Question: 

The visualization of inflammatory processes still poses a serious challenge. Recently, ¹⁹F MRI using perfluorocarbon emulsions as contrast agent has been applied for the specific detection of inflammatory foci. Emulsified perfluoro-15-crown-5 ether (PFCE) exhibits ideal imaging properties but due to its long biological half life it is not applicable for repetitive application. In the present study, we have developed a matrigel-based lipopolysaccharide (LPS) release model for the future evaluation of perfluorocarbons with short tissue retention times. We have characterized the immune cell infiltration into the matrigel plug from the acute to the healing phase and we also demonstrate the feasibility of this model for ¹H/¹⁹F MRI inflammation imaging using PFCE.

Methodology: 

Matrigel/LPS (50 ml matrigel with LPS: 0, 1, 10, or 50 mg) was injected subcutaneously into the neck of male C57BL/6 mice. ¹H/¹⁹F MRI was performed at a Bruker DRX 9.4T NMR spectrometer. For ¹⁹F labelling, PFCE emulsion (500 ml of 10% perfluoro-15-crown ether w/v) was injected intravenously via the tail vein 24 hrs after matrigel implantation and ¹⁹F MRI was performed 4d later. Immune cell infiltration was determined by flow cytometry, H&E staining and immunohistochemistry.

Results: 

Subcutaneous implantation of matrigel resulted in a defined ellipsoid structure which could be localized by ¹H MRI. Addition of 50 mg of LPS to the matrigel resulted in a macroscopically visible local inflammation, which decreased over time from d1 to d20. Control plugs did not show any signs of inflammation. Immune cell analysis in LPS-containing plugs showed a high amount of neutrophil granulocytes in the early phase which strongly decreased up to d20 whereas the amount of macrophages increased with time. H&E staining as well as immunohistochemistry demonstrated predominant localization of CD11b⁺ immune cells in the boundary area of the plug next to the normal tissue. ¹⁹F MRI after PFCE application revealed a specific ¹⁹F signal in the inflamed but not in the control plug. The ¹⁹F signal was proportional to the amount of LPS applied and was restricted to areas in the periphery of the matrigel plug.

Conclusion: 

Matrigel releasing LPS is a robust and quantitative model for induction of subcutaneous inflammation which covers the full course of inflammation from the acute to the healing phase. The matrigel/LPS model is suitable to assess the performance of clinically relevant perfluorocarbon emulsions for noninvasive visualization of inflammation by ¹⁹F MRI.