The effect of a highly purified betulin-based triterpene extract on human keratinocytes and epidermal Langerhans cells ex vivo and in vivo

A highly purified triterpene extract (TE) from the outer bark of birch has proven an effective treatment of solar keratoses in vivo. Here we have assessed the effect of TE on cell proliferation and apoptosis of human epidermal cells ex vivo and in vivo. The effects of TE on living skin were examined ex vivo in skin explants treated for 24 hours with TE, and in vivo in punch biopsies obtained from patients with actinic keratoses treated twice daily with TE oleogel for 3 months. Skin explants and biopsy specimens were stained for apoptotic DNA fragments, the proliferation marker Ki67 and the Langerhans cell marker CD1a using specific antibodies. Short-term incubation of skin explants with TE resulted in a significantly increased expression of Ki67 in basal keratinocytes, increased DNA fragmentation of distal stratum granulosum (DSG) keratinocytes, and a decrease of epidermal Langerhans cells. In contrast, after 3 months treatment of actinic keratoses with TE oleogel, no increase of apoptotic cells could be detected. However, we observed a reduced expression of Ki67 in dysplastic keratinocytes and a significant increase of CD1a positive cells in the epidermis. We hypothesize that TE induces the proliferation of basal keratinocytes eventually leading to terminal differentiation and DNA fragmentation of DSC keratinocytes. After initial depletion of epidermal Langerhans cells there is a repopulation with increased numbers of CD1a+ cells which might indicate an increased immunosurveillance in the epidermis. Taken together these findings provide immunohistological evidence of differentiation promoting and immunomodulatory effects of TE ex vivo and in vivo.