Alveolar macrophages acquire 1 alpha-hydroxylase activity in inflammation, and thereby metabolize 25 hydroxyvitamin D3 (25 D3) to the active metabolite, 1 alpha,25-dihydroxyvitamin D3 (1,25 D3, calcitriol). Calcitriol is a potent differentiation agent that modulates mononuclear phagocyte activation