The aging of neutrophils is actively involved in the cardio-metabolic consequences of high fat diet
Authors: Baragetti A , Da Dalt L , Moregola A , Parolini A , Terenghi O , Mattavelli E , Svecla M , Uboldi P , Catapano A L , Norata G D 
Published: May 2024

Abstract

Funding Acknowledgements
Type of funding sources: Public grant(s) – EU funding. Main funding source(s): Italian Ministry of Health - Progetti di Rilevante Interesse Nazionale (PRIN); European Commission - PNRR Missione 4 (Progetto CN3 - National Center for Gene Therapy and Drugs based on RNA Technology); European Commission - PNRR Missione 4 (Progetto MUSA- Multilayered Urban Sustainability Action; European Commission - PNRR Missione 6

Background
High Fat Diets (HFDs) promote inflammation in cardio-metabolic diseases by epigenetically activating the inflammasome and inducing an unfavorable oxidative metabolic skewing in long-living hematopoietic cells (1,2). Whether these mechanisms also activate the short-living neutrophils, which rely on other mechanisms of action and metabolic demands (3), is less clear. Yet, multiple neutrophil entities co-exist in blood and tissues and "age" during their short-term half-life, by virtue of a reciprocal regulation of CXCR4, that promotes a "fresh" status when these cells leave the bone marrow, and CXCR2, that accelerates their aging when they are committed against pathogens (4).

Purpose
Derailed aging is critical for the innate functions of neutrophils and we study whether it also contributes to exacerbate the metabolic and inflammatory consequences of HFD.

Methods
We immunophenotyped neutrophils and characterized the metabolic responses in physiology (wild-type mice, WT) and in mice with either constitutively aged neutrophils (MRP8 driven conditional deletion of CXCR4; herein CXCR4fl/flCre+) or with constitutively fresh neutrophils (MRP8 driven conditional deletion of CXCR2; CXCR2fl/flCre+), following 20 weeks of HFD feeding (45% Kcal from fat).

Results
After 20 weeks HFD, the gluco-metabolic profile of CXCR4fl/flCre+ mice was comparable to that of WT mice, while CXCR2fl/flCre+ mice were protected from obesity and insulin resistance.
At 20 weeks CXCR4fl/flCre+ accumulated more in liver but CXCR2fl/flCre+ neutrophils infiltrated less. Also, CXCR4fl/flCre+, but not CXCR2fl/flCre+ neutrophils, infiltrated more in the liver two hours after an intragastric gavage with olive oil versus fasting. As this finding was not observed when these mice models were fed 20 weeks a standard fat diet, these findings suggest that HFD feeding primes a preferential relocation of aged neutrophil in the liver. Consequently, the liver of CXCR4fl/flCre+ mice presented with enriched oxidative metabolism and pathways related to NETosis/inflammation, while pathways related to physiological patrolling and resolutive inflammation were more enriched in the liver of the CXCR2fl/flCre+ counterpart.
Finally, CXCR4fl/flCre+, mice also accumulated more in the visceral adipose tissue (VAT), and this resulted into altered lymphocytic cell architecture. By contrast, the reduced infiltration of CXCR2fl/flCre+ neutrophils in VAT correlated with less amount of pro-inflammatory macrophages.
In humans, increased plasma levels of Cxcl1 (ligand of CXCR2) correlated with visceral obesity and metabolic syndrome.

Conclusions
Neutrophil aging might contribute to the cardio-metabolic consequences of HFD (Picture 1). Future studies are needed to understand the mechanism and propose the aging as a therapeutic target over and above the pathways targeted by the actual "anti-inflammasome" therapies approved for the treatment of cardiovascular diseases.