Mechanisms of resolution of allergic contact dermatitis.

Photoallergic and allergic contact dermatitis are examples of type IV hypersensitivity reactions that involve T cell-mediated immune responses against haptens that come into contact with the skin. These two types of allergies differ in that for routine contact allergens, the hapten is usually a chemically reactive species that readily couples to host proteins; for photoallergic reactions, UV light (320 - 400 nm) is necessary to generate ("photoactivate") the chemically reactive hapten. From this point on, both photoallergic and allergic contact dermatitis are likely to proceed along the same pathways. For both types of cutaneous delayed-type hypersensitivity, there are naturally occurring mechanisms that terminate this type of T cell-mediated inflammation (tolerance induction). An important tolerance mechanism in the skin involves the induction of T-cell clonal anergy by "amateur" antigen-presenting cells such as keratinocytes. Advances in the understanding of the molecular pathways of T-cell activation and inactivation by antigen-presenting cells have identified critical signaling molecules such as B7/BB-1 antigen. The overexpression of these signaling molecules by the keratinocytes of transgenic mice disrupts the normal kinetics of resolution of murine contact hypersensitivity. These animals have prolonged contact hypersensitivity reactions that resemble some chronic dermatologic conditions in humans. This animal model may be a useful tool to better understand chronic allergic and photoallergic contact dermatitis.