C/EBPb Is Involved in the Amplification of Early Granulocyte Precursors during Candidemia-Induced “Emergency” Granulopoiesis

Granulopoiesis is tightly regulated to meet host demands during both " steady-state " and " emergency " situations, such as infections. The transcription factor CCAAT/enhancer binding protein b (C/EBPb) plays critical roles in emergency granulopoiesis, but the precise developmental stages in which C/EBPb is required are unknown. In this study, a novel flow cytometric method was developed that successfully dissected mouse bone marrow cells undergoing granulopoiesis into five distinct subpopulations (#1–5) according to their levels of c-Kit and Ly-6G expression. After the induction of candidemia, rapid mobilization of mature granulocytes and an increase in early granulocyte precursors accompanied by cell cycle acceleration was followed by a gradual increase in granulocytes originating from the immature populations. Upon infection, C/EBPb was upregulated at the protein level in all the granulopoietic subpopulations. The rapid increase in immature subpopulations #1 and #2 observed in C/EBPb knockout mice at 1 d postinfection was attenuated. Candidemia-induced cell cycle acceleration and proliferation of hematopoietic stem/ progenitors were also impaired. Taken together, these data suggest that C/EBPb is involved in the efficient amplification of early granulocyte precursors during candidemia-induced emergency granulopoiesis. N eutrophilic granulocytes are the major cell type at the front line of host defense (1, 2). Granulocytes are continuously produced in the bone marrow (BM) and supplied to the peripheral blood or tissues, where they fight microorganisms via their bactericidal activities (1, 2). A shortage of granulocytes rapidly causes fatal infections, whereas an excess of granulocytes triggers conditions such as acute respiratory distress syndrome and sepsis-related other acute organ dysfunctions (1, 3). Therefore, the number of granulocytes must be tightly regulated to meet host demands. Granulopoiesis, the process of granulocyte production in the BM, is under the control of both intrinsic and extrinsic cellular factors. In mice deficient in the transcription factor CCAAT/ enhancer binding protein (C/EBPa), granulocytes are completely absent, suggesting a central role for C/EBPa in " steady-state " gran-ulopoiesis (4, 5). During " emergencies " such as infection, the supply of granulocytes from the marginal and BM pool increases, and the cells are mobilized to the site of infection. In addition, BM granulopoiesis is increased in response to various cytokines including IL-3, GM-CSF, and G-CSF (6–9). We and others have previously shown that, in contrast to steady-state granulopoiesis, which is dependent on C/EBPa, emergency granulopoiesis is dependent on the transcription factor C/EBPb (8, 10). Although C/EBPa plays a critical role in the transition …