We have examined the cytological and functional characteristics of mast cells grown in tissue culture from the bone marrow of mice and rats and compared them with mast cells isolated from the peritoneal cavity of these animals. In both species, bone marrow-derived mast cells (considered to be a model of mucosal mast cells) have fewer cytoplasmic granules and lower histamine content than peritoneal mast cells. Sprague-Dawley rat peritoneal mast cells were responsive to various endogenous peptides and to compound 48/80. However, peritoneal mast cells isolated from BDF₁ mice (a strain widely used to obtain bone marrow-derived mast cells) were not responsive to the same secretagogues. Rat and mouse bone marrow-derived mast cells obtained from Sprague-Dawley rats and BDFI₁ mice were also hyporesponsive to calcium ionophore as compared to peritoneal mast cells and unresponsive to compound 48/80 and peptides. Despite the similarity of the functional characteristics of mouse and rat bone marrow-derived mast cells, mouse bone marrow-derived mast cells could not be used as a model of responsiveness to peptides for rat mucosal mast cells because of the differences in responsiveness between the peritoneal mast cells in the two species. Obtention of homogeneous rat bone marrow-derived mast cells may provide a useful tool to study the functional heterogeneity in an intraspecies system since most of our knowledge on mast cell physiology and pharmacology is derived from studies on rat peritoneal mast cells.

The binding of human myeloma IgE immunoglobulin on rat mast cells was studied by three independent techniques. A mixed agglutination reaction with anti-IgE-coated Sephadex granules demonstrated that only human IgE-coated rat mast cells were clearly agglutinated. This binding is strong (50% agglutination) in 3 min and progresses for 30 min (95% agglutination). Autoradiographic studies with ¹²⁵I-labelled human serum proteins demonstrated the selective formation of grains on mast cells incubated with labelled IgE. Upon action of anti-IgE antiserum on IgE-coated rat mast cells, the mast cells released up to 47.5%) of their total histamine content in a fluorometric histamine assay. A relationship was established between sensitizing doses of human IgE and histamine release. These results bring evidence for a binding of human IgE on rat mast cells and imply the existence of receptors for this immunoglobulin on mast cell membrane.

Peritoneal mast cells from immunized rats can form rosettes with antigen-coated sheep red blood cells. The receptor responsible for this active rosette formation is shown to be IgE cytophilic antibody: rosettes are inhibited by previous contact of mast cells with antigen, or with anti-IgE antiserum; the kinetics of mast cell rosettes following a primary immunization with ovalbumin and Bordetella pertussis vaccine is similar to the kinetics of reaginic antibody response. Furthermore, a reaginic serum can induce passive rosette formation. There is no correlation between cell-bound and circulating IgE.

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