Upregulated surface expression of intracellularly sequestered Igepsilon receptors (FcepsilonRII/CD23) following...

We investigated the regulation, secretion, and surface expression of the low-affinity FcepsilonRII receptor (CD23) in eosinophils isolated from human blood using multiple monoclonal antibodies (mAbs) directed at different epitopes of human CD23. Substantial surface expression of CD23 was not demonstrated in the resting state. Mean fluorescence intensity (MFI) measured by flow cytometry was 7. 1 +/- 0.8 for 9P25 mAb (p = NS) and 15.7 +/- 3.8 for BU38 mAb (p <. 04) versus 5.3 +/- 1.0 for IgG1 isotype control Ab. By contrast, MFI using BU38 mAb was 154 +/- 18 for JY-B lymphocytes (p <.0001 versus eosinophils). Despite weak surface expression, eosinophil permeabilization demonstrated substantial intracellular expression of CD23; MFI was 33.6 +/- 5.2 for 9P25 mAb versus 4.4 +/- 0.43 for IgG control (p <.001). Western blot analysis using both positive and negative controls demonstrated immunological identity with CD23 on JY-B lymphocytes. Activation of eosinophils caused rapid translocation of CD23 to the surface membrane (160 +/- 33 MFI; p <. 005), which was maximal within 30 sec. Secretory CD23 was detected within the perfusate also at 30 sec and was fully reinternalized at 10 min. This is the first demonstration of the presence of intracellular CD23 in human eosinophils. Our data indicate that eosinophils rarely express CD23 on their surface but are capable of transient high-level expression and secretion with rapid reuptake of intracellular stores of CD23.