Infectious Entry of Merkel Cell Polyomavirus

Merkel Cell Polyomavirus (MCPyV) is a small, non-enveloped tumor virus associated with an aggressive form of skin cancer, the Merkel cell carcinoma (MCC). MCPyV infections are highly prevalent in the human population with MCPyV virions being continuously shed from human skin. However, the precise host cell tropism(s) of MCPyV remains unclear: MCPyV is able to replicate within a subset of dermal fibroblasts, but MCPyV DNA has also been detected in a variety of other tissues. However, MCPyV appears different from other polyomaviruses as it requires sulfated polysaccharides such as heparan sulfates and/or chondroitin sulfates for initial attachment. Like other polyomaviruses, MCPyV engages sialic acid as a (co-)receptor. To explore the infectious entry process of MCPyV, we analyzed the cell biological determinants of MCPyV entry into A549 cells, a highly transducible lung carcinoma cell line, in comparison to well-studied simian virus 40 and a number of other viruses. Our results indicate that MCPyV enters cells via caveolar/lipid raft-mediated endocytosis but not macropinocytosis, clathrin-mediated endocytosis or glycosphingolipid-enriched carriers. The viruses internalized in small endocytic pits that led the virus to endosomes and from there to the endoplasmic reticulum (ER). Similar to other polyomaviruses, trafficking required microtubular transport, acidification of endosomes, and a functional redox environment. To our surprise, the virus was found to acquire a membrane envelope within endosomes, a phenomenon not reported for other viruses. Only minor amounts of viruses reached the ER, while the majority was retained in endosomal compartments suggesting that endosome-to-ER trafficking is a bottleneck during infectious entry.