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The discovery of the thymus was perhaps the major milestone of contemporary immunology. Here Anthony Davies reflects on the events surrounding this discovery and puts into perspective the significance of those experiments today.

The blood lymphocytes from a case of prolymphocytic leukaemia were subjected to a battery of different tests in order to establish as certainly as possible their T or B cell type of origin. The results of the tests for surface markers indicated the T-cell origin of the leukaemic cells in this patient, and this provided a good opportunity to determine the participation of T cells in the various tests proposed for measuring human lymphocyte function.

Lymphocytes from tumour-bearing lymph nodes in 15 patients with Hodgkin's disease, and lymphocytes from 6 spleens infiltrated by Hodgkin's tissue and 6 tumour-free spleens were studied using T and B lymphocyte markers. For comparison, lymphocytes from 16 tumour-free lymph nodes were similarly assessed. It was found that the tumour-bearing nodes showed a predominance of T lymphocytes compared with the control nodes, whereas in involved splenic tissue the proportion of T lymphocytes was slightly reduced compared with uninvolved spleens.

Experimental infection of normal CBA mice with the parasite Plasmodium berghei yoelii (P. yoelii) resulted in a mild, non-fatal and self-limiting infection which lasted for 15-17 days. Animals which recovered from the primary infection were immune to reinfection though parasites could be detected in the kidneys of such mice 4 weeks after recovery from infection. (No plasmodia were demonstrated in the peripheral blood and other tissues examined.) In T cell-deprived mice, P. yoelii infections resulted in a progressive parasitaemia and proved fatal in 35-40 days. Studies of fluorescent antibody levels and morphological changes in the spleens of infected normal and T cell-deprived mice showed that while normal mice produced high levels of IgG1, IgG2 and IgM antiplasmodial antibodies and developed a strong and sustained germinal centre response, in T cell-deprived animals the production of IgG1 antibodies was almost completely abolished and the germinal centre response severely impaired. Reconstitution of T cell-deprived mice with syngeneic thymus grafts resulted in partial restoration of immunological responsiveness. P. yoelii infections in these reconstituted animals ran a self-limiting course akin to that seen in normal CBA mice; the level of protective immunity and the germinal centre response correlated with the degree of reconstitution achieved.

Injection of syngeneic, but chromosomally distinguishable, lymph node or spleen cells into adult mice resulted in both T- and B-lymphocyte chimaerism. In spleen-injected mice haematopoietic chimaerism was also established. It appeared that the donor T lymphocytes were added to the host recirculating T-cell pool so that a hyperlymphoid state was produced. The percentage of donor T lymphocytes declined very slowly in normal mice, but remained stable in adult-thymectomized animals. There was no evidence of a homeostatic mechanism involving destruction of excess peripheral T lymphocytes or grossly affecting the flow of T lymphocytes from the thymus into the recirculating T-cell pool. A preliminary model of the T-lymphocyte system is proposed.

T-cell-deprived CBA mice were given thymus graft implants during the course of an infection with Trypanosoma musculi and at a time when the parasitaemia was high. As a consequence of the grafts, most of the mice survived the infection whereas control ungrafted mice died. In grafted animals there was generally a period of 60-80 days after grafting during which time parasitic numbers remained high, and a further period of 200 or more days when trypanosomes were still detectable in the circulation. The mice were highly variable in their responses and possible reasons for this are considered.

50 cases of primary acute lymphoid leukaemia (A.L.L.) were analysed for the presence of T and B membrane markers on bone-marrow and/or peripheral-blood cells. 26% of cases were predominantly T-cell in type, 4% were B, the remaining 70%, without detectable membrane markers, were classified as "null" cell A.L.L. Of particular interest is the correlation between this immunological classification and the prognosis, since T-cell and B-cell A.L.L. were associated with a poorer prognosis than null-cell A.L.L. in terms of both median length of first complete remission and median survival. With one exception the T-cell cases were, according to the W.H.O. classification, of either the prolymphocytic or macrolymphoblastic type of A.L.L. and were more extensive than the comparable null-cell A.L.L. In contrast, cases of the W.H.O. prolymphoblastic and microlymphoblastic types were all found to be null-cell A.L.L. and were associated with the worst and best prognosis respectively. The correlation found between the immunological classification of A.L.L. and the prognosis means that patients with a poor prognosis can be selected for more intensive therapy.

To study the function of different lymphocyte populations in the Moloney strain of murine sarcoma virus (M-MuSV) tumorigenesis, we gave M-MuSV injections to CBA mice selectively deprived of thymus (T) lymphocytes by thymectomy, X-rradiation, and syngeneic bone marrow injection. Although no tumors appeared in the control group, 80% of the derived mice had tumors that grew progressively and ultimately killed them. In deprived mice, grafted with a syngeneic thymus (reconstituted mice) before or after an M-MuSV injection, tumors regressed or did not develop. Histologically, the lymph nodes and spleens of reconstituted mice, compared to those of deprived animals, showed repopulation of the thymus-dependent areas and prominent follicles in the cortex. Moreover, tumor tissue of reconstituted mice was extensively infiltrated by lymphocytes. To evaluate the number of lymphoid cells needed to prevent or regress M-MuSV tumors, we injected varying amounts of lymphoid cells into deprived mice. Even low lymphocyte numbers (10(6) cells) were sufficient to exert, in some cases, protection against M-MuSV tumorigenesis. This effect was not abolished by subsequent splenectomy or antilymphocyte serum treatment. Finally, deprived mice, given repeated injections of antiserum (hyperimmune) against M-MuSV, had tumors which appeared only after a prolonged latency. From these results, it is concluded that T-cell population integrity is important in affording total host protection against the M-MuSV tumors.

In normal CBA/H mice implanted under the kidney capsule with eight CBA/H.T6T6 neonatal thymus lobes it was observed that the percentage of marked thymus-graft derived T cells in the periphery, after building up to a peak, showed a biphasic exponential decline. The initial decline was very rapid and appeared to be due to loss of the thymus-graft derived cells from the system. The later decline was slower and was the same as that of an introduced cohort of lymph-node lymphocytes. The second rate of decline was, however, considerably more rapid than that of lymph-node cohort in non thymus-grafted mice. We conclude that in multiply thymus-grafted mice the flow of cells through the T-cell pool is more rapid than in normal mice and that in this sense the thymus can be thought to drive the lymphoid system.

Lance and Taub (1) showed that when radioactively labeled lymphocytes were injected into a syngeneic mouse and the lymph node cells of this animal transferred to a second syngeneic recipient, the proportion of radioactivity found in the lymph node relative to the amount present in the spleen of the secondary recipient had increased markedly. The interpretation of this result was that some lymphocytes have the capacity to "home" to their organ of origin. The purpose of the experiments described here was to test the "homing" copacity of T cells by a method that did not involve radioactive labeling. It has been shown elsewhere that some or all mouse T cells are stimulated to divide in culture by the mitogens phytohemagglutinin (PHA) and concanavalin A (Con A) (2). We therefore elected to inject karyotypically distinct lymphocytes into syngeneic recipients and to follow their subsequent distribution by culture of lymph node and spleen cells of the recipient with PHA or Con A. In this manner the homing capacities of spleen and lymph node T cells could be determined, and furthermore, the effects of labeling with chromium-51 ((51)Cr) could be assayed with respect to the persistence of mitogen responsiveness in the injected cells.

Peripheral lymphoid tissues of mice which have been thymectomized at 2 or 4 weeks of age, that is, before they achieve adult body weight, have been shown to be lacking in cells responsive to the T-cell mitogen, phytohaemagglutinin, when the animals became adult, and these mice have also been shown to have a deficient immune response against sheep erythrocytes. It is suggested these effects of pre-adult thymectomy are consequent upon removal of the prime source of T cells prior to the animal having acquired complete T-cell populations of the adult. Spleens and lymph nodes of mice thymectomized at 8 weeks of age were found to have reduced numbers of cells susceptible to the cytotoxic effects of anti-Thy-1 serum as early as 4 weeks after the operation, whereas the number of lymphocytes responsive to T-cell mitogens in these lymphoid tissues was not reduced at this time. The number of spleen-borne antibody-producing cells in a primary or secondary response was not affected by 8-week thymectomy, either when the response was tested in the operated animal, or after transfer of cells from such an animal to an irradiated recipient. The results are discussed with respect to other work on the effects of thymectomy of mice during the post-neonatal and pre-adult period.

Following the implantation of syngeneic chromosomally marked thymuses under the kidney capsule of normal adult mice, donor T cells were found in the blood and other lymphoid organs. The mice were found to have increased numbers of Thy.1-positive cells in spleen and lymph nodes, increased peripheral blood lymphocyte levels and also augmented immune responses to sheep erythrocytes. The contribution made by an individual graft to the donor T cell pool was independent of the number of thymuses grafted as was the growth of the grafts themselves. The results reaffirm that there is no homeostatic feedback control of thymus growth in adult mice.

CBA/H and CBA/HT676 radiation chimeras were prepared by lethal irradiation and subsequent reconstitution with bone marrow of the host karyotype and thymocytes of the opposite karyotype. After T- and B-cell chimerism had been established, the animals were inoculated with SEWA or TA3Ha ascites tumor--donor cell hybrids were isolated following the explantation of TA3Ha tumor in selective HAT medium and by selecting for adherent cells from SEWA. The T6T6 chromosomal marker served to distinguish between the type of cell involved in the fusion. In all hybrids the donor component was a nonthymus-derived cell.