Friday, January 25, 2008

A periodic table of microbes?

There's a great article from Melinda Wenner in this week's Nature News that expands on a theme I've been writing about lately: microbe and human interactions.

The article discusses recent results by Leonid Margolis. Margolis was attempting to grow HIV in some tonsil tissue infected with Human Herpes Virus 6 (HHV-6). In HIV patients, HHV-6 seems to hasten HIV progression, and no one knows why.

However in the tonsil tissue, Margolis and colleague, Jean-Charles Grivel, couldn't get the HIV to thrive. He repeated his experiments. Same result.

“We couldn't believe our eyes,” Margolis says. HHV-6, at least in this situation, seemed to protect against HIV.

Wenner writes, "Margolis uncovered why HHV-6 prevents HIV replication under certain conditions. A subtype of HIV, most often found in early infection, generally gains entry into the cell by binding the receptor CCR5. When HHV-6 infects first, however, it instigates production of an immune chemical that binds to CCR5 receptors, blocking HIV's access. HIV can develop resistance to this chemical over time and HHV-6 co-infection may exert selective pressure on HIV to become immune-resistant, or switch to a different co-receptor — a change accompanied by increased HIV virulence. This explains the often poor prognosis of patients infected by both viruses."

These results emphasize the importance of doing experiments under conditions that closely resemble real world cases, rather than in sterile lab cultures.

“If you study a pathogen such as HIV in a cell-culture model where there is no immune system, there is no effect from other microbes that are in the normal state in an infected human,” says Jack Stapleton, director of the University of Iowa's Helen C. Levitt Center for Viral Pathogenesis and Disease in Iowa City. “You're really missing important factors that will influence the pathogenesis.”

Margolis fantasizes about creating, "a table, not unlike Mendeleev's table of the elements, for infectious agents. Each cell in the table might feature the name of the microbe, the immune factors it affects, the receptors it uses and the signalling systems it incorporates. Just as the elemental version predicts how two substances react with one another, the periodic table of microbes would predict how two microbes interact in the human body."

Photo: HIV attacking CD4 cell from NIH.

1 comment:

  1. You can put anything in a table, but there's an underlying reason why the table of chemical elements is periodic. For microbes, a phylogenetic tree is probably as good an overall guide to their properties as you can get.