Studying Triggers of the Immune Response

Posted July 27th, 2009 at 7:00 am.

Stephanie Eisenbarth

Vaccination has protected people from a wide variety of dire maladies over the last several decades, but immunologists still are trying to determine how exactly how the body generates an immune response to a particular vaccine. The inclusion of an adjuvant, a substance mixed with the immunogen to boost its efficacy, helps to trigger the response, though researchers long have struggled to pinpoint why.

Working with one of the world’s most common adjuvants, aluminum hydroxide, or alum, Stephanie C. Eisenbarth ’96 is uncovering the secrets behind the immune response.

Toll and Nod

Eisenbarth, a postdoctoral fellow in the departments of immunobiology and laboratory medicine at the Yale University School of Medicine, coauthored a study in last June’s edition of Nature setting forth a molecular basis for alum’s role in activating the immune system. Previous research had described the importance of the immune system’s Toll-like receptors (TLRs) in detecting pathogens and telling the body to fight them off. TLRs are important receptors in the evolutionarily older and more general “innate” immune system. Eisenbarth’s study was the first to note the key role played by the system’s Nod-like receptors (NLRs)—specifically, the receptor known as Nalp3—in the adjuvant activity of alum. Nalp3 forms a complex called an “inflammasome” upon activation, and it turns out that alum can initiate that process. The paper also observed that when the Nalp3 inflammasome is removed, cells do not produce interleukins, a part of the immune system usually triggered by alum, and antibody and T-cell responses fall.

“Toll-like receptors are one class; these Nod-like receptors are a second class, and what they induce in the immune response is a little bit different, which makes sense,” Eisenbarth says. “They’re signaling and telling the immune system that something has happened, and they’re doing it in a slightly different way.”

What led Eisenbarth to explore alum’s role in the immune response was her dissatisfaction with the decades-old hypothesis that attempted to explain its importance since alum’s discovery in the 1920s. Scientists had theorized simply that alum attached to antigens and released them slowly, activating the immune response in the process. To Eisenbarth, the hypothesis was not nearly specific enough, especially given advances in the field of immunology over the last decade.

“It was kind of like a hand-waving, general argument to say you’re giving something in a form that may help something,” she says. “We know a whole lot more than we did in the 1920s about immunology, and none of that was incorporated into this explanation. One of the most important discoveries was made 10 years ago—the identification of Toll-like receptors, which help regulate when we have an immune response and when we don’t. That’s critical, because you don’t want to have an immune response against yourself—that’s autoimmunity. These receptors and these cells help the body realize what is a virus it needs to respond to, and what is self-DNA, which should not be responded to. That’s a critical distinction.”

New Questions

Eisenbarth emphasizes that while her team’s discovery of Nod-like receptors’ significance is important, there is much more work to do. The NLR pathway, uncovered by the Yale researchers’ use of both in-vitro and in-vivo testing, is a key explanation, but likely not the only one, for alum’s efficacy. As often happens, answering one question, while certainly satisfying, has also raised a new set of questions, giving Eisenbarth new directions for her work.

In particular, she observes, researchers now need to fill in the blanks between alum’s activation of the NLR pathway and its significance within the immune response. Among the new goals is determining whether the body’s production of the signaling molecules interleukin-1 and interleukin-18 is the important factor initiated by the pathway, resulting in the new generation of immunity. Additionally, scientists need to apply these discoveries to human models. Eisenbarth’s work was done on mice; while their cells have the same Nod-like and Toll-like receptors as human cells, confirming and extending the conclusions with respect to humans will help in the development of new and better ways to prevent infections.

“There are diseases already known and associated with multiple NLRs, so we know that they are active in human beings, but it would be very nice to study the role of these actual NLRs in the human response to alum,” she says. “That’s something that’s harder to address. It’s always harder to go into humans, but it’s important to do, to say alum can and does trigger this pathway in human beings.”

As a graduate student at Yale, where Eisenbarth earned her Ph.D. and M.D. degrees, she studied allergies and asthma. Her post-doctoral work has been primarily in vaccine research.

“The ability to prevent infectious disease is one of the most important interventions in all of human health. Vaccines have had a tremendous impact on global health. The immune system has the ability to do a lot of good, but it also causes very severe diseases, like autoimmunity. That’s something I would also like to study, because it’s interesting to understand why the immune response goes awry. It presents a lot of really interesting opportunities that have direct implications for human health, both good and bad. Understanding both sides of the coin may help us intervene more directly.”

Tom Durso writes about science, health care, and business for a variety of publications, including the Philadelphia Business Journal and Family Business magazine.

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