The Role Of Vitamin D Deficiency In Pulmonary Aspergillus Infection In Mice

It is increasingly recognized that vitamin D plays an important role in pulmonary defense, immunity, and inflammatory processes. Immune effects of vitamin D include increased secretion of the antimicrobial peptide cathelicidin, decreased proinflammatory cytokine and chemokine production, inhibition of dendritic cell activation, and alteration of T-cell activation. The active vitamin D generating enzyme, la-hydroxylase, is expressed by the airway epithelium, alveolar macrophages (AMs) and dendritic cells, indicating that active vitamin D can be produced locally within the lungs, and it acts in an autocrine or paracrine fashion, and is responsible for many 6mmune-modulatory actions. These cellular effects of vitamin D are important for host responses against infection and impaired vitamin D status would compromise the immune activity of vitamin D.Vitamin D deficiency and insufficiency is a global issue which has significant implications for health. Multiple studies have identified a correlation between vitamin D deficiency and various respiratory infections, especially increased risk of tuberculosis, influenza and viral respiratory tract infections. Aspergillus fumigatus(A. fumigatus) remains the most common Aspergillus species that cause respiratory infections, in particular in the immunocompromised patient population. However, little has been done to assess the impact of impaired vitamin D status on the host control of pulmonary A. fumigatus infection. Only in cystic fibrosis (CF) patients, vitamin D deficiency has been proposed as a risk factor for allergic bronchopulmonary aspergillosis (ABPA).Based on the fact that local synthesis of active vitamin D in lung can act to support immunomodulation at this barrier site, we hypothesized that vitamin D deficiency may compromise pulmonary resistance to A. fumigatus.In the current study, we sought to test this hypothesis by inducing pulmonary A. fumigatus infection in mice raised on vitamin D sufficient (VitD+) or vitamin D deficient (VitD-) diets, and evaluated parameters of susceptibility and resistance to the infection in terms of mortality, weight change, fungal growth, inflammatory pathology. AMs were stimulated with A. fumigatus conidia in vitro. We preliminary explored the possible mechanisms of defective resistance to A. fumigatus in lung of vitamin D deficient mice through analysis of the kinetic expression of pattern recognition receptors (PRRs) and inflammatory mediators in vivo and in vitro.Upon A. fumigatus infection, vitamin D deficient mice showed higher mortality, greater fungal load and more weight loss than its sufficient counterparts. More aggravated and sustained inflammation was observed histologically in vitamin D deficient mice. Compared to VitD+mice, VitD-mice had overall higher levels of IL-1β, IL-6, TNF-α and in lung and AMs at baseline. Upon A. fumigatus exposure, vitamin D deficiency led to enhanced and sustained expression of TNF-α, IL-1β, IL-6, CXCL1 and CCL3 both in vivo and in vitro. These cytokines increased significantly in VitD+lung and AMs upon A. fumigatus infection. Vitamin D deficiency significantly enhanced their expression in lung and AMs, delayed their peak time, and prolonged the time interval when they returned to baseline.The initial sensing of fungal infection is mediated by pattern recognition receptors (PRRs), which trigger intracellular signaling cascades, leading to transcriptional expression of inflammatory mediators. TLR2, TLR4 and β-glucan receptor dectin-1 are key PRRs known to be involved in A. fumigatus recognition. Vitamin D deficiency increased TLR2, TLR4 and dectin-1 expression in lung and AMs at baseline. Upon A. fumigatus challenge, these PRRs expression were up-regulated significantly both in vivo and in vitro, while VitD-lung and AMs still had higher levels of them than th p.i., with a delayed peak time and a prolonged time interval until return to baseline.The active vitamin D generating enzyme CYP27B1 is expressed in lung, and the active form of vitamin D,1,25-dihydroxyvitamin D3 can be produced locally within the lungs, that it acts in an autocrine or paracrine fashion to modulate immune actions. We observed the inhibited expression of CYP27B1 and VDR in lung and AMs of VitD-mice at basal level, as well as post A. fumigatus challenge, which, together with vitamin D deficiency, would impair the ability to synthesize and recognize the active form of vitamin D in lung, and influence the local immune modulation.In conclusion, we have identified an important role of vitamin D for host defense against A. fumigatus in an immunocompentent host. By modifying the pro-inflammatory cytokines and by exerting its influence on the major PRRs involved in A. fumigatus recognition, namely TLR2, TLR4 and dectin-1 both in vitro and in vivo, vitamin D deficiency causes an aggravated and prolonged inflammatory response in response to A. fumigatus infection. Therefore, the relatively higher baseline of pro-inflammatory cytokines and PRRs in VitD-mice indicates that vitamin D deficiency might lead to a hyper-inflammatory response, which might be one possible mechanism for the exaggerated and prolonged inflammation, and cause defective host resistance to A.fumigatus. In addition, the impaired synthesis of active vitamin D, and the inhibition of vitamin D recognition might cause the enhanced basal expression of pro-inflammatory cytokines and PRRs in VitD-mice.The key determinant of the pathogenicity of Aspergillus species, and the reason for the diversity of host outcomes, is hypothesized to be the nature of the immune response of the host. Serum vitamin D concentrations needed to elicit an optimal immune response. Our study indicates that vitamin D deficiency might lead to an aberrant immune response to A.fumigatus. We hope that our study may shed some light on the role of vitamin D deficiency in susceptibility to Aspergillus in immunosuppressed patients.