Lower Levels of Whole Blood LPS-Stimulated Cytokine Release Are Associated with Poorer Clinical Outcomes in Surgical ICU Patients

Abstract

In vitro pretreatment of human monocytes with lipopolysaccharide (LPS) induces "endotoxin tolerance" with blunted TNF and IL-6 release to rechallenge with LPS. The pro-inflammatory cytokines TNF and IL-6 are important mediators in sepsis. A high IL-6 concentration has been used as a marker of infection severity, but IL-6 may also have beneficial effects as an acute-phase protein. We sought to address two questions: (a) What is the relationship between TNF and IL-6 release? (b) Is the clinical outcome different for intensive care unit (ICU) patients with ex vivo characteristics of endotoxin tolerance (low levels of ex vivo LPS-stimulated cytokine release)? Heparinized whole blood was obtained from 62 surgical ICU patients and 15 control subjects and incubated for 3 h at 37 degrees C in the presence or absence of 10 ng/mL LPS. Concentrations of TNF and IL-6 were measured in plasma samples using an enzyme-linked immunosorbent assay (pg/mL). Clinical data on ICU length of stay (LOS), ventilator days, white blood cell count (WBC), and documented clinical infection were obtained by chart review. Outcome parameters for patients with low ex vivo LPS-stimulated cytokine release (low = IL-6 < 3000 pg/mL and TNF <2100 pg/mL) were compared to patients with Normal/High concentrations of cytokines. Cytokines were essentially undetectable in ICU patients or controls without LPS stimulation, however a range of values was measured for LPS-stimulated release in both ICU patients (IL-6, 7847 +/- 857 pg/mL; TNF, 4390 +/- 457 pg/mL) and controls (IL-6, 7704 +/- 793 pg/mL; TNF, 6706 +/- 715 pg/mL). There were no differences in age between High/Normal concentrations of cytokines compared to the Low cytokine group, however there were significant differences in WBC, cytokine concentrations, ICU LOS, incidence of clinical infection, and mortality. The Low group also required an average of 6.9 more days of mechanical ventilation (p < 0.05). LPS-stimulated TNF release seemed to correlate better with the observed mortality than did IL-6 release. The data suggest that ICU patients with characteristics of endotoxin tolerance (low LPS-stimulated cytokine release capacity) have significantly poorer clinical outcomes. Ex vivo LPS-stimulated whole blood cytokine production may be useful to identify ICU patients with severe sepsis.