Studies Shed Light on Gene Response to Bacterial Infections

CAMBRIDGE, Mass. — Making a medical diagnosis today often relies on symptomology, bacterial cultures, stain tests, experience—and luck. But new research by systems biologists at Whitehead Institute for Biomedical Research aims to offer physicians new diagnostic tools by uncovering important differences in the way immune cells respond to bacteria that cause botulism, diphtheria, strep throat, staph and a range of other infections.

The work focuses on molecules called “Toll-like receptors,” positioned on immune cell surfaces, that have evolved to serve as docking clamps for specific molecules on the surfaces of bacterial cells. In research published recently in the Journal of Immunology, Whitehead scientist Richard Young led a team that applied microarray technology to study how genes in human immune cells are modified when they encounter bacteria. It’s a first step, Young said, not only toward the development of a bacterial infection diagnostic tool, but also “an approach to dissecting out the molecular details of the pathways by which these cells are fighting the infection.”

Studies of the human genome reveal 10 Toll-like receptors densely packed on the surfaces of immune cells called macrophages. When bacterial microbes attack these cells, signals sent through the Toll-like receptors change the level of protein production by genes in the macrophage. The scientists found 101 genes whose protein expression changed significantly when exposed to the two major classes of bacteria, Gram-positive and Gram-negative.

Using microarray (“gene chip”) technology that’s become a key tool in genetics research, Young and former postdoctoral researcher Gerard Nau studied the expression of hundreds of genes activated by a single bacterium, a view that offered a detailed display of the complexities of gene response to infection.

Their results focused on a molecule found in the cell walls of a Gram-negative bacteria called lipopolysaccharide—LPS for short—which helps give the cell wall membrane its fluidity. LPS joins with a Toll-like receptor called TLR4, forming a pair the microarray analysis identified as crucial to the differences in gene expression seen in the presence of Gram-negative and Gram-positive bacteria.

The detection of this unique gene expression profile could help physicians identify the Gram status of the bacterial agent immediately and prescribe the most effective antibiotic.

“If I could tell that a patient coming into the emergency room had, with a 99 percent certainty, a Gram-negative infection, that would skew how I would approach the patient,” said Nau, now an assistant professor at the University of Pittsburgh who doubles as both an immunology researcher and an infectious disease physician. “That could be a useful diagnostic test.”

It’s a view shared by Douglas Golenbock, chief of the Division of Infectious Diseases and Immunology at the University of Massachusetts Medical School. “I think this research represents a basic science advance that may soon have a real impact on our ability to take care of patients,” Golenbock said.

The research was supported by the National Institutes of Health, Corning, Inc., Affymetrix, Inc., Millennium Pharmaceuticals, Inc. and Bristol-Myers Squibb Co.

Citation

Nau, G. J., Schlesinger, A., Richmond, J. F., & Young, R. A. (2003). Cumulative Toll-like receptor activation in human macrophages treated with whole bacteria. The Journal of Immunology, 170(10), 5203-5209.