Introduction
Dyslipidemia is one of the main modifiable risk factors for the
development of cardiovascular diseases (CVD), being the ischemic heart
disease the leading cause of mortality in the world. Every year, more
people die from CVD than from any other reason, according to data from
the World Health Organization, it is estimated that 17.9 million people
died from this cause in 2019, which represents 32% of all registered
deaths in the world . In addition to high blood pressure, diabetes,
obesity, and smoking, dyslipidemia is one of the main cardiovascular
risk factors. The latter it is defined as disorders in blood lipids
characterized by an increase in cholesterol and/or triglyceride levels
called hypercholesterolemia and triglyceridemia, respectively .
While most of the triglyceride and cholesterol content is obtained from
dietary sources, de novo lipogenesis contributes significantly to
serum lipid content in people who have a high-carbohydrate diet. The
metabolic pathways by which the macromolecules obtained through the diet
are processed, such as glycolysis, Krebs cycle, oxidative
phosphorylation, beta oxidation, among others, have as their main
function the generation of energy, and an imbalance in these can promote
pathological processes, such as dyslipidemia . These metabolic pathways
not only provide energy for cellular homeostasis, but also control
immune cell functions. Immune cells at rest, use processes such as Krebs
cycle and oxidative phosphorylation for ATP generation, but cells with
pro-inflammatory phenotype such as M1 macrophages and activated T
lymphocytes tend to change to aerobic glycolysis, while M2 macrophages
and regulatory T lymphocytes induced in the periphery continue with
oxidative phosphorylation. Reprogramming of the metabolic state of
immune cells influences the generation of epigenetic changes which lead
to functional changes. This cellular metabolic state is affected by
systemic metabolism, either by nutrients availability or by signalling
pathways induced by each metabolite . These concepts are the basis of
innate immunological memory, this phenomenon, also called immune
training, is defined by metabolic changes originated after priming with
pathogens or sterile stimuli that lead to sustained functional changes
orchestrated mainly by epigenetic reprogramming, which are sustained
changes in gene expression and cellular physiology, which does not imply
permanent changes .
An immune cell type that attracts more attention in the immunometabolism
area is the macrophages population. Macrophages are phagocytic cells of
innate immunity with a broad functional spectrum, from pro-inflammatory
to anti-inflammatory phenotypes representing the extremes. Monocytes,
cells that develop from bone marrow precursors, travel in bloodstream
for a few days, then they migrate to tissue and become macrophages with
different phenotypes . Tissue-resident macrophages are long-lived cells
derived mostly from erythro-myeloid progenitors that emerge from the
yolk sac . The first to emerge are the primitive macrophages, which are
not derived from monocytes and seed every tissue. When erythro-myeloid
progenitors seed the fetal liver, they generate fetal monocytes that
differentiate into macrophages, and represent the most abundant
tissue-resident macrophage population . Furthermore, monocytes derived
from hematopoietic stem cells emerge from the fetal liver and
differentiate into long-lived macrophages, while adult hematopoiesis
begins in the bone marrow. Bone marrow -derived monocytes contribute to
the different populations of postnatal tissue resident macrophages .
Monocytes/ macrophages are recognized because their important roles in
regulating homeostasis and immune defense through their inflammatory or
tissue repair properties . The importance of metabolism in immune cells
for the programming of macrophages with their different functional
spectra suggests that metabolic pathways may play a role for long-term
functional changes in monocytes and macrophages during immune training .
The role of these cells is widely described in obesity, being the main
population present in the adipose tissue stromal vascular fraction,
where there is an increase in the proliferation of macrophages coupled
with the recruitment of circulating monocytes to this tissue. Due to the
production of cytokines such as IL-1β, IL-6 and TNF-α, M1 macrophages
participate in the low-grade chronic inflammation that characterizes
obesity .
In past years, it has been shown that when treating monocyte derived
macrophages with high concentrations of insulin, glucose and palmitate,
characteristic of metabolic syndrome, a different pro-inflammatory
phenotype is induced. These cells present surface markers and
transcription factors different from classical macrophages and were
called metabolically activated macrophages (MMe) . MMe present surface
molecules such as CD36, which binds to long chain fatty acids and
facilitates their transport in the cell, participating in the use of
lipids in muscle, storage of adipose energy and absorption of intestinal
fat ; ABCA1 is a cholesterol efflux pump in the elimination pathway of
cellular lipids that are then collected by apoA-I, forming high-density
lipoproteins (HDL) ; and PLIN2, which is a protein expressed on the
lipid droplet membrane . These MMe have been described in metabolic
syndrome and have been found in adipose tissue during obesity,
performing beneficial and detrimental functions during diet-induced
obesity in mice , and in mammary adipose tissue promoting tumorigenesis
during obesity . MMe produce pro-inflammatory cytokines, although in a
lesser extent than classic M1 macrophages. The expression of their
characteristic surface markers, as well as the attenuated inflammatory
response, is mainly mediated by the transcription factor PPAR-γ , that
could be contributing to the chronic low-grade inflammatory state
present in metabolic syndrome and obesity.
Due to dietary overload, the metabolites produced by the different
metabolic pathways can be used for alternative pathways in organs and
tissues, such as adipose tissue, modifying and defining systemic
metabolic responses It is not completely clear whether the change from a
healthy systemic metabolic state to a pathological one, such as the
dyslipidemic state, lead to changes causing immune training influencing
polarization to different cell types.
The aim of this study was to evaluate if high cholesterol and
triglycerides levels, main feature of dyslipidemia, are promoting immune
training in peripheral blood cells, functioning as a first stimulus,
conditioning monocytes to present a metabolic phenotype and leading them
to polarization into metabolically activated macrophages. We found that
monocytes with metabolic phenotype expressing CD36, ABCA1 and PLIN2, are
present in systemic circulation. In vitro stimulation showed that
MMe from patients with dyslipidemia, play a dynamic role with production
of pro- and anti-inflammatory cytokines.