BRITTANY, a 4-year-old girl living in New York*, woke up one morning in 1983 with a fever and a rash. Her parents had heard about an outbreak of meningitis in the neighbourhood and, fearing the worst, took her to hospital. In the car Brittany started to deteriorate rapidly, and to her parents鈥� horror she began to lose consciousness.
By the time Brittany reached hospital, she had developed severe sepsis, a mysterious syndrome that kills one in three of its victims. Sepsis is defined as the response of the body to invading pathogens or injury, involving abnormalities of body temperature, heart rate, breathing rate and other problems. If the response escalates and major organs such as the heart, kidneys and liver start seriously malfunctioning, this is known as severe sepsis.
As Brittany was rushed into the intensive care unit, her heart stopped beating. A team of doctors and nurses battled for 4 hours to revive her, but Brittany could not be brought back to life.
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One of the doctors struggling to save her was Brett Giroir, who has since become a colleague of mine. He was profoundly affected by that day鈥檚 events. Ask any doctor or nurse, and the chances are they will be able to recount a tale of a patient they cannot forget 鈥� often young, and caught unawares by a disease or injury that hijacked their lives with the speed and shock of a violent crime. Often the care-givers can remember every excruciating detail, down to the colour of the tiles on the emergency room floor or the smell of the anti-bacterial cream applied to a patient鈥檚 wounds.
My own never-to-be-forgotten encounter came 20 years ago, when I was training to become a surgeon at what was then called the New York Hospital (now the NewYork-Presbyterian Hospital), and it too involved a child with severe sepsis. I found the experience so frustrating and perplexing that it changed the course of my career. I chose to become a researcher in the field of severe sepsis, and investigate the questions that so baffled us. What is severe sepsis? Why does it develop? And how can it be treated? Fortunately during the past 20 years there has been a sea change in our understanding of the underlying cause of severe sepsis. There are now reasons for hoping that effective medicines may one day be developed.
Although in Brittany鈥檚 case severe sepsis developed as a complication of meningitis, in most patients the trigger is a more commonplace illness, such as pneumonia, appendicitis, cancer, heart disease or a skin infection. I have seen cases in children after an insect bite, and in healthy adults who received an initially trivial wound. Frustratingly, the treatments currently available are inadequate. Severe sepsis is the third most common cause of death for hospitalised patients in the developed world, surpassed only by heart disease and cancer.
In the west, most patients with severe sepsis spend weeks in an intensive care unit, which can cost more than $50,000 a day. Severe sepsis costs an estimated $17 billion annually in the US alone, but the condition is neither widely known nor championed by fund-raising groups in the same way as higher-profile diseases like cancer and AIDS. Doctors may not even use the term severe sepsis when talking to patients or their relatives, focusing instead on the original cause of the problem, such as pneumonia or kidney infection.
鈥淧owerful evidence was emerging that the toxins came from the patient鈥檚 own immune system鈥�
For many years, the prevailing theories about severe sepsis focused on bacteria and the poisonous molecules they produce. We thought these bacterial toxins were directly poisonous to the body鈥檚 cells and organs. But in the 1980s there came hints that this might not be the whole explanation. Lloyd Old and colleagues at Memorial Sloan-Kettering Cancer Center in New York isolated a new immune-cell signalling molecule (a cytokine, in other words) that caused mouse tumours to shrivel up and die. They named this chemical tumour necrosis factor (TNF).
Independently, a group led by Tony Cerami at Rockefeller University across the street from Old鈥檚 lab was investigating another cytokine that caused a syndrome of severe weight loss or 鈥渃achexia鈥� in patients with chronic illnesses such as terminal cancer. They named this chemical cachectin, but it soon emerged that it was, in fact, the same substance as Old鈥檚 compound, TNF. In an experiment using an animal model of severe sepsis, in which mice were injected with a lethal dose of bacterial toxins, Cerami鈥檚 team showed that pretreating mice with antibodies to TNF protected them against this type of poisoning.
At the time I was working as a research fellow in surgery at Cornell University Medical College at Steve Lowry鈥檚 lab, next to Rockefeller University. Cerami鈥檚 experiment on mice set me wondering whether TNF could be one of the mysterious toxins that contribute to severe sepsis. Collaborating with Cerami and his colleague Bruce Beutler, I discovered that infusing TNF into rats caused a syndrome of shock and tissue injury that was indistinguishable from septic shock, a complication that occurs in some patients with severe sepsis, in which their blood pressure and heart rate suddenly drop. In 1986 we reported in Science that TNF was sufficient to cause septic shock (vol 234, p 470).
Our next step was to investigate whether it might be possible to use this as the basis of a treatment for septic shock. To ensure we were producing true septic shock, we infused live bacteria, rather than isolated bacterial toxins, into the veins of baboons. In 1987 my team reported in Nature that TNF antibodies could prevent septic shock in baboons (vol 330, p 662). Now we knew that TNF was both necessary and sufficient to cause septic shock. The antibodies had to be administered before the bacteria and the onset of shock, however. Like closing the barn door after the horse has bolted, the antibodies could not reverse the damage once TNF had been released.
Friend or foe
Together these studies were powerful evidence that at least some of the toxins involved in severe sepsis were produced not by invading bacteria, but by the patient鈥檚 own immune system. A typical, uncomplicated injury or infection sets off a series of beneficial immune responses to kill invading pathogens and promote healing. A normal immune response is a carefully orchestrated battle fought with cytokines that travel into and out of the front lines to stimulate the activity of some immune cells, suppress the activity of others, and recruit new troops. The TNF research showed that in addition to their beneficial activities, cytokines were also potentially dangerous.
At this stage the biotech industry started carrying out human trials of TNF antibodies in patients with severe sepsis. Unfortunately the treatment failed to significantly improve survival. This made headlines in the science and business press, but for several reasons perhaps this should not have come as a great surprise. Firstly, most of the patients in the trials did not have septic shock, but other forms of severe sepsis without shock. Secondly, the patients who did have septic shock were often treated after their immune cells had stopped producing TNF. Our finding that the antibodies needed to be administered very early in the disease, before high levels of TNF developed, was not incorporated into the clinical trial design. It later turned out that most of the patients in the trials did not have significantly raised levels of TNF at the time of treatment.
Satisfyingly, researchers have managed to develop a use for TNF antibodies that has significantly improved the quality of life for millions of people. These are not patients with septic shock or severe sepsis, but people with rheumatoid arthritis or inflammatory bowel disease. Both are autoimmune diseases in which there is an overproduction of cytokines, although at lower levels than those necessary to cause shock.
Meanwhile, my group at the Institute for Medical Research in Manhasset, New York, was continuing to pursue other avenues that might lead to successful treatments for severe sepsis. In the early 1990s I began to wonder whether other cytokines might be involved in the development of severe sepsis without shock. To investigate this we began by exposing cultured immune cells to bacterial products, and looking at the molecules they released that could be cytokines. We were hoping to find one that was released later in the disease course than TNF, so there would be a wider 鈥渢herapeutic window鈥� in which antibodies could be used as a treatment.
After several years we homed in on a compound called 鈥渉igh mobility group B1鈥� (HMGB1), a protein already known to be present in the cell nucleus, where it seemed to be involved in maintaining the structure of chromosomes. We found that it also acted as a cytokine. It does not cause septic shock, but in animals it does cause a syndrome of progressive organ failure that kills in a way that looks very similar to severe sepsis. We also found it rises to extremely high levels in the bloodstream of people with severe sepsis.
In 1999 my colleague Haichao Wang and I reported in Science that antibodies to HMGB1 significantly reduced the number of mice that died after being exposed to normally lethal levels of bacterial toxin (vol 285, p 248). And last year Huan Yang and I completed the critical step of showing that HMGB1 antibodies protect mice with lethal peritonitis, an intra-abdominal infection that is a widely used model of severe sepsis (Proceedings of the National Academy of Sciences, vol 101, p 296).
Out of control
TNF and HMGB1 are unlikely to be the end of the story. I have little doubt that we will find more molecules that contribute to the development of septic shock and severe sepsis. Their relative importance probably varies from patient to patient, and even at different times in the illness of a given patient.
The big challenge now is to use this new knowledge of the importance of cytokines in severe sepsis to develop treatments for the condition. A biotech firm I co-founded called Critical Therapeutics, based in Lexington, Massachusetts, is carrying out safety tests of HMGB1 antibodies on animals, with a view to running human trials. These antibodies may have an additional use. In a striking parallel to the story of TNF, Ulf Andersson at the Karolinksa Institute in Stockholm, Sweden, has shown that they also suppress arthritis in animals.
A major question that I have been investigating with my colleagues is why the regulatory controls on the immune system sometimes fail, so causing severe sepsis. Some years ago we discovered a previously unrecognised controlling step. In 2000 we reported in Nature that the vagus nerve, a major nerve trunk that descends from the brain to control every beat of your heart and every breath you take, also controls the immune system from minute to minute (vol 405, p 458). Just as the vagus nerve controls the vital activities of the heart, liver and other organs by releasing the neurotransmitter acetylcholine, it also uses this chemical to block the immune system鈥檚 production of TNF, HMGB1 and other cytokines. Although the details are still emerging, we think that the cytokine-producing immune cells are sited in these major abdominal organs, and so can be affected by the vagus nerve鈥檚 release of acetylcholine.
鈥淎n intriguing idea is that with training, people could learn to turn down their immune system鈥�
In 2003 we found that the specific receptor on immune cells that responds to acetylcholine was the alpha 7 subunit of the nicotinic acetylcholine receptor family, or alpha 7 for short (Nature, vol 421, p 384). Alpha 7 was already known as a receptor on the surface of neurons, but this was the first time it had been shown to have a major role in controlling immune cells. We showed that in animals at least, chemicals that stimulate alpha 7 significantly suppressed the release of TNF and HMGB1. As might be expected, they also prevented the onset of acute shock, and reversed severe sepsis and experimentally induced arthritis. Compounds that stimulate alpha 7 are being developed as potential medicines by Critical Therapeutics.
Another intriguing idea arises from the fact that with suitable training, sometimes called biofeedback, people can learn to reduce their heart rate by increasing activity of the vagus nerve. Could patients with arthritis and other autoimmune diseases such as Crohn鈥檚 disease learn to alleviate their symptoms by increasing their vagus nerve activity? We are investigating this possibility, although our research is still at a very early stage.
In clinical trials, the main criteria currently used to classify patients with severe sepsis and septic shock are their clinical signs and symptoms, such as their blood pressure, heart rate and breathing rate. I believe it is time to rethink this approach, focusing also on the underlying causative role of cytokines. It is crucial that we learn more about the different cytokines involved in sepsis so that in future we can be guided by what I call 鈥渢he cytokine theory of disease鈥�. The treatment of individual patients would be primarily guided by the activity levels of specific cytokines, their molecular receptors, and the presence or absence of factors that modify the cellular responses to these cytokines.
The path to our present understanding of severe sepsis has been a meandering one: from patients with shock and sepsis, to animals, to humans with arthritis and inflammatory bowel disease, and back to studies of the nervous system in animals and humans. Innumerable investigators in laboratories and clinics around the world have dedicated immeasurable time and resources to this field.
The specific details as we understand them now are sure to change as new cytokines are discovered. As our knowledge grows, I hope that the focus on cytokines will one day produce effective new therapies for severe sepsis.
