Background: The IgE receptor (FcepsilonRI) may exist as a tetramer (alphabetagamma2) or a trimer (alphagamma2) because FcepsilonRIbeta is dispensable for membrane expression of FcepsilonRIalpha. FcepsilonRIbeta amplifies signaling of FcepsilonRI so that regulation of FcepsilonRIalpha:beta stoichiometry would affect cellular responsiveness.
Objective: We examined basophils from a variety of donors for differences in their expression of FcepsilonRIalpha and FcepsilonRIbeta protein.
Methods: Enriched blood basophils were assessed at baseline and after IL-3 culture for FcepsilonRIalpha and FcepsilonRIbeta protein by Western blotting, surface FcepsilonRIalpha by flow cytometry, and FcepsilonRIbeta mRNA by real-time PCR. Basophil functional response was measured by allergen-triggered histamine release.
Results: For the FcepsilonRIalpha subunit, 2 protein bands with molecular weights of 50 kd and 60 kd were identified by Western blots. The 60-kd band correlated to surface-expressed FcepsilonRIalpha detected by flow cytometry (Spearman R = 0.78, P <.01). Surface FcepsilonRIalpha also correlated with FcepsilonRIbeta protein (Spearman R = 0.92, P <.01). FcepsilonRIbeta protein levels increased disproportionately with higher surface FcepsilonRIalpha expression. The ratio of FcepsilonRIbeta to FcepsilonRIalpha varied 10-fold among donors and correlated with surface FcepsilonRIalpha. Basophil 50-kd alpha protein levels were similar despite a 10-fold range in surface FcepsilonRIalpha expression, implying stores of this protein such as those found in eosinophils. Unlike eosinophils, the basophil 50-kd protein was lost with culture and was absent from supernatants. Levels of beta protein and mRNA were enhanced by IL-3 culture, whereas FcepsilonRIalpha expression (by flow cytometry and 60 kd) was not.
Conclusion: These findings demonstrate variable stoichiometry of FcepsilonRIalpha:beta in whole cells and that this stoichiometry can be altered by IL-3 culture. With the assumption that all detected beta protein is surface expressed, these findings suggest a variable stoichiometry for FcepsilonRIalpha:beta that is also related to FcepsilonRIalpha surface expression.