The present study demonstrates that while mice exposed to the aeroallergen HDM for 7 weeks develop increases in wall thickness and muscularization of pulmonary arterioles, this was not associated with significant increases in RVSP. However, when HDM exposure was extended out to 20 weeks, arterial remodeling was more severe and RVSP was significantly increased, although mild. Despite these increases in RVSP and arterial remodeling, Bmpr2 ΔE2 mice showed similar responses to WT mice, suggesting that reductions in BMPR-II signaling do not predispose for more severe PAH with chronic allergen-induced inflammation. Although vascular changes were similar between Bmpr2 ΔE2 and WT mice, AHR in Bmpr2 ΔE2 mice was more severe after 20 weeks HDM which suggests a potential role for this pathway as a modifier of reactive airway disease.
Previous studies have examined the role of inflammation and chronic hypoxia in the development of PAH in Bmpr2 heterozygous mice (Bmpr2+/−
). Zhang et. al. treated Bmpr2+/−
mice with adenovirus-delivered 5-lipoxygenase (5-LO), which is known to facilitate inflammation and is increased in patients and animal models of PAH 
. These studies demonstrated increased RVSP and vascular remodeling in Bmpr2+/−
mice compared to WT; however, the degree of muscularization in the 5-LO treated mice was mild 
. A subsequent study demonstrated that Bmpr2+/−
mice treated with combined 5-LO and monocrotaline developed sustained increases in RVSP, thickening of small pulmonary arteries, and perivascular inflammation of the remodeled vessels that was more severe in Bmpr2+/−
mice compared to WT 
. Another study utilizing Bmpr2+/−
mice, found that a combination of hypoxia and chronic serotonin infusion increased RVSP, right ventricular hypertrophy, and pulmonary remodeling 
. In the present study, we utilized HDM, as many people develop allergic responses this common aeroallergen 
. In addition, Erjefalt et. al. had previously demonstrated that chronic HDM exposure could cause pulmonary vascular modeling in mice after 20 weeks of HDM; however, no direct measures of PAH were reported 
. Unlike reports mentioned earlier, in the present study, PAH severity was not enhanced in BMPR2 mutant mice compared to WT mice. This discrepancy may be due to the different types of inflammation induced in the previous studies since they primarily elicit Th1 responses and little to no eosinophils 
. In contrast to previous models, HDM exposure induces the release of Th2 cytokines (interleukin 4 (IL-4), IL-5, and IL-13) and an influx of eosinophils during acute exposures and neutrophils during chronic exposures 
. A role for Th2 cytokines in the etiology of vascular remodeling was demonstrated by Grunig et. al. as pulmonary arterial remodeling in OVA and Aspergillus fumigatus
exposed mice was reduced in IL-4 deficient mice. Comparing the findings of our study to others, suggests that the type of stimuli may be important in determining whether PAH is more severe or not in BMPR2 mutant mice. In addition, in some of these studies, more than one insult was required for animals with reduced BMPR2 to show any PAH phenotype 
, suggesting that the development of the disease may be multi-factorial. This is consistent with additional genetic and/or environmental factors being necessary to trigger PAH in patients with BMPR2 mutations since the penetrance of the disease is low.
In the present study, we assessed vascular remodeling and RVSP after 7 and 20 weeks of HDM exposure to determine if the changes in arterial remodeling were associated with PAH. In addition, we examined the question of whether a Bmpr2 signaling deficiency in HDM exposed mice would cause more severe pulmonary arterial remodeling changes, since mutations in this gene are found in 70% of patients with heritable PAH. Although increases in arterial muscularization in HDM exposed mice were observed after 7 weeks, RVSP was not increased. However, after 20 weeks of HDM exposure, RVSP was similarly increased in both WT and Bmpr2 ΔE2 mice in conjunction with more severe arterial remodeling. Our data agrees with that of two previous studies which showed that persistent allergic inflammation could induce pulmonary vascular remodeling 
, however; in addition, we were able to detect PAH, although mild. The development of PAH in our model might be partly due to a higher percentage of fully muscularized pulmonary arteries at 20 weeks compared to 7 weeks; however, factors such as vasoconstriction could also be playing a role in the pathophysiology.
Although an increase in the number of inflammatory cytokines, macrophages, T cells, B cells, and chemokines have been observed in patients with PAH, the exact role of these molecules in the disease process is unclear 
. As previously mentioned, Grunig et al. 
, exposed mice to ovalbumin and Aspergillus fumigatus
for extended periods of time, and found that both of these allergens caused pulmonary arterial remodeling; however, increased muscularization of the pulmonary arteries did not correlate with RVSP measurements. A more recent study by Morrell et al. reported severe pulmonary arterial remodeling in the presence of Th1 and Th2 inflammatory responses in a mouse model of schistosomiasis, which is thought to be a common cause of pulmonary hypertension 
. PAH was not observed in these mice, even though extensive pulmonary arterial remodeling was detected. In contrast to the studies above, we were able to detect increases in RVSP after inducing chronic inflammation in our model of vascular remodeling. Recent reports indicate that both mouse strain and sex can affect the inflammatory responses 
. It is possible that the strain of mice used (Balb/c/Byj) in this study may have influenced the response to chronic inflammation differently compared to the other studies since mice of a different background were used (C57BL/6) 
. Although differences in sex have previously been shown to affect the inflammatory response, we were unable to detect any differences in inflammation or RVSP between males and females in our model; however, there was a trend towards higher RVSP in females.
In addition to vascular remodeling and PAH, we also assessed AHR in our model since a study reported decreased BMPR2 in the airways of asthmatic patients, providing evidence for a potential role for the BMPR2 pathway in allergic asthma 
. Interestingly, in our study, more severe AHR was observed in Bmpr2 ΔE2 mice after 20 weeks of HDM exposure compared to WT mice. These findings offer further support that this pathway may be a modifier of the asthmatic response, although additional studies are needed to address this more extensively. Prior to this study, there had been little evidence suggesting any common features between PAH and asthma. One case report showed both increased pulmonary pressures and reactive airway disease in two patients with congenital heart disease 
. After long-term treatment of these patients for the asthma symptoms, pulmonary pressures decreased; however, whether these two diseases are linked is unclear. One explanation for the lack of information regarding any connection between these two diseases may be due to the fact that the PAH patient population is small.
In summary, we demonstrated that chronic HDM exposure causes arterial remodeling and PAH in mice, although we were unable to detect any differences in the response between WT and Bmpr2 ΔE2 mice. This suggests that chronic HDM exposure causes vascular remodeling and PAH through mechanisms that may be independent of the BMPR-II signaling pathway. To our knowledge, this is the first study to establish that chronic allergen exposure causes PAH, albeit mild. Additionally, we observed more severe AHR in Bmpr2 ΔE2 mice compared to WT after chronic HDM exposure, suggesting a potential role for Bmpr2 in allergic airway disease.