The role of Langerhans cells and natural killer cells in cancer immune surveillance

The most common skin tumors are non-melanoma skin cancers like basal cell carcinoma (BCC). These epithelial tumors develop in the basal layers of the epidermis due to UV exposure or chemical carcinogenesis. Next to these tumors Langerhans cells (LC), a subset of skin dendritic cells (DC), populate the epidermis. The main function of skin DC and LC is to screen the peripheral tissue for invading pathogens. Their functional role in immunosurveillance of skin tumors is incompletely understood. The aberrant activation of the hedgehog signaling pathway has been detected as one of the major genetic causes for BCC. A transgenic mouse model with inducible expression of an activating allele of the hedgehog protein Smoothened (R26-SmoM2) develops skin alterations similar to BCC. I have attained first results using this mouse model showing that natural killer cells (NK) and LC disappear from the skin during tumorigenesis, in contrast other immune cells seem to be unaffected. Mice developing BCC were crossed with a mouse model that allows depletion of langerin- positive skin DC, which led to a higher tumor burden. These observations indicate that NK cells and LC are important for immunosurveillance early in tumorigenesis. The loss of these two cell types allows the tumor to evade immune recognition. Thus, the aim of the proposed project will be to investigate whether (i) the interaction between NK cells and LC is required for tumor immunity in BCC and (ii) the tumor or cells of the tumor-microenvironment lead to apoptotic cell death in NK cells and LC. It is important to understand how tumor cells evade destruction by the immune system and if the elimination of NK cells and LC is one of the major tumor escape mechanism. Furthermore, I plan to restore tumor immunity by adoptive NK cell transfer directly into the tumor. The obtained results will give new insights into the importance of innate immunity in BCC and will show how the tumor can alter tumor immunity. This knowledge is crucial for the improvement and the development of novel immunotherapeutical strategies against skin cancer. 1

Genetic activation of hedgehog/glioma-associated oncogene homolog (HH/GLI) signaling causes basal cell carcinoma (BCC), a very frequent nonmelanoma skin cancer. Small molecule targeting of the essential HH effector Smoothened (SMO) has proven an effective therapy of BCC, though the frequent development of drug resistance poses major challenges to anti-HH treatments. In light of recent breakthroughs in cancer immunotherapy, we analyzed the possible immunosuppressive mechanisms in HH/GLI-induced BCC in detail. Using a genetic mouse model of BCC, we identified profound differences in the infiltration of BCC lesions with cells of the adaptive and innate immune system. Epidermal activation of Hh/Gli signaling led to an accumulation of immunosuppressive regulatory T cells, and to an increased expression of immune checkpoint molecules including programmed death (PD)-1/PD-ligand 1. Anti-PD-1 monotherapy, however, did not reduce tumor growth, presumably due to the lack of immunogenic mutations in common BCC mouse models, as shown by whole-exome sequencing. BCC lesions also displayed a marked infiltration with neutrophils, the depletion of which unexpectedly promoted BCC growth. The study provides a comprehensive survey of and novel insights into the immune status of murine BCC and serves as a basis for the design of efficacious rational combination treatments. This study also underlines the need for predictive immunogenic mouse models of BCC to evaluate the efficacy of immunotherapeutic strategies in vivo.