Rhodococcus equi: Saskatchewan Researchers Accept the Challenge

Ask Marianela Lopez, DVM, PhD, how long she has been studying Rhodococcus bacteria and she says, laughing, "Since I was born? Well, truthfully, since I began my PhD at Washington State University in 1996--it does seem a lifetime ago."

Lopez is a member of a new team studying Rhodococcus equi infection of foals, supported by an award from the Heather Ryan and L. David Dubé Veterinary Health Research Fund at the Western College of Veterinary Medicine (WCVM). Led by Hugh Townsend, BSc, DVM, MSc, the team is plumbing the mysteries of the newborn foal's immune response to develop ways to protect neonates from infection.

A problem in eastern Canada, the United States, and internationally, R. equi is a soil bacterium that causes severe pneumonia in foals. "Currently, the key to managing this disease is early diagnosis, but this is time consuming and expensive," said Townsend, a professor in WCVM's Department of Large Animal Clinical Sciences.

"The problem is that foals don't tend to show signs of the disease until it has advanced to the point where it's difficult, or sometimes impossible, to treat," Lopez added. "Catching foals at an early stage of the disease requires repeated examination along with ultrasound, radiographs and laboratory tests."

In all species of mammals, Lopez explained, the immune system of newborns has specific characteristics that lead to disease susceptibility. In the case of R. equi, young foals are vulnerable to disease but become immune once they're four to five months old. Adult horses don't become ill unless they have an immunological deficiency.

"We're interested in these characteristics because we want to see if we can change them in order to protect (foals) from disease," said Lopez. She added that the knowledge gained could potentially be applied to many diseases affecting the young of other species, including humans.

Team R. equi Revs Up

Lopez joined the University of Saskatchewan Vaccine and Infectious Disease Organization (VIDO) after meeting Townsend at a Rhodococcus conference in the United States. Before her arrival in 2002, research scientists at the University of Saskatchewan were not focusing on this particular problem in horses.

Projects were soon underway with grants and new collaborations in Rhodococcus research. In 2004, Lopez received a two-year postdoctoral fellowship from the Saskatchewan Health Research Foundation to study the bacteria. A collaborative partnership with Mary Hondalus, DVM, PhD, at the Harvard School of Public Health also allowed Townsend and Lopez to develop an R. equi model in foals, to undertake some limited work on developing a vaccine, and to begin forming a larger research team.

"We needed to form an effective team of investigators to gather as much information as possible during our studies," said Townsend. WCVM team members include Drs. Katharina Lohmann, Fernando Marqués, Steve Manning, Kristin Poirier, Navjot Kaur and, Baljit Singh along with VIDO collaborators Drs. George Mutwiri and Volker Gerdts.

The team's aim is to understand the immune response of the foal as well as the genetic variations that may affect immunity in individual foals. Ideally, Townsend said the project will lead to a vaccine that will protect foals early in their lives after a single vaccination. However, it's more likely that foals and perhaps their dams will need to be vaccinated more frequently.

The Type 2 Bias Factor

The crux of the problem is a characteristic of the newborns' immune systems that generate responses with what is called a "Type 2 bias." This bias, which is replaced by a more balanced Type 1/Type 2 immune response later in life, exists because the fetal immune response must avoid rejection of maternal cells--something that would harm the fetus.

"The Type 2 bias keeps the fetus from reacting adversely to its environment--in this case, the mother's womb--so the fetus can develop normally until it's born," explained Lopez.

But there's a catch: the fetus, and therefore the newborn foal's immune system, tends not to react very vigorously to foreign cells (including bacteria) that are capable of causing infections. So, the Type 2 bias predisposes newborns to a variety of infections, such as R. equi, that a more mature immune system could battle effectively. For this reason, part of the team's strategy will be to try and formulate vaccines that will stimulate an immune response in foals that's more like that of an adult horse's response.

Once R. equi gain access to the lung, the bacteria actually take up residence inside one of the cell types that make up the host's immune system. A key focus of the team's investigation, which is under Singh's direction, "is trying to understand why cells within the lung that are responsible for protecting against full-fledged infection are unable to do so," Townsend said. These cells, called macrophages, should "gobble up" the organism and digest it. But in the foal, these cells have difficulty doing this task. Once R. equi get inside these defending cells, the bacteria make the macrophages their hosts--not only surviving, but multiplying.

Harnessing the Maternal Immunity

Another challenge the researchers face in vaccine development is a phenomenon known as "maternal antibody interference." Mares that have been exposed to an infectious organism produce antibodies that are then transferred to the foal in their colostrum. The antibodies, meant to protect the baby early in life, can interfere with or block the vaccine's ability to generate an immune response in the foal.

Researchers have wondered whether they could capitalize on maternal immunity by vaccinating the mare to protect the foal. "But so far, results of various studies suggest that maternal immunity stimulated in this way doesn't fully protect foals against R. equi infection," said Townsend.

What the team hopes to achieve is to develop a vaccine strategy that will stimulate an effective immune response in the face of maternal antibodies, effectively combining the two strategies of vaccinating mare and foal.

Solving this problem is a challenge that applies to many vaccines that could be given to young animals. Currently, because maternal antibodies and other characteristics of newborns' immune system limit immune responses early in life, most vaccines aren't generally given to foals until they're six to nine months old. However, because R. equi only affects foals early in life, waiting to vaccinate isn't an effective strategy.

Townsend pointed out another complexity: "A vaccine that would only work in foals with no maternal antibodies ... wouldn't work in environments in which this disease is common. This is because mares in these environments are naturally exposed to the organism, likely resulting in maternal antibody transfer to their foals."

Now midway through the project, the team has refined the R. equi disease model and continues to carry out laboratory analyses. The researchers are beginning to uncover the ways in which the immune system of the newborn differs from the adult, particularly in the response of cells in the lung. They hope to adjust these factors using specific compounds capable of modifying the immune response that can be added to a vaccine. Team members are analyzing these formulations in the lab to determine which combination will generate the most protective immune response. Going forward, the team will apply this knowledge to the development of vaccine candidates.

Working with large animals such as horses means very expensive, logistically challenging experiments. In addition to the grant from the Ryan and Dubé fund, the project involves work funded by the federal Advancing Canadian Agriculture and Agri-Food Program and a pharmaceutical industry partner. Townsend continues to seek additional funding: "Although our studies are likely to inform us on many levels, this is going to be a long road."

He guesses that the R. equi problem likely has multi-faceted causes that could be environmental, genetic--or both. However, Lopez and Townsend feel the collaborative nature of the project is what gives it its strength. "People coming from different disciplines see the problem differently, and that gives us a broader understanding," he said. The team will also provide a foundation for future collaborations.

A non-linear approach is at play on another level as well. Lopez recalls how former VIDO director Lorne Babiuk, PhD, DSc, used to say that researchers investigating the workings of the immune system should try to think about the "big picture" effect of anything they learned.

"You're always trying to visualize the immune system in action versus seeing things on paper, you're always trying to make a movie in your mind of how things work. Because there are several components to the cause of this disease, it's critical to have a multi-disciplinary team of people playing that same movie from all different angles," explained Lopez.

"Once we understand how all of those little 'scenes' or pieces work together, the big picture will make sense."--Tess Laidlaw

A Saskatoon freelance writer and communication instructor, Tess Laidlaw worked for six years in science communication and is pursuing a degree exploring the ways science is presented through the media.

Reprinted with permission of Horse Health Lines, publication for the Western College of Veterinary Medicine's Equine Health Research Fund. Visit ehrf.usask.ca for more information.

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