Subject disposition

In total, 463 subjects were enrolled into the FOOTPRINTS® study (Fig. 1). Of these, 123 subjects were categorised as GOLD stage 1, 130 as GOLD stage 2, and 129 as GOLD stage 3. Nineteen subjects had COPD and homozygous ZZ A1ATD and 62 were EwAL. Of the 19 subjects in the A1ATD group, 1 subject (5.3%) met the criteria for GOLD stage 1, 5 (26.3%) for GOLD stage 2, 12 (63.2%) for GOLD stage 3, and 1 (5.3%) for GOLD stage 4.

Fig. 1

CONSORT flow diagram. A1ATD alpha-1-antitrypsin deficiency, GOLD Global Initiative for Chronic Obstructive Lung Disease

All demographics and analyses are based on the per-protocol set, which comprised 456 subjects (Table 1). Seven subjects were excluded from the per-protocol set due to newly added anti-inflammatory treatment or change to therapy prior to Visit 1, presenting with an immunocompromising condition, significant pulmonary disease other than COPD, or A1ATD subjects with a genotype other than ZZ.

Table 1 Subject demographicsBaseline characteristics

Selected baseline characteristics are shown in Table 1. The majority of subjects were male (n = 294, 64.5%) and the mean (± SD) age was 60.7 (6.9) years. Mean (± SD) body mass index was 26.6 kg/m2 (3.8) and mean (± SD) smoking pack history was 43.1 pack-years (19.5). Of note, a higher proportion of subjects in the A1ATD group were male (84.2%) compared with subjects in the GOLD 1–3 groups (59.8–69.7%) and EwAL group (62.3%). The mean age was lower in the A1ATD group (52.8 ± 9.0) and EwAL group (53.9 ± 6.8) than in the GOLD 1–3 groups (61.8 ± 5.4–62.9 ± 5.6). Smoking pack history was also lower in the A1ATD group (25.2 ± 12.3) and EwAL (32.9 ± 14.6) than in the GOLD 1─3 groups (41.5 ± 17.6–49.2 ± 21.6).

As disease severity increased, the proportion of subjects with multiple comorbidities increased (Table 1); 27.0% of GOLD 1 subjects had ≥ 3 comorbidities compared with 39.4% of GOLD 2 and 40.2% of GOLD 3 subjects. The most common comorbidities (> 10% prevalence) in GOLD 1, 2 and 3 subjects were, respectively, systemic arterial hypertension (36.1%, 40.9%, 49.6%), hyper-/dyslipidaemia (20.5%, 31.5%, 29.1%), pneumonia (4.9%, 19.7%, 22.8%), gastroesophageal reflux disease (14.8%, 15.7%, 15.7%), depression (9.0%, 17.3%, 15.7%), osteoarticular disorders (10.7%, 18.1%, 14.2%), diabetes (13.1%, 13.4%, 11.0%), anxiety (6.6%, 14.2%, 14.2%) and obesity (4.9%, 12.6%, 12.6%). Further details are given in the supplementary material (Additional file 1). Of the subjects in the A1ATD group, 21.1% had ≥ 3 comorbidities, but, as noted above, this was a younger population, with subjects being on average 10 years younger.

The percentage of prior COPD exacerbations (defined as an exacerbation that had occurred at least once during the lifetime of a subject) increased with increasing disease severity (as per GOLD classification): 24.6%, 37.8% and 55.9% of GOLD 1, GOLD 2 and GOLD 3 subjects, respectively, had experienced a previous COPD exacerbation. Overall, 40.8% of COPD subjects had a documented COPD exacerbation in their medical history.

Further information on baseline characteristics are available in Additional file 1.

Symptom questionnaires and 6MWT

Symptom questionnaires were conducted for subjects in the GOLD 1–3 and A1ATD groups, but not for the EwAL group. As shown in Table 2, increasing disease severity was associated with an increase in symptoms. Symptom scores in the A1ATD group were intermediate between those of the GOLD 1 and 3 groups. Increasing disease severity was associated with increases in the mean CAT score (GOLD 1: 11.2 [SD 5.7]; GOLD 2: 14.7 [SD 6.2]; GOLD 3: 16.6 [SD 7.2]; A1ATD: 13.4 [SD 7.4], P < 0.0001), the proportion of subjects with a CAT score ≥ 10 (GOLD 1: 62.3%; GOLD 2: 78.0%; GOLD 3: 79.5%; A1ATD: 63.2%, P = 0.0335), the proportion of subjects with an mMRC dyspnoea score ≥ 2 (GOLD 1: 11.5%; GOLD 2: 32.3%; GOLD 3: 60.6%; A1ATD: 47.4%, P < 0.0001) and the proportion of subjects with an SGRQ score ≥ 25 (GOLD 1: 50.8%; GOLD 2: 69.3%; GOLD 3: 83.5%; A1ATD: 73.7%, P < 0.0001).

Table 2 Symptom questionnaires and 6-min walk test

Increasing disease severity was associated with a reduction in distance covered in the 6MWT (GOLD 1: 489.3 m [SD 91.7 m]; GOLD 2: 458.0 m [SD 101.3 m]; GOLD 3: 403.8 m [SD 121.7 m], P < 0.0001). The 6MWT distance for subjects with A1ATD (508.6 m [SD 114.4 m]) was numerically higher than in any of the GOLD groups, but lower than in the EwAL group (573.1 m [SD 100.3 m]).

Lung function and lung volumeAirflow limitation

At baseline, patients with the greatest disease severity (as defined by the GOLD 1–4 classification system) had the poorest lung function (Table 3; Fig. 2). Patients with COPD and A1ATD also had worse lung function than EwAL. Lung function parameters for the A1ATD group were similar to the GOLD stage 2/3 groups, with the exception of FVC.

Table 3 Post-bronchodilator lung function and lung volume measurementsFig. 2

Lung function parameters. A1ATD alpha-1-antitrypsin deficiency, FEV1 forced expiratory volume in 1 s, FVC forced vital capacity, GOLD Global Initiative for Chronic Obstructive Lung Disease

Across GOLD groups 1–3, mean FEV1 was highest in GOLD 1 (2.66 L [SD 0.54 L]) and lowest in GOLD 3 subjects (1.17 L [SD 0.29 L]). Mean FEV1 was lower in GOLD groups 1–3 and subjects with A1ATD (1.85 L [SD 0.85 L]) than in EwAL (3.48 L [SD 0.77 L]). Mean percent predicted FEV1 was 87.4% [SD 9.4%] in GOLD 1 subjects and decreased with increasing disease severity to 39.6% [SD 7.6%] in GOLD 3 subjects (P < 0.0001). The mean percent predicted FEV1 was 50.4% (SD 20.3%) in subjects with A1ATD and 102.1% (SD 12.2%) in the EwAL group. Mean FVC also decreased with increasing disease severity (GOLD 1: 4.22 L [SD 0.85 L]; GOLD 2: 3.66 L [SD 0.90 L]; GOLD 3: 3.16 L [SD 0.89 L], P < 0.0001), and, overall, was highest in subjects with A1ATD (4.54 L [SD 1.38 L]. The EwAL control group had a mean FVC of 4.42 L (SD 0.97 L). The mean FEV1/FVC ratio was 63.4% (SD 5.4%) in GOLD 1 subjects and reduced with increasing disease severity to 38.2% (SD 9.0%) in GOLD 3 subjects (P < 0.0001). Subjects with A1ATD had a mean FEV1/FVC ratio of 40.6% (SD 13.4%) and the EwAL control group FEV1/FVC ratio was 78.8% (SD 5.7%).

Plethysmography

Table 3 shows post-bronchodilator lung function and lung volume measurements at baseline. Increasing disease severity was associated with increasing mean residual volume (GOLD 1: 2.51 L [SD 0.66 L]; GOLD 2: 2.91 L [SD 0.96 L]; GOLD 3: 3.64 L [SD 1.02 L], P < 0.0001). Residual volume was high in subjects with A1ATD (3.63 L [SD 0.81 L]) and comparable with GOLD 3 COPD subjects, whereas it was lowest in EwAL (2.27 L [SD 0.54 L]).

TLC values generally increased with increasing disease severity (GOLD 1: 6.70 [SD 1.29]; GOLD 2: 6.60 [SD 1.44]; GOLD 3: 7.02 [1.58], P = 0.0217). The highest mean TLC was observed in the A1ATD group (8.16 L [SD 1.53 L]), whereas TLC for EwAL was 6.81 L (SD 1.27 L).

Additional lung function tests

Increasing disease severity and A1ATD were associated with reduced diffusing capacity of the lung for carbon monoxide (DLCO), (P < 0.0001) (Table 3). The mean percent predicted DLCO was 79.8% (SD 21.3%) in GOLD 1 subjects, 63.4% (SD 18.1%) in GOLD 2 subjects, 51.5% (SD 17.5%) in GOLD 3, subjects and 51.1% (SD 12.7%) in subjects with A1ATD.

The mean DLCO/alveolar volume ratio was highest in GOLD 1 subjects (2.25 mmol/min/kPa [SD 1.36 mmol/min/kPa]) and decreased with disease severity (GOLD 3: 1.61 mmol/min/kPa [SD 1.20 mmol/min/kPa]). Overall, mean DLCO/alveolar volume was lowest in the A1ATD group (0.83 mmol/min/kPa [SD 0.19 mmol/min/kPa] vs 1.88 mmol/min/kPa [SD 1.20 mmol/min/kPa] in EwAL). Further DLCO parameters are available in Additional file 1.

Soluble biomarkers

For the majority of biomarkers presented in Fig. 3, the mean expression levels or concentration increased with disease severity. A linear increase in the level of WBC (P < 0.0001), fibrinogen (P = 0.0249), hsCRP (P = 0.0002) and plasma SP-D (P = 0.0194) was observed whereas sRAGE levels linearly decreased with disease severity (P < 0.0001), indicative of more severe emphysema. In all GOLD groups (GOLD 1–3), levels of WBC, fibrinogen, hs-CRP and plasma SP-D were numerically higher compared with the EwAL group. Further soluble biomarker parameters are available in Additional file 1.

Fig. 3

Distribution of inflammatory biomarkers at baseline by diagnosis group: a WBC, b fibrinogen, c hs-CRP, d IL-6, e sRAGE, f plasma SP-D. All biomarker values (except for IL-6) are log2 transformed. WBC, sRAGE: P ≤ 0.0001; hs-CRP: P = 0.0002; plasma SP-D: P = 0.0194; fibrinogen: P = 0.0249, linear regression for subject groups ordered by disease severity (Kruskal–Wallis test used for fibrinogen). No association was observed between positive expression of IL-6 and subject groups (chi-square test, P = 0.0701). A1ATD alpha-1-antitrypsin deficiency, Abs absolute, EwAL ex-smokers without airflow limitation, GOLD Global Initiative for Chronic Obstructive Lung Disease, hs-CRP high-sensitivity C-reactive protein, IL-6 interleukin 6, SP-D surfactant protein D, sRAGE soluble receptor for advanced glycation end product, WBC white blood count

No association was detected between IL-6 positive expression and disease severity (P = 0.0701). IL-6 was expressed less often in the EwAL subjects compared with GOLD 1–3 and A1ATD subjects, of whom 13%–26% expressed IL-6 (Fig. 3d).

Qualitative CT data

Increasing disease severity was associated with a greater prevalence of centrilobular emphysema (Cochran–Mantel–Haenszel test, P < 0.0001) (Fig. 4). Emphysema was present in all subjects in the A1ATD group, and in 41.0% of the EwAL group. Among GOLD subjects, the prevalence of advanced destructive emphysema was higher in GOLD 3 (GOLD 1: 9.0%; GOLD 2: 13.4%; GOLD 3: 31.5%), as well as the prevalence of confluent emphysema (GOLD 1: 13.1%; GOLD 2: 24.4%; GOLD 3: 26.8%). The Fleischner classification does not provide a separate category for panlobular emphysema [15], which would be graded as either confluent or advanced destructive; these findings were present in 14 (82%) of 17 subjects with A1ATD.

Fig. 4

Centrilobular emphysema on CT (A) and CT images showing typical emphysema findings for each category i) absent, ii) mild, iii) moderate, iv) confluent, v) advanced destructive (B). Percentages do not total 100% due to missing data (missing: EwAL, 3.3%; GOLD 1, 2.5%; GOLD 2, 1.6%; GOLD 3, 3.9%; A1ATD, 10.5%)

Quantitative CT data

When assessing lung volumes by quantitative CT, a linear increase in expiratory volume and a linear decrease in PD15 and ALD were observed with increasing disease severity (P < 0.0001 for all comparisons) (Table 4). Mean expiratory volume increased from 2.83 L (SD 0.67 L) in EwAL subjects to 4.78 L (SD 1.24 L) in GOLD 3 subjects. PD15 decreased from 91.2 g/L (SD 19.4 g/L) in EwAL subjects to 47.8 g/L (SD 24.9 g/L) in GOLD 3 subjects. PD15 was 44.4 g/L (SD 21.1 g/L) in A1ATD subjects. Similarly, ALD decreased from 88.50 g/L (SD 13.90 g/L) in EwAL subjects to 54.8 g/L (SD 23.1 g/L) in GOLD 3 subjects. ALD was 50.9 g/L (SD 18.2 g/L) in A1ATD subjects. A linear increase was also established for inspiratory volume with increasing disease severity (EwAL: 5.80 L [SD 1.30 L]; GOLD 1: 6.42 L [SD 1.28 L]; GOLD 2: 6.31 L [SD 1.35 L]; GOLD 3: 6.63 L [SD 1.45 L], P < 0.0001). Subjects in the A1ATD group had higher mean inspiratory (8.07 L [SD 1.73 L]) and expiratory (5.33 L [SD 1.51 L]) volumes than subjects in the GOLD 1–3 groups and the EwAL group.

Table 4 Qualitative and quantitative CT scans

The mean FRC/TLC ratio, a measure of air trapping, was 0.570 (SD 0.118) in GOLD 1 subjects versus 0.722 (SD 0.097) in GOLD 3 subjects. Ex-smoker controls had the lowest FRC/TLC ratio (0.499 [SD 0.112]). Values for GOLD 2 subjects (0.629 [SD 0.102]) and A1ATD subjects (0.653 [SD 0.121]) were between the values for GOLD 1 and GOLD 3 subjects.