Development of mast cells in vitro. II. Biologic function of cultured mast cells

The role of IgE in reaginic hypersensitivity is known to be related to its high affinity for certain target cells such as mast cells and basophil granulocytes. The recent discovery of IgE secreting tumors in rat strain and the isolation of rat monoclonal IgE made the rat mast cell system one of the most useful tools for the studies of sensitization of the cells with IgE. The present experiments were undertaken to study the biologic function of cultured mast cells. The cells were obtained by long term culture of rat thymus cells on rat embryonic fibroblast monolayers. Pure mast cell preparations obtained by culture were incubated with ¹²⁵I labeled rat E myeloma protein to study receptors for IgE on their surface. When the cells were obtained after 35 to 45 days culture, the average of receptors per mast cell was 100,000 to 400,000. An equilibrium constant of the binding reaction between their receptor and rat IgE was in the order of 10⁸ M^-1. The histamine content of the cultured mast cells was 0.2 to 5 μg/10⁶ cells. The measurement of histamine content in mast cells recovered after different periods of culture suggested that the histamine content increased with maturation. Even after 45 to 50 days culture, the histamine content of cultured mast cells was significantly lower than that in rat peritoneal mast cells. The cultured mast cells were passively sensitized in vitro with rat IgE antibody against Nippostrongylus brasiliensis. The sensitized cells released histamine upon incubation with the antigen. It was also found that cultured mast cells released histamine upon exposure to compound 48/80. These results indicated that cultured mast cells have physiologic functions similar to those of normal rat mast cells, but they have not reached full maturation.