Is autophagy in macrophages important to the pathogenesis of Buruli ulcer?

Buruli ulcer is a neglected tropical disease, meaning that it affects some of the most disadvantaged communities on earth and receives relatively little research attention. It is a serious skin infection that causes large, progressively growing, ulcers that can cause lifelong disfigurement and disability, even when treated successfully with antibiotics. While the ulcers are painless, an unfortunate result of this is that many patients do not seek treatment until the later stages when the ulceration is widespread. Advanced infections can cover 15% of the body's surface area and take over a year to heal. People can sometimes need to have major surgery or limbs amputated in very severe cases.

The disease is most commonly found in teenagers in West Africa but is also spreading in other areas, including Australia. It is caused by Mycobacterium ulcerans, a bacterium related to the one that causes tuberculosis. M. ulcerans has evolved a clever way of avoiding attack by the immune system. It produces a potent chemical toxin called mycolactone which stops immune cells from making many of the proteins they need to communicate with each other and activate the immune response.

Macrophages are an important part of the innate immune system that provides the earliest defence against infection. They are a type of immune cell which eat bacteria and send out signals to alert the body to invading pathogens. They patrol skin tissue and can engulf M. ulcerans but the bacteria are able to live and grow inside macrophages. Eventually the toxin kills these cells, weakening the body's natural defences. However, the macrophage response does still seem to offer some protection against Buruli ulcer, since some inherited genetic traits controlling macrophage function can alter the severity of the disease.

Autophagy (derived from the Greek words for "self" and "eating") is a process used by cells to recycle their components and it helps them cope with stressful conditions such as starvation. The autophagy system can also be used to remove bacteria and other invading pathogens from cells. We have evidence that this system is involved in the response of macrophages to M. ulcerans bacteria and mycolactone, and this involves one of the genetic traits that alter BU disease severity. We think people with the variant trait can control the infection better than those with the 'normal' version.

In this project we will investigate exactly how M. ulcerans bacteria interact with macrophages and induce autophagy and ask how one genetic trait protects against Buruli ulcer. This will help us develop new approaches to treating this devastating disease.

Buruli ulcer (BU) is a necrotizing skin infection caused by Mycobacterium ulcerans (Mu). Virulence depends on an exotoxin, mycolactone, which suppresses immune responses by inhibiting the Sec61 translocon, blocking production of most cytokines, chemokines and immune cell receptors. The bacteria are extracellular but can be found inside some macrophages within lesions and evidence is accumulating that macrophages may play an important role in controlling the infection. Macrophages have been shown to phagocytose Mu and also to produce IL-1 beta in response. We recently discovered that both mycolactone and Mu bacilli induce an increase in macroautophagy in macrophages. Interestingly, genetic studies have identified an SNP in a key autophagy gene associated with Crohn's disease, ATG16L1, that reduces BU severity. Macrophages expressing the SNP variant protein, ATG16L1-T300A are less able to clear intracellular bacteria by autophagy but, since ATG16L1 also regulates inflammasome activity, these cells show an enhanced IL-1 beta response during bacterial infection. We hypothesise that the interaction between Mu and macrophages is a deciding factor in the outcome of Mu infection and that the SNP in ATG16L1 could protect against ulceration in BU by enhancing IL-1 beta production. In our laboratory at the University of Surrey, in collaboration with scientists at Imperial College London, the Crick Institute and Harvard, we will define the autophagic response of macrophages to Mu and the impact of the ATG16L1-T300A polymorphism on survival of both macrophage and bacteria in vitro, disentangling the effects of the bacterium and mycolactone. We will investigate the effect of ATG16L1-T300A expression on global macrophage responses and cytokine production induced by Mu using RNAseq and cytokine array. Finally, we will determine the role of the polymorphism in Mu infection and establish the importance of IL-1 in an in vivo model.