New Approaches to Understanding TB Can Help Inform Development of Better Vaccine

With upwards of 2 billion individuals infected with Mycobacterium tuberculosis (Mtb) each year, the infection continues to pose a serious challenge on a global scale. Although advances have been made in the fight against the disease, much is left to be desired when it comes to diagnostic, treatment, and prevention options.

In the Monday plenary at the 25th Conference on Retroviruses and Opportunistic Infections (CROI), JoAnne Flynn, PhD, from the University of Pittsburgh, stressed the importance of gleaning a better understanding of the pathogenesis of tuberculosis (TB) in order to develop new, effective vaccines that will be able to offer much needed protection against the disease.

There are several reasons why TB has remained such a serious global problem. Firstly, the diagnostics available to detect the disease are incredibly outdated, and, as such, many individuals never receive a diagnosis despite having active TB, according to Dr. Flynn. Secondly, although effective treatment is available, the regimen is long—at least 6 months minimum—and is known to cause several side effects. Finally, of course, is the growing problem of resistance, with many strains of the disease becoming resistant to drugs used in the standard care regimen. It doesn’t help that the ultimate preventive weapon in our arsenal, Bacillus Calmette—Guérin (BCG) or is not very effective either. “It is still given to a lot of babies worldwide, but I will tell you that most people who die of TB were vaccinated with BCG. So, clearly, we need to do better in terms of a vaccine,” shared Dr. Flynn. Poor access to health care is another contributing factor, as “this is a disease of poor people,” she added.

Within 2 years of being infected by having inhaled the bacterium through the air, a small percentage of individuals will develop active tuberculosis, what Dr. Flynn refers to as symptomatic disease. “However, most people will contain the infection through their immune response and have latent TB, which is simply clinically asymptomatic infection,” Dr. Flynn stressed. “Those people will most likely remain infected for the rest of their lives.” When the disease is asymptomatic, individuals may never learn of their infections but nonetheless, they run the risk of reactivation in time which could result in the active form of the disease, which is transmissible.

“We don’t know what leads to this risk of reactivation. Is it bacterial burden? Is it host response? There are a lot of things that we still don’t know about that,” Dr. Flynn admitted.

What do we know? For one, it’s better to prevent infection in the first place than concentrate on treating an individual once they are infected. The ultimate form of prevention is a better vaccine.

Although there are no definitive criteria on what would be required to develop a better vaccine against TB, according to Dr. Flynn there are at least a few aspects that need to be taken into consideration that we already know:

• T cells are important; they are needed to activate macrophages (which house the infection) to kill the bacteria. Thus, cell-mediated immunity is important. We need to know what kind of T cells are needed and where they need to be.
• Although not typically thought of when it comes to TB, antibodies may offer a potential way to vaccinate against the disease. However, currently, there is little data to suggest this is possible.
• The current vaccine is an attenuated version of the Mycobacterium species that’s missing some of the immunodominant antigens from TB. We need to learn more about whether or those antigens matter.

“We don’t have a vaccine that is truly effective, and we have no correlates as to how to make that vaccine,” Dr. Flynn lamented. “Therefore, in the HIV world, you guys are much further along than we are in the TB world.”

Perhaps most importantly, in order to create a better vaccine, a better understanding of the pathogenesis of tuberculosis is especially important, and it is for that reason that Dr. Flynn provided conference goers with a crash course into what happens in the lung when someone is infected with the disease. “The granuloma is the pathological hallmark of TB,” she said. “[It] is basically the host response to infection. What we know from our studies is that each individual bacillus that you inhale will form an individual granuloma in the lungs,” some of which will work, and some of which won’t, which further complicates the understanding of TB.

In order to study how they granulomas form, Dr. Flynn and colleagues use animal models, particularly using different macaques, as they “recapitulate the entire spectrum of infection outcomes seen in humans—everything from latent to active TB,” she said.

In order to watch how infection spreads in real-time, the Dr. Flynn and her team developed an imaging modality called PET/CT, which uses fluorodeoxyglucose (FDG) as a probe, just like in cancer studies. As each granuloma takes up some of the FDG, it allows the investigators to measure the level of inflammation or metabolic activity for each granuloma.

Dr. Flynn exemplified how dynamic and heterogeneous granulomas are through the use of a few of the scans; they showed that while some granulomas in 1 lung lobe essentially went away, granulomas in another lung lobe grew worse, became inflamed, and grew in size. “These 2 things are happening simultaneously,” she explained. “[That evidence,] along with a lot of other data [tell us] that each granuloma has its own trajectory and it doesn’t care at all what the other granulomas in the lungs are doing.”

The team also used the macaque models to further their understanding of reactivation. They gave anti-tumor necrosis factor (TNF) to 26 monkeys with latent TB for the duration of 8 weeks. “Anti-TNF drugs are used for a variety of inflammatory diseases and we’ve known from mouse models that TNF is a very important cytokine in TB,” Dr. Flynn explained.

They performed PET/CT imaging before giving the anti-TNF, and subsequently at 4 weeks and 8 weeks after anti-TNF. “We defined reactivation very strictly,” said Dr. Flynn. “We said reactivation is going to be the formation of a new granuloma after anti-TNF. When we defined it that way, it turned out that half of the monkeys reactivated in 8 weeks and half of the monkeys did not.” To predict which subjects would reactivate and which would not, they created a “simple algorithm” which looked at the total amount of FDG, or the total inflammation in the lungs. “When we did that, we can see that the monkey who would reactivate had prior inflammation in the lungs, compared to those who would not, prior anti-TNF,” she said.

Looking at latent controls, the team sought out to predict which monkeys would reactivate using the algorithm they defined. “We wanted to see, what’s the difference between monkeys given no anti-TNF at all?” she said.

Although there were many differences, Dr. Flynn highlighted 1 in particular: monkeys who were not given anti-TNF, who were at high-risk for reactivation had a granuloma with high levels of bacteria. “What that means is, only 1 granuloma will put you at risk for reactivation,” she explained.

What about reinfection? There is currently no way to know if someone who has latent TB is reinfected because diagnostics for this aren’t available, according to Dr. Flynn. After considering past research suggesting that Mtb infection may have protective qualities, the investigators decided to conduct another study which found “robust concomitant immunity in TB,” where fewer granulomas establish, the bacteria establish don’t grow to the same level, and the TB in the reinfection granulomas are killed more efficiently,” according to Dr. Flynn. “All of this results in a 10,000-fold protection of bacterial burden compared to naïve animals. And the protection is far superior to boosted BCG. What this tells us is that a vaccine may have to mimic TB to become very effective.”

Although there are many challenges to understanding TB, the team highlighted that it’s important to understand that heterogeneity is a very important aspect of the disease. One bad granuloma could result in dissemination and reactivation, according to Dr. Flynn, and in order to prevent disease, a vaccine would need to succeed in all granulomas. However, when these clues and research are taken into consideration, improved vaccines and interventions may soon be underway.