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