Japan. J. Med. Sci. Biol., 29, 289-321, 1976 REVIEW SOME ENDOCRINE ASPECTS OF THE THYMUS GLAND Louis V. CASO Department o f Anatomic Sciences, Temple University, Health Sciences Center, School of Dentistry, Philadelphia, Pennsylvania (Received: May 8, 1976) CONTENTS I. Introduction ........................................................ 289 II. Active Factors Present in the Thymus Gland ........................ 290 Lymphopoietin Lymphocyte Stimulating Hormone Other Stimulators of Lymphatic Tissue III. Thymectomy and the Immune Response.............................. 293 Endocrine Effects Immunological Effects IV. Action of Thymic Extracts on the Immune Response.................. 295 Humoral Antibodies Cell-mediated Antibodies V. The B and T Cell Systems of Immunocompetence .................... 298 Action of Thymic Extracts on the Maturation of T Lymphocytes VI. Inhibitory Action of the Thymus Gland .............................. 301 Growth Inhibitory Factor from Thymus Gland VII. Cellular Origin of Thymic Hormones................................. 303 VIII. Role of the Thymus in Neoplastic Disease............................ 304 Thymus in Relation to Immunity in Aging Studies of Aging Mice The Thymus in Relation to the Neoplasms of the Immune System The Thymus in Relation to Tumor Growth in Experimental Animals Immunosuppression and Immune Surveillance Thymic Humoral Factor Alpha Globulins Other Immunosuppressive Thymic Fractions IX. Summary .......................................................... 311 I. INTRODUCTION Research on the thymus gland has revealed an organ of unexpected com- plexity and diversity of function. The mechanisms of the immune response have been associated with the thymus gland, and the intricacies of the differentiation of immunocompetent cells and their activation by antigens are at present the subject of elaborate investigation. Much of this research has been carried out in the mouse, and Metcalf (1964) has described the structure of the mouse thymus, which consists of 90% by weight 289 290 CASO Vol. 29 of lymphocytes, which in turn account for 99.9% of observed mitoses in the gland. These lymphocytesare not unique to the thymus and are interchangeable with those from other organs. Thymic grafts have been found to be infiltrated by lymphocytes from spleen and bone marrow, but not from thymus or lymph nodes. Thymic lymphocytes in the mouse have been shown to be replaced nor- mally and continually by stem cells from the circulation. The lymphocytes are embedded in a meshwork of reticular epithelial cells, derived from endoderm, and are closely packed in the outer region to form a cortex, while the inner region or medulla has fewer lymphocytes in a meshwork of reticular epithelial cells and mesenchymal reticular cells. Radiating from the medulla are arteries which end in capillaries in the subcapsular region of the cortex. These radial arteries are surrounded by reticular epithelial cells which enclose a space around the arteries, forming a double-wall barrier about the blood vesselsin the cortex. This barrier is incomplete in the subcapsular region. Lymphocytes form a cull around the radial arteries, and in thymic grafts many primitive lymphocytes are found along the arteries. The thymus of the intact mouse, however, contains primitive cells not along blood vessels but in clusters scattered through the subcapsular zone and the middle and inner regions of the cortex (Metcalf, 1964). There is evidence that mesenchymal reticular cells, scattered among the epithelial cells in the outer cortex or along the double wall of the radial vessels, stimulate mitosis in adjacent lymphocytes. These mesenchymal reticular cells are PAS-positiveand phagocytic in contrast to the reticular epithelial cells of the vascular sheath and medulla, which are PAS-negative and non-phagocytic and do not show direct association with mitosis in adjacent lymphocytes (Clark, 1963; Hoshino, 1963). Metcalf (1964) postulates that the PAS-positive mesenchymal cells trigger off mitoses in adjacent lymphocytes after initial stimulation of mitosis by the reticular epithelial cells. In addition, he notes the presence of PAS-positive granules in the cytoplasm of some of the reticular epithelial cells of the medulla, a fact which suggests that they may be secretory even though mitoses are infrequent in this region. II. ACTIVEFACTORS PRESENT IN THE THYMUSGLAND Lymphopoietin: Lymphoid cells exhibit intense proliferation on entering the thymus, the primitive lymphocytes and mitoses being concentrated in the cortex. The stimulus appears to be intrinsic to the thymus. It is notable that antigenic stimulation is without effect in stimulating thymic lymphopoiesis, the blood-thymic barrier of reticular epithelial cells, though incomplete, perhaps being the major obstacle. Evidence for the production of a lymphocyte-stimulating factor within the thymus, active on thymic lymphocytes, comes from the behavior of thymic tissue in organ grafts. The grafts are only successful if a minimal proportion of medullary tissue is present in the graft. Only the epithelial and mesenchymal 1976 SOME ENDOCRINE ASPECTS OF THE THYMUS GLAND 291 reticular cells survive the graft, while lymphoid cells are replaced by lympho- cytes from the host (Metcalf and Wakonig-Vaartaja, 1964). Thymic graft growth is determined by the age, strain and mitotic pattern of the donor mouse and not the recipient, despite genetic differences between the donor and recipient animals and migration of lymphocytes into the thymic graft (Metcalf et al., 1961). The grafts are not affected by the host's thymus, by thymectomy in the host, or other thymic grafts in the host. In turn the thymus of the host is not affected by a thymic graft. Metcalf (1964) presents the hypothesis that lympho- poietin is produced by the epithelial reticular cells of the cortex and medulla. This substance would be active in the context of PAS-positive mesenchymal reticular cells or epithelial reticular cells in the cortex. In addition, the medulla may secrete a separate substance which is necessary for the maintenance of thymic epithelial cells, lymphoid cells or phagocytic (mesenchymal) cells. Recent work by Goldstein (1975) has produced two purified polypeptides from calf thymus. These have been found to be closely related to each other in activity and have been named thymopoietin I and II by Goldstein. One effect is the in vivo induction of neuromuscular block similar to that found in myas- thenia gravis (Goldstein, 1968; Goldstein and Manganaro, 1971). The other effects indicate that this substance can induce the appearance of antigens found on thymic lymphocytes, TL and Thy-1 (ƒÆ), when the extract is incubated with bone marrow hematopoietic cells (Back and Goldstein, 1975). It fails, however, to induce lymphopoiesis in thymic lymphocytes with or without mitogens such as Concanavalin A, and therefore would not appear to be the substance respon- sible for the in vivo proliferation of lymphocytes in the thymus. These experi- ments were carried out on thymus cell suspensions, whereas Metcalf envisaged the hormone acting in the structural context of the thymic cortex. Thymo- poietin does, however, induce lymphopoiesis in suspensions of spleen cells by enhancing the proliferation caused by Concanavalin A. lymphocyte Stimulating Hormone: One effect of thymectomy in neonatal animals is a depletion of lymphoid cells in the spleen and lymph nodes. This is accompanied by marked lymphopenia. Metcalf (1964) prepared an extract from mouse thymus gland, heat labile and non-diffusible, which when injected into neonatally thymectomized mice produced temporary lymphocytosis and an elevated lymphocyte/polymorphonuclear leukocyte ratio (L/P). Extracts from spleen, lymph node, liver and other organs were ineffective. He called this thymic extract lymphocytosis stimulating factor (LSF), and further studies revealed its presence in supernatants from thymus gland growing in tissue culture, and from serum of patients with chronic lymphoid leukemia and lymphosarcoma. It was also found in supernatants from serum of high leukemic strains of mice (AKR and C58). The sera from normal persons, or from patients with acute leukemia or chronic myeloid leukemia, were negative for LSF (Met- calf, 1966). Thymic extracts from calf, rat or isologous mouse caused increased lymph node weight, increased uptake of tritiated thymidine by DNA of lymph nodes 292 CASO Vol. 29 and increased uptake of 14C-labeled glycine into protein of lymph nodes (Klein , Goldstein and White, 1965). While thymic extracts gave only partial replacement in thymectomy so that lymphopenia and lymphoid organ atrophy were not corrected completely , grafts of isogeneic or allogeneic thymus in neonatally thymectomized mice prevented these effects if the grafts were done prior to extensive deterioration of the lym- phatic system (Leuchars et al., 1964; Miller, 1965). Moreover, these authors found that some dividing lymphocytes in the host spleen and lymph nodes , after antigenic stimulation, can be demonstrated to be derived from the thymus graft; while in thymectomized animals receiving thymus graft but no antigenic stimulation, most cells from the host are found in both the graft and the host's lymphoid organs (Miller, 1962). That a humoral factor from the thymus is responsible for the maintenance of lymphoid tissue in the spleen and lymph nodes was demonstrated by the use of millipore diffusion chambers (0.45 to 0.1ƒÊm pore size) which contain thymus grafts. These preparations prevented