Plasmacytoid dendritic cells (pDCs) display surface but not membrane-bound IgE across a broad range of total serum IgE levels

IntroductionPlasmacytoid dendritic cells (pDCs) are unique antigen presenting cells that may be implicated in allergic disease because they bind IgE on their surface and modulate important Th1/Th2 cytokine responses. While conducting in vitro experiments using excess omalizumab to capture IgE, we discovered that this immunoglobulin is not readily removed from pDC to the same extent observed for basophils suggesting that a portion of the IgE on pDC is membrane bound. MethodsBasophils and pDC were prepared from leukopacks using established protocols. In order to isolate PBMCs, blood was also drawn from consenting donors with a wide range in total serum IgE levels. B cells and pDCs were identified by flow cytometry by gating on CD19/CD27 and BDCA2/CD123, respectively. Quilizumab, a mouse anti-human monoclonal IgG1 antibody with specificity only for membrane-bound IgE, was used to detect this molecule in both cell types. ResultsWhen used in vitro at 1.5 mg/ml, omalizumab removed [~]80-90% of the IgE expressed by basophils. In contrast, IgE expression decreased only 30-40% on pDC treated likewise. Upon analyzing pDC for membrane-bound IgE, there was no significant difference in number of target-bound cells and mean fluorescence between the mouse isotype control and quilizumab among pDCs (P = 0.125, 0.165). However, the ratio of the proportion of target-bound CD19+27+ B cells compared to other cells was 32:1 for isotype (P < 0.001) and 54:1 for quilizumab (P = 0.015). ConclusionOverall, this study demonstrates that while pDCs express significant levels of Fc{varepsilon}RI that binds IgE to the surface, there is no appreciable amount of membrane-bound IgE noted. The reason for omalizumabs poor ability to remove IgE from pDCs remains unknown.