Résumé
Objective To investigate the dynamic variation of subsets of myeloid derived suppressor cells (MDSC) and their ratio in septic mice, and to discuss their role in the development of sepsis. Methods Male C57BL/6 mice were randomly divided into sepsis model group and sham group according to random number table. Polymicrobial sepsis was induced by using cecal ligation and puncture (CLP), while mice in sham group only underwent laparotomy and laparorrhaphy without CLP. Thirty mice in each group were used to observe living condition, and the 20-day survival rate was compared between the two groups. In addition, subsets of MDSC in peripheral blood, spleen and bone marrow were analyzed with flow cytometry for other 60 mice (12 mice at each time point, as 0, 3, 7, 12 and 20 days). Spleens were harvested at 7 days for weighing, and single cell suspension of spleen tissue was prepared for splenocyte counting. Histopathologic changes in spleen tissue and liver tissue were observed under light microscope after hematoxylin and eosin (HE) stain. Results ① No mice died in sham group within 20 days after the operation. On the other hand, 10 mice in model group died within 20 days, and the difference in survival rate between the two groups was statistically significant (100.0% vs. 66.7%, χ2 = 11.861, P = 0.001). ② The spleens in model group showed obvious enlargement and significantly outweighed as compared with those in sham group (mg: 413.33±41.63 vs. 111.67±17.56, t = 11.564, P = 0.000), and the total count of splenocytes was significantly higher than that in sham group (×109/L: 21.20±2.43 vs. 1.87±0.06, t = 13.578, P = 0.005). ③ Pathological sections with HE staining showed that the liver tissue and spleen tissue remained normal in sham group. In model group, the hepatic tissue showed acute inflammatory reaction, including tissue disruption, capillary congestion, infiltration of neutrophils, marked edema of hepatocytes and focal hepatocellular necrosis. Abnormalities were also found in the spleen tissue: the red pulp and white pulp were disordered, splenic sinus was congested with numerous red cells, the splenic capsule thickened, immature myeloid cells with circular nuclei proliferated in the subcapsular region and perivascular region, splenic cord and splenic sinus were infiltrated with a large number of hematopoietic cells. ④ No significant changes in the monocytic MDSC (M-MDSC) and granulocytic MDSC (G-MDSC), and their ratio were found in peripheral blood, spleen and bone marrow at every time point in sham group. On the other hand, in model group, the ratio of M-MDSC and G-MDSC was continuously increased in peripheral blood, spleen and bone marrow, and M-MDSC only slightly decreased at 20 days. On the other hand, the ratio of M-MDSC/G-MDSC rose at first followed by a decrease. The ratio of M-MDSC/G-MDSC in peripheral blood was higher than 1 from 3 days after the operation, reaching the peak at 12 days (compared with 0 day: 4.16±0.53 vs. 0.79±0.11, P < 0.05), while the ratio of M-MDSC/G-MDSC in spleen and bone marrow after CLP were lower than 1 at all time points, reaching the peak on 7 days after the operation (compared with 0 day: 0.70±0.06 vs. 0.25±0.02 in spleen, 0.39±0.06 vs. 0.11±0.01 in bone marrow, both P < 0.05), and then gradually decreased afterwards. Conclusion Subgroups of MDSCs were continuously aggregated in the peripheral blood, spleen and bone marrow, and their ratio rose first and decreased afterwards along with the development of sepsis, and the changes may reflect the change of immune status at different stages of sepsis.