Macrophages, immune cells that are part of the first line of defense, recognize and engulf microbes in a vigilant effort to keep the body healthy. The researchers found that macrophages respond to a broad range of bacteria by sending off an alarm to the rest of the immune system and transforming into a cell primed to mount an immune response.
Further study revealed that the macrophage didn’t have to "see" the whole bacteria to send off its alarm signal, but the presence of specific bacterial components, such as proteins and sugars, induced activation. "These findings will help researchers design therapeutics that will stimulate the immune system in a targeted manner, perhaps with fewer side effects," says Young, lead author on the study, which will appear in the February 5 issue of Proceedings of the National Academy of Sciences.
"The interplay between a person’s immune mechanisms and a microbe’s attempts to circumvent these defenses represents a complex relationship. DNA arrays help researchers dissect this struggle by measuring the activity of many genes in the immune cells as they respond to pathogens. As a result, researchers gain invaluable information about the strengths and vulnerabilities of the microbes and our own immune system during an infection," explains Gerard Nau, a first co-author on the study and a researcher in the Young lab. Ann Schlesinger, postdoctoral fellow in the Young lab, and Joan Richmond, former Young lab member, were also first authors on the paper.
DNA array studies improve our understanding of macrophage defenses, provide insights into disease development, and suggest targets for therapeutic intervention. The researchers found, for instance, that Mycobacterium tuberculosis, the bacteria that cause tuberculosis, fail to activate critical macrophage genes that are involved in fighting bacteria. M. tuberculosis are unique in that they can somehow survive inside the macrophage, only later to escape and cause disease.
It turns out that M. tuberculosis doesn’t trigger the macrophage response to produce IL-12 and IL-15, like other bacteria do. IL-12 plays a fundamental role in activating another arm of the immune system called T helper responses and is critical for host resistance to tuberculosis infection in mice and humans. The lack of strong IL-12 response in macrophages indicates that this may be one way M. tuberculosis evades host defenses and supports the use of both IL-12 and IL-15 in clinical tuberculosis therapies, as suggested by animal models.
The researchers also found interesting specifics about the shared macrophage activation program, which was induced by a wide range of bacteria, including Gram-positive, Gram-negative, and mycobacteria. While the activation program seemed to trigger a generic alarm, which called on other antibacterial defenses of the immune system, the majority of changes triggered in the macrophage involved cell surface proteins and signaling molecules. Such changes generate new functions in the macrophage, suggesting a maturation process similar to that observed with other immune cells.
Interestingly, the presence of certain bacterial components was enough to activate the alarm signal. This suggests promising new adjuvants (compounds that make a vaccine more potent by increasing an immune response) that could be used in vaccine development. The list includes heat shock proteins, which supports their use in preclinical and clinical vaccine trials.
The bacterial components that set off the alarm signal activated a family of proteins called Toll-like receptors (TLRs), suggesting that small molecule drugs designed to activate this receptor may provide a new therapeutic approach.
"DNA microarray data is providing us with unprecedented details about our own immune defense cells as they orchestrate a response to attacking bacterial pathogens that are responsible for some of the major diseases of mankind," says Young. "This information should lead to new therapeutic strategies against these diseases."
Journal
Proceedings of the National Academy of Sciences