Fibrocyte localization to the airway smooth muscle is a feature of asthma

Abstract

Background: Airway smooth muscle (ASM) hyperplasia is a hallmark of asthma that is associated with disease severity and persistent airflow obstruction.

Objectives: We sought to investigate whether fibrocytes, a population of peripheral blood mesenchymal progenitors, are recruited to the ASM compartment in asthma.

Methods: We assessed the number of fibrocytes in bronchial biopsy specimens and peripheral blood from subjects with mild-to-severe refractory asthma versus healthy control subjects. In vitro we investigated potential mechanisms controlling fibrocyte migration toward the ASM bundle.

Results: Fifty-one subjects with asthma and 33 control subjects were studied. In bronchial biopsy specimens, the number of fibrocytes was increased in the lamina propria of subjects with severe refractory asthma (median [interquartile range] number, 1.9/mm(2) [1.7/mm(2)]) versus healthy control subjects (median [interquartile range] number, 0/mm(2) [0.3/mm(2)], P < .0001) and in the ASM bundle of subjects with asthma of all severities (subjects with severe asthma, median [interquartile range] number, 3.8/mm(2) [9.4/mm(2)]; subjects with mild-to-moderate asthma, median [interquartile range] number, 1.1/mm(2) [2.4/mm(2)]); healthy control subjects, (median [interquartile range] number, 0/mm(2) [0/mm(2)]); P = .0004). In the peripheral blood the fibrocyte number was also increased in subjects with severe refractory asthma (median [interquartile range] number, 1.4 x 10(4)/mL [2.6 x 10(4)/mL]) versus healthy control subjects (median [interquartile range] number, 0.4 x 10(4)/mL [1.0 x 10(4)/mL], P = .002). We identified that in vitro ASM promotes fibrocyte chemotaxis and chemokinesis (distance of migration after 4.5 hours, 31 microm [2.9 microm] vs 17 microm [2.4 microm], P = .0001), which was in part mediated by platelet-derived growth factor (mean inhibition by neutralizing antibody, 16% [95% CI, 2% to 32%], P = .03) but not by activation of chemokine receptors.

Conclusion: This study provides the first evidence that fibrocytes are present in the ASM compartment in asthma and that ASM can augment fibrocyte migration. The importance of fibrocytes in the development of ASM hyperplasia and airway dysfunction in asthma remains to be determined.

FIG 1

A, Representative photomicrographs of bronchial biopsy specimens showing negative isotype control (i, magnification ×200), fibrocytes (highlighted by arrows) from 2 asthmatic subjects in the lamina propria (ii-iii) and ASM bundle (iv-vi) stained for CD34 (ii and v), collagen I (iii and vi), and α-SMA (iv; magnification ×400). B and C, Enumeration of fibrocytes in the lamina propria (Fig 1, B) and ASM bundle (Fig 1, C) in bronchial biopsy specimens from asthmatic subjects versus healthy control subjects. Horizontal bars indicate medians (interquartile ranges). P < .05, Kruskal-Wallis test. P values for the Dunn post test are shown in the figure.

FIG 2

A, Representative photomicrographs of cultured fibrocyte morphology (magnification ×200) and collagen I, CD34, α-SMA, CCR3, CCR7, CXCR3, and CXCR4 expression. Insets in the collagen I and α-SMA panels indicate rabbit and mouse isotype controls, respectively (magnification ×200). B, Flow cytometric analysis showing example dot plots of CD34⁺ collagen I–positive fibrocytes with corresponding isotype controls and example histograms for CD13⁺, HLA-DR⁺, α-SMA⁺, CCR7⁺, and CXCR4⁺ fibrocytes (black lines) with matched isotype controls (gray lines).

FIG 3

The proportion of PBMCs identified as fibrocytes in culture is increased in refractory asthma compared with that seen in healthy control subjects. Dot plots of the number of peripheral blood fibrocytes in subjects with or without asthma are shown. Horizontal bars indicate medians (interquartile ranges). P < .05, Kruskal-Wallis test. P values for the Dunn post test are given in the figure.

FIG 4

A, Representative line graphs showing migration paths of fibrocytes. The arrow represents migration toward ASM ± PDGF. B, fibrocyte migration alone and in the presence of ASM cells, with the addition of PDGF (C) or PDGF neutralizing antibody (D). E, Distance of fibrocyte migration above spontaneous movement in the presence of mast cells ± ASM. Data are presented as means ± SEMs, 1-way ANOVA was used across groups, and post hoc Tukey comparisons were used between groups.