Herein, we focus on key immune cells (i.e., dendritic cells, innate lymphoid cells, B and T cells, basophils and mast cells) involved in allergic disease and its resolution to further understand the effect of AIT on the phenotype, frequency or polarization of these cells. We strive to discriminate differences in immune responses between responders and non-responders to AIT, and discuss the eligibility of a non/low-responder subset for dose adaptation. A differential behavior in immune cells is clearly observed in responders, highlighting the importance of conducting clinical trials with large cohorts of well-characterized subjects to decipher the immune mechanism of AIT. We conclude that there is a need for designing new clinical and mechanistic studies to support the scientific rationale of dose adaptation in the interest of patients who do not properly respond to AIT.

What's self got to do with it: Sources of heterogeneity among naive T cells

There is a long-standing assumption that naive CD4⁺ and CD8⁺ T cells are largely homogeneous populations despite the extraordinary diversity of their T cell receptors (TCR). The self-immunopeptidome plays a key role in the selection of the naive T cell repertoire in the thymus, and self-peptides are also an important driver of differences between individual naive T cells with regard to their subsequent functional contributions to an immune response. Accumulating evidence suggests that as early as the β-selection stage of T cell development, when only one of the recombined chains of the mature TCR is expressed, signaling thresholds may be established for positive selection of immature thymocytes. Stochastic encounters subsequently made with self-ligands during positive selection in the thymus imprint functional biases that a T cell will carry with it throughout its lifetime, although ongoing interactions with self in the periphery ensure a level of plasticity in the gene expression wiring of naive T cells. Identifying the sources of heterogeneity in the naive T cell population and which functional attributes of T cells can be modulated through post-thymic interventions versus those that are fixed during T cell development, could enable us to better select or generate T cells with particular traits to improve the efficacy of T cell therapies.

Thymic changes as a contributing factor in the increased susceptibility of old Albino Oxford rats to EAE development

The study was aimed to examine putative contribution of thymic involution to ageing-associated increase in susceptibility of Albino Oxford (AO) rats to the development of clinical EAE, and vice versa influence of the disease on the progression of thymic involution. To this end we examined (i) the parameters of thymocyte negative selection efficacy, the thymic generation of CD4+CD25+Foxp3+ T regulatory cells (Tregs) and thymic capacity to instruct/predetermine IL-17-producing T-cell differentiation, and thymopietic efficacy-associated accumulation of "inflammescent" cytotoxic CD28- T cells in the periphery, and (ii) the key underlying mechanisms in young and old non-immunised AO rats and their counterparts immunised for EAE (on the 16^th day post-immunisation when the disease in old rats reached the plateau) using flow cytometry analysis and/or RT-qPCR. It was found that thymic involution impairs: (i) the efficacy of negative selection (by affecting thymocyte expression of CD90, negative regulator of selection threshold and the expression of thymic stromal cell integrity factors) and (ii) Treg generation (by diminishing expression of cytokines supporting their differentiation/maturation). Additionally, the results suggest that thymic involution facilitates CD8+ T-cell differentiation into IL-17-producing cells (previously linked to the development of clinical EAE in old AO rats). Furthermore, they confirmed that ageing-related decrease in thymic T-cell output (as indicated by diminished frequency of recent thymic emigrants in peripheral blood) resulted in the accumulation of CD28- T cells in peripheral blood and, upon immunisation, in the target organ. On the other hand, the development of EAE (most likely by increasing circulatory levels of proinflammatory cytokines) contributed to the decline in thymic output of T cells, including Tregs, and thereby to the progression/maintenance of clinical EAE. Thus, in AO rats thymic involution via multi-layered mechanisms may favour the development of clinically manifested autoimmunity, which, in turn, precipitates the thymus atrophy.

Age-Associated Changes to Lymph Node Fibroblastic Reticular Cells

The decreased proportion of antigen-inexperienced, naïve T cells is a hallmark of aging in both humans and mice, and contributes to reduced immune responses, particularly against novel and re-emerging pathogens. Naïve T cells depend on survival signals received during their circulation among the lymph nodes by direct contacts with stroma, in particular fibroblastic reticular cells. Macroscopic changes to the architecture of the lymph nodes have been described, but it is unclear how lymph node stroma are altered with age, and whether these changes contribute to reduced naïve T cell maintenance. Here, using 2-photon microscopy, we determined that the aged lymph node displayed increased fibrosis and correspondingly, that naïve T-cell motility was impaired in the aged lymph node, especially in proximity to fibrotic deposition. Functionally, adoptively transferred young naïve T-cells exhibited reduced homeostatic turnover in aged hosts, supporting the role of T cell-extrinsic mechanisms that regulate their survival. Further, we determined that early development of resident fibroblastic reticular cells was impaired, which may correlate to the declining levels of naïve T-cell homeostatic factors observed in aged lymph nodes. Thus, our study addresses the controversy as to whether aging impacts the composition lymph node stroma and supports a model in which impaired differentiation of lymph node fibroblasts and increased fibrosis inhibits the interactions necessary for naïve T cell homeostasis.

Age-related thymic involution: Mechanisms and functional impact

The thymus is the primary immune organ responsible for generating self‐tolerant and immunocompetent T cells. However, the thymus gradually involutes during early life resulting in declined naïve T‐cell production, a process known as age‐related thymic involution. Thymic involution has many negative impacts on immune function including reduced pathogen resistance, high autoimmunity incidence, and attenuated tumor immunosurveillance. Age‐related thymic involution leads to a gradual reduction in thymic cellularity and thymic stromal microenvironment disruption, including loss of definite cortical‐medullary junctions, reduction of cortical thymic epithelial cells and medullary thymic epithelial cells, fibroblast expansion, and an increase in perivascular space. The compromised thymic microenvironment in aged individuals substantially disturbs thymocyte development and differentiation. Age‐related thymic involution is regulated by many transcription factors, micro RNAs, growth factors, cytokines, and other factors. In this review, we summarize the current understanding of age‐related thymic involution mechanisms and effects.

A Proposed Link Between Acute Thymic Involution and Late Adverse Effects of Chemotherapy

Epidemiologic data suggest that cancer survivors tend to develop a protuberant number of adverse late effects, including second primary malignancies (SPM), as a result of cytotoxic chemotherapy. Besides the genotoxic potential of these drugs that directly inflict mutational burden on genomic DNA, the precise mechanisms contributing to SPM development are poorly understood. Cancer is nowadays perceived as a complex process that goes beyond the concept of genetic disease and includes tumor cell interactions with complex stromal and immune cell microenvironments. The cancer immunoediting theory offers an explanation for the development of nascent neoplastic cells. Briefly, the theory suggests that newly emerging tumor cells are mostly eliminated by an effective tissue immunosurveillance, but certain tumor variants may occasionally escape innate and adaptive mechanisms of immunological destruction, entering an equilibrium phase, where immunologic tumor cell death "equals" new tumor cell birth. Subsequent microenvironmental pressures and accumulation of helpful mutations in certain variants may lead to escape from the equilibrium phase, and eventually cause an overt neoplasm. Cancer immunoediting functions as a dedicated sentinel under the auspice of a highly competent immune system. This perspective offers the fresh insight that chemotherapy-induced thymic involution, which is characterized by the extensive obliteration of the sensitive thymic epithelial cell (TEC) compartment, can cause long-term defects in thymopoiesis and in establishment of diverse T cell receptor repertoires and peripheral T cell pools of cancer survivors. Such delayed recovery of T cell adaptive immunity may result in prolonged hijacking of the cancer immunoediting mechanisms, and lead to development of persistent and mortal infections, inflammatory disorders, organ-specific autoimmunity lesions, and SPMs. Acknowledging that chemotherapy-induced thymic involution is a potential risk factor for the emergence of SPM demarcates new avenues for the rationalized development of pharmacologic interventions to promote thymic regeneration in patients receiving cytoreductive chemotherapies.

Central tolerance is impaired in the middle-aged thymic environment

One of the earliest hallmarks of immune aging is thymus involution, which not only reduces the number of newly generated and exported T cells, but also alters the composition and organization of the thymus microenvironment. Thymic T‐cell export continues into adulthood, yet the impact of thymus involution on the quality of newly generated T‐cell clones is not well established. Notably, the number and proportion of medullary thymic epithelial cells (mTECs) and expression of tissue‐restricted antigens (TRAs) decline with age, suggesting the involuting thymus may not promote efficient central tolerance. Here, we demonstrate that the middle‐aged thymic environment does not support rapid motility of medullary thymocytes, potentially diminishing their ability to scan antigen presenting cells (APCs) that display the diverse self‐antigens that induce central tolerance. Consistent with this possibility, thymic slice assays reveal that the middle‐aged thymic environment does not support efficient negative selection or regulatory T‐cell (Treg) induction of thymocytes responsive to either TRAs or ubiquitous self‐antigens. This decline in central tolerance is not universal, but instead impacts lower‐avidity self‐antigens that are either less abundant or bind to TCRs with moderate affinities. Additionally, the decline in thymic tolerance by middle age is accompanied by both a reduction in mTECs and hematopoietic APC subsets that cooperate to drive central tolerance. Thus, age‐associated changes in the thymic environment result in impaired central tolerance against moderate‐avidity self‐antigens, potentially resulting in export of increasingly autoreactive naive T cells, with a deficit of Treg counterparts by middle age.

Specificity of Adaptive Immune Responses in Central Nervous System Health, Aging and Diseases

The field of neuroimmunology endorses the involvement of the adaptive immune system in central nervous system (CNS) health, disease, and aging. While immune cell trafficking into the CNS is highly regulated, small numbers of antigen-experienced lymphocytes can still enter the cerebrospinal fluid (CSF)-filled compartments for regular immune surveillance under homeostatic conditions. Meningeal lymphatics facilitate drainage of brain-derived antigens from the CSF to deep cervical lymph nodes to prime potential adaptive immune responses. During aging and CNS disorders, brain barriers and meningeal lymphatic functions are impaired, and immune cell trafficking and antigen efflux are altered. In this context, alterations in the immune cell repertoire of blood and CSF and T and B cells primed against CNS-derived autoantigens have been observed in various CNS disorders. However, for many diseases, a causal relationship between observed immune responses and neuropathological findings is lacking. Here, we review recent discoveries about the association between the adaptive immune system and CNS disorders such as autoimmune neuroinflammatory and neurodegenerative diseases. We focus on the current challenges in identifying specific T cell epitopes in CNS diseases and discuss the potential implications for future diagnostic and treatment options.

Effect of puberty on the immune system: Relevance to multiple sclerosis

Puberty is a dynamic period marked by changing levels of sex hormones, the development of secondary sexual characteristics and reproductive maturity. This period has profound effects on various organ systems, including the immune system. The critical changes that occur in the immune system during pubertal onset have been shown to have implications for autoimmune conditions, including Multiple Sclerosis (MS). MS is rare prior to puberty but can manifest in children after puberty. This disease also has a clear female preponderance that only arises following pubertal onset, highlighting a potential role for sex hormones in autoimmunity. Early onset of puberty has also been shown to be a risk factor for MS. The purpose of this review is to overview the evidence that puberty regulates MS susceptibility and disease activity. Given that there is a paucity of studies that directly evaluate the effects of puberty on the immune system, we also discuss how the immune system is different in children and mice of pre- vs. post-pubertal ages and describe how gonadal hormones may regulate these immune mechanisms. We present evidence that puberty enhances the expression of co-stimulatory molecules and cytokine production by type 2 dendritic cells (DC2s) and plasmacytoid dendritic cells (pDCs), increases T helper 1 (Th1), Th17, and T follicular helper immunity, and promotes immunoglobulin (Ig)G antibody production. Overall, this review highlights how the immune system undergoes a functional maturation during puberty, which has the potential to explain the higher prevalence of MS and other autoimmune diseases seen in adolescence.

Quantification of dendritic cell subsets in human thymus tissues of various ages

Background

Dendritic cells (DCs) in the thymus are involved in central tolerance formation, but they also have other functions in the thymus, such as pathogen recognition. The density changes of human thymic DCs have been hardly investigated. In this study, human thymus samples of various ages were collected for tissue sectioning and staining. The thymic cortex and medulla area as well as the densities of various subsets of thymic DCs were calculated.

Results

All common DC subsets were found in the human thymus of various ages. Most DCs had accumulated in the human thymic epithelial space, especially the medulla. We also found that the human thymic cortex had atrophied relatively faster than the medulla, which led to a gradual increase of the area ratio of the medulla to cortex with the increase of age. The densities of DC subsets in the human thymus showed various changes with increasing age, which contributed to the composition changes of DC subsets. The density of plasmacytoid DCs (pDCs) in the human thymus had increased gradually with aging, which suggested that pDCs plays another essential role in the thymus in addition to central tolerance.

Conclusions

Inconsistent with the shrinking of the epithelial space in the thymus, the densities of DC subsets in the epithelial space of the thymus are maintained at a constant level with aging to preserve highly efficient autoreactive thymocyte screening. An increasing density of the thymic pDCs with aging implies an extra function of DCs in the thymus beyond central tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12979-021-00255-8.