The Florey lecture, 1989. Self-tolerance: the key to autoimmunity

Self-nonself discrimination and tolerance in T and B lymphocytes

The immune system must not only fight off infections, but also ensure that it does not react against its own body tissues. Since clones of lymphocytes have predetermined reactivities, some will be self-reactive and have the potential to cause damage. They should therefore be neutralized in some way. In a system as complex and important as that governing self-tolerance, many mechanisms must exist to neutralize autoaggressive lymphocytes. They may be classified under two main groups. In one the tolerant state arises from the physical or functional silencing of potentially autoaggressive lymphocytes after antigen encounter. This may involve clonal deletion, clonal abortion or clonal anergy. In the second, regulatory mechanisms of the immune system itself may hold autoreactive lymphocytes in check, for example through the operation of idiotypic network interactions and the action of specialized suppressor cells. Much evidence has accumulated for the physical deletion of autoreactive T cells as they mature in the thymus. The fate of any that escape thymus censorship has been the subject of recent research and is discussed here. Under certain conditions, self-tolerance must also be imposed at the B-cell level to prevent the production of potentially damaging autoantibodies. Although the mechanisms which silence self-reactive lymphocytes are very efficient, self-tolerance can break down, and autoimmunity will thus ensue. The main factors responsible for this are briefly described here.

The Croonian Lecture, 1992. The key role of the thymus in the body's defence strategies

For centuries the thymus has remained a mysterious organ with largely unknown functions. The first demonstration of its crucial role in the development of the immune system was reported in 1961, when it was found that mice thymectomized at birth had poorly developed lymphoid tissues, impaired immune reactivities, and an inordinate susceptibility to develop infections. Although thymus lymphocytes were for a long time deemed immunoincompetent, it was shown in 1967 that they could respond to antigen by proliferating to give rise to a progeny of cells which did not secrete antibody (T cells), but which had a remarkable ability to induce bone marrow cells (B cells) to become antibody formers. This was the first unequivocal demonstration of a major division of labour among mammalian lymphocytes. Tremendous progress in our understanding of the function of the thymus and of the T cells derived from it followed. Distinct T cell subsets were characterized and shown to have an essential role in initiating and regulating a variety of immune responses. The ontogenetic events which occurred during their differentiation were mapped, and this allowed studies of the selection of the T cell repertoire. The major histocompatibility complex and associated peptides were shown to govern T cell selection and antigen activation, and the antigen-specific T cell receptor and the genes which code for it were characterized. Future studies should allow some insight into how to activate T cells more effectively for vaccination purposes, and how to switch them off to prevent autoimmune reactions and to induce tolerance to transplanted tissues.

Limiting dilution analysis in leprosy

Peripheral blood mononuclear cells (PBM) obtained from leprosy patients and healthy controls were cultured with Mycobacterium leprae and the control antigens, BCG and SKSD. Parallel cultures were supplemented with additional interleukin-2 (IL-2). On the basis of the level of response to M. leprae, leprosy patients could be divided into low, intermediate and high responders. The addition of IL-2 resulted in enhanced proliferation to antigen only by cells from intermediate responders. This effect was neither antigen specific nor was it confined to cells from leprosy patients. When limiting dilution analyses were performed on cells from 26 patients across the leprosy spectrum, no M. leprae-reactive lymphocytes were detected in cells from subjects with lepromatous disease. The precursor frequency for cultures containing M. leprae plus IL-2 was no greater than that of cultures containing IL-2 alone, thereby excluding the possibility of clonal anergy reversible with IL-2. This was observed in both untreated patients and those on long-term treatment, which made sequestration of antigen-reactive cells within leprosy lesions an unlikely explanation. On the other hand, M. leprae-reactive lymphocytes were detected in patients with tuberculoid and borderline tuberculoid disease and in two subjects with borderline lepromatous leprosy in type I reversal reaction. IL-2 reactive cells were detected in all patients regardless of clinical classification. Three 'suppressor' curves were obtained but were not confined to cells from lepromatous patients. Taken together, these findings suggest that the non-responsiveness to M. leprae characteristic of the great majority of multibacillary patients is due to an absence of antigen-sensitive T cells.