Heterologous expression of ABIN-1 (A20 Binding and Inhibitor of NF-kB-1, gene name Tnip1), like A20, was suggested to restrict TNF-induced inflammation and cell death. ABIN-1's physiological function was unknown. To interrogate ABIN-1's roles in vivo, we generated ABIN-1-deficient mice. ABIN-1 was required for successful embryonic development. TNF-deficiency rescued this lethality, demonstrating that ABIN-1 restricts potentially lethal TNF-induced signals. ABIN-1-/- TNF-/- mice develop a striking immune phenotype, suggesting that ABIN-1 restricts TNF-independent signals. ABIN-1-/- TNF-/- RAG-1-/- mice have splenomegaly, which supports that ABIN-1 expression in innate cells preserves immune quiescence. ABIN-1-/- fetal liver chimera develop partially cell-intrinsic widespread immune activation, suggesting that ABIN-1-/- hematopoietic cells are sufficient for exaggerated immune activation.

Aberrant immune activation contributes to the pathogenesis of multiple autoimmune and inflammation-exacerbated diseases. The Tnip1 gene is strongly associated with susceptibility to psoriasis in humans. Psoriasis is a chronic, inflammatory skin disease caused by a combination of environmental and genetic factors. We demonstrated that mice lacking ABIN-1 specifically in dendritic cells (DCs), ABIN-1Flox CD11c-Cre mice, exhibited perturbed immune homeostasis. ABIN-1 deficient DCs displayed exaggerated NF-kB and MAP kinase signaling and produced more IL-23 than normal cells in response to TLR ligands. Challenge of ABIN-1Flox CD11c-Cre mice with topical TLR7 ligand lead to greater numbers of TH17 and gamma delta T cells and exacerbated development of psoriaform lesions. These phenotypes were reversed by DC-specific deletion of the TLR adaptor MyD88. These studies link ABIN-1 with IL-23 and IL-17, and provide cellular and molecular mechanisms by which ABIN-1 regulates susceptibility to psoriasis. They support that ubiquitin sensor ABIN-1 regulates cell survival and restricts proinflammatory signals in DC to protect against psoriasis.

Ubiquitination is a process used by cells to tightly control signaling pathways by covalently attaching a ubiquitin molecule to a target substrate. This process is reversible by the activity of deubiquitinating enzymes. Otubain 1 (Otub1) is a novel deubiquitinating enzyme whose physiologic relevance in an in vivo setting had not been described. In this dissertation, we describe two novel functions of Otub1, discovered after creating and analyzing Otub1-deficient (Otub1-/-) mice.

We first found that Otub1-/- mice die during late embryonic development. To examine whether Otub1 plays a role in the functioning of immune cells, we reconstituted irradiated recipients with Otub1-/- fetal livers to create chimera. At baseline in the absence of overt stimulation, Otub1-/- chimera had more memory-phenotype CD8+ T cells than wildtype and T cell homeostasis was disrupted in a cell-intrinsic manner. Otub1-/- T cells also hyperrespond to homeostatic signals, particularly IL7 stimulation.

While studying Otub1's role in T cell homeostasis, we also discovered it had a role in regulating programmed cell death. Using mouse embryonic fibroblasts (MEFs) and a human cell line, we found that Otub1-deficiency or Otub1 knockdown sensitizes the cell to undergo more TNF-induced apoptosis and necroptosis. Specifically, Otub1-deficiency led to increased formation of the necrosome, the Rip3-containing complex that drives necroptosis. This function may in part be driven by an accelerated loss of cellular inhibitor of apoptosis protein-1 (c-IAP1) in Otub1-/- cells as compared to wildtype. Together, we have shown that Otub1 has important in vivo roles in regulating T cell homeostasis and programmed cell death. Along with other studies, this work establishes Otub1 as a critical regulator of diverse signaling pathways and opens up several future avenues of investigation.