Integration-defective lentiviral vectors (IDLVs) are being increasingly deployed 
in both basic and preclinical gene transfer settings. Often, however, the IDLV 
transgene expression profile is muted when compared to that of their 
integration-proficient counterparts. We hypothesized that the episomal nature of 
IDLVs turns them into preferential targets for epigenetic silencing involving 
chromatin-remodeling histone deacetylation. Therefore, vectors carrying an array 
of cis-acting elements and transcriptional unit components were assembled with 
the aid of packaging constructs encoding either the wild-type or the class I 
mutant D116N integrase moieties. The transduction levels and transgene-product 
yields provided by each vector class were assessed in the presence and absence 
of the histone deacetylase (HDAC) inhibitors sodium butyrate and trichostatin A. 
To investigate the role of the target cell replication status, we performed 
experiments in growth-arrested human mesenchymal stem cells and in post-mitotic 
syncytial myotubes. We found that IDLVs are acutely affected by HDACs regardless 
of their genetic makeup or target cell replication rate. Interestingly, the 
magnitude of IDLV transgene expression rescue due to HDAC inhibition varied in a 
vector backbone- and cell type-dependent manner. Finally, investigation of 
histone modifications by chromatin immunoprecipitation followed by quantitative 
PCR (ChIP-qPCR) revealed a paucity of euchromatin marks distributed along IDLV 
genomes when compared to those measured on isogenic integration-competent vector 
templates. These findings support the view that IDLVs constitute preferential 
targets for epigenetic silencing involving histone deacetylation, which 
contributes to dampening their full transcriptional potential. Our data provide 
leads on how to most optimally titrate and deploy these promising episomal gene 
delivery vehicles.