Advancing Immunotherapy: Experimental Strategies Against Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a neuroendocrine skin cancer with a five-year survival rate of less than 71 %. Thus, the development of new treatment strategies for MCC is necessary. In most cases, the tumor is caused by the expression of a truncated form of the Large T antigen (LT) of the Merkel cell polyomavirus (MCPyV). Dr. Sauerer evaluated previously a combination therapy of a therapeutic monocyte-derived dendritic cells (moDCs)-based vaccination and the immune checkpoint inhibitor Avelumab, and discovered that Avelumab induces lysis of moDCs by NK cells. I showed that Avelumab induced lysis via antibody-dependent cellular cytotoxicity (ADCC), as Atezolizumab and Durvalumab — despite similar binding to moDCs — did not cause lysis due to their modified fragment crystallizable (Fc) region. However, DC numbers in Avelumab-treated MCC patients were similar to healthy controls, suggesting low clinical relevance of the ADCC in Avelumab monotherapy. Therefore, simultaneous DC vaccination and Avelumab is not advisable, but a sequential approach remained under investigation. Although T-cell priming against the tumor antigen LT was successful, MCC cell lines were resistant to both LT-specific T-cell lysis and Avelumab-mediated ADCC, likely due to low PD-L1 and MHC I expression on the tumor cells. IFNγ, but not EGF, upregulated PD-L1 and HLA-ABC expression. However, since this strategy would require IFNγ pre-treatment before vaccination and Avelumab, its clinical feasibility is questionable, and the triple therapy was not pursued further. To enhance tumor-specific T-cell activation, artificial CD80 expression on tumor cells was explored as a treatment strategy for various cancers, including MCC. CD80-encoding mRNA was delivered using nanoparticles based on poly-beta amino-ester (pBAE) polymers. Although MCC cell lines were successfully transfected, efficiency was lower than with electroporation, likely due to their suspension growth. Regardless of CD80 expression, the MCC cell line WaGa failed to activate antigen-specific T cells. These findings indicated that CD80 introduction is not a viable treatment for MCC and highlighted the cancer’s strong immune evasion mechanisms. Since MCC tumor cell lines rely on continuous LT expression for proliferation, understanding LT’s impact on their transcriptional program may help identify new therapeutic targets for MCPyV-induced MCC. Thus, a knockdown of LT in MCC cell lines by siRNAs was established which lasted at least 72 h on protein level. Oxford Nanopore sequencing was not robust enough to assess LT’s impact on the cell transcriptome. In contrast, next-generation sequencing (NGS) produced high-quality data but showed only few differentially expressed genes upon LT knockdown. Gene set enrichment analysis confirmed the data’s reliability, suggesting LT has limited influence on the MCC transcriptome. While no novel targets were identified, the high-quality NGS data may help uncover LT-independent therapeutic targets for MCC.