The Basic Leucine Zipper Transcription Factor Nfil3 Directs the Development of 13 a Common Innate Lymphoid Cell Precursor 14 15

40 41 42 43 44 2 45 Abstract 46 Innate lymphoid cells (ILCs) are recently identified lymphocytes that limit infection and promote tissue 47 repair at mucosal surfaces. However, the pathways underlying ILC development remain unclear. Here we 48 show that the transcription factor NFIL3 directs the development of a committed bone marrow precursor 49 that differentiates into all known ILC lineages. NFIL3 was required in the common lymphoid progenitor 50 (CLP), and was essential for the differentiation of αLP, a bone marrow cell population that gives rise to 51 all known ILC lineages. Clonal differentiation studies revealed that CXCR6 + cells within the αLP 52 population differentiate into all ILC lineages but not T-and B-cells. We further show that NFIL3 governs 53 ILC development by directly regulating expression of the transcription factor TOX. These findings 54 establish that NFIL3 directs the differentiation of a committed ILC precursor that gives rise to all ILC 55 lineages and provide insight into the defining role of NFIL3 in ILC development. 56 57 58 3 Introduction 59 Innate lymphoid cells (ILCs) are a recently identified family of lymphocytes that perform a 60 variety of immune functions at barrier surfaces (Spits and Cupedo, 2012). Although ILCs share a 61 common developmental origin with Band T-cells, they lack antigen-specific receptors. Instead, they 62 exert their immune functions through cytokine secretion in a manner similar to T helper cells (Spits et al., 63 2013). Despite their important contributions to immunity, the pathways that regulate ILC development 64 remain poorly understood. 65 There are three known ILC groups. ILC1, which include conventional NK (cNK) cells, require 66 the transcription factors T-BET and/or EOMES, produce interferon-γ (IFNγ) (Fuchs et al. important for the defense of barrier surfaces as they promote anatomical containment of commensal 74 bacteria (Sonnenberg et al., 2012), regulate CD4 + T cell responses to commensal bacteria (Hepworth et al., 75 2013; Qiu et al., 2013), and stimulate epithelial cells to produce antibacterial proteins (Sanos et al., 2011). 76 All ILC differentiate from the common lymphoid progenitor (CLP), which resides in the bone 77 marrow and also gives rise to Band T-lymphocytes (Hoyler et al., 2012; Possot et al., 2011). Committed 78 ILC progenitors that are positioned developmentally downstream of the CLP have been identified, and 79 give rise to various ILC subsets. For example, an Id2 (inhibitor of DNA binding 2)-expressing progenitor, 80 known as …