Obese asthmatic patients have decreased surfactant protein A levels: Mechanisms and implications

Abstract

Background: Eosinophils are prominent in some patients with asthma and are increased in the submucosa in a subgroup of obese patients with asthma (OAs). Surfactant protein A (SP-A) modulates host responses to infectious and environmental insults.

Objective: We sought to determine whether SP-A levels are altered in OAs compared with a control group and to determine the implications of these alterations in SP-A levels in asthmatic patients.

Methods: Bronchoalveolar lavage fluid from 23 lean, 12 overweight, and 20 obese subjects were examined for SP-A. Mouse tracheal epithelial cells grown at an air-liquid interface were used for mechanistic studies. SP-A^-/- mice were challenged in allergen models, and exogenous SP-A therapy was given after the last challenge. Eosinophils were visualized and quantitated in lung parenchyma by means of immunostaining.

Results: Significantly less SP-A (P = .002) was detected in samples from OAs compared with those from control subjects. A univariable regression model found SP-A levels were significantly negatively correlated with body mass index (r = -0.33, P = .014), whereas multivariable modeling demonstrated that the correlation depended both on asthma status (P = .017) and the interaction of asthma and body mass index (P = .008). Addition of exogenous TNF-α to mouse tracheal epithelial cells was sufficient to attenuate SP-A and eotaxin secretion. Allergen-challenged SP-A^-/- mice that received SP-A therapy had significantly less tissue eosinophilia compared with mice receiving vehicle.

Conclusions: SP-A functions as an important mediator in resolving tissue and lavage fluid eosinophilia in allergic mouse models. Decreased levels of SP-A in OAs, which could be due to increased local TNF-α levels, might lead to impaired eosinophil resolution and could contribute to the eosinophilic asthma phenotype.

Keywords: IL-6; Surfactant; TNF-α; asthma; eosinophils; eotaxin; epithelial cells; lung function; obesity; surfactant protein A.

Figure 1

(A) SP-A expression as detected by Western blot is significantly decreased in obese asthma (OA) as compared to lean asthma (LA) (^#p=0.002) and lean normal (LN) (^*p=0.014) subjects. (B) SP-A levels are negatively correlated with increasing BMI, R= −0.33, p=0.013. (C) Representative Western blot of lean normal (LN), lean asthma (LA), obese normal (ON) and obese asthma (OA) samples. (D) SP-A levels measured via ELISA in a subset of samples demonstrate similar trends among groups (p=0.034).

Figure 2

(A) Representative Western blot for SP-A expression in cell-free BAL from lean and obese mice. (B) Obese mice that are either leptin deficient (Ob/Ob) or leptin receptor deficient (Db/Db) were significantly heavier than WT mice (lean) and (C) have significantly lower SP-A levels in BAL than lean mice (p<0.01).

Figure 3

(A) Model in which murine tracheal epithelial cells were cultured at ALI for 2 weeks and then treated with vehicle, TNF-α, or leptin for either 24 hr. or 5 days. (B) Trans-epithelial resistance was not altered by TNF-α or leptin challenges. TNF-α did not affect (C) SP-A mRNA levels by MTECs at 24 hr. but did cause a significant reduction in (D) secretion of SP-A into the apical supernatants. **p<0.001 versus vehicle, pre-treatment and leptin treated. Leptin did not have any effect on SP-A transcription or secretion. (E) TNF-R−/− MTECs did not have altered levels of SP-A secretion upon TNF challenge.

Figure 4

Mice lacking SP-A were challenged in a standard (A) OVA or (E) HDM protocol. Twenty-four hours after the last challenge in either model, mice were given an SP-A rescue treatment (or vehicle) and lung tissue was harvested 6 days later for analysis of tissue eosinophilia by anti-MBP staining. In both the OVA (B,C) and HDM (F,G) models, SP-A rescue treatment led to significantly lower tissue and BAL eosinophilia as compared to the vehicle treated. *p<0.05, ***p<0.001. (D) Representative lung sections from OVA mice that received vehicle (upper) or SP-A treatment (lower). MBP⁺ stained eosinophils denoted by arrows; V denotes a vessel.

Figure 5

MTECs were cultured at ALI for 2 weeks prior to experimental analysis. (A) MTECs from SP-A^−/− mice demonstrated a significant reduction in eotaxin secretion in both the apical and basolateral supernatants as compared to WT MTECs. n=3 independent experiments, 3–6 transwells/experiment, ***p<0.01. (B) WT and TNF-R−/− were challenged with TNF-α or vehicle for 24 hr. and Eotaxin was assessed in the basolateral compartment. **p<0.01, n=3–4 transwells/group. C) Eotaxin levels in BAL from OVA treated WT and SP-A−/− mice as determined by ELISA. N=2 experimental sets combined and represented as fold relative to WT, *p<0.05.

Figure 6

Proposed model includes elevated levels of TNF-α in obese individuals, in part leads to an attenuation of SP-A secretion. Loss of functional SP-A is a contributing factor to the enhanced tissue eosinophilia, which has been observed in several studies of obese asthmatics. In vitro studies support the concept that SP-A promotes movement of eosinophils from the tissue indirectly by enhancing eotaxin production from airway epithelial cells.