Cholesterol biosynthesis in human macrophages and induction of immune tolerance

Abstract

Endotoxin tolerance in monocytes or macrophages is characterized by impaired cytokine production capacities. This phenomenon is understood to convey a ‘protective function’ against the potentially harmful effects of excessive cytokine production, or hyperinflammation. However, sustained tolerance, which is advocated as a potential mechanism of immunosuppression, could increase vulnerability to infections. Recent studies provided evidence that endotoxin tolerance in innate immune cells was concomitant with elevated expression of genes involved in the cholesterol biosynthesis pathway. Direct evidence on the influence of the cholesterol biosynthesis pathway in establishing endotoxin tolerance is limited. In the proposed study, monocytes will be purified from whole blood samples obtained from 4 healthy volunteers to develop an in-vitro model of monocyte-derived-macrophage immune tolerance. The influence of Fluvastatin, a cholesterol biosynthesis blocker, will be tested on the induction of tolerance and inflammatory responses by measuring cytokine production and assessing gene expression patterns of pro- and anti-inflammatory genes against those of genes related to the cholesterol biosynthesis pathway. This line of research could prove invaluable in identifying new treatment avenues to potentially reverse the deleterious effects of immunosuppression. Mann Whitney U test-based comparisons between groups showed significantly higher levels of TNF upon Fluvastatin pre-treatment and LPS stimulation compared to RPMI medium controls (p value = 0.020) or Fluvastatin pre-treated only cells (p-value = 0.018). Lower levels of TNF were noted in LPS-stimulated cells subject to Fluvastatin pre-treatment relative to non- Fluvastatin treated LPS stimulated counterparts, however the difference was not statistically significant. LPS restimulation yielded lower levels of TNF compared to a single stimulation, which was not prevented by Fluvastatin pre-treatment; neither intervention (restimulation with and without Fluvastatin pre-treatment) yielded statistically significant results. Relative SQLE expression was significantly induced in monocytes stimulated by LPS and this induction was impaired by Fluvastatin pre-treatment. There were no reliable and conclusive comparisons with regard to SQLE expression in LPS stimulated and re-stimulated samples. Fluvastatin pre-treatment did not prevent the dampening of TNF levels associated with induction of immune tolerance. The role of SQLE in induction of immune tolerance remains inconclusive.