Molecular characterization of the low-affinity IgE receptor Fc epsilonRII/CD23 expressed by human eosinophils

CD23/Fc epsilonRII, the low-affinity receptor for IgE, is a pluripotent molecule with pleiotropic effects on cell activation and proliferation, antigen presentation and IgE synthesis. Initial investigations have suggested that CD23 expression was restricted to B lymphocytes and macrophages, but a much wider cell distribution is now acknowledged. Despite experimental evidence suggesting that human eosinophils could express the low-affinity IgE receptor Fc epsilonRII/CD23 with biological functions, no molecular cloning data have been reported until now. Whereas in situ hybridization confirmed the expression of CD23 mRNA in eosinophils, RT-PCR analysis of human eosinophil cDNA derived from a cDNA library revealed the presence of CD23, totally homologous with the CD23 a and b sequences. Eosinophils from different hypereosinophilic patients as well as the eosinophilic leukemia cell line EoL-3, analyzed by RT-PCR, expressed both CD23 a and b isoforms. In situ RT-PCR confirmed that mRNA corresponding to CD23 a and b isoforms was detected in human eosinophils. Finally, immunocytochemistry allowed us to show a differential expression of Fc epsilonRII/CD23 and Fc epsilonRI by subpopulations of eosinophils, with a preferential expression of Fc epsilonRII/CD23 in the hypodense population. These results provide definitive evidence that the low-affinity IgE receptor (Fc epsilonRII) synthesized by human eosinophils is identical to the CD23 molecule expressed on B cells, and that the two CD23 isoforms a and b can be expressed by eosinophils.