ABOVE: A microscopic image of a biopsied lymph node of a person with untreated HIV Flickr, NIAID
Vaccines might work better if they know where to go: to small sacks inside lymph nodes called lymphoid follicles. According to research published in Science on January 27, vaccines that rapidly seek shelter inside follicles are more likely to trigger a strong immune response. In comparison, vaccines that dawdle around elsewhere in the body can get degraded, leading to mediocre protection. The discovery that a vaccine’s target location influences its effectiveness could help scientists design better protein-based vaccines.
“I’m certainly excited by the work and the quality of the study,” says Jason Cyster, an immunologist at the University of California, San Francisco, who didn’t work on the project. “It’s an important advance.”
Vaccines introduce the body to antigens—bits of weak or inactive viruses that launch a protective response against the real thing. As with most foreign substances that pierce the skin, vaccines ooze to the lymph nodes. Before antigens get there, though, they encounter protein-cleaving enzymes called proteases, which are present throughout the bloodstream and notoriously mangle antigens. So, by the time antigens reach the B cells that produce pathogen-detecting antibodies, they can be almost unrecognizable. This means immune cells will make antibodies against a protein that looks nothing like the original vaccine.
For that reason, a continual, fresh supply of antigens sometimes results in better immunity, explains study coauthor Aereas Aung, an immunologist and biomedical engineer at the University of Toronto who previously worked as a postdoc at MIT in fellow study author Darrell Irvine’s lab. Inside the body, follicular dendritic cells are thought to provide B cells, the other immune cells in the follicle, with a continual antigen supply. Follicular dendritic cells churn out exact copies of the antigen, but it’s unclear how they do so.
Aung and Irvine are in the business of building better HIV vaccines. As part of his efforts to do so, Aung was curious about how stable vaccine antigens are throughout their journey through the lymphatic system, and how their degradation might lead to poorer protection.
Aung and his colleagues started off by testing antigen stability inside lymph nodes in a mouse model. Using the fluorescence resonance energy tomography (FRET) technique, they vaccinated a mouse with a fluorescent antigen designed to stop fluorescing once it degrades. After 48 hours, they peered at the mouse’s lymph nodes through a microscope. The outside compartments of the lymph node darkened, while the follicles remained bright, suggesting that the follicle might be a sanctuary for antigens, protecting them from degradation.
Suspecting that proteases caused this degradation, the authors wanted to look at protease activity directly. To do that, they used a combination of RNA sequencing, histology, and imaging techniques in mouse lymph nodes in vitro. In one experiment, researchers visualized protease activity throughout the lymph node by applying a fluorescent peptide probe in the shape of a hairpin. The probe was specifically designed to stick to cells that produce proteases, which target and slice the probe in half. With these techniques, the team confirmed that there was significantly less protease activity in the extracellular environment inside follicles than throughout the rest of the lymph node. This again suggested that the follicle might act as a protease-free haven for antigens.
For decades, scientists “have known that these [follicular dendritic cells] were able to hold on to intact antigen,” says Aung, but not how. “What we’ve added to that story . . . is that the microenvironment certainly does play a role. That’s something I found to be awesome.”
Encouraged by their findings, the authors thought they might be able to improve vaccines by sending them straight to follicles. They created a nanoparticle-based HIV vaccine designed to home in on proteins on the surface of follicular dendritic cells. Compared to a more traditional vaccine, the targeted vaccine led to higher numbers of B cells targeting the intact, undamaged antigen. The team also found larger numbers of germinal centers, which make B cells produce potent antibodies, enhancing protection against infection.
“We’ve known for a long time that the extracellular environment is full of aggressive proteases. . . . But I think the vaccine community hasn’t considered that enough in terms of the degree to which this is diminishing the quality of the . . . antibody response,” Cyster says.
Aung says that this suggests that delivering a more targeted vaccine might lead to better protection against pathogens. “You have to make sure you get it to the follicle, or else your vaccine won’t be as effective,” he says.
Cyster agrees, adding that if researchers can “build more protease-resistant vaccines, that will improve the odds of making a protective antibody response.”
