Phd in IINNOVATION IN IMMUNO-MEDIATED AND HEMATOLOGICAL DISORDERS

Thesis title: Dissecting the roles of STAT4 in effector differentiation of liver ILC1

The murine liver is patrolled by innate lymphoid cells (ILCs) belonging to ILC1 and NK cell lineages, which are defined by type I immune features and IFN-g production. In this thesis, we showed that ILC1 were endowed with high expression of the signal dependent transcription factor (TF) STAT4. To evaluate the cell-intrinsic effect of STAT4 on innate lymphocytes, we generated a novel mouse model allowing conditional deletion of Stat4 gene in ILC1 and NK cells. We observed that genetic ablation of Stat4 in NKp46+ ILCs led to a contraction of the homeostatic pool of liver ILC1, while total NK cells remained unaffected. The observed contraction of Stat4-deficient liver ILC1 was associated with a decreased expression of CXCR6, a chemokine receptor involved in liver localization, as well as with dysregulated gene expression of several transcripts, at steady state, as evaluated by using scRNA-seq approach. Liver ILC1 devoid of Stat4 exhibited a more effector-like phenotype and a skewing toward cytotoxic ILC1. In this regard, IL-2-stimulated Stat4-deficient ILC1 were more potent granzyme producers as their Stat4-sufficient counterparts, implying that Stat4 assumed a suppressor role in effector differentiation of ILC1. In contrast to ILC1, STAT4 was required for the terminal differentiation of NK cells, and its lack affected the reservoir of terminally differentiated NK cells throughout tissues. Moreover, STAT4 activating cytokine IL-12 had a distinctly stronger suppressive effect on transcriptome of liver ILC1, compared to NK cells, further emphasizing that STAT4 plays diverging roles in liver ILC subsets. Altogether, our results show unexpected homeostatic roles for STAT4 in regulating effector functions of type 1 innate lymphocytes.