ScienceDaily (Aug. 1, 2012) — A new study has discovered one more way the human immunodeficiency virus (HIV) exploits the immune system. Not only does HIV infect and destroy CD4-positive helper T cells -- which normally direct and support the infection-fighting activities of other immune cells -- the virus also appears to use those cells to travel through the body and infect other CD4 T cells. The study from Massachusetts General Hospital (MGH) investigators, which will appear in the journal Nature and has received advance online release, is the first to visualize the behavior of HIV-infected human T cells within a lymph node of a live animal, using a recently developed "humanized" mouse model of HIV infection.
"We have found that HIV disseminates in the body of an infected individual by 'hitching a ride' on the T cells it infects," says Thorsten Mempel, MD, PhD, of the MGH Center for Immunology and Inflammatory Diseases, who led the study. "Infected T cells continue doing what they usually do, migrating within and between tissues such as lymph nodes, and in doing so they carry HIV to remote locations that free virus could not reach as easily. There are drugs that can manipulate the migration of T cells that potentially could be used to help control the spread of virus within a patient."
When HIV is introduced into blood or tissues, the virus binds to CD4 molecules on the surface of helper T cells, injecting its contents into cells and setting off a process that leads to the assembly and release of new virus particles. It has long been assumed that these free virus travel by diffusion through tissue fluids to encounter new cells that can be infected. But recent studies have suggested that HIV can also pass directly from cell to cell when structures called virological synapses form during long-lasting interactions between T cells. Since CD4 T cells usually migrate quickly and form only transient contacts with other cells, the current study was designed to examine whether HIV alters the migration of infected T cells, allowing the kind of persistent contact that facilitates the spread of infection.