Immunity Bites

Unleashed, immune cells can turn on their masters



T cells (green) and B cells (red) fight invaders, but when not needed must be kept in check to prevent them from attacking the body’s own tissues and causing autoimmune disorders. Two “leash” proteins and a communication protein appear to help keep these trigger-happy cells from erupting into friendly fire.




Measuring messenger RNA, which acts like an order slip for building a copy of a gene’s protein, gives scientists a feel for a gene’s activity level in a cell. This in turn allowed Peng’s group to highlight genes with distinct differences in activity levels in the mice with lupus-like symptoms.




In addition to their work with immune cell leashes, Peng and his colleagues recently connected lupus in mice to a protein that is involved in immune system communications.



SCIENTISTS ARE UNCOVERING NEW COMPLEXITIES in the innermost workings of the human immune system that could make big differences for patients with autoimmune diseases. Thousands of Americans are diagnosed with these disorders each year as cells in their bodies that normally attack invaders like bacteria and viruses instead turn their fury on the body’s own tissues. This about-face causes conditions such as lupus, myasthenia gravis, allergies, psoriasis, diabetes, Graves’ disease, rheumatoid arthritis and multiple sclerosis.

For decades, scientists assumed that these disorders were caused mostly by bad instructions to the cells that serve as the immune system’s attack dogs. These cells, which are collectively referred to as lymphocytes and include B cells and T cells, rely on a complex signaling and detection system that tells them when, where and what to attack. If immune attack cells were assaulting the wrong targets, researchers reasoned, something had to be going awry in that signaling system.

Thanks to the work of researchers like Stanford Peng, MD, PhD, assistant professor of medicine in rheumatology and of pathology and immunology, a much more complicated picture of the causes of autoimmune diseases is beginning to emerge. Expanded insights into these causes may soon be offering scientists new frontiers for developing drugs that can ease or prevent such disorders.

One of the biggest new developments in autoimmune theory focuses on what immune attack cells are like when they’re not on the job battling invaders. Scientists previously assumed that mature, unused versions of T and B cells were “sleeping” or dormant.

But a new theory starting to gain widespread acceptance suggests that the cells are constantly spoiling for a fight, and healthy immune systems have to constantly work to restrain them, in effect putting a “leash” on the attack dogs.

In both T and B cells, Peng has identified the first-ever examples of these leashes — proteins that actively work within the cells to keep them quiet when they’re not needed.

Stanford Peng, MD, PhD, reviews lab results with, from left, graduate students Stephanie Lathrop and Barbara Schraml and laboratory technician Ling Lin.

Peng specializes in the study of lupus, an autoimmune condition that afflicts approximately 1.5 million Americans with a range of symptoms including arthritis, prolonged fatigue, skin rashes, kidney damage, anemia and breathing pain.

Through selective breeding, scientists have developed several mouse models that exhibit one or more lupus-like symptoms. To identify the gene leashes, Peng’s research group compared levels of messenger RNA for various genes in normal mice and a lupus mouse model.

Measuring messenger RNA, which acts like an order slip for building a copy of a gene’s protein, gives scientists a feel for a gene’s activity level in a cell. This in turn allowed Peng’s group to highlight genes with distinct differences in activity levels in the mice with lupus-like symptoms.

The first leash they found, a protein called Foxj1, had never previously been linked to immune system functions. Based on messenger RNA levels, though, the gene appeared to be much less active in lupus mice than in normal mice. When Peng and colleagues disabled the gene for the protein in normal mice, the mice developed lupus-like symptoms.

“These symptoms included inflammation in multiple organs like their lungs, their salivary glands, their kidneys, and other organs, which is very characteristic of lupus,” Peng explains.

Scientists had previously identified Foxj1 as a transcription factor, a protein that can bind to DNA to increase or decrease the activity of other genes. Further investigation by Peng’s group showed that decreased Foxj1 activity led another transcription factor, NF-κB, to increase its activity.

“This protein belongs to a family of transcription factors heavily implicated in various types of inflammation, including the inflammation caused by infections and by allergies,” Peng says. “So our thinking is that without Foxj1, more NF-κB is activated, possibly triggering the inappropriate activation of T cells and leading to organ inflammation and other lupus symptoms.”

Inappropriately activated T cells also are involved in multiple sclerosis and in diabetes, suggesting that Foxj1 also might be a contributing factor in these conditions, Peng notes.

The second leash recently identified by Peng’s group is known as microphthalmia-associated transcription factor (MITF). Microphthalmia is a genetic condition that causes abnormally small eyes and impaired vision.

Like the Foxj1 protein, Peng’s group became interested in MITF when messenger RNA studies suggested the gene was unusually inactive in a mouse lupus model. Peng and colleagues lowered activity levels of the protein in normal mice, and close examination of those mice showed that B cells were spontaneously turning themselves on and making antibodies, clumps of proteins that are normally designed to attack invaders. The new antibodies in the mice were autoantibodies — antibodies targeted to the body’s own tissues that are a characteristic symptom of lupus.

“This is the first transcription factor we’ve found that has to be active in the resting B cell to keep it that way,” Peng says.

MITF’s sphere of influence is proving a little harder to define than that of Foxj1. It appears to restrain interferon regulatory factor 4 (IRF4), a transcription factor previously linked to the activation of B cells. But it appears to have that effect by proxy through its influence on several other genes that in turn act to keep IRF4 in check.

“We’ve been focusing our efforts to develop new treatments for autoimmune disease on pathological targets — genes that are overused or are used inappropriately, leading to immune system attacks on self,” Peng says. “Another concept we should keep in mind is that the loss of one of these regulatory genes that keep the immune system in check also may be a primary contributing factor.”

In addition to their work with immune cell leashes, Peng and his colleagues recently connected lupus in mice to a protein that is involved in immune system communications.

The protein, SLAM-associated protein (SAP) appears to be involved in exchanges between B cells and T cells. Scientists have long known that T cells “talk” to B cells to help them produce antibodies meticulously customized to destroy the last scattered remnants of a persistent invader. But they’ve had a hard time determining the details of how those interactions take place.

“SAP may give us an important first insight into how this occurs,” Peng says. “But even more importantly, it may provide us with a target for new lupus treatments that don’t widely suppress the immune system.”

Earlier research had shown that higher levels of SAP were present in animals with autoimmune conditions than in normal animals.

Peng affirmed the SAP-autoimmunity connection through work with a lupus model created by exposing mice to a hydrocarbon oil. Such exposures cause normal mice to develop kidney disease, arthritis and other conditions similar to lupus. However, mice with genetically disabled SAP stayed healthy even after exposure.

To their surprise, researchers found that most immune system functions appeared to be working normally in mice lacking SAP.

“We have identified other immune system proteins that are potential targets for new autoimmune disease treatments, but they all affect large portions of the immune system, making weakened immune function a potential side effect of any new drug,” Peng explains. “Our early experiments suggest targeting SAP for treatment may avoid that risk.”

Peng cautions that errors in any one gene are unlikely to be the sole cause of an acute autoimmune disorder like lupus. “It’s very clear now that no single gene or even couple of genes are sufficient to explain lupus,” he notes. “You probably need multiple malfunctions in different genes to cause such a severe autoimmune syndrome.”

The multiple causes of lupus are likely reflected in the multiple mouse models of the disorder, says Peng.

“Each of the animal models has slightly different clinical aspects to it, probably because they represent a slightly different facet of the human disease,” he explains. “It’s therefore going to be very interesting to test if these are findings that can apply to lupus generally or if they’re limited to subsets of lupus.”

Peng’s group recently identified another leash protein from the same family of genes as Foxj1. They are currently working in the lab to further understand the activity of all the proteins and also have begun studying human lupus patients to see if they can detect signs of abnormal activity in these proteins.