Dynamic changes in epidermal Ia-positive cells in allergic contact sensitivity reactions in mice

In mice sensitized with trinitrochlorobenzene, serial changes in epidermal Ia-positive cells were studied at various times after challenge. Until 3 days post-challenge, the Ia-positive cells consisted only of dendritic Langerhans cells; their number was decreased but they were significantly enlarged, with extending dendrites. Some Langerhans cells were also found surrounding a hair follicle, extending their dendrites toward the follicle like the spokes of a wheel. From 3 to 9 days after challenge, keratinocytes also began to express Ia antigens in the epidermis in addition to Langerhans cells, whose size diminished. This suggests that there are two phases in the response of the epidermal Ia antigens in contact sensitivity reactions, i.e. an early phase in which enlarged Langerhans cells are the only Ia-positive cells in the epidermis, and a late phase in which keratinocytes take over as the major Ia-positive cells, while Langerhans cells resume their original size. Ia antigen expression on keratinocytes in this late phase probably plays a crucial role in completely eliminating allergens deposited on the keratinocytes.