Increased and pathologic emperipolesis of neutrophils within megakaryocytes associated with marrow fibrosis in GATA-1low mice

Deletion of megakaryocytic-specific regulatory sequences of GATA-1 (Gata1(tm2Sho) or GATA-1(low) mutation) results in severe thrombocytopenia, because of defective thrombocytopoiesis, and myelofibrosis. As documented here, the GATA-1(low) mutation blocks megakaryocytic maturation between stage I and II, resulting in accumulation of defective megakaryocytes (MKS) in the tissues of GATA-1(low) mice. The block in maturation includes failure to properly organize a granules because von Willebrand factor is barely detectable in mutant MKS, and P-selectin, although normally expressed, is found frequently associated with the demarcation membrane system (DMS) instead of within granules. Conversely, both von Willebrand factor and P-selectin are barely detectable in GATA-1(low) platelets. Mutant MKS are surrounded by numerous myeloperoxidase-positive neutrophils, some of which appear in the process to establish contact with MKS by fusing their membrane with those of the DMS. As a result, 16% (in spleen) to 34% (in marrow) of GATA-1(low) MKS contain 1 to 3 neutrophils embedded in a vacuolated cytoplasm. The neutrophil-embedded GATA-1(low) MKS have morphologic features (high electron density and negativity to TUNEL staining) compatible with those of cells dying from para-apoptosis. We suggest that such an increased and pathologic neutrophil emperipolesis may represent one of the mechanisms leading to myelofibrosis by releasing fibrogenic MK cytokines and neutrophil proteases in the microenvironment. (C) 2004 by The American Society of Hematology.