In vivo dynamics of intraepidermal CD8+ T cells and CD4+ T cells during the evolution of fixed drug eruption

Summary Background  Although a severe form of fixed drug eruption (FDE) clinically and histologically mimics toxic epidermal necrolysis (TEN), subsequent evolution of the two conditions is quite different. It remains unknown, however, which factors determine whether these lesions resolve spontaneously or subsequently progress to TEN. Objectives  Because epidermal injury in TEN can be locally reproduced in the evolving FDE lesions, we sought to investigate how epidermal damage can be induced in the evolving FDE lesions and how disease progression to TEN can be prevented, by analysing the FDE lesions induced by clinical challenge with the causative drug. Methods  We immunohistochemically investigated in vivo dynamics of T‐cell trafficking and activation that occur in the evolving FDE lesions using sequential biopsy specimens obtained at multiple time points from the FDE lesions. Results  Intraepidermal CD8+ T cells, which are resident in the lesional epidermis as a stable homogeneous population of memory T cells, transiently acquire a natural killer‐like phenotype and express cytotoxic granules upon activation. The influx into the epidermis of CD4+ T cells including Foxp3+ regulatory T cells (Tregs) during the evolution serves to ameliorate epidermal damage induced by activation of the intraepidermal CD8+ T cells. Interleukin‐15 derived from the lesional epidermis could maintain the survival of the intraepidermal CD8+ T cells even in the absence of antigenic stimulus over a prolonged period of time (> 4 years). Conclusions  Whether Tregs could migrate to the lesions upon activation of intraepidermal CD8+ T cells would determine whether the inflammation becomes resolved spontaneously or progresses to TEN.