VOLUME 12, NUMBER 16 • FEBRUARY 23, 2001

Nussenzweig Lab Finds Many Immune Cells Are Fine-tuned to Prevent "Friendly Fire"

Michel Nussenzweig is the Sherman Fairchild Professor and head of the Laboratory of Molecular Immunology. He is also an HHMI investigator.

Despite the remarkable precision with which the immune system matches invading antigen with unique antibodies, the overall defense is based on randomness. The body creates 1 trillion antibody molecules at will, none of them made to order for a particular threat. Not until the antigen comes in contact with its corresponding B cell does a specific antibody assume particular importance.

This arrangement, while ensuring diversity, does have drawbacks. Random production inevitably creates antibodies that recognize and attack the body’s own proteins, meaning that a significant number of immune cells are potentially harmful. To address this threat, the body employs various mechanisms to prevent this from happening. So far, four methods have been discovered by which the immune system removes this hazard–a phenomenon scientists refer to as "tolerance."

In some cases, the body simply destroys the B cell–a process called "central deletion." This method was first proposed by Rockefeller President Emeritus Joshua Lederberg in 1959. More recent research has uncovered three other strategies. Some cells undergo a process called "anergy," in which the cells are disabled and die a slow death. In rare cases, called "ignorance," the cells simply fail to respond to the antigen for which they were designed.

The fourth mechanism, "receptor editing," involves altering the gene of the B cell receptor so that it codes for a slightly modified antibody. Rather than taking the B cell out of action, receptor editing allows the cell to mature and carry on its normal defense role. The modification to the cell in receptor editing is not extensive. But since antibodies are so highly specialized, a minor change can still have a big impact–much the way a slight imperfection on a key can render it useless.

The editing itself is haphazard in that the alteration is not guided so that a specific sequence is inserted when the new gene is patched together. Rather, the body splices in a random sequence and sees if it works. If the cell is still self-reactive with its new receptor, it once again is subject to one of the body’s four protective mechanisms.

Although researchers were aware that receptor editing took place, they did not know how often it occurs in live animals. The prevailing assumption of how the immune system achieves tolerance has been that the troublesome immune cells are taken out of action–the body simply eliminates or enfeebles the entire cell. There was no evidence that receptor editing made a large contribution to antibody production.

Now, however, researchers in the Laboratory of Molecular Immunology, headed by Sherman Fairchild Professor Michel Nussenzweig, have found that receptor editing represents a major force in shaping the antibody repertoire. In a paper published in today’s issue of Science, the researchers report that one-quarter of the body’s antibodies’ are created through receptor editing. The idea that so many B cells are fine-tuned and allowed to progress will cause immunologists to revise the model by which they understand the system.

"This is the first experiment designed to measure how often receptor editing occurs in a live animal," says lead author Rafael Casellas, a graduate student in the laboratory. "Until now, all we knew is that editing can happen. This study proves that it is happening to a surprising extent."

Casellas and his colleagues conducted their experiment with genetically modified mice that allowed them to determine whether a specific DNA stretch had been changed during immune system development. They inserted a human DNA sequence into a precise spot in the mouse genome and waited to see how often it showed up in the B cells after the animals had developed.

They found that 25 percent of the light chains on the surface of the B cells’ antibody molecules are produced by receptor editing. Further, that editing was shown to occur in the B cells during a two-hour delay in development, at a stage in which they are normally recombining their light-chain genes.

"The body senses that the B cell is self-reactive, so it halts the cell to be modified during that early stage in development," Casellas says.

In some ways it makes biological sense for the body to rely on receptor editing to such a large degree. Modifying potentially harmful immune cells rather than destroying them conserves energy while maintaining an immune system that is both strong and safe.

"It is quite striking that what is being targeted so often is not the B cell per se, but a specific part of the antibody–the receptor–that the cell produces," Casellas says. "It is a much finer distinction, one that the body makes more often than most of us suspected."

This type of experiment became possible only through the development of innovative genetic manipulations that allow researchers to target precise DNA locations to ask specific questions.

"The technology is allowing us to develop sophisticated systems aimed at answering very specific questions about immunity and tolerance," says Michel Nussenzweig. "Use of these ‘knock-in’ genes should lead us to a much clearer understanding of how the body codes for its antibodies.