Loss of TCM cells during chronic SIV infection (bottom) depletes pathogen-fighting TEM cells and triggers AIDS.

HIV and its simian counterpart, SIV, replicate rapidly after infection but often take many years to subdue the immune system and cause AIDS. Okoye et al. (page 2171) now find that these slow burner viruses only cause disease after they deplete the precursor cells that give rise to pathogen-fighting effector cells.

The effector memory T (TEM) cells that battle these viruses at infection sites develop from a pool of CD4+ central memory (TCM) cells that are stored in secondary lymphoid organs. Although the virus infects both types of cells simultaneously, TEM cells are the first casualties. This early depletion of TEM cells is not completely devastating; the TCM population quickly cranks out more TEM cells. But this...

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