Circulating mature dendritic cells homing to the thymus promote thymic epithelial cells involution via the Jagged1/Notch3 axis

Multiple proinflammatory conditions, including chemotherapy, radiotherapy, transplant rejection, and microbial infections, have been identified to induce involution of the thymus. However, the underlying cellular and molecular mechanisms of these inflammatory conditions inducing apoptosis of thymic epithelial cells (TECs), the main components of the thymus, remain largely unknown. In the circulation, mature dendritic cells (mDCs), the predominant initiator of innate and adaptive immune response, can migrate into the thymus. Herein, we demonstrated that mDCs were able to directly inhibit TECs proliferation and induce their apoptosis by activating the Jagged1/Notch3 signaling pathway. Intrathymic injection of either mDCs or recombinant mouse Jagged1-human Fc fusion protein (rmJagged1-hFc) into mice resulted in acute atrophy of the thymus. Furthermore, DAPT, a γ-secretase inhibitor, reversed the effects induced by mDC or rmJagged1-hFc. These findings suggest that acute or aging-related thymus degeneration can be induced either by mass migration of circulating mDCs in a short period of time or by a few but constantly homing mDCs.

Image-guided intrathymic injection of multipotent stem cells supports lifelong T-cell immunity and facilitates targeted immunotherapy

Key Points Image-guided intrathymic injection of cells or drugs permits implementation of clinically relevant strategies to improve thymic function. Intrathymic injection of hematopoietic stem cells generates long-lasting antigen-specific T-cell immunity.

Development of thymic NK cells from double negative 1 thymocyte precursors

The differentiation of natural killer (NK) cells and a subpopulation of NK cells which requires an intact thymus, that is, thymic NK cells, is poorly understood. Previous in vitro studies indicate that double negative (CD4−CD8−, DN) thymocytes can develop into cells with NK cell markers, but these cells have not been well characterized. Herein, we generated and characterized NK cells differentiating from thymic DN precursors. Sorted DN1 (CD44+CD25−) CD122−NK1.1− thymocytes from Rag1−/− mice were adoptively transferred into Rag1−/−Ly5.1 congenic mice. After intrathymic injection, donor-derived cells phenotypically resembling thymic NK cells were found. To further study their differentiation, we seeded sorted DN1 CD122−NK1.1− thymocytes on irradiated OP9 bone marrow stromal cells with IL-15, IL-7, Flt3L, and stem cell factor. NK1.1+ cells emerged after 7 days. In vitro differentiated NK cells acquired markers associated with immature bone marrow–derived NK cells, but also expressed CD127, which is typically found on thymic NK cells. Furthermore, we found that in vitro cells generated from thymic precursors secreted cytokines when stimulated and degranulated on target exposure. Together, these data indicate that functional thymic NK cells can develop from a DN1 progenitor cell population.

TCR-dependent differentiation of thymic Foxp3+ cells is limited to small clonal sizes

Numerous studies have highlighted the importance of high-affinity interactions between T cell receptors (TCRs) and their ligands in the selection of Foxp3+ regulatory T cells (T reg cells). To determine the role of the TCR in directing T cells into the Foxp3+ lineage, we generated transgenic (Tg) mice expressing TCRs from Foxp3+ cells. Initial analyses of the TCR Tg mice crossed with RAG-deficient mice showed that the percentage of Foxp3+ cells was very low. However, intrathymic injection and bone marrow chimera experiments showed a saturable increase of the Foxp3+ population when T reg TCR Tg cells were present in low numbers. Furthermore, when analyzing whole thymi of T reg TCR Tg RAG-deficient mice, we found significantly more Foxp3+ cells than in conventional T cell TCR Tg mice. Our results indicate that although the TCR has an instructive role in determining Foxp3 expression, selection of Foxp3+ individual clones in the thymus is limited by a very small niche.