Empowering the Liver’s Shock Troops
It is a protection system that works consistently over decades: A toxin, virus or bacteria attacks the liver. Immune-system cells flood in to repel the raiders and repair the damage, but their wound-healing work scars the soft liver tissue. So, hard on the heels of the first wave of immune-system cells come others that eat away the fibrous scar tissue.
With all three pounds of liver thus cleared, it can continue at full capacity with its more than 500 essential daily tasks—among them, digesting, supplying muscle fuel, regulating cardiac blood flow and churning out over 100 different types of vital enzymes.
This finely balanced system was first mapped just over a decade ago by a team from Hadassah–Hebrew University Medical Center and Mount Sinai Hospital in New York; they showed not only how this immune modulation of the liver operates but also how it can blunder.
In the years since, their map has enabled them to point to a genetic signature responsible for the development of deadly liver disease and to suggest how future medications may combat it. A Hadassah team found that overexpression of a gene known as Neuroligin 4, or NLGN4, in certain immune-system cells in the livers of cirrhosis patients activates a mechanism that prevents these cells clearing scar tissue from the liver—a finding that was recognized by the American Association for the Study of Liver Diseases with their Young Researcher prize in 2012.
Immune modulation of the liver, a field that rapidly went global, was born in the early 2000s, when Dr. Rifaat Safadi, an Arab-Israeli gastroenterologist from the Hadassah–Hebrew University Medical Center in Jerusalem, spent a two-year fellowship at Mount Sinai Hospital. He was specializing in hepatology under the renowned Dr. Scott L. Friedman.
“We started out with the idea that inflammation is seen in all chronic liver disease, whether caused by alcohol, fat or viruses,” says Dr. Safadi, who directs Hadassah’s Liver Unit and chairs the Israel Association for the Study of the Liver. “Where there is inflammation, there will be immune cells at work, and we decided to investigate their role. That is when we found there are two dis-tinct kinds of immune cell involved: CD8 T-cells that do the wound healing but scar the liver; and natural killer (NK) cells—the immune system’s shock troops—that attack anything foreign and clean away the scar tissue.”
The team knew that the scar removal work of the NK cells is as vital as the healing. The scars are bundles of nonfunctioning fibrous tissue that impede blood flow through the liver and slow its proc-essing of nutrients, hormones and drugs. Depositions of tissue accumulate and ultimately shrink the liver, making it hard and knobby, turning it from a healthy pinkish-brown to a sickly yellow. This is the progressive and ultimately fatal condition called cirrhosis, the final phase of advanced liver disease.
Incidence of liver disease has exploded in recent years, driven by increasing affluence and Western lifestyles. Liver disease kills some 35 million people each year, according to World Health Organization figures. Nonalcohol-associated liver disease is spiking, often linked with unhealthy diet and obesity. And while alcohol-associated cirrhosis has long been known, drinking levels are high and rising, in Israel as in Europe and the United States.
Much of the drinking, believes Dr. Safadi, is underreported or hidden in all Israel’s major commu-nities—Jewish, Muslim and Christian. “Even social drinkers are susceptible to the effects of alcoholism and liver scarring,” he says.
Back in Israel from his fellowship in 2002, Dr. Safadi thought about the NK cells. If their job in the liver is removing scar tissue and thus preventing cirrhosis, why is the illness the 12th-leading cause of death in the United States, and higher still in much of hard-drinking Europe? Even in Israel, where incidence of cirrhosis is far lower, why is it on the rise? “I could think of only two possible reasons,” he says. “Either there aren’t enough NK cells to do the job and they are overwhelmed, or they’re malfunctioning.”
He put together a research team and got funding from the United States-Israel Binational Science Foundation. “Hadassah’s model is that of the physician-researcher,” he says. “That is what first attracted me to Hadassah in 1983, that is why I have invested my career in this insti-tution and stayed here, even though my wife and children live in Nazareth.”
He appointed as team leader Hadassah researcher Johnny Amer, Ph.D. “Within two years, we had our answer as to why NK cells fail to do their job,” says Amer. “We compared how they performed in three different groups: in patients with fatty liver disease, in patients with cirrhosis and in healthy volunteers.”
The team found that in the early stages of fatty liver disease, the NK cells function reasonably well. In its later stages, however, they are exhausted and function very poorly. And in the cirrhotic patients, they no longer function at all. “This finding, incidentally, sheds light on why cirrhosis patients are at higher risk for cancer,” says Amer. “NK cells perform multiple tasks, and one of those tasks is helping defend the body against carcinogenic cells.”
The team’s discovery, published in The Journal of Hepatology in February 2006, coincided with similar findings by researchers at the United States National Institutes of Health. The Hadassah and American teams have since cooperated.
Their answer, however, brought more questions. Many people suffer liver damage, the Israeli team mused. Why are some more susceptible to cirrhosis than others?
“It was back to the lab,” says Dr. Safadi. “One line of investigation was a gene array analysis of the NK cells we had isolated from the per-ipheral blood of our cirrhosis patients and from our healthy controls. This revealed a gene—NLGN4—that is overexpressed in the NK cells of cirrhosis patients. We saw that its overexpression activates a mechanism that prevents the cells clearing the scars.”
Mutations in NLGN4 were already known to be linked to other disorders. NLGN4 is involved in brain neuron com-munication and is completely non-functional in autism spectrum disorder and other neurodevelopmental impairments. It had also been linked to diabetes, with its expression increased in insulin resistance, a precursor to the illness. Until the Hadassah team’s discovery, however, NLGN4’s role in the immune system and the liver was unknown. The team announced its findings in Novem-ber 2012 at a conference of the American Association for the Study of Liver Diseases in Boston, where Amer was awarded the conference’s Young Researcher prize.
“Our next step,” Dr. Safadi says, “is to design a therapy for advanced liver disease that targets the overexpression of NLGN4, reduces it and thus allows the NK cells to eliminate scar tissue on the liver. In animal and tissue culture models, we’ve tried two different approaches. One stimulates NK cells with antibodies so that they function at par. The other is creating recombinant proteins that mimic NLGN4, so that it’s correctly expressed. We’ve achieved good results with both.”
While these encouraging early results suggest that pharmacy shelves will one day stock a therapy for the millions of people battling liver disease, that day is still years away.
“The important first stage is behind us—we have proved the concept,” says Dr. Safadi, who has protected the team’s work by privately patenting NLGN4, as medical center funding is limited. “Ahead lies the long series of trials and possible set-backs, advances and approvals. We’re working as quickly as we can to design a safe, practical and effective therapy to combat this worldwide epidemic of liver disease.”