Progress and prospects: biological properties and technological advances of herpes simplex virus type 1-based amplicon vectors.

The last two years have seen significant advances in our understanding of the cellular innate responses elicited or activated by the entry of amplicon particles, which may, in part, explain the transient nature of transgene expression often observed in cells infected with helper-free amplicon stocks. At the technological level, the most consistent progress has been in strategies to enhance the stability of transgene cassettes, either through integration into host chromosomes or through the conversion of the amplicon genome into a replication-competent extrachromosomal element.