Some people hit retirement age and take up golf. Not Klaus Rajewsky. In 2001, when Rajewsky, a prominent molecular immunologist and expert on B cell development, reached the age of mandatory retirement in his native Germany, he decamped to the Immune Disease Institute to be able to continue his research. Now an IDI senior investigator and the Fred S. Rosen Professor of Pediatrics and Professor of Pathology at Harvard Medical School, Rajewsky is making good use of his anti-retirement. He recently captured a major German science prize, and his lab is churning out significant discoveries on the hot topic of genetic regulation by microRNAs. In two prominent research publications this spring, Rajewsky and his colleagues have established how microRNAs contribute to normal B cell development. Their results shed light on how cells make the life and death decisions that keep autoimmune disease and cancer at bay.

On May 28, Rajewsky was honored in Marburg, Germany, with the Emil von Behring Prize for his outstanding work on the origins and molecular workings of antibody-producing B cells. The prize, consisting of a medal and $25,000 euros, is one of most prestigious awards given in Germany. It commemorates Emil von Behring, the winner of the first Nobel Prize for Physiology and Medicine in 1901. Behring is credited with discovering antibodies, which just happen to be the subject of much of Rajewsky's work.

In his research, Rajewsky has outlined the process by which B cells develop from their earliest stages in the bone marrow into full-fledged antibody-producing cells. Before joining the IDI, he was a professor at the University of Cologne and head of the European Monterontondo Research Center in Rome. In Cologne, he developed a technique for genetically tailoring mouse embryonic stem cells to create animals with conditional mutations. This technique, which uses the Cre/loxP recombination system, allows researchers to eliminate, mutate or add a gene of choice in selected tissues at will. The resulting mice reveal the critical functions of the targeted genes in health and disease.

Collaborating with Jamey Marth at Vancouver, Rajewsky and colleague Hua Gu unveiled Cre/loxP-based conditional gene targeting in a paper in Science in 1994. The method is now widely used by biomedical researchers to understand normal development and to make models of human disease. Rajewsky applied conditional gene targeting mainly to immunology and generated various mouse models of immune diseases. Presently he uses genetically engineered mice mostly to probe the lives of B cells, the antibody-producing cells of the immune system that sometimes transform into lymphomas. His discovery of the genes involved in normal development of B cells in mice has yielded many insights into autoimmune disease and cancer in people.

Since joining the IDI, Rajewsky has continued his pioneering mouse work, moving forward into an entirely new line of research focused on microRNAs. These novel regulators of gene expression are arguably the most exciting discovery in basic biology of the last decade. In every cell, the DNA in genes is copied into a messengerRNA (mRNA), which is then translated into a protein. Studies in simple worms in the 1990's unexpectedly revealed a class of small RNA molecules that regulate the translation step. These small, or microRNAs bind to target sequences on mRNAs and physically block translation or trigger degradation, or both. These new gene suppressors were soon found to be highly conserved throughout evolution and critical for organisms ranging from plants to humans to develop and function properly. Their relatives, small interfering RNAs, are being avidly pursued as highly targeted genetic medicines for a large number of diseases.

Even so, the exact physiological functions of miRNAs have not been so easy to figure out. Each microRNA can target and inhibit hundreds of mRNAs, and hundreds of microRNAs have been found in mammalian cells. To sort out microRNAs regulatory networks in T and B lymphocytes, Rajewsky and his colleagues have used conditional knockouts and other genetic manipulations to produce mice who express too much or too little of certain microRNAs to study their effects.

In one study, they Rajewsky and colleagues ablated the gene for dicer, an enzyme critical for producing microRNAs. When dicer is missing from all tissues, animals do not survive. However, by selectively removing dicer only from B cells, the researchers, led by postdoctoral fellows Sergei Koralov and Stefan Muljo, produced mice that were normal, except for a nearly total lack of B cells. The results, which were published last March in the journal Cell, indicated that developing B cells lacking microRNAs were dying before they could mature.

To figure out why, Rajewsky tapped the expertise of his son, Nikolaus, a professor at the Max Delbruck Center for Molecular Medicine and a prominent bioinformatics specialist who has developed high-throughput computer methods to identify the targets of microRNAs. The younger Rajewsky's analysis revealed that genes whose expression increased in dicer-minus B cells often carried recognition sequences for a particular group of microRNAs, known as the miR17-92 cluster. One of the genes most strongly regulated by miR17-92 was BIM, a protein that controls programmed cell death in immune cells. In agreement with the computer work, the IDI researchers found that B cells lacking dicer, and therefore low in miR17-92, had increased levels of BIM protein, which acted to prevent B cell maturation by triggering excessive and premature cell death.

A second study published at the same time in Nature Immunology shows the flip side of miR17-92-when cells have too much of the miRNAs, they survive too well. In that study, which started out as an independent investigation, postdocs Changchun Xiao and Lakshmi Srinivasan found that boosting the expression of mir17-92 cluster microRNAs in immune cells of resulted in overgrowth of B and T cells. The animals developed a precancerous blood profile and lethal autoimmune kidney disease. Immune cells from these mice had lower levels of BIM and a related protein PTEN, another miR-17~92 target. Both proteins contributed to the cells' escape from normal physiological constraints on growth.

"Since microRNAs were discovered, we have tried to find out if miRNA regulation would explain some of the processes in leukocyte differentiation which so far we could not explain," Rajewsky says. The new work moves the investigators closer to that goal, but also gives a broader understanding of the wider role of microRNAs in mammalian cells in general, he says.

The idea that miR17-92 microRNAs regulate survival pathways for immune cells applies to other tissues as well. Many kinds of kinds of tumors have high levels of miR17-92 microRNAs, as a result of gene amplification. Very recent work from other labs has implicated miR17-92 microRNAs in heart and lungs development, and in cancer growth.

As Johns Hopkins University researcher Joshua Mendell writes in a recent perspective in Cell, "The available evidence places the miR17-92 cluster at the nexus of critical pathways that regulate cellular life and death decisions during normal development and in malignancy." Because of this, Mendell speculates, if microRNAs-based drugs become reality, the miR17-92 cluster will be the first to be targeted for new cancer treatments.

Looking back on his lab's accomplishments of the last several years, Rajewsky reflects, "It was really very nice how everything came together-my connection with my son, our papers and the other investigators' papers. That is very unusual, and very pleasant."

Despite these recent successes, Rajewsky says, researchers have only just scratched the surface of what there is to know about microRNAs. And that is enough to make the prospect of retirement every bit as unattractive to him now, as it was seven years ago.