Synergy of TL1A and IL-18 promotes GM-CSF-dependent thymic granulopoiesis in mice

Rheumatology and Immunology Medicine Basic Medical Sciences Hematology Life Sciences Immunology
TL1A IL-18 thymic neutrophils acute thymic atrophy cytokine synergy extramedullary granulopoiesis

Background Introduction

In acute systemic inflammatory conditions, the immune system undergoes significant changes, typically characterized by increased myeloid cell generation and suppressed lymphocyte production. In the thymus specifically, systemic inflammation leads to acute thymic atrophy and subsequent impairment of T lymphocyte generation. However, the mechanisms by which systemic inflammation affects the thymus, beyond suppressing T cell development, are not fully understood. In this study, we investigated how the synergistic action of two cytokines, TL1A and IL-18, inhibits T lymphocyte generation while promoting thymic myeloid cell production.

Research Source

This study was conducted by Mario Ruiz Pérez, Christian Maueröder, Wolf Steels, and others from Ghent University and affiliated research institutions in Belgium. The paper was published in the academic journal “Cellular & Molecular Immunology” in 2024.

Research Process

  1. Experimental Model and Cytokine Injection

The study began by injecting TL1A and IL-18 into newborn mice to observe their effects on thymic development. Experimental groups included PBS injection (control group), TL1A injection alone, IL-18 injection alone, and combined TL1A and IL-18 injection. Subsequently, a neonatal thymic organ culture (NTOC) model was established to analyze the mechanisms of these cytokines in more detail in vitro.

  1. Detection of Thymic Cells and Receptor Expression

Flow cytometry was used to characterize the expression of DR3 and IL-18Rα receptors in the thymus of newborn and adult mice, distinguishing different subgroups of thymic cells. Results showed that DR3 was widely expressed in both neonatal and adult thymi, while IL-18Rα expression was mainly limited to ILC1, ILC2, and γδT cells.

  1. Cytokine Treatment and Effect Observation

In the NTOC model, combined treatment with TL1A and IL-18 resulted in acute atrophy of both neonatal and adult mouse thymi, as analyzed by flow cytometry, with a significant decrease in T cell numbers and a marked increase in myeloid cells and neutrophils. Transmission electron microscopy (TEM) further verified these changes, showing notable morphological alterations in stromal cells after combined TL1A and IL-18 treatment.

  1. Development and Maturation of Thymic Neutrophils

The NTOC system provided an isolated platform for studying thymic development. Based on single-cell RNA sequencing (scRNA-seq) and cell cycle analysis data, the study found that thymic neutrophils could proliferate and mature from myeloid progenitor cells in the in vitro culture system. Further trajectory inference analysis (Slingshot) revealed the developmental trajectory of neutrophils from pre-neutrophils to immature neutrophils to mature neutrophils.

  1. Relationship Between Notch Signaling Pathway and Neutrophil Expansion

Using the γ-secretase inhibitor LY411575 to block the Notch signaling pathway in the NTOC system, results showed that Notch signaling has a limiting effect on inhibiting neutrophil development, consistent with previous research findings.

Research Results

  1. Synergistic Effect of TL1A and IL-18

In in vivo experiments, newborn and adult mice injected with combined TL1A and IL-18 showed significant growth retardation and acute thymic atrophy, further demonstrating the synergistic effect of these two cytokines. In the NTOC model, combined treatment also led to significant expansion of myeloid cells and neutrophils, while T cell numbers decreased, consistent with in vivo experimental results.

  1. Fate Exploration of Thymic Progenitor Cells

Using Rag1-Cre and Ms4a3-Cre genetic tracing tools, it was found that most thymic neutrophils originate from Rag1-positive thymic myeloid progenitor cells (GMPs) rather than bone marrow-derived cells. This suggests the existence of a myeloid cell generation pathway in the thymus independent of the bone marrow.

  1. Functional Assessment

Further functional experiments showed that TL1A and IL-18-induced thymic neutrophils possess capabilities similar to peritoneal neutrophils in terms of oxidative burst, phagocytosis, migration, and formation of neutrophil extracellular traps (NETs). High expression of metalloproteases (such as MMP8 and MMP9) suggests these cells may have specific tissue functions in matrix remodeling.

  1. GM-CSF Dependent Mechanism

Measurement of cytokine levels in NTOC supernatants revealed that TL1A and IL-18 synergistic treatment significantly upregulated the release of GM-CSF, IL-17A, and IFN-γ. Single-cell RNA sequencing and flow cytometry confirmed that GM-CSF is mainly produced by ILC1s, and anti-GM-CSF receptor antibodies significantly reduced neutrophil numbers in the NTOC system, indicating that GM-CSF is a key factor driving thymic myeloid cell generation.

Research Significance and Value

This study reveals the synergistic role of TL1A and IL-18 in thymic development, promoting myeloid cell generation through a GM-CSF-dependent mechanism while inhibiting T lymphocyte generation. These findings not only deepen our understanding of thymic biology but also provide new potential therapeutic targets for addressing immune system changes induced by systemic inflammation.

Research Highlights

  1. First Revelation of TL1A and IL-18 Synergy

The study first revealed how TL1A and IL-18 regulate thymic myeloid cell generation through GM-CSF, providing new insights.

  1. Unique Thymic Myeloid Cell Generation Pathway

Through genetic tracing models, the study confirmed the existence of an independent myeloid cell generation pathway in the thymus, advancing research on thymic function.

  1. Comprehensive Functional Assessment

The study evaluated the functions of TL1A and IL-18-induced thymic neutrophils in various aspects, validating their important role in the immune system.

Conclusion

The research demonstrates that TL1A and IL-18 synergistically promote acute thymic atrophy and drive extramedullary myeloid cell generation under the control of Notch and GM-CSF signaling pathways. This discovery provides a new perspective on understanding the profound impact of systemic inflammation on the thymus, while pointing out potential therapeutic targets that may help improve patients’ immune system function and health status.

Questions Worthy of Further Research

  1. Tissue-Specific Functions of Thymic Neutrophils Further research needs to explore the specific roles of thymic neutrophils in tissue-specific functions, especially their contributions to thymic remodeling and immune regulation.

  2. Potential of Anti-TL1A and IL-18 Therapy Further investigation into whether anti-TL1A and IL-18 therapy can effectively prevent or reverse thymic atrophy clinically, thereby enhancing immune system function.

Epilogue

This study not only makes significant contributions to elucidating the mechanisms of thymic development and function but also provides innovative perspectives and potential solutions for treating systemic inflammation, holding important scientific and clinical application value.