200 Million Thymocytes and I: A Beginner's Survival Guide to T Cell Development

Protective effects of Ganoderma lucidum spores on estradiol benzoate-induced TEC apoptosis and compromised double-positive thymocyte development

Backgroud: Thymic atrophy marks the onset of immune aging, precipitating developmental anomalies in T cells. Numerous clinical and preclinical investigations have underscored the regulatory role of Ganoderma lucidum spores (GLS) in T cell development. However, the precise mechanisms underlying this regulation remain elusive. Methods: In this study, a mice model of estradiol benzoate (EB)-induced thymic atrophy was constructed, and the improvement effect of GLS on thymic atrophy was evaluated. Then, we employs multi-omics techniques to elucidate how GLS modulates T cell development amidst EB-induced thymic atrophy in mice. Results: GLS effectively mitigates EB-induced thymic damage by attenuating apoptotic thymic epithelial cells (TECs) and enhancing the output of CD4+ T cells into peripheral blood. During thymic T cell development, sporoderm-removed GLS (RGLS) promotes T cell receptor (TCR) α rearrangement by augmenting V-J fragment rearrangement frequency and efficiency. Notably, biased Vα14-Jα18 rearrangement fosters double-positive (DP) to invariant natural killer T (iNKT) cell differentiation, partially contingent on RGLS-mediated restriction of peptide-major histocompatibility complex I (pMHCⅠ)-CD8 interaction and augmented CD1d expression in DP thymocytes, thereby promoting DP to CD4+ iNKT cell development. Furthermore, RGLS amplifies interaction between a DP subpopulation, termed DPsel-7, and plasmacytoid dendritic cells (pDCs), likely facilitating the subsequent development of double-negative iNKT1 cells. Lastly, RGLS suppresses EB-induced upregulation of Abpob and Apoa4, curbing the clearance of CD4+Abpob+ and CD4+Apoa4+ T cells by mTECs, resulting in enhanced CD4+ T cell output. Discussion: These findings indicate that the RGLS effectively mitigates EB-induced TEC apoptosis and compromised double-positive thymocyte development. These insights into RGLS's immunoregulatory role pave the way for its potential as a T-cell regeneration inducer.

Speed and navigation control of thymocyte development by the fetal T-cell gene regulatory network

T-cell differentiation is a tightly regulated developmental program governed by interactions between transcription factors (TFs) and chromatin landscapes and affected by signals received from the thymic stroma. This process is marked by a series of checkpoints: T-lineage commitment, T-cell receptor (TCR)β selection, and positive and negative selection. Dynamically changing combinations of TFs drive differentiation along the T-lineage trajectory, through mechanisms that have been most extensively dissected in adult mouse T-lineage cells. However, fetal T-cell development differs from adult in ways that suggest that these TF mechanisms are not fully deterministic. The first wave of fetal T-cell differentiation occurs during a unique developmental window during thymic morphogenesis, shows more rapid kinetics of differentiation with fewer rounds of cell division, and gives rise to unique populations of innate lymphoid cells (ILCs) and invariant γδT cells that are not generated in the adult thymus. As the characteristic kinetics and progeny biases are cell-intrinsic properties of thymic progenitors, the differences could be based on distinct TF network circuitry within the progenitors themselves. Here, we review recent single-cell transcriptome data that illuminate the TF networks involved in T-cell differentiation in the fetal and adult mouse thymus.

Direct regulation of TCR rearrangement and expression by E proteins during early T cell development

γδ T cells are widely distributed throughout mucosal and epithelial cell-rich tissues and are an important early source of IL-17 in response to several pathogens. Like αβ T cells, γδ T cells undergo a stepwise process of development in the thymus that requires recombination of genome-encoded segments to assemble mature T cell receptor (TCR) genes. This process is tightly controlled on multiple levels to enable TCR segment assembly while preventing the genomic instability inherent in the double-stranded DNA breaks that occur during this process. Each TCR locus has unique aspects in its structure and requirements, with different types of regulation before and after the αβ/γδ T cell fate choice. It has been known that Runx and Myb are critical transcriptional regulators of TCRγ and TCRδ expression, but the roles of E proteins in TCRγ and TCRδ regulation have been less well explored. Multiple lines of evidence show that E proteins are involved in TCR expression at many different levels, including the regulation of Rag recombinase gene expression and protein stability, induction of germline V segment expression, chromatin remodeling, and restriction of the fetal and adult γδTCR repertoires. Importantly, E proteins interact directly with the cis-regulatory elements of the TCRγ and TCRδ loci, controlling the predisposition of a cell to become an αβ T cell or a γδ T cell, even before the lineage-dictating TCR signaling events. This article is categorized under: Immune System Diseases > Stem Cells and Development Immune System Diseases > Genetics/Genomics/Epigenetics.

Liver X receptor β is required for the survival of single-positive thymocytes by regulating IL-7Rα expression

Liver X receptors (LXRs) are known as key transcription factors in lipid metabolism and have been reported to play an important role in T-cell proliferation. However, whether LXRs play a role in thymocyte development remains largely unknown. Here, we demonstrated that LXRβ deficiency caused a reduction in single-positive (SP) thymocytes, whereas the transitions from the double-negative to SP stage were normal. Meanwhile, LXRβ-null SP thymocytes exhibited increased apoptosis and impairment of the IL-7Rα-Bcl2 axis. In addition, the LXR agonist T0901317 promoted the survival of SP thymocytes with enhanced IL-7Rα expression in wild-type mice but not in LXRβ-deficient mice. Mechanistically, LXRβ positively regulated the expression of IL-7Rα via direct binding to the Il7r allele in SP thymocytes, and forced expression of IL-7Rα or Bcl2 restored the survival of LXRβ-defective SP thymocytes. Thus, our results indicate that LXRβ functions as an important transcription factor upstream of IL-7Rα to promote the survival of SP thymocytes.

Immature CD8 Single-Positive Thymocytes Are a Molecularly Distinct Subpopulation, Selectively Dependent on BRD4 for Their Differentiation

T cell differentiation in the thymus proceeds in an ordered sequence of developmental events characterized by variable expression of CD4 and CD8 coreceptors. Here, we report that immature single-positive (ISP) thymocytes are molecularly distinct from all other T cell populations in the thymus in their expression of a gene profile that is dependent on the transcription factor BRD4. Conditional deletion of BRD4 at various stages of thymic differentiation reveals that BRD4 selectively regulates the further differentiation of ISPs by targeting cell cycle and metabolic pathways, but it does not affect the extensive proliferation that results in the generation of ISPs. These studies lead to the conclusion that the ISP subpopulation is not a hybrid transitional state but a molecularly distinct subpopulation that is selectively dependent on BRD4.

Control of Intra-Thymic αβ T Cell Selection and Maturation by H3K27 Methylation and Demethylation

In addition to transcription factor binding, the dynamics of DNA modifications (methylation) and chromatin structure are essential contributors to the control of transcription in eukaryotes. Research in the past few years has emphasized the importance of histone H3 methylation at lysine 27 for lineage specific gene repression, demonstrated that deposition of this mark at specific genes is subject to differentiation-induced changes during development, and identified enzymatic activities, methyl transferases and demethylases, that control these changes. The present review discusses the importance of these mechanisms during intrathymic αβ T cell selection and late differentiation.

Hints on T cell responses in a fish-parasite model: Enteromyxum leei induces differential expression of T cell signature molecules depending on the organ and the infection status

BACKGROUD

Enteromyxum leei is a myxozoan parasite that produces a slow-progressing intestinal disease. This parasite invades the paracellular space of the intestinal epithelium and progresses from the posterior to the anterior intestine. The aim of the present study was to gain insights into fish T cell responses in the gilthead sea bream-E. leei infection model using a PCR-array with 30 signature molecules for different leukocyte responses in head kidney, spleen, anterior and posterior intestine.

RESULTS

The PCR-array results suggest that E. leei induced migration of T cells from head kidney to intestines where T_H1, CTL and T_H17 profiles were activated and kept in balance by the upregulation of regulatory cytokines. These results were partially validated by the use of cross-reacting antibodies and BrdU immunostaining to monitor proliferation. Zap70 immunostaining supported the increased number of T cells in the anterior intestine detected by gene expression, but double staining with BrdU did not show active proliferation of this cell type at a local level, supporting the migration from lymphohaematopoietic tissues to the site of infection. Global analyses of the expression profiles revealed a clear separation between infected and exposed, but non-infected fish, more evident in the target organ. Exposed, non-infected animals showed an intermediate phenotype closer to the control fish.

CONCLUSIONS

These results evidence a clear modulation of the T cell response of gilthead sea bream upon E. leei infection. The effects occurred both at local and systemic levels, but the response was stronger and more specific at the site of infection, the intestine. Altogether, this research poses a promising basis to understand the response against this important parasite and establish effective preventive or palliative measures.

Immune dysregulation may contribute to disease pathogenesis in spinal muscular atrophy mice

Spinal muscular atrophy (SMA) has long been solely considered a neurodegenerative disorder. However, recent work has highlighted defects in many other cell types that could contribute to disease aetiology. Interestingly, the immune system has never been extensively studied in SMA. Defects in lymphoid organs could exacerbate disease progression by neuroinflammation or immunodeficiency. Smn depletion led to severe alterations in the thymus and spleen of two different mouse models of SMA. The spleen from Smn depleted mice was dramatically smaller at a very young age and its histological architecture was marked by mislocalization of immune cells in the Smn^2B/- model mice. In comparison, the thymus was relatively spared in gross morphology but showed many histological alterations including cortex thinning in both mouse models at symptomatic ages. Thymocyte development was also impaired as evidenced by abnormal population frequencies in the Smn^2B/- thymus. Cytokine profiling revealed major changes in different tissues of both mouse models. Consistent with our observations, we found that survival motor neuron (Smn) protein levels were relatively high in lymphoid organs compared to skeletal muscle and spinal cord during postnatal development in wild type mice. Genetic introduction of one copy of the human SMN2 transgene was enough to rescue splenic and thymic defects in Smn^2B/- mice. Thus, Smn is required for the normal development of lymphoid organs, and altered immune function may contribute to SMA disease pathogenesis.