On August 1, 1945, three-year-old Tasuku Honjo escaped the firebombing of Toyama City on his mother's back after an unexploded incendiary bomb landed in their garden shelter. Raised in Ube, his early fascination with science began at Kamihara Elementary School, where he observed the rings of Saturn through a portable telescope. Although he initially planned to pursue astronomy, a biography of microbiologist Hideyo Noguchi given by his mother shifted his ambition toward medical science and biochemistry.
Early Breakthroughs in the Laboratory
Honjo entered Kyoto University Medical School around 1960 and joined the laboratory of biochemist Osamu Hayaishi for graduate studies from 1967 to 1971. In the summer of 1967, he focused his research on why diphtheria toxin requires the metabolic cofactor NAD to block cellular protein synthesis. According to the biography published by the Nobel Foundation, Honjo used labeled forms of NAD available in the lab to demonstrate that diphtheria toxin acts as an enzyme. He proved that the toxin catalyzes the transfer of the ADP-ribose moiety from NAD to elongation factor 2 (EF-2), establishing his early reputation in molecular biology.
Improvised Tools and the Train Ride Epiphany
Seeking international training, Honjo moved to the United States to join Philip Leder's laboratory at the National Institutes of Health (NIH), studying the genetics of antibody diversity. When Honjo returned to Japan in 1974 to establish his own research line at the University of Tokyo, he faced severe material constraints and a lack of laboratory funding. To build essential apparatuses, he visited the Akihabara electronics district to purchase thick rubber plates, hoses, and mesh, assembling his own vacuum gel dryer for protein electrophoresis.
In 1978, while riding the train during his 90-minute commute from Yokohama to Tokyo, Honjo analyzed experimental data on constant-region gene loss in myeloma cells. Porings over the patterns on the train, he formulated the DNA deletion hypothesis model for class switch recombination, detailing how B cells change the class of antibodies they produce.
Unlocking the Machinery of Immunity
Honjo expanded his focus to the molecular signals driving immune adaptation. By expressing cDNA libraries in amphibian oocytes, his research group isolated interleukin-4 (IL-4) and interleukin-5 (IL-5), key cytokines responsible for regulating class switching and cellular differentiation. In 1999, Honjo and Masamichi Muramatsu identified activation-induced cytidine deaminase (AID), an enzyme expressed specifically in activated B cells within germinal centers. Subsequent gene-knockout experiments confirmed that AID is the essential enzyme required for both class switch recombination and somatic hypermutation.
The Mechanics of Immune Adaptation
By validating the DNA deletion model and isolating the AID enzyme, Honjo unlocked the primary genetic mechanisms explaining how the human immune system adapts to resist diverse pathogens. His findings demonstrated how targeted DNA rearrangements allow a limited set of inherited genes to generate millions of protective antibody variants.
AI-generated editorial illustration; not a documentary photograph.
Source: Nobel Prize
Tasuku Honjo
SIGNAL TITANS
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