The breaking of tolerance against the lipopolysaccharide from E. coli 055:B5 was studied. It was found that immune responsiveness recovered very slowly in vivo, tolerance still existing 3 wk after the last tolerizing injection. However, if spleen cells from tolerant mice were transferred into irradiated syngeneic recipients, the tolerant state was readily broken. Spleen cells transferred 3 days after the last tolerance-maintaining dose did not respond, whereas cells transferred on day 5 or 7 responded equally well as normal spleen cells. It was also possible to break tolerance by incubating tolerant spleen cells, which did not respond after transfer, for 20 hr in vitro before transfer into irradiated recipients. The results suggest that there exist reversibly inactivated cells in tolerant animals and that these cells can be reactivated upon removal of the cells to a neutral environment.

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