In response to HIV infection, the body deploys troops of combatants known as CD4+ T cells to carry out a counter attack. Somewhere down the line, however, HIV gains the upper hand, dismantling the T-cell system and leaving the body defenseless against the opportunistic infections that characterize AIDS. Exactly how HIV brings the immune system to its knees has been the focus of much research. Findings reported today in the journal Immunity provide important insight.

CD4+ T cells fall into two different categories: naive cells and memory cells. Naive cells become memory cells when exposed to a pathogen. These memory cells then lead other immune system cells into battle against invaders. Scientists have long thought that HIV infects only memory T cells, based on studies of T cells isolated from blood. But the new research, conducted by Mark A. Goldsmith of the University of California, San Francisco, and colleagues, indicates that HIV does, in fact, infect naive T cells. Unlike investigators before them, this team focused on T cells from lymphoid organs such as the spleen and lymph nodes. For research purposes, these cells hold an important advantage over those from blood in that they can be cultured without the use of artificial stimulants, which can alter the cells' behavior.

Without healthy naive cells to replace the defeated memory cells, the T cell population declines precipitously. "The collapse of the CD4+ T-cell system is the cardinal feature of AIDS," Goldsmith observes, "so understanding which subsets of T cells can be infected is important in providing a clear picture of how HIV reduces the number of T cells."