HYDROXYUREA BEFORE ORAL ANTIGEN BLOCKS THE INDUCTION OF ORAL TOLERANCE

1. Treatment with hydroxyurea (HU, 1 mg/g ip, 2 doses applied 7 h apar t) eliminates the majority of cells undergoing mitosis (cycling cells) without affecting non-cycling cells. Oral tolerance, induced by a sin gle gavage with 20 mg of ovalbumin, results in a drastic inhibition of anti-Ova antibody responses in young adult mice. Oral tolerance is ac tively maintained by the presence of specific suppressor T cells which may adoptively transfer the tolerance to naive syngeneic recipients. Under the clonal selection hypothesis, the induction of oral tolerance should be blocked by HU treatment applied soon after oral exposure to the antigen by the elimination of specific clones of lymphocytes acti vated by tolerogenic presentation of the antigen. 2. However, treatmen t with HU initiated 3, 6 or 24 h after oral exposure to ovalbumin had no effect on the induction of oral tolerance in B6D2F1 mice. However, treatment with HU 24 h before antigen exposure, totally blocked the in duction of tolerance. Treatment with HU 72 h before ovalbumin had no e ffect. 3. In animals treated with HU 24 h before, the adoptive transfe r of normal thymus, bone marrow or spleen cells partially restored the susceptibility to the induction of oral tolerance. 4. The results sug gest that cycling cells, which may be totally regenerated within 72 h after treatment with HU, and are present in normal thymus, bone marrow and spleen, am crucially important for the induction of oral toleranc e.