Mast cells retain their responsiveness upon continuous and repetitive exposure to antigen.

To assess the effects of prolonged and repetitive exposure of antigen on mast cells (MC) we utilized our co-culture system in which rat peritoneal MC are kept viable and functionally active on 3T3 fibroblasts. MC were presensitized with IgE anti-dinitrophenyl-human serum albumin (anti-DNP-HSA), activated with the relevant antigen (DNP-HSA) (69.4% histamine release) and incubated for 7 days in medium alone or medium containing the antigen with or without Ca2+ ions. During the prolonged incubation MC released only small amounts of histamine (20-26 ng/plate/day) under all the different culture conditions. Moreover, these MC were not activated after each medium replacement with fresh antigen. When activated MC cultured in medium containing antigen and Ca2+ were presensitized prior to a second challenge performed on days 1, 2, 5 and 7, they released 35-45% histamine but were still partially desensitized compared to initially challenged MC. Immunofluorescence studies demonstrated that the MC unresponsiveness to a second challenge could be partially attributed to lack of membrane-associated IgE antibodies. Repetitive activation (four times) of MC at daily intervals did not abrogate their releasing capacity although the intracellular histamine content was diminished. When repetitive activation was performed 2 and 4 hr after the first challenge, a more intense MC refractoriness was observed (release of 17.3% and 21.1% histamine respectively versus 67.7%). It is suggested that MC activation occurs even after continuous exposure to antigen and that MC can respond to repetitive immunological stimulation.