Enno Christophers

Pustules are among the most common lesions produced in human skin. Infections by pathogens and drug-induced reactions are frequent causes of pustule formation. In recent years immune-mediated pustular diseases have drawn attention. It is proposed to classify pustular diseases according to the initiating events and sites: purely epidermal pustules, follicular pustules or pustules noted in autoinflammatory syndromes. The unifying pathology in all of the three categories is a microinvasion of activated neutrophils into epidermal or adnexal epithelia. Formation of pustules involves established IL-17 / IL-23, IL-36 / IL-36RN driven pathology, or IL-1 /caspase-activated autoinflammation. Pathophysiology demonstrates an intriguing synergy of keratinocytes with neutrophils. This is called keratinocyte-myeloid synergy (KMS). Non-infectious pustules are formed by IFN alpha controlling the production of chemoattractants (IL-8, LTB4) or induced by IL-1-regulated inflammasomes and caspase/ IFN beta-induced chemotaxins. The presence of physical barriers, for example, cornified cell layers (str. corneum), is instrumental in establishing chemotactic gradients and blocking migrating neutrophils. In follicular KMS-driven pustular disorders, in contrast to epidermal pustules, neutrophil-mediated toxicity propagates lasting and expanding ulcerating diseases with increased levels of circulating immunoglobulin A (IgA). Complexed IgA is suggested to propagate ongoing pustular diseases. These are prerequisites essential for developing pustules in burdensome human skin diseases.

More often as compared to other barrier systems (gastrointestinal, urogenital, and respiratory linings) human skin over millions of years has been subject to fundamental changes in structure and function. When life on land started, the first changes consisted in the formation of a coherent impermeable stratum corneum. Two‐legged locomotion was followed by loss of body hair and formation of sweat glands. Major changes took place after the agricultural revolution, investigating settlements with domestication of animals and plants. Living together after giving up nomadic life, hairless skin became a battlefield for pathogens, members of the skin microbiome, and arthropod visits. Human skin became exceptional in showing a boosted, highly developed immune system which is much more complex as compared to the "skins" of other species. A recently found skin disinfection system ("Cationic Intrinsically Disordered Antimicrobial Peptides, CIDAMPs") dates back to the origins of life and still is active in present‐day integuments. As a skin‐restricted and effective principle, keratinocyte‐ myeloid synergy (KMS) is recognized. As a consequence of such highly developed immune defense, the basic contributions of KMS ‐ cells (keratinocytes, neutrophils, macrophages) in regulating innate immunity is emphasized. Antimicrobial peptides and chemokines became major keratinocyte products. The formation of impermeable str. corneum membrane has enabled KMS ‐ cells to accumulate within upper skin levels and cause a special group of human skin diseases, pustular dermatoses.