Rat basophil
leukemia (RBL) cells were sensitised with varying proportions of monoclonal
IgE anti-
ovalbumin (OVA) and anti-DNP
antibodies, and
serotonin release was measured after challenge with aggregated OVA or dinitrophenylated
human serum albumin (
DNP-HSA). Highly aggregated OVA was shown to provoke the degranulation of RBL cells that had been sensitised with an
IgE preparation containing 2%
IgE anti-OVA
antibodies. Highly substituted DNP32-HSA induced degranulation of RBL cells sensitised with just 0.5%
antigen-specific
IgE. When cells were sensitised with high percentages of specific
IgE, maximum degranulation was seen at concentrations of 2 micrograms/ml (aggregated OVA) and 50 ng/ml (
DNP-HSA), while moderate degranulation was still seen at
antigen concentrations as low as 50 and 2 ng/ml, respectively. Low-molecular weight aggregates of OVA and low-valency DNP4-HSA only stimulated degranulation when high percentages of RBL
Fc epsilon receptor were occupied by
antigen-specific
IgE. The sensitising abilities of two anti-DNP
monoclonal antibodies of differing affinities were compared. When challenged with low-valency
antigen, only cells sensitised with the higher-affinity
monoclonal antibody exhibited moderate levels of degranulation. Degranulation required exposure to high
antigen challenge doses (5 micrograms/ml). Cells sensitised with either
monoclonal antibody responded strongly when challenged with a wide range of concentrations (1-250 ng/ml) of high-valency DNP32-HSA, although greater sensitivity was always seen with the higher-affinity antibody. These results suggest that
antigen valency is a critical parameter for mast cell function, and that low-affinity antibody may be capable of sensitising mast cells to high-valency
antigen.