Idiopathic Pulmonary Fibrosis (IPF) is a progressive and fatal disorder of excessive collagen deposition by fibroblasts resulting in reduced lung elasticity and poor survival. Despite ongoing efforts and new pharmacologic agents, only limited efficacy has been achieved to delay disease progression. This likely reflects the complex nature of the disease including vital cell-cell interactions. Recently, we identified critical interactions between macrophages and fibroblasts which led to the development of pulmonary fibrosis. Our data demonstrate that interleukin-1 receptor associated kinase (IRAK)-M, a negative regulator of Toll-like receptor signaling, was elevated in macrophages from human IPF patients and murine macrophages after bleomycin-induced pulmonary fibrosis and lead to enhanced collagen expression by fibroblasts. Using our experimental murine model of fibrosis, we demonstrate that mice deficient in IRAK-M are protected from pulmonary fibrosis and this effect is dependent on IRAK-M expression in macrophages. The goal of this proposal is to investigate the mechanism by which macrophage expression of IRAK-M regulates the development of pulmonary fibrosis. In order to accomplish this, we propose two specific aims.
In specific aim 1, we will investigate the role of IRAK-M in regulating monocyte trafficking to the lung during pulmonary fibrosis. Lung macrophage are either derived embryonically from the yolk-sac or are bone marrow-derived from circulating monocytes. We will interrogate the role of IRAK-M in regulating monocyte trafficking using adoptive transfer, competitive bone marrow transplantation model, and cell-specific IRAK-M knockout mice. In addition, we will investigate the role of IRAK-M in regulating expression of profibrotic genes and CCR2, the chemokine receptor involved in monocyte recruitment, using macrophages isolated from bleomycin challenged mice as well as primary macrophages from IPF patients and normal, donor controls. To show translational relevance, we will use our characterized human samples to generate a humanized IPF mouse model with cells from IPF patients.
In specific aim 2, we will determine the role of IRAK-M in regulating macrophage function, specifically their ability to identify, take up and degrade collagen in the lung during pulmonary fibrosis. We will assess the role of IRAK-M in the uptake of collagen fragments after bleomycin challenge. Macrophage and monocyte populations from our murine models as well as cells isolated from IPF patients and normal donor controls will be flow sorted and we will measure the expression of IRAK-M, collagen uptake receptors, and collagen degradation enzyme expression and activity. Finally, we will establish a macrophage-fibroblast co-culture system to investigate the bidirectional interactions that these two cell types exert on each other. These studies will provide the foundation for the development of novel biomarkers of disease as well as new interventions based on specific molecular targets that should improve the human health of IPF patients.
IPF affects roughly 128,000 people a year in the United States with a medium life expectancy of only 3 years after initial diagnosis; although novel therapeutic options are emerging and being tested in patients diagnosed with IPF, many patients still require orthotropic lung transplantation as a rescue treatment strategy. To develop molecular targets for the diagnosis and treatment of IPF, additional insight into the mechanism of disease is required. We propose to examine the role for a previously under-appreciated cell type, the macrophage, in pulmonary fibrosis in order to identify biomarkers of disease and novel targets for therapeutics.