Diagnosing Primary Sjögren’s Syndrome
For the thousands of Americans who will be evaluated this year for the autoimmune disorder primary Sjögren’s syndrome, their doctors will likely test for two antibodies that are often associated with the condition. The problem is today's standard blood tests detect the more strongly associated antibody, called SSB, only about half the time, making the meaning of a negative result uncertain.
But these numbers could one day improve. Scientists at the National Institute of Dental and Craniofacial Research (NIDCR), part of the National Institutes of Health, report online in the journal Autoimmunity that a rapid, automated test now under development called LIPS identified the SSB antibody correctly three out of four times and with perfect accuracy. It also detected a second antibody, SSA, about as well as today's standard assays in the group's initial study of 82 people, 57 of whom had well-characterized primary Sjögren's syndrome.
"This is just step one in our work to improve antibody detection for Sjögren’s syndrome,” said Peter Burbelo, Ph.D., lead author on the paper and a scientist at NIDCR. “With further refinements, the percentages will only get better."
Burbelo and colleagues also reported using LIPS to profile additional antibodies of interest that may be related to other largely unrecognized manifestations of the syndrome. These first-pass antibody profiles have the potential to help scientists define for the first time a range of clinical subtypes of primary Sjögren’s, which could greatly improve diagnoses and possibly better target treatment.
Primary Sjögren’s syndrome is a chronic autoimmune disorder that affects about 4 million Americans, 90 percent of whom are women. Scientists have long thought the syndrome is triggered when white blood cells mistakenly attack the body's moisture-producing glands, typically causing chronic dry mouth, dry eyes, and arthritis.
But other Sjögren's-related health problems continue to be reported in the medical literature, including lymphoma, thyroid dysfunction, painful peripheral neuropathy, and gastrointestinal problems. These seemingly phantom symptoms remain poorly defined, diagnostically controversial, and suggest a more systemic and yet-to-be-defined underlying pathology.
That’s where LIPS enters the picture. Short for luciferase immunoprecipitation technology, LIPS is a traditional bait-and-bind diagnostic assay. The bait is any recombinant laboratory-made antigen, or segment of a protein known to elicit an antibody response, that is then fused to an enzyme similar to the light-producing luciferase that produces the flash in fireflies.
According to Burbelo, this hybrid antigen is incubated in solution with a drop of a person's serum. If the antibody of interest is present, it will bind to the bait. Thereafter, the entire antibody-antigen complex attaches to small beads that are then measured by its firefly-like light emission. The greater the intensity of the light flash, the more target antibody there is bound to the antigen.
What makes LIPS different from other antibody tests is its extreme sensitivity, or ability to correctly detect an antibody of interest. Burbelo said this stems from selecting mammalian cells and its biological machinery as the recombinant cloning vector to make the antigens. They more closely resemble human cells than the non-mammalian sources typically used today to make test antigens, such as the bacterium E. coli.
"Antibodies must see the natural three-dimensional shape of an antigen to recognize it," said Burbelo. "With today's tests, the assumption is an antigen's three-dimensional shape will be retained once it's produced and affixed to plastic. But typically that's not the case. Antibodies will recognize a stretch of amino acids here and maybe there. They won't see other pieces of the protein that have degraded and lost their natural conformations."
"With LIPS, more of the three-dimensional shape is retained, and that heightens the sensitivity," he continued. "For example, an ELISA [Enzyme-Linked ImmunoSorbent Assay] typically has a dynamic range of between 5 and 15,000 units of signal. A unit of signal is the measure of light intensity. In other words, how bright is the test’s green or red signal? With LIPS, we see samples that range from zero to sometimes over a million units of signal.”
This zero-to-million diagnostic window allows Burbelo and colleagues to more objectively measure any antibody response associated with an autoimmune disorder. It also suggests LIPS may have the potential to detect most developing autoimmune disorders before they become symptomatic, more closely monitor the ups and downs of a patient's antibody counts over time, and track the immunological outcome of an autoimmune treatment.
To date, Burbelo and colleagues have published intriguing data involving LIPS and a range of autoimmune conditions, from type I diabetes to herpes simplex virus. In the current study, they began by applying their new tool to the Sjögren's-associated antibodies, SSA and SSB. But they did so with a diagnostic twist.
"Traditionally, the SSA antigen consists of two proteins - Ro52 and Ro60 - that are conjoined and anchored on the plastic diagnostic assays," said Mike Iadarola, Ph.D., an NIDCR scientist and senior author on the study. "We teased apart the two proteins to present their full three-dimensional shape and were able to measure the antibody response to each separately."
The group found that 42 of the study's 57 previously diagnosed Sjögren's patients had antibody reactions against a segment of the Ro52 antigen that does not elicit a response in today's ELISA tests, a sign of the LIPS assay's greater sensitivity. Overall, LIPS detected antibody responses against each protein in about 60 percent of patients, roughly equal to an ELISA, but with a far greater dynamic range and perfect specificity.
The scientists then decided to test for other conditions not traditionally linked to Sjögren's syndrome but that occasionally - and bewilderingly - have been reported in some patients. "We hypothesized that if we made antigens for the thyroid, stomach, and peripheral nervous system, LIPS could pick out the subset of patients with antibodies against these tissues," said Iadarola.
And it did. The scientists found 14 percent of their Sjögren's patients had antibodies against the thyroid antigen, 16 percent had antibodies against an antigen associated with autoimmune gastritis, and four percent had antibodies linked to an autoimmune attack of the eye's peripheral nerve.
These findings suggest that LIPS may be used as a substitute for more invasive and expensive procedures typically used to diagnose these peripheral conditions. "Again, this study is just a start," said Burbelo. "All of these tests can be performed on the automated robotic machine in my laboratory, and we can do thousands a day. There's more data to come."