Atopic Dermatitis Studies through In Vitro Models

Abstract

Atopic dermatitis (AD) is a complex inflammatory skin condition that is not fully understood. Epidermal barrier defects and Th2 immune response dysregulations are thought to play crucial roles in the pathogenesis of the disease. A vicious circle takes place between these alterations, and it can further be complicated by additional genetic and environmental factors. Studies investigating in more depth the etiology of the disease are thus needed in order to develop functional treatments. In recent years, there have been significant advances regarding in vitro models reproducing important features of AD. However, since a lot of models have been developed, finding the appropriate experimental setting can be difficult. Therefore, herein, we review the different types of in vitro models mimicking features of AD. The simplest models are two-dimensional culture systems composed of immune cells or keratinocytes, whereas three-dimensional skin or epidermal equivalents reconstitute more complex stratified tissues exhibiting barrier properties. In those models, hallmarks of AD are obtained, either by challenging tissues with interleukin cocktails overexpressed in AD epidermis or by silencing expression of pivotal genes encoding epidermal barrier proteins. Tissue equivalents cocultured with lymphocytes or containing AD patient cells are also described. Furthermore, each model is placed in its study context with a brief summary of the main results obtained. In conclusion, the described in vitro models are useful tools to better understand AD pathogenesis, but also to screen new compounds in the field of AD, which probably will open the way to new preventive or therapeutic strategies.

Keywords: atopic dermatitis; epidermal keratinocytes; in vitro; model; reconstructed epidermis; skin equivalents.

Figure 1

Hypothetic simplified representation of the pivotal role played by thymic stromal lymphopoietin (TSLP) in the pathogenesis of acute atopic dermatitis (AD) and the interplay between barrier alterations and Th2 immune dysregulation. TSLP released from challenged keratinocytes is able to activate migration of Langerhans cells toward draining lymph nodes (35) where they subsequently initiate allergic response by promoting differentiation of naïve CD4⁺ T cells into inflammatory Th2 effectors. TSLP can also directly act on these Th2 effectors to induce their proliferation (–45). Th2 effector cells produce IL-4, IL-5, IL-13, TNFα, while downregulating IFNγ (35), possibly explaining the link observed between barrier alterations and inflammatory conditions. Indeed, among these cytokines, IL-4 and IL-13 are more particularly known as being able to alter the expression of proteins involved in the epidermal barrier like filaggrin (19, 20, 49), loricrin (LOR) and involucrin (IVL) (50), desmoglein 1 and 3 (component of desmosomes) (49, 51), sphingomyelinase and glucocerebrosidase (genes implicated in the synthesis of ceramides) (52), or the tight junction component claudin-1 (53). Every cytokine-induced alteration of the epidermal barrier allows conditions more favorable for penetration of allergens/pathogens that can then be recognized by dendritic antigen-presenting cells. Processed allergens/pathogens are next presented to naive B cells and activate them to eventually produce specific immunoglobulin E (IgE). Such IgE then bind to the high affinity receptor FcεRI expressed on basophils, eosinophils, and mast cells. After re-exposure to a previously encountered antigen, the body can respond directly and more strongly. Indeed, when IgE are produced in greater amounts, basophils, eosinophils, and mast cells are ready to mediate inflammatory reactions. This immune inflammatory response is known as the allergic response. Basophils are able to promote Th2 cytokine response, while mast cells can massively degranulate, notably releasing histamine which induces edema and pruritus. IL-4 is known to modulate this allergic response and to induce overproduction of IgE. In addition, TSLP could also play a role in the itch symptoms observed in AD patients. Indeed, TSLP activates mast cells to release histamine upon binding to its specific receptor. Even further, keratinocytes directly interact with dermal sensory neurons via TSLP, thereby inducing itch (42). Moreover, TSLP may also promote basophil and eosinophil responses. Basophils promote Th2 cytokine response, while TSLP induces recruitment of eosinophils to Th2 cytokine-associated inflammation sites (54).

Figure 2

Summary of published models developed for in vitro studies of atopic dermatitis (AD) features. Different types of in vitro models can be used in regards to the research focus, from the most simple ones consisting of only one cellular type (named here 2D models) to more complex models reconstituting the different layers of the epidermis (called here 3D models). 2D models are used in order to characterize behavior of one given cellular type whereas 3D models are more commonly used with the aim of trying to reproduce some features of the pathology. Within these two types of models, immortalized cells can be used, but in order to be closer to the in vivo situation, primary cells are often used. Cells derived directly from patients are probably the most relevant but also the more complicated to obtain and the most limited. While performing 3D models, two different types of models are commonly used: human skin equivalents (HSE) or reconstructed human epidermis (RHE). HSE reproduce the epidermis on top of a dermis equivalent, whereas RHE only reproduce the different layers of the epidermis. HSE are more complicated to produce but display the advantage to present a dermal compartment whereas RHE allow evaluation of keratinocyte-specific type of responses and analysis of released or secreted molecules in the culture medium. HSE or RHE can both be challenged by interleukin cocktails and genetically modified. And cocultures with immune cells could be performed in both cases.