Role of delayed cellular hypersensitivity and adhesion molecules in amoxicillin-induced morbilliform rashes

Morbilliform rashes induced by amoxicillin are though to be caused by a delayed cell-mediated immune reaction. The importance of amoxicillin skin tests is not well defined. A better understanding of the mechanisms of amoxicillin-induced morbilliform rashes can be obtained by performing cutaneous immunohistological studies on specimens from amoxicillin-induced morbilliform rashes and positive amoxicillin skin test results.

Skin biopsy specimens were obtained from 5 patients who had developed an amoxicillin-induced morbilliform rash. All patients underwent amoxicillin prick, patch, and intradermal tests. Similar immunohistological investigations were performed on amoxicillin-induced morbilliform rashes and positive skin test biopsy specimens, with a special focus on the expression of adhesion molecules. Three of the 5 patients developed delayed positive results to intradermal and patch tests and 2 patients developed delayed positive results to prick tests. Amoxicillin-induced morbilliform rashes were well reproduced by skin tests, with similar immunohistological results in amoxicillin-induced morbilliform rashes and skin test biopsy specimens. Keratinocytes were activated and expressed CD54 (intercellular adhesion molecule 1); perivascular lymphocytes were mostly CD2+, CD3+, and CD4+ and exhibited CD11a through CD18 (leukocyte function-associated antigen 1) and often HLA-DR and/or CD62L (leukocyte endothelial cell adhesion molecule 1); and endothelial cells were activated with a strong expression of CD54 (intercellular adhesion molecule 1), CD62E (endothelial leukocyte adhesion molecule 1), and CD31 (platelet endothelial cell adhesion molecule 1) in lesser amounts.

Findings of this clinical and immunohistochemical study support the theory of a T-cell-mediated immune reaction in patients with amoxicillin-induced morbilliform rashes, with a strong involvement of adhesion molecules both on endothelial and infiltrating cells. Our findings emphasize the importance of delayed readings of amoxicillin prick, intradermal, and patch tests.