Background
Inflammation forms an important part of the human innate immune system and is largely dependent on the activation of the "classical" NF-κB pathway through Toll-like receptors (TLRs). Understanding this has allowed researchers to explore roles of therapeutic targets in managing conditions such as sepsis. Recapitulating an inflammatory response using lipopolysaccharide (LPS), a "sterile" technique, can provide information that is dissimilar to the clinical condition. By examining NF-κB activation (through immunoblotting of the p65 subunit) in two separate cell lines (murine and human) and analyzing two murine models of sepsis (intraperitoneal [IP] LPS and IP stool inoculation), an evaluation of the translational disconnect between experimental and clinical sepsis can be made.
Conclusion
Our results support previous observations that translation of murine models into the human clinical setting suffers from considerable limitations including species-associated differences in LPS response seen at a molecular level. Furthermore, the histopathological changes during clinical sepsis cannot be adequately reproduced by injection of LPS. Therefore, the so-called translational disconnect that exists between murine LPS models and human sepsis involves NF-κB activation at a molecular level and is further augmented by the use of LPS as a stimulus for infectious responses in vivo.
Methods
THP-1 (human) cells and RAW 264.7 (murine) cells were dosed with concentrations of LPS (human, 1 pg/mL to 100 ng/mL; murine, 30 pg/mL to 1,000 ng/mL) and nuclear actin and p65 were immunoblotted to measure changes in nuclear density. In vivo, C57BL/6 mice received either IP injection of stool suspension (5 µL/g) or LPS (25 mg/kg) or saline (1 mL/kg). Animals were culled at 6 hours and tissues were analyzed.
Results
An increase in basal p65:actin density in THP-1 cells (mean 0.214, standard error of the mean 0.024) was seen at doses as small as 0.1 ng/mL (0.519±0.064). In contrast to RAW 264.7 cells, basal increases (0.170±0.025) were only seen when a dose of 3 ng/mL (0.387±0.078) was used. Dose-response analysis of p65:actin ratio showed that THP-1 cells respond to lower doses of LPS than RAW 264.7 cells and lower doses produce a greater fold increase in the nuclear p65 density. Both in vivo models showed evidence of neutrophil (NL) recruitment into tissues (which was more intense after LPS treatment). IP stool inoculation resulted in an acute suppurative peritonitis and more substantial evidence of NL recruitment into adipose tissue and skeletal muscle.