After hemorrhagic shock and during sepsis, antigen-presenting cells loose the capacity to respond to bacteria or bacterial products with the secretion of pro-inflammatory cytokines. This malfunction of antigen-presenting cells like macrophages and dendritic cells is associated with the development of immunosuppression and might lead to multi-organ failure and death. GM-CSF is a cytokine that is known to modulate the cytokine secretion of antigen-presenting cells and additionally acts as growth factor on myeloid cells. In the present study, the capacity of GM-CSF to restore the secretion of pro-inflammatory cytokines by macrophages and dendritic cells was investigated in animal models for hemorrhagic shock and polymicrobial sepsis. Additionally, it was tested in vitro, whether GM-CSF might mediate the differentiation of competent dendritic cells from bone marrow during sepsis. GM-CSF restored the suppressed LPS-induced TNF-α release from the splenic macrophages after hemorrhagic shock and during sepsis up to levels observed for the respective LPS-stimulated cells from sham mice. GM-CSF partially but not completely restored the suppressed LPS-induced TNF-α production from the peritoneal macrophages after hemorrhagic shock. The GM-CSF-mediated restoration of the LPS-induced TNF-α secretion by peritoneal macrophages was verified on the level of IκBα phosphorylation. In addition GM-CSF effects were also associated with increased levels of the activated forms of the signaling molecules P38MAPK and ERK1/2, possibly suggesting an involvement of one of these MAPK in the stimulatory effects of GM-CSF. In contrast, GM-CSF was inefficient in restoring the LPS-induced cytokine response by dendritic cells from septic animals. Analyses of the phenotype of dendritic cells that differentiated from bone marrow progenitors of septic mice in the presence of GM-CSF showed that these dendritic cells display an aberrant cytokine response and fail to polarize Th lymphocytes towards Th1. This malfunction of in vitro generated dendritic cells from septic mice is associated with an increased secretion of IL-10 in response to immunostimulatory bacterial oligonucleotides and resembles the deviated cytokine response from splenic dendritic cells during sepsis. The absence of IL-10 or LPS during the GM-CSF-dependent differentiation of dendritic cells suggests that another mechanism was responsible for the development of the abnormal phenotype from BMDC from septic mice. A reduced population of CD4+ dendritic cell progenitors with so far unknown function was found in the bone marrow of septic mice and might play a role in the aberrant differentiation of dendritic cells during sepsis. In summary, GM-CSF therapy might reinforce the macrophages’ antibacterial defence mechanisms during hemorrhagic shock and sepsis-associated immunosuppression. However, differentiation of dendritic cells from bone marrow during sepsis might result in an aberrant phenotype of dendritic cells that prevent the development of a protective Th1 response and, thereby, further aggravates immunosuppression.