Acute lung injury (ALI) and the more severe acute respiratory distress syndrome (ARDS) are common, devastating clinical syndromes of acute respiratory failure in the critically ill person. The incidence of ALI is 79 per 100,000 person years with a mortality rate of 30% to 65% . Survivors of ARDS experience a significant reduction in health-related quality of life, with 46% reported to be unable to return to work within 12 months.
ALI is the final common pathway of response to a variety of direct pulmonary insults, such as bacterial /viral pneumonia and gastric aspiration, or indirect insults, such as abdominal sepsis or battlefield trauma. Only a relatively small proportion of patients develop ALI, with research suggesting that genetic, demographic (age), social (smoking, alcohol abuse) and other factors play a role in determining who develops ALI [1, 2]. There are no current readily available tests that can clearly identify those who are at high risk of ALI and no therapeutic interventions proven to prevent its occurrence.
One lung ventilation (OLV) as a model for ALI/ARDS
To allow access to the oesophagus during surgery (using the transthoracic technique), one of the lungs is deflated and the subject is ventilated through the other lung. This is known as one-lung ventilation (OLV). There is a high postoperative incidence of ALI/ARDS [3–5] following OLV and unlike most insults leading to lung injury the delivery of OLV is predictably timed, thereby allowing serial studies to be carried out throughout the period of stimulus and development of the condition. Preoperative risk factors including age, respiratory function and cigarette smoking have been found to be related to the incidence of postoperative pulmonary complications [3, 6–8]. It is unclear at present why only a percentage of patients undergoing OLV develop lung injury or why the lung injury typically occurs 24 to 48 hours after the cessation of OLV. Our data show that the development of lung injury is, however, associated with a doubling of in-hospital stay and elevated mortality.
We have extensively modeled the local and systemic inflammatory response to transthoracic oesophagectomy in 50 patients undergoing OLV. After OLV, patients have a neutrophilic alveolitis, with a significant alveolar and systemic inflammatory response. This is associated with the release of markers of both endothelial and alveolar epithelial dysfunction and an increase in the permeability of the alveolar barrier. This manifests clinically as increased extravascular lung water and a fall in oxygenation.
Alveolar levels of surfactant protein D and bronchoalveolar lavage fluid (BALF) protein permeability index are highest in those who develop ALI within 72 hours of OLV suggesting that peri-operative alveolar epithelial damage is a risk factor for the subsequent development of ALI. Immediate post-operative plasma markers of neutrophilic activation (myeloperoxidase, and matrix metalloproteinase-9 (MMP-9)) as well as the receptor for advanced glycation end-products (RAGE, a type I epithelial cell marker) are similarly raised in those who develop ALI within 72 hours of OLV. Proposed causative mechanisms for this injury include the ischaemic/reperfusion insult suffered by the collapsed lung, as well as oxidative stress and barotrauma causing epithelial injury to the ventilated lung . These mechanisms are important in the pathogenesis of ALI making OLV a valid model for studying the pathogenesis of ALI in humans and exploring therapeutic strategies for preventing lung injury in a predefined subject population [10, 11].
Vitamin D biology
Vitamin D3, or cholecalciferol, is mainly formed in the skin after exposure to sunlight, then hydroxylated in the liver to 25-hydroxyvitamin D3 (25(OH)D3) and subsequently in the kidney to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). When 25(OH)D3 is sufficiently available, 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) is formed in the kidney which is further catabolised. Vitamin D metabolites are bound in the circulation to vitamin D binding protein (VDBP) which has a high affinity for 25(OH)D3, 24,25(OH)2D3 and 1,25(OH)2D3 and, therefore, regulates free circulating concentrations of vitamin D metabolites. The biologically active metabolite 1,25(OH)2D3 can also be generated locally within tissues due to induction of extra-renal cyp27b1 (25(OH)D-1-alpha hydroxylase) and binds to the vitamin D receptor (VDR) resulting in modified gene expression.
Epidemiological studies have suggested a role for low vitamin D status in the risk of developing both viral and bacterial infection [12, 13]. A recent study has further demonstrated the pleiotropic anti-inflammatory effects of vitamin D in patients with pulmonary tuberculosis . Published data suggest that vitamin D deficiency is common in critically ill patients , and recent prospective studies suggest an association with increased morbidity and mortality [16–18]. Literature on acute vitamin D supplementation in critical illness is lacking but serious adverse events attributable to vitamin D supplementation are rare [18, 19].
Is severe vitamin D deficiency a driver of post-OLV ALI?
Vitamin D has profound effects on human immunity acting as an immune system modulator, preventing excessive expression of inflammatory cytokines and increasing the ‘oxidative burst’ potential of macrophages, thereby enhancing bacterial killing. Vitamin D also stimulates the release of antimicrobial peptides such as LL-37 (cathelicidin) within the lung. LL-37 can also bind to and neutralize lipopolysaccharide (LPS), and functions as a chemoattractant for neutrophils, monocytes and T cells through a formyl peptide receptor-like molecule .
Respiratory epithelial cells convert 25(OH)D3 to 1,25(OH)2D3 and activate VDR responsive genes increasing the production of hCAP18 from which LL-37 is cleaved within 24 hours . In terms of the pathophysiology of ALI this could be important as LL-37 may drive epithelial repair responses as well as being an anti-microbial peptide . Elevating local LL-37 may also be important as a downstream immunomodulator of vitamin D since it has recently been shown to reduce Toll-like receptor (TLR) agonist-mediated neutrophil-derived increases in IL-1β, IL-6, IL-8 and tumour necrosis factor-alpha (TNF-α) in addition to stimulating bacterial phagocytosis . The ability of 1,25(OH)2D3 to directly inhibit nuclear factor-kappaB (NF-kb) signalling and suppress macrophage TLR expression suggests that vitamin D may also play a key role as a feedback regulator of macrophage responses [24, 25].
The only study looking at vitamin D levels in patients with severe sepsis suggests that these patients have a lower serum vitamin D level than healthy control patients. This was associated with lower plasma levels of LL-37, suggesting that this deficiency is of functional importance in vivo. IL-1 and TNF production induced by TLR3 agonists from monocyte derived macrophages are inhibited to the same extent by 25(OH)D3 and 1,25(OH)2D3 after 24 hours suggesting that the vitamin D metabolites may have a rapid anti-inflammatory action and that local intracrine activation of 25(OH)D3 can be anti-inflammatory .
Recent data have further implicated vitamin D in adaptive immunity because of its influence upon the differentiation of T cells between the regulatory T cell (Treg) and the pro-inflammatory T helper 17 (Th17) subsets [28–30]. Th17 cells are known to stimulate tissue inflammation and neutrophil chemotaxis, both of which are seen in ALI, predominantly by IL-17 production. It also appears that expression of markers of Treg cells (Foxp3) or Th17 cells (IL-17) by T cells may not be stable and that there is a greater degree of plasticity in their differentiation than previously appreciated . Recent evidence has further suggested Treg cells are important in the resolution of experimental ALI. This suggests that local lung regulation of the balance between Treg and Th17 cells may be a determinant of resolution/persistence of neutrophilic inflammation which is known to be associated with a poor prognosis in human ALI.
Although the above suggest a potentially beneficial effect of vitamin D, we must exercise some caution as some studies have shown potentially adverse effects of vitamin D. In low dose nasal LPS challenge, cellular inflammation is actually lower in vitamin D receptor deficient (VDR KO) mice due to toll-receptor hyporesponsiveness. In addition the chemotactic effects of LL-37 could in theory increase neutrophil recruitment to the lung; albeit in ALI, CXCL-8 and ENA-78 are the main chemokines driving neutrophil recruitment. More recently it has been shown that the plasma LL-37 concentration was decreased in vitamin D supplemented patients with tuberculosis, possibly representing a global suppressive effect of vitamin D supplementation on markers of acute phase response or an indirect response to enhanced microbial killing . Despite these reservations, the predominant biological effects of vitamin D led us to hypothesise that vitamin D deficiency may be a risk factor for ALI, causing elevated inflammation which results in exaggerated epithelial damage in at risk, vitamin D deficient individuals.
The VINDALOO trial is a three centre randomised double blind, placebo-controlled trial aiming to define the safety and effectiveness of a single high dose of vitamin D in preventing ALI in a group of patients at high risk of developing the condition.