Previously uncharacterized eosinophil chemotactic activity of apparent m.w. 1500 to 2500 by Sephadex G-25 gel filtration was present preformed in rat peritoneal mast cells, was associated with the mast cell granules, and was released by challenge of the mast cells with rabbit anti-rat F(ab′)2 antiserum. Further purification of this activity by Dowex-1 chromatography revealed two peaks of eosinophil chemotactic activity eluting at pH 5.4 to 4.4 and pH 3.1 to 2.1, respectively. On Sephadex G-50 gel filtration both activities filtered just behind the α chain of insulin (m.w. 2540). The more acidic material gave a major peak of eosinophil chemotactic activity on high pressure liquid chromatography whereas the less acidic material separated into two equal peaks of eosinophil chemotactic activity of different hydrophobicity. Both the less acidic and the more acidic intermediate m.w. activities required a concentration gradient for expression of eosinophil chemotactic activity and were capable of rendering eosinophils unresponsive to homologous or heterologous chemotactic stimuli. Checkerboard assays of the more acidic factor over a range of concentrations did not reveal chemokinetic activity in the chemotactic dose-range and neither the more acidic nor the less acidic factors enhanced the spontaneous migration of eosinophils when placed on the cell side alone of the chemotactic chamber. The mast cell serves as a source of several eosinophil chemotactic factors differing in size and charge, which could possibly stabilize the chemotactic gradient and sustain an influx of eosinophils to the site of mast cell activation.

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