T lymphocytes and altered keratinocytes express interferon-γ and interleukin 6 in lichen planus

Abstract Lichen planus is asumed to represent a delayed hypersensitivity reaction, in the course of which cytokines control the proliferation and differentiation of cytotoxic T lymphocytes which attack the epidermis and cause apoptosis of undifferentiated keratinocytes. Since interferon-γ and interleukin 6 are known to be markedly generated in lichen planus, we investigated the cellular localization of these cytokines in affected skin/oral mucosa biopsy specimens using in situ hybridization for interferon-γ and in situ reverse transcription-polymerase chain reaction for interleukin 6 mRNA. In the upper subepithelial connective tissue interferon-γ mRNA was noted within proliferating CD3⁺ T lymphocytes. In this tissue compartment interleukin 6 mRNA was detected in infiltrating CD4⁺ and CD8⁺ T lymphocytes. In the epithelium, expression of interferon-γ mRNA and interleukin 6 mRNA was observed in the basal and suprabasal keratinocytes of altered skin/oral mucosa. In contrast, normal skin did not reveal any interferon-γ or interleukin 6 expression, although a few CD4⁺ and CD8⁺ T lymphocytes were noted in the dermis as well as the epidermis. These findings indicate that in lichen planus the proinflammatory cytokines interferon-γ and interleukin 6 are produced not only by activated T lymphocytes but also by altered keratinocytes, and suggest that stimulated keratinocytes may amplify the course of lichen planus.