IgE interacts with FcεRI to induce FcεRI up-reglation on Human Basophils (HB)

FcεRI expression on HB is altered by the presence or absence of IgE Ab. It is not clear whether this alteration in cell surface expression is coordinated by an interaction between IgE Ab and FcεRI itself or through another IgE binding protein on HB. Four possibilities were examined, IgE interacting through CD32 (IgG receptor on HB), CD23, εBP or FcεRI. HB (40-99%) were cultured for 7 days (IMDM media/10 ng/ml IL-3) ±IgE (1μg/ml), which caused a 3-10 fold up-regulation of FcεRI. FcεRI expression was detected by flow cytometry using the monoclonal anti-FcεRIα Ab, 22E7. Previous studies excluded a role for CD32 as IgG did not cause or influence IgE-induced FcεRI expression. α-Lactose had no influence on the ability of IgE to upregulate FcεRI, excluding a role for εBP. A role for CD23 was excluded by 4 different studies. Purified HB were examined by flow cytometry for CD23 expression; no difference between MHM6 (anti-CD23) and isotype control IgG was found. Purified HB were also not found to express mRNA for CD23 as determined by RT-PCR while the same PCR primers could detect mRNA in lymphocytes (32 cycles). Anti-CD23 (MHM6) was not found to induce or alter IgE-induced up-regulation of FcεRI. Finally, two recombinant IgE-Fc's, one wild type (IgE-Fc(WT)), one mutant (IgE-Fc(R334S)), were used to upregulate FcεRI. The IgE-Fc(WT)and IgE-Fc(R334S) have been previously shown to have equivalent binding characteristics to CD23 but IgE-Fc(R334S) has a 33 fold lower affinity for binding to FcεRI. When these fragments were used to up-regulate FcεRI on HB, IgE-Fc(WT) was found to up-regulate expression with 30 fold greater potency than IgE-Fc(R334S) (EC50 of 1.7 × 10^-10 M vs 4.9 × 10^-9 M for IgE-Fc(WT) and IgE-Fc(R334S), respectively). Collectively these data indicate no role for CD23 and indicate that IgE interacts with FcεRI to induce its up-regulation.