Genetic live vaccines mimic the antigenicity but not pathogenicity of live viruses.

The development of an effective HIV vaccine is both a pressing and a formidable problem. The most encouraging results to date have been achieved using live-attenuated immunodeficiency viruses. However, the frequency of pathogenic breakthroughs has been a deterrent to their development. We suggest that expression libraries generated from viral DNA can produce the immunologic advantages of live vaccines without risk of reversion to pathogenic viruses. The plasmid libraries could be deconvoluted into useful components or administered as complex mixtures. To explore this approach, we designed and tested several of these genetic live vaccines (GLVs) for HIV. We constructed libraries by cloning overlapping fragments of the proviral genome into mammalian expression plasmids, then used them to immunize mice. We found that inserting library fragments into a vector downstream of a secretory gene sequence led to augmented antibody responses, and insertion downstream of a ubiquitin sequence enhanced cytotoxic lymphocyte responses. Also, fragmentation of gag into subgenes broadened T-cell epitope recognition. We have fragmented the genome by sequence-directed and random methods to create libraries with different features. We propose that the characteristics of GLVs support their further investigation as an approach to protection against HIV and other viral pathogens.