| Abstract |
Pulmonary arterial hypertension (PAH), primary or secondary to coexisting condition, is a
devastating disease, characterized by vasoconstriction, vascular wall remodeling, with resultant
right ventricular hypertrophy and failure. Despite significant progress in this field, the
mechanisms underlying the development of PAH are still obscure.
Lung inflammation has been increasingly implicated in the development of pulmonary
hypertension. Tissue hypoxia, a well-known stimulus for pulmonary hypertension, has been
reported by our group to induce an inflammatory response that precedes the development of
hypoxia-induced pulmonary hypertension. Also, lung-specific constitutive expression of the
cytoprotective enzyme heme oxygenase-1 (HO-1) can suppress both the lung inflammation and
the later development of hypertension. In the present study, we utilized a bitransgenic mouse
model, that was generated by crossing of mice that express the reverse tetracycline
transactivator (rtTA) under the control of Clara cell secretory protein promoter (CC10) with
mice that harbor the human HO-1 transgene under the control of the bacterial tetracycline
response element (TRE) and expressed HO-1 in a lung-specific, inducible way (tetOn system).
By turning on and off HO-1 activity and modulating lung inflammatory response, our goal was
to shed light on the nature of this hypoxia-induced inflammatory response and its role in the
later development of the disease.
Soon after hypoxic exposure, and in the absence of doxycycline (dox), we demonstrated
significant monocyte/macrophage accumulation (F4/80, CD11c positive cells) and
cytokine/chemokine production (FGFb, IL-1b, MIP-1a, IP-10, IL-13, IL-4, IL-17 and IL-2) in the
bronchoalveolar lavage (BAL). HO-1 overexpression, in the presence of dox, suppressed the
accumulation of cells in the BAL and the cytokine/chemokine elevation, presumably through its
end product carbon monoxide (CO). Hypoxic alveolar macrophages manifested an in vivo
phenotype consistent with alternative activation, characterized by the expression of found in
inflammatory zone (Fizz-1), mannose receptor C type 1 lectin (CD206), chitinase 3 like 3 (CHI3L3
or Ym1), arginase-1 and galectin-3 (Lgals3) while they failed to express markers of classical
activation such as inducible nitric oxide synthase (iNOS), tumor necrosis factor-alpha (TNF-a)
and interleukin 12 (IL-12p40). Mice with upregulated HO-1 levels did not exhibited elevated
The role of alternatively activated macrophages in pulmonary hypertension
alternative activation markers. Moreover, mRNA levels of two recently recognized inducers of
alternative macrophage polarization, CCL2 (previously known as monocyte chemoattractant
protein -1, MCP-1) and interleukin-6 (IL-6), were upregulated in the hypoxic lungs, an effect
abrogated by the expression of HO-1.
In order to pinpoint the role of the above inflammatory response in the development of
disease, we treated the mice with dox transiently. Upon removal of dox after short term
administration (2 days), prominent macrophage accumulation and activation ensued and this
was accompanied by pulmonary hypertension one to two weeks later. However, one week
treatment with dox was adequate to inhibit the accumulation and activation of macrophages
and prevented the later development of PAH.
These findings suggest that macrophage recruitment and alternative activation plays a
pivotal role in the development of hypoxic pulmonary hypertension. HO-1 confers
cytoprotective effects in the lung vasculature not only via its vasodilating and antiproliferative
properties but also via its anti-inflammatory effects. Understanding of the cellular and
molecular pathways that contribute to the progression of pulmonary hypertension will lead to
the development of targeted therapies and may raise the hope for the improvement of the
prognosis that, in spite of significant recent success, remains too dismal.
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The role of alternatively activated macrophages in the pathogenesis of pulmonary arterial hypertension
