Macrophage differentiation, polarization, and cholesterol biosynthesis during immune tolerance

Abstract

Immune tolerance refers to the process by which immune cells, such as monocytes and macrophages, produce a dampened immune response upon secondary exposure to a particular antigen. Macrophages are heterogenous and highly plastic immune cells that are regarded as key elements in the innate immune system. Their ability to polarize into different functional phenotypes (for example, M1 and M2) has been implicated in the progression of the host response to sepsis, defined as the abnormal response to infection leading to lethal organ dysfunction. M1 macrophages are understood to be pro-inflammatory macrophages, whilst M2 are anti-inflammatory macrophages with tissue repair properties. Recent evidence has suggested that monocyte differentiation and polarisation may be skewed in community acquired pneumonia (CAP) patients relative to non-infectious subjects. Moreover, an increased expression of genes related to the cholesterol biosynthesis pathway has been implicated in the determination of tolerance in monocytes of CAP patients. The influence of cholesterol biosynthesis on peripheral monocytes in terms of their differentiation and polarisation into M1 or M2 macrophages is unclear. In this study, enzyme-linked immunosorbent assays (ELISA) and flow cytometric techniques were used to assess the effects of consecutive endotoxin stimulations on peripheral blood mononuclear cells (PBMCs). Using the ELISA technique, it was found that average TNF- α concentration decreased upon secondary exposure with LPS, whilst the addition of Fluvastatin, a cholesterol biosynthesis inhibitor, presented no significant difference in the induction of immune tolerance. Flow cytometric analysis of stimulated cells showed an expansion of an HLA-DR high macrophage population when incubated in Fluvastatin alone, indicating that this may have an influence on macrophage polarization.