The Skin and Intestinal Microbiota and Their Specific Innate Immune Systems

Abstract

The skin and intestine are active organs of the immune system that are constantly exposed to the outside environment. They support diverse microbiota, both commensal and pathogenic, which encompass bacteria, viruses, fungi, and parasites. The skin and intestine must maintain homeostasis with the diversity of commensal organisms present on epithelial surfaces. Here we review the current literature pertaining to epithelial barrier formation, microbial composition, and the complex regulatory mechanisms governing the interaction between the innate immune system and microbiota in the skin and intestine. We also compare and contrast the skin and intestine-two different organ systems responsible creating a protective barrier against the external environment, each of which has unique mechanisms for interaction with commensal populations and host repair.

Keywords: AMPs; innate immunity; intestine; microbiome; skin.

Figure 1

Skin-microbial interactions promote innate immune function. The skin is an active immune organ whose function is augmented by the presence of commensal microbiota. The epidermis is made up of numerous keratinocytes. The stratum corneum is sealed via intracellular lipids, and other epidermal keratinocytes are connected via tight junctions. Dermal appendages include sweat glands, hair follicles, and sebaceous glands, all of which contribute to immune function. Keratinocytes and dermal appendages release antimicrobial peptides and proteins (AMPs), which provide defense against pathogenic microbes. A number of bacteria species are commensal colonizers of the skin surface. The top three bacterial species for each skin site are shown (4). Dry and sebaceous sites are colonized predominantly by Cutibacterium acnes, whereas moist sites and the foot are colonized chiefly by Corynebacterium tuberculostearicum.

Figure 2

Microbiota augment intestinal innate immunity. Intestinal epithelial cells, which make up the physical barrier of the intestine, secrete antimicrobial peptides and proteins (AMPs). Goblet cells secrete mucus which forms an additional layer of protection against pathogens. Dendritic cells present antigen to B cells within Peyer's patches, stimulating them to secrete IgA. The intestine provides unique niches in which commensal bacteria thrive. Bacteroides and Firmicutes species comprise the majority of luminal bacteria, whereas segmented filamentous bacteria and Helicobacter pylori can penetrate into the mucus layer of the intestine (8, 9). Alcaligenes species are able to inhabit Peyer's patches (10).