Volume 116, Issue 5 , Pages 1171-1174, November 2005
How the immunologic function of the thymus was discovered
Article Outline
The thymus has only comparatively recently yielded the secrets of its function. Indeed, before the early 1960s, it was considered to be an organ that had become redundant during the course of evolution and perhaps just a graveyard for dying lymphocytes. Even in the late 1950s, when its lymphopoietic function had been well established, immunologists were unwilling to attribute to it any immune function. Thus unlike recirculating small lymphocytes and unlike lymphocytes from the spleen and lymph nodes, thymus lymphocytes could not initiate significant immune reactions after adoptive transfer to appropriate recipients. Antibody-forming plasma cells and germinal centers, so prominent in the spleen and lymph nodes, never appeared in normal thymus tissue. Mice thymectomized in adult life were perfectly able to respond immunologically, a fact that had led some groups to conclude that “the thymus gland does not participate in the control of the immune response.”1
In 1958, having recently graduated in Medicine from the University of Sydney, Australia, and being totally ignorant about the thymus and its function, I arrived in London with a scholarship that enabled me to study for the PhD degree at the Chester Beatty Research Institute, an institute for cancer research. Many of the scientists there were busy searching for new chemical carcinogenic compounds. Adding more compounds to an ever-growing list of such agents did not interest me because I would rather have worked on some model in which pathogenetic mechanisms had to be investigated. Hence I felt rather frustrated. I was then told that the institute had 2 satellites outside greater London, one being at a place called Pollards Wood at Chalfont St Giles in Buckinghamshire. There Dr R. J. C. Harris was studying the development of sarcomas in turkeys caused by the Rous sarcoma virus, a line of investigation that interested me. So I visited him. Instead of joining his group, he suggested that I might be willing to investigate the pathogenesis of lymphocytic leukemia induced in mice by what was presumed to be a virus that had recently been discovered by Ludwig Gross in the United States. This suited me perfectly. As a PhD student, I was under his supervision, although my official supervisor had to be a full professor of the University of London, which in my case was Professor Sir Alexander Haddow, the director of the Chester Beatty Research Institute, who for 1 year appeared quite unaware of my existence. Six months after my arrival, Harris was appointed director of the Division of Virology of the Imperial Cancer Research Funds at Mill Hill, London. I was therefore left without an immediate supervisor but very fortunate to acquire his animal space and a small shack (Fig 1).
Fig 1.
The Shack at Pollards Wood. In the late 1950s and early 1960s, many of the experiments with neonatally thymectomized mice were planned and done in this shack and in adjoining converted horse stables. Reprinted from Immunology: The Making of a Modern Science. Gallagher RB, Gilder J, Nossal GJV, Salvatore G, editors. London: Academic Press; 1995. p. 75-84. With permission from Elsevier.
Gross had claimed that extracts of mouse leukemic tissues contained a virus responsible for causing the disease. To induce a high incidence of leukemia in low leukemic strain mice by the Gross virus in those early days, it was essential for the extracts to be given at birth. I was able to show that the disease failed to develop in neonatally inoculated mice when thymectomized at 4 to 5 weeks of age but did so when subsequently grafted with syngeneic thymus tissue from newborn donors. What was most interesting to me was the finding that implanting the thymus as late as 6 months after adult thymectomy still allowed leukemic transformation.2 Evidently, the virus must have remained latent, and I next showed that it could be recovered from the healthy nonleukemic tissues of neonatally inoculated mice thymectomized at 5 to 6 weeks of age.3 I wondered why the virus had to be given at birth and not later and in what tissue it multiplied. If leukemic transformation occurred only if it could first multiply in the developing thymus, this might explain why it had to be given at birth. Thymectomy performed 5 weeks later might have removed the source of the malignant cells but not the virus that would have disseminated widely and thus become available to invade a subsequently grafted neonatal thymus and transform its cells. If this were true, I expected that neonatal mice lacking a thymus from birth would not be able to support virus multiplication and hence would not have leukemia when grafted later with thymus tissue. To test such a hypothesis, I had to adapt my technique of adult thymectomy to neonatal mice. With practice, I was able to do so with little surgical mortality, although cannibalism by the mothers caused a heavy toll.
The survivors seemed healthy at first, but many lost weight and died 1 to 3 weeks after weaning, irrespective of whether they were inoculated with virus. By contrast, mice subjected to adult thymectomy had never shown any signs of ill health. As a result of my findings on neonatally thymectomized mice, I boldly concluded that rather than being a useless organ, “the thymus at birth may be essential to life.”4 What followed was quite simple: I obviously had to examine histologically the tissues of these mice. The results were clear: there was a marked deficiency of lymphocytes in blood and lymphoid tissues and lesions in the liver that suggested infection by some type of hepatitis virus. Because Gowans had recently shown that circulating small lymphocytes were immunologically competent cells able to reject foreign skin, it seemed obvious that I should test the immune response of my mice. They were grafted with skin from foreign (H-2 incompatible) strains of mice and from rats. They failed to reject such skin and failed to do so even when grafted before the onset of wasting. The results were so spectacular that I concluded, in a preliminary report in which my results were published, that “during embryogenesis the thymus would produce the originators of immunologically competent cells many of which would have migrated to other sites at about the time of birth. This would suggest that lymphocytes leaving the thymus are specially selected cell.”5 Such a bold conclusion was indeed contrary to the prevailing dogma. This was the very first publication providing data indicating an immune function for the thymus. Before the end of 1961, I sent much more detailed data to the Proceedings of the Royal Society, Series B, showing (1) the effects of neonatal thymectomy on body weights, lymphocyte populations, cellular immune responses, and antibody formation; (2) the restoration of immune competence by implanting syngeneic thymus grafts; (3) the presence of thymus graft–derived cells in the lymphoid tissues of neonatally thymectomized recipients of thymus tissue bearing chromosomally marked cells; and (4) the acquisition of immunologic tolerance to donor tissue by means of the implantation of allogeneic thymuses. These results were received by the journal on January 5, 1962, and published in May of that year.6 I also showed that even the adult thymus was still required to allow recovery of the immune system after it had been damaged by irradiation.7
What was the reaction of the immunologic community to such new and revolutionary findings? Most could not find flaws in my data, but many regarded my conclusions with some degree of skepticism. The famous immunologist and Nobel Laureate, Sir Peter Medawar, was not convinced, as shown in a letter he wrote to me in 1961 (Fig 2), and as he later reiterated in the discussion of a paper I presented in London: “We shall come to regard the presence of lymphocytes in the thymus as an evolutionary accident of no very great significance.”8 Nevertheless, he was very supportive of my work and urged me to continue. He favored the idea that the thymus was an epithelial endocrine organ able to exert some noncellular humoral influence on peripheral lymphopoiesis and hence on the development of the immune system. Other immunologists were less inclined to accept my concept of thymus function. There were many strange and trivial criticisms. For example, some claimed that what I had observed could occur only in the strain of mice I had been using or that my mice must have been in such poor health that any surgical stress would prejudice their immune competence or that whatever the thymus might have been doing in my mice, it could not possibly do in human subjects. Others even claimed that they had done my experiments, and their mice remained perfectly health, were not susceptible to infection, and rejected foreign skin grafts.
Fig 2.
Peter Medawar's letter in reply to one in which I had briefly summarized the effects of neonatal thymectomy. The letter predates the first publication of the effects of neonatal thymectomy.5 Reprinted from Immunology: The Making of a Modern Science. Gallagher RB, Gilder J, Nossal GJV, Salvatore G, editors. London: Academic Press; 1995. p. 75-84. With permission from Elsevier.
In 1963, I was awarded an Eleanor Roosevelt International Fellowship, enabling me to work for 1 year at the National Institutes of Health, in Bethesda, Maryland, in Dr Lloyd Law's department. There I had access to germ-free mice and thymectomized them at birth to test their immune competence in the absence of any possible infection. These mice remained healthy after weaning but were still unable to reject foreign skin grafts.9 I was thus able to squash any suggestion that my findings pertained only to mice that were unhealthy. In addition, with Law and collaborators, I consolidated some earlier observations I had made and published in Nature, indicating that mice lacking a thymus from birth were much more prone to experience neoplasms, thus adding weight to Burnet's hypothesis of immunologic surveillance.
My results on the induction of specific tolerance in neonatally thymectomized mice after the implantation of allogeneic thymus tissue6 had far-reaching implications: lymphocytes, developing in the thymus graft in the presence of foreign cells (the epithelium of the graft does not completely degenerate after grafting) and with reactivity for the antigens on those cells, must have been deleted (a “selective immunological thymectomy,”6 as I stated). Hence by implication, the thymus might well be the site at which self-nonself discrimination occurs and self-tolerance is imposed. This idea received strong support from Nobel Laureate Sir Macfarlane Burnet, who had personally visited me in London and was in fact one of the few immunologists who believed in the role of the thymus in immunity.
In the late 1950s and early 1960s, Professor Koller's team at the Chester Beatty Research Institute in South Kensington was investigating the effects of heavy doses of irradiation on the regeneration of the hemopoietic system after an intravenous injection of syngeneic or allogeneic bone marrow cells. I approached him and persuaded him and his group to collaborate with me to test the hypothesis that in “lethally” irradiated mice thymectomized as adults, only the hemopoietic tissues and not the lymphoid tissues would regenerate after bone marrow injection. The experiments were done, and as predicted, adult thymectomized, lethally irradiated, and bone marrow–protected mice were immunoincompetent in contrast to sham-thymectomized, irradiated, and marrow-protected mice, which regained immunocompetence in less than 2 weeks after irradiation.10 I found it strange that some time later Burnet wrote in his book Auto-immunity and Auto-immune Disease (published in 1972)11 that he had reservations “about the significance of results obtained in such biological monstrosities as pure line mice thymectomized, lethally irradiated, and salvaged by injection of bone marrow from another mouse.” Was he wondering whether such complicated experimental manipulations could ever relate to normal physiology? For, of course, never in the history of human ailment (until then) had any disease been seen to mimic the effects of adult thymectomy combined with irradiation. Evidently Burnet did not foresee the coming of the disease AIDS! In spite of Burnet's rather strongly worded reservations, the technique of adult thymectomy, irradiation, and marrow protection has been used continuously for numerous experiments in cellular immunology, and a great deal has been learned about the immune system from its use. It was in fact by using such monstrosities that my first PhD student and I discovered that lymphocytes were not a single uniform population of cells, as previously widely believed, but belonged to 2 separate cell populations with distinct functions, now known as T and B cells. This will be the subject of a subsequent Allergy Archives report.
References
- . The role of the thymus in antibody production: an experimental study of the immune response in thymectomized rabbits. Transpl Bull. 1956;4:21–22
- . Fate of subcutaneous thymus grafts in thymectomized mice inoculated with leukaemic filtrates. Nature. 1959;184:1809–1810
- . Recovery of leukaemogenic agent from non-leukaemic tissues of thymectomized mice. Nature. 1960;187:703
- . Analysis of the thymus influence in leukaemogenesis. Nature. 1961;191:248–249
- . Immunological function of the thymus. Lancet. 1961;2:748–749
- . Effect of neonatal thymectomy on the immunological responsiveness of the mouse. Proc R Soc Lond B Biol Sci. 1962;156B:415–428
- . Immunological significance of the thymus of the adult mouse. Nature. 1962;195:1318–1319
- . Role of the thymus in the origin of immunological competence. In: Wolstenholme GEW, Knight J editor. The immunologically competent cell: its nature and origin. London: Ciba Foundation Study Group; 1963;p. 70
- . Pathogenesis of the post-neonatal thymectomy wasting syndrome. Nature. 1964;204:151–155
- . Role of the thymus in the recovery of the immune mechanism in the irradiated adult mouse. Proc Soc Exp Biol Med. 1963;112:785–792
- Burnet FM. Auto-immunity and auto-immune disease. Lancaster, United Kingdom: MTP; 1972. p. 45.
PII: S0091-6749(04)03117-3
doi:10.1016/j.jaci.2004.11.039
Volume 116, Issue 5 , Pages 1171-1174, November 2005
