Allergen-induced histamine release in rat mast cells transfected with the alpha subunits of Fc epsilon RI.

A rat mast cell histamine assay (RMCHA) has been developed to quantitate the biological activity of a recombinant humanized, monoclonal anti-IgE antibody (rhuMAbE25). Rat mast cells (RBL 48), transfected with the alpha subunit of the high affinity human IgE receptor (Fc epsilon RI), were presensitized for 2 h with human plasma containing IgE specific for ragweed and challenged with ragweed allergen in the presence of 50% D2O. Histamine release plateaus at 0.1 micrograms/ml of ragweed. The release of histamine was time, temperature and Ca2+ dependent. This ragweed-induced histamine release could be inhibited by rhuMAbE25 in a dose-dependent fashion with an IC50 of 1.19 +/- 0.31 micrograms/ml (n = 25). Other humanized MAbs and recombinant human growth factors neither trigger histamine release nor inhibit ragweed-induced histamine release. This RMCHA correlates well with the human basophil histamine assay (HBHA) (Fei et al., 1994) with a correlation coefficient of 0.93 (n = 59, p < 0.0001). Histamine was also released when the cells were presensitized with human plasma containing the respective allergen-specific IgE and then challenged with standardized mite, D. farinae, house dust mix, standardized cat pelt, or Alternaria tenuis. Comparison of allergen-induced histamine release showed a good correlation between RMCHA and HBHA with a correlation coefficient of 0.69 (n = 37, p = 0.0001). We conclude that RMCHA provides a useful tool to confirm allergen-specific IgE in allergic patients and can be used to evaluate the biological activity of any anti-IgE monoclonal antibody. Moreover, RMCHA provides an unique opportunity to study the mechanism of IgE-mediated histamine release in the absence of interfering proteins and growth factors normally present in whole blood.