Cell Competition, the Kinetics of Thymopoiesis, and Thymus Cellularity Are Regulated by Double-Negative 2 to 3 Early Thymocytes

Detecting ectopic thymus in thymoma-associated myasthenia gravis through flow cytometry analysis of CD3mediumTCRvβmediumCD4+CD8+ T cells and its clinical significance

Background

Traditional histological methods for identifying ectopic thymus (ET) have significant limitations including high risk of false negatives. This study aims to evaluate the effectiveness of flow cytometry in detecting ETs in patients undergoing total thymectomy.

Methods

We analyzed 864 samples from 103 patients using flow cytometry and hematoxylin and eosin (H&E) staining. ETs were identified by the presence of CD3mediumTCRvβmediumCD4+CD8+ T cells in flow cytometry or Hassall's corpuscles in H&E staining.

Results

In the discovery set, flow cytometry detected ETs in 69.2% of samples, compared to 23.6% by histological methods. The validation set showed a higher incidence of ETs in myasthenia gravis (MG) patients than in non-MG patients (73.5% vs. 58.0%, P < 0.0001) and in those with thymic epithelial tumors versus normal thymus (68.1% vs. 58.1%, P = 0.0088). MG patients exhibited a higher prevalence of active ETs, characterized by a high proportion of CD4+CD8+ T cells, indicating robust thymopoiesis, compared to those without MG (P = 0.0001). Specific regions, such as the left cervical root, areas along the right and left phrenic nerves, and the left innominate vein, showed significantly higher activity (P < 0.05). Additionally, ETs were more frequently found in the cervical region than in the mediastinum (75.0% vs. 60.8%, P = 0.0012), and in patients aged 40 years or younger compared to those older than 40 years (73.0% vs. 60.6%, P = 0.0027).

Conclusions

Flow cytometry is a viable alternative for ET detection, providing a novel distribution map that enhances surgical decision-making in MG treatment.

High-resolution mapping of cell cycle dynamics during steady-state T cell development and regeneration in vivo

Control of cell proliferation is critical for the lymphocyte life cycle. However, little is known about how stage-specific alterations in cell cycle behavior drive proliferation dynamics during T cell development. Here, we employed in vivo dual-nucleoside pulse labeling combined with the determination of DNA replication over time as well as fluorescent ubiquitination-based cell cycle indicator mice to establish a quantitative high-resolution map of cell cycle kinetics of thymocytes. We developed an agent-based mathematical model of T cell developmental dynamics. To generate the capacity for proliferative bursts, cell cycle acceleration followed a "stretch model" characterized by the simultaneous and proportional contraction of both G1 and S phases. Analysis of cell cycle phase dynamics during regeneration showed tailored adjustments of cell cycle phase dynamics. Taken together, our results highlight intrathymic cell cycle regulation as an adjustable system to maintain physiologic tissue homeostasis and foster our understanding of dysregulation of the T cell developmental program.

T Cell Development: From T-Lineage Specification to Intrathymic Maturation

T cell development occurs in the thymus in both mice and humans. Upon entry into the thymus, bone marrow-derived blood-borne progenitors receive instructive signals, including Notch signaling, to eliminate their potential to develop into alternative immune lineages while committing to the T cell fate. Upon T-lineage commitment, developing T cells receive further instructional cues to generate different T cell sublineages, which together possess diverse immunological functions to provide host immunity. Over the years, numerous studies have contributed to a greater understanding of key thymic signals that govern T cell differentiation and subset generation. Here, we review these critical signaling factors that govern the different stages of both mouse and human T cell development, while also focusing on the transcriptional changes that mediate T cell identity and diversity.

Mechanisms Underlying the Development of Murine T-Cell Lymphoblastic Lymphoma/Leukemia Induced by Total-Body Irradiation

Exposure to ionizing radiation is associated with an increased risk of hematologic malignancies in myeloid and lymphoid lineages in humans and experimental mice. Given that substantial evidence links radiation exposure with the risk of hematologic malignancies, it is imperative to deeply understand the mechanisms underlying cellular and molecular changes during the latency period between radiation exposure and the emergence of fully transformed malignant cells. One experimental model widely used in the field of radiation and cancer biology to study hematologic malignancies induced by radiation exposure is mouse models of radiation-induced thymic lymphoma. Murine radiation-induced thymic lymphoma is primarily driven by aberrant activation of Notch signaling, which occurs frequently in human precursor T-cell lymphoblastic lymphoma (T-LBL) and T-cell lymphoblastic leukemia (T-ALL). Here, we summarize the literature elucidating cell-autonomous and non-cell-autonomous mechanisms underlying cancer initiation, progression, and malignant transformation in the thymus following total-body irradiation (TBI) in mice.

Overexpression of Lmo2 initiates T-lymphoblastic leukemia via impaired thymocyte competition

Cell competition has recently emerged as an important tumor suppressor mechanism in the thymus that inhibits autonomous thymic maintenance. Here, we show that the oncogenic transcription factor Lmo2 causes autonomous thymic maintenance in transgenic mice by inhibiting early T cell differentiation. This autonomous thymic maintenance results in the development of self-renewing preleukemic stem cells (pre-LSCs) and subsequent leukemogenesis, both of which are profoundly inhibited by restoration of thymic competition or expression of the antiapoptotic factor BCL2. Genomic analyses revealed the presence of Notch1 mutations in pre-LSCs before subsequent loss of tumor suppressors promotes the transition to overt leukemogenesis. These studies demonstrate a critical role for impaired cell competition in the development of pre-LSCs in a transgenic mouse model of T cell acute lymphoblastic leukemia (T-ALL), implying that this process plays a role in the ontogeny of human T-ALL.

Cell competition in development, homeostasis and cancer

Organ development and homeostasis involve dynamic interactions between individual cells that collectively regulate tissue architecture and function. To ensure the highest tissue fidelity, equally fit cell populations are continuously renewed by stochastic replacement events, while cells perceived as less fit are actively removed by their fitter counterparts. This renewal is mediated by surveillance mechanisms that are collectively known as cell competition. Recent studies have revealed that cell competition has roles in most, if not all, developing and adult tissues. They have also established that cell competition functions both as a tumour-suppressive mechanism and as a tumour-promoting mechanism, thereby critically influencing cancer initiation and development. This Review discusses the latest insights into the mechanisms of cell competition and its different roles during embryonic development, homeostasis and cancer.

Thymosin Beta 15 Alters the Spatial Development of Thymic Epithelial Cells

The thymus is the most sensitive organ under various pathophysiological conditions, such as aging, starvation, and infection. As a key stromal cell for T cell development, it is well-known that thymic epithelial cells (TECs) play an important role in the thymus response to the external environment. Thymosin beta 15 (Tβ15) is a G-actin binding protein secreted by TECs, it plays an important role in maintaining the dynamic balance of actin, angiogenesis, axonal formation, and wound healing, but the relationship between Tβ15 and TECs is not clear yet. Here, we show the impact of Tβ15 on the TEC's spatial development, as well as the T-cell differentiation and thymic output. As a result, TEC is the main effector cell of Tβ15 in the thymus. Tβ15 OX inhibits the chemotaxis of TECs to the medulla and subsequently blocks the positive selection of thymocytes from CD3⁺TCRβ⁺CD4⁺CD8⁺ double positive cells to CD3⁺TCRβ⁺CD4⁺CD8^- single-positive (CD4SP) cells. Tβ15-knockdown accelerates the reticular differentiation of astral TECs and medullary TECs. Importantly, mice implanted with Tβ15-knockdown iTECs show high thymic output but low peripheral T cell maturity and activity. In a word, our results explain the role of Tβ15 on the differentiation and function of TECs and provide a new perspective for understanding the process of thymus development and degeneration.

TUMOR CELL MALIGNANCY: A COMPLEX TRAIT BUILT THROUGH RECIPROCAL INTERACTIONS BETWEEN TUMORS AND TISSUE-BODY SYSTEM

Since the discovery of oncogenes and tumor suppressor genes in the late past century, cancer research has been overwhelmingly focused on the genetics and biology of tumor cells and hence has addressed mostly cell-autonomous processes with emphasis on traditional driver/passenger genetic models. Nevertheless, over that same period, multiple seminal observations have accumulated highlighting the role of non-cell autonomous effectors in tumor growth and metastasis. However, given that cell autonomous and non-autonomous events are observed together at the time of diagnosis, it is in fact impossible to know whether the malignant transformation is initiated by cell autonomous oncogenic events or by non-cell autonomous conditions generated by alterations of the tissue-body ecosystem. This review aims at addressing this issue by taking the option of defining malignancy as a complex genetic trait incorporating genetically determined reciprocal interactions between tumor cells and tissue-body ecosystem.

Peptidylprolyl isomerase C (Ppic) regulates invariant Natural Killer T cell (iNKT) differentiation in mice

Peptidyl‐prolyl cis‐trans isomerase C (Ppic) is expressed in several bone marrow (BM) hematopoietic progenitors and in T‐cell precursors. Since the expression profile of Ppic in the hematoimmune system was suggestive that it could play a role in hematopoiesis and/or T lymphocyte differentiation, we sought to test that hypothesis in vivo. Specifically, we generated a Ppic‐deficient mouse model by targeting the endogenous locus by CRISPR/Cas9 and tested the requirement of Ppic in hematopoiesis. Several immune cell lineages covering BM progenitors, lymphocyte precursors, as well as mature cells at the periphery were analyzed. While most lineages were unaffected, invariant NKT (iNKT) cells were reduced in percentage and absolute cell numbers in the Ppic‐deficient thymus. This affected the most mature stages in the thymus, S2 and S3, and the phenotype was maintained at the periphery. Additionally, immature transitional T1 and T2 B lymphocytes were increased in the Ppic‐deficient spleen, but the phenotype was lost in mature B lymphocytes. In sum, our data show that Ppic is dispensable for myeloid cells, platelets, erythrocytes, αβ, and γδ T lymphocytes in vivo in the steady state, while being involved in B‐ and iNKT cell differentiation.

Modeling the Dynamics of T-Cell Development in the Thymus

The thymus hosts the development of a specific type of adaptive immune cells called T cells. T cells orchestrate the adaptive immune response through recognition of antigen by the highly variable T-cell receptor (TCR). T-cell development is a tightly coordinated process comprising lineage commitment, somatic recombination of Tcr gene loci and selection for functional, but non-self-reactive TCRs, all interspersed with massive proliferation and cell death. Thus, the thymus produces a pool of T cells throughout life capable of responding to virtually any exogenous attack while preserving the body through self-tolerance. The thymus has been of considerable interest to both immunologists and theoretical biologists due to its multi-scale quantitative properties, bridging molecular binding, population dynamics and polyclonal repertoire specificity. Here, we review experimental strategies aimed at revealing quantitative and dynamic properties of T-cell development and how they have been implemented in mathematical modeling strategies that were reported to help understand the flexible dynamics of the highly dividing and dying thymic cell populations. Furthermore, we summarize the current challenges to estimating in vivo cellular dynamics and to reaching a next-generation multi-scale picture of T-cell development.

Self-renewal of double-negative 3 early thymocytes enables thymus autonomy but compromises the β-selection checkpoint

T lymphocyte differentiation in the steady state is characterized by high cellular turnover whereby thymocytes do not self-renew. However, if deprived of competent progenitors, the thymus can temporarily maintain thymopoiesis autonomously. This bears a heavy cost, because prolongation of thymus autonomy causes leukemia. Here, we show that, at an early stage, thymus autonomy relies on double-negative 3 early (DN3e) thymocytes that acquire stem-cell-like properties. Following competent progenitor deprivation, DN3e thymocytes become long lived, are required for thymus autonomy, differentiate in vivo, and include DNA-label-retaining cells. At the single-cell level, the transcriptional programs of thymopoiesis in autonomy and the steady state are similar. However, a new cell population emerges in autonomy that expresses an aberrant Notch target gene signature and bypasses the β-selection checkpoint. In summary, DN3e thymocytes have the potential to self-renew and differentiate in vivo if cell competition is impaired, but this generates atypical cells, probably the precursors of leukemia.