In vivo Macrophage Imaging Using MR Targeted Contrast Agent for Longitudinal Evaluation of Septic Arthritis

Summary

We demonstrate how to perform macrophage MR imaging using ultrasmall superparamagnetic contrast agent (USPIO) in septic arthritis, allowing an initial and longitudinal in vivo non-invasive evaluation of macrophages infiltration and an assessment of therapy efficacy.

Abstract

Macrophages are key-cells in the initiation, the development and the regulation of the inflammatory response to bacterial infection. Macrophages are intensively and increasingly recruited in septic joints from the early phases of infection and the infiltration is supposed to regress once efficient removal of the pathogens is obtained. The ability to identify in vivo macrophage activity in an infected joint can therefore provide two main applications: early detection of acute synovitis and monitoring of therapy.

In vivo noninvasive detection of macrophages can be performed with magnetic resonance imaging using iron nanoparticles such as ultrasmall superparamagnetic iron oxide (USPIO). After intravascular or intraarticular administration, USPIO are specifically phagocytized by activated macrophages, and, due to their magnetic properties, induce signal changes in tissues presenting macrophage infiltration. A quantitative evaluation of the infiltrate is feasible, as the area with signal loss (number of dark pixels) observed on gradient echo MR images after particles injection is correlated with the amount of iron within the tissue and therefore reflects the number of USPIO-loaded cells.

We present here a protocol to perform macrophage imaging using USPIO-enhanced MR imaging in an animal model of septic arthritis, allowing an initial and longitudinal in vivo noninvasive evaluation of macrophages infiltration and an assessment of therapy action.

Introduction

Magnetic resonance imaging (MRI) is considered to be the imaging modality of choice for the demonstration of infectious synovitis due to its high spatial resolution and soft tissue contrast. Signal changes observed on MRI in arthritis low T1 and high T2 signal reflecting the increased presence of extracellular water content, and a marked enhancement after gadolinium based-contrast agent administration, consistent with the histologically findings of increased vascularity owing to vasodilation and angiogenesis¹. Nevertheless, MRI is often unable to demonstrate resolution of infection during antibiotic therapy as persistent enhancement can be observed in the j....

Protocol

All procedures involving animal subjects have been approved by the University Hospital Institutional Animal Care committee.

1. Intraarticular Bacterial Inoculation

1. Before injection, anesthetize rabbits by means of intramuscular injection of ketamine (30 mg/kg of body weight) mixed with xylazine (4 mg/kg) within Erector Spinae muscle. Ensure animals are fully anesthetized by checking that they fail to respond to paw pinch. This protocol provides sufficient anesthesia for the intraarticular injection of the knee joint. Place the animals under oxygen mask during anesthesia.
2. Shave animals' knee. Prepare the knee for as....

Results

On unenhanced images, the synovium of infected knees presents a diffuse swelling of intermediate signal that is non distinguishable from surrounding soft tissue, while femur and patella appear as low signal structures (Figure 1).

On the USPIO-enhanced images, 24 hr after the contrast agent administration, synovial area containing USPIO-loaded macrophages will demonstrate signal loss (Figures 1 and 2).

Macrophage inf.......

Discussion

While nonspecific gadolinium-based contrast agents provide information about the volume and perfusion of the extracellular space, macrophage imaging by USPIO-enhanced MR allows a precise anatomical localization and a qualitative evaluation of macrophage infiltration within infected synovium without the need of tissue sampling⁹. Due to their high sensitivity, USPIO can demonstrate even subtle cellular infiltrate present in the early phases of infection. Under successful antibiotic therapy, as the pathogens are .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Name of Reagent/Material	Company	Catalog Number	Comments
P904	Guerbet		Dose: 150 μmol Fe/kg
Ketamine (500 mg/ml ketamine)	Virbac		Dose: 30 mg/kg
Rompun (20 mg/ml Xylazine)	Axience		Dose: 4 mg/kg
Buprenorphine	Vetoquinol		Dose: 0.1 mg/kg/8 hr
BD 22 G, 1 inch	BD Biosciences	381423
BD 25 G, 5/8 inch	BD Biosciences	305122