MMP12 and pulmonary emphysema: Products of activated leukocytes, involving MMP-12 and neutrophil-derived elastase, can destroy elastic fibers and additional elements of the alveolar septa in the vicinity of these small airways, ultimately leading to lobar central emphysema.398 In vitro and in vivo studies indicate that indole-3-acetic acid produced by the respiratory microbiome reduces emphysema and reduces lung function through IL-22-mediated macrophage-epithelial cell interactions.