COMMENTARYhttp://immunol.nature.com • june 2002 • volume 3 no 6 • nature immunology 501César Milstein, the father of modern immunologyTimothy A. SpringerCenter for Blood Research, Harvard Medical School, Boston, MA 02115, USA. ([email protected])César Milstein was born in 1927 at Bahia Blanca, Argentina, to immi-grants from Russia involved in the secular, intellectual Jewish cultureof the time. César was an adventurous youth who went to college inBuenos Aires, where he majored in chemistry and was active in poli-tics. It was through politics that he met his lifelong love, Celia, andafter graduation and marriage, the couple hitchhiked through Europeon a year-long honeymoon.After returning, César carried out enzyme research under Stoppanifor his Doctor en Química degree at the Universidad de Buenos Aires,while he and Celia scraped together just enough money to supportthemselves by moonlighting as clinical bio-chemists. In 1958, César received a prestigiousBritish Council fellowship and sailed with Celiato England. Milstein published papers on kineticson his own and on the amino acid sequence ofenzyme active sites with Fred Sanger and receiveda second Ph.D. degree in 1960. Sanger had com-pleted the amino acid sequences of the insulin Aand B chains in 1951 and 1953 and the assign-ment of their intrachain and interchain disulfidebonds in 1955. Milstein helped Sanger celebratehis first Nobel prize in 1958. The Milsteinsreturned to Buenos Aires in 1961, Celia to com-plete her Ph.D. studies and César to whatappeared to be a promising position as head of theDivision of Molecular Biology at the InstitutoNacional de Microbiología. However, fundingwas cut off after a military coup and an ensuingvendetta against liberals and intellectuals at theinstitute. Finding the situation impossible, Césarwrote to Fred Sanger. In 1963, César joinedSanger and others under the chairmanship of MaxPerutz at the recently created Medical ResearchCouncil (MRC) Laboratory of Molecular Biology.The generation of antibody diversityMilstein has cited both a research student he inherited in Argentina whowas working on antibodies, and the suggestion of Fred Sanger, as stim-uli for his decision to work on the antigen-combining sites of antibod-ies. Shortly after Milstein’s death, Sanger with typical modesty deniedhis own involvement in this decision and noted simply that antibodydiversity was a hot topic at the time. Rodney Porter was undoubtedyanother influence. Porter had already begun studies on antibodies in1946 as Sanger’s first Ph.D. student, and the Porters and Milsteins laterbecame lifelong friends, taking annual walking vacations in Europe. In1958 Porter had described the antibody Fab and Fc fragments, and in1959 Gerald Edelman described dissociation into heavy and lightchains, enabling the modern view of antibodies as Y-shaped moleculeswith two Fab fragments and one Fc fragment to emerge in the early1960s. The question of whether antibody diversity was a consequenceof sequence variation had become a soluble problem.Milstein first approached this problem by determining thesequence of disulfide-bonded peptides in Bence-Jones light chainsand obtained evidence for both variable and constant sequences.Milstein also defined the inter-heavy chain disulfide bridges thatcharacterize each immunoglobulin (Ig) subclass. Milstein became anadvocate of somatic mutation and, with SydneyBrenner, published a paper on this topic in1966. As is often the case, the advocates of theopposing schools of germline diversity andsomatic mutation both turned out to be right,with a combined mechanism far more complexthan imagined by anyone.César’s interest in the mechanism of somaticmutation drove his research for the rest of hislife and was the impetus that led him to inventmonoclonal antibodies. It is a tribute toMilstein’s taste and instincts that of all themechanisms for antibody diversification,somatic mutation continues to be the mostenduring research problem and the most rele-vant to the issue of affinity maturation. It is atribute to his enthusiasm and tenacity that hecontributed a paper on this topic the very weekbefore he died.As a scientist, César was open, approachableand loved to discuss scientific issues. His earlyactivity in politics seemed to have developedhis ability to see a problem from all possibleangles, gnawing on it until the best approachesfor solving it were found and carried out, andthen clearly arguing all the evidence in support of a conclusion. Hewas interested in the big picture, but also in the smallest detail thatcould shed light on it. Furthermore, he was not interested in anapproach if others were taking it, but sought unique ways of attack-ing scientific problems.From myelomas to hybridomasIn the 1970s, César turned from the sequencing of myeloma proteinsto mRNA sequencing and work with myeloma cell lines (antibodysecreting tumors) cultured in vitro. RNA sequencing required hugeamounts of cells and 32P. When I first met him in 1977, much of hisCelia and César Milstein, with Tim and NoahSpringer, in front of the Fogg Art Museum atHarvard, 1992.Photo credit: Tim SpringerIn memoriam: César Milstein, who with the late Georges Köhler invented monoclonal antibodies,died on 24 March 2002.Their invention sprang from basic research on antibody diversity and speci-ficity, and spawned revolutionary advances in biology, medicine and industry.© 2002 Nature Publishing Group http://immunol.nature.comnature immunology • volume 3 no 6 • june 2002 • http://immunol.nature.comCOMMENTARYtime was spent with myeloma and hybridoma cells, and I wasimpressed that they were almost like children to him. Cells were main-tained in spinner flasks out in the lab, with ingenious methods fordrawing off exponentially growing cells when needed for experimentsand for continuous infusion of media. With George Brownlee in 1972,César translated mRNA from the myeloma cells, discovering a signalsequence on the light chain precursor. Their sequencing efforts alsorevealed in the mRNA the junction between the variable and constantregions, which showed that their joining preceded protein synthesis.The development of recombinant DNA and DNA sequencing openedup new approaches, but they were taken by many others, includingTerry Rabbitts, whom César had hired at the MRC. Therefore, in char-acteristic fashion, César decided to embark