One of the more interesting findings of this study is that while overall viral titers from CIITA-expressing cells decrease the infectivity of these particles is significantly enhanced. Increased infectivity was due to improved virion maturation in a viral protease-dependent manner. Not only was processing to capsid p24 more complete in CIITA expressing cells, but vector-only control cells and those only expressing HLA-DR contained increased levels of processing intermediates. The presence of higher levels of processing intermediates in virus produced in these cells may help explain the reduced infectivity, as studies in both MLV [65] and HIV [7] demonstrate that partially cleaved Gag products act transdominantly to reduce virion infectivity through reduced reverse transcription of viral RNA, despite the presence of functional Reverse Transcription polymerase [7] .