Toll-like Receptor 4‑Independent Effects of Lipopolysaccharide Identified Using Longitudinal Serum Proteomics

Sepsis remains one of the most lethal
and costly conditions treated
in U.S. hospitals, with approximately 50% of cases caused by Gram-negative
bacterial infections. Septic shock is induced when lipopolysaccharide
(LPS), the main component of Gram-negative outer bacterial membrane,
signals through the Toll-like receptor 4 (TLR4) complex. Lethal endotoxemia,
a model for septic shock, was induced in WT C57BL6 and TLR4–/– mice by administration of Escherichia coli LPS. WT LPS treated mice showed high morbidity, while PBS treated
LPS and treated TLR4–/– mice did not. ANOVA
analysis of label-free quantification of longitudinal serum proteome
revealed 182 out of 324 proteins in LPS injected WT mice that were
significantly changed across four time points (0, 6, 12, and 18 h).
No significant changes were identified in the two control groups.
From the 182 identified proteins, examples of known sepsis biomarkers
were validated by ELISA, which showed similar trends as MS proteomics
data. Longitudinal analysis within individual mice produced 3-fold
more significantly changed proteins than pair-wise comparison. A subsequent
global analysis of WT and TLR4–/– mice identified
pathways activated independent of TLR4. These pathways represent possible
compensatory mechanisms that allow for control of Gram-negative bacterial
infection regardless of host immune status.