Endosomes, exosomes and Trojan viruses

Retroviruses are enveloped viruses that are generally assumed to bud at the plasma membrane of infected cells. Recently it has become apparent that some of these viruses use the endocytic pathway to coordinate their assembly and release. In addition, these and some other enveloped viruses exploit the machinery that generates the internal membranes of multivesicular bodies (MVB). These observations and others have led to the suggestion that retroviruses be regarded as 'viral exosomes'. Here we discuss this concept and the emerging evidence that compartments of the endocytic pathway play important roles in the biogenesis of both the internal vesicles of MVB and viruses.