Immunology and the skin. Current concepts

Immunity at the surface: homeostatic mechanisms of the skin immune system

The coordinated function of multiple epidermal and dermal cell populations allows the skin immune system to respond rapidly and effectively to a wide variety of insults occurring at the interface of the organism and its environment. Keratinocytes are the first line of defense in the skin immune system, and keratinocyte-derived cytokines are pivotal in mobilizing leukocytes from blood and signaling other cutaneous cells. Cytokine-mediated cellular communication also enables dermal fibroblasts and endothelial cells lining the cutaneous vasculature to participate in immune and inflammatory responses. Skin is an important site for antigen presentation, and both epidermal Langerhans cells and dermal dendritic cells play pivotal roles in T cell-mediated immune responses to antigens encountered in skin. Proinflammatory signaling pathways are necessarily balanced by a variety of regulatory pathways that help maintain the homeostatic functioning of the skin immune system.

The role of cytochrome P450 enzymes in hepatic and extrahepatic human drug toxicity

The human cytochrome P450 enzyme system metabolises a wide array of xenobiotics to pharmacologically inactive metabolites, and occasionally, to toxicologically active metabolites. Impairment of cytochrome P450 activity, which may be either genetic or environmental, may lead to toxicity caused by the parent compound itself. In practise, this usually only applies to drugs that have a narrow therapeutic index and when their clearance is critically dependent upon the fraction normally metabolised by that pathway. P450 enzymes may also convert the drug to a chemically reactive metabolite, which, if not detoxified, may lead to various forms of hepatic and extrahepatic toxicity, including cellular necrosis, hypersensitivity, teratogenicity, and carcinogenicity, depending on the site of formation and the relative stability of the metabolite, and the cellular macromolecule with which it reacts. Variation in the regulation and expression of the drug metabolising enzymes may play a key role in both interindividual variation in sensitivity to drug toxicity and tissue-specific damage. Avoidance of toxicity may be possible in rare instances by prediction of individual susceptibility or by designing new chemical entities that are metabolised by a range of enzymes (both cytochromes P450 and others) and do not undergo bioactivation.

Does prolactin play a role in skin biology and pathology?

Despite its extensive repertoire of biological activities, which include the growth- and osmoregulation of epithelial tissues as well as immunoregulatory properties, the potential significance of the pituitary hormone prolactin (PRL) for human skin biology and pathology has yet to be fully appreciated. In this essay, the hypothesis is presented that PRL acts as a neuroendocrine modulator of skin epithelial cell proliferation and of the skin immune system by forming a 'prolactin-circuit' between the central nervous system and the skin. Binding to specific skin receptors, modulation of cytokine release in the skin, and stimulation of somatomedin release by mesenchymal cells are among the suggested pathways by which PRL could affect epithelial cell growth in the skin. Potential feedback signals, arising from the skin and modifying pituitary PRL release, are briefly outlined. Centering on the role of PRL in both psoriasis and hair growth as models for studying the proposed PRL-skin connection, clinical and experimental evidence in support of this theory is discussed in the context of a 'neuroimmune-dermatological' perspective.

The oral mucosa of the MRL/I mouse: a synoptic picture of systemic autoimmune disorders

The oral manifestations of systemic autoimmunity were investigated in a kinetic study of the MRL/1 mouse. Lesions in the epithelium, connective tissue and minor salivary glands were characterized in serial sections of the soft palate and the cheeks with respect to 1) the type of inflammatory cells present, 2) the presence and type of vasculitis, 3) the presence of necrosis, 4) the occurrence of deposits. By the age of 16 wk, 100% of our animals had developed mild to severe lesions in at least one compartment of the mucosa. Between 16 and 32 wk of age, pathologic manifestations affected the epithelial and subepithelial tissues, the striated muscle tissue, the vascular system and, much less frequently, the minor salivary gland network.

Uptake of antigen by afferent lymph dendritic cells mediated by antibody

Dendritic cells isolated from sheep afferent lymph were examined for their ability to bind soluble protein and peptide antigens labeled with fluorescein both in in vitro assays and following intradermal injection of antigen in vivo. Analysis of dendritic cells by flow cytometry revealed weak direct binding of proteins and peptide antigens. However, the degree of uptake was greatly enhanced in the presence of specific antibody in vitro or if antigen was injected intradermally into antigen-primed sheep. About 60% of dendritic cells possessed the ability to take up antigen in both the in vitro and in vivo experiments. The uptake of antigen occurred very rapidly, reaching maximum values in terms of cell numbers and fluorescence intensity in less than 5 min in vitro and 20-40 min following in vivo challenge. Both sheep IgG subclasses could mediate this effect, but F(ab')2 fragments were ineffective. Procedures adopted to remove complement components from the in vitro test mixtures did not result in any reduction in the binding of antigen by dendritic cells. Two-color flow cytometry analysis of the dendritic cell population further showed that 43% of cells taking up the antigen/antibody complexes were CD1+, suggesting a relationship between these cells and Langerhans' cells or other dendritic cells in skin. The results, thus, indicate that approximately two thirds of sheep afferent lymph dendritic cells bind antigen/antibody complexes via an Fc receptor, a mechanism which could be important in the accentuated accessory function of these cells known to occur following secondary antigen challenge.