Master of microevolution  pg. 2

Transmitted across mucosal barriers through sexual contact and by direct blood-to-blood contact, HIV relies primarily on a “Trojan horse” approach to reach its target host cell, the helper T cell.

Fluorescence microscopy captures the migration of green-labeled HIV particles to the junction between a dendritic cell (left) and a smaller T cell. Intact virus crosses the “infectious synapse” between the cells undetected, after which it takes over the T cell's machinery as a factory for its own reproduction.
Image courtesy of Thomas Hope, Ph.D., and David McDonald, Ph.D., University of Illinois at Chicago
The Trojan horses are the scavenger cells, including macrophages and dendritic cells, which patrol the mucosal borders. These “border guards” normally ingest and break down viruses or bacteria, and present peptide pieces of the foreign invaders on their cell surfaces to the helper T cells. When these viral antigens dock to cell surface molecules including the CD4 receptor, helper T cells begin proliferating and activate the immune response.

Somehow HIV escapes digestion by the scavenger cells and is presented intact—and thoroughly infectious—to the helper T cell. It hooks, and fuses with, the cell membrane and pulls itself inside. Then, like others in its family of “retroviruses,” it hijacks the cell’s reproductive machinery to make new copies of itself. (See “Secrets of a Deadly Virus).

HIV replicates faster in helper T cells that are proliferating. “The virus is so in tune with how our immune system works that it’s evolved to thrive in the exact circumstances that the immune system uses to try to beat it back,” says D’Aquila.

Eventually, and it may take years, the population of helper T cells declines—some are killed by the virus; others by mechanisms that are not well understood. When the generals are taken out of the action, immune responses gradually become uncoordinated and ineffective even against weak invaders, called “opportunistic” infections, that otherwise would not gain a foothold in the body. The result: acquired immune deficiency syndrome (AIDS).

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