Group 2 innate lymphoid cells (ILC2) are tissue-resident innate lymphocytes that play a crucial role in allergic inflammation. ILC2 are derived from common lymphoid progenitor (CLP) and IL-7 and Notch signals are essential for their differentiation. There are two critical steps for differentiation into ILC2; the lineage commitment step from CLP into the ILC committed progenitor (ILCP), and the differentiation step after lineage commitment, in which ILCP develop into mature ILC2. While ILCP and transcription factors essential for ILC2 differentiation have been well studied, optimal microenvironments such as external factors and supporting cells that regulate these steps are unknown.
To clarify the external factors that regulate the lineage commitment step, CLP from fetal liver were co-cultured with TSt4-DLL1 stromal cells. We found that concentration of IL-7 differentially regulated T and ILC2 differentiation. Furthermore, we established TSt4 Tet-off DLL stromal cells in which expression levels of DLL can be controlled by Doxycyclin in a dose-dependent manner. Using these cells, we revealed that strength and duration of Notch signals divide the commitment to T, B and ILC2.
Next, to identify the microenvironment that supports the ILC2 differentiation step after lineage commitment, we investigated cell populations in the mesentery, which is the most abundant source of ILC2, and found that ILCP and immature ILC2 that lack potential to produce IL-5 and IL-13 in response to IL-33 exist in the fetal mesentery. We also elucidated that STAT5 activators such as IL-2, IL-7, and TSLP support their maturation after birth. Furthermore we found that PDGFRα+gp38+ mesenchymal cells support ILC2 differentiation from ILCP in the mesentery.
Our results demonstrate that microenvironmental factors such as concentration of IL-7 and strength and duration of Notch signals regulate lymphocyte lineage commitment from CLP and peripheral tissues provide an optimal microenvironment for terminal differentiation of ILC2.