Macrophage Responses to Epithelial Dysfunction Promote Lung Fibrosis in Aging

Lay Abstract Idiopathic pulmonary fibrosis (IPF) is characterized by the progressive replacement of the delicate spongy tissue of the lung with scar tissue in the absence of an apparent cause. Despite the emergence of novel therapies that slow its progression, IPF universally results in progressive shortness of breath and eventual death from respiratory failure. For reasons that we do not understand, pulmonary fibrosis is a disease that is much more common in older patients. A careful search for genetic mutations in families with pulmonary fibrosis, and genetic screens (genome wide association studies) in people with non-familial pulmonary fibrosis has revealed that people with mutations in a handful of genes are at increased risk for the development of lung fibrosis. Most of these genes encode proteins found exclusively in the lining cells (epithelial cells) of the lung, suggesting that changes in the lung lining cells drive the development of fibrosis. However, even in patients with these genetic mutations, the onset of disease is typically delayed until older ages (50s or 60s) and many people who have the genetic mutation never develop disease. For example, the highest risk gene for non-familial pulmonary fibrosis is present in 20% of the Caucasian population. This suggests that even in patients with a strong genetic predisposition to fibrosis, another insult must occur for clinical disease to develop. This “second hit” is most likely driven by the environment and does not develop until the individual is older. In this application, we suggest that this “second hit” comes from the loss of healthy lung macrophages and their replacement with a “second-best” stop-gap macrophage. Alveolar macrophages are the most common patrolling immune cell in the lung; one macrophage is assigned to every 3 alveoli. Exciting studies from several groups have shown that in mice, these cells live an extremely long time, perhaps as long as 1/3 the entire lifespan, and are capable of renewing themselves by cell division. We have used genetic/surgical techniques to label these cells and have found that with age, these cells gradually die off and are replaced by cells that come from the blood (monocyte-derived alveolar macrophages). This process is accelerated by agents that cause lung injury like influenza A virus infection or the administration of the chemotherapy drug bleomycin. The macrophages recruited from the blood look much like the original macrophages we are born with, however, when we examined them carefully at the level of gene expression, we found marked differences. Specifically, while the original macrophages tend to inhibit inflammation and fibrosis, the recruited monocyte derived macrophages promote them. We used advanced murine genetic techniques to induce mutations that prevented the recruitment of these monocyte derived macrophages into the lung and found they prevented lung fibrosis in mice. In the first two aims of this proposal, we are asking two broad questions. First, we want to know whether the loss of the macrophage pool we are born with is a part of normal aging and whether it can be accelerated by injuries over the lifespan—viral infections and pollution exposure. Second, we want to know whether renewing the macrophage pool by transferring young macrophages into old mice, can reduce the increased susceptibility of old mice to fibrosis. We plan to ask these same questions in normal mice and in mice who harbor a genetic defect that increases the risk of lung fibrosis. In the third aim of the applicatio