Epidemiological studies support the importance of adequate vitamin D status for the maintenance of pulmonary health and function; low levels of serum 25-hydroxyvitamin D 3 are associated with an increased incidence or poor control of asthma, respiratory infection and chronic obstructive pulmonary disease (COPD). 1–4 Vitamin D is proposed to support respiratory health through promoting antimicrobial functions necessary for efficient clearance of pathogens, and dampening inflammation with the potential to damage lung structure and impair gaseous exchange. 4 However, the role of its major carrier protein, vitamin D-binding protein (VDBP), is less well understood. A paper by Wood et al in Thorax 5 together with independent studies, highlight the potential of VDBP to influence respiratory function both by determining vitamin D bioavailability and via direct effects on innate cell function.
VDBP is a glycosylated α-globulin protein synthesised by many tissues including the liver, kidneys, gonads and fat, and also by neutrophils. It binds circulating 25(OH)-vitamin D and 1,25(OH) 2-vitamin D with high affinity. The levels of this protein far exceed circulating levels of these vitamin D metabolites, although VDBP has a rapid turnover rate. VDBP has three distinct domains which confer additional functions beyond carriage of vitamin D (reviewed by Chishimba et al 6). These functions include augmentation of the monocyte and neutrophil chemotactic response to the complement anaphylotoxin C5a, 7 8 scavenging of actin molecules released from necrotic cells 9 and, of interest to the current article, formation of a dimeric molecule with macrophage activating factor (DBP-MAF) which drives macrophages towards a more phagocytic phenotype with increased superoxide generation. 10 These activities of VDBP may be of …
