Vitamin D (1,25(OH)2D3) induces α-1-antitrypsin synthesis by CD4+ T cells, which is required for 1,25(OH)2D3-driven IL-10

Studies to identify novel immune-regulatory functions of active vitamin D (1,25(OH)(2)D3) in human CD4(+) T cells revealed that 1,25(OH)(2)D3 potently induced expression of the gene SERPINA1, encoding the anti-protease alpha-1-antitrypsin. We confirmed alpha-1-antitrypsin protein expression by 1,25(OH)(2)D3-treated CD4(+) T cells, but not in CD8(+) T cells or monocytes. alpha-1-Antitrypsin promotes anti-inflammatory IL-10 synthesis in other immune cell populations. We therefore investigated its immune-regulatory effects in CD4(+) T cells. Plasma-derived alpha-1-antitrypsin drove IL-10 synthesis by CD4(+) T cells, which was not dependent on anti-protease activity, but appeared to require a serum-binding factor, since this could not be achieved with recombinant protein. alpha-1-Antitrypsin is reported to bind complement components, which regulate T cell function. A role for this interaction was therefore probed. Plasma-derived, but not recombinant alpha-1-antitrypsin contained C3a. Surface Plasmon Resonance and Microscale Thermophoresis demonstrated alpha-1-antitrypsin binding to C3a. Addition of C3a to CD4(+) T cells cultured with recombinant alpha-1-antitrypsin restored induction of IL-10, whereas neutralisation of C3a abrogated IL-10 induced by plasma-derived alpha-1-antitrypsin. To interrogate an endogenous role for the alpha-1-antitrypsin-C3a axis in 1,25(OH)(2)D3-driven CD4(+) T cell IL-10 synthesis, we treated cells from healthy or alpha-1-antitrypsin-deficient individuals (which transcribe SERPINA1 but do not secrete protein) with 1,25(OH)(2)D3. A significant correlation was identified between SERPINA1 and IL10 gene expression in healthy donor CD4(+) T cells, which was absent in cells from alpha-1-antitrypsin-deficient individuals. Therefore, alpha-1-antitrypsin is required for 1,25(OH)(2)D3-induced IL-10 expression in CD4(+) T cells, interacting with C3a to drive IL-10 expression.