Tuberculosis, the forgotten plague

On March 24, 1882, German microbiologist Robert Koch announced the discovery of the pathogen of tuberculosis: Mycobacterium tuberculosis. In June 1921, French bacteriologist Albert Calmette and his assistant Camille Guérin cultivated bovine Mycobacterium and developed a tuberculosis vaccine that was first used on humans. This was the Bacille Calmette-Guérin vaccine (BCG) named after them.

Today, BCG is still the only vaccine approved to fight tuberculosis, more than a century after its introduction. But the demon of tuberculosis has never been completely defeated. For decades, scientists have been working to develop better options. After testing dozens of methods and experiencing multiple clinical failures, researchers say a second tuberculosis vaccine is expected to be on the market. Abundant channels and a large number of candidate vaccines make the victory over tuberculosis full of hope.

By Anthony King

Compiled by | Xian Jie, Idobon

On July 18, 1921, a baby was born in Paris. His mother died of Mycobacterium tuberculosis (Mtb) infection shortly after giving birth, and his grandmother who took care of him also had tuberculosis (TB). In order to protect the newborn, the doctor vaccinated him with a dose of live bovine Mycobacterium bovis. This was the first baby in human history to be vaccinated with BCG.

Today, more than 100 million newborns are vaccinated with BCG every year, mainly in developing countries. This move can save tens of thousands of lives. However, the protection provided by BCG is still incomplete, and tuberculosis remains the number one infectious killer on the planet. It is estimated that tuberculosis has claimed the lives of about 1 billion people in the past 200 years, and 1.4 million people died of it in 2019 alone. Now that doctors have antibiotics that can treat tuberculosis, this "huge burden" has become even more unfortunate - because we seem to have begun to forget that tuberculosis is a plague.

Disadvantages of BCG

Tuberculosis in children often occurs outside the lungs and can present as miliary tuberculosis and tuberculous meningitis. The former affects multiple organs and is almost always fatal if not treated with antibiotics in time; the latter is caused by infection of the membranes surrounding the brain and spinal cord. The biggest advantage of BCG is that it can prevent these infections. Vaccinating newborns with BCG reliably prevents the spread of tuberculosis bacteria between children.

More than 90% of TB cases occur in adolescents and adults and present as what is known as pulmonary TB. This is a serious lung disease that can cause chest pain or coughing up blood. BCG offers more diverse protection for the lungs, but for some reason it is less effective against pulmonary TB than other types of TB.

Curiously, there are regional differences in the protection provided by BCG in adolescents and adults: it works well in Scandinavia and other high-latitude regions, but it protects less well in people closer to the equator. One of the most popular explanations is that there are different species of mycobacteria in the equatorial region that trigger immune memory in the body, allowing the body to recognize BCG and reduce its replication. There is strong epidemiological evidence for this explanation: areas of the world where BCG does not work well also tend to have the highest rates of exposure to nontuberculous mycobacteria.

To complicate matters further, Mycobacterium tuberculosis can lie dormant in the human body for decades. Around 2 billion people worldwide may be infected with the bacterium, but when the right moment comes, such as when the immune system becomes suppressed (as is common in people with AIDS), the bacterium can reactivate, make a person sick, and can easily spread through the air and infect others.

The key to understanding these dynamics is to understand the immune response. Mycobacterium tuberculosis has coexisted with humans for thousands of years, and it has evolved a set of "molecular tricks" to evade our immune radar and even reduce the immune response to protect itself when detected. As people breathe, TB bacteria in the air enter the lungs and attract immune cells that "patrol" the lungs: alveolar macrophages.

Mycobacterium tuberculosis has a thick waxy layer of mycolic acid in a capsule-like structure that is not easily recognized by the immune system. When macrophages recognize and engulf Mycobacterium tuberculosis, the Mycobacterium tuberculosis hiding in the "wax capsule" can prevent the phagolysosome from maturing, so that they will not be killed and can even replicate. At the same time, Mycobacterium tuberculosis will also interfere with antigen presentation, making it impossible for "front-line" immune cells to transmit information about Mycobacterium tuberculosis to helper T cells, delaying the response of protective T cells, and limiting the effect of T cells after activation.

It can be said that macrophages inadvertently provide the tuberculosis bacteria with the "shelter" they desire - within the macrophages, the tuberculosis bacteria can be protected from the attack of antibodies.

At this point, the human body eventually isolates Mycobacterium tuberculosis in a group of immune cells called "granuloma", like a "time bomb", waiting for the best time for your defense system to collapse.

Of course, not all cases are serious. Most people infected with tuberculosis will not develop the disease, and only 5%-15% will develop tuberculosis. After all, tuberculosis and humans have experienced such a long struggle and game that a balance of wins and losses has been established.

A futile search: What are the indicators of tuberculosis immunity? Although BCG has been used for a hundred years, scientists have not fully figured out how it mobilizes the immune system to protect children, nor what kind of immune response adults need to produce to be considered a successful vaccination. Like most vaccines, BCG is much better at stimulating antibody production than it is at stimulating a strong T-cell response. Almost all vaccines work by producing neutralizing antibodies, but for tuberculosis, neutralizing antibodies seem to be far from enough.

Correlates of protection (CoP) are a set of indicators for evaluating vaccine effectiveness. These indicators can only be mined and developed when placebo-controlled trials (i.e., Phase 3 clinical trials) involving thousands of people are successful. However, in the blood of tuberculosis patients, there is no indicator that "this patient has been vaccinated and protected by the vaccine."

For many years, scientists have believed that the strong response of T cells triggered by BCG is the key to fighting tuberculosis, as it can destroy cells infected with Mycobacterium tuberculosis. A large number of studies have also shown that CD4+ T cells (also called TH1 cells) are essential for controlling infection and preventing communicable diseases. However, the more CD4+ T cells there are, the stronger the ability to fight tuberculosis is, so it is difficult to use its abundance as an indicator to evaluate the effectiveness of vaccines. In addition, in recent years, it is believed that helper T cells 17 (also called Th17 cells) that produce pro-inflammatory cytokines can also protect the human body.

In order to develop a vaccine that can effectively recruit and activate T cells, Helen McShane's team at the University of Oxford used a modified Ankara cowpox virus (MVA) as a vector to deliver tuberculosis antigens, named "MVA85A vaccine". According to McShane's recollection, in early clinical trials, the MVA85A vaccine stimulated the ideal correct immune response-CD4+ T cells were induced and secreted interferon γ, tumor necrosis factor and interleukin-2. However, a South African infant trial showed that the addition of the MVA85A vaccine did not show a more effective anti-tuberculosis effect than BCG vaccination alone. This may mean that the researchers went in the wrong direction and found the wrong CoP indicator. In the end, McShane's team concluded that the level of T cell response induced by the MVA85A vaccine was not enough to enhance the protective effect after BCG vaccination.

"We shouldn't lose sight of the fact that TB is primarily a respiratory infection, but current research almost always looks at what happens in the blood," said Hazel Dockrell, an immunologist at the London School of Hygiene and Tropical Medicine. "That doesn't help us understand what happens in the lungs, where there may be special cells that are key to fighting TB."

BCG

The century-old BCG vaccine is a live, weakened strain of Mycobacterium bovis, a related bacterium to tuberculosis (Mtb). Vaccination with BCG draws “frontline” immune cells to the injection site. Dendritic cells and other antigen-presenting cells display the parts of the bacteria in the BCG vaccine on their surfaces, driving T-cell responses to fight future pathogen infection and B-cells to produce antibodies. Both BCG and Mtb enter vesicles called phagosomes, but the difference is that BCG is eventually degraded, while Mtb can survive for a long time inside macrophages.

Tuberculosis vaccines in development

There are more than a dozen TB vaccines in clinical trials, and while new TB vaccines may not yet prevent infection, some can already prevent carriers from developing TB. Granted, preventing infection has always been the holy grail of vaccine success, but that’s a very, very high bar, so it’s not a bad idea to bend the bar a little bit—for example, preventing infected people from developing TB. After all, in almost all TB vaccine trials, subjects have been vaccinated with BCG and have latent TB.

Leading vaccines under development

Two vaccine candidates are currently in Phase 3 clinical trials, with eight more closely behind in Phase 2. Further back, three are in Phase 1 trials (not shown), and a handful of preclinical candidates are working toward human testing.

Live vaccines

VPM1002: a live attenuated BCG vaccine containing a pore-forming protein from another bacterium that allows antigens and mycobacterial DNA to enter the cytoplasm from the phagosome

BCG revaccination: another shot of BCG

MTBVAC: live, genetically weakened Mycobacterium tuberculosis with virulence gene mutations (the first and only vaccine of its kind to enter clinical trials)

Protein subunit vaccines

M72 + ASO1: a recombinant fusion protein consisting of two Mycobacterium tuberculosis antigens and an adjuvant

H56:IC31: a protein vaccine consisting of two early secretory proteins, a latent protein and an adjuvant

ID93/GLA-SE: A fusion of four Mycobacterium tuberculosis toxic antigens and an adjuvant

GamTBVac: a subunit vaccine combining two Mycobacterium tuberculosis antigens and an adjuvant

Whole cell vaccines

DAR-901: an inactivated formulation of Mycobacterium obuense that does not cause disease

MIP: A killed vaccine consisting of Mycobacterium indicus pranii, which grows rapidly and does not cause disease.

Vector-based vaccines

TB/Flu04L: Nasal administration, containing two antigens of Mycobacterium tuberculosis, attenuated, live influenza virus

Most TB vaccine candidates are being tested in adolescents and adults. However, there are two vaccines worth paying attention to, namely the MTBVAC vaccine in Phase 2A clinical trials and the VPM1002 vaccine in Phase 3 clinical trials. They can be tested in adults as well as infants and newborns, and may be particularly useful for immunocompromised children with AIDS. Of course, if we can prevent adolescents and adults from contracting TB, we don’t have to worry about the need for TB vaccines in children.

The candidate that almost everyone is talking about right now is the protein subunit vaccine M72 being developed by GlaxoSmithKline (GSK). The vaccine consists of a fusion of two proteins from the bacterium Mycobacterium tuberculosis, using an adjuvant called AS01 (which GSK has used in its blockbuster shingles vaccine and its industry-leading malaria candidate). Although M72's effects in nonhuman primates have not looked promising, it reduced the incidence of tuberculosis by 54% over three years in a recent trial involving more than 3,000 adults in Kenya, South Africa, and Zambia.

BCG is a live, replicable vector that is similar to a "slow-progressing version" of naturally infected Mycobacterium tuberculosis. Although the protective effect of BCG weakens during adolescence, it is still effective against tuberculosis throughout childhood. The M72 vaccine is mainly composed of some proteins, so it was doubted by some scholars in the early days. They believe that vaccines like BCG that can provide a long enough immune period to avoid natural infection are the most suitable tuberculosis vaccines. But the M72 vaccine provides protection for 3 years, proving the limitations of this idea.

Thomas Hawn, an infectious disease scientist at the University of Washington, was not involved in the development of the M72 vaccine, but he believes that the effectiveness of the M72 vaccine is due to the vaccine's adjuvant. The AS01 adjuvant can trigger an innate immune receptor called toll-like receptor 4 (TLR4), which can produce a powerful immune response, summoning T cells in addition to antibody-producing B cells. "In the past, we didn't have such highly effective adjuvants. Now, with new adjuvants, our ability to trigger different immune responses in a more subtle way has been greatly improved."

M72 protein subunit vaccine

The two mycobacterial recombinant proteins that make up BCG are fused together for injection. The fusion protein is recognized by immune cells and then displayed on their surface, triggering an immune response against the antigen, while a patented adjuvant (AS01) from GlaxoSmithKline enhances the immune response.

However, it cannot be ignored that the current trial scale of M72 is very small, with only 26 tuberculosis patients in the placebo group and 13 tuberculosis patients in the vaccine group. Undoubtedly, if you want to seek emergency use authorization from regulators, you must first conduct a trial of 10,000 people.

Meanwhile, the Danish National Serum Institute is developing two subunit vaccines, and its main candidate vaccine H56 (composed of three antigens and a new adjuvant) is undergoing Phase 2B clinical trials in Tanzania and South Africa. Another subunit vaccine called H107 contains eight specific antigens of Mycobacterium tuberculosis. Since these antigens are not shared by BCG, the H107 vaccine will not cross-react with BCG, which means that the two can be used in combination.

But a number of adjuvanted protein subunit vaccines have died halfway. They looked promising in early and mid-stage trials, but ultimately failed. Some scientists have lost interest in subunit vaccines. One of them is Andreas Kupz, a vaccinologist at James Cook University in Queensland, Australia, who has turned to live vaccines made using the original BCG method. For example, the Bill & Melinda Gates Medical Research Institute is conducting a trial to give booster doses of BCG to people aged 10 to 18 in South Africa. At first, everyone thought this approach would never work, but the results showed that it prevented 45% of secondary TB infections in South African teenagers, and even those who were infected recovered within 6 months. It is worth mentioning that this method does not prevent infection with the tuberculosis bacillus (positive blood tests), but more people eventually turned negative for TB and the infection was eventually cleared.

There are also some new live vaccines under development, and the one at the forefront is the VPM1002 vaccine. This is a genetically modified BCG vaccine that is currently in phase 3 clinical trials led by Stefan Kaufmann of the Max Planck Institute for Infection Biology in Berlin. Kaufmann said that they do not think that BCG is bad, but they hope to improve the neonatal BCG vaccine so that it can also be used in adults and adolescents. They added the gene of a pore-forming protein (listeriolysin O) produced by Listeria to the vaccine. Once the tuberculosis bacillus is engulfed by "frontline" immune cells (such as macrophages), it is sequestered into the phagosome, and listeriolysin O can penetrate the phagosome membrane and leak molecules from VPM1002. These molecules appear on the cell surface, induce CD8+ T cells to attack infected cells, and allow DNA to escape, triggering pro-inflammatory pathways.

VPM1002 whole cell vaccine

Unlike BCG, this is a genetically modified BCG vaccine. After injection, macrophages carry it into phagosomes, where they produce an enzyme that creates pores in the phagosome membrane, allowing the antigen to leak into the cytoplasm and activate inflammasomes in a similar way to TB bacteria.

Another candidate live vaccine is more different from BCG because it uses the M. tuberculosis bacterium itself. There is a view that more directly mimicking natural infection may be the way to success for vaccines. Researchers at the University of Zaragoza in Spain deleted two pathogenic genes of M. tuberculosis to make it safer, thus developing a modified version of the MTBVAC vaccine, which is now in Phase 2a trials. The process simulated by MTBVAC may be closer to real tuberculosis infection than BCG and can induce a more similar memory immune response, thereby providing better protection.

Remember the regional differences in BCG protection mentioned at the beginning of this article? As mentioned above, the best evidence so far is that the greater number of mycobacteria near the equator seems to reduce the effectiveness of BCG in adults. This means that the same phenomenon is likely to be repeated with these new live mycobacterial vaccines. Indeed, there is evidence that immune responses to mycobacteria in the environment can interfere with the efficiency of BCG, so the question now is to what extent this will be a problem for MTBVAC and VPM1002? Will these vaccines be given to elderly people who have been exposed to mycobacteria for a lifetime? If cross-reactivity between mycobacteria does affect vaccine effectiveness, then protein subunit vaccines or vector-based vaccines (such as the ones currently being studied by McShane's team) may be better options. After all, "don't put all your eggs in one basket."

Eliminating tuberculosis

To revive the tuberculosis vaccine, researchers must consider that BCG not only prevents tuberculosis, but also stimulates the immune system to resist a wider range of respiratory diseases and even sepsis. Of course, researchers observed this nonspecific protective effect shortly after BCG was widely used: at that time, BCG vaccination not only reduced tuberculosis deaths in children, but also reduced deaths from other causes. Currently, BCG is also used in immunotherapy for early bladder cancer: injecting it directly into the bladder can trigger the patient's immune system to attack the tumor. Correspondingly, the VPM1002 vaccine has also shown potential for treating bladder cancer in clinical trials.

Over the past decade, researchers have gradually uncovered the mechanism of these nonspecific effects, finding that it involves genetic reprogramming of innate immune cells, a phenomenon known as trained immunity. Essentially, BCG leaves an epigenetic imprint on cells so that they can respond to other subsequent infections. Some scientists have even speculated that this universal mechanism of immune defense is enough to (basically) explain why BCG fights tuberculosis. While most tuberculosis vaccines currently in development are being tested specifically against pulmonary tuberculosis, Nigel Curtis, a tuberculosis vaccine researcher at the Murdoch Children's Research Institute and the University of Melbourne, is leading a multinational trial to investigate health workers who have received BCG to see if it can also affect COVID-19 infection.

Vaccination

In the 1920s, BCG was taken orally; soon doctors began injecting it under the skin (a standard practice for nearly a century). But now researchers are considering new ways to deliver the vaccine, such as directly into the lungs and intravenously.

Helen McShane, a vaccine developer at the University of Oxford, said they are trying to get the vaccine into the lungs because that is where tuberculosis enters the body. Some animal experimental data show that getting the vaccine into the lungs is the best way to protect against it, and when the tuberculosis vaccine is administered via aerosol, the immune response in the lungs and blood is stronger.

Andreas Kupz, a vaccine scientist at James Cook University in Queensland, Australia, also said that when BCG enters the lungs directly through the nose or spray, resident memory cells will be activated - local memory T cells and possibly memory B cells, which are always ready to respond to the reappearance of specific pathogens.

These cells also appear to be stimulated by intravenous administration of BCG, with a recent study in Nature reporting that rhesus macaques vaccinated intravenously had better CD4+ and CD8+ T cell responses compared with monkeys given nasal and intradermal administration. McShane believes that intravenous BCG can teach us more about which immune responses are necessary so that we can design vaccines specifically to induce those responses.

Currently, the development of tuberculosis vaccines has been complicated by the COVID-19 pandemic. Experts interviewed by Science magazine pointed out that some tuberculosis vaccine trials have been delayed or slowed down. If it were not for the COVID-19 outbreak, the M72 vaccine trial should have been completed. In addition, during the pandemic, it is likely that tuberculosis cases were not reported or even delayed in treatment. The global blockade against the new coronavirus has undermined the "End TB" plan.

In fact, according to the Stop TB Partnership, a non-governmental organization, nine of the countries with the most TB cases saw a sharp decline in the diagnosis and treatment of TB infections in 2020, ranging from 16% to 41%. The organization estimates that the 12 months of the COVID-19 outbreak have offset 12 years of progress made in the global fight against TB. The official number of TB cases and deaths may decline in 2020 due to the disruption of the COVID-19 pandemic, but the number of TB deaths in 2020 may exceed that of COVID-19 because those who have undiagnosed TB will not receive the treatment they deserve.

More than 95% of tuberculosis deaths worldwide occur in low- to middle-income countries, and a study by the London School of Hygiene and Tropical Medicine estimates that basic social support could reduce the tuberculosis burden by 85%. Humans have adapted to the presence of tuberculosis, but poor living conditions, poverty, overcrowding, and low immunity due to HIV all lead to more disease. The World Health Organization says there is a $3.3 billion funding gap to implement existing tuberculosis interventions.

There is no doubt that vaccines can be a key part of the global strategy to combat TB. If we had a vaccine that could prevent a recipient from developing TB disease after being infected with the tuberculosis bacillus, or even prevent infection altogether, that would be a remarkable achievement.

Although the COVID-19 pandemic has temporarily hampered the development of tuberculosis vaccines, some "chemical reactions" are expected to occur in the development of COVID-19 vaccines. For example, some researchers believe that mRNA vaccines, which strongly stimulate the immune system, are not only very flexible but also highly immunogenic, and are suitable for targeting tuberculosis.

We need a better TB vaccine for the No. 1 killer among infectious diseases. But wherever the answer lies, researchers are determined to find it and move forward with clinical programs.

This article is authorized to be translated from TheScientist online magazine, link address:

https://www.the-scientist.com/features/tuberculosis-the-forgotten-pandemic-68894

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