Infection International

IT'S THE PHAGOSOME

Russell accepted a position at New York University and started collaborating with local researchers. He soon realized that Leishmania's success had little to do with which pathway the pathogen exploited to get into the macrophage. "The real key to whether the parasite survived was how it dealt with the environment inside the host cell," he says. The insight won him his first National Institutes of Health grant on Leishmania-macrophage interactions, and laid the foundation for the work he does to this day.

With grant in hand, Russell pulled up stakes and moved to Washington University in St. Louis, where he intended to develop a detailed biochemical understanding of the intracellular phase of Leishmania life cycle. He began by isolating phagosomes. "Of course, once we'd started those experiments it soon became clear that we didn't have a clue about what the hell we were doing," says Russell. No one had ever published an account of the molecular composition of a phagosome, he and his team didn't know what they should be looking for.

That's where M. tuberculosis came in. Russell decided that the best way to learn something meaningful about phagosome biology was to do a "compare and contrast" type experiment: He would examine vacuoles filled with Leishmania versus those stuffed with mycobacteria and look for differences that were pathogen-specific. In the process, he noticed that vacuoles containing live tuberculosis bacteria don't acidify. Under normal circumstances, when a macrophage ingests an invading bacterium, the phagosome containing the bugs acidifies, and eventually fuses with a lysosome – the intracellular sac of digestive enzymes that spells doom for most infections. Mycobacteria somehow disrupt this process, keeping phagosomes a fairly pleasant place to live.

The discovery made the cover of Science and made Russell's career. But not before it shaved a few years off his life. To start with, the manuscript was initially rejected. "Two reviewers just trashed it," says Russell. "I didn't even consider an appeal." Instead, Russell sent the paper to someone connected with the Proceedings of the National Academies of Science. Then Science called and asked for another look, and within three weeks "the paper went from being a "rejected piece of crap to being a report in Science with a front cover and a Perspective," he recalls.

That's when one of his grad students announced that she couldn't reproduce a key part of the experiment. "I said, 'It's no problem at all. If it's wrong, we'll just retract the paper,"' says Russell. "But inside I'm absolutely dying. Here's my big break and the rug is being pulled out from under me." Fortunately, the researchers were able to sort out the problem before the paper came out.

With that publication, Russell shifted his attention entirely to the study of mycobacterium, and returned the unused portion of his Leishmania grant. "He actually gave the monies back to NIH," says Alexander. "That's unheard of. But it shows David is completely honest."

"I decided I had to make a clean break and move on," explains Russell. "I reached a point where I really wanted to write a grant for what I wanted to do rather than subvert funding from a different source."

THE JOYS OF TINKERING

While Russell admits that he likes grant writing, he concedes that the revisions can become a little tedious. "But I find it forces you to really think rigorously and creatively about the project." It's that rigor and creativity – and a willingness to develop techniques to answer a question – that make Russell such a strong scientist, says Beth Rhoades of the Lovelace Respiratory Research Institute in Albuquerque, N. Mex., a former postdoc and current collaborator. "In science it's so easy to say, 'here are the things I know how to do' and then go find the questions you can answer with those techniques," says Rhoades. "David has the opposite approach. He likes to tinker."

That technical flexibility allows Russell to tackle problems creatively, think unconventionally, and discover things that are novel, says Ulrich Schaible of the Max Planck Institute in Berlin, another former postdoc. Take, for example, the fluorescent markers Russell and his colleagues have designed to track in real time the maturation of intracellular compartments in infected macrophages. They took substrates traditionally used to measure the activity of enzymes (e.g., proteinases and lipases) and coupled them to carrier particles and fluorescent dyes such that they light up only when they're in the presence of functional enzymes. "They're really beautiful," says Eric Rubin, of the Harvard School of Public Health. And they allow Russell to assess whether bugs face a hostile environment inside the phagosome, and to monitor how those conditions change over time. "That's something which is really new," notes Schaible.

"I love playing games with techniques," Russell explains. "And I have a grad student who's of a similar mindset, so both of us go into flights of fancy about what things we can design to do dynamic readouts."

One of Russell's most elegant experiments was one he performed to isolate mutants that don't fare well after being engulfed and quickly find themselves relegated to the lysosomal pits. He started by loading lysosomes with iron. "Macrophages are stupid," says Rubin. "If you feed them iron filings, they eat them." And the particles wind up sitting, undigested, in lysosomes. Russell then offered the metallic macrophages a meal of mutagenized mycobacteria, and used a magnet to pull out the lysosomes, which contained bugs that had lost the ability to manipulate the system. "I love that experiment," says Rubin. "It's just a very neat, very clever experiment."

Through experiments like these, Russell determined that mycobacteria that lack a metabolic enzyme called isocitrate lyase have difficulty surviving inside the activated macrophages that are often present during a chronic infection. Because humans don't possess this enzyme, it provides a good target for the development of new antibiotics.

AFRICA AND ADMINISTRATION

At the same time, Russell is collaborating with Henry Mwandumba at the University of Liverpool to study alveolar macrophages collected from the lungs of individuals infected with M. tuberculosis in Malawi, an African nation rife with TB and HIV. "You always worry that what you find in animal models might not translate into what is going on in human disease," says Mwandumba. Together the researchers confirmed that the natural strains of mycobacteria that infect people in the real world wind up in vacuoles that don't acidify.

Russell is also exploring what drives mycobacterium during the late stage of disease to trigger the massive tissue damage that allows their transmission via the coughing of infected sputum, which is a critical part of the bug's life cycle. An infected human will "pop his clogs" – in other words, die – before transmitting the infection to another individual, he says. Preliminary work suggests that the secret might lie in the way the bacteria present a particular lipid on their surface. "That's our big hand-waving model," adds Russell. "It could cave in on us, but that's OK. I'm enthusiastic about it."

He's also enthusiastic about being chair of his department at Cornell. "I wanted to foster a strong community; one I'd enjoy working in," he says. And he wanted to improve the graduate education program to guarantee that students would have access to resources and faculty would have access to students. Since arriving in 2000, Russell has recruited a half-dozen new faculty members and presided over a tripling of NIH funding in his department. "He's a dream," says Donald Smith, dean of Cornell's College of Veterinary Medicine. "The college is different because he's here." Perhaps Russell, like his mycobacteria, has found his niche.

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