Studies on the role of the thymus and spleen in the immune system of the clawed toad (xenopus laevis, daudin)

Abstract

1) A thymus-independent alioimmune system emerges early in the life of the clawed toad, independent of changes in the lymphoid system occurring at metamorphosis. Studies in Chapter 2 on first and second-set allograft rejection, both in larvae blocked from metamorphosing and in normally developing toads, show that there is no qualitative difference between the larval and adult allo-immune response. 2) In the intact adult, the spleen is shown to be a major site of cellular reactivity following allografting. Spleen transfer experiments between mutually tolerant partners demonstrate that this reactivity has an immunological basis. Preliminary studies in Chapter 3 implicate the spleen as a site of alioimmune memory.3) Work in Chapter 4 employs the sensitive ICA technique for investigating the role of thymus and spleen in humoral immunity. RFC kinetics in the spleen following administration of different doses of sheep erythrocytes reveal a dose response curve more similar to that described in mammals than in other amphibian species. Higher immunegen doses result in more rapid and more elevated RFC production. Morphological characterisation of rosettes failed to distinguish separate functional populations (i.e. "helper" and antibody producing) of lymphocytes in Xenopus. Early thymectomy abrogates responsiveness to injected SRBC as measured by cellular proliferation and RFC levels.4) Chapter 5 identifies a critical role of the thymus throughout much of larval life in the maturation of humoral immunity. Thymectomy up to stage 53 abrogates RFC production to SRBC, and antibody production is impaired by thymectomy as late as stage 57. Thymectomy later at stage 52 has no effect on allograft responses. The larval thymus controls development of both alioimmunity and humoral immunity but commands the maturation of the latter for a more prolonged period of development. Adult thymectomy has no effect on SRBC responsiveness up to 250 days post-surgery. 5) Chapter 6 provides new insight into the ultrastructure of developing thymus and emphasises the rudimentary nature of this organ at the time when thymectomy is routinely performed.