TL1A and IL-18 synergy promotes GM-CSF dependent thymic emergency granulopoiesis
The current model of hematopoiesis only resolves the thymus around the production and establishment of the peripheral T cell pool. However, the role and in situ development of other immune subsets has been overlooked. We aim to demonstrate that during TL1A and IL-18 induced inflammation, the thymus is capable of producing other cell types such as neutrophils, monocytes, and macrophages and that these potentially have other functions aside from supporting T cell development as scavengers. Here we have proven that ex vivo and in vivo treatment with TL1A and IL-18 results in acute thymic atrophy by a massive loss of DP T cells and shrinking of the thymic lobe. By electron microscopy, flow cytometry and single cell we demonstrate that neutrophils are able to maturate inside the thymus lobe in a NOTCH-independent manner. We used fate-mapping tools to elucidate the origin of thymic neutrophils since we observed a great expansion in our culture that is isolated from the influx of BM progenitors. The Rag1-Cre Rosa26-YFP fate mapping model revealed that neutrophils share a common progenitor with T cells, while monocytes/macrophages do not. Furthermore, we found that most of the thymic GMPs (defined as Lin-Sca-1-c-Kit+CD16/32+CD34+) show history of Rag1 expression in comparison with bone marrow GMPs. Additionally, by using Ms4a3-Cre Rosa26Tdtomato fate mapping model we observed that there was not difference in Tdt labeling between the thymus and bone marrow, suggesting that thymic neutrophils still undergo conventional neutrophils pathway arising from GMPs. Moreover, we show that thymic-derived neutrophils are functional and are capable of forming extracellular neutrophils traps (NETs) similarly to benchmark peritoneal neutrophils. We found that the expansion of thymic neutrophils is GM-CSF dependent by using Csf2rb KO mice.  Additionally, we identified DR3+ and IL-18Ra+ expressing subsets of ILCs and gdT cells as the cellular source GM-CSF. Lastly, in vivo treatment with TL1A+IL-18 lead to emergency granulopoiesis and an increase of neutrophils in all the organs investigated, including the thymus. Overall design: There are two sets of samples. The first set consists of two samples both originating from murine neonatal thymus organ cultures 6 days post treatment, one treated with TL1A, the other sample treated with TL1A + IL-18.The second set consists of seven 5' scRNA sequencing samples all derived from murine neonatal thymus organ cultures that were treated with mock or a combination of cytokines across three timepoints. The first is a reference sample taken on day 0. Samples 2 to 7 are all taken from the lobes of a neonatal thymus organ culture.The treatments of samples 2,3 and 4 are, respectively, mock treatment, IL-12+IL-18 and TL1A+IL-18. The samples were taken 1.5 days post treatment. Samples 5,6 and 7 have the same respective treatments but were taken 3 days post treatmentSample 8 is taken from the supernatant of a neonatal thymus organ culture 3 days post treatment with IL-12+IL-18.