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B lymphocytes with receptors specific for the hapten fluorescein (FLU) were prepared from the spleens of mice of various ages. For most experiments, a one-step fractionation procedure based on the adherence of FLU-specific cells to FLU-gelatin was used. For some experiments, a subset of higher FLU-binding capacity was prepared from the FLU-gelatin binding population through the use of the fluorescence-activated cell sorter (FACS). FLU-specific B cells were placed into microculture with either FLU(3.6)-human gamma globulin (FLU(3.6)HGG) or FLU(12)HGG usually for 24 h at 37 degrees C. The tolerogen was then removed and 0.1 The results showed that with FLU(3.6)HGG, hapten-specific newborn B cells could be tolerized at 1,000-fold lower tolerogen concentrations than adult splenic B cells of equal antigen-binding capacity. The high-avidity subset was even more susceptible to tolerance induction. Tolerance could be induced within 8 but not within 2 h, and at lower tolerogen concentrations, longer periods of tolerogenesis were required for a given effect. Using a 24-h tolerogenesis phase, 50 percent reduction in clone frequency among newborn FLU-gelatin fractionated cells was achieved at 0.08 FLUI(12)HGG proved a more powerful tolerogen than FLU(3.6)HGG. It had an effect on adult cells, 50 percent reduction in clone frequency being noted at around 1 
g/ml of a T-independent antigen, FLU-polymerized flagellin, was substituted. 3 days later, cells were harvested from the microcultures and assayed for FLU-specific plaque-forming cells to determine any reduction in clonable hapten-specific B cells which the tolerogenesis treatment might have induced. g/ml of FLU(3.6)HGG. Tolerance induction in immature B cells was inhibited by the concomitant presence of a polyclonal B-cell activator, Escherichia coli lipopolysaccharide (LPS) but tolerance once induced, was stable to challenge with LPS. Tolerogenesis was hapten specific. The proportion of tolerizable cells in spleens decreased with increasing age, reaching 50 percent at around 9 days. g/ml. However, and in contrast to results claimed for other T- independent systems, there still was a major difference between immature and mature B cells, the immature cells displaying much greater sensitivity to tolerogenesis.
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