Impossible Vaccine Tames Staphylococcus aureus

Image: Courtesy of Ali Fattom THE END IS NEAR: S. aureus attached to tissue If Scottish surgeon Alexander Ogsten ever daydreamed that discovering Staphylococcus aureus would win acclamation, it was before he crossed paths with the British Medical Journal and came away the worse for it, squashed like a cockroach caught scurrying across a tray of tea and crumpets. Upbraiding the upstart for daring to step beyond his place, the editor dismissed Ogsten's 1881 paper on the bacterium, jotting in swift strokes of ink that "little of any worth comes from Scotland." Let us pause a moment to savor this naked insult, those of us who admit to guilty pleasure in watching scientists fight, for here the curtains opened upon a field in which interesting acrimony has often flourished right alongside interesting science. Credit this state of delight to the molecular properties of S. aureus, being as it is somewhat different from those of its brethren, and so a continual source of disagreement and rancor. S. aureus does not welcome the faint of heart or thin-skinned. With this as background, it is understandable that some observers of the field may have found a double meaning in recent news from the New England Journal of Medicine1 that a polysaccharide-based vaccine protects patients undergoing hemodialysis from S. aureus infection. It was, of course, a fine achievement for medicine. But in some corners there may have been a tinge of the bittersweet in knowing that a dispute over whether a staph vaccine was even possible, a dispute that more than once erupted in bouts of sputtering rage between opposing researchers, was at last being laid to rest. Who would have guessed when it began that the controversy would last the better part of 30 years? The fight was over polysaccharides: Did S. aureus have them? Could they be used for a vaccine? Year after year, the weight of the data said no. That the truth turned out otherwise--that it was found at all--"comes down to the persistence of a small group of people," says Ali Fattom, who directed research on the vaccine at the Rockville, Md., laboratories of Nabi Biopharmaceuticals. He ought to know, as a battle-scarred defender of a vaccine few thought would ever work. A COAT OF A DIFFERENT COLOR Staphylococcus aureus is a Gram-positive bacterium found in several types of farm animals, but the privilege of being its chief reservoir belongs to humans; one in four people offers it home and shelter on skin and nasal passages. (Streaked on agar plates, its colonies are intensely yellow, hence the Latin aureus.) Mostly its presence is of no concern. But in some--neonates, immunosuppressed patients, patients undergoing surgery--it can cause serious infections including pneumonia, osteomyelitis, endocarditis, toxic shock syndrome, and sepsis, and it can be lethal. Often these infections occur in hospitals. Nearly a quarter of nosocomial, or hospital-acquired, infections are due to S. aureus. And increasingly, they are antibiotic-resistant. Half of hospital- isolated S. aureus strains resist methicillin, the preferred treatment, and since 1997 physicians have watched with alarm as strains emerged resistant to vancomycin, the only antibiotic that works when methicillin does not. In the 1950s and 1960s it became apparent that for some pathogens, polysaccharide-based vaccines offered alternatives to antibiotics. Polysaccharides are the major components of the outer layer, or capsule, of bacteria. Polysaccharides are poor antigens, and for this reason capsules increase bacterial resistance to antibody-mediated ingestion (opsonization) and destruction (phagocytosis) by macrophages and other immune cell scavengers. Nevertheless, even poor antigens can be turned to medical purpose; vaccinologists found that killed whole bacteria and large doses of purified polysaccharides sometimes produced antipolysaccharide antibodies capable of protecting against infection. Capsules are associated with Gram-positive bacteria, so it seemed reasonable that the capsular polysaccharides of S. aureus might serve as the basis for a vaccine. But the idea was not long-lived, as intensive efforts to find its polysaccharides failed. By the mid-1960s researchers drew the logical conclusion--S. aureus did not have polysaccharides--and vaccine efforts halted. "YOU'RE WASTING YOUR TIME" Here matters might have rested, with never a harsh word spoken, had not Walter Karakawa, heedless of all better judgment, resumed the polysaccharide hunt in 1974. Karakawa noticed that the earlier investigations used laboratory strains of S. aureus, not organisms cultured from infectious isolates. S. aureus polysaccharides were undetectable by the standard methods (microscopy with India ink staining; morphological studies of bacterial colonies with and without polysaccharides), so Karakawa pinned his hopes on a serological approach. Beginning at Pennsylvania State University, Karakawa collected antisera from animals injected with S. aureus obtained from clinical isolates. By painstakingly tabulating how many isolates each antisera preparation recognized, over the course of 15 years he discovered eight capsular types, of which types 5 and 8 accounted for 85% of infections. There was punishment, though, for researching a settled question: Karakawa was denied grant funding for his work. What kept him going was a patron: The eminent vaccinologist John B. Robbins of the National Institutes of Health took Karakawa under his wing. Robbins had worked on the first Pneumococcus vaccine, the first Meningococcus vaccine, and the first Hemophilus influenza B vaccine, and in the early 1980s was pioneering conjugate vaccines, which transform weak polysaccharide antigens into antigens of power. With Karakawa's help, Robbins intended to buck consensus opinion and add a conjugate Staphylococcus aureus vaccine to his string of successes. A conjugate vaccine links a capsular polysaccharide to a carrier protein in order to trick helper T cells into recognizing polysaccharides as antigens, not something they would otherwise do. When the antigen-presenting cells process the carrier protein for T-cell recognition, the attached polysaccharide necessarily gets the same treatment. While B cells do not require help from T cells to produce antipolysaccharide antibodies, stimulation by T helper cells activated by polysaccharide antigens makes all the difference--antibodies with dramatically higher affinity are produced, as well as surging titers in instances of reinfection. Antibodies against capsule types 5 and 8 were not crossprotective, so an effective vaccine required a component for each type, separately tested for safety and immunogenicity, then combined and tested for efficacy. The project began in earnest in 1986, when Fattom, arriving in Robbin's lab fresh from his doctoral work at Hebrew University on microbial life in the Dead Sea, was assigned the job of making the vaccine. Fattom gradually realized that with his responsibility and rising seniority came unsought status as a lightning rod for criticism leveled at the vaccine. Increasingly he would be the one to defend it in what he refers to as "hot discussions" at meetings. At that time, most researchers conceded that Karakawa had proven the polysaccharides existed; the chemical structures had been worked out, and various chemical properties explained the failure of the India ink and colony morphology studies. But the concession did not extend to the existence of capsules. Rather, S. aureus became known for its microcapsules, small patches of polysaccharide coating discernable only with an electron microscope. Whether microcapsule polysaccharides could be used for a vaccine was debatable. By 1990, types 5 and 8 vaccines were ready for safety testing in humans. Fattom had attached the polysaccharides to a nontoxic, recombinant carrier protein derived from Pseudomonas aeruginosa exoprotein A. Also around that time, he left NIH for Univax Biologics, of Rockville, Md. (since acquired by Nabi, of Boca Raton, Fla.), to work on vaccine manufacture and clinical testing. When Univax announced three years later that Phase I trials showed both vaccines were safe and elicited type-specific antibodies in humans, the academic community responded pretty much as always--unenthusiastically. Fattom was still the go-nowhere guy with the go-nowhere vaccine, although now for a different reason. "Ok, so you have antibodies," is the typical comment he remembers. "They won't protect against infection." That view was based on failure of animal experiments to prove antibodies against types 5 and 8 prevented S. aureus infections from being lethal. Evidence from humans seemed to indicate the same thing: In the course of the safety trials it was discovered that practically everyone has antibodies against types 5 and 8, simply because of normal environmental exposure, and clearly those antibodies do not protect against bacteremia. Although Fattom recalls widespread support at Univax, he was not working in a cloister. Choruses of "You're wasting your time" were reaching the ears of company president Thomas Stagnero, forcing him to decide whether to keep the project or kill it. "He could easily have said, 'Let's drop it,'" recalls Fattom. Had Stagnero been a scientist rather than a businessman, he might have done just that. Instead, he expressed confidence that Karakawa, Robbins, and Fattom were on the right track, and told them to press ahead. This brush with termination marked a turning point, for it convinced Fattom that he needed to demonstrate with an animal model that antibodies against polysaccharides protected against infection. This had not been done earlier because the NIH and Univax researchers did not develop an animal model. Robbins' goal had been to get the vaccine into the clinic quickly and safely, not to research the molecular basis of virulence. So animal model development had been deferred. With his own animal model Fattom could address what he saw as flaws in other researchers' experiments. As a postdoc, he had discovered that culture conditions made the difference between capsules and microcapsules, and many previous studies used poor culture conditions. Polysaccharide production peaks in late logarithmic and stationery growth phases--after the point at which most researchers harvest bacteria. And, commonly used broths contain too much phosphate, reducing polysaccharide production; decrease the phosphate level to that of blood, and polysaccharide production soars. Fattom faulted other experiments for testing low-potency, whole-cell vaccines, or for challenging animals with huge innoculums that overwhelmed the immune response. The naturally acquired antitype 5 and 8 antibodies were low-titer, low-affinity products of B cells unstimulated by T cells. Titers of so-called T cell-independent antibodies do not reach high titers upon reinfection, which is why surgery can result in S. aureus bacteremia--surgical wounds can allow in enough bacteria to overwhelm low-level antibody protection. In 1996 Fattom finally developed a mouse model in which a reasonable innoculum caused infection. With this model he was the first to demonstrate that conjugate vaccines protected against lethal injections of Staphylococcus aureus. Knowing he would need corroboration, he then invited Jean C. Lee of Harvard to test his vaccines in her endocarditis model with rats. A year later the results were just as he predicted--the vaccines protected Lee's rats.2 Now, at last, skeptics started to come around. Maybe the vaccine would work. THE TRIAL The randomized, double-blind clinical trial reported in the New England Journal of Medicine tested the efficacy of StaphVax, the combined types 5 and 8 conjugate vaccine, in patients on hemodialysis due to end-stage renal failure. It is a group "notorious for S. aureus bacteremia," says Fattom; using dialysis equipment several times a week greatly heightens the risk of bacteria entering through breaks in the skin. The clinical trial demonstrated that between three and 40 weeks, S. aureus bacteremia developed in 11 of 892 vaccinated patients and 26 of 906 control patients. The lower rate of infection in the vaccinated group was statistically significant, indicating an estimated vaccination efficacy of 57%. Even so, protection disappeared after 10 months due to unexpected declines in antibody titers by one-half in only six months. (In healthy people the drop would have been 10-15%.) In hindsight, the decline might have been foreseen, Fattom says. Between dialysis treatments uricemia develops, and high uric acid levels impair white blood cells. In addition, half the subjects had diabetes, which also causes white-cell impairment. And, they were old (median age, 59). Unless white blood cells stay in good shape, he says, "antibodies are not efficient in protecting you." Eventually, another major market for the vaccine will be patients scheduled for surgery, people whose white blood cells are usually healthy. If StaphVax administered two weeks before surgery prevents S. aureus bacteremia (a 1-4% risk), the vaccine will likely be offered to millions of people each year. Arguably Nabi could control a billion-dollar market, with no competitor; belief that the vaccine was impossible was so strong that no other company tried to make one. The vaccine remains a few years away from the market; the Food and Drug Administration has requested that Nabi repeat the trial before asking for market approval. Although something could always go wrong next time, the weight of opinion now is that the vaccine is going to make it. Fattom has been touched by the many congratulations he has received from colleagues and their comments on how much the vaccine is needed. He feels vindicated. But he is also the survivor of an ordeal: "I am today a different man than the one who started this whole thing," he says, "getting into fights." He knows the battles were about data, not personalities. It was because he believed in his data that he kept pushing. "If you believe in something," he says, "and you have the scientific basis to move things forward, there's always a chance for you to do it." Ogsten would undoubtedly agree. After all, one way or another, doesn't almost everything of any worth come from Scotland? Tom Hollon (thollon@starpower.net) is a freelance writer in Rockville, Md. References 1. H. Shinefield et al., "Use of a Staphylococcus aureus conjugate vaccine in patients receiving hemodialysis," New England Journal of Medicine, 346:491-6, Feb 14, 2002. 2. J.C. Lee et al., "Protective efficacy of antibodies to the Staphylococcus aureus type 5 capsular polysaccharide in a modified model of endocarditis in rats," Infection and Immunity, 65:4146-51, 1997.

Impossible Vaccine Tames Staphylococcus aureus

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Image: Courtesy of Ali Fattom
THE END IS NEAR: S. aureus attached to tissue