Choreographing Immunity in the Skin Epithelial Barrier.

The skin interfaces with the external environment and is home to a myriad of immune cells that patrol the barrier to ward off harmful agents and aid in tissue repair. The formation of the cutaneous immune arsenal begins before birth and evolves throughout our lifetime, incorporating exogenous cues from microbes and inflammatory encounters, to achieve optimal fitness and function. Here, we discuss the context-specific signals that drive productive immune responses in the skin epithelium, highlighting key modulators of these reactions, including hair follicles, neurons, and commensal microbes. We thus also discuss the causal and mechanistic underpinning of inflammatory skin diseases that have been revealed in recent years. Finally, we discuss the non-canonical functions of cutaneous immune cells including their burgeoning role in epithelial regeneration and repair. The rapidly growing field of cutaneous immunity is revealing immune mechanisms and functions that can be harnessed to boost skin health and treat disease.

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Eavesdropping on the conversation between immune cells and the skin epithelium

The skin epithelium covers our body and serves as a vital interface with the external environment. Here, we review the context-specific interactions between immune cells and the epithelium that underlie barrier fitness and function. We highlight the mechanisms by which these two systems engage each other and how immune–epithelial interactions are tuned by microbial and inflammatory stimuli. Epithelial homeostasis relies on a delicate balance of immune surveillance and tolerance, breakdown of which results in disease. In addition to their canonical immune functions, resident and recruited immune cells also supply the epithelium with instructive signals to promote repair. Decoding the dialogue between immunity and the epithelium therefore has great potential for boosting barrier function or mitigating inflammatory epithelial diseases.

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Stem cells repurpose proliferation to contain a breach in their niche barrier.

Adult stem cells are responsible for life-long tissue maintenance. They reside in and interact with specialized tissue microenvironments (niches). Using murine hair follicle as a model, we show that when junctional perturbations in the niche disrupt barrier function, adjacent stem cells dramatically change their transcriptome independent of bacterial invasion and become capable of directly signaling to and recruiting immune cells. Additionally, these stem cells elevate cell cycle transcripts which reduce their quiescence threshold, enabling them to selectively proliferate within this microenvironment of immune distress cues. However, rather than mobilizing to fuel new tissue regeneration, these ectopically proliferative stem cells remain within their niche to contain the breach. Together, our findings expose a potential communication relay system that operates from the niche to the stem cells to the immune system and back. The repurposing of proliferation by these stem cells patch the breached barrier, stoke the immune response and restore niche integrity.

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The Microbiome in Atopic Dermatitis

As an interface with the environment, the skin is a complex ecosystem, colonized by many microorganisms that coexist in an established balance. The cutaneous microbiome inhibits colonization with pathogens such as S. aureus and is a crucial component for function of the epidermal barrier. Moreover, crosstalk between commensals and the immune system is now recognized, as microorganisms can modulate innate, as well as adaptive, immune responses. Host-commensal interactions also have an impact on the developing immune system in infants and subsequently the occurrence of diseases such as asthma and atopic dermatitis. Later in life, the cutaneous microbiome contributes to the development and course of skin disease. Accordingly, in patients with atopic dermatitis, a decrease in microbiome diversity correlates with disease severity and increased colonization with pathogenic bacteria such as S. aureus. Early clinical studies suggest that topical application of commensal organisms (e.g., S. hominis or R. mucosa) reduces atopic dermatitis severity and support an important role for commensals in decreasing S. aureus colonization in patients with atopic dermatitis. Advancing knowledge of the cutaneous microbiome and its function in modulating the course of skin disorders such as atopic dermatitis may result in novel therapeutic strategies.

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Commensal-dendritic cell interactions specifies a unique protective skin immune signature.

Naik S*, Bouladoux N*, Linehan J, Han SJ, Harrison OJ, Wilhelm C, Conlan S, Himmelfarb S, Byrd A, Deming C, Quinones M, Brenchley JM, Kong H, Tussiwand R, Murphy KM, Merad M, Segre JA and Belkaid Y. Commensal-dendritic cell interactions specifies a unique protective skin immune signature. Nature. 520(7545): 104-108 April 2015.

Here we show that defined commensals dominantly affect skin immunity and identify the cellular mediators involved in this specification.This interaction may represent an evolutionary means by which the skin immune system uses fluctuating commensal signals to calibrate barrier immunity and provide heterologous protection against invasive pathogens. These findings reveal that the skin immune landscape is a highly dynamic environment that can be rapidly and specifically remodelled by encounters with defined commensals, findings that have profound implications for our understanding of tissue-specific immunity and pathologies.

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