Myeloperoxidase-positive mononuclear cells predominate over CD3-positive T-lymphocytes in the epidermis of Stevens-Johnson syndrome and toxic epidermal necrolysis.

Stevens-Johnson syndrome and toxic epidermal necrolysis (SJS/TEN) are life-threatening mucocutaneous conditions traditionally regarded as severe T cell-mediated hypersensitivity reactions. However, recent evidence has highlighted the potential role of myeloperoxidase (MPO), primarily produced by neutrophils, in the pathogenesis of SJS/TEN. Here, we examined the expression of MPO in skin biopsy specimens from patients with SJS/TEN (n = 15) to evaluate its diagnostic utility, comparing the findings with those from patients with erythema multiforme (EM) (n = 8) and graft-versus-host disease (GVHD) (n = 10). MPO-positive mononuclear cells were present in all SJS/TEN cases, often outnumbering CD3-positive T cells (86.7%). A significantly higher number of cases with an MPO/CD3 ratio greater than 1 was observed in patients with SJS/TEN compared with that in patients with GVHD (p < 0.01). However, no such difference was observed between SJS/TEN and EM. These findings suggest that an MPO/CD3 > 1 is a pathologic feature of SJS/TEN; however, its specificity may be limited.