Under pressure: Stem cell-niche interactions coordinate tissue adaptation to inflammation

Stem and progenitor cells (SCs) are emerging as key drivers of tissue adaptation to inflammation caused by microbes, injury, noxious agents, and other onslaughts. These pressures are most acutely experienced in epithelial tissues such as the skin and gut that interface with the external environment. Thus, here we review how epithelial SCs of the skin and intestine, along with their supportive niches, sense and respond to inflammation for the sake of preserving tissue integrity. We highlight inflammation-induced plasticity in SCs and their progeny and the lasting memory that forms thereafter. The burgeoning area of SC responses to inflammatory stressors may expand therapeutic perspectives in epithelial inflammatory conditions, wound repair, cancers, and aging.

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Baby’s First Bacteria: Discriminating Colonizing Commensals from Pathogens.

At birth, microbes rapidly colonize our epithelial surfaces. In this issue of Cell Host & Microbe, Leech et al. (2019) uncover how the neonatal immune system discriminates between a colonizing commensal and pathogen to selectively generate tolerance to commensal species.

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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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