Bcl-2 expression by eosinophils in a patient with hypereosinophilia

Abstract

The hypereosinophilic syndrome (HES) represents a heterogenous group of disorders characterized by hypereosinophilia in the blood and bone marrow, as well as eosinophilic tissue infiltration. The mechanisms of eosinophilic accumulation in HES are largely unknown. It has been recently suggested that the tissue eosinophilia observed in allergic disorders is, at least partially, caused by inhibition of eosinophil apoptosis by T cell‐, mast cell‐, and eosinophil-derived cytokines.1 Therefore we hypothesized that abnormalities in the process of eosinophil apoptosis may play a part in the etiology of HES. Members of the B-cell lymphoma-2 (Bcl-2) family of proteins are important regulators of apoptosis in many cellular systems.2 The first member of the family, Bcl-2, was originally cloned from the breakpoint of a t(14;18) translocation present in many human B-cell lymphomas. Increased production of Bcl-2 protein as a result of t(14;18) translocations contributes to neoplastic B-cell expansion by preventing B-cell death. 2 In this article we describe an unusual case of HES with blood and tissue eosinophils that expressed, in contrast to normal eosinophils, the Bcl-2 gene. Because it was impossible to induce the Bcl-2 gene by IL-5 or GM-CSF in normal eosinophils, it is unlikely that the observed eosinophil Bcl-2 expression in this subject was the result of stimulation with eosinophil hematopoietins in vivo. Moreover, overexpression of Bcl-2 was associated with delayed death of blood eosinophils in vitro, suggesting that Bcl-2 did act as an apoptosis repressor in this cellular system.