MHC class II antigen expression is increased in different forms of urticaria.

Urticarial reactions encompass a variety of inflammatory and immunological reactions. In order to clarify specific aspects of these processes, we analyzed the distribution and sequential expression of major histocompatibility complex II (MHC class II) molecules in tissue sections from different types of whealing reactions. Using immunohistochemical techniques and monoclonal antibodies, expression of HLA-DR, HLA-DP, and HLA-DQ was examined on resident and infiltrating cells in different skin cell compartments, comparing early with longer-lasting wheals and lesional with uninvolved skin. Sequential biopsies were studied in cold urticaria (CU). No increase of MHC class II molecule expression was found in early prick test wheals to common inhalant allergens. In CU, however, sequential biopsies demonstrated an up-regulation of MHC class II molecules within 30 min after elicitation. This was more pronounced in longer-lasting urticaria lesions of acute, chronic recurrent and delayed pressure urticaria, with HLA-DR and, to a lesser degree, HLA-DP and HLA-DQ being noted on cell infiltrates, on vascular endothelia and around nerves and sweat glands. Nonelesional skin in these types of urticaria also showed increased MHC class II expression. Longer-lasting urticarial wheals are thus associated with up-regulation of MHC class II molecules on resident and infiltrating cells, suggesting an involvement of these molecules in the pathomechanisms of these types of urticarial lesions.