Turning on our immune sentries

Melbourne, Monash, UQ and the synchrotron find what sends our MAITs into action to protect our gut from invaders.

Our guts, lungs and mouths are lined with mysterious immune cells that make up to ten per cent of the T cells in our immune system. Last year Australian researchers showed that these cells act as sentinels against invading bacteria and fungi. Now they’ve identified the precise biochemical key that wakes up these sentries and sends them into action.

The patented work, published in Nature today, provides the starting point to understanding our first line of defence, and what happens when it goes wrong.  It will lead to new ways of diagnosing and treating inflammatory bowel disease, peptic ulcers and even TB. It could also lead to novel protective vaccines.

The discovery is the result of national and international collaboration between the universities of Melbourne, Monash, Queensland and Cork. It also depended on access to major facilities including the Australian Synchrotron and the Bio21 Institute.

Last year members of the research team won an Australian Museum Eureka Prize for determining that these immune cells, known as mucosal-associated invariant T cells (MAITs), detect reactive intermediates in the synthesis of vitamin B2 (riboflavin) that is made by many invasive bacteria and fungi. The latest discovery narrows the trigger down to a small group of compounds produced by specific bacteria and fungi, which may be associated with several diseases.

“We have unlocked a secret that will enable our team to investigate the role that MAIT cells play in health and disease, which is exciting,” says Dr Alexandra Corbett, a lead author on the study from the University of Melbourne. “However, there are major international laboratories with whom we have to compete.”

“This is an excellent example of how collaborative research in Australia can bring groups with expertise in different areas together to make significant advances,” says joint leader Prof Jim McCluskey, Deputy Vice-Chancellor (Research) of the University of Melbourne.

“To get from the first observation to today’s discovery required not just smart people but access to Melbourne’s Bio21 Institute platforms, dozens of visits to the Australian Synchrotron, and a global research network including our Irish colleagues who provided access to mutant bacterial strains,” says Prof Jamie Rossjohn, one of the senior authors and NHMRC Australia Fellow of Monash University. “All that coming together allowed us to beat our international competitors and secure the patent,” he says.

The finding that these human immune cells have evolved to detect bacterial synthesis of riboflavin, but not the actual vitamin in our diet, may be a valuable clue to disease pathology and new drug development strategies,” adds senior author, Prof David Fairlie of the Institute for Molecular Bioscience at the University of Queensland.

The work is an early win for the recently announced ARC Centre of Excellence in Advanced Molecular Imaging. “We want to unravel the complex molecular interactions that define how we fight disease,” says Prof Rossjohn. “This remarkable research collaboration shows us how to do it.”

“MAIT cells are a discovery so recent that they have not even made it into the textbooks,” McCluskey says. “Most doctors know nothing about them. Yet they constitute about one cell in ten of the body’s T cells and half of all the T cells in the liver.”

Very little was known about their role, beyond the fact that they had an association with bacteria. Then, the Australian researchers revealed they had discovered that MAIT cells interact with certain precursor molecules which are the building blocks of vitamin B. In particular, when MAIT cells are exposed to the precursors of vitamin B2, they initiate immune system action against foreign invaders.

The significance of this finding is in the fact that humans and other mammals use, but do not make, riboflavin; only bacteria and fungi do. This means that only bacteria and fungi are associated with riboflavin precursors of the type which send our MAITs into action. That makes our MAITs a useful guard against infection in our gut, mouth and lungs.

Now the team has taken things a significant step further. In the work published in Nature today, the researchers have narrowed down the biochemical trigger for MAIT cells to a small group of compounds which form when the riboflavin precursor molecules interact with specific bacterial metabolites. This reaction is only possible in certain, but by no means all, bacteria and fungi. And that means the diseases and microbes targeted by our MAITs can now be traced.

For interviews:

• Professor Jamie Rossjohn, Monash University, jamie.rossjohn@monash.edu; Work phone: +61 3 9902 9236
• Professor Jim (James) McCluskey, DVC Research, The University of Melbourne, jamesm1@unimelb.edu.au
• Professor David Fairlie, Institute for Molecular Bioscience, The University of Queensland, d.fairlie@imb.uq.edu.au, Work phone: +61 7 3346 2989

Media contacts:

• Niall Byrne, +61 417 131 977, niall@scienceinpublic.com.au (for the ARC Imaging Centre)
• Lucy Handford, lucy.handford@monash.edu (for Monash University)
• Bronwyn Adams, b.adams@imb.uq.edu.au; Work phone: +61 7 3346 2134; Mobile:  +61 418 575 247 (for UQ)
• Annie Rahilly, +61 432 758 734, anne.rahilly@unimelb.edu.au (for the University of Melbourne).

Background information

Summary

Mucosal associated invariant T cells (MAIT cells) are an abundant population of T cells, that are prominent in the liver and the lining of the gut. We identified a family of compounds to which MAIT cells bind. They are derived from the building blocks of bacterial vitamin B. This is a major breakthrough as it allows us to work out the function of MAIT cells and how they operate. Our findings have implications for immunity, and the dietary intake of vitamin-B containing foods. Our fundamental discovery has very real clinical potential. It should pave the way to unravelling the role of MAIT cells in health and immunopathology—particularly in chronic inflammatory diseases—and to providing diagnoses and treatments for human diseases.

Research overview

Our research has resulted in a fundamental advance in understanding immunity, by defining the particular compounds that activate mucosal associated invariant T-cells (MAIT). Found in the liver and at mucosal surfaces, these cells comprise 10% of all the T-cells in the blood. Our success at determining the compounds that trigger MAIT cells provides us with a major clue as to how they contribute to immunity – knowledge that will help us forge new clinical pathways to treating chronic inflammatory diseases like TB, peptic ulceration, periodontal disease, and inflammatory bowel disease.

MAIT cells are activated by antigen compounds bound to MR1 proteins. We showed that the structure of MR1 was suited to binding antigens originating from the building blocks of vitamins. These antigens are only found in bacteria and fungi. In contrast to the MHC and CD1 families of proteins, MR1 can present folic acid (vitamin B9) and riboflavin (vitamin B2) metabolites, the latter of which activate MAIT cells. We showed that the immune system is triggered by these vitamin building blocks, which represent a new class of antigen for T-cell surveillance. This has implications for way diet can influence MAIT activity, and for whether other microbial compounds can affect and change MAIT cell function. Our findings will be pivotal in understanding the role of MAIT cells in the body, and in promoting their possible use in clinical practice.

Implications

While MAIT cells comprise a remarkable 10% of all T-cells in the blood, they are unknown in clinical practice. Their role in immune protection and in immunopathology is not established, particularly in chronic inflammatory diseases like TB, peptic ulceration, periodontal disease and in inflammatory bowel disease. The identification of a family of compounds derived from bacterial vitamin metabolites to which MAIT binds is a major breakthrough, as it allows assessment of their role. Defining the nature of the compounds that bind to MAIT cells will pave the way to a detailed understanding of their role in immunity. Directing MAIT cell activity becomes possible through the development of vaccines and antagonists that can manipulate MAIT cell functions according to their role. Our present work has very real clinical potential in unravelling the role of MAIT cells in health and disease, and in determining potential diagnostic and therapeutic compounds to combat human disease.

Abstract

T-cell activation by transitory neo-antigens derived from distinct microbial pathways

For a full copy of the paper:

T cells discriminate between foreign and host molecules by recognizing distinct microbial molecules, predominantly peptides and lipids.  Riboflavin precursors found in many bacteria and yeast also selectively activate mucosal-associated invariant T (MAIT) cells, an abundant population of innate-like T cells in humans. However, the genesis of these small organic molecules and their mode of presentation to MAIT cells by the Major Histocompatibility Complex related protein, MRI, are not well understood.We show here that MAIT cell activation requires key genes encoding enzymes that form 5-amino-6-D-ribitylaminouracil (5-A-RU), an early intermediate in bacterial riboflavin synthesis. While 5-A-RU does not bind MR1 or activate MAIT cells directly, it does form potent MAIT-activating antigens via non-enzymatic reactions with small molecules, such as glyoxal and methylglyoxal, which are derived from other metabolic pathways.

The MAIT antigens formed by the reactions between 5-A-RU and glyoxal/methylglyoxal were simple adducts, 5-(2-oxoethylideneamino)-6-D-ribitylaminouracil (5-OE-RU) and 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU) respectively, which bound to MR1 as shown by crystal structures of MAIT TCR ternary complexes.  Although 5-OP-RU and 5-OE-RU are unstable intermediates, they became trapped by MR1 as reversible covalent Schiff base complexes. Mass spectra supported the capture by MR1 of 5-OP-RU and 5-OE-RU from bacterial cultures that activate MAIT cells, but not from non-activating bacteria, indicating that these MAIT Ags are present in a range of microbes.

Thus, MR1 is able to capture, stabilize and present chemically unstable pyrimidine intermediates, which otherwise convert to lumazines, as potent antigens to MAIT cells. These pyrimidine adducts are microbial signatures for MAIT cell immunosurveillance.

Senior authors

• Prof Jamie Rossjohn, Department of Biochemistry and Molecular Biology, Monash University & ARC Centre of Excellence in Advanced Molecular Imaging
• Dr David P. Fairlie, Division of Chemistry & Structural Biology, Institute for Molecular Bioscience, The University of Queensland & ARC Centre of Excellence in Advanced Molecular Imaging
• Prof James McCluskey, Department of Microbiology & Immunology, The University of Melbourne
• Dr Lars Kjer-Nielsen, Department of Microbiology & Immunology, The University of Melbourne

Lead author

• Dr Alexandra Corbett, Department of Microbiology & Immunology, The University of Melbourne

About the ARC Centre of Excellence in Advanced Molecular Imaging

The ARC Centre of Excellence in Advanced Molecular Imaging integrates physics, chemistry and biology to unravel the complex molecular interactions that define immunity.

The Centre will develop new imaging methods to visualise atomic, molecular and cellular details of how immune proteins interact and affect immune responses.

It will enable Australia to be an international leader in biological imaging, train the next generation of interdisciplinary scientists, and provide new insights into combating common diseases that afflict society.

About the Australian Synchrotron

The Australian Synchrotron is a source of highly intense light ranging from infrared to hard X-rays used for a wide variety of research purposes. The intense light it produces is filtered and adjusted to travel into experimental workstations, where the light reveals the innermost, sub-microscopic secrets of materials under investigation, from human tissue to plants to metals and more.

With the new knowledge that synchrotron science provides about the molecular structure of materials, researchers can invent ways to tackle diseases, make plants more productive and metals more resilient.

Officially opened in July 2007, the Australian Synchrotron is one of fewer than 40 similar facilities around the world.