The link between the skin microbiome and the immune system

Picture this image for a moment; healthy, blushing toddlers running in the playground, climbing peat blocks, playing, and digging in the soil. Not a too common scenario nowadays, right? Is it then a mere coincidence that the number of atopic dermatitis and psoriasis cases have drastically increased in the last decade? Or that the severity of these conditions continues to rise? — Science says no. [1] [2] [3]

“Today it is known that the skin microbiome and the immune system are deeply connected and communicate between each other.”

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Atopic dermatitis and psoriasis belong to a class of skin diseases modulated by the immune system, hence the name Immune-Mediated Skin Diseases (IMSDs). Like other autoimmune conditions, IMSDs are caused by the inappropriate activation of the immune system, leading to the assault of healthy tissue. [4] Most of the available treatments are not effective or come with side effects. [5] [6] The bright side? Novel treatments are being developed thanks to the available knowledge of the microbiome. Today it is known that the skin microbiome and the immune system are deeply connected and communicate between each other. [3] [7]

Our immune system has the capacity to change the skin microbiome by modifying the skin environmental conditions. For instance, the increased activity of Interleukins 4 and 13, small proteins which regulate the immune response, cause the decreased expression of the protein filaggrin, which has been linked to a higher risk of Staphylococcus aureus infections. The presence of these pathogenic bacteria is the hallmark of atopic dermatitis. [8]

Our skin function and activities drop as we age due to hormonal changes, such as during puberty and menopause, and through the remodelling of the immune system (immunosenescence). [9] These events cause alterations in the microbial composition of the skin. It is been shown that women and men above 60 have reduced sebum production. [10] Sebum is a lipid-rich secretion produced by the sebaceous glands that lubricates our hair and skin. A decline in sebum correlates with the reduction of Cutibacterium bacteria, the predominant species in sebaceous sites. [11] Cutibacterium aids in the maintenance of skin balance and prevents the colonization of opportunistic bacteria.

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“[…] the skin microbiome has the ability to influence and even control the immune system.”

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On the other hand, the skin microbiome has the ability to influence and even control the immune system. Recent studies have demonstrated that microbes thrive not only on the skin surface (stratum corneum) but also on the skin appendages, dermis and extend deep into the dermal adipose tissue. [12] The appendages encompass the sebaceous glands, hair follicles, and sweat glands. These ducts maximize the microbial attachment to the skin and act as an interface for the conversation between the microbiome and the host-immune system. [13] Microbes communicate with the immune system by activating receptors in the surface of skin cells or by producing molecules named metabolites.

“Besides these intrinsic factors, extrinsic factors such as pollution, sun exposure, diet, antibiotics, cosmetics and the environment also influence the microbial composition of the skin.”

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Besides these intrinsic factors, extrinsic factors such as pollution, sun exposure, diet, antibiotics, cosmetics and the environment also influence the microbial composition of the skin. [14] Let us recall the image of toddlers playing with soil. A trial was conducted in Finland where the yards of urban day-care centres were re-furnished with forest floor and turf, just for children to play with, for a period of 28 days. After the trial, the results revealed that the toddlers had more diverse Proteobacterial and Gammaproteobacterial communities in their skin and gut than children in regular urban day-care facilities. [15) More importantly, the intervention stimulated the immune system of the children by increasing the ratio of Interleukins and T cells, the superstars of the immune response, in the blood. This simple trial demonstrated how environmental diversity on the commensal skin microbiome of young children can stimulate the immune system and augment the host-defenses. As it is well-known the immune system is moulded during the very first years of life, now there is enough evidence to assume that the skin microbiome encounters faced by young children may have a long-term implication in their immune response and the microbiome interactions. [16] [17]

‍“[…] they have a complex, dynamic two-way communication and that both have the ability to greatly influence each other.”

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The study of the link between skin microbiome and the immune system is still in its early stages. However, what is known is that they have a complex, dynamic two-way communication and that both have the ability to greatly influence each other. Numerous gaps about these communication pathways need to be filled to better understand and bring down the incidence of IMSDs. The bottom line? Remember to never stop playing outside in nature. Your microbiome and immune system will thank you.

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