Challenge of mast cells with increasing amounts of antigen induces desensitization

In vivo rush desensitization is a procedure widely employed to quickly desensitize allergic patients by administering increasing doses of the offending antigen at short intervals. The mechanism(s) underlying this process and the possible role of mast cells in it have not been well delineated.

To define an ex vivo model for rush desensitization utilizing the mast cell-fibroblast coculture system.

Rat peritoneal mast cells were cultured on 3T3 fibroblasts and were preincubated with saturating or suboptimal concentrations of IgE anti-DNP antibodies. Mast cells were then repeatedly challenged at 30 min intervals, with increasing amounts (0.001-100 ng/mL) of the antigen DNP-HSA, in the presence of calcium ions. Parallel control cultures were stimulated only once by each antigen concentration. In another set of experiments, mast cells were repeatedly activated with increasing concentrations (0.1-1000 ng/mL) of compound 48/80. Supernatants and cell sonicates were assessed for histamine content and the percentage of histamine released was calculated.

When saturating concentrations of IgE anti-DNP antibodies were used, mast cells challenged repeatedly with DNP-HSA did not release significant amounts of histamine up to an antigen concentration of 1 ng/mL. At this stage they were partially desensitized, releasing only 108.3 +/- 17.1 ng histamine/plate (7.9 +/- 0.8%). A marked desensitization was observed at optimal antigen concentration (100 ng/mL), where experimental mast cells released only 45.6 +/- 10.9 ng/plate, compared with 661.9 +/- 48.2 ng/plate in firstly challenged cultures. Desensitization was probably not due to mast cells histamine depletion, since the cells still contained large amounts of histamine (579.5 +/- 108.6 ng/plate) at the end of the procedure. A similar pattern of desensitization was observed when mast cells were preincubated with a suboptimal concentration of IgE antibodies. Activation of mast cells with increasing amounts of the IgE-independent secretagogue, compound 48/80, did not cause desensitization since at each concentration both repetitively challenged and control cultures released similar amounts of histamine. Furthermore, challenge of antigen-desensitized mast cells with compound 48/80 caused the release of 75.9 +/- 4.9% histamine comparable to the percentage of histamine released from controls (79.5 +/- 6.7).

Repeated exposure of mast cells to gradually increasing amounts of antigen induces their refractoriness. This observation would suggest a role for mast cells in rush desensitization procedure in vivo. Our coculture system may serve as a useful model for studying this process.