The mAb AA4 binds to novel derivatives of the
ganglioside Gd1b on rat basophilic
leukemia (RBL-2H3) cells. Some of the
gangliosides are located close to the high affinity
IgE receptor (
Fc epsilon RI), and binding of mAb AA4 inhibits
Fc epsilon RI-mediated histamine release. In the present study, mAb AA4 was found to bind exclusively to mast cells in all rat tissues examined. In vitro, within 1 min of mAb AA4 binding, the cells underwent striking morphologic changes. They lost their normal spindle shaped appearance, increased their ruffling, and spread over the surface of the culture dish. These changes were accompanied by a redistribution of the cytoskeletal elements, actin,
tubulin, and
vimentin, but only the actin was associated with the membrane ruffles. Binding of mAb AA4 also induces a rise in intracellular
calcium, stimulates
phosphatidyl inositol breakdown, and activates PKC. However, the extent of these changes was less than that observed when the cells were stimulated with
antigen or antibody directed against the
Fc epsilon RI. None of these changes associated with mAb AA4 binding were seen when the cells were exposed to nonspecific
IgG,
IgE, or four other anti-cell surface
antibodies, nor were the changes induced by binding mAb AA4 at 4 degrees C or in the absence of extracellular
calcium. Although mAb AA4 does not stimulate histamine release, it enhances the effect of the
calcium ionophore A23187 mediated release. The morphological and biochemical effects produced by mAb AA4 are similar to those seen following activation of the cell through the
IgE receptor. Therefore, the surface
gangliosides which bind mAb AA4 may function in modulating secretory events.