Fibrocyte Contribution to Systemic Fibrosis in Chronic Kidney Disease

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Exposure to gadolinium-based magnetic resonance imaging (MRI) contrast is a major risk factor in the acquisition of nephrogenic systemic fibrosis (NSF), a severely debilitating disease first recognized in patients with acute or chronic renal impairment. MRI contrast agents heretofore were considered safe in these patients and thus indiscriminately used. Gadolinium-based contrast agents remain a mainstay for MR imaging and alternatives are still lacking. The applicant's preliminary data show that MRI contrast elicits a direct proliferative effect on human fibroblasts with an increase in extracellular matrix synthesis in vitro. Rats with renal insufficiency exposed to MRI contrast demonstrate significant skin abnormalities, including recruitment of bone marrow-derived cells. MRI contrast-treated mice similarly demonstrate skin fibrosis with dermal hypercellularity. Furthermore, gadolinium has been detected in affected rat and mouse skin as well as other organs (such as the kidneys and livers) from MRI contrast-treated animals. The overall objective of this proposal is to identify candidate mechanisms that trigger the aberrant pattern of fibrosis after exposure to gadolinium-based MRI contrast in the setting of renal insufficiency. The central hypothesis is that in the setting of renal insufficiency, MRI contrast exposure triggers a pro-fibrotic state in target organs, such as the skin. Subsequent generation of specific chemokines activate and recruit bone marrow-derived and circulating mesenchymal precursor cells, or fibrocytes, to the affected areas. These cells compound the fibrotic process by synthesizing and reducing the generation of extracellular matrix and inducing the proliferation of resident cells. Experiments will address the following Specific Objectives: 1a: Determine the mechanism by which gadolinium-based contrast induces NSF. The applicant has proven that bone marrow-derived cells constitute around 44% of the dermal cellularity in skin lesions. Whether gadolinium deposition in the skin elicits the generation of pro-fibrotic cytokines that lead to fibrocyte recruitment or if gadolinium stimulation in other sites, such as the bone marrow, programs fibroblast precursors to hone to specific sites is unknown. The applicant will test this by "priming" bone marrow donor (transgenic, "tagged") rats with exposure to MRI contrast and then transplanting this bone marrow into lethally-irradiated animals. 1b: Demonstrate that gadolinium-based contrast induces an increase in circulating fibrocytes. The effect of MRI contrast on the number of circulating fibrocytes at specific time points will be quantified in rats. 2a: Determine the chemokines and receptors responsible for recruitment of bone marrow-derived cells to specific tissues. The hypothesis is that the monocyte chemoattractant protein- (MCP-) 1/chemokine receptor 2 (CCR2) axis modulates the recruitment of fibrocytes in MRI contrast-induced systemic fibrosis. The chemical mediators that lead to the recruitment of circulating cells to NSF lesions is not well-explored. Resistance to NSF will be examined in animals with a genetic deficiency CCR2 will be explored. The role of this protein will be validated by using bone marrow transplant experiments where CCR2-deficient bone marrow is transplanted to normal (wild-type) mice and vice-versa. 2b: Test the hypothesis that biodistribution of gadolinium differs between normal renal function and renal insufficiency in mice and rats. Even though prior publications definitively indicate that the liver is a major reservoir for gadolinium in MRI-contrast treated rats and mice, the skin is invariably involved in NSF and to a far greater extent than any liver pathology in experimental animals found to date by the applicant. If the deposition of gadolinium is the nidus for triggering the disease, perhaps biodistribution of gadolinium differs between the states of renal sufficiency and insufficiency. Gadolinium accumulation in organs potentially affected in NSF (skin, lungs, heart, diaphragm, liver, spleen, bone, muscle) will be compared in animals with renal insufficiency (subtotal nephrectomies) and sham-treated controls. The findings will lead to a better understanding of how systemic fibrosis occurs and why certain organs are targeted.