Airway Surfactant Protein D Deficiency in Adults With Severe Asthma

Abstract

Background: Surfactant protein D (SP-D) is an essential component of the innate immune defense against pathogens within the airways. SP-D also regulates allergic inflammation and promotes the removal of apoptotic cells. SP-D dysregulation is evident in several pulmonary diseases. Our aim was to investigate whether airway and serum levels of SP-D are altered in treatment-resistant severe asthma.

Methods: SP-D concentrations were measured in matched serum and BAL samples collected from 10 healthy control subjects (HC) and 50 patients with asthma (22 with mild asthma [MA] and 28 with severe asthma [SA]). These samples were also evaluated by using Western blot analysis to investigate variations in SP-D size.

Results: SP-D levels in BAL samples were significantly lower in SA compared with HC and MA (P < .001) and inversely correlated with BAL eosinophil cationic protein concentrations in SA (P < .01). Serum SP-D was significantly increased in SA compared with HC and MA (P < .001), and BAL/serum ratios were significantly lower in SA compared with HC and MA (P < .001). Reduced SP-D levels in BAL samples, with concomitant increases in serum in SA, were associated with degraded fragments of SP-D in the serum and increased BAL neutrophil counts and lipopolysaccharide levels.

Conclusions: These findings suggest defective innate immunity within the airways in SA, as reflected by low BAL SP-D concentrations and altered bacterial presence with airway neutrophilia. Furthermore, BAL SP-D leakage into the serum in patients with SA may provide a peripheral blood biomarker, reflecting increased epithelial damage and/or epithelial permeability within the peripheral airways.

Keywords: asthma; biomarkers; bronchoalveolar lavage; eosinophilic inflammation; immunology (lung); immunology asthma; neutrophilic inflammation; severe asthma; surfactant protein D.

Figure 1

A-C, BAL inflammatory cell counts. Percentages of total cell count for (A) macrophages, (B) neutrophils, and (C) eosinophils from HC (n = 10), MA (n = 22), and SA (n = 28) subjects. HC = healthy control; MA = mild asthma; NS = not significant; SA = severe asthma.

Figure 2

A-C, BAL inflammatory mediator concentrations. Concentrations in BAL of (A) MPO, (B) IL-8, and (C) ECP from HC (n = 10), MA (n = 22), and SA (n = 28) subjects. ECP = eosinophil cationic protein; MPO = myeloperoxidase. See Figure 1 legend for expansion of other abbreviations.

Figure 3

A-C, Surfactant protein D (SP-D) concentrations, in matched (A) BAL and (B) serum samples from HC (n = 10), MA (n = 22), and SA (n = 28) subjects as well as the (C) log ratio BAL:serum SP-D concentrations for the same groups. ln = natural logarithm. See Figure 1 legend for expansion of other abbreviations.

Figure 4

Correlation between serum SP-D and ECP concentrations in patients with severe asthma (Spearman ρ, r² = 0.352; P < .05). See Figure 2, Figure 3 legends for expansion of other abbreviations.

Figure 5

A and B, Western blots showing the structural integrity of SP-D. In patients 1 through 7 with SA, matched patient (A) BAL and (B) serum samples. See Figure 1 legend for expansion of abbreviation.

Figure 6

A-C, BAL NE and LPS levels and relationship to sSP-D. (A) NE levels in BAL from HC (n = 10), MA (n = 22), and SA (n = 28) subjects. (B) LPS (EU/mL) concentrations in BAL from MA (n = 16) and SA (n = 19). (C) Linear regression plot of sSP-D against both BAL NE and BAL LPS identifying significant inverse correlations (Spearman’s ρ, r² = 0.163 and r² = 0.162, respectively; both, P < .05). LPS = lipopolysaccharide; NE = neutrophil elastase; sSP-D = serum surfactant protein D. See Figure 1 legend for expansion of other abbreviations.