Vitamin D deficiency induces Th2 skewing and eosinophilia in neonatal allergic airways disease

Asthma is a chronic airway inflammatory disease which commonly begins in early life. Growing evidence suggests that both genetic and environmental factors contribute to the development and severity of asthma, and exposures during the peri-natal period are highly influential (1). Vitamin D is primarily derived from skin synthesis in response to sunlight in humans, and deficiency has been implicated in many diseases including pulmonary diseases involving both infection and allergy (2). An association between vitamin D receptor (VDR) gene polymorphisms and asthma susceptibility is reported (3, 4); however, the relationship between serum vitamin D levels and asthma remains controversial (5). An inverse association between serum vitamin D and total IgE, lung function and asthma exacerbations in physician diagnosed asthmatic children have been described (6–8). Low serum vitamin D in childhood is thought to be predictive of asthma in adolescent males (9). Conversely, increased asthma was reported in adulthood after a period of vitamin D supplementation during the first year of life (10). A recent unselected cohort of over 2000 children observed no association between serum vitamin D and asthma or lung function; however, higher serum vitamin D was associated with a greater risk of wheeze (11). Serum vitamin D levels have been associated with disease severity in children and were reduced in severe compared to moderate asthmatics (12). Additionally, lower serum vitamin D was associated with poorer lung function and increased sensitization to aeroallergens (12, 13). Maternal vitamin D status during pregnancy may also determine risk of asthma development rather than the child's own vitamin D levels. Reduced maternal vitamin D during pregnancy has been associated with an increase in early wheeze, but this association does not extend to asthma at school age (14–16). Cord blood vitamin D levels inversely correlated with wheeze (17), while a U-shaped relationship with total and antigen-specific IgE has been observed in a population of children in Arizona (18). Conversely, maternal vitamin D status has been reported to have no associations with asthma or wheeze (19, 20). The variation in reported findings, and reliance on associations, rather than proof of causation, has prevented any firm conclusions about the impact of childhood or maternal vitamin D status on asthma development in children. The role of vitamin D as an immunomodulator is better defined, with vitamin D skewing the immune system towards regulation. Treatment of murine CD4+ T cells with vitamin D promotes IL-10 gene expression (21) and protein (22), while treatment of human CD4+ T cells with vitamin D also results in IL-10 secretion (23). Multiple studies have reported a positive correlation between serum vitamin D and FoxP3+ regulatory T-cell numbers in both the periphery and airways of adults (24) and children (25, 26). As diet is the only source of vitamin D for rodents (without a contribution from ultraviolet light), we used a vitamin D-deficient diet to alter vitamin D status. Pregnant females were fed a vitamin D-deficient diet starting from day-16 gestation (last trimester) to focus on immune-mediated mechanisms of disease development in offspring and to limit any effects of in utero vitamin D deficiency on foetal lung growth and development (27). To determine whether vitamin D deficiency exacerbates allergic airways disease (AAD), and the mechanisms underlying this, we used a neonatal model of inhaled house dust mite (HDM) exposure (28). We hypothesized that pups experiencing in utero and early-life vitamin D deficiency would develop more severe HDM-induced airway hyper-responsiveness (AHR), eosinophilic inflammation and remodelling due to a reduction in regulatory T cells in the lung.