1521 Contrasting roles of necroptosis in wound recovery and skin inflammation

Apoptotic cell death is vital for inflammation resolution in skin injury and infection. In contrast, the importance of the more inflammatory necroptotic cell death in skin injury is largely unknown. Here we explore how loss of the necroptotic effectors, RIPK3 and MLKL, affect healing in three models of skin injury; 1) Smac-mimetic-induced Toxic Epidermal Necrolysis (TEN), 2) Tamoxifen-induced loss of cFLIP and 3) 5mm excision wounds with and without prior bone marrow transplant. Necroptotic KO mice respond as strongly as WT mice upon induction of TEN or loss of epidermal cFLIP; however, they recover from the skin lesions faster. Rapid healing also occurs upon mechanical injury. On day 6 post-excision the wounds on necroptotic KO mice had closed 15-20% more than those on WT mice (p=0.0017 for Mlkl-/-, p=0.0003 for Ripk3-/-). Necroptotic KO mice transplanted with WT bone marrow prior to wounding showed the same rapid healing suggesting that the beneficial effect of necroptosis loss is skin-localized. Interestingly, while Mlkl-/- mice had faster wound closure, we also observed more frequent and severe dressing-associated dermatitis in this group. This complication was reduced in Mlkl-/-mice that received WT bone marrow, suggesting a contrasting role for circulating immune cell necroptosis in reducing inflammatory skin reactions. This work shows that loss of necroptotic effectors in the skin is beneficial to wound healing, while loss of the same effecters in the haematopoietic systems may be harmful. The healing effect in both immune-mediated and mechanical injuries indicates considerable therapeutic potential for local targeting of these proteins in acute cutaneous injury.