Skin in vitro models offer much promise for research, testing drugs, cosmetics, and medical devices, reducing animal testing and extensive clinical trials. There are several in vitro approaches to mimicking human skin behavior, ranging from simple cell monolayer to complex organotypic and bioengineered 3-dimensional models. Some have been approved for preclinical studies in cosmetics, pharmaceuticals, and chemicals. However, development of physiologically reliable in vitro human skin models remains in its infancy. This review reports on advances in in vitro complex skin models to study skin homeostasis, aging, and skin disease.

Psoriasis and atopic dermatitis are chronic inflammatory skin diseases characterized by keratinocyte (KC) hyperproliferation and epidermal acanthosis (hyperplasia). The milieu of disease-associated cytokines and soluble factors is considered a mitogenic factor; however, pinpointing the exact mitogens in this complex microenvironment is challenging. We employed organotypic human epidermal equivalents, faithfully mimicking native epidermal proliferation and stratification, to evaluate the proliferative effects of a broad panel of (literature-based) potential mitogens. The KC GF molecule, the T-helper 2 cytokines IL-4 and IL-13, and the psoriasis-associated cytokine IL-17A caused acanthosis by hyperplasia through a doubling in the number of proliferating KCs. In contrast, IFN-γ lowered proliferation, whereas IL-6, IL-20, IL-22, and oncostatin M induced acanthosis not by hyperproliferation but by hypertrophy. The T-helper 2‒cytokine‒mediated hyperproliferation was Jak/signal transducer and activator of transcription 3 dependent, whereas IL-17A and KC GF induced MAPK/extracellular signal‒regulated kinase kinase/extracellular signal‒regulated kinase‒dependent proliferation. This discovery that key regulators in atopic dermatitis and psoriasis are direct KC mitogens not only adds evidence to their crucial role in the pathophysiological processes but also highlights an additional therapeutic pillar for the mode of action of targeting biologicals (e.g., dupilumab) or small-molecule drugs (e.g., tofacitinib) by the normalization of KC turnover within the epidermal compartment.

Atopic dermatitis (AD) is a common T-helper 2 (Th2) lymphocyte-mediated chronic inflammatory skin disease characterized by disturbed epidermal differentiation (e.g., filaggrin (FLG) expression) and diminished skin barrier function. Therapeutics targeting the aryl hydrocarbon receptor (AHR), such as coal tar and tapinarof, are effective in AD, yet new receptor ligands with improved potency or bioavailability are in demand to expand the AHR-targeting therapeutic arsenal. We found that carboxamide derivatives from laquinimod, tasquinimod, and roquinimex can activate AHR signaling at low nanomolar concentrations. Tasquinimod derivative (IMA-06504) and its prodrug (IMA-07101) provided full agonist activity and were most effective to induce FLG and other epidermal differentiation proteins, and counteracted IL-4 mediated repression of terminal differentiation. Partial agonist activity by other derivatives was less efficacious. The previously reported beneficial safety profile of these novel small molecules, and the herein reported therapeutic potential of specific carboxamide derivatives, provides a solid rationale for further preclinical assertation.