No abstract

Objective-In mice, homeostatic erythropoiesis occurs primarily in the bone marrow. However, in response to acute anemia, BMP4 dependent stress erythropoiesis occurs in the adult spleen. BMP4 can also regulate stress erythropoiesis in the fetal liver. In humans, erythropoiesis occurs in the bone marrow. However, in certain pathological conditions, extramedullary erythropoiesis is observed where it can occur in several organs including the liver. Given these observations, we propose to investigate whether the BMP4 dependent stress erythropoiesis pathway can regulate extramedullary erythropoiesis in the livers of splenectomized mice.Methods-Using splenectomized wildtype and flexed-tail (f) mice, which have a defect in BMP4 signaling, we compared their recovery from Phenylhydrazine induced hemolytic anemia and characterized the expansion of stress BFU-E in the livers of these mice during the recovery period.Results-Our analysis indicates that in the absence of a spleen, stress erythropoiesis occurs in the murine liver. During the recovery, stress BFU-E are expanded in the livers of splenectomized mice in response to BMP4 expressed in the liver. f/f mice, which exhibit a defect in splenic stress erythropoiesis do not compensate for this defect by up-regulating liver stress erythropoiesis. Furthermore, splenectomized f/f mice exhibit a defect in liver stress erythropoiesis, which demonstrates a role for the BMP4 dependent stress erythropoiesis pathway in extra-medullary erythropoiesis in the adult liver.Conclusions-Our data indicate that the BMP4 dependent stress erythropoiesis pathway regulates extra-medullary stress erythropoiesis, which occurs primarily in the murine spleen or in the case of splenectomized mice, in the adult liver.

The Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) is an adaptor molecule critical for immunoreceptor and integrin signaling in multiple hemopoietic lineages. We showed previously that SLP-76 is required for neutrophil function in vitro, including integrin-induced adhesion and production of reactive oxygen intermediates, and to a lesser extent, FcγR-induced calcium flux and reactive oxygen intermediate production. It has been difficult to determine whether SLP-76 regulates neutrophil responses in vivo, because Slp-76−/− mice exhibit marked defects in thymocyte and vascular development, as well as platelet and mast cell function. To circumvent these issues, we generated mice with targeted loss of SLP-76 expression within myeloid cells. Neutrophils obtained from these animals failed to respond to integrin activation in vitro, similar to Slp-76−/− cells. Despite these abnormalities, SLP-76-deficient neutrophils migrated normally in vivo in response to Staphylococcus aureus infection and efficiently cleared micro-organisms. Interestingly, SLP-76-deficient neutrophils did not induce a robust inflammatory response in the localized Shwartzman reaction. Collectively, these data suggest that disruption of integrin signaling via loss of SLP-76 expression differentially impairs neutrophil functions in vivo, with preservation of migration and killing of S. aureus but reduction in LPS-induced tissue damage and vascular injury.