Background Hemorrhagic shock is usually associated with complicated immune and inflammatory responses,which are sometimes crucial for the prognosis.As regulators of the immune and inflammatory system; proliferation,migration,distribution and activation of myeloid-derived suppressor cells (MDSCs) are intimately linked to the inflammation cascade.Methods In a model of severe hemorrhagic shock,thirty-five rats were randomly divided into control,sham,normal saline resuscitation (NS),hypertonic saline resuscitation (HTS),and hydroxyethyl starch resuscitation (HES),with seven in each group.M DSCs were analyzed by flow cytometric staining of CD11b/c+Gra+ in peripheral blood mononuclear cells (PBMC),spleen cell suspensions,and bone marrow nucleated cells (BMNC).Simultaneously,the expressions of arginase-1 (ARG-1) and inducible nitric oxide synthase (iNOS) mRNA in MDSCs were evaluated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR).Results In the early stage after hemorrhagic shock,fluid resuscitation and emergency treatment,the MDSCs in the PBMC of NS,HTS and HES groups markedly increased,and MDSCs in BMNC of these groups decreased accordingly,significantly different to the control group.In hemorrhagic shock rats infused with HTS at the early resuscitation stage,MDSCs in PBMC increased about 2 and 4 folds,and MDSCs in BMNC decreased about 1.3 and 1.6 folds,as compared to the sham group respectively,with statistically significant difference.Furthermore,compared to the NS and HES groups,the MDSCs in PBMC of HTS group increased 1.6 and 1.8 folds with statistically significant differences; the MDSCs decrease in BMNC was not significant.However,there was no statistically significant difference in MDSCs of spleen among the five groups.In addition,compared to the control,sham,NS and HES groups,the ARG-1 and iNOS mRNA of MDSCs in PBMC,spleen and BMNC in the HTS group had the highest level of expression,but no statistically significant differences were noted.Conclusions In this model of rat with severe and controlled hemorrhagic shock,small volume resuscitation with HTS contributes to dramatically early migration and redistribution of MDSCs from bone marrow to peripheral circulation,compared to resuscitation with NS or HES.