E proteins are important for restraining the ILC2 differentiation potential in the thymus

Innate lymphoid cells (ILCs) have gained recognition as important regulators in various immune responses. Current paradigm states that all ILCs originate from common lymphoid progenitors (CLP) in the bone marrow through an array of downstream ILC progenitors. Id2, a member of the Id family that inhibits E protein transcription factors, is expressed in these ILC progenitors and is indispensable for ILC differentiation. Interestingly, we found that down-regulating E protein function in the thymus by Id1 overexpression or deletion of two E protein genes resulted in massive expansion of ILC2s in the thymus as well as other organs where ILC2 normally reside. The phenotypic thymic ILC2s expressed ILC2 signature genes and down-regulated T lineage-specific genes. Moreover, Id1 transgenic mice exhibited spontaneous infiltration of eosinophils and heightened type 2 immune responses to the allergen, papain, in the lung. Furthermore, these mice had increased ability in the expulsion of helminth parasites, N. Brasiliensis. These results raise an interesting question as to whether the thymus is a natural site for ILC2 production and the phenotypes of the mutant mice simply highlight such capacity. Indeed, we found that wild type DN1 and DN3 thymocytes can differentiate into IL-5 and IL-13-producing ILC2 cells in vitro. Ablation of E proteins in DN3 cells accelerated the differentiation while gain of E protein function inhibited the process. Together, our data reveal a new ILC2 differentiation program occurring in the thymus and a critical role of E proteins in restraining such a differentiation potential while ensuring proper T cell production.