HLA-DR expression is induced on keratinocytes in delayed hypersensitivity but not in allergen induced late-phase reactions

Summary In view of increasing evidence suggesting an active immunoregulatory role of the skin keratinocytes and the observation that the differentiation of allergen specific T lymphocytes is critical in the development of allergy, we evaluated epidermal expression of HLA‐DR antigen in skin reactions induced with an atopen (house dust mite) and with an non‐atopic antigen (Hemocyanin). Two groups of patients with house dust mite (Dermatophagoides pteronyssinus [Der p]) allergy were compared, one group was skin tested with Der p, the other group was immunized and subsequently skin tested with Helix pomatia Hemocyanin (HPH), Biopsy specimens taken at 48 h after the HPH (n= 11) and Der p (n= 11) tests were analysed immunohistologically. Reactions in both groups were comparable in size. Immunohistological analysis showed domination by CD4 + lymphocytes. Expression of HLA‐DR antigen by epidermal keratinocytes was observed in six out of 11 of the HPH induced reactions, but in none of the Der p induced reactions. Eosinophils were spotted only throughout the Der p induced reactions, showing a good correlation with the number of CD4 positive lymphocytes. The lack of HLA‐DR expression by keratinocytes during the allergen‐induced reaction, compared with the Hemocyanin induced reaction can be the result of a difference in cytokine profile of the lymphocytes dominating the dermal infiltrate. On the other hand evidence exists that defective HLA‐DR expression by keratinocytes enhances antigen induced lymphocyte activation, and may thus contribute to the development of allergen‐specific T‐lymphocytes.