Finding Cinderella’s Slipper: The Discovery Story of the Shared Epitope in Rheumatoid Arthritis
Rheumatoid arthritis is an old disease—it was first described in the early 19th century and received its formal name in the 1850s, with the term “rheumatoid” derived from the Greek concept of “flowing current” in the body. Yet despite this long history, scientists continued to struggle to understand key concepts about the disease as recently as the second half of the 20th century. In particular, it seemed apparent that a person could inherit a gene or set of genes that increased their risk of rheumatoid arthritis (thereby explaining why the disease runs in some families), but the nature of such genetic markers was not known.
Dr. Robert Winchester, Professor of Medicine, Pathology and Cell Biology, and Pediatrics in the Division of Rheumatology at the Columbia University Vagelos College of Physicians and Surgeons, became interested in this question early in his career. By the 1960s, researchers had discovered that every cell in a person’s body carries a protein on its surface called human leukocyte antigen (HLA)
and that this protein displays parts of the cell to circulating components of the immune system (known as T cells) while they are on patrol. As immune cells mature, they use the bits of cells shown to them by the HLA molecules to learn how to differentiate between self and foreign material.
When Dr. Winchester began his research studies, the only type of HLA that was known at the time is what we now call “HLA class I molecules.” These molecules were discovered when researchers made use of a handy natural phenomenon: during normal pregnancy, women produce harmless antibodies directed against the paternal HLA antigens expressed in the fetus. Using blood from pregnant patients as “typing reagents,” scientists could show that HLA class I molecules were expressed on all lymphocytes (a type of white blood cell). However, Dr. Winchester was able to employ a different methodology—one that involved fluorescent antibodies—and demonstrated that a second type of HLA molecule now called “class II HLA” was expressed only on specific types of white blood cells named B cells and monocytes. As Dr. Winchester explains, “We stumbled on this finding in the control of an experiment looking for why patients with systemic lupus erythematosus had defective mixed lymphocyte responses. This led to the identification of different genetic variants, or ‘allotypes,’ of HLA and we were able to show that patients with lupus and rheumatoid arthritis were distinguished by these distinct allotypes.” Specifically, patients with lupus had what are now referred to as HLA DR2 and DR3 while patients with rheumatoid arthritis had HLA DR4. This work gained considerable attention and Dr. Winchester was honored by being designated a Fellow of the American Association for the Advancement of Science for the “discovery of human class II major histocompatibility complex (MHC) molecules.”
Yet the story is complex, as Dr. Winchester points out: some patients with rheumatoid arthritis had HLA DR4, but not all of them. In certain labs, Dr. Winchester’s work on HLA DR4 in rheumatoid arthritis could be replicated, but in labs in Jerusalem and Bombay, this was not the case. In places where HLA DR4 did not correlate with rheumatoid arthritis, a different variant—HLA DR1—was found to associate with the disease. Even so, Dr. Winchester and his colleagues had identified several pregnancy sera that reacted with the blood of nearly all patents with rheumatoid arthritis, suggesting that the HLA molecules in these patients shared a structure in common, but what could that structure be? “It was all depressingly complex and made little sense,” Dr. Winchester recalls. By that time, he had left the Rockefeller University, where he had been conducting his research up until that point, and had started a new Rheumatology unit with a major laboratory at the NYU Hospital for Joint Diseases (he would later join Columbia in 1991).
Remaining convinced that some yet-to-be found amino acid configuration underlay the complex story of rheumatoid arthritis genetics revealed by the experiments with blood from pregnant patients, Dr. Winchester enlisted the help of several rheumatology fellows to identify this enigmatic structure—which they called the “shared epitope”—at the level of its DNA sequence. Principal among these fellows was Dr. Peter Gregersen, who helped lead an effort to sequence two variants of HLA DR4: one that was representative of the type that was associated with rheumatoid arthritis and another that was not associated with the disease. This work led to the identification of a motif of four amino acids which, if encoded by a DR gene, resulted in susceptibility to developing rheumatoid arthritis. In essence, Dr. Winchester and Dr. Gregersen had discovered “Cinderella’s slipper”—that is, they had found a binding pocket structure into which peptides could potentially fit and go on to drive the T cell immune response underlying rheumatoid arthritis. The identity of this peptide, which they called “Factor X,” would only be identified many years later by several investigators including Dr. Lars Klareskog, a Swedish physician and researcher who showed that citrullinated proteins might “fit into the slipper” and spur the formation of antibodies. Today, antibodies to citrullinated proteins are one of the most widely used tests to help diagnose patients with rheumatoid arthritis. In 2013, all three scientists—Drs. Winchester, Gregersen, and Klareskog—were awarded the Crafoord Prize in Polyarthritis by the Royal Swedish Academy of Sciences, the same body that selects winners of the Nobel Prize.
Dr. Winchester’s discoveries have been some of the most important in the annals of rheumatology, yet, as is true of much of science, this is not the end of the story. While a portion of the shared epitope is in the part of the HLA-DR molecule that binds peptides like citrullinated protein, a different part of the molecule is in a non-peptide binding area. This section interacts directly with T cell receptors, but exactly how this direct interaction plays into susceptibility to rheumatoid arthritis is not completely known and is a current area of research in Dr. Winchester’s lab. In the words of Dr. Winchester, “One senses that, with the powerful new developments in immunology now available, the pace of our understanding will palpably accelerate and that in the relatively near future the processes and molecular interactions responsible for rheumatoid arthritis will be defined, leading to a way of putting this disease in the realm of past history.”