Final Report Summary - A20 IN ALLERGY (The role of A20 in dendritic cells and mast cells in the allergic immune response) Asthma is a chronic inflammatory airway disease wherein the transcription factor NF-?B plays a crucial role. NF-?B controls activation of dendritic cells (DC) and mast cells (MC) that contribute to disease pathogenesis. Activation of NF-?B is tightly regulated by ubiquitination and deubiquitination. Studies have identified several deubiquitinating enzymes like A20 and its binding-partners which regulate NF-?B activation. In view of the crucial role of TLR signalling and NF-?B activation in asthma and preliminary data implying deubiquitination of NF-?B signalling partners as a crucial downregulatory mechanism to prevent overt inflammation, we hypothesize that the function of A20 critically controls allergic airway inflammation. Here, we studied the effect of cell-specific deletion of A20 on allergic inflammation. For this we will use conditional A20-deficient animals, in which the A20 gene has been floxed, to allow for Cre recombinase-mediated knock-out of A20fl alleles in mice transgenically overexpressing Cre under a DC- or MC- specific promotor. Preliminary asthma experiments have surprisingly shown that DC-specific A20 deletion in mice (DC-A20KO mice) did not contribute to disease pathology, whereas MC-specific deletion of A20 (MC-A20KO mice) exhibited an exacerbation of the disease. In the DC-A20KO mice elevated levels of IL-17 produced by T cells could be found, and this lead us to the idea that these mice could develop auto-immunity with age. DCs are crucial regulators of both immunity and tolerance. We have observed that A20 (TNFAIP3) determines the activation threshold of DCs, via control of canonical NF-kB activation. DC-A20KO mice lacking A20 in DCs demonstrated spontaneous proliferation of CD4+ and CD8+ and double negative T cells, their conversion to IFNgamma-producing effector cells, and expansion of plasma cells. They developed ds-DNA antibodies, nephritis, and the anti-phospholipid syndrome -features of systemic lupus erythematosus- as well as lymphosplenomegaly and extramedullary haematopoiesis. A20-deficient DCs were resistant to apoptosis, caused by increased sensitivity to CD40L and RANKL pro-survival signals and upregulation of Bcl-2 and Bcl-x. They captured injected apoptotic cells more efficiently, resisted the inhibitory effects of apoptotic cells and induced self-reactive effector lymphocytes. As genetic polymorphisms in TNFAIP3 are associated with human autoimmune disorders, these findings identify A20-mediated control of DC activation as a crucial checkpoint in development of systemic autoimmunity. A revised manuscript of this work has been submitted to Immunity. The MC-A20KO mice showed increased inflammation and sensitisation to allergens compared to their WT littermates. MCs play a crucial role in the early allergic reaction through release of preformed (histamine, serotonin, hexosaminidase) and newly synthesized (cytokines, prostaglandins, leukotriens) mediators of inflammation upon crosslinking of the high affinity receptor for IgE (FcepsilonRI) by allergen and specific IgE. The production of cytokines by mast cells following IgE triggering depends amongst others on proper activation of NF-kB signalling. In addition to IgE receptor crosslinking, mast cells are also activated by TLR and cytokine signals, again implying NF-kB. This increased sensitization appears to be due to an increased susceptibility to IL-1 and/or TNFalpha signalling (both cytokines) and not to increased susceptibility of MCs to TLR signalling. The MC-A20KO mice show increased vascular permeability, which could lead to increased influx of inflammatory cells to the lungs of these mice. Besides this, the MC-A20KO mice show in a MC-dependent model of sensitisation, that the adaptive response is increased and not the innate response. Experiments are still on-going to further unravel the pathways by which lack of A20 in mast cells induces stronger sensitisation and allergic airway inflammation.