Inhibitor of APoptosis proteins (IAPs) and SHARPIN regulate the immune response in the skin to limit inflammation and maintain homeostasis

Abstract

The skin is a remarkable organ, a barricade between our vulnerable insides and a constantly changing environment full of physical, chemical, and biological aggressors. Maintenance of barrier integrity, immune surveillance, and rapid response are fundamental, and this multifaceted protection is orchestrated by the epithelial barrier and immune cells. Acute and chronic inflammatory skin diseases can arise due to abnormal over-reactions to the changing environment. A number of these diseases have been associated with genetic aberrations of the TNF super family and innate receptors signalling. My PhD studies have focused on the role of particular E3 ligases in regulating inflammatory signalling in skin homeostasis and inflammation.

Inhibitor of APoptosis proteins (IAPs) and the Linear Ubiquitin-chain Assembly Complex (LUBAC) are E3 ubiquitin ligases that play crucial roles in innate immunity by regulating cell death and survival pathways from the TNF and pattern recognition receptor families. Genetic or pharmacological disruption of the IAPs or LUBAC member SHARPIN induce dermatological phenotypes with interesting parallels to a variety of human skin diseases.

To investigate the contribution of immune cells to the Sharpincpdm cutaneous phenotype I utilised the transgenic Diphtheria Toxin Receptor (DTR) system to specifically ablate particular immune cell subsets in-vivo. I have found that Langerhans cells play a pivotal role in the cell death mediated skin disease that arises in Sharpin mutant mice, placing them as a potential cellular source of pathogenic TNF in the Sharpincpdm skin, and highlighting a T-cell independent role for Langerhans cells in driving skin inflammation.

Epidermal specific genetic deletion of the cellular IAPs (cIAPs) resulted in early post-natal lethality due to widespread dermatoses. Pharmacological loss of cIAP1, cIAP2 and XIAP by subcutaneous injection of an IAP antagonist drug (smac-mimetic; SM) into mice induced a Toxic Epidermal Necrolysis (TEN) like local inflammatory lesion characterised by keratinocyte cell death, immune cell infiltration, and increased production of pro-inflammatory cytokines. Both the genetic and pharmacological phenotypes can be ameliorated by the loss of a single allele of RIPK1. I have conducted a screen injecting SM into a panel of knock-out and mutant mouse strains in order dissect the complex set of interactions initiated by injection of SM and leading to the TEN like lesional response. I found that disruption of IAPs leads to a breakdown in immune tolerance to commensal microorganisms, which can then initiate inflammatory responses in the skin. A full response to SM depends on interactions between innate immune signalling pathways, immune cells, and the microbiota, nicely highlighting the multifaceted processes involved in skin inflammation and cell death.