Development of adaptive human skin immunity

The main task of the immune system is to protect the body from a diverse range of foreign substances and pathogens, which very often use the skin or mucous membranes as an entrance port. The first contact with these immune-stimulatory molecules happens shortly after birth. Little is known about this crucial phase during the development of the skin immune system. The skin, as a border between internal and external environment, comprises all the instruments of the immune system. Non-immune and immune cells determine whether and how the skin immune system responds to infectious threats. T cells play an important role in this process. Numerous studies reported on the existence and function of T cells in the skin of adult humans. Interestingly, T cells are found already in fetal human skin, albeit in minute numbers. However, nothing is known about their precise phenotype and function. In this project, we are therefore planning to learn more about their nature using standard as well as cutting-edge technology. Successful preliminary experiments have shown that it is technically possible to isolate sufficient T cells from fetal skin facilitating meaningful phenotypic and functional experiments. The investigation of the isolated T cells with a special flow cytometer (ImageStream) represents an attractive possibility to capture and explore cells simultaneously by flow cytometry as well as microscopy and is therefore planned to be applied in this project. Furthermore, a recently developed new technology (Chromium controller) will be used to unravel the molecular and functional repertoire of prenatal skin T cells at selected gestational time points. These data will be validated by in situ evaluations using immunohistochemistry and immunofluorescence and will be complemented by expansion cultures. Indeed, preliminary experiments have shown that a multiplication of these cells is possible with the help of various factors and thus further research work will be feasible. Our study will contribute to a better understanding of the immune system of the skin before birth, which may lead to a superior and substantially earlier therapeutic/prophylactic influence on many skin diseases caused by inflammatory T cells and thus close this knowledge gap.

The demands on the developing human immune system are enormous. During pregnancy, it must selectively defend against infections while avoiding excessive immune reactions. Additionally, the transition from the sterile womb environment to one with numerous microorganisms after birth must be managed smoothly. Thus, the fetal immune system has distinct functions compared to adults and is controlled by mechanisms that are only partially understood. The skin, our largest visible protective organ, harbors numerous T cells in healthy adults. These white blood cells play a decisive role in the body's immune defense. They recognize foreign substances (= antigens) through receptors, consisting of either or chains, ensuring an efficient immune response but can also trigger inflammation and autoimmune diseases. T cells have been identified in fetal skin, but our understanding of their phenotype and function has been limited due to their low frequency and the lack of appropriate examination methods. In this project we focused on developing methodologies to isolate sufficient quantities of T cells from fetal tissues, facilitating comprehensive analyses utilizing cutting-edge molecular and cell biology techniques. By implementing and refining specific methodologies, we have effectively isolated T cells from fetal skin in adequate amounts. Their analysis has led to the identification of a unique T cell subset characterized by the co-expression of and T cell receptors. Thus far, this novel cell type has been exclusively detected in our study in fetal skin and the intestine, but not in adult skin. Analysis of gene signatures, cytokine profiles, and in silico receptor-ligand interactions of positive T cells suggests their involvement in early skin development. Functional in vitro studies have demonstrated their capacity to actively respond to foreign proteins. Nonetheless, conducting extensive functional studies with freshly isolated fetal skin T cells posed a challenge. By adapting established methodologies, we achieved ex vivo expansion of these cells without compromising their phenotype. This pivotal advancement allowed for an in-depth exploration of their features, unveiling their complexity. Our project has made significant progress in better understanding the immune system in fetal skin. We demonstrated for the first time that a unique subset of T cells potentially plays a key role in the development of human skin. Furthermore, these cells demonstrated the ability to elicit a protective immune response outside their typical environment. This indicates specific regulatory mechanisms in the skin that maintain a tolerant environment during development in the womb. Our findings align with a growing number of studies suggesting that the fetal immune system is not simply an immature or less active version of the adult immune defense. Instead, it fulfils independent tasks and exhibits specific functionalities executed by specialized cells.