Alterations of Thymic Epithelial Cells in Lipopolysaccharide-induced Neonatal Thymus Involution

Background： Vascular endothelial growth factor （VEGF） in the thymus was mainly produced by the thymic epithelial cells （TECs）, the predominant component of the thymic microenvironment. The progression of TECs and the roles of VEGF in the neonatal thymus during sepsis have not been reported. This study aimed to explore the alterations of TECs and VEGF level in the neonatal thymus involution and to explore the possible mechanisms at the cellular level. Methods： By establishing a model of clinical sepsis, the changes of TECs were measured by hematoxylin-eosin staining. confocal microscopy, and flow cytometry. Moreover, the levels of VEGF in serum and thymus were assessed based on enzyme-linked immunosorbent assay and Western blotting. Results： The number ofthymocytes and TECs was significantly decreased 2411 after lipopolysaccharide （LPS） challenge. （2.40 ± 0,46）× 10^7 vs. （3.93 ± 0.66）× 10^7 and （1.16 ± 0.14）× 10^5 vs. （2.20 ± 0.19）× 10^5, P 〈 0.05, respectively. Cortical TECs and medullary TECs in the LPS-treated mice were decreased 1.5-fold and 3.9-fold, P 〈 0.05, respectively, lower than those in the controls. The number of thymic epithelial progenitors was also decreased. VEGF expression in TECs was down-regulated in a time-dependent manner. Conclusion： VEGF in thymic cells subsets might contribute to the development of TECs in neonatal sepsis.

Thymic epithelial cell development and differentiation: cellular and molecular regulation

Thymic epithelial cells (TECs) are one of the most important components in thymic microenvironment supporting thymocyte development and maturation. TECs, composed of cortical and medullary TECs, are derived from a common bipotent progenitor, mediating thymocyte positive and negative selections. Multiple levels of signals including intracellular signaling networks and cell-cell interaction are required for TEC development and differentiation. Transcription factors Foxn1 and autoimmune regulator (Aire) are powerful regulators promoting TEC development and differentiation. Crosstalks with thymocytes and other stromal cells for extrinsic signals like RANKL, CD40L, lymphotoxin, fibroblast growth factor (FGF) and Wnt are also definitely required to establish a functional thymic microenvironment. In this review, we will summarize our current understanding about TEC development and differentiation, and its underlying multiple signal pathways.

Insm1 regulates mTEC development and immune tolerance

The expression of self-antigens in medullary thymic epithelial cells(mTECs)is essential for the establishment of immune tolerance,but the regulatory network that controls the generation and maintenance of the multitude of cell populations expressing self-antigens is poorly understood.Here,we show that Insm1,a zinc finger protein with known functions in neuroendocrine and neuronal cells,is broadly coexpressed with an autoimmune regulator(Aire)in mTECs.Insm1 expression is undetectable in most mimetic cell populations derived from mTECs but persists in neuroendocrine mimetic cells.Mutation of Insm1 in mice downregulated Aire expression,dysregulated the gene expression program of mTECs,and altered mTEC subpopulations and the expression of tissue-restricted antigens.Consistent with these findings,loss of Insm1 resulted in autoimmune responses in multiple peripheral tissues.We found that Insm1 regulates gene expression in mTECs by binding to chromatin.Interestingly,the majority of the Insm1 binding sites are co-occupied by Aire and enriched in superenhancer regions.Together,our data demonstrate the important role of Insm1 in the regulation of the repertoire of self-antigens needed to establish immune tolerance.

The complicated role of NF-κB in T-cell selection

The nuclear factor （NF）-κB transcription factor family plays important roles in the immune system. Aberrant NF-κB signaling is frequently associated with inflammation and autoimmune diseases but the underlying mechanisms are not fully understood. Recent studies show that NF-κB plays a critical role in T-cell central tolerance. Two NF-κB signaling pathways have been identified： the canonical pathway and the alternative pathway. In the establishment of T-cell central tolerance, the alternative pathway appears to be the key signaling component in thymic stromal cells for their development and function, while the canonical pathway exerts its function more in autonomous T-cell selection. This review intends to summarize the current understanding of the role of NF-κB in establishing T-cell central tolerance and highlight unsolved intriguing questions for future work.