UittrekselMelanoma, the deadliest form of skin cancer, is less common than other types of cancer but is most likely to grow and spread to other organs like lymph nodes, lungs and brain. Treatment for advanced disease has long been insufficient but recent advances within the field of immunotherapy give patients a new chance of survival. In 2015, Talimogene laherpavec (T-VEC), the first oncolytic virus, was approved for treatment of metastatic melanoma. This is a herpes simplex virus-1 (HSV-1) derived oncolytic virus which has been genetically modified to replicate specifically within cancer cells and contains the GM-CSF gene.
The aim of our experiments was to examine the immunogenicity of the cell death induced by and the maturation of dendritic cells by the exposure to pathogen and danger associated molecular pattern (PAMPs and DAMPs) after treatment of melanoma cells with T-VEC.
First, we studied the effect of T-VEC on the human melanoma cell lines 624-mel and 938-mel and determined that T-VEC infects and replicates within these cells. By using the IncuCyte® image analyzing software, we determined that T-VEC kills the cells between 32h and 48h post-infection. The Annexin-V/DAPI assay indicated that 938-mel died partially apoptotic, while we could not see this for 624-mel. This suggests apoptosis is not the only cell death mechanism involved.
To evaluate whether T-VEC induces an immunogenic type of cell death, we studied different markers of immunogenic cell death (ICD) in response to T-VEC infection. An increase in calreticulin exposure, acting as an ‘eat-me’ signal, was detected for both 624-mel and 938-mel. Next to that, we saw an increase in ATP release in 938-mel acting as a ‘find-me’ signal.
Both BDCA-1+ dendritic cells (DCs) and monocyte-derived DCs showed an increased expression of different maturation markers such as CD80, CD83, CD86, PD-L1, HLA-ABC when exposed to T-VEC and T-VEC treated melanoma cells. When combining both PAMPs and DAMPs in a coculture experiment, a significant increase in PD-L1 expression could be observed. This indicates important potential in the combination of checkpoint inhibitors and T-VEC.
Production of IL-12p70 remained under the detection limit.
In conclusion, we showed that T-VEC effectively kills and infects melanoma cells by inducing an immunogenic type of cell death. We also showed that T-VEC, either directly or upon infection of tumor cells, induces maturation of DCs.
Further research is needed to specify the type of cell death induced by the virus and to assess whether the DC activation leads to enhanced T-cell responses.