Titolo della tesi: Lipid restriction amplifies type I interferon response in monocytes: lessons learnt from familial hypolipidemia
Monocytes are members of the mononuclear phagocyte system (MPS) together with macrophages and dendritic cells. They play a key role in health and in disease. In particular, they may play beneficial or detrimental functions in hyperlipidemia and cardiovascular disease. Cell functions are deeply influenced by systemic and intracellular metabolism. Although much is known about the metabolism of immune cells, the effect of systemic metabolism and nutrient availability on immune cell function requires further investigations.
FHBL2 subjects, carrying a mutation in the ANGPTL3 gene, display profound hypolipidemia and are protected against cardiovascular disease: therefore they represent a unique contest to study and understand how systemic metabolism influences intracellular metabolism of innate immune cells, and whether immunometabolic phenomena contribute to the protection from cardiovascular risk.
In this study, we performed an ex vivo analysis of FHBL2 subjects and controls, and observed that monocytes of FHBL2 subjects display lower intracellular lipid content and higher sensitivity to type I IFN compared to control groups.
We developed an in vitro system that resembled the phenotype observed ex vivo, by culturing monocytes from healthy controls in normal serum, lipid deprived serum or lipid deprived serum plus LDL. We took advantage of this system to dissect the metabolic mechanism behind the link between lipid deprivation and IFN response that we observed in FHBL2 subjects, since this system allowed us to study some major metabolic pathways. We observed that, in vitro, lipid deprivation reduced mitochondrial fitness and glycolytic metabolism, and interestingly, also reduced ER stress response
In this in vitro setting, we could observe that not only lipid deprivation synergized with exogenous IFN in amplifying the transcription of interferon-stimulated genes, but also induced a spontaneous low-level type I IFN production. Indeed, we could detect induction of IRF3 mRNA and higher IFNa content, under lipid restriction, even in the absence of exogenous type I IFN supplementation.
Since lipid restriction may activate endogenous cholesterol biosynthesis through the mevalonate pathway, which is linked to type I IFN response, we analyzed the expression of genes encoding for the most representative enzymes of the mevalonate pathway in monocytes cultured in different conditions of lipid exposure: we found that the mevalonate pathway is induced in lipid deprivation, and that protein prenylation sustains the increased type I IFN responsiveness that we observed in lipid deprived setting.
Finally, we performed a functional analysis, to verify whether lipid restriction in monocytes controlled the production of inflammatory cytokines, which play well recognized roles in atherosclerotic disease: in lipid deprived condition, we observed a specific suppression of IL-1beta cytokine whose production is inflammasome-driven) which was mediated by protein prenylation and by type I IFN signaling.
In conclusion, this study shows that hypolipidemia establishes a peculiar immune profile, characterized by a shift from inflammasome to IFN response in monocytes, which may contribute to protect FHBL2 subjects from cardiovascular disease.