Marrow mast cell hyperplasia in hairy cell leukemia.

Marrow mast cells, frequently elevated in chronic B-lymphoproliferative disorders, were counted per high magnification field (HMF) on toluidine blue stained marrow biopsies from 34 patients diagnosed with hairy cell leukemia (HCL); similar counts were performed on splenic sections from nine of these patients. Biopsies from 28 normal marrow transplant donors and 10 normal spleens served as controls. Mast cells were distributed irregularly throughout normal and HCL marrows, but tended to be more concentrated about the hairy cells in cases focally involved by HCL. HCL marrows averaged 12.7 mast cells/HMF compared to 1.1 for controls. Although most normal marrows (86%) averaged < 2 mast cells/HMF, 88% of HCL cases averaged > or = 2 mast cells/HMF. Splenic mast cells averaged < 1/HMF for both HCL and control cases. By electron microscopy, marrow mast cells in HCL demonstrated normal substructure with numerous granules. The cell surfaces of mast cells showed filopodia that often came in contact with those of hairy cells. These results indicate most HCL marrows, but not spleens, are associated with varying degrees of mast cell hyperplasia. Furthermore, this study suggests a biologic interaction between mast cells and hairy cells. The significance of marrow mast cell hyperplasia in HCL is unknown, but these mast cells may contribute to the increased reticulin fiber network observed in HCL marrows.