Introduction: Pulmonary inflammation has been proposed as a key pathogenic driver of pulmonary hypertension (PH), vascular disease with increasing global significance. Perivascular macrophages have been observed in the lungs of PH patients. However, the mechanisms of expansion of lung macrophages and the role of blood-borne monocytes in PH are poorly understood.
Hypothesis: We hypothesize that increased monocyte recruitment and differentiation in the lungs augment inflammatory macrophage content and vascular remodeling in PH.
Methods: We recapitulated PH in both hypoxic mouse and monocrotaline injected rat models. We enumerated blood and lung myeloid cell populations in mouse and rat as well as in PH patients using multicolor flow cytometric analysis. To track the origin of lung macrophages, we surgically joined congenically different hypoxic mice (parabiosis). To evaluate the importance of monocyte recruitment to the lungs we reduced monocyte number in two separate rodent models of PH - via genetic deficiency of cx3cr1 or ccr2 in chronically hypoxic male mice and by pharmacologic inhibition of Cx3cl1 in monocrotaline-exposed rats.
Results: Flow cytometric analysis of lungs in mouse, rat, and human with PH showed an increase in interstitial macrophages and monocytes (interstitial macrophages, control, 6.3x104±3.1.104; PH, 5.7x105±5.105, monocytes, control, 7.6x104±3.1x104; PH, 4.1x105±3.7x105). Additionally, mRNA expression of chemokines including ccl2 and cx3cl1, responsible for monocyte recruitment, was elevated in the lungs, indicating that increased monocyte recruitment results in higher macrophage content in the lungs. Congruently, parabiosis data demonstrated that most of inflammatory perivascular macrophages originated from blood monocytes. Deficiency of cx3cr1 or ccr2 in mouse and pharmacological inhibition of Cx3cl1 in rats exhibited decreased monocyte accumulation (WT normoxia, 221±58; WT hypoxia, 920±259; ccr2 KO hypoxia, 153±0), inflammation and lung remodeling.
Conclusions: We conclude that a) inflammatory perivascular lung macrophages are derived from circulating monocytes, b) monocyte recruitment is crucial for vascular remodeling in pulmonary hypertension.