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Front Page

T Cell Misfits May Spell Autoimmunity

Autoreactive Cells Alter Receptors To Escape Elimination

Image courtesy of Kai Wucherpfennig

Misfits to rebels. Pair A shows the crystal structure of a misfitting T cell receptor taken from a multiple sclerosis patient. This autoreactive receptor (yellow and red) sits off to the side of the peptide–MHC complex (blue and green), leaving large portions of the peptide exposed. In pair B, a normal T cell receptor sits squarely over the microbial peptide–MHC complex.

For a would-be T cell, the journey from cradle to grave is likely to be a brief one. After leaving the bone marrow, where it is born, the immature immune cell travels directly to the thymus, where it undergoes a fierce winnowing process. To become a mature T cell, it must learn to attack alien proteins and not those peptides that are produced by the body. Precursors that fail this task—because they have a strong affinity for self-peptides—are summarily eliminated.

The thymus is scrupulous in carrying out the selection process; the vast majority of precursors die. But occasionally an autoreactive T cell will slip by unnoticed and travel to the periphery, where it can cause disease. In multiple sclerosis, for example, T cells leave the thymus, travel to the brain, and attack a protein found in the myelin sheath surrounding nerve fibers. Researchers have long wondered how the rogue T cells are able to avoid elimination in the exacting environment of the thymus. It now appears that autoreactive T cells can disguise their presence by altering the way their receptors interact with their target.

Michael Hahn, Kai Wucherpfennig, and their colleagues revealed the
trick by capturing and crystallizing a T cell receptor in the act of binding to a self-peptide along with the MHC complex that holds the peptide in place. This is the first time a human autoreactive T cell receptor has been crystallized and imaged. The result, which appears in the April 10 Nature Immunology, presents a sharp contrast to previous crystal structures, which have shown T cell receptors binding only to foreign, microbial peptides. In those images, the T cell receptor always sits squarely over the microbial peptide–MHC complex. The newly crystallized autoreactive T cell receptor, taken from a patient with multiple sclerosis, lies off to one side of the self-peptide–MHC complex. In fact, it is perched so far off center that large portions of the peptide are exposed (see figure above).

It makes intuitive sense that off-kilter contact between T cell receptor and peptide might result in a weaker signal.

How does this eccentric positioning enable the T cell to avoid being eliminated in the thymus? The researchers, who are at the Dana–Farber Cancer Institute, believe that the misfitting receptors may literally muffle the T cells’ presence. Normally, when T cell receptors strongly bind to self-antigen, they send a robust signal to the nucleus. Within the thymus, this strong signal acts as a suicide command. T cell receptors that have only a loose affinity for self-antigen produce a much weaker signal, which acts as a survival message. Hahn, an HMS research associate in pathology; Wucherpfennig, an HMS associate professor of neurology; and colleagues believe the misfitting T cell receptor mimics the latter, thereby triggering a weak signal that calls into play a survival program.

It makes intuitive sense that off-kilter contact between T cell receptor and peptide might result in a weaker signal. In fact, the researchers have concrete support for this scenario. To send signals, T cell receptors require the help of nearby surface proteins, the CD4 coreceptors. Hahn, Wucherpfennig, and colleagues found that the autoreactive T cell receptor exhibited an altered relationship with CD4, which could affect the signaling process.

T Cell Puzzle
The discovery is still a long way from leading to therapies. To begin, the altered T cell receptor has so far been characterized in only one patient. But it does open up the possibility that odd-fitting T cell receptors may be responsible for other cases of multiple sclerosis, and even of other autoimmune diseases. “It may not be this exact topology, but other arrangements could lead to the same result—low affinity, partial binding,” Wucherpfennig said.

Photo by Steve Gilbert

"We think the off-centering of the T cell receptor over the peptide–MHC complex changes signaling in the thymus so that the signal is not strong enough for the autoreactive T cells to be deleted," said Kai Wucherpfennig (left). He is shown with co-authors (clockwise from back) Jason Pyrdol, Michael Hahn, and Melissa Nicholson.

It might seem obvious that an answer to the mystery of how T cells elude detection in the thymus lies with the T cells themselves. Yet the image of the T cell receptor stolidly perched over its peptide target has been entrenched in researchers’ minds. “Everybody was quite confident that this was the general situation,” said Wucherpfennig. It now seems clear that the very thing that allows the autoreactive T cell to avoid destruction in the thymus—its low affinity for the self-peptide—has also allowed it to elude being discovered by researchers.

Wucherpfennig spent years trying to crystallize the autoreactive T cell receptor, but it would not stay bound to the self-peptide, myelin basic protein, and the associated MHC molecule. He and colleagues broke the complex down and tried crystallizing its components. In 1998, he managed to crystallize the peptide–MHC portion. What he saw surprised him. Normally, T cell receptors contact the peptide–MHC complex at a fairly circumscribed set of residues. But this peptide–MHC complex showed signs of contact beyond those limits. “That was the strongest hint that this interaction was unusual,” he said.

When Hahn came to the lab five years ago, he tried to generate a T cell receptor–peptide–MHC complex that could be crystallized, but with no luck. He then expressed the two components independently and mixed them in the crystallizing process. “He got crystals with only MHC and no T cell receptor, which was probably due to the low affinity with which the T cell receptor binds the MHC complex,” Wucherpfennig said. Hahn switched tacks. Borrowing a method developed by colleagues, he tried artificially tethering the parts together using a flexible linker. “The T cell receptor could dissociate, but did not diffuse away,” said Wucherpfennig.

The next hurdle was making crystals that would diffract. Hahn and his colleagues screened 440 before they found a few good enough to determine the structure. Their first electron density map showed what Wucherpfennig had suspected years before, namely that the T cell receptor was oddly shifted over the peptide–MHC complex. But even he did not expect such a dramatic rearrangement. “I could never have guessed this particular structure,” Wucherpfennig said.

—Misia Landau

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