Cell death is an essential process that plays a vital role in restoring and maintaining skin homeostasis. Apoptosis of immune cells is critical for resolving acute inflammation associated with injury and infection. In contrast, lytic forms of cell death such as necroptosis, pyroptosis and NETosis can directly promote inflammation, ensuring an adequate tissue response. However, the effects of RIPK3 and MLKL-mediated necroptosis on wound recovery have not been established.
Loss of apoptosis regulation in the skin by either pharmacological disruption of IAPs or inducible genetic loss of cFLIP causes excessive apoptotic epidermal cell death and Toxic Epidermal Necrolysis like skin lesions. I found that RIPK3 and MLKL knock-out mice have as severe reactions as wild-type mice upon induction of these disease models; however, they recover from the lesions faster. This healing phenomenon also occurs upon induction of a mechanical injury (5mm punch biopsy), suggesting it is not simply an artefact of innate immune signalling disruption.
I performed bone marrow reconstitutions before inducing excision wounds to determine what cell types are responsible for this effect. Restoration of RIPK3 expression in the haematopoietic system of RIPK3 knock-out mice further improved the recovery speed, suggesting that the loss of necroptotic effectors in the skin is advantageous to wound healing but that loss of the same effecters in the haematopoietic systems may be harmful. Furthermore, MLKL epidermal knock-outs retain the healing advantage upon wound induction, strongly suggesting that loss of necroptotic effector proteins in keratinocytes can drive accelerated epidermal recovery.
This work suggests that the activity of RIPK3 and MLKL impairs cutaneous recovery from acute injury. The healing effect in both immune-mediated and mechanical injuries suggests that this impairment may be common upon cutaneous disruption, indicating considerable therapeutic potential for targeting these proteins in acute cutaneous injuries.