Keratinocyte glucocorticoids enable local immune resilience to preserve skin homeostasis.

Endogenous glucocorticoids (GCs) are pivotal in controlling inflammation. Keratinocyte-derived GCs contribute to local skin homeostasis as deletion of the GC-producing enzyme 11ß-hydroxylase (Cyp11b1) in keratinocytes exacerbated skin inflammation. Since local tamoxifen-induced knockout (KO) induction may contribute to skin irritation, we implemented intraperitoneal injections to induce a systemic skin GC depletion preventing experimental skin irritation in order to reveal the importance of skin GC in steady-state. Both, local and systemic skin GC deficiency models exhibited reduced skin GC levels and increased migration of skin antigen-presenting cells to draining lymph nodes. However, systemic skin GC ablation did not result in pronounced skin inflammation as seen in local model. Interestingly, systemic skin GC deficiency elevated systemic inflammatory markers and provoked adrenal GC synthesis. RNA sequencing of keratinocytes revealed distinct gene expression patterns between local and systemic KOs. Local skin GC ablation showed a stronger inflammatory and apoptotic response, while systemic skin GC deficiency triggered several compensatory regulatory pathways, mitigating extensive skin inflammation. These findings underscore the critical role of local GCs in skin immune resilience against minor skin irritations and highlight the interplay between skin and adrenal GC levels. Overall design: To investigate the effect of systemic skin GC deficiency and its difference to local skin GC deficiency, K14-CreERTam x Cyp11b1-L2/L2 mice (conditional knockout, cKO) and Cre-negative Cyp11b1-L2/L2 littermate control mice (floxed control, ctrl_fl) were treated with tamoxifen for five consecutive days, either by intra-peritoneal (i.p.) injection (systemic), or by topical application (local) to induce knockout of Cyp11b1 in keratinocytes and thereby ablate skin GC synthesis. For systemic administration, mice were daily i.p. injected with tamoxifen in oil-ethanol solution, for local application, back hair was gently dry-shaved, and dorsal and ear skin was treated daily with tamoxifen in ethanol. On day six, keratinocytes were isolated from ear and dorsal skin and enriched by flow cytometry sorting. Total RNA of sorted keratinocytes were isolated and gene expression was analyzed by bulk RNA sequencing. Five mice per experimental group (ctrl_fl local, cKO local, ctrl_fl systemic, cKO systemic) were used.