May 31, 2016

May's featured paper is "Endosomal MR1 Trafficking Plays a Key Role in Presentation of Mycobacterium tuberculosis Ligands to MAIT Cells," published in PLoS Pathogens. The paper is published by a team led by Drs. Melanie Harriff and David Lewinsohn.

Tuberculosis (TB) is among the world's oldest and most active infectious diseases, known since antiquity.

About one-third of the world's population is infected with Mycobacterium tuberculosis (Mtb), the bacterium that causes TB. Of those people infected, most have a dormant form of the disease known as latent TB and don't develop symptoms.

Nonetheless, a small proportion develops active TB and becomes sick. Scientists have yet to understand why one person gets sick and another person doesn't, but have found that young children or people with weakened immune systems are at higher risk of developing active TB. 

TB remains a global health concern, with an estimated 9 million new cases and 1.5 million deaths each year. TB can usually be treated with a combination of medications administered over the course of six to nine months. If not treated properly, it can be fatal. 

Nonetheless, drug-resistant strains of the disease are now widespread. All told, TB remains the leading single cause of death due to infection in the world. 

Defense against TB

Hope for eradicating TB lies in understanding exactly how Mtb interacts with a host person's body at a basic intracellular level, knowledge that can lead to improved or novel approaches to prevention and treatment. 

Enter Melanie Harriff, Ph.D., research microbiologist at the VA Portland Health Care System, and research assistant professor of medicine in the Division of Pulmonary/Critical Care Medicine in the OHSU School of Medicine, and David Lewinsohn, M.D., Ph.D., staff physician in the VA Portland Health Care System and professor of medicine in the Division of Pulmonary/Critical Care Medicine in the OHSU School of Medicine. Their research team is contributing to this worldwide effort by looking at a particular subset of white blood T-cells called MAIT cells. The team's focus is the role MAIT cells play in detecting the presence of Mtb in the body and how it participates in orchestrating a broader immune response. 

This month's featured paper is a recent study by the team, published in the journal PLoS Pathogens, which elucidates the finely-tuned cellular processes and mechanisms that allow MAIT cells to detect the presence of Mtb infection.  

"As someone who in grade school was required to have a chest x-ray at the start of every school year because of TB exposure, I've always been fascinated by our faculty's work on TB," said Mary Heinricher, Ph.D., associate dean for basic research in the OHSU School of Medicine. "This paper looks at the other side –defenses against TB, and a very specific and tightly-regulated way that the immune system responds to the TB bacterium." 

Mapping MAIT cells

TB is spread through airborne droplets caused by the coughing of an infected person. The bacterium enters a person's respiratory tract and travels into the lungs. 

The body's first line of defense is a type of resident immune cell called a macrophage, which serves as a cellular sentinel. Scientists have known about macrophages for some time. These cells recognize bacterial infection and inhibit growth of Mtb while recruiting other cells to mount an immune response. 

But recently, other cellular sentinels have been found. "We and others have identified a subset of bacteria-reactive T-cells called MAIT cells, mucosal-associated invariant T-cells, which are enriched in mucosal sites such as the lung and gut," said Dr. Harriff. "Because these cells are present in the mucosa, they are poised to serve as sentinels of bacterial infection."

"We know that MAIT cells recognize and respond to unique molecules called vitamin-B metabolite ligands –ligands are molecules that bind to other molecules -- produced by many types of bacteria and fungi, including Mtb," said Dr. Harriff. 

Some Mtb ligands can be antigens. Antigens are foreign or toxic substances that trigger the body's immune response. Mtb antigens are presented on a special molecule called MRI, an MHC-I molecule. Researchers hypothesize that this presentation is a tightly-choreographed dance. To be successful, the process must allow for recognition of intracellular infection but avoid inappropriate T-cell activation that might lead to autoimmunity.  

So how exactly does MRI recognize the Mtb ligand and initiate that careful, tightly-regulated response? The mechanisms for presenting these unique ligands were unknown. In their study, Dr. Harriff and the team set out to identify the processing and presentation pathways for the Mtb MR1 ligands.

Roles in trafficking

The team's previous studies demonstrated that human airway epithelial cells are very efficient at presenting ligands from Mtb to MAIT cells. 

"Furthermore, presentation of these small molecule ligands on MR1 was not dependent on processing pathways that have been described for other MHC-I molecules that present peptide ligands," said Dr. Harriff. "Work by others in the field suggested that MR1 trafficking in late endosomes was important to MAIT cell activation."

Using lentiviral shRNA gene silencing, the researchers identified trafficking molecules that are specifically involved in regulating MAIT cell activation in response to Mtb-infected airway cells. Among these, the team showed that Syntaxin 18, Rab6 and VAMP4 have distinct roles in trafficking of MR1 in the context of Mtb infection or addition of exogenous ligand. 

"Together, our data suggest that endosome-mediated trafficking of MR1 is key to selective sampling of the intracellular environment, and demonstrate that there is a distinct pathway for loading of exogenous ligands," said Dr. Harriff. "We demonstrated that, in contrast to other Class I molecules, MR1 localizes to endosomal compartments and translocates to the cell surface upon addition of ligand."

Next steps

The work has led to projects that address a number of questions, say the researchers. 

"First, while MAIT cells are known to recognize vitamin B metabolites from bacteria such as E. coli, our work has shown that MAIT cells can also selectively recognize small molecules derived from Mtb," said Dr. Harriff. "In work funded by the Bill and Melinda Gates Foundation, we are working to identify these Mtb-specific ligands. This, in turn, would facilitate the development of vaccination strategies that would target MAIT cells. We are continuing to more precisely characterize the endosomal recycling pathway required for presentation of Mtb ligands."

She added, "In parallel, we are also using a similar screening strategy to identify trafficking molecules that are critical to MAIT cell recognition of extracellular pathogens. These findings will enable us to better define strategies to develop MAIT cell vaccines, or conversely to block MAIT cells if they are associated with autoimmunity. Our research may also contribute to an increased understanding of the mechanisms underlying inappropriate MAIT cell activation that can lead to autoimmune disorders."

"Finally, we have found that smoking alters the prevalence and bacterial-dependent responses of MAIT cells," said Dr. Harriff. "As a result, we are determining how smoking alters MR1 trafficking and MAIT cell recognition of bacterial lung pathogens."

Thanks to the team's work in better understanding these intracellular interactions in Tb infection, stopping this ancient infectious disease is one step closer. 

Resources

• Read the paper
  
• Read about the Oregon Tuberculosis Lab
  

Citation

Harriff MJ, Karamooz E, Burr A, Grant WF, Canfield ET, Sorensen ML, et al. (2016) Endosomal MR1 Trafficking Plays a Key Role in Presentation of Mycobacterium tuberculosis Ligands to MAIT Cells. PLoS Pathog 12(3): e1005524. doi:10.1371/journal.ppat.1005524

More Published Papers 

• Previous OHSU School of Medicine Papers of the Month
• Recent OHSU published papers 

Pictured above, from left to right: Michelle Sorensen, Dr. Melanie Harriff, Dr. David Lewisohn and Dr. Elly Karamooz.