Pathogen-induced apoptosis in phagocytic cells “Stimulation of innate immunity in tuberculosis - fro

Boosting the body’s resistance to TB New co-infections, such as HIV, and increased levels of drug resistance mean tuberculosis remains a threat to health, particularly in poverty-stricken areas of the world. Apoptotic cells play a major role in stimulating the innate immune response, an area being addressed by Professor Olle Stendahl Around 2 billion people across the world are infected by mycobacterium tuberculosis, the bacteria that causes tuberculosis, and while relatively few will go on to develop the disease this still represents a significant threat to public health. More than 1.5 million people die every year from TB, a context in which the work of Professor Olle Stendahl’s research group at Linkoping University takes on real significance. “The main questions we are addressing are what role the body’s innate immune response plays in the host response to TB and also how you can boost the innate immune response in order to facilitate clearing of the bacteria and enhance resistance to the infection. We are looking at the role of apoptotic cells in boosting the innate immune response in macrophages in the lung and are also interested in the role of nitric oxide in the lung as a resistance factor to TB, and in boosting the production of nitric oxide,” he says. While in most infected people the bacteria is latent and causes no ill-effects, some events can trigger the onset of the disease. “If you get a co-infection with HIV for instance, or perhaps with worms, then that can initiate the growth of the bacteria, because the immune response is impaired or changed,” explains Professor Stendahl. “That’s one reason TB has become a very threatening disease over the last couple of years, although we thought we had controlled it.”

Phagocytic cells The role of phagocytic cells in ingesting harmful foreign particles makes them an important part of the body’s response to infection. There are two primary types of phagocytic cells – monocytes and neutrophils – which respond to chemical signals by accumulating at sites where a pathogen has invaded the body. “The monocytes and neutrophils will accumulate and then phagocytose and kill the bacteria,” explains Professor Stendahl. In more chronic cases of infection, where the bacteria rest inside cells, macrophages – cells produced by the differentiation of

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monocytes in tissues – are able to kill bacteria over the longer-term; enhancing this process will offer improved protection against infection. “The BCG vaccine was developed in the 1930s. This vaccine is effective in protecting children against TB, but it doesn’t work very well in adults. So we need a better vaccine,” continues Professor Stendahl. “But there is no real established drug that will boost the immune response outside vaccines. There have been clinical

“The main questions we are addressing are what role the body’s innate immune response plays in the host response to TB and also how you can boost the innate immune response in order to facilitate clearing of the bacteria and enhance resistance to the infection” trials – people have shown for instance that vitamin D boosts the immune response and the function of the macrophages. We have shown that giving patients arginine – an amino-acid – will boost the activity of the macrophages to produce nitric oxide, and that will kill the bacteria. We can see that treatment is more effective in the presence of this compound.”

These attributes have made macrophages an important research focus, with Professor Stendahl’s group looking at how their function can be boosted and the immune response in the lung enhanced. This also requires an active phagocytic cell that can phagocytose and kill the bacteria; the process of apoptosis holds real importance

in this regard. “Apoptosis means cell death.

But it’s not that the cell just dies in an uncontrolled way and goes into pieces – it’s what we used to call Programmed Cell Death, it’s a more regulated way for the cells to die,” explains Professor Stendahl.

All cells have a limited lifespan; granulocytes live for only around 24 hours, after which they die in a regulated way.

“When these cells go into apoptosis they have to disappear. They don’t just lie there and leak out a lot of enzymes that will damage the tissue – they are being phagocytosed by other cells,” continues Professor Stendahl. “The macrophages will phagocytose the apoptotic cells. The idea in our project is that the neutrophils will accumulate in the infectious area, in the lung and then phagocytose some of the bacteria. They will go into apoptosis and then these apoptotic cells will be recognised by the macrophages, so it will phagocytose the apoptotic cell, and together with that the bacteria.”

Heat shock protein The bacteria have certain virulence factors that specifically trigger apoptosis in the neutrophils, but there are also situations where they might impair the normal apoptotic process in macrophages. One of the main goals of Professor Stendahl’s research is to find a way to stimulate macrophages to be more active; the group have found that apoptotic cells or bodies can stimulate macrophages, the next step is to analyse the underlying mechanisms at the molecular level. “We have found some molecules – so-called heat shock proteins, in particular heat shock protein 72 – which will signal to the macrophage to be more active,” outlines Professor Stendahl. This could eventually lead to the development of more powerful, effective methods of treating TB than are currently available. “The problem today is that treatment takes a long time – you usually have to treat people for six months with multiple drugs.

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Over the last 10-15 years we have seen increased levels of resistance to certain antibiotics, more and more bacteria are resistant to these multiple drugs,” explains Professor Stendahl. “If the patients you treat do not follow this challenging regiment, there is a high risk they will develop resistance to certain antibiotics. We need to find new drugs and ways to reduce the time it takes to treat TB, this is one of our most important goals.”

Alongside this basic research into apoptotic cells at Professor Stendahl’s lab in Linkopping the group is also pursuing extensive clinical studies into a large cohort of household contacts, looking at how people who are already infected with the bacteria respond to its presence.

Researchers are looking at the immunological state of the household contacts and how that affects each individual’s chances of being infected.

“Can we understand why just one individual is infected and not all the people sleeping in the same room? We are looking at the function of phagocytic cells, the production of nitric oxide production in the lung, and also at some other immunological parameters on patients,” explains Professor Stendahl.

Living conditions are another important factor in terms of an individual’s susceptibility to infection. “In Sweden the prevalence of TB was about 400 per 100,000 at the beginning of the twentieth century. Now it’s around 6 in 100,000, so the frequency has gone down by a factor of around 100. Most of that reduction can be explained by better living conditions,” says Professor Stendahl.

“Sub-Saharan Africa in particular is going through a period of urbanisation and together with HIV, that will increase the risk of people contracting tuberculosis. People living in bad, crowded conditions are at greater risk of contracting tuberculosis.”

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At a glance Full Project Title Pathogen-induced apoptosis in phagocytic cells “Stimulation of innate immunity in tuberculosis from bench to bedside”

A model for how apoptotic cells and nutritional additives, arginine and vitamin D, enhance the innate immune response to M. tuberculosis.

Drug resistance Concern over the increased threat posed by TB has led to an intense focus on vaccine development. However, with no new vaccine likely to emerge within the next five to ten years, Professor Stendahl believes it is important to also explore alternatives. “If we can boost the innate immune response of infected individuals in a cheap and effective way then that will definitely have an impact in the future,” he says. The group will continue its research into the underlying mechanisms behind the immune system’s response to infection. “The challenge is to understand how the apoptotic cells activate the macrophages. We have some leads for that,” outlines Professor Stendahl. “We also want to understand how co-infections affect the immune response to TB, and how this affects latent TB infections. We think that nitric oxide is important and want to find a robust way to increase nitric oxide production in lung tissue – we believe that would enhance resistance to infection. We have preliminary data showing that people that can generate more nitric oxide are not infected, while the group that cannot generate so much have a bigger chance of being infected by people in their family.”

Project Objective The main objective is to understand how innate immune activation stimulates macrophages, thereby facilitating early host response against and eradication of Mycobacterium tuberculosis.

Project Funding 5 million SEK (550k per annum) Contact Details Project Coordinator, Olle Stendahl, MD, PhD Div Medical Microbiology, Dept of Clinical and Experimental Medicine Faculty of Health Sciences, Linköping University 581285 Linköping, Sweden T: +46 101 032 050 E: cid:123112b2-8b84-401a-bb884de0f7202eb1@ad.liu.se] W: http://www.hu.liu.se/ike/forskning/ medmikro/stendahl?l=en&sc=true

Olle Stendahl, MD, PhD

Project Coordinator

Olle Stendahl, MD, PhD, is professor of medical microbiology at Linköping University, where he leads a translational research team focusing on host response against tuberculosis in Sweden and Ethiopia. OS received his MD and PhD from Linköping University Medical School in 1973, and has been a visiting scientist at Harvard Medical School, University of Geneva, and University of California at San Francisco (UCSF).

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