Novel function of human beta-defensin 2 – protecting epidermal barrier against pathogenic proteases
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Atopic Dermatitis (AD) is a common chronic relapsing inflammatory skin disease affecting 15 - 20% of children and 2 - 10% of adults worldwide, with significant morbidity. A hallmark of AD is disruption of the critical barrier function of upper epidermal layers, causatively linked to environmental stimuli, genetics and infections. Another typical feature of AD is skin infections, especially from Staphylococcus aureus (S. aureus), which closely relates with the disease severity. Although not a normal flora, S. aureus is found on 75-100% of AD lesions (<30% on healthy skin). S. aureus secrete a range of virulence factors, including extracellular toxins and proteases which contribute to disease pathogenesis. S. aureus serine protease A (SspA/V8) is a well-characterised extracellular protease widely expressed among different S. aureus strains. The pathogenic effect of V8 protease has been demonstrated in vivo, damaging murine skin integrity via effects on the stratum corneum (SC), but the targets for this V8-mediated damage remains unclear. The capacity of proteases to induce barrier dysfunction has been proposed as a key driving force in the initiation and exacerbation of AD. Thus, understanding the host factors that maintain barrier function is a priority in developing novel therapeutic approaches. This thesis therefore aimed at detecting host factors which can combat the barrier dysfunction caused by pathogenic proteases, assessing their relevance in vitro and ex vivo and elucidating the underlying mechanisms. Firstly, an in vitro skin barrier integrity model was developed, using both immortalized and primary keratinocytes, to evaluate the barrier damage mediated by pathogenic proteases. The results revealed that S. aureus protease SspA/V8 is the dominant secreted factor (in laboratory and AD clinical strains of S. aureus) inducing barrier integrity impairment. In addition, studies demonstrated that V8 protease itself was sufficient to induce barrier disruption, and this phenotype was not dependent on cell death, but rather on breaking down of cell-cell junctions. Key tight junction proteins including claudin-1 and occludin were found to be degraded by V8 protease. Next, a wide range of host and bacterial factors were investigated to determine whether they could promote protection of keratinocytes against V8 damage. Several factors, including IL-1β, TNF-α, heat-killed Staphylococcus epidermidis (which is the main skin normal flora), were found to induce protection against V8 protease, with IL-1β having the strongest effect. In addition, data indicated that this IL-1β-mediated protection was independent of effects on claudin-1 but occurred via secretion of a transferrable host factor. Induction of keratinocyte expression of the antimicrobial/host defence peptide human beta-defensin 2 (hBD2) was found to be the mechanism underpinning this IL-1β- induced protective effect. Endogenous hBD2 expression was required and sufficient for protection against V8 protease-mediated integrity damage, and exogenous application of hBD2 was also protective. An ex vivo model using human skin tissue was also established to address this novel function of hBD2, and preliminary data indicated that exogenous hBD2 protected against V8-mediated damage in this system. Overall, my data reveal a novel function for the antimicrobial/host defence peptide hBD2. This modulatory property of hBD2, independent of its antibacterial effects, gives new significance to the defective induction of hBD2 in the barrier-defective skin lesions of AD and indicates therapeutic potential to prevent S. aureus-mediated aggravation of skin barrier dysfunction in AD.