If B cells were your coworkers, you would be tempted to advise them to get a life outside of work. Each of these essential immune cells is coated with a different receptor that gives them enviable job security while they kill time waiting for the single matching invading pathogen, such as a virus, that they were born to fight. Their molecular equipment also keeps them alive. Take off their microbedetecting receptors, and B cells die.
Now scientists have discovered why B cells’ infection fighting and survival are so tightly intertwined. The receptor does double duty in B cells by also triggering a low-level activation of PI3-kinase (PI3K), a crucial signaling center for all healthy cells that goes into overdrive in many cancers. The new finding comes from the same laboratory that first reported 12 years ago the surprising news that the B cells’ antigen receptors are necessary for cell survival.
“Nobody knew what the survival signal was,” said senior author Klaus Rajewsky, the Fred S. Rosen professor of pediatrics at HMS, the Immune Disease Institute and Children’s Hospital Boston. “That’s what this paper is about.”
The work has important implications for treatment of leukemia, lymphoma and other hematological malignancies, disorders that often arise from B cells, and for autoimmune diseases, such as rheumatoid arthritis and lupus, which may develop in part from misdirected B cell antibodies, said David Fruman, who studies B cell activation, signaling and tumorigenesis at the University of California, Irvine.
“Every major pharmaceutical maker is trying to block the PI3K pathway for treating inflammatory diseases, autoimmune diseases and cancer,” said Fruman, who was not involved in the study. “These findings suggest these drugs may help kill off unwanted B cells or those that are growing out of control. Of all the different B cell pathways, this may be the most promising.”
The earlier study, in 1997, showing that B cells die without their antigen receptors, left Rajewsky and other scientists with a big problem. “We had our ideas,” he said, “but we didn’t know how to address the hypothesis.”
The central quandary involved how to study the signaling in a mature resting B cell, which by definition does not signal much. If the researchers activated the cells through the receptor to amplify the signal, then they would be studying the receptor’s role in fighting infection, not its survival as a resting cell.
The answer came from a fancier version of the same genetic approach first used to eliminate the receptor, which knocked out the molecule when the B cell matured in mice. “We had found out cells died without the receptor,” Rajewsky said. “Now they had to be rescued.”
This time, the team, led by postdoctoral fellow Lakshmi Srinivasan, combined the conditional receptor knockout with activation of a candidate gene. One by one, Srinivasan and her colleagues tested single copies of five genes in mice, selecting the same signaling pathways the receptor used to activate the cell, but at levels too low to turn on the infection-fighting machinery.
Just one, the PI3K gene, resulted in a dramatic rescue of the B cells missing their antigen receptors. Even without antigen receptors, the B cells with PI3K thrived and populated the lymph nodes and spleen of the mice in just the right places and in the right numbers as if they were normal.
“It was so striking that we knew that was it,” said Rajewsky, who happened to be walking through the lab when Srinivasan was examining the early evidence. “We had to do a lot of work to really nail it, but that was the ah-ha moment. We still had to prove that we were not looking at a transgenic artifact.”
Srinivasan, who had trained as a molecular biologist, systematically examined the full PI3K pathway. Deleting a downstream gene (Pten), whose protein holds PI3K activity in check, also ramped up PI3K activity enough to rescue B cells without antigen receptors.
Even further downstream in the signaling pathway, B cells missing antigen receptors could be partially restored by a gene deletion mimicking PI3K function. “It’s a technical tour de force,” said Fruman. “The technical hoops they had to jump through to make a system of simultaneously knocking out the receptor while maintaining the PI3K signaling pathway were prodigious.”
Interestingly, another leading contender for the B cell survival signaling, NF-kappaB, failed to restore survival of B cells missing their antigen receptors.
Rajewsky, Srinivasan and their colleagues are looking at the impact of PI3K signaling on lymphomas to learn what impact it has on the cancer physiology and pathology. Many lymphomas feature B cells that retain their receptors, which could be helping them survive and spread.
Students may contact Klaus Rajewsky at rajewsky@idi.harvard.edu for more information.
Conflict Disclosure: The authors declare no conflicts of interest
Funding Sources: The National Institutes of Health, Leukemia and Lymphoma Society, European Union through MUGEN, the Joint Program in Hematology and Transfusion Medicine at HMS, the Portuguese Foundation for Science and Technology, and the Robert A. and Renee E. Belfer Foundation. The authors are solely responsible for the content of this work.
