Background: The dual role of the renin-angiotensin-aldosterone system (RAAS), leukocyte infiltration and increased oxidative stress linked to elevated reactive oxygen species (O₂^-) production in the vascular system support the development of arterial hypertension, one predictive factor for cardiovascular disease (CVD) and chronic kidney disease (CKD).The stress-inducible heme-oxygenase isoform (HO-1) degrades heme to yield bilirubin and carbon monoxide. It is vascular protective by suppressing inflammation and NOX2 mediated oxidative stress. Preclinical evidence shows a strong association of HO-1 activity to hypertension. 

However, role of myeloid cell specific HO-1 in the detrimental effects of vascular dysfunction has not been explored. 

Methods: Hypertension was induced in 8-13 weeks old male mice with selective depletion and over-expression of HO-1 (Ho-1^fl/flLysM^Cre/wt and Ho-1^indLysM^cre) in myelomonocytic cells, via AngII infusion (1mgAngII/Day/Kg). Blood pressure in ATII versus sham-mice was recorded by using tail-cuff with the Coda^TM system (Kent Scientific). Bilirubin levels were quantified in plasma by HPLC method (standard 5µM/L). The quantification of adherent and rolling leukocytes in the arterial vascular endothelium was detected by epifluorescence intravital video microscopy (IVM) by injection of 100µl of acridine orange (0.5mg ml^-1; Sigma Aldrich) via jugular catheter (0.28 mm ID, 0.61mm OD, Smiths Medical, Deutschland GmbH). In addition, to evaluate endothelial function of isolated aortic rings (~4mm) organ chamber method was performed to test the endothelium-dependent relaxation in response to Acetylcholine (ACh).

Results: In the first part of our study, AngII-infused Ho-1^fl/fl LysM^Cre/wt versus LysM^Cre/wt showed no significant differences. However, the over-expression of HO-1 in myelomonocytic cells attenuated blood pressure increase in response to AngII. IVM experiments highlighted decreased leukocytes recruitment in line with transcriptome analysis that underline reduction of the expression of vascular cell adhesion molecule 1 (VCAM-1) in AngII-infused Ho-1^ind LysM^Cre/wt versus LysM^Cre/wt.  ACh mediated vasodilator experiments revealed the restoration of endothelial function in Ho-1^ind LysM^Cre/wt, which was associated with an increase of bilirubin plasma level that can mediate ROS scavenging.

Conclusion: AngII promotes vascular dysfunction, inflammation and tissue damage. Our results indicate the over-expression of the HO-1 enzyme in myeloid cells provides protection from oxidative stress in AngII induced arterial hypertension. Increase of bilirubin can be one of the main decisive elements as it has antioxidant and anti-inflammatory activity and it can assume future clinical relevance for CVD treatment.