Background and aims

Pneumonia is associated with an increased incidence of adverse cardiovascular events impacting clinical long-term outcome. However, the underlying pathomechanisms of how pneumonia increases the cardiovascular risk are poorly understood. In this study, we investigated the effect of bacterial pneumonia on vascular regeneration, immune cell differentiation and plasma proteome profile in a murine carotid injury model.

Methods and results

C57BL/6 mice were infected with Streptococcus pneumoniae (S.pn.) TIGR4 or PBS and treated with ampicillin, starting 24 hours post infection (p.i.). On day 7 p.i. carotid artery injury (CI) was induced using a bipolar microforceps. Three days post-CI (10 days p.i.) vascular regeneration and re-endothelialization were determined by Evans blue staining. Our results showed that infection with S.pn. significantly impaired wound healing as compared to control animals. Ten days after infection the spleen, bone marrow and whole blood samples were collected for immune cell phenotyping by flow cytometric analysis. In particular, a significant increase of pro-atherogenic Th1 cells was detected in the circulatory blood of infected mice. Furthermore, proteome profiling (Olink, Sweden) of plasma showed a significant increase in glucagon levels in mice with bacterial infection which was further confirmed by ELISA. 

To lower plasma glucagon levels, mice were additionally treated with liraglutid, a GLP1-agonist, (100µg/kg bw; 0h, 24h and 48h p.i.), which resulted in improved vascular regeneration. 

To further investigate the effect of glucagon on endothelial inflammation in vitro, primary human aortic endothelial cells (HAECs) were treated with glucagon (10nM and 100nM) and expression of the cellular adhesion-molecules ICAM-1, E-Selectin and P-Selectin were measured by flow cytometry and qPCR. We observed a significant increase in ICAM-1, E-Selectin and P-selectin as well as increased monocyte adhesion upon glucagon stimulation. Moreover, measurement of the mitochondrial respiration by Seahorse mito stress test in vitro, showed reduced maximal respiration and spare capacity after glucagon stimulation, indicating impaired mitochondrial function. 

Conclusions

Our findings demonstrate impaired endothelial repair after injury upon bacterial pneumonia. This was accompanied by increased Th1 cell differentiation in the blood of infected mice and elevated plasma glucagon levels. Lowering plasma glucagon levels upon treatment with the GLP-1 agonist resulted in improved endothelial repair. These findings suggest a distinct immuno-metabolic pathway, which may contribute to the development of vascular disease and elevated pneumonia-related cardiovascular risk.