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In vitro thymus suppression of hemagglutinin production in Xenopus laevis: location, drug and temperature sensitivity.

Suppression of splenic hemagglutinin (HA) production by allothymuses in vitro was further studied in Xenopus laevis, the South African clawed toad. Since thymic capacity to suppress splenic HA secretion into the culture medium is retained in animals previously exposed to N-methyl-N-nitrosourea (NMU), which destroys the thymus cortex in this species, suppressor function must be located in the thymus medulla. Reciprocal thymus-spleen combinations showed that normal thymus can suppress immunised spleen fragments from NMU-treated animals. Since Xenopus exposed to NMU are also devoid of helper function, thymus suppression acts directly on the antibody forming system. Cyclophosphamide removed thymic suppression and enhanced spleen fragment antibody production, while Cyclosporin A enhanced thymic suppression and blocked HA production. Therefore, antigenic stimulation of thymic suppression and antibody production are inversely related in this species. Long-term storage of Xenopus in the cold increased lymphoid cellularity of the thymus medulla, but thymuses from cold-stored animals could not suppress normal spleen fragments and their spleens were not suppressed by normal thymuses. Thus, ectotherms may retain immunological capacity when subjected to prolonged cold by a loss of thymic suppression.