Inhibition of human antigen-specific memory B cell response in vitro by a diphtheria toxin-related interleukin 2 fusion protein

Recombinant diphtheria toxin-related interleukin-2 fusion protein (DAB 486IL-2) is specifically cytotoxic for cells bearing the high-affinity IL-2 receptor (p55/75). We evaluated the effects of DAB 486IL-2 on the generation of tetanus toxoid(TT)-specific IgG antibody-forming cells in 6-day cocultures of human splenocytes and TT-coupled Sepharose beads. The results indicate that a significant portion (30–75%) of the anti-tetanus toxoid IgG response in vitro was susceptible to inhibition by 10 −10 M DAB 486IL-2. The inhibition required both the IL-2 portion of the fusion protein and an active toxin moiety and was greater when the IL-2 toxin was added on Day 3 as compared with Day 0 of culture. The induction of the p55 (Tac) subunit of the IL-2R was demonstrable by two-color flow cytometry on a small percentage (5%) of B cells and on a higher percentage (10%) of non-B cells 3 days after exposure to TT-coupled Sepharose. Short-term (2 hr) treatment of T and B cell subpopulations separated on Day 3 of culture followed by remixing indicated that while activated T helper cells were most strongly inhibited by DAB 486IL-2, up to 50% of the TT-specific IgG response was inhibited by treatment of B cells alone with DAB 486IL-2. Our results suggest that a strategy of eliminating human memory B cells by a combination of antigen activation and properly timed administration of a recombinant lymphokine-toxin fusion protein is feasible.