King's College London

Alpha-1 antitrypsin (AAT) is the most abundant serine protease inhibitor in the circulation, and is crucially protective in controlling pulmonary proteases including neutrophil elastase. AAT deficiency (AATD) is strongly associated with chronic obstructive pulmonary disease (COPD), primarily due to protease-antiprotease imbalance. However, recent studies also suggest immunomodulatory properties of AAT including the induction of IL-10 by dendritic cells and monocytes. Our group recently demonstrated that SERPINA1, the gene encoding AAT, is one of the most highly upregulated genes in vitamin D treated CD4+ T cells, and that AAT also upregulates IL-10 synthesis by these cells, in a complement C3a dependent manner.
CD8+ T cells are implicated in the pathogenesis of COPD. Therefore the initial aim was to assess whether vitamin D increases AAT synthesis in CD8+ T cells. Vitamin D significantly upregulated SERPINA1/AAT in CD4+ T cells, with a much smaller effect in CD8+ T cells. Addition of exogenous TGF-β, previously shown to increase AAT secretion by other cell types, dose-dependently enhanced vitamin D induction of AAT in CD8+, but not CD4+, T cells. TGF-β with vitamin D increased VDR and RXRα in CD8+ T cells, whilst neutralization of TGF-β reduced the capacity of vitamin D to enhance both SERPINA1 and VDR in CD4+ T cells. These data imply that TGF-β modifies the response of vitamin D to human T cells. Data from a TGF-β bioassay suggested no difference in baseline secretion of bioactive TGF-β between the 2 cell types, but increased secretion by CD4+ T cells following vitamin D exposure, providing a potential explanation for the requirement of exogenous TGF-β in CD8+ T cell cultures.
The next aim was to study AAT functions in T cells. Firstly, effects of wild-type (MM) and mutant (ZZ) AAT on CD8+ T cell immuno-regulatory function were assessed. However, little effect on cytokine responses including IL-10 induction was observed. As a result, gene expression microarrays were performed to further investigate AAT functions. These highlighted a low number of genes/pathways regulated by wild-type AAT, suggesting that AAT synthesised by CD8+ T cells may act on accessory cells, and/or that effects on CD8+ T cells may be indirect, requiring the presence of accessory cells in culture.
The potential of vitamin D to increase AAT levels in AATD patients carrying the PiZZ mutation was next evaluated. The capacity of vitamin D +/- TGF-β to induce SERPINA1/AAT was compared in T cells from PiZZ patients, healthy control subjects and non-AATD COPD patients. The previously observed SERPINA1 induction by vitamin D +/- TGF-β in healthy individuals was reduced in PiZZ CD8+ but not CD4+ T cells. However, this response was significantly impaired at the protein level in both PiZZ CD4+ and CD8+ T cells, suggesting that T cells from AATD patients may exhibit a secretion defect.
The IL-17 family cytokines are over-produced in severe and chronic respiratory disease. Since AAT may reduce IL-17A in animal models, evidence of dysregulated cytokine responses in PBMC cultures, following T cell receptor ligation, from AATD patients was investigated. Strikingly, an increased frequency of cells co-expressing IL-17A, with IL-17F and IL-22 was observed in PBMC cultures from PiZZ patients, in comparison to healthy individuals or older COPD patients. There was no difference in Th1 or Th2 cytokines including IFN-γ and IL-13 or IL-10 between the groups. Vitamin D increased IL-13 and decreased IFN-γ, IL-17A, IL-17F and IL-22. This enhanced Th17 response may be explained by the lack of AAT, although direct suppression by vitamin D is also possible, and this warrants further study.
These findings suggest an interaction between vitamin D and AAT and support the concept that both are important mediators in the maintenance of respiratory health. Understanding how those mediators exert protective effects in the airways is critical in optimizing therapeutic strategies for AATD, where the only approved therapy has many limitations.