788
12. Ladenheim ML, Kotler TS, Pollock BH, Berman Incremental prognostic power of clinical
DS, Diamond GA. history, exercise
electrocardiography and myocardial perfusion scintigraphy suspected coronary artery disease. Am J Cardiol 1987;
in 59:
270-77. 13. Dunn RF, Freedman
B, Bailey IK, Uren R, Kelly DT. Noninvasive prediction of multivessel disease after myocardial infarction.
Circulation 1980; 62: 726-36. 14. Gibson RS, Taylor GJ, Watson DD, et al. Predicting the extent and location of coronary artery disease during the early postinfarction period by quantitative thallium-201 scintigraphy. Am J Cardiol 1981; 47: 1010-19. 15. Legrand V, Albert A, Rigo P, Kulbertus HE. Complementary role of thallium-201 scintigraphy to predischarge exercise electrocardiography for patients stratification after a first myocardial infarction. Eur Heart J 1986; 7: 644-53. 16. Abraham RD, Freedman B, Dunn RF, et al. Prediction of multivessel coronary artery disease and prognosis early after acute myocardial infarction by exercise electrocardiography and thallium-201 myocardial perfusion scanning. Am J Cardiol 1986; 58: 423-27. 17. Gruppo Italiano per lo Studio Streptochinasi nell’infarto Miocardico (GISSI). Long-term effects of intravenous thrombolysis in acute myocardial infarction: final report of the GISSI Study. Lancet 1987; ii; 871-74. 18. Schroder R, Nehaus K, Linderer T, Leizorovicz A, Wegscheider K, Tebbe U. Risk of death from recurrent ischemic events after intravenous streptokinase in acute myocardial infarction: results from the Intravenous Streptokinase in Myocardial Infarction (ISAM) Study. Circulation 1987; 76 (suppl II); 44-51. 19. Boucher CA, Brewster DC, Darling RC, Okada RD, Strauss HW, Pohost GM. Determination of cardiac risk by dipyridamole-thallium imaging before peripheral vascular surgery. N Engl J Med 1985; 312: 389-94. 20. Leppo J, Plaja J, Gionet M, Tumolo J, Paraskos JA, Cutler BS. Noninvasive evaluation of cardiac risk before elective vascular surgery. JACC 1987; 9: 269-76. 21. Mangano DT, London MJ, Tubau JF, et al. Dipyridamole thallium-201 scintigraphy as a preoperative screening test: a reexamination of its predictive potential. Circulation 1991; 84: 493-502.
The thymus at
thirty
The familiar comment that "half the scientists who have ever lived are alive today" almost certainly understates the case with respect to immunologistsfor this particular breed the proportion extant must be closer to 90%. Although clinical immunology can trace its origins back some two hundred years to Edward Jenner, and the formal scientific study of immune phenomena began a century ago with Metchnikoff, it is only very recently that the subject has gained the status of a major discipline, spawning institutes, university departments, international associations, and numerous publications. Most immunologists now working must have been born before any function had been assigned to the lymphocyte1-the equivalent, in bacteriological terms, of being Pasteur’s contemporary-yet it evokes no surprise to hear the topic of circulating CD4 cells discussed knowledgeably on popular radio programmes about AIDS or to find that many members of the public are tolerably well informed about immunosuppression in the context of organ transplants and even about monoclonal antibodies as possible therapeutic agents. The clearing of immunological mists can be attributed to a series of critical discoveries allied to some remarkable insights that have placed the experimental observations in a
An outstanding example is our understanding of the role of the thymus which, thirty years ago, was highlighted in The Lancet by a preliminary communication from J. F. A. P. Miller.22 Before 1960 the thymus evoked little interest. It was known to undergo progressive involution in adult life; in children with respiratory distress, demonstration of a large thymic shadow radiographically often led to the unnecessary removal or irradiation of the "enlarged" gland. Working for his PhD at the Chester Beatty Institute in London, Miller was initially interested in the cellular origins of a virally induced mouse
coherent
context.
lymphatic leukaemia and undertook thymectomy in young animals with subsequent reimplantation of syngeneic thymus in some.2,3 He was able to show that the neonatal thymus was the site of leukaemogenic virus/cell interaction, but he became more fascinated by the pathological consequences he observed when thymectomy was carried out immediately after birth. Sheer technical difficulties had prevented any previous experiments of this type and, although several forms of congenital thymic aplasia with associated defects of immune function are now well recognised in man,4they had not been described at that time. Miller recognised that his neonatally thymectomised mice were unusually susceptible to that infections and surmised they were When he took immunologically impaired. special care to protect them from pathogens so that they remained apparently healthy, he found that they were tolerant of skin grafts from mice of unrelated strains and even from rats. Removal of the thymus from slightly older animals had no such dramatic effects, suggesting that in early life the thymus was a site of education of cells passing through it. Careful necropsy of his animals showed that neonatal thymectomy led to a deficiency of lymphocytes in the blood and peripheral lymphoid tissues.z2 This work established that the thymus was an important organ in the development of a functioning immune system. Furthermore, the timing of these discoveries was propitious-not only did they follow close on the heels of Gowans’ observation that the small lymphocyte is capable of mediating immune responses,s but also they came just as the division of lymphoid cells into anatomically and functionally distinct subsets was being recognised in birds.6 Subsequent experiments in mice by several groups, who manipulated thymic, thoracic duct, and marrowderived lymphocyte populations, confirmed that these subdivisions apply also in mammals and led ultimately to our current understanding of the intricate cellular interactions that underlie virtually all immune reactions.7,8 At least three main lines of immunological inquiry may be said to stem, directly or indirectly, from the discovery that thymus-derived lymphocytes have distinctive properties: (a) identification of cell-surface antigens that denote direction and stage of lymphocyte differentiation; (b) study of molecular events
789
underlying the presentation of antigen and its specific recognition by B or T cell clones; and (c) analysis of intercellular signalling via lymphokines, their receptors, and other trans-membrane molecules in the course of lymphocyte maturation and as an essential component of integrated immune reactions.8,9 The thymus has not yielded all its secrets. Preoccupations in 1991 include the developmental and functional significance of the subdivision of thymic lymphocytes according to whether their T-cell receptors are the products of rearranged ap or yo genes,1O,11 and also the question of epithelial/lymphoid cell interactions during the phase of intrathymic differentiation.12 It is probably of little comfort to those wrestling with such complexities to recall that Miller’s seminal discoveries arose from a research project whose original aim was only distantly related to its outcome. 1. Yoffey JM, Courtice FC, eds. The lymphocyte. Lymphatics, lymph and lymphoid tissue. 2nd ed. London: Edward Arnold, 1956: 391-439. 2. Miller JFAP. Immunological function of the thymus. Lancet 1961; ii: 748-49. 3. Miller JFAP. The discovery of the immunological function of the thymus. Immunol Today 1991; 12: 42-45. 4. Rosen FS, Cooper MD, Wedgwood RJP. The primary immunodeficiencies. N Engl J Med. 1984; 311: 300-10. 5. Gowans JL. The immunological activity of lymphocytes. In: Wolstenholme GEW, O’Connor M, eds. Biological activity of the leucocyte. London: Churchill, 1961. 10: 32-44. 6. Szenberg A, Warner NL. Dissociation of immunological responsiveness in fowls with a hormonally arrested development of lymphoid tissue. Nature 1962; 194: 146-47. 7. Claman HN, Chaperon EA, Triplett RF. Thymus-marrow cell combinations-synergism in antibody production. Proc Soc Exp Biol Med 1966; 22: 1167-71. 8. Miller JFAP. Three decades of T-ology. Thymus. 1990; 16: 131-42. 9. Nossal GJV. Triumphs and trials of immunology in the 1980s. Immunol Today 1988; 9: 286-91. 10. Haas W, Kaufman S, Martinez-AC. The development and function of &agr;&bgr; T cells. Immunol Today 1990; 11: 340-43. 11. Nikolic-Zugic J. Phenotypic and functional stages in the intrathymic development of T cells. Immunol Today 1991; 12: 65-70. 12. Boyd RC, Hugo P. Towards an integrated view of thymopoiesis. Immunol Today 1991; 12: 71-79.
Flanagan and colleagues3 at Stanford University Medical School, USA, who have identified a single site of action for both FK506 and cyclosporin. During T-cell activation, which takes place after the T-cell receptor complex binds antigen, there is activation of a tyrosine kinase, phospholipase, and protein kinase C. A factor that is normally absent in resting T cells-nuclear factor of activated T cells, NF-ATbecomes detectable after T-cell receptor-mediated (and phospholipase-mediated) phosphoinositide hydrolysis and is responsible for the early events of T-cell-gene activation. Induction of NF-AT activity is dependent on cyclosporin and FK506 sensitive steps in the lymphocyte. NF-AT has two inducible nuclear factor and a preexisting cytosolic moiety that is transported to the nucleus, in the presence of calcium, to combine with its nuclear partner. This translocation step is blocked by cyclosporin and Fez3 Liu and colleagues4 have shown that a drugimmunophilin complex binds to calcineurin (a protein phosphatase). DeFrancos suggests that cyclosporin and FK506 might inhibit the phosphatase activity of calcineurin, thereby preventing an essential dephosphorylation step in cytosolic NF-AT processing, which in turn might prevent both translocation of cytosolic NF-AT to the nucleus and subsequent interleukin-2 gene transcription.33 Delineation of this network of membrane, cytosolic, and nuclear interactions should provide fertile ground for the design of safer immunosuppressive drugs. components:
an
1. Kahan BD. Drug 1725-38. 2. Moore JM, Peattie
therapy: cyclosporine. N Engl J Med 1989; 321:
DA, Fitzgibbon MJ, Thomson JA. Solution structure of the major binding protein for the immunosuppressant FK506. Nature 1991; 351: 248-50. 3. Flanagan WM, Corthesy B, Bram RJ, Crabtree GR. Nuclear association of a T-cell transcription factor blocked by FK506 and cyclosporin A. Nature 1991; 352: 803-07. 4. Liu J. Calcineurin is a common target of cyclophilin-cyclosporin A and FKBP-FK506 complexes. Cell 1991; 66: 807-15. 5. DeFranco AL. Immunosuppressants at work. Nature 1991; 352: 754-55.
Unmasking immunosuppression of cyclosporin and FK506 to prevent transplant organ rejection preceded identification of their precise molecular targets. Cyclosporin is a ligand for cyclophilin A, an immunophilin with peptidyl-prolyl cis-trans isomerase (rotamase or protein-folding) activity that is abundant in all cells. Earlier this year Moore et all identified the major
Clinical
use
FK506-binding protein, FKBP, as another immunophilin, also with rotamase activity. The ubiquitous nature of the immunophilins suggests that selective inhibition of T-cell activation by cyclosporin and FK506 is
unlikely. Nevertherless, other data inhibitory effect of cyclosporin on an event early in T-cell activation that is dependent on calcium influx into the lymphocyte. Perhaps the drugimmunophilin complex has immunosuppressive
point to an
activity itself. Some of this confusion has
now
been settled
by
Cholecystectomy practice transformed Laparoscopic cholecystectomy is the best method of removing a gallbladder containing stones in patients who are not jaundiced. Even a year ago that pronouncement would have shocked many surgeons, whose reactions to the operation were no more than curious interest or guarded acceptance. Pioneers in France/-3 the USA,4and the UKS,6 stimulated a wave of interest in the procedure, and the results of over 1500 cholecystectomies carried out by members of the US Southern Surgeons’ Clubmust cause even the greatest sceptic to ponder. The average hospital stay was 1-2 days and there were few complications from operations carried out in various hospitals by both university and staff surgeons. The European
retrospective survey8 of seven centres (three French,
12. Ladenheim ML, Kotler TS, Pollock BH, Berman Incremental prognostic power of clinical
DS, Diamond GA. history, exercise
electrocardiography and myocardial perfusion scintigraphy suspected coronary artery disease. Am J Cardiol 1987;
in 59:
270-77. 13. Dunn RF, Freedman
B, Bailey IK, Uren R, Kelly DT. Noninvasive prediction of multivessel disease after myocardial infarction.
Circulation 1980; 62: 726-36. 14. Gibson RS, Taylor GJ, Watson DD, et al. Predicting the extent and location of coronary artery disease during the early postinfarction period by quantitative thallium-201 scintigraphy. Am J Cardiol 1981; 47: 1010-19. 15. Legrand V, Albert A, Rigo P, Kulbertus HE. Complementary role of thallium-201 scintigraphy to predischarge exercise electrocardiography for patients stratification after a first myocardial infarction. Eur Heart J 1986; 7: 644-53. 16. Abraham RD, Freedman B, Dunn RF, et al. Prediction of multivessel coronary artery disease and prognosis early after acute myocardial infarction by exercise electrocardiography and thallium-201 myocardial perfusion scanning. Am J Cardiol 1986; 58: 423-27. 17. Gruppo Italiano per lo Studio Streptochinasi nell’infarto Miocardico (GISSI). Long-term effects of intravenous thrombolysis in acute myocardial infarction: final report of the GISSI Study. Lancet 1987; ii; 871-74. 18. Schroder R, Nehaus K, Linderer T, Leizorovicz A, Wegscheider K, Tebbe U. Risk of death from recurrent ischemic events after intravenous streptokinase in acute myocardial infarction: results from the Intravenous Streptokinase in Myocardial Infarction (ISAM) Study. Circulation 1987; 76 (suppl II); 44-51. 19. Boucher CA, Brewster DC, Darling RC, Okada RD, Strauss HW, Pohost GM. Determination of cardiac risk by dipyridamole-thallium imaging before peripheral vascular surgery. N Engl J Med 1985; 312: 389-94. 20. Leppo J, Plaja J, Gionet M, Tumolo J, Paraskos JA, Cutler BS. Noninvasive evaluation of cardiac risk before elective vascular surgery. JACC 1987; 9: 269-76. 21. Mangano DT, London MJ, Tubau JF, et al. Dipyridamole thallium-201 scintigraphy as a preoperative screening test: a reexamination of its predictive potential. Circulation 1991; 84: 493-502.
The thymus at
thirty
The familiar comment that "half the scientists who have ever lived are alive today" almost certainly understates the case with respect to immunologistsfor this particular breed the proportion extant must be closer to 90%. Although clinical immunology can trace its origins back some two hundred years to Edward Jenner, and the formal scientific study of immune phenomena began a century ago with Metchnikoff, it is only very recently that the subject has gained the status of a major discipline, spawning institutes, university departments, international associations, and numerous publications. Most immunologists now working must have been born before any function had been assigned to the lymphocyte1-the equivalent, in bacteriological terms, of being Pasteur’s contemporary-yet it evokes no surprise to hear the topic of circulating CD4 cells discussed knowledgeably on popular radio programmes about AIDS or to find that many members of the public are tolerably well informed about immunosuppression in the context of organ transplants and even about monoclonal antibodies as possible therapeutic agents. The clearing of immunological mists can be attributed to a series of critical discoveries allied to some remarkable insights that have placed the experimental observations in a
An outstanding example is our understanding of the role of the thymus which, thirty years ago, was highlighted in The Lancet by a preliminary communication from J. F. A. P. Miller.22 Before 1960 the thymus evoked little interest. It was known to undergo progressive involution in adult life; in children with respiratory distress, demonstration of a large thymic shadow radiographically often led to the unnecessary removal or irradiation of the "enlarged" gland. Working for his PhD at the Chester Beatty Institute in London, Miller was initially interested in the cellular origins of a virally induced mouse
coherent
context.
lymphatic leukaemia and undertook thymectomy in young animals with subsequent reimplantation of syngeneic thymus in some.2,3 He was able to show that the neonatal thymus was the site of leukaemogenic virus/cell interaction, but he became more fascinated by the pathological consequences he observed when thymectomy was carried out immediately after birth. Sheer technical difficulties had prevented any previous experiments of this type and, although several forms of congenital thymic aplasia with associated defects of immune function are now well recognised in man,4they had not been described at that time. Miller recognised that his neonatally thymectomised mice were unusually susceptible to that infections and surmised they were When he took immunologically impaired. special care to protect them from pathogens so that they remained apparently healthy, he found that they were tolerant of skin grafts from mice of unrelated strains and even from rats. Removal of the thymus from slightly older animals had no such dramatic effects, suggesting that in early life the thymus was a site of education of cells passing through it. Careful necropsy of his animals showed that neonatal thymectomy led to a deficiency of lymphocytes in the blood and peripheral lymphoid tissues.z2 This work established that the thymus was an important organ in the development of a functioning immune system. Furthermore, the timing of these discoveries was propitious-not only did they follow close on the heels of Gowans’ observation that the small lymphocyte is capable of mediating immune responses,s but also they came just as the division of lymphoid cells into anatomically and functionally distinct subsets was being recognised in birds.6 Subsequent experiments in mice by several groups, who manipulated thymic, thoracic duct, and marrowderived lymphocyte populations, confirmed that these subdivisions apply also in mammals and led ultimately to our current understanding of the intricate cellular interactions that underlie virtually all immune reactions.7,8 At least three main lines of immunological inquiry may be said to stem, directly or indirectly, from the discovery that thymus-derived lymphocytes have distinctive properties: (a) identification of cell-surface antigens that denote direction and stage of lymphocyte differentiation; (b) study of molecular events
789
underlying the presentation of antigen and its specific recognition by B or T cell clones; and (c) analysis of intercellular signalling via lymphokines, their receptors, and other trans-membrane molecules in the course of lymphocyte maturation and as an essential component of integrated immune reactions.8,9 The thymus has not yielded all its secrets. Preoccupations in 1991 include the developmental and functional significance of the subdivision of thymic lymphocytes according to whether their T-cell receptors are the products of rearranged ap or yo genes,1O,11 and also the question of epithelial/lymphoid cell interactions during the phase of intrathymic differentiation.12 It is probably of little comfort to those wrestling with such complexities to recall that Miller’s seminal discoveries arose from a research project whose original aim was only distantly related to its outcome. 1. Yoffey JM, Courtice FC, eds. The lymphocyte. Lymphatics, lymph and lymphoid tissue. 2nd ed. London: Edward Arnold, 1956: 391-439. 2. Miller JFAP. Immunological function of the thymus. Lancet 1961; ii: 748-49. 3. Miller JFAP. The discovery of the immunological function of the thymus. Immunol Today 1991; 12: 42-45. 4. Rosen FS, Cooper MD, Wedgwood RJP. The primary immunodeficiencies. N Engl J Med. 1984; 311: 300-10. 5. Gowans JL. The immunological activity of lymphocytes. In: Wolstenholme GEW, O’Connor M, eds. Biological activity of the leucocyte. London: Churchill, 1961. 10: 32-44. 6. Szenberg A, Warner NL. Dissociation of immunological responsiveness in fowls with a hormonally arrested development of lymphoid tissue. Nature 1962; 194: 146-47. 7. Claman HN, Chaperon EA, Triplett RF. Thymus-marrow cell combinations-synergism in antibody production. Proc Soc Exp Biol Med 1966; 22: 1167-71. 8. Miller JFAP. Three decades of T-ology. Thymus. 1990; 16: 131-42. 9. Nossal GJV. Triumphs and trials of immunology in the 1980s. Immunol Today 1988; 9: 286-91. 10. Haas W, Kaufman S, Martinez-AC. The development and function of &agr;&bgr; T cells. Immunol Today 1990; 11: 340-43. 11. Nikolic-Zugic J. Phenotypic and functional stages in the intrathymic development of T cells. Immunol Today 1991; 12: 65-70. 12. Boyd RC, Hugo P. Towards an integrated view of thymopoiesis. Immunol Today 1991; 12: 71-79.
Flanagan and colleagues3 at Stanford University Medical School, USA, who have identified a single site of action for both FK506 and cyclosporin. During T-cell activation, which takes place after the T-cell receptor complex binds antigen, there is activation of a tyrosine kinase, phospholipase, and protein kinase C. A factor that is normally absent in resting T cells-nuclear factor of activated T cells, NF-ATbecomes detectable after T-cell receptor-mediated (and phospholipase-mediated) phosphoinositide hydrolysis and is responsible for the early events of T-cell-gene activation. Induction of NF-AT activity is dependent on cyclosporin and FK506 sensitive steps in the lymphocyte. NF-AT has two inducible nuclear factor and a preexisting cytosolic moiety that is transported to the nucleus, in the presence of calcium, to combine with its nuclear partner. This translocation step is blocked by cyclosporin and Fez3 Liu and colleagues4 have shown that a drugimmunophilin complex binds to calcineurin (a protein phosphatase). DeFrancos suggests that cyclosporin and FK506 might inhibit the phosphatase activity of calcineurin, thereby preventing an essential dephosphorylation step in cytosolic NF-AT processing, which in turn might prevent both translocation of cytosolic NF-AT to the nucleus and subsequent interleukin-2 gene transcription.33 Delineation of this network of membrane, cytosolic, and nuclear interactions should provide fertile ground for the design of safer immunosuppressive drugs. components:
an
1. Kahan BD. Drug 1725-38. 2. Moore JM, Peattie
therapy: cyclosporine. N Engl J Med 1989; 321:
DA, Fitzgibbon MJ, Thomson JA. Solution structure of the major binding protein for the immunosuppressant FK506. Nature 1991; 351: 248-50. 3. Flanagan WM, Corthesy B, Bram RJ, Crabtree GR. Nuclear association of a T-cell transcription factor blocked by FK506 and cyclosporin A. Nature 1991; 352: 803-07. 4. Liu J. Calcineurin is a common target of cyclophilin-cyclosporin A and FKBP-FK506 complexes. Cell 1991; 66: 807-15. 5. DeFranco AL. Immunosuppressants at work. Nature 1991; 352: 754-55.
Unmasking immunosuppression of cyclosporin and FK506 to prevent transplant organ rejection preceded identification of their precise molecular targets. Cyclosporin is a ligand for cyclophilin A, an immunophilin with peptidyl-prolyl cis-trans isomerase (rotamase or protein-folding) activity that is abundant in all cells. Earlier this year Moore et all identified the major
Clinical
use
FK506-binding protein, FKBP, as another immunophilin, also with rotamase activity. The ubiquitous nature of the immunophilins suggests that selective inhibition of T-cell activation by cyclosporin and FK506 is
unlikely. Nevertherless, other data inhibitory effect of cyclosporin on an event early in T-cell activation that is dependent on calcium influx into the lymphocyte. Perhaps the drugimmunophilin complex has immunosuppressive
point to an
activity itself. Some of this confusion has
now
been settled
by
Cholecystectomy practice transformed Laparoscopic cholecystectomy is the best method of removing a gallbladder containing stones in patients who are not jaundiced. Even a year ago that pronouncement would have shocked many surgeons, whose reactions to the operation were no more than curious interest or guarded acceptance. Pioneers in France/-3 the USA,4and the UKS,6 stimulated a wave of interest in the procedure, and the results of over 1500 cholecystectomies carried out by members of the US Southern Surgeons’ Clubmust cause even the greatest sceptic to ponder. The average hospital stay was 1-2 days and there were few complications from operations carried out in various hospitals by both university and staff surgeons. The European
retrospective survey8 of seven centres (three French,
