Global Defects in Neutrophil and Monocyte Function Offer New Targets to Improve Sepsis Survival in Obese, Diabetic Mice

Author(s):
Lynn Frydrych; Guowu Bian; Katherine He; Peter Ward; Matthew Delano

Background:

Sepsis is the leading cause of death in the critically ill population, with an overall mortality rate of 20%. Mortality rates double in the obese, type II diabetic (T2D) population due to infectious complications. Neutrophils (PMNs) and monocytes are essential for bacterial eradication and sepsis survival. The PMN and monocyte functional defects that contribute to the worse sepsis mortality in obesity and T2D are unknown. PMN and monocyte defects are important therapeutic targets to improve sepsis survival, since obesity and T2D is no longer just a disease of high-income countries, but is rapidly becoming a worldwide pandemic.

Hypothesis:

We hypothesize that obesity and T2D create a functional immune deficiency by altering PMN and monocyte cellular function, which inhibits bacterial clearance and promotes sepsis mortality.

Methods:

30 week old C57BL/6 (lean) and Diet Induced Obese (DIO) mice underwent cecal ligation and puncture (CLP), (LD₂₀), or sham procedure. At serial time points from 1 to 14 days, n=5 mice/group were euthanized. Peritoneal fluid was analyzed for bacterial counts. Monocytes and PMNs were isolated by MACS separation. Phagocytic ability and reactive oxygen species (ROS) generation were assessed by flow cytometry. Cytokine analysis was done with Luminex^TM technology. Genomic analysis of PMN phagocytic pathways was completed with RT² Profiler Arrays.

Results:

Compared to lean mice, DIO mice had significantly less survival after CLP (p<0.05). DIO mice failed to eliminate bacteria from the peritoneal cavity when compared to lean mice (p<0.01). DIO PMNs are immature and expressed less CD11b and CXCR3 following sepsis compared to lean PMNs (p<0.01). PMN and monocyte phagocytic and ROS ability in DIO mice was dramatically reduced (p<0.01) up to 14 days after sepsis. Genomic analysis revealed significantly less Mertk and Axl transcripts in DIO PMNs, which regulate phagocytosis of necrotic and apoptotic cells. DIO mice also produced far less plasma MIP1A and MCP-1 cytokine levels (p<0.05) and hence recruited significantly less M1, M2A, and M2B monocytes into the peritoneal cavity when compared to lean mice (p<0.05).

Conclusions:

DIO mice demonstrate global defects in PMN and monocyte phagocytosis and ROS generation, which hinder bacterial eradication and worsen sepsis mortality. Augmentation of Mertk and Axl transcripts, and plasma MIP1A and MCP-1 levels may counteract these defects through improved PMN function and monocyte recruitment respectively.