THE ROLE OF LYMPHOID TISSUE IN MORPHOSTASIS

69 ÆETIOLOGICAL FACTORS IN

Hypothesis

BENZOTHIADIAZINE HYPERGLYCÆMIA HYPERGLYCaeMIC activity of benzothiadiazines has not previously been reported in normal laboratory animals, but has been seen in obese hypertensive patients, usually with a family history of diabetes, by Finnerty,l Goldner et al. and Shapiro et al.3 Since the discovery by Langdon and Wolff4 and Wolff et al. that the sodiumretaining thiadiazine, diazoxide, plus trichlormethiazide can produce diabetes in normal dogs, we have examined the hyperglycasmic activity of a series of benzothiadiazines and a phthalimidine in Sprague-Dawley rats. We have given 70 times the recommended dose per kg. This proportion between rat and man is not unusual and was found also applicable to studies with oral sulphonylureas (table l). PAIRS OF RATS RECEIVING DAILY DRUGS BY MOUTH: BLOOD-SUGAR ESTIMATED TWICE WEEKLY

THE ROLE OF LYMPHOID TISSUE IN MORPHOSTASIS "What a piece of work is a man!... And yet, to me, what is this quintessence of dust ?" Hamlet, Act 2, Scene 2.

THE unity, or the integrity of the body depends not only upon the interdependence of different tissues and organs but also, and perhaps equally importantly, upon the interrelationship between the cells of any one differentiated tissue and cells of a similar differentiation, wherever they may lie in the body. Although knowledge concerning this interrelationship is only fragmentary, evidence has been accumulating in recent years that the growth of at least some of the differentiated tissues of the body is controlled by tissue-specific products released from specialised cells into extracellular fluids, and that these substances may act locally or through the blood-stream.1-3 It would seem, however, that any equilibrium established between tissuespecific components contained within specialised cells and related substances in extracellular fluid would provide too rigid a model to be operative in a biological system. A buffer " mechanism could add plasticity; and lymphoid tissue seems well suited to perform this function. "

We wish

to

report here the observations that this

hyperglycxmic activity can be nullified by the coincidental oral administration of tolbutamide (0-5 g. per kg.) (table 11), and also by the administration of potassium chloride (750 mg. per kg.) (table ill). This last finding bears out the report of Reutter and Labhart,6 who have drawn attention to the connection between the hyperglycaemic and potassium-depleting effect of chlorthalidone in man. Our studies have established the hyperglycaemic activity of benzothiadiazines in normal laboratory animals and its antagonism by tolbutamide as well as by potassium salts. It is of interest that potassium antagonised the hyperglycxmic action not only of the sodium and potassium-depleting drug, hydrochlorothiazide, but alsc of the sodium-retaining compound, diazoxide, which does not give rise to potassium loss.7 The therapeutic implications of these findings cannot be judged from our preliminary experiments. More extensive studies are in progress and will be reported PSPW1PTP.

FREDERICK W. WOLFF Department of Medicine (Division of Clinical Pharmacology), The Johns Hopkins University, School of Medicine, Baltimore, Maryland 1.

M.D.

Durh.

WILLIAM W. PARMLEY M.D. Johns Hopkins

Finnerty, F. A. In Hypertension: the First Hahnemann Symposium on Hypertensive Disease (edited by J. H. Moyer); p. 653. Philadelphia, 1959.

2. 3. 4. 5. 6. 7.

Goldner, M. G., Zarowitz, H., Akgun, S. New Engl. J. Med. 1960, 262, 403. Shapiro, A. P., Benedek, T. G., Small, J. L. ibid. 1961, 265, 1028. Langdon, R. G., Wolff, F. W. Brit. med. J. 1962, ii, 926. Wolff, F. W., Hollander, C., Langdon, R. G., Ruebner, B., Skoglund, R. Diabetes (in the press). Reutter, F., Labhart, A. Helvetia med. acta, 1961, 28, 487. Rubin, A. A., Roth, F. E., Taylor, R. M., Rosenkilde, H. J. Pharmacol. 1962, 136,

344.

The thesis of this paper is that in the mammal an essential function of lymphoid tissue is to establish and maintain morphostasis for many of its differentiated tissues. The term morphostasis was introduced by Weiss, who, in discussing differentiation in certain Protozoa, defined it as " ... the steady state condition which maintains a particular pattern". In the Metazoa, the concept of morphostasis has been adopted by other workers to signify the morphological counterpart of homoeostatic mechanisms which become evident in regeneration.5 But here the word is used to define the morphological expression in the mammal of ill-understood homoeostatic mechanisms which lead to a balance of differentiated tissues, and which act throughout life, through growth, maturity, and senescence to death. This is contrary to the current general view that the essential function of lymphoid tissue is to eliminate foreign substances which enter the extracellular fluids of the organism from the external environment, and, perhaps also, from its own cells and tissues. However, Burnet 6 has written: "

Sooner or later immunology must be incorporated into the pattern of biology and each theory as it emerges must be in line with current developments in biochemistry and genetics ... but it must also be influenced by and influence the coming development of what is as yet an almost untouched biological field, embryonic differentiation, the population dynamics of body cells and the relation to both of somatic

general

"

mutation."

Although antibody production has been accounted for in a number of ways,’ immunology still awaits incorporation into the general pattern of biology. Weiss, P. Quart. Rev. Biol. 1950, 25, 177. Druckrey, H. in Ciba Foundation Symposium on Carcinogenesis (edited by G. E. W. Wolstenholme and C. M. O’Connor). London, 1959. 3. Bullough, W. S. Biol. Rev. 1962, 37, 307. 4. Weisz, P. B. Amer. Nat. 1951, 85, 293. 5. Needham, A. E. in Fundamental Aspects of Normal and Malignant Growth (edited by W. W. Nowinski); chap. 7. Amsterdam, 1960. 6. Burnet, F. M. in Mechanisms of Antibody Formation; p. 15. Prague, 1. 2.

1960.

7.

Burnet, F. M. The Clonal Selection Theory of Acquired Immunity. Cambridge, 1959.

70 THE H

CONTROL

OF

GROWTH

auto-antibody concept of growth and differentiation in which cells were regarded as being made up of constituents that bear the same sort of relation as antigen does to antibody. Weissextended this concept by postulating that the specialised cells of an organ synthesise templates and " antitemplates ". He’suggested that normally the templates were confined within the cell but that they could be transferred to similar cells where they would promote growth. He supposed that the antitemplates which could inactivate templates were diffusible compounds which accumulated in the common humoral pool; when the antitemplates reached a Tyler

suggested

an

"

"

critical concentration in this pool, growth of the organ ceased. Druckrey2 suggested that growth of a differentiated tissue is regulated by a two-stage feedback, acting at the cellular level as well as through a higher " regulation centre ". The first level of feedback occurs within each differentiated cell, where tissue-specific components specifically block: (a) functional units ", which produce the tissue-specific components; and (b) proliferation units ", which control the mitotic activity of the tissue. The second level of feedback, Druckrey suggested acts through a regulation centre " the function of which is to stimulate the formation of tissue-specific components. Tissuespecific substances released from differentiated cells inhibit the regulation centre; hence the stimulus to the formation of new tissue-specific components is removed (negative feedback). Bullough 3 has recently reviewed the evidence that growth in the tissues of adult mammals is controlled by a series of tissue-specific mitotic inhibitors (chalones) : any reduction of inhibitor concentration, either local or general, leads to greater Burnet 9 suggested that immunological mitotic activity. could be derived from an aspect of the process by recognition which all multicellular animals succeed in maintaining a characteristic morphological and functional unity. "

"

"

My view of the growth control of differentiated tissues emerged from studies of the reaction of lymph-nodes to allogenic (homologous) tissue grafts: the results were summarised in a recent paper. 10 During the course of this work it became apparent that current concepts could not account for the pattern of lymphatics and of lymphoid tissue throughout the human body, and the curious nature and life history of the small lymphocyte, as revealed particularly in laboratory animals. These prevailing concepts can be summarised as follows: 1. The primary function of lymphatics is to provide a route whereby proteins which have escaped from the blood-capillaries into extravascular spaces are returned to the blood." 2. The primary function of lymphoid tissue is to form antibodies against the constant challenge of foreign substances entering the animal from its environment.

On my

hypothesis

the function of

lymphatics and

lymphoid tissue can be integrated into the economy of the body. It does, however, call for a new concept of antibody formation which will be discussed elsewhere. THE

1. The

HYPOTHESIS

specialised cells, and particularly the mitotic

cells, of most differentiated tissues of mammals, from time time release some of their components into extracellular fluids. These are termed " equilibratory factors ", and they comprise principally " !’MU!Ma/-pec!/!C/aco?’ " and " tissue-specific factors ". 2. The equilibratory factors are neutralised wholly or in part in (a) local extracellular fluid, (b) in lymph-nodes, after entry into the lymphatics, and (c) in lymphoid tissue (excluding the thymus) throughout the body, after entry into the blood-stream.

to

8. 9. 10. 11.

Tyler, A. Growth, 1946, suppl. 10, 7. Burnet, F. M. The Integrity of the Body. London, 1962. Burwell, R. G. Ann. N.Y. Acad. Sci. 1962, 99, 821. Yoffey, J. M., Courtice, F. C. Lymphatics, Lymph and Lymphoid Tissue. London, 1956.

3. Individual-specific factors are rapidly neutralised, perhaps by the action of both macrophages and certain endothelial cells. Tissue-specific factors are neutralised more slowly by conjugation with globulins, most of which are cell-bound to lymphocytes. Some globulins, which are complementary to certain tissue-specific factors, circulate freely in the plasma. These humoral and cell-bound natural auto-antibodies are produced in lymphoid tissue by cells of the lymphocytic series. Lymphocytes (large, medium, and small) exist as clones which become conditioned " or induced during development to produce globulins which are complementary to the many tissue-specific factors released from the differentiating tissues. 4. The growth control of many differentiated tissues is influenced by two equilibria: (a) One equilibrium is established between tissue-specific "

components contained within specialised cells and tissuespecific factors of the same specificity in their local extracellular fluid. Some of the tissue-specific factors released from a differentiated tissue may act (perhaps after conjugation with hormones) as mitotic inhibitors to cells having that differentiation.3 (b) A second equilibrium is established between tissuespecific factors in extracellular fluid and lymphocytes which carry globulins complementary to those tissue-specific factors; these lymphocytes circulate between the lymphoid tissue (excluding the thymus), the blood, and the extracellular fluids of many of the differentiated tissues of the body. However, it must be remembered that extracellular fluid and lymphocytes are virtually absent from the normal central nervous system.

5. Equilibratory factors are released from specialised cells as they enter mitosis. This is assumed, because (a) there is morphological de-differentiation of cells in mitosis and (b) the mitotic activity of differentiated tissues 12 and their degree of lymphatic drainage can often be correlated. 11 6. The mitotic proliferation of a differentiated tissue, through the release of tissue-specific factors into the extracellular fluid (a) increases the entry of small lymphocytes into lymphoid tissue from the blood-stream by recruiting lymphocytes which carry globulins complementary to those tissue-specific factors; and (b) reduces the output from lymphoid tissue of humoral and cell bound globulins belonging to the same clones. By such means, less globulins of these clones become available throughout the body to neutralise tissue-specific factors in the extracellular fluid of the organ in question, where the concentration of tissue-specific factors increases, mitosis is impaired, and the equilibria are re-established. 7. In compensatory hypertrophy, the excision of an organ (e.g., kidney) or a part of an organ (e.g., liver) removes a source of tissue-specific factors the effect of which in lymphoid tissue is to (a) reduce the entry from the blood-stream of small lymphocytes bearing globulins complementary to those tissue-specific factors; and (b) increases the output from lymphoid tissue of humoral and cell-bound globulins belonging to these same clones. Through the availability of more such globulins throughout the body, this lowers the tissue-specific factors in the extracellular fluid of the remaining tissue having that differentiation, and leads in turn to mitotic proliferation of its component cells and re-establishment of the equilibria. 8. A persistent rise, from any cause, in the concentration of tissue-specific factors in the extracellular fluid of the specialised cells which synthesise them will cause a

71

specific inhibition of the capacity of such specialised cells to synthesise the corresponding tissue-specific components, leading ultimately to the mitotic and metabolic impairment of those cells. EVIDENCE

FOR

THE

HYPOTHESIS

and chick embryos 22 liberate substances into the common humoral pool which could inhibit the differentiation 21 and the growth 22 of tissues. Weiss and Kavanau 23 constructed a mathematical model oi Weiss’s theory of growth control. They concluded that: The parameters of the general solution have been evaluated for chick growth, and the resulting specific solution has yielded wholly reasonable values for the parameters and predictions for other characteristics of a biological system and its growth." Applying this mathematical model Kavanau 24 found that the compensatory growth curves for the total mass of an organ are modulatory; and this is consistent with what is known of liver growth after partial hepatectomy in the rat. The damped oscillation of liver size indicates a non-critically-damped feedback mechanism. 3. Evidence from studies concerning the control of mitotic activity in adult mammalian tissues.-Bullough 3 has recently considered the mechanisms which could influence and control mitotic activity in adult mammalian tissues. He proposes that it is in the nature of cells to undergo mitosis when they are not prevented. Further, he summarises the evidence that mitosis is controlled in a differentiated tissue by tissue-specific inhibitory substances (chalones) which are inhibitory to further mitosis in that tissue. In the liver and kidney, he writes, the chalones, if present, must be in the blood-stream; in the skin, the evidence indicates, that they can diffuse only about 1 mm. from the vicinity of the cells in which they are produced. In terms of my hypothesis, this could enable epidermal chalones to reach the most superficial lymphatics of the dermis, and hence to be carried to the regional lymph-nodes. The tissuespecific chalones of Bullough could, therefore, be identified, at least in part, with the tissue-specific factors of the present "

The Presence

of Specific

and

Non-specific Components in

Differentiated Cells Many differentiated cells have been shown by immunological techniques to contain components which are heterogenetic, species-specific, individual-specific, tissuespecific, and components which are not individualspecific but are common to most cells of an individual. 12-14 The biochemical structure of all such components will be directed by the base sequences of the operator-structural genes. If differentiated cells release equilibratory factors into extracellular fluids, the factors probably belong to the class of proteins or protein complexes.

of Individual-specific and Tissue-specific Substances Cells Differentiated from 1. Evidence from the transplantation of foreign tissues.Because skin transplanted orthotopically as an allogenic graft evokes an actively acquired immune response in the draining lymph-nodes, Medawar 15 suggested that, in normal everyday life, skin liberates a large number of genetically-distinguishable substances which enter the lymphatic capillaries and so are transported to the regional lymph-nodes. In a subsequent The Release

at a time when he and his colleagues believed that desoxyribonucleoproteins were the transplantation antigens, Medawar 16 postulated a systemic circulation of nucleic acids from differentiated tissues; in this model, the lymphocyte was regarded as essentially the vector of nucleic acid or nucleoprotein. In the light of later findings relating to the chemical composition of transplantation antigens,17 this suggested cycle of individual-specific factors would imply, not nucleoproteins, but carbohydrates with lipid or lipoprotein attachments. In renal transplants, the transplantation antigens apparently enter the blood-stream directly,’" and, in normal everyday life, individual-specific factors from both kidneys are assumed to

paper,

do likewise. On reaching

hypothesis. THE

PRESENCE

OF

TISSUE-SPECIFIC

Latterly, several

INDIVIDUAL-SPECIFIC AND SUBSTANCES IN PLASMA

groups of inherited

serum-proteins

have been discovered in

humans; these include the many blood-group substances, transferrins, haptoglobins,25 and paraproteins.2" It has been impossible, however, to elicit transplantation immunity or immunological tolerance by injecting allogenic plasma into animals 15 ls; hence Ii

regional lymph-node the transplantation a fresh allogenic tissue graft inserted into the antigens leaving rabbit’s ear are apparently seeded to multiple sites in the cortex of the lymph-node by means of a double shunt: (a) from lymph to blood (across the endothelial cells of sub-marginal capillaries); and (b) from blood to lymphoid tissue (across the endothelium of post-capillary venules).10 In the light of the hypothesis, this observation reveals the route in lymph-nodes which equilibratory factors released from differentiated cells would normally traverse. Further, in post-capillary venules, some of the equilibratory factors are assumed to be taken up by small lymphocytes which have been shown to migrate across the vessel walls and enter lymph-nodes from the blood at this a

those

individual-specific factors which can evoke transplantation immunity or immunological tolerance (if present in plasma) are there seemingly only in low concentration. But substances released from many differentiated cells do enter and persist in the bloodstream. These include: (a) proteins with specific physiological properties, such as albumin, blood-coagulation factors, hormones, and antibodies; (b) tissue-specific enzymes’ 27 28; (c) tissue-specific antigens 29 ; and (d)

site. 19

y-globulins.3O

Preliminary observations have indicated also that, after the transplantation of fresh allogenic grafts of pulmonary tissue

Such substances evidently circulate in the blood-stream mixture of plasma-proteins. Hence, if a mechanism of growth control, such as the one suggested here, is to factors of a given specificity function, "then tissue-specific " must be recognised not only by (a) components in differentiated cells having that specialisation but also by (b) the complementary globulins which, it is supposed, are carried mainly by lymphocytes.

and renal tissue to a subcutaneous site in rats, tissue-specific substances are detectable in the corresponding regional

lymph-nodes.2O 2. Evidence from experiments designed to test Weiss’s theory of growth control.- There is some support for the suggestion that at least some of the differentiated tissues of frog embryos 21 12. 13. 14.

Furth, J., Kabat, E. A. J. exp. Med. 1941, 74, 247. Henle, W., Chambers, L. A., Groupe, V. ibid. p. 495. Nairn, R. C. in Fluorescent Protein Tracing; chap. 9 (edited by R. C. Nairn). Edinburgh, 1962. 15. Medawar,P. B. Harveian Lecture. Series LXX, 144. New York, 1956-57. 16. Medawar, P. B. Ann. N.Y. Acad. Sci. 1957, 68, 255. 17. Brent, L., Medawar, P. B., Ruszkiewicz, M. Brit. J. exp. Path. 1961, 42, 464. 18. Hume, D M., Egdahl, R. H. Surgery, 1955, 38, 194. 19. Gowans, J. L. Ciba Foundation Study Group no. 10. Biological Activity of the Leucocyte (edited by G. E. W. Wolstenholme and M. O’Connor); p. 108. London, 1961. 20. Burwell, R. G., Carrick, B. M. Unpublished, 1962. 21. Rose, M. Biol. Rev. 1957, 32, 351.

as a

Weiss, P. Science, 1952, 115, 487. Weiss, P., Kavanau, J. L. J. gen. Physiol 1957, 41, 1. Kavanau, J. L. Proc. Nat. Acad. Sci. 1960, 46, 1658. Blumberg, B. S. Arch. environm. Hlth, 1961, 3, 612. Ossermann, E. F. Amer. J. Med. 1961, 31, 671. Fishman, W. H. in The Plasma Proteins (edited by F. W. Putnam) vol. 2. New York, 1960. 28. Wilkinson, J. H. An Introduction to Diagnostic Enzymology. London

22. 23. 24. 25. 26. 27.

1962. 29. 30.

Yeh, S. D., Serp, W. F., Burch, C., Barnes, F. W. Arch. intern. Med. 1959, 103, 933. Miller, L. L., Bly, C. G., Bale, W. F. J. exp. Med. 1954, 99, 133.

72 THE

INTERRELATIONSHIP OF LYMPHOID TISSUE AND CELLS OF OTHER DIFFERENTIATED TISSUES

THE

In their studies of immunological tolerance, immunohave shown that an interrelationship must exist, at least in the developing organism, between differentiating cells and the animal’s own lymphoid tissue.7 31 That lymphoid tissue in the mature mammal has a general (or intrinsic) function in the body (as well as its special immunological function in response to foreign antigenic determinants) cannot, as yet, be regarded as firmly established. But a great deal of evidence favours this view: . (1) The presence and the pattern of distribution of

logists

lymphatics throughout the body." (2) The correlation which can be drawn in many instances between the mitotic activity of differentiated tissues,32 and their degree of lymphatic drainage." (3) The " double-shunt in lymph-nodes which is traversed apparently by tissue derivatives, but not ’by classical antigens. 10 (4) The changes after the excision of lymph-nodes .33 34 (5) The shortened life of neonatally thymectomised mice.35 (6) The effects of splenectomy upon bone-marrow." (7) The cause of death in some animals dying from secondary disease.36 37 (8) The degree of development of lymphoid tissue in germ"

"

free animals.3a (9) The difference in the survival-time of lymphocytes: (a) when cultured in vitro alone (about a week 39) and with certain other differentiated cells (several weeks 40), and (b) in vivo (possibly several months 39 41). (10) The recirculation of lymphocytes.42 (11) The entry of lymphocytes into differentiated tissues," and into the cytoplasm of differentiated cells.39 (12) The surface interaction in vitro of lymphocytes and certain other differentiated cells.39 43 (13) Lymphocytes have been shown to turn ’into monocytes and macrophages,39 and perhaps into haemocytoblasts.44 (14) The growth-promoting properties of lymphocytes in vitro together with circumstantial evidence appertaining to small lymphocytes, has been used to formulate the concept that lymphocytes have a trephocytic function.37 45 .

Each of these observations could be explained if an essential role of lymphocytes in tiie body is trophic, the lymphocyte serving as a vector of materials required by other cells. But this explanation ignores two further pieces of evidence: (a) that small lymphocytes, or a proportion of them, are immunologically-competent cells 46 47; and (b) that substances which may be natural auto-antibodies to bodv components have been detected in Dlasma.49-56 31. Medawar, P. B. Nature, Lond. 1961, 189, 14. 32. Leblond, C. P., Walker, B. E. Physiol. Rev. 1956, 36, 255. 33. Sanders, A. G., Florey, H. W. Brit. J. exp. Path. 1940, 21, 275. 34. Turner, M. L., Hall, V. E. Anat. Rec. 1943, 85, 401. 35. Parrott, M. V., East, J. Nature, Lond. 1962, 195, 347. 36. Loutit, J. F., Micklem, H. S. Brit. J. exp. Path. 1962, 43, 77. 37. Louitt, J. F. Lancet, 1962, ii, 1106. 38. Miyakawa, M. Ann. N.Y. Acad. Sci. 1959, 78, 221. 39. Trowell, O. A. Int. Rev. Cytol. 1958, 7, 235. 40. Pulvertaft, R. J. L. Proc. Roy. Soc. Med. 1959, 52, 315. 41. Shelton, E. J. cell. Biol. 1962, 12, 652. 42. Gowans, J. L. J. Physiol. 1959, 46, 54. 43. Humble, J. G., Jayne W. H. W., Pulvertaft, R. J. V. Brit. J. Hœmat. 1956, 2, 283. 44. Yoffey, J. M. Lancet, 1962, i, 206. 45. Kelsall, M. A., Crabb, E. D. Lymphocytes and Mast Cells. London, 1959. 46. Gowans, J. L., Gesner, B. M., McGregor, D. D. in Ciba Foundation Study Group no. 10. Biological Activity of the Leucocyte; p 32 (edited by G. E. W. Wolstenholme and C. M. O’Connor). London, 1961. 47. Porter, K. A., Cooper, E. H. J. exp. Med. 1962, 115, 997 48. Gitlin, D., Gross, P. A. M., Janeway, C. A. New Engl. J. Med 1959, 260, 21 49. Walsh, L. S. N. J. Immunol. 1925, 10, 803. 50. Dacie, J. V. in The Hæmolytic Anaemias. Part 2. The Auto-Immune Hæmolytic Anæmias. London, 1962. 51. Milgrom, F. in Recent Progress in Microbiology. Symposium on Tissuespecific Antibodies; p. 200 (edited by G. Tunevall). Oxford, 1959. 52. Terasaki, P. I., Chamberlain, C. C. J. exp. Med. 1962, 115, 439. 53. Deckers, C., Maisin, J. Nature, Lond. 1963, 197, 397. 54. Gajdusek, D. C. Arch. intern. Med. 1958, 101, 9. 55. Mackay, I. R., Perry, B. T. Aust. Ann. Med. 1960, 9, 84. 56. Kidd, J. G., Friedewald, W. F. J. exp. Med. 1943, 76, 543, 557.

PRESENCE IN PLASMA OF NATURAL AUTOTO CERTAIN BODY COMPONENTS

ANTIBODIES

THE

With certain exceptions, such as the iso-agglutinins, it has not yet been possible to establish whether some of the y-globulins in plasma are " normal " rather than immune globulins.48 However, a few reports suggest that substances, some of which may be natural auto-anti. bodies to certain body components, are present in the plasma of, at least some, apparently healthy animals, including man. These body components include those contained in spermatozoa,49 erythrocytes, 50 51 epidermal cells,52 possibly liver and kidney cells, 53 54 thyroid,55 and a component present in a wide variety of tissues.56 In this connection some serum-paraproteins have been shown to have binding affinities to certain tissue cells.26 These " natural auto-antibodies are probably proteins -perhaps globulins-and they are produced by cells in lymphoid tissue. If this is so, then the suggestion that information appertaining to the specificity of differentiated tissues could be carried by lymphocytes seems reasonable. Of particular interest, then, is the recent observation that a population of small lymphocytes can carry the property of immunological tolerance to sheep-erythrocytes injected into irradiated adult rats 57; this could imply that the lymphocytes which could have produced antibody to sheep-erythrocytes injected into the irradiated host had been eliminated in the tolerant host 7; or that lymphocytes, " as tolerant cells ", could carry " information " appertainto the antigenic specificity of a differentiated tissue.57 "

DISCUSSION

Individual-specific

Factors

Since there is evidence that some mammalian differentiated tissues, and particularly mitotic tissues, normally release individual-specific factors capable of evoking transplantation immunity into their regional lymphatics or into the blood-stream, individual-specific factors should be present in plasma. But allogenic plasma injected into mice and rabbits has been found to elicit neither transplantation immunity, nor immunological tolerance,15 16 which suggests that if individualspecific factors are present in plasma, they are there only in low concentration. These findings imply that if factors are released from normal individual-specific differentiated tissues, they are neutralised in large part either before, or shortly after, they enter the bloodstream.

With regard to the stability of iso-antigens outside the body, the iso-antigens which evoke transplantation immunity have been found to be relatively unstable not only to the action of some physical and chemical treatments but also to the action of certain enzymes 59 If then, as the evidence suggests, individual-specific factors also are unstable in the intact animal, their breakdown could be due to the action of enzymes; and there are some data implicating the endothelial cells of postcapillary venules in lymph-nodes,Io as well perhaps as the littoral cells of lymph-nodes, spleen, and bone-marrow in this presumed disruption of individual-specific factors.

Tissue-specific Factors At first sight there seems to be little advantage for the body in the possible neutralisation of its own tissuespecific factors. However, a clue to the understanding of 57.

58. 59.

Gowans, J. L., McGregor, D. D., Cowen, D. M., Ford, C. E. Nature, Lond. 1962, 196, 651. Medawar, P. B. in Biological Problems of Grafting (edited by F. Albert and P. B. Medawar); p. 6. Oxford, 1959. Kandutsch, A. A. Plast. reconstr. Surg. 1961, 27, 135.

73

the fate of individual-specific factors within the body, may lie in the observation that transplantation antigens leaving differentiated cells, may do so, not alone, but in the Hence the company of the tissue-specific factors. 20 neutralisation of individual-specific factors is perhaps in some way obligatory for mechanisms which the body may have for dealing with tissue-specific factors released from its own differentiated cells. The detection of tissuespecific substances in the blood suggests that some tissuespecific factors circulate and remain unneutralised for at least some time after their release from differentiated cells. The neutralisation of tissue-specific factors, however, seems desirable to avoid the potential danger, after negative feedback, of the accumulation of tissue-specific factors impairing the function of all cells having that

differentiation.

Tissue-specific factors seem unlikely to be destroyed entirely by enzyme activity; for this would need the presence in lymphoid tissue of cells containing enzymes capable of destroying tissue-specific factors released from any differentiated cell in the body. Neutralisation could, however, be achieved by conjugating tissue-specific factors with molecules that are complementary to their own structure. Conjugation would have to happen throughout the body; for autoplastic grafts transplanted to anyone of various heterotopic sites in the same animal (mammal) usually become, for a short time at least, a component part of that individual. Hence, if there is such a conjugating-inactivating system, a mobile store of such complementary molecules could well be free in the plasma, particularly for avascular differentiated tissues: this would account for the " natural humoral autoantibodies " to certain body components such as epidermal cells and spermatozoa. Most of the " natural autoantibodies ", however, may be entirely cell-bound. Of the blood-cells small lymphocytes and of the molecules, y-globulin seem most fitted for the function of conjugation. This conjectural model of the growth control of differentiated tissues implies that small lymphocytes in the adult mammal exist as tissue-conditioned lymphocytes, perhaps as clones, in which each clone carries a specific molecule which is complementary to one or more of the multiple tissue-specific factors released from many of the differentiated tissues of the body. On this reasoning, any condition, such as the mitotic proliferation of a differentiated tissue leading to the entry of increased amounts of tissue-specific factors into extracellular fluid, would reduce the output, in lymphoid tissue, of small lymphocytes which carry globulins complementary to the tissue-specific factors of that differentiated tissue. If these small lymphocytes are regarded as being the differentiated (or effector) cells of the lymphocytic series, then tissuespecific factors could produce this effect by (a) stimulating mitosis in large and medium lymphocytes (which in this connection would be regarded as being the relatively undifferentiated cells of the lymphocytic series), and at the same time (b) by impairing the differentiation of these cells into small lymphocytes.60 With less small lymphocytes of the correct clones available for neutralising the complementary tissue-specific factor, the concentration of tissue-specific factors in the extracellular fluid of the particular organ would increase; mitosis there would be

impaired; and, ultimately, the particular equilibria between (a) tissue-specific components and tissuespecific factors and (b) tissue-specific factors and comple60.

Burch,

P. R. J. Personal communication.

mentary clones of established.

lymphocytes, would become

re-

Contrariwise, any condition, such as the partial removal of liver or kidney, leading to the entry of reduced amounts of tissue-specific factors into extracellular fluid, is presumed to cause in lymphoid tissue, an increased output of small lymphocytes which carry globulins complementary to the tissue-specific factors of the differentiated tissue (such as liver or kidney). This change in lymphocytic output could result from the lowered concentration of tissue-specific factor in lymphoid tissue that permit the complementary clones of lymphoid cells to differentiate into small lymphocytes. 60 With more small lymphocytes available for neutralising the complementary tissuespecific factors, the concentration of tissue-specific factors in the extracellular fluid of the particular organ would decrease, mitosis there would increase, and ultimately would lead to the re-establishment of the particular equilibria between that tissue, its tissue-specific factors, and its complementary clones of lymphocytes. The suggestion that any tendency for the concentration of tissue-specific factors in lymphoid tissue to increase (such as during the mitotic proliferation of a differentiated tissue) is counterbalanced by a recruitment (from the blood-stream) of small lymphocytes carrying globulins complementary to those tissue-specific factors, implies the existence of lymphocytic chemotropism. The experimental evidence, in this connection, does not deny the possibility that such chemotropism of lymphocytes to tissue-specific factors (released after the enzymatic disruption of individual-specific factors) may take place. Although lymphocytes have been found not to undergo chemotaxis to tissues (autolysing, partially-digested, or virus-infected),39they have been reported, nevertheless, to show taxis to cells in motisis, to macrophages, to megakaryocytes, and to certain malignant cells .40 43 61 The concept that small lymphocytes (or effector) cells are differentiated does not conflict with recent work which shows that under some laboratory conditions (in vivo 46 47 and in vitro 62 63), small lymphocytes can change rapidly into dividing cells with new morphological characteristics. In a later paper the view will be put forward that antibody-forming cells are derived from the dictionary of tissue-conditioned lymphocytes in the body; furthermore, the present hypothesis does not exclude the possibility that a proportion of circulating small lymphocytes may not be conditioned to tissue-specific factors and may exist for a period in the blood-stream as tissue-unconditioned lymphocytes, each of which could have the capacities of a pluripotential lymphocytic stem-cell. Burnethas argued that certain blood-group substances may cause the specific elimination during development of the clone of globulin-producing cells which eventually would have produced the iso-agglutinin in question. In this way, tissue-specific factors released from widely distributed specialised cells in the body which undergo mitosis, could perhaps cause the specific elimination of their complementary clones of globulinproducing cells. Thus, endothelial cells and fibroblasts could be expected to eliminate their complementary clones of lymphocytes. In the case of endothelial cells, this would ensure that no circulating lymphocytes would adhere too strongly to the walls of blood-vessels. The 61. 62. 63.

Sharp, J. A., Burwell, R. G. Nature, Lond. 1960, 188, 474. Carstairs, K. Lancet, 1962, i, 829. Quaglino, D., Hayhoe, F. G. J., Flemans, R. J. Nature, Lond. 1962, 196, 338.

74 of tissue-specific factors released from endothelial cells could be performed by macrophages. In the case of fibroblasts, mast cells 45 64 could undertake this equilibratory function and produce histamine and heparin as by-products. In the light of this hypothesis lymphoid tissue plays a role similar to that suggested for the " regulation centre " in Druckrey’s model. The " natural " globulins function like the antitemplates in Weiss’s model, but Weiss suggests that the antitemplates act entirely at the cellular level; on my assumption, the globulins act principally on tissuefluid, though specific factors contained within extracellular the serum " natural auto-antibodies " may act also at the cellular level. Burch,65 however, suggests that the cellof my hypothesis act bound " natural auto-antibodies The at the cellular level. current mathematical entirely human autoimmune diseases of Burch 65 of various studies of are revealing that the number parent-cells or stem-cells that could produce such postulated tissue-conditioned lymphocytes remains constant from the first year of life onwards. Because the absolute mass of lymphoid tissue (and hence the number of small lymphocytes) does not remain constant with age,l1 the stem-cells of tissueconditioned small lymphocytes cannot be the small lymphocytes themselves, but could be large lymphocytes, primitive reticular cells, or thymic epithelial (reticular) cells. 66 Further, because the thymus may be the primordium of the anti-body-forming system, 61 6 11 the absolute number of parent-cells or stem-cells could be determined in the thymus during prenatal or early postnatal developThe blood-thymic barrier in the adult 69 could ment. protect such cells which persist in the thymus from the hazards of foreign antigenic determinants. Thymic lymphocytes may be of endodermal origin 70; hence such a postulated model of the thymic origin of the parentcell or stem-cells of tissue-conditioned small lymphocytes would be analogous to that found in the ovaries of some mammals, in which the absolute number of oocytes may be determined shortly after birth and may be derived from primordial germ-cells which have migrated from the region of the extra-embryonic endoderm into the organ

equilibration

"

during development. 71 The implications of this hypothesis of growth control for interpretations of antibody formation, the concept of horror autotoxicus, auto-immunity, development growth, ageing, and cancer will be discussed in subsequent papers. Suffice it to say that auto-immunity may represent an unmasking by disease or experimental means of mechanisms which are normally concerned in the homoeostasis of tissue components. CONCLUSION

A hypothesis is proposed which relates the primary function of lymphoid tissue to the establishment and maintenance of a steady-state condition of many differentiated tissues (morphostasis), a function which may be operative throughout the major part of life through growth, maturity, and senescence to death. Experiments are in progress to test the hypothesis. It is a pleasure to record my indebtedness to Prof. A. Durward for his constant encouragement and criticism. I have also enjoyed the 64. Riley, J. G. Lancet, 1962, ii, 40. 65. Burch, P. R. J. ibid. 1963, i. 1253. 66. Clark, S. L. Amer. J. Anat. 1963, 112, 1. 67. Miller, J. F. A. P. Proc. roy. Soc. B, 1962, 156, 415. 68. Burnet, F. M. Science, 1961, 133, 307. 69. Marshall, A. H. E., White, R. G. Brit. J. exp. Path, 1961, 42, 379. 70. Auerbach, R. Develop. Biol. 1961, 3, 336. 71. Franchi, L. L., Mandl, A. M., Sir Solly Zuckerman in The Ovary (edited by Sir Solly Zuckerman); chap. 1. London, 1962.

great advantage of innumerable discussions with my colleagues and in particular with Dr. J. A. Sharp, Dr. B. Ballantyne, Dr. P. R. J. Burch, Dr. H. M. Anthony, and Dr. G. Gowland of the National Institute for Medical Research. I should like to thank Prof. P. B. Medawar, F.R.S., for advice and criticism in the preparation of this work

Department of Anatomy,

University of Leeds

R. GEOFFREY BURWELL M.D., B.SC. Leeds, F.R.C.S.

Reviews of Books Schizophrenia: Chemistry, Metabolism, and Treatment J. R. SMYTHIES, M.SC., M.D., M.R.C.P., D.P.M., senior lecturer, department of psychological medicine, University of Edinburgh. Springfield, Illinois: Charles C. Thomas. 1963. Pp. 86.$4-75, A PLAIN guide has long been needed to the biochemistry of schizophrenia, and none was better qualified to write it than Dr. Smythies, who is a one-man " multidisciplinary " team. This monograph will be essential reading for every postgraduate student of psychiatry. Most biochemical writing completely fails to communicate with those lacking a detailed and recent knowledge of the subject. Dr. Smythies, on the other hand, avoids jargon and explains these very complex matters in straightforward language. Applying high scientific standards, he finds most work from before the last decade fit only for the waste-paper basket. He points out that little was known then of the base-line of normal brain chemistry, that there was no hypothesis to suggest what to look for, that many vital aspects

left uncontrolled, and that research was determined mainly by the techniques available-crude and misleading as they often were. Even much recent work (for example, that on the toxicity of schizophrenic body-fluids) needs to be repeated with much stricter control, particularly of diet and exercise. Dr. Smythies explains the two main approaches-the examination of patients’ metabolism for faults, and the study of the mode of action of psychomimetic drugs. In each case, there is the problem of separating primary biochemical disorders from secondary or irrelevant ones, and of determining whether the fault is specific or common to any state of stress. His suggestions for future work outline some of the ways in which this might be done, and should be noted by those who allocate research funds. Though much remains confused, it seems certain that a toxic factor is localised in the globulin fraction of the serum, and that this may be linked with abnormal immune responses. There are also newly discovered hallucinogenic derivatives of tryptamine, separated by a very narrow chemical gap from substances which are known to be active in brain biochemistry. Dr. Smythies’ brief chapter on the chemical treatment of schizophrenia emphasises how little is known in this subject. He points out, however, that the drugs concerned have close connections with hydroxytryptamine and adrenaline, which are key chemicals in the brain and which must have some important influence on the activity of the autonomic nervous system. There is also an ably balanced presentation of the xtiology of the disease, which emphasises that biochemistry alone can never provide the whole answer. In a field where speculation has been endless, and where many theories have been built on shaky foundations, this book is a model of scientific objectivity. were

Immunology

for Students of Medicine

J. H. HUMPHREY, M.D., F.R.S., head of the division of immunology, National Institute for Medical Research, Mill Hill, London; R. G. WHITE, D.M., university reader in bacteriology, The London Hospital. Oxford: Blackwell Scientific Publications. 1963.

Pp.

450. 45s.

THE study of immunology has greatly expanded and changed its character during the past two decades. From being the province of the blood-group serologist, bacteriologist, and virologist, immunology has come to be recognised as an important branch of biological science. Investigations of immune responses of vertebrates to various challenges have