Bone marrow eosinophils in plasma cell disorders

In experimental studies, eosinophils have been shown to promote the survival, proliferation, and retention of plasma cells in the bone marrow (BM). The clinical significance of eosinophils in plasma cell disorders (PCDs) in humans is largely unknown. This study focuses on the frequency and phenotype of eosinophils in the BM and peripheral blood (PB) in patients with untreated PCD compared with healthy controls. The number of eosinophils per se did not correlate with the number of BM plasma cells or disease stage. The expression of chemokine receptor 4, which is important in the homing capacity to bone marrow stromal cells, was significantly higher in patient eosinophils and increased with disease stage. BM eosinophils from patients, especially from those with manifest disease, were more activated. Another finding in this study was that eosinophils in PB and BM from both patients and healthy controls expressed CD80 (B7-1). We discuss probable immunomodulatory consequences of surface expression of CD80 by eosinophils in conditions with marked T-cell exhaustion (e.g., multiple myeloma). Finally, we found that patients treated with corticosteroids had low levels of circulating eosinophils but preserved levels of eosinophils in the BM.