Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
HIV infection in a cell can lead to the formation of "syncytia", which is when cells fuse together to form large cells with many nuclei. Here's what happens:
Remember the env protein which sticks out of the virus particle (through the membrane) and binds to CD4 proteins (which stick out of a variety of cells)? When an infected cell starts producing HIV proteins, the HIV env proteins migrate to the cell membrane - and then they might poke out of the cell, just as if they were poking out of a HIV particle. This means that the infected cell now has the dubious honour of being able to bind to other cells with CD4 protein. So an infected CD4 cell can join with a healthy CD4 cell, and the two merge in the same way that the HIV particle originally merged into the cell: the membranes fuse and become one. This repeats, and eventually you have one large, HIV-infected, CD4+ cell with many nuclei - as many as 500.
The formation of syncytia doesn't always happen in HIV-infected people (about 50% of people after 5 years). However, autopsies have found syncytia on the spleens of some patients, and they frequently find CD4 syncytia in the brains of patients who have had serious neurological complications resulting from AIDS.