Exosomes are small vesicles that bud from the endosome membrane into its lumen. Following endosome fusion with the plasma membrane, the exosomes are released into the extracellular space.
Booth et al. found that the plasma membranes of cultured T cells have discrete domains enriched in proteins typically found in endosomes. The same sites were enriched for exosomal lipids, and small exosome-like vesicles were found just outside of these membrane sites, suggesting that exosomes can bud not only from endosomes but from the plasma membrane itself.
When T cells were engineered to express HIV Gag, which encodes the viral capsid proteins, the viral proteins were sorted to these membrane domains and budded from these sites in exosome-like vesicles.
The team thinks retroviruses have co-opted this endogenous cellular pathway for viral budding. They point out that, in agreement with their data, HIV is released initially into endosomes in some cell types and directly from the plasma membrane in others.