Reduced airway levels of fatty-acid binding protein 4 in COPD: relationship with airway infection and disease severity

Background For still unclear reasons, chronic airway infection often occurs in patients with Chronic Obstructive Pulmonary Disease (COPD), particularly in those with more severe airflow limitation. Fatty-acid binding protein 4 (FABP4) is an adipokine involved in the innate immune response against infection produced by alveolar macrophages (M phi). We hypothesized that airway levels of FABP4 may be altered in COPD patients with chronic airway infection. Methods In this prospective and controlled study we: (1) compared airway FABP4 levels (ELISA) in induced sputum, bronchoalveolar lavage fluid (BALF) and plasma samples in 52 clinically stable COPD patients (65.2 +/- 7.9 years, FEV1 59 +/- 16% predicted) and 29 healthy volunteers (55.0 +/- 12.3 years, FEV1 97 +/- 16% predicted); (2) explored their relationship with the presence of bacterial airway infection, defined by the presence of potentially pathogenic bacteria (PPB) at >= 10(3) colony-forming units/ml in BALF; (3) investigated their relationship with the quantity and proportion of M phi in BALF (flow cytometry); and, (4) studied their relationship with the severity of airflow limitation (FEV1), GOLD grade and level of symptoms (CAT questionnaire). Results We found that: (1) airway levels of FABP4 (but not plasma ones) were reduced in COPD patients vs. controls [219.2 (96.0-319.6) vs. 273.4 (203.1-426.7) (pg/ml)/protein, p = 0.03 in BALF]; (2) COPD patients with airway infection had lower sputum FABP4 levels [0.73 (0.35-15.3) vs. 15.6 (2.0-29.4) ng/ml, p = 0.02]; (3) in COPD patients, the number and proportion of M phi were positively related with FABP4 levels in BALF; (4) BALF and sputum FABP4 levels were positively related with FEV1, negatively with the CAT score, and lowest in GOLD grade D patients. Conclusions Airway FABP4 levels are reduced in COPD patients, especially in those with airway infection and more severe disease. The relationship observed between M phi and airway FABP4 levels supports a role for FABP4 in the pathogenesis of airway infection and disease severity in COPD.