Opinion: Milan Hašek and the discovery of immunological tolerance

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PERSPECTIVES

interest in natural sciences. During theSecond World War, Hasek escaped deporta-tion to the German Reich by working as alabourer in nearby factories. After the war, heenrolled at Charles University, Prague, tostudy medicine, where he met ProfessorBohumil Sekla, the head of the Departmentof Biology, a staunch proponent ofMendelian genetics, and leading opponent ofthe pervasive, politically enforced Lysenkoistanti-Mendelian doctrine (BOX 1). I personally

remember from 1953 how Sekla bravelytaught the Mendelian laws to packed lecturetheatres of medical students, who wereexcited by the experience of publiclyexpressed ‘dissident’ views, at a time whenexpression of such opinions was quiteunprecedented (FIG. 1).

Sekla quickly recognized Hasek’s talents,allowing him to carry out research and assistwith teaching in his department right fromthe first year of his undergraduate medicalstudies, and by appointing him as a lecturerimmediately after his qualification in 1950.However, being an active member of theCommunist party, Hasek moved within afew months to the Institute of Biology of thenewly formed Czechoslovak Academy ofSciences, led by Ivan Malek, who was animportant member of the CommunistParty. This political association provided theyoung Hasek with superior funding for staffand facilities, as well as administrative and

“Science is like an amoeba, sending outpseudopodia in all directions, but able tomove in only one way”(From M. Hasek’s letter on 25 Dec 1970)

“A discovery is not a fact, but the statementof a fact” 1

This year is the 50th anniversary of thepublication of a paper by Milan Hasek, inwhich he showed the phenomenon ofimmunological tolerance by the selectivefailure of chimaeric chick-embryo parabiontsto produce antibodies against the red bloodcells of each other. The discovery oftolerance was credited by the Nobel prize,but excluded Hasek, because hemisinterpreted his original experimentalresults. Hasek exuded an impressivepersonality and a much admired joie devivre. With the benefit of hindsight, thisarticle assesses the background of theperiod and the circumstances that led tothis important discovery. I discuss Hasek’sexperimental ingenuity, the influence of theLysenkoist genetic doctrine, Hasek’sacceptance of the immunological theory toexplain his work and his role in establishinga successful institute at the CzechoslovakAcademy of Sciences.

Milan Hasek was born on 25 October 1925 inPrague and went to school in the nearby townof Pardubice. During this time, under theinfluence of Emil Hadac, who was later a mem-ber of the Academy of Sciences and Professorin ecology and botanics, he developed his

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Milan Hasek and the discovery of immunological tolerance

Juraj Ivanyi

O P I N I O N

Box 1 | Lysenkoism: political misuse of the concept of epigenetic inheritance

The concept of inheritance of acquired traits, originally attributed to J. B. Lamarck, hasconclusively been replaced by the genetic laws that were formulated by Gregor Mendel.Nevertheless, even in the middle of the twentieth century, a few scientists still believed in‘epigenetic inheritance’ — that is, of environmental conditions that result in heritable changes ofgene expression3. The Lysenkoist doctrine proposed that traits that are acquired in response toan environmental factor would become “assimilated into the expressed genetic programmethrough selected enrichment”, and wrongly suggested that such genetic assimilation iscontradictory to Mendelian genetics. Unfortunately, this concept was grossly misused by T. D. Lysenko for political aims, with devastating effects on genetic research in general and inmedicine in particular. When the Soviet Union suffered food shortages in the 1930s, Stalinturned to scientists to remedy this situation using communist ideology. Whereas others feltunable to meet the agricultural targets and therefore declined, Lysenko developed a strategy forproducing grafted plants that improved the properties and yields in parts of the country with aharsh climate. He overinterpreted the grafting procedures that were successfully developed by I. V. Michurin and included them in a genetic doctrine. After Lysenkoism was adopted as ‘partypolicy’, it was repressively implemented, first in the Soviet Union and also, from 1948, in itsneighbouring countries, including Czechoslovakia. It has been suggested that the past anti-Mendelian influence of an important Czech geneticist, Vladislav Ruzicka, could have contributedto the presumed enthusiasm with which Lysenkoism was embraced by some geneticists41-43.However, this seems unlikely, as Ruzicka died in 1934 and anti-Mendelian sentiments did notexist between established geneticists in the1950s. So, Ruzicka’s pupil Bohumil Sekla was a leadingopponent of Lysenkoism and similar to all other geneticists who publicly rejected the doctrine,he was persecuted in various ways.

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environmentally induced genetic change,alternative molecular mechanisms by whichenvironmental conditions can cause heritablechanges have been elucidated only recently.For example, gene expression has been shownto involve stress proteins5, and this subject has been reviewed in a wider and historicalcontext6. This research is, however, an exten-sion of Mendelian genetics, in contrast to pre-viously misguided anti-Mendelian researchbetween 1920 and 1960, which was particularlydistorted under the Lysenkoist doctrine.

Hasek’s ingenious experimental designHasek set out to prove the inheritance ofacquired traits in vertebrates by developing anexperimental model of VEGETATIVE HYBRIDIZATION.This term was used by I. V. Michurin and subsequently in the doctrine of Lysenko withthe contention that grafting in plants led toheritable traits.

However, one of Michurin’s principles ofplant grafting turned out to be valuable:Michurin showed that the development ofmetabolism is most amenable to change byenvironmental factors in young plants7. It isconceivable that this knowledge led Hasek tomake the crucial decision to choose chickenembryos as his experimental model. Althoughthis model was chosen to maximize the envi-ronmental influences on the developingmetabolism, paradoxically, it also turned outto be crucial for the discovery of immunolog-ical tolerance. Other Lysenkoist scientists atthat time were changing the source of the eggwhite as the food for an embryo in attemptsto achieve vegetative changes. By contrast,Hasek opted for a new approach, that is tochange the source of the blood supply to theembryo. He was apparently aware of earlierstudies that showed that anastomosis of bloodvessels of the placenta in some twin cattle —an example of EMBRYO PARABIOSIS — resulted in hormonal influences. This led him to postulate that blood mediated the “mutualmetabolic assimilation between parabionts”— which he called ‘‘rapprochement’’ or‘‘approximation’’ — that might result in theinheritance of desired traits. He suggested thatparabionts, owing to the exchange of bloodcirculation during embryonic life, woulddevelop marked changes in their physiologyor metabolism8 (FIG. 2).

Having set out to develop a method forproducing chick-embryo parabionts with aclear purpose in mind, a crucial obstacle washow to join the blood vessels of two embry-onated eggs. Following the precedent set bygrafting in plants and knowing the powerfulangiogenic properties of embryonic tissue,Hasek initially implanted a small piece of

supervision. Although Hasek embraced theLysenkoist doctrine with outspoken enthusi-asm between 1950 and 1953, he is alsoremembered as having the audacity to expressin private and public his doubts about thereliability of some of the data from Lysenko’sgroup, and his intention to carry out a criticalexperimental assessment of their findings (P. Koldovsky, personal communication).

It seems, in retrospect, that apart from hispolitical interests, Hasek probably did have adeep-seated, although vague, hope from theoutset of his research until the end of his lifethat there might be some way by which theenvironmentally influenced gene expressionof somatic cells could be inherited. Such acontention is supported by recollections ofconversations with Hasek about EPIGENETICS3

by Avrion Mitchison, who worked with himin 1956, as well as my own recollectionsbetween 1963 and 1967, when I was a Ph.D.student in Hasek’s laboratory. And muchlater, Vladimir Holán — the last person withwhom Hasek collaborated before his death in1984 — remembers him taking a keen inter-est in Gorzcynski and Steele’s claim about the inheritance of IMMUNOLOGICAL TOLERANCE4

and of their failure to reproduce these data (V. Holán, personal communication). Itcould be argued that Hasek’s intuitive incli-nation to this subject was too far ahead of thedevelopment of the necessary technologies.Whereas mutagenesis is a well-recognized

research autonomy from his university peerSekla. This was representative of the generaltrend in the country, in which privileged fund-ing for research was given to the Institutions ofthe Academy, whereas the universities werereduced mainly to undergraduate teaching.

Hasek’s motivation to epigeneticsIt is entirely understandable, considering thepolitical atmosphere at the time, that inreturn for his privileged position, Hasek’sresearch aimed to advance the Lysenkoistgenetic doctrine, which was given top priority by the new communist regime inCzechoslovakia (BOX 1). With hindsight, it iseasy to imagine Hasek’s enthusiasm to achievesuch an objective that was considered to bepolitically important.

The main objective of Hasek’s researchwas to show the inheritance of environmen-tally acquired somatic traits in vertebrates.This revival of the Lamarckian mode ofinheritance was politically motivated againstMendelian genetics. It was formulated withthe claim that such evidence had already beenprovided in plants by T. D. Lysenko and hisassociates2. Hasek’s position in those ‘revolu-tionary’ times seems to be strikingly unconven-tional by today’s standards. Although he had only just started working towards hisCandidate of Sciences (C.Sc.) (equivalent toPh.D.) thesis, he was already leading a smallgroup of staff entirely without senior academic

Figure 1 | Photograph of Bohumil Sekla. Sekla was an uncompromising proponent of Mendelian geneticsat a time when this was forbidden. His brave lectures to medical students on Mendel’s laws were anexpression of political dissent. He was Hasek’s original mentor, but later adversary, when Hasek joined theLysenkoist party establishment. This photograph shows him in 1953, giving a lecture to medical students.

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politically promoted, but scientifically longdisproved, Lamarkian genetic concept; sec-ond, his choice of embryos as an experimentalmodel; third, the application of a plant-basedrationale for grafting of a blastoderm-tissuebridge for joining the eggs; and fourth, hisawareness of immunological monitoring todetermine the relatedness between individu-als, which led him to carry out immunologicaltesting of the supposed ‘‘mutual metabolicassimilation-approximation’’ between the surviving chick-embryo parabionts.

How have Hasek’s discoveries contributedto the subsequent elucidation of the mecha-nisms of tolerance? Perhaps the most impor-tant aspect, which was postulated neither byFrank McFarlane Burnet’s theory nor byMedawar’s work, was that inducibility oftolerance diminishes with the taxonomicdistance between the parabiotic partners11−13

— that is, tolerance induction depends onthe relatedness between the tolerogen andhost-protein structure14. This has direct rele-vance for xenogeneic-organ transplantation(xenotransplantation), in which induction oftolerance is one of the main obstacles.Considerable progress has been made recentlyin both fundamental and applied aspects ofxenotransplantation15.

Hasek’s interpretation of his dataHasek’s original publication8 was printed inCzech, but strangely first reached Westernreaders in an edition of the same journal thatwas published in Russian with a summary inGerman16. The crucial finding of this paperwas succinctly recorded with the words: ‘‘thepresence of partner’s agglutinogens duringparabiosis led to the lack of antibody responsein adult age’’. He described this finding as‘‘extraordinary’’, indicating his awareness of itsimportance among several other unremark-able and distracting results in the same paper.Despite this entirely correct description of the

blastoderm tissue, which was removed fromembryos that had been incubated for 20−40hours, onto the chorioallantoic membrane ofeggs that had been incubated for 8−12 days.With the blastoderm tissue engrafted, he usedit to make a bridge between the egg pairs. Theuse of the blastoderm bridge was one of thecrucial technical elements that led to the suc-cessful hatching of chick-embryo parabionts.

Hasek knew that genetic similarity couldbe measured by the speed of graft rejection9.This knowledge might explain why he decidedto test the immune responsiveness of hatchedchick parabionts against constituents of theother as evidence of achieving metabolicapproximation. So, immunological testing wasan important element of his initial experi-mental design. He cross-immunized thehatched parabionts with blood from the part-ner chick and showed their failure to produceantibodies specific for their partner, but not

for red blood cells from an unrelated chick.This was essentially an indication of immuno-logical tolerance. However, having a differentaim at the outset, Hasek initially interpretedthe observed specific lack of immune respon-siveness as evidence of a ‘‘rapprochement ofmetabolism’’ between the developing para-bionts. Hasek’s crucial paper8 was submitted tothe journal of Czechoslovak Biology (in Czech)five months before, but published two monthsafter, the publication, in Nature, of the firstpaper that described immunological tolerancefrom Peter Medawar’s laboratory10 (TIMELINE).

Although Hasek noted the importance ofthe marked specific immunological featuresof parabionts, he failed to interpret them inan immunological context. In retrospect,it might be concluded that Hasek’s unequivo-cal demonstration of the phenomenon ofimmunological tolerance resulted from severalcoincidences: first, his intention to prove the

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Egg 1 Egg 2

Embryo

Egg shell

Blastoderm bridge

Extraembryonalblood vessels

Scheme parabiosis

Figure 2 | Embryo parabiosis. Embryo parabionts were obtained by vascular anastomosis between apair of chick embryos. After making a small opening in the egg shells, the chorioallantoic membranes ofembryos that had been incubated for 8−12 days were joined by a bridge of tissue from a 20−40-hour oldblastoderm. Rapid vascularization of the bridge by blood vessels from both embryos leads to theinterchange of blood between the embryos. This results in a higher degree of immunological tolerancethan injection of allogeneic cells into the chorioallantoic vein. Parabiosis is interrupted physiologically afterhatching, when the embryonic circulation is separated from the embryos29. The photograph shows thesealing of the joined eggs with paraffin after insertion of the blastoderm bridge17 between two eggs.Reprinted from Milan Hasek’s thesis.

1950 1952 1953 1954 1955 1960 1961

Timeline | History of events related to Hasek’s research on immunological tolerance

Hasek qualifies inmedicine andembarks on hisresearch project

Hasek’s method ofembryo parabiosispaper submitted

Publication of Hasek’s keypaper showing the specificlack of antibody responsein chick parabionts

Publication of the key paperon immunological toleranceby Billingham et al. in Nature

Publication of Hasek’s firstimmunological interpretationof specific immune tolerancein chick parabionts

The Nobel Prize for the discovery of immunologicaltolerance awarded only toMedawar and Burnet.

Hasek receives the prestigious ‘State Award’, organizes the first International Symposium on theMechanisms of Immunological Tolerance in Liblice(near Prague) and his group is expanded as a separate Institute of the Academy of Sciences.

Publication of the firstpaper of Hasek & Hrabain English in Nature.

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This indicates that Hasek accepted and imple-mented the immunological theory for theongoing research in his laboratory and inhis publications as early as the first half of1954. However, he continued to proclaim hisloyalty to Lysenkoism on politically chargedoccasions — for example, in defence of apublished criticism of his book20,21 — for atleast another one or two years. This carefulapproach would have been necessary to sus-tain the official backing and substantialfunding for his steadily expanding group,which was consequently upgraded in 1961to a relatively large Institute at the Academyof Sciences. During the following years,Hasek and his group gained internationalrecognition, which is evident by the organi-zation of an important conference on themechanisms of tolerance in November1961 in Liblice near Prague22 and by Hasekbeing invited to write an authoritativereview article for Advances in Immunology23.At home, the achievements of his team wereacknowledged by presentation of the presti-gious State Award. Despite this recognition,Hasek had to continue to use great politicalskills in precarious circumstances, as theLysenko doctrine remained politicallyinfluential in Czechoslovakia as late as1966. Even at that time, it had taken couragefor Hasek to sign a petition against a dog-matic article propagating the Lysenkoistdoctrine in the leading Czech newspaperRude Pravo.

to specific immune unresponsiveness fol-lowing the inoculation of lymphoid cells tomouse, and also chick, embryos in a paper by Rupert Billingham, Lesley Brent andMedawar10. This historical publication iswidely quoted for the experiments in mice,whereas their use of chick embryos was some-how forgotten. The paper also had a markedinfluence on Hasek, who only ten monthsafter its publication (July 1954) submitted hisnext paper to Czechoslovak Biology in whichhe essentially adopted the immunologicalinterpretation for his published data on theacceptance of skin grafts between chick para-bionts18. This paper contained up-to-dateimmunological concepts, with no referenceto the genetic doctrine, and was entitled‘‘vegetative rapprochement’’ instead of ‘‘vege-tative hybridization’’. This indicates thatHasek had already accepted the immunolog-ical interpretation eight months beforemeeting Medawar in April 1955 (FIG. 3).Therefore, his turning point must haveoccurred before that meeting, although thepersonal encounter undoubtedly had furtherpositive influence on Hasek’s thinking16. Inthe concurrent first publication in an inter-national journal with Tomas Hraba, no ref-erence was made to vegetative hybridizationand the authors avoided giving any name tothe experimental results19. Eventually, a yearlater, Hasek used the term tolerance for thefirst time12, while formulating both papers inexclusively immunological terms.

results, the paper interpreted this crucial find-ing in terms of vague ‘‘metabolic’’ effects,rather than specific immunological mecha-nisms. Interestingly, heritability — the corner-stone of Lysenko’s doctrine — althoughinferred in the misleading title of the paper,‘‘vegetative hybridization’’, and in the intro-duction, was not discussed in association withthe obtained experimental results. The doc-trine was more forcefully presented, however,in Hasek’s Ph.D. thesis17. Here, he quotedextensively the papers by Lysenko, but did notquote the epigenetic research that has beencarried out in the West3. However, Hasek nei-ther attempted, nor even indicated any inten-tion, to mate his parabionts to find out iftheir rapprochement would be inherited bytheir progeny. His failure to do so is surpris-ing, considering that this would have beentechnically entirely feasible. It remains unclearwhether he did not attempt this due to possiblescepticism about the experimental prospectsfor genetic transmission of the observedimmunological traits. Furthermore, he empha-sized in his paper the metabolic assimilationor HETEROSIS manifestations, rather than the‘‘presumed’’genetic implications from his para-bionts. This leaning of Hasek’s original inter-pretations towards the physiology of acquiredtraits could be referred to as Michurinist,rather than Lysenkoist, as only the latteremphasized the inheritance of the acquiredtraits (A. McLaren, personal communication).

The discovery of immunological toler-ance is generally attributed to Medawar andhis colleagues, for showing the acceptanceor prolonged survival of skin allografts owing

Glossary

EPIGENETICS

The study of heritable changes in gene expression thatare not caused by changes in the DNA sequence itself.

IMMUNOLOGICAL TOLERANCE

Lack of immune responsiveness to a specific antigen,due to an active process by the immune system whenpre-exposed to antigen under suitable conditions —for example, during early ontogeny.

VEGETATIVE HYBRID

A chimaeric animal with some acquired cells thatcontain alleles different from host cells. It is distinctfrom a sexual hybrid (heterozygote), which has twoalleles of each gene pair in each cell.

EMBYRO PARABIOSIS

The joining of blood vessels during embryonic life,leading to the exchange of blood between geneticallydifferent embryos. First achieved by Hasek using ablastoderm bridge for the joining of eggs.

HETEROSIS

The superiority of a heterozygous phenotypecompared with that of the homozygous parents.Also known as hybrid vigor.

Figure 3 | Photograph of Peter Medawar, J. Z. Young, Milan Hasek and Leslie Brent (from left toright) at the Embryological Conference in Brussels, 4–7 April 1955. Here, Hasek and Medawar met for thefirst time.

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very wrong that you are not sharing in thisprize’’16. However, he qualified Hasek’s work asmerely having ‘‘reproduced Owen’s phenome-non in chickens’’, although acknowledging inhis much later memoirs that Hasek ‘‘discov-ered tolerance independently of ourselves’’30.Despite an obvious friendship between thetwo men, it seems that on occasions thatmattered, Medawar did not doubt his viewthat there could not be any degree of sharedrecognition.

Impact of Hasek’s group on immunologyAfter his original discovery, Hasek encouragedhis colleagues to carry out research that led toimportant contributions to a wide range ofdisciplines, including general immunology,immunology of transplantation, immunologyof cancer, virology and genetics31. The broadrealm of his influence reflected his personalinclination and style, which involved the selec-tion of spirited and imaginative scientists, giv-ing them the freedom to follow their owninterests and enjoying intellectual discoursewith them on a broad range of research topics.Apart from the main application in tissuetransplantation, the discovery of tolerance toRous sarcoma virus made a fundamental con-tribution to research on the mechanisms ofmalignant transformation of cells32. The confi-dence in his group was uplifted by visits fromseveral important scientists from the West.McLaren and Michie, having heard aboutHasek’s work from Jaroslav Sterzl (Head of aseparate department at the Academy inPrague), were the first to make Medawaraware of Hasek’s experiments and visited himin 1954 (FIG. 4). Mitchison spent six months inPrague in 1956. He remembers Hasek as a lov-able person who was full of ideas, easy to talkto and quite unclouded by politics. Hasek builtthe institute at the Academy of Sciences (fullyfunded by the government), having attractedseveral talented young scientists. When askedhow such a conglomerate came into existence,Jan Klein wrote in his obituary on Hasek: ‘‘Myanswer is simple: it was all Hasek’s doing. Hehad the magnetic personality and his enthusi-asm for science was boundless’’33. The facilitiesof the institute supported an extensive breed-ing programme of inbred and MHC-congenicstrains of mice and chickens, which matchedor even exceeded similar progammes in theUnited States and United Kingdom. Finally, itshould be noted that by having a strong posi-tion in the Communist Party, Hasek couldappoint scientists on the basis of their acade-mic merits and ignore the political rules ofsocial-class discrimination that existed at thetime. I had the good fortune of being recruitedto the privileged environment of the Academy,

The Nobel Prize for toleranceIn recognition of the importance of the phe-nomenon of immunological tolerance, theNobel Prize in Physiology or Medicine wasawarded in 1960 to Medawar for reportingthe acceptance of skin allografts in mice andchickens that had been pre-inoculated duringembryonic development with allogeneic lym-phoid cells10, and to Burnet for first proposingthat exposure to antigens before the develop-ment of immune responsiveness, actively andspecifically abrogates the capacity to respondto that antigen in later life24. Both Burnet’stheory and Medawar’s experiments were cru-cially influenced by the earlier observation ofred-cell chimaerism — that is, tolerance of agenetically different tissue — in dizygotictwin cattle by Ray Owen25. Paradoxically,Hasek misinterpreted Owen’s findings interms of vegetative hybridization17, but it isdifficult to know whether he became aware ofOwen’s work before or after he carried out thechick parabiosis experiments. As it wasBurnet who formulated the immunologicalexplanation for Owen’s experiments (thoughfailed to prove it experimentally26), it is possi-ble that theoretical scientific inventivenesswas more highly merited by the NobelCommittee than merely the description ofnew observations. L. D. Felton’s earlier indi-cation of immunological paralysis in adultanimals induced using large doses of polysac-charides should also be acknowledged27. Thismight have been considered at the time ofthe award to be a different phenomenon,

although subsequent research showed thatthe mechanisms of neonatally induced toler-ance and paralysis in adults are interrelated,and the term paralysis was later substituted bythe almost universally used term tolerance.Several possible reasons why Hasek did notget the Nobel Prize have been discussed previ-ously28, and it has been tenuously suggestedthat the political atmosphere that surroundedthe cold war might not have been to hisadvantage29. It could also be speculated thatthe committee found it difficult to choosebetween Owen and Hasek, but contentiousissues raised by the referees and/or membersof the Nobel Committee will have to waituntil 2011, when the Nobel Medical Archives,which are kept in confidence for 50 years, canbe opened.

Medawar first heard about Hasek’s workfrom personal contacts between British andCzech scientists, Anne McLaren and DonaldMichie, in the autumn of 1953. He then firstmet Hasek at an Embryology Conference inApril 1955 in Brussels (FIG. 3), and invited himto the first conference that was dedicated toimmunological tolerance in London the fol-lowing year. However, Medawar quoted Hasekin his 1958 Croonian lecture only in a foot-note and not at all in a later introductory lec-ture22. On his way to receive the Nobel Prize inStockholm, Medawar travelled throughPrague, where a post-celebratory drive inHasek’s car was terminated by crashing into aroadworks barrier. Medawar gave high creditto Owen when writing to him: ‘‘I think it is

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Figure 4 | Anne McLaren and Donald Michie visiting Milan Hasek’s department in Prague-Dejvicein 1956. McLaren and Michie, from the United Kingdom, were the first Western scientists to learn aboutHasek’s work from Jaroslav Sterzl, and they reported on it verbally to Peter Medawar in the autumn of1953. McLaren retained her research interest in the epigenetic mechanisms of inheritance. In thebackground, Hasek is seen talking to Jan Svoboda, who later became director of the Institute (1991–1999). The photograph was taken by Ludek Martinek, who was Hasek’s research assistant for the early parabiosis experiments and for many years subsequently.

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discovery for clinical organ transplantationmight be realized. Although funding for pre-sent research sometimes favours proposalsthat are too feasible, with quasi-predictable‘deliverables’, the history of Hasek’s entry to sci-ence is an example of how important discover-ies might sometimes be made on the ground offalse premises30.

Juraj Ivanyi is at the Guy’s Campus of KingsCollege London, 28 Floor Guy’s Tower,Guy’s Hospital, London SE1 9RT, UK.

e-mail: juraj.ivanyi@kcl.ac.uk

doi:10.1038/nri1133

1. Grmek, M. D., Cohen, R. S. & Cimino, G. On scientificdiscovery. in The Erice lectures 3 (Dordrecht, 1981) .

2. Glushchenko, I. E. Vegetative Hybridization of Plants(Moscow, 1951) (in Russian).

3. Waddington, C. H. Canalization of development and theinheritance of acquired characters. Nature 150, 563–565(1942).

4. Gorczynski, R. M. & Steele, E. J. Inheritance of acquiredimmunological tolerance to foreign histocompatibilityantigens in mice. Proc. Natl Acad. Sci. USA 77,2871–2875 (1980).

5. Rutherford, S. L. & Lindquist, L. Hsp90 as a capacitatorfor morphological evolution. Nature 150, 563–565 (1998).

6. McLaren, A. Too late for the midwife toad. Stress,variability and hsp90. Trends Genet. 15, 169–171 (1999).

7. Vinogradova, T. V. Basics of Michurinist Biology (TextbookPublisher for Ministry of Education, Leningrad, 1950) (inRussian).

8. Hasek, M. Vegetative hybridization of animals by jointblood circulation during embryonal development. Cs. Biol. 2, 265–277 (1953) (in Czech).

9. Harris, M. Specificity and mode of action of cytotoxinsproduced against alien transplants in rats. J. Exp. Med.107, 439–449 (1948).

10. Billingham, R. E., Brent, L. & Medawar, P. B. Activelyaquired tolerance of foreign cells. Nature 172, 603–606(1953).

11. Hasek, M. & Hraba, M. The significance of phylogenickinship in immunological approximation duringembryogenesis. Folia Biol. 1, 1–10 (1955).

12. Hasek, M. Tolerance phenomena in birds. Proc. R. Soc.B 146, 67–77 (1956).

13. Hasek, M., Hort, J., Lengerova, A. & Vojtiskova, M.Immunological tolerance in the heterologous system.Folia Biol. 9, 1–30 (1963).

14. Ivanyi, J. & Valentova, V. The immunological significanceof taxonomic origin of protein antigen in chickens. FoliaBiol. 12, 36–48 (1966).

15. Sebille, F., Dorling, A. & Lechler, R. I. The cellular rejectionof xenografts: recent insights. Xenotransplantation 10,4–6 (2003).

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22. Hasek, M., Lengerova, A. & Vojtiskova, M. Mechanismsof Immunological Tolerance (Czechoslovak Academy ofSciences, Prague, 1962).

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90% of HLA-mismatched kidney grafts.However, the need to avoid the serious sideeffects that are associated with generalimmunosuppression led to the recent revival ofresearch aiming to achieve antigen-specificimmune tolerance37. The clinical protocols thatare being developed use refined combinationsof antibodies, such as CamPath1 (specific forCD52), and drugs, such as rapamycin, whichallows the selective deletion of T cells38−40. Thehigh level of interest in this approach is indi-cated by the initiation of coordinated collabo-rative clinical trials that include the ImmuneTolerance Network and the National Institutesof Health initiative in the United States, whichare targeted at CD28 and CD40 co-receptorblockade. Despite outstanding hurdles in prov-ing that principles from rodent models can betransferred to humans without undue sideeffects, the common strategy behind thisresearch is to partially deplete T cells withimmunosuppressive agents and thereby inducespecific immunological tolerance.

Concluding remarksHasek’s experimental approach combinedintellectual ingenuity with technical excel-lence, and he rapidly adopted the correctinterpretation of his discovery. He proved hisleadership capacity by harnessing and exploit-ing both his research achievements and hispolitical influence for obtaining governmentfunding to build a substantial and successfulresearch group. Both before and after severalmembers of his group moved around theworld, they have made a considerable impactin the field of immunology. Following severaldecades of basic and clinical research efforts,there is now hope that the legacy of this

despite being rejected from university onpolitical grounds. Subsequently, I have realizedthat coming from Hasek’s ‘stable’ has helpedopen doors for me — first in the United Statesand later in the United Kingdom, at theNational Institute for Medical Research underMedawar’s directorship in 1968.

After the invasion of Czechoslovakia bythe Soviet Union in 1968, a large number ofHasek’s colleagues were dispersed around theworld, whereas others stayed and continuedto work at home under difficult circum-stances, in which most of the previously lib-eral atmosphere had gone. Notably, 24 of thescientists who emigrated, all with establishedpositions in different countries, met in theNetherlands in 1988. Still bound by theirtraining experiences and warm reminiscencesof Hasek (FIG. 5), they presented lectures, aswell as personal tributes to his impressive per-sonality and anecdotes, both published in theproceedings from the meeting34.

In personal letters, Hasek wrote to me in1970: ‘‘I think matters cannot get worse. Herethey are managing a lot and often stupidly; Iwas expelled from the Party, demoted fromdirectorship and the Council of the Academy;instead of chasing large numbers of publica-tions we get the opportunity to think overmatters from an appropriate perspective’’.

Clinical application of toleranceAn article on the history of the discovery ofimmunological tolerance would be incom-plete without addressing its impact in the lightof present knowledge.Although the inductionof immunological tolerance was originallyassociated with the immature immune systemduring early ontogeny, it soon became clearthat it can also be achieved in adult animalsthat have been immunocompromised by sub-lethal irradiation and given a suitably highdose of antigen35. The expected application ofimmunological tolerance for transplantationof human organs has taken longer to succeedthan was originally envisaged, owing to thetime it took to resolve the fundamental mech-anisms of allograft immunity, mainly throughresearch in genetically controlled strains ofmice, and the difficulties in translating thesefindings in rodents to large animals andhumans. Nevertheless, in an excellent review ofthe subject, Robert Lechler and his colleaguesconvey an optimism about the prospects, andexplain why this is greater now than at anytime during the intervening five decades36.

Clinical organ transplantation has suc-ceeded with the use of non-specific immuno-suppressive drugs — initially, azathioprine andprednisolone and, later, cyclosporine — whichenable long-term survival of approximately

Figure 5 | Photograph of Milan Hasek.

© 2003 Nature Publishing Group

P E R S P E C T I V E S

AcknowledgementsI would like to thank V. Hasková, A. McLaren, M. Havlik, P. Koldovsky, A. Mitchison, J. Klein, P. Ivanyi, R. Lechler, V. Holán,J. Sekerák, J.Sterzl, I. Hilgert and L. Martinek for valuable help inwriting this article.

Online Links

DATABASESThe following terms in this article are linked online to:LocusLink: http://www.ncbi.nlm.nih.gov/LocusLink/CD28 | CD40 | CD52

FURTHER INFORMATIONPeter Brian Medawar Nobel Lecture: http://www.nobel.se/medicine/laureates/1960/medawar-lecture.htmlImmune tolerance network:http://www.immunetolerance.orgImmune tolerance: http://www.niaid.nih.gov/publications/discovery/immune.htmAccess to this interactive links box is free online.

27. Felton, L. D. The significance of antigen in animal tissues.J. Immunol. 61, 107 (1949).

28. Matalova, A. in Biology Integrating ScientificFundamentals (ed. Hoppe, B.) 423–431 (Institute furGeschichte der Naturwissenschaften, Munich, 1997).

29. Havlik, M. Milan Hasek — the discoverer ofimmunological tolerance. Thesis in History Gymnasium ofJan Keppler (Prague, 1998) (in Czech).

30. Medawar, P. B. Memoir of a thinking radish. AnAutobiography (Oxford University Press, 1984).

31. Hasek, M. Research report 1959−1966 of the Institute ofExperimental Biology and Genetics. Folia Biol. 13,208–244 (1967).

32. Svoboda, J. in Mechanisms of Immunological Tolerance(ed. M. Hasek, A. Lenerova and M. Vojtiskova) 199–210(Czechoslovak Academy of Sciences, Prague, 1961).

33. Klein, J. In memoriam Milan Hasek (1925−1984).Immunogenetics 21, 105–108 (1985).

34. Ivanyi, P. Realm of Tolerance (Springer–Verlag, Berlin, 1989).35. Michie, D. & Woodruff, M. F. A. Induction of specific

immunological tolerance of homografts in adult mice bysublethal irradiation and injection of donor type spleen cellsin high dosage. Proc. R. Soc. B 156, 280–288 (1962).

36. Lechler, R. I., Garden, O. A. & Turka, L. A. Thecomplementary roles of deletion and regulation intransplantation tolerance. Nature Rev. Immunol. 3,147–158 (2003).

37. Thomas, F., Ray, P. & Thomas, J. M. Immunologicaltolerance as an adjunct to allogeneic tissue grafting.Microsurgery 20, 435–440 (2000).

38. Li, Y. et al. Blocking both signal 1 and signal 2 of T-cellactivation prevents apoptosis of alloreactive T cells andinduction of peripheral allograft tolerance. Nature Med.5, 1298–1302 (1999).

39. Kahan, B. D. The matrix of clinical immunosuppression fortransplantation. Transplant Proc. 33, 3035–3037 (2001).

40. Jonker, M. et al. Long-term kidney graft survival bydelayed T cell ablative treatment in rhesus monkeys.Transplantation 73, 874–880 (2002).

41. Sekerák, J. So-called vegetative hybridization as ameans of achieving genetic change tested on animals inPrague. Folia Mendeliana 81, 29–32 (1996−1997).

42. Janko, J. Anti-Mendelism in Bohemia and Moravia. FoliaMendeliana 81, 17–27 (1996−1997).

43. Sekerák, J. Lysenkoism in Czechoslovakia. FoliaMendeliana 83, 79–88 (1998−1999).

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