HIV (red arrows; tiny circles) occupies vacuoles that are continuous with but separated from the extracellular space.

HIV assembles in a compartment that lies deep within macrophages yet is continuous with the extracellular environment, report Deneka et al. (page 329).

HIV forms membrane-encapsulated particles that assemble at, and bud off from, the surface of infected T cells. In macrophages, however, the majority of virus particles are found intracellularly and have been reported to associate with structures containing an endosomal marker called CD63.

Deneka and colleagues' data, however, indicate that HIV does not enter macrophages by endocytosis. Upon HIV infection, the virus did not colocalize with any other endosome markers besides CD63. It instead associated with a newly identified vesicular structure marked by the transmembrane receptors CD81, CD9, and CD53.

The CD81/9/53 structures were shown by electron microscopy to be connected to the outside of...

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