The Endocrine Thymus

OWEN M. RENNERT, M.D.

Department o f Pediatrics, Oklahoma Children s Memorial Hospital, Department o f Biochemistry and Molecular Biology, University of Oklahoma, Oklahoma City, OK 73126

ABSTRACT

The mediastinal, lympho-epithelial organ was termed thymus to denote a ductless, gland-like body which has the appearance of a “warty excre scence” (Greek, thymos). Early investigators had ascribed a functional role to this gland including the production of humoral factors which modulated cell growth, maturation and exerted influence on the process of mineraliza tion. Investigators of the past decade have vindicated these observations and identified that the thymus is, indeed, an organ which “sets in motion, and spurs on” (Gr. hormaein) target tissues.

Introduction Until the early 1960’s, the available in formation dealt with embryological In 1942, Maximow and Bloom, in their development, a description of histologic textbook of histology,15 summarized the components and a recognition of aberra knowledge available on the function of tions of morphology. This organ is derived the thymus gland. They wrote: “The func from the third and fourth branchial clefts tions of the thymus are unknown, except which yield its epithelial components, for its ability to form lymphocytes, a few while;the lymphoid structures develop plasma cells and myelocytes. That the later in fetal life. Its cortex contains change from a very large organ in the closely packed thymic lymphocytes sur embryo, infancy and childhood, into a rounding a medulla composed of lympho gradually disappearing organ with cytes and epithelial reticular cells includ development of sexual maturity, has led ing Hassall’s corpuscles. many authors to ascribe an endocrine function to this gland. The claims that the Changes in structure were reported in injection of extracts of the thymus into certain endocrine disturbances and in parent rats causes a precocious growth of malignant transformation; there was no their progeny which is cumulative in suc direct evidence that the organ produced ceeding generations, and that thy any internal secretion. Atrophy of the mectomy causes a retardation in growth of gland was a physiologic process occurring the progeny have not been substan after puberty or could occur precociously tiated.” following serious illnesses or infections 195 0091-7370/79/0500-0195 $01.20 © Institute for Clinical Science, Inc. 196 RENNERT in childhood. Hyperplasia of the gland the development of a “runting” syn occurred when associated with endocrine drome. Shortly after the description of the disorders such as thyrotoxicosis, acro effects of thymic ablation, laboratory in megaly and, in some instances, Addison’s vestigations demonstrated reversal of this disease. Historically, marked enlarge symptom complex by subcutaneous im ment of the thymus, status thymico plantation of thymic tissue (or intraperi- lymphaticus, was said to produce respira toneally) and finally by the administration tory compression symptoms of dyspnea, of thymic cell free extracts.14 cyanosis, etc., in the vulnerable infant. These experimental data established an This concept of a constitutional abnormal endocrine role of the thymus gland and ity characterized by enlargement of the related its action to the ontogenesis, func thymus was subsequently recognized to tion and senescence of the T-lymphatic play no role in the disease manifestations; system. It was predictable, based on these however, the functional significance of observations and the functional role of immunoincompetence in such children the T cell system, that defects would be warrants reinvestigation.23 Neoplasms of encountered in man relating thymic ab this gland are relatively rare; however, normalities with immunodeficiency, au malignancies of the epithelial and toimmune or neoplastic diseases. Sub lymphoid components have been sequent investigations by Goldstein and documented. collaborators,27 M etcalf6 and others1 have The rapid advances in the study of defined abnormalities in thymic endo immunobiology in man, the delineation of crine secretory function in a variety of dis the T and B cell systems, and the associa orders. tion of thymic tumors with abnormalities Essential to a discussion of thymic hor of neuromuscular transmission3 have re mone physiology and chemistry is a re sulted in a reexamination of the hormonal view of certain aspects of T-cell immuno function of the thymus. biology. T-cells are the thymic modified or derived mononuclear cells which are Thymic Hormone Physiology functional in the defense against fungal and Chemistry and viral infections, in the host-versus- Earlier investigators had reported that graft response and in the development of thymectomy affected mineralization and delayed hypersensitivity or cellular im the growth of experimental animals.10 munity. The progenitor T-lymphocytes Evidence had been accumulated identify originate from the bone marrow and mi ing a relationship between defective grate to the thymus gland where they sub mineralizations of eggs , in doves which sequently undergo immunologic trans was corrected by treating these birds with formations. Following their passage a thymic extract.20 through the thymus gland, they undergo a Definitive experimental data establish process of differentiation resulting in the ing the endocrine function of the thymus development of immunocompetence. was the result of studies in experimental Subséquently, they migrate to peripheral animals and as a consequence of investi lymphoid organs where they coexist with gation of the pathophysiology of human B-lymphocytes. Animal experiments diseases. Hallmark experiments9,17 on employing thymectomy and thymus im neonatal thymectomy in mice and other plantation, or administration of thymic animal species produced profound im cell-free extracts, identified that the gland pairment of the immune system, lympho itself or its products are necessary for the cyte deficiency in the peripheral blood maturation of T-lymphocytes.25 The and reticuloendothelial system, as well as thymus may be viewed as regulating the THE ENDOCRINE THYMUS 197 maturation of the marrow precursor cell to recognition of a quantitative increase in a thymocyte and, subsequently the differ cell surface antigens of T-cells as they ma entiation of the thymocyte into a mature ture. Those cell surfaces antigens used as immunocompetent cell. The in vitro and an index of immunologic competence are: in vivo monitoring of these maturational THY-1 antigens, TL-antigens, LY- changes is dependent upon the applica antigens (in the mouse the H2 locus) and tion of a methodology which identifies a the HLA locus in man. Investigations of T-cell and assesses its immuno- the structural characteristics of these cell competence. surface antigens has been carried out by The major assays for the recognition of many investigators.26 immunocompetent cells are: blastogénie The cardinal experiment which estab transformation, mixed lymphocyte reac lished the existence of thymic humoral tion, the in vivo mixed lymphocyte test or factors was the restoration of immuno graft-versus-host reaction, measurement logical competence in neonatally thy- of thymic membrane antigens and the mectomized mice by cell free extracts.18 demonstration of immunoglobulins on Evidence for the in vivo production and the surface of thymocytes. function of these humoral factors was the Blastogénie transformation of T-cells25 observation18 that neonatally thymecto- is assessed through the use of two mito mized female mice had restoration of gens, phytohemagglutinin and conca- normal immunological manifestations navalin A. Following addition of either during pregnancy which could only be mitogen to a suspension of mononuclear explained by the transfer of “thymic” fac cells the incorporation of ?H-thymidine tors produced by the fetus. The described into macromolecular desoxyribonucleic effects of these hormones include: (1) an acid (DNA) is monitored. This assay gives increase in the level of circulating lym an index of the maturation of T-cells since phocytes as well as increased lympho immunoincompetent cells do not show cytes in certain areas of the reticuloendo increased DNA synthesis. The second thelial system, (2) restoration or repair of methodology, the mixed lymphocyte reac cellular immunity, (3) regulation of anti tion,25 evaluates the competence of body production to certain thymus- immunocompetent T-cells to undergo dependent antigens, and (4) maintenance blastogénie transformation following ex of resistance to infection, particularly posure to lymphocytes (mononuclear those of viral origin. The macromolecular cells) from another subject. Here too, thymic factors whose identity or func DNA synthesis is monitored by the incor tional significance is now fairly well es poration of 3H-thymidine. In vivo assess tablished are shown in table I. ment of immunocompetent T-cells is The best studied thymic hormone is .measured in the graft-versus-host25 reac thymosin, having a molecular weight of tion (in vivo mixed lymphocyte test). approximately 10,000 and composed of Donor T-cells are injected into an immuno- 108 amino acids, half of which are di- suppressed experimental animal (immuno carboxylic amino acids.12 This poly logic immaturity in the newborn animal peptide was administered to thymecto- or artificial suppression of the host im mized animals, produces restoration of mune system). Induction of splenomegaly the graft vs. host reaction and accelerates occurs in the host as a consequence of the the immunological maturation of T-cells. presence of immunocompetent donor Presumptive data imply that thymosin T-cells. exerts its activity through a cAMP- A technological advance in the study of mediated* mechanism. The specificity of immunocompetence occurred with the * Adenosine 3', 5'-cyclic monophosphate. 198 RENNERT

TABLE I

Biological Activity of Other Thymic Factors

LYMPHOCYTOSIS MAINTENANCE OF LYMPHOID TISSUE

(LSHr, LSHh) LYMPHOCYTOSIS

HOMEOSTATIC THYMIC HORMONE STIMULATE ANTIBODY SYNTHESIS

THYMIC HUMORAL FACTOR RESTORES ANTIBODY SYNTHESIS

THYMOSIN ACCELERATES SKIN ALLOGRAFT REJECTION

RESTORES GRAFT V, HOST REACTION

THYMIC LIPID FRACTION STIMULATES ANTIBODY FORMATION

(Fr. B, Fr. B III) MODIFIES TUMOR GROWTH

STIMULATES TUMOR CELL GROWTH

thymosin action is questioned since a purified fractions is assayed by the induc number of chemicals which increase tion of rosette formation following incuba intracellular cAMP levels may induce tion of sheep erythrocytes in the presence differentiation of precursor T-cells in of spleen cells or T-cells bearing the vitro. THY-1 antigen. Specificity is given to this Thymopoietin is yet another peptide bioassay since rosette formation is in hormone(s) isolated from this gland.8 Two hibited when THY-1 antigen-bearing molecular species of this peptide hor cells are incubated in the presence of mone have been isolated from mouse azathioprine. The in vitro thymic rosette- thymus. These macromolecules, of inducing effect is exerted within a short molecular weight 5500 and 1600, respec period of time and at nanogram concentra tively, increase the responsiveness of tions of the purified peptide. precursor T-cells to lymphoblastic trans Experimental details on the structure, formation with concanavalin A and phyto preparation and biological activity of the hemagglutinin. In vitro incubation of pre other thymic factors (table I) may be found cursor cells with these peptide hormones in the recent literature.2 Recent investiga results in the acquisition not only of tions support the concept that the thymus T-mitogen responsiveness but also the produces several factors involved in the acquisition of the THY-1 antigen. Though maturation of precursor cells into these thymopoietins have similar biologi immunocompetent T-cells. Advances in cal activities, thymopoietin 2 appears to the study of the structure and character be only 3 percent as biologically active as ization of thymic surface antigens and thymopoietin 1. Most important is the their interactions with thymic humoral demonstration that thymopoietin 1 and 2, factors will lead to a better understanding when injected into mice, induced im of the regulation of T-cell immune func pairment of neuromuscular transmission tion. The characterization of thymic pep similar to that observed in myasthenia tide hormones (factors) has resulted in gravis. their potential therapeutic application to Thymic humoral factor has been iso reconstitute immunodeficient patients. lated from human and murine serum. Its Therapeutic trials in progress have met biological activity in crude serum and with some success. The major unresolved THE ENDOCRINE THYMUS 199

query to be answered by these clinical ble fibroblasts. In this variant Hassall’s trials is whether or not the patient with corpuscles are commonly observed and a cellular immune deficiency, who shows vascular or lymphangiometous appear increased T-cell markers following thymic ance of the tumor is frequently observed. hormone treatment, does appear to be Only 1 percent of all thymomas recog clinically improved. nized occur in the pediatric age group. The long term survival and the clinical Thymic Tumors characteristics of thymoma in childhood Primary tumors of the thymus generally are significantly worse. The ten year sur occur during the third and fourth decades vival for thymomas in adults is 60 to 70 of life, 0.5 to 2 percent of reported cases percent4 whereas in several studies the occur in children; usually in children survival amongst children was under two > 10 years of age or in adolescence.4 years, and systemic metastases were Though these neoplasms are relatively common. rare, they have been the subject of much In the context of a discussion of thymic study because of the wide spectrum of endocrine function, two aspects of associated disorders involving the thymomas merit special discussion. It has neuromuscular, hematologic, immuno recently been recognized that carcinoid logic and endocrinologic systems. Collec and oat cell carcinomas of the thymus are tively tumors of thymus are referred to as associated with endocrine manifestations thymomas and they account for 20 percent in affected patients. Carcinoid tumors of the tumor-like lesions of the medias may occur scattered throughout the body tinum .13 and present in organs particularly of epi The morphologic and histologic char thelial nature derived from primitive di acteristics of these neoplasms define the gestive canal. These tumor cells have three major classifications of tumor. Usu been classified as: diffuse endocrine epi ally they are circumscribed, encapsulated thelial organ,6 peripheral endocrine or lesions varying in size up to 15 to 20 cm in paracrine system5 or amine precursors up diameter; more aggressive, malignant le take and decarboxylation system sions penetrate the capsule. Rarely is (APUD).21 All tumors of this type are po extrathoracic spread found, and approxi tentially malignant, and they have the mately one-third of the primary thymic capacity to invade and metastasize de tumors display malignant characteristics. pendent upon their anatomic location and Histologically, three general cell-patterns size. These tumors display morphologic, are found within these tumors. Type histochemical and behavioral properties 1—this consists of small cells, lympho which are a function of their site of origin. cytes, with no classical pattern and in Thymic carcinoid tumors, in common which Hassall’s corpuscles are rare or ab with lung, stomach pancreas and duo sent. This morphologic pattern occurs in denum, are foregut derivatives which 15 percent of thymomas. Type 2—proto may: (a) be associated w ith C ushing’s plasmic cell type is characterized by large syndrome or type 1 multiple endocrine epithelial cells with acidophilic cyto adenomatosis, (b) be arranged in trabecu plasm occurring as islands or clusters lar or gland-like structures, (c) be argyro- against a lymphoid background. Distinc phil, not argentaffil or (d) contain small tive Hassall’s corpuscles are seen in this uniformally round granules on electron variety. Its incidence accounts for approx microscopy.24 imately 60 percent of thymomas. Type Oat cell carcinomas of the thymus may 3—This is a spindle cell pattern in which be viewed as an undifferentiated form of the epithelial cells histologically resem carcinoid; endocrine disturbances as 200 RENNERT sociated with this tumor are related to its gravis and are suggestive of a break in hormonal secretion. This tumor contains immunotolerance. neurosecretory cytoplasmic granules A variety of clinical situations has been which have a high serotonin content. associated with the occurrence of Twenty-four cases have been recorded in thymomas in patients. Red cell aplasia the literature. The reported patients and systemic lupus erythematodes have ranged in age from 20 to 70 years, one was been associated with malignant thy a boy nine years of age, and there appears momas, in addition to the more com to be a male preponderance.24 The clinical monly recognized association with myas mode of presentation has varied from thenia gravis. The concurrence of asymptomatic lesions, localized medias thymomas with these disease states has tinal symptoms and manifestations of been studied too infrequently to allow any Cushing’s syndrome, multiple endocrine conclusions about the significance of adenomatosis and myopathy. These these correlations.7’19,22 tumors usually are all of low grade malig Investigations on the functional signifi nancy; however, they may metastasize to cance of the thymus and its endocrine ac regional lymph nodes or to bone with tivity have opened a new vista of scientific osteoblastic lesions. activity. These frontiers have their scien The association of thymomas with the tific origins in observations from the turn neuromuscular disorder myasthenia of the century. They epitomize Voltaire’s gravis merits special attention. Approxi views on originality. “Originality is noth mately 30 percent of patients with myas ing but judicious imitation—the most thenia gravis have associated thymomas. original writers borrowed one from This neurologic disorder is characterized another. The instruction we find in books by reduced muscle action potentials fol is like fire. We fetch it from our neighbors, lowing nerve stimulation which may be kindle it at home, communicate it to corrected by treatment with cholines- others, and it becomes the property of all.” terase inhibitors. It had been suggested for some time that a humoral factor binds References to the post-synaptic membrane and, thus, 1. 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