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The antigen stimulation of thymidine uptake by immune rabbit lymph node cells in vitro was studied. The kinetics of the response varied depending on the time between immunization and culture. Cultures set up early after immunization showed a peak response over day 1-2 of culture while those set up late after immunization showed a peak response over day 3-4. Studies using the metabolic inhibitor BUDR suggested that this is due at least in part to a larger recruitment of cells into the response during the third day of culture when lymph node cells taken late after immunization were used. Removal of antigen from cultures after brief exposure of cells at 4° reduced the magnitude but did not eliminate the proliferative response, suggesting that some antigen is specifically bound to the cells in the cold. Readdition of antigen restored normal reactivity. Holding cells at 4° for 4 hours without antigen had no effect on their response to subsequent addition of antigen. However, if cells were held at 4° for 3 hours with antigen present a severe degree of depression of subsequent thymidine incorporation was observed in some but not all experiments. This depression of responsiveness was interpreted as an in vitro phenomenon comparable to immunologic tolerance. [ABSTRACT FROM AUTHOR]

Immunization of HSA-tolerant animals with sulphanil-HSA induces the formation of antibodies that react with native HSA. To test whether the anti-HSA response does actually represent complete restoration of the original reactivity, the antibodies produced following immunization of tolerant animals with sulphanil-HSAS were compared to those produced by normal rabbits immunized with the native antigen. Gel-diffusion analysis of the precipitins disclosed that the patterns of the two types of antibody were not completely identical. The antibodies of the 'restored' tolerant rabbits were directed mainly towards an antigenic determinant of the native HSA, to which normal rabbits immunized with HSA did not respond. When the 'restored' rabbits were challenged with native HSA after a prolonged rest period, the antibodies formed thereafter were completely identical with anti-HSA produced by normal HSA-immunized rabbits. Thus, complete restoration of the original immunological reactivity to HSA was achieved following an intermediary stage of atypical anti-HSA elicited by the conjugated protein. Attempts to break down natural tolerance to RSA by treatment with sulphanil-RSA failed to give any evidence of an autoimmune elimination of RSA. To test whether this inability to break tolerance to RSA was due to the presence of an excess of native protein, animals that had been made tolerant to HSA were immunized with a mixture of sulphanil-HSA and native HSA. Under such conditions, the termination of acquired tolerance was inhibited. The possible relevance of these observations to the cellular basis of immunological tolerance is discussed. [ABSTRACT FROM AUTHOR]