In another example of how the AIDS virus exploits its opponents, scientists have found that HIV uses immune system proteins to hitch rides on the antibody factories known as B cells.
Since B cells interact closely with T cells, the immune cells that HIV normally infects, the hitchhiking strategy may help the virus find potential host cells.
HIV infects a subset of T cells and, over time, reduces their number so much that a person’s immune system can’t fight off other infections. Consequently, scientists generally characterize AIDS as a T cell disease.
Yet scientists have long recognized that B cells also go awry in people with a full-blown HIV infection. “The B cells make a lot of antibodies . . . not targeted to anything,” says Susan Moir of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. The cells “are going into overdrive. It’s almost like an allergic reaction,” she says.
In test-tube experiments, Moir and her colleagues had found evidence that HIV infects B cells as well as T cells. To determine if that occurs in the body, they began studying the B cells of HIV-infected people. It turned out that HIV didn’t readily enter the B cells but did stick to their surface.
In the Sept. 5 Journal of Experimental Medicine, Moir’s team reports those results and describes how HIV hitches onto B cells. In a process that normally helps the body get rid of viruses, an immune system protein—part of the so-called complement system—coats HIV’s surface when it circulates in the bloodstream. But B cells have surface proteins that bind to this complement molecule.
Presto, B cells and HIV unite.
B cells circulate through the blood and into lymphoid tissues where T cells mature, so sticking onto B cells is a smart strategy for the AIDS virus, says Moir. In lymphoid tissue, she explains, B cells and T cells crowd together so that “you have an excellent opportunity for passage of virus from the B cell to the T cell.”
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The researchers suspect that the binding of HIV-complement complexes to B cells also explains why these immune cells go into overdrive. The B cell surface proteins that interact with complement help regulate the production of antibodies. HIV’s coat of complement may be constantly stimulating the B cells through these docking stations, says Moir.