Langerhans Cells Facilitate UVB-Induced Epidermal Carcinogenesis.

UVB light is considered the major environmental inducer of human keratinocyte (KC) DNA mutations, including within the tumor-suppressor gene p53, and chronic exposure is associated with cutaneous squamous cell carcinoma formation. Langerhans cells (LCs) comprise a dendritic network within the suprabasilar epidermis, yet the role of LCs in UVB-induced carcinogenesis is largely unknown. Herein we show that LC-intact epidermis develops UVB-induced tumors more readily than LC-deficient epidermis. Although levels of epidermal cyclopyrimidine dimers following acute UVB exposure are equivalent in the presence or absence of LCs, chronic UVB-induced p53 mutant clonal islands expand more readily in association with LCs, which remain largely intact and are preferentially found in proximity to the expanding mutant KC populations. The observed LC facilitation of mutant p53 clonal expansion is completely αß and γδ T-cell independent and is associated with increased intraepidermal expression of IL-22 and the presence of group 3 innate lymphoid cells. These data demonstrate that LCs have a key role in UVB-induced cutaneous carcinogenesis and suggest that LCs locally stimulate KC proliferation and innate immune cells that provoke tumor outgrowth.

Mechanisms of chemical cooperative carcinogenesis by epidermal Langerhans cells.

Cutaneous squamous cell carcinoma (SCC) is the most prevalent invasive malignancy with metastatic potential. The epidermis is exposed to a variety of environmental DNA-damaging chemicals, principal among which are polyaromatic hydrocarbons (PAHs) ubiquitous in the environment, tobacco smoke, and broiled meats. Langerhans cells (LCs) comprise a network of dendritic cells situated adjacent to basal, suprabasal, and follicular infundibular keratinocytes that when mutated can give rise to SCC, and LC-intact mice are markedly more susceptible than LC-deficient mice to chemical carcinogenesis provoked by initiation with the model PAH, 7,12-dimethylbenz[a]anthracene (DMBA). LCs rapidly internalize and accumulate DMBA as numerous membrane-independent cytoplasmic foci. Repopulation of LC-deficient mice using fetal liver LC-precursors restores DMBA-induced tumor susceptibility. LC expression of p450 enzyme CYP1B1 is required for maximal rapid induction of DNA-damage within adjacent keratinocytes and their efficient neoplastic transformation; however, effects of tumor progression also attributable to the presence of LC were revealed as CYP1B1 independent. Thus, LCs make multifaceted contributions to cutaneous carcinogenesis, including via the handling and metabolism of chemical mutagens. Such findings suggest a cooperative carcinogenesis role for myeloid-derived cells resident within cancer susceptible epithelial tissues principally by influencing early events in malignant transformation.