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Aug 18, 2010

Researchers Discover How Some Pathogens Evade the Immune System

  • A set of sugars found on some disease-causing pathogens helps those pathogens fight the body's natural defenses as well as vaccines, say two Iowa State University researchers. The findings are published in the current online issue of the Journal of the American Chemical Society.

    Nicola Pohl, Ph.D., professor of chemistry, and Christine Petersen, Ph.D., assistant professor of veterinary pathology, began studying persistent pathogens such as tuberculosis and the parasite Leishmania five years ago when they noticed that some types of the parasite can make people sick while others do not.

    "One of the things I was curious about was that pathogenic strains of Leishmania have a different sugar coating on them than nonpathogenic strains," Dr. Pohl remarks. Their aim was to find out whether just the sugar coating is enough to make something pathogenic or nonpathogenic.

    Normally, when a disease-causing agent enters the body, cells called macrophages engulf and start to destroy the agent. Leishmania-type diseases are resistant to this process.

    To test the theory on the resistance effect of the sugar coating, Drs. Pohl and Petersen developed an experiment that required creating small beads measuring one micron in diameter to mimic the pathogens. One group of beads was then coated with a type of sugar that is similar to that of Leishmania. Another set of beads was coated with a lactose-type sugar that isn't harmful to the cell. A third had no coating.

    The beads were then introduced into macrophages. The cells noticed the uncoated beads and started an immune response, as they should. When the lactose-covered beads were introduced, they were also recognized and removed. When the Leishmania-sugar covered beads were introduced, the macrophages took a much longer time to recognize their presence. Then, the immune defense system slowed down or dampened the attacks.

    This dampening, Drs. Petersen and Pohl showed, is due to an interaction between the sugar on the bead and toll-like receptor 2 within the macrophage. "There is something inherent about the sugars themselves and the difference in these sugars that dampens your normal response to the pathogen," explains Dr. Pohl.

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