Activation pattern of Langerhans cells in the afferent and efferent phasesof contact hypersensitivity

Langerhans cells are MHC class II: (Ia) positive antigen-presenting cells t hat play a crucial role in the induction of contact hypersensitivity (CHS). The topical application of a hapten modifies the cell. surface moieties of Langerhans cells, and activates Langerhans cells to increase their size an d Ia intensity. The haptenated and activated Langerhans cells emigrate from the epidermis and thus the in situ density of Langerhans cells usually dec reases during 24-48 h after the hapten application in CHS. To determine whe ther the early activation pattern of Langerhans cells is different between the afferent phase and the efferent phase of CHS, we compared the density a nd morphologic changes of Langerhans cells in CHS to trinitrochlorobenzene using nonsensitized and sensitized mice. We found that the application of a hapten induces more significant enlargement of Langerhans cell size in the afferent phase than in the efferent phase, whereas the reduction of Langer hans cell density is more marked in the efferent than in the afferent phase of CHS. Moreover, topical immunosuppressive drugs inhibit the in situ acti vation of Langerhans cells.