Stability of cytokine, cellular and clinical response to the intravenous LPS challenge repeated after one year: a healthy volunteer trial

Whether the magnitude of individual cytokine, cellular, and clinical responses to the intravenous lipopolysaccharide (LPS) challenge is constant in individuals over extended time periods is unknown. Nine healthy volunteers received an intravenous LPS injection of 2 ng/kg bodyweight twice at intervals of at least one year. Circulating cytokines and leukocyte subsets were quantified using a multiplex immunoassay and cytometry by time-of-flight, respectively. Self-reported symptoms and vital signs were also assessed. We observed moderate to strong intra-individual correlations in the responsiveness of most cytokines (IL-6 [AUC_{0 - 10}]: R = 0.93, p < 0.001; CRP [mg/dL]: R = 0.88, p = 0.004; IL-8 [AUC_{0 - 10}]: R = 0.71, p = 0.031; TNF-alpha [AUC_{0 - 10}]: R = 0.67, p = 0.047; IL-10 [AUC_{0 - 10}]: R = 0.42, p = 0.26) and cellular subsets (CD8 T lymphocytes: R = 0.9, p = 0.002; B lymphocytes [G/L]: R = 0.89, p = 0.003; CD4 T lymphocytes: R = 0.84, p = 0.001; neutrophils: R = 0.80, p = 0.017; monocytes: R = 0.16, p = 0.710) between the 1st and 2nd LPS challenges. Vital signs and symptoms were not reproducible. While the average cellular and clinical response was similar between the two LPS challenges, we found a significantly attenuated AUC_{0 - 10} of IL-6 (percent difference, -41.9% [95% CI -73.0 - -10.7]) and TNF-alpha (percent difference, -35.7% [95% CI -70.0 - -1.6]) at the 2nd LPS challenge. Individual cytokine and cellular responses to intravenous LPS showed a significant degree of correlation when measured more than one year apart. These correlations did not translate to the reproducibility of clinical symptoms and vital signs, which showed greater variability and were not constant over time. The partly reduced cytokine release in the 2nd LPS challenge might be interpreted as an indicator of a long-lasting tolerance to endotoxin.