Probing the heart for infection

Identifying acute endocarditis, a rapidly progressing infection of the heart valves, currently requires a myriad of tests and a fair amount of detective work. As such, the mortality rate is shockingly high, not because of a lack of treatment but rather because of the amount of time it takes to localize the infection and to identify the offending organism. A new imaging probe, however, developed by researchers at the Center for Systems Biology may soon enable this potentially fatal infection to be easily diagnosed. By identifying and targeting a specific product released by Staphylococcus aureus, the most deadly bacterial cause of acute endocarditis, the researchers could simultaneously confirm the presence of this bacteria as well as the site of colonization in a mouse model of the disease. With such a tool in hand, it may soon be possible to accurately diagnose patients and to treat them promptly. The report appears in the September issue of Nature Medicine.

Staphylococcus aureus, or S. aureus, is a bacterial species frequently found on the skin and in the nose. Whilst most bacteria that enter the bloodstream are rapidly killed by the immune system, some may still survive. If there is also some pre-existing damage to the heart valve or to any other part of the heart lining, these circulating bacteria can stick to and colonize these sites. In time, ‘vegetations’ consisting of bacteria and various blood clotting components begin to form. “Within these vegetations, S. aureus secretes something called staphylocoagulase,” explains Matthias Nahrendorf, MD, PhD, who is joint first author on the Nature Medicine paper. “This factor essentially hi-jacks the blood coagulation system to shield itself from destruction by the immune system.”

In activating the blood clotting mechanism, this enzyme—staphylocoagulase—results in the activation of prothrombin, a protein which leads to the deposition of fibrin, a key component of blood clots. As the bacteria multiply, the amount of staphylocoagulase released increases, which causes more blood clotting and in turn, growth of the bacterial vegetation. Eventually, these vegetations prevent the affected valves from functioning properly and fragments of the vegetations may also break off and travel in the bloodstream to other organs, such as the kidneys, lungs and brain. If left untreated, the infection can be rapidly fatal.

“Especially in acute endocarditis, the mortality can be very high, up to 47%, and it can happen very quickly,” says Nahrendorf, who is also an Assistant Professor of Radiology at the Center for Systems Biology. “It’s important to start the patient on the right antibiotics as soon as possible, but to do this it is also important to find out what bacterium is causing the endocarditis.”

Taking the detective work out of diagnosis

“Diagnosis of acute endocarditis is difficult because you can’t see the infection,” says Nahrendorf. Symptoms are also typically vague, indicative of a general infection rather than specific for endocarditis.

Acute endocarditis is typically first suspected after a physician sees a mysterious growth on a patient’s echocardiogram—essentially an ultrasound of the heart. “But this doesn’t tell you whether it is caused by S. aureus or if it is even endocarditis,” says Nahrendorf.

To support a diagnosis of endocarditis, a patient is then usually checked for a fever and for the presence of a heart murmur. Blood cultures are also done to identify the infective agent, but these are not always successful. “The problem is that once you have done all of this, days have passed, and the patient may already have reached a critical condition,” says Nahrendorf.

In a bid to develop a faster and more accurate method for diagnosing acute endocarditis, the research group investigated the possibility of creating an imaging probe that could enable visualization of the infection in situ. “We basically looked a little closer at the factor [staphylocoagulase] that is secreted by S. aureus,” says Nahrendorf. “We studied where this factor is expressed and where it is made and we developed a molecular imaging agent that could target it.”

Given its pivotal role in staphylocoagulase’s activation of the clotting cascade, a prothrombin analog was developed as the imaging probe and its ability to detect S. aureus vegetations was subsequently examined in a mouse model of acute endocarditis. “We induced endocarditis in mice by basically mimicking what sometimes happens to patients undergoing intravenous catheterization in the ICU,” says Nahrendorf. Namely, a thread was passed down the right carotid artery of mice, to cause a small amount of damage to the aortic heart valve. The mice were then injected with S. aureus twenty-four hours later, following which over 85% developed endocarditis.

Initial experiments using a fluorescent version of the probe under an optical imaging microscope, revealed that the agent primarily targets the peripheral edge of vegetations. The researchers subsequently discovered that only the bacteria on the outside of the vegetation produce staphylocoagulase, particularly in larger lesions; this finding is consistent with the enzyme’s role in vegetation growth. A series of molecular studies were then performed to determine exactly how and where the probe binds staphylocoagulase, and how the enzyme actually anchors itself to the vegetation.

By understanding precisely how staphylocoagulase works, the investigators were then able to develop a radiolabeled version of the probe, which could identify S. aureus vegetations with PET, an imaging modality routinely used in the clinic. Thus, with optimization and FDA-approval, this probe could hopefully be adapted for use in patients.

“Ultimately, we developed a tool that could be used to look at, and specifically diagnose, S. aureus endocarditis,” says Nahrendorf. “This was not possible before.”

Furthermore, because PET imaging provides quantifiable data, this “tool” could be used to assess the severity of disease, compare patients in clinical trials and also evaluate the effectiveness of treatment.

“Right now, what we really want to do is optimize this probe so that it is streamlined for clinical use,” says Nahrendorf. “To get it into patients, that is what we are hoping for.”

Written by Yvonna Fisher-Jeffes, PhD

Panizzi P, Nahrendorf M, Jose-Luiz Figueiredo JL, Panizzi J, Marinelli B, Iwamoto Y, Keliher E, Maddur AA, Waterman P, Kroh HK, Leuschner F, Aikawa E, Swirski FK, Pittet MJ, Hackeng TM, Fuentes-Prior P, Schneewind O, Bock PE, Weissleder R
In vivo detection of Staphylococcus aureus endocarditis by targeting pathogen-specific prothrombin activation
Nature Med. 2011;17(9):1142-6 – PMID: 21857652 – PMCID: PMC3169740

Press Coverage

MGH press Release (pdf)
Nature: New imaging probe may ID heart valve infection – Cardiovascular Business (pdf)
Labeled Prothrombin to Detect Bacterial Endocarditis – MedGadget (pdf)
Detecting heart-valve infection – Harvard Gazette
Imaging the Infected Heart -Science Translational Medicine Focus (pdf)
Imaging Heart Infection – Science Editor’s Choice