Macrophage Infiltration Detected at MR Imaging in Rat Kidney Allografts: Early Marker of Chronic Rejection?

Ferumoxytol Is Not Retained in Kidney Allografts in Patients Undergoing Acute Rejection

PURPOSE

To evaluate whether ultrasmall superparamagnetic iron oxide nanoparticle (USPIO)-enhanced magnetic resonance imaging (MRI) can detect allograft rejection in pediatric kidney transplant patients.

PROCEDURES

The USPIO ferumoxytol has a long blood half-life and is phagocytosed by macrophages. In an IRB-approved single-center prospective clinical trial, 26 pediatric patients and adolescents (age 10-26 years) with acute allograft rejection (n = 5), non-rejecting allografts (n = 13), and normal native kidneys (n = 8) underwent multi-echo T2* fast spoiled gradient-echo (FSPGR) MRI after intravenous injection (p.i.) of 5 mg Fe/kg ferumoxytol. T2* relaxation times at 4 h p.i. (perfusion phase) and more than 20 h p.i. (macrophage phase) were compared with biopsy results. The presence of rejection was assessed using the Banff criteria, and the prevalence of macrophages on CD163 immunostains was determined based on a semi-quantitative scoring system. MRI and histology data were compared among patient groups using t tests, analysis of variance, and regression analyses with a significance threshold of p < 0.05.

RESULTS

At 4 h p.i., mean T2* values were 6.6 ± 1.5 ms for native kidneys and 3.9 ms for one allograft undergoing acute immune rejection. Surprisingly, at 20-24 h p.i., one rejecting allograft showed significantly prolonged T2* relaxation times (37.0 ms) compared to native kidneys (6.3 ± 1.7 ms) and non-rejecting allografts (7.6 ± 0.1 ms). Likewise, three additional rejecting allografts showed significantly prolonged T2* relaxation times compared to non-rejecting allografts at later post-contrast time points, 25-97 h p.i. (p = 0.008). Histological analysis revealed edema and compressed microvessels in biopsies of rejecting allografts. Allografts with and without rejection showed insignificant differences in macrophage content on histopathology (p = 0.44).

CONCLUSION

After ferumoxytol administration, renal allografts undergoing acute rejection show prolonged T2* values compared to non-rejecting allografts. Since histology revealed no significant differences in macrophage content, the increasing T2* value is likely due to the combined effect of reduced perfusion and increased edema in rejecting allografts.

CD163-Macrophages Are Involved in Rhabdomyolysis-Induced Kidney Injury and May Be Detected by MRI with Targeted Gold-Coated Iron Oxide Nanoparticles

Macrophages play an important role in rhabdomyolysis-acute kidney injury (AKI), although the molecular mechanisms involved in macrophage differentiation are poorly understood. We analyzed the expression and regulation of CD163, a membrane receptor mainly expressed by anti-inflammatory M2 macrophages, in rhabdomyolysis-AKI and developed targeted probes for its specific detection in vivo by MRI. Intramuscular injection of glycerol in mice promoted an early inflammatory response, with elevated proportion of M1 macrophages, and partial differentiation towards a M2 phenotype in later stages, where increased CD163 expression was observed. Immunohistological studies confirmed the presence of CD163-macrophages in human rhabdomyolysis-AKI. In cultured macrophages, myoglobin upregulated CD163 expression via HO-1/IL-10 axis. Moreover, we developed gold-coated iron oxide nanoparticles vectorized with an anti-CD163 antibody that specifically targeted CD163 in kidneys from glycerol-injected mice, as determined by MRI studies, and confirmed by electron microscopy and immunological analysis. Our findings are the first to demonstrate that CD163 is present in both human and experimental rhabdomyolysis-induced AKI, suggesting an important role of this molecule in this pathological condition. Therefore, the use of probes targeting CD163-macrophages by MRI may provide important information about the cellular composition of renal lesion in rhabdomyolysis.

In vivo delivery, pharmacokinetics, biodistribution and toxicity of iron oxide nanoparticles

Iron oxide nanoparticles (IONPs) have been extensively used during the last two decades, either as effective bio-imaging contrast agents or as carriers of biomolecules such as drugs, nucleic acids and peptides for controlled delivery to specific organs and tissues. Most of these novel applications require elaborate tuning of the physiochemical and surface properties of the IONPs. As new IONPs designs are envisioned, synergistic consideration of the body's innate biological barriers against the administered nanoparticles and the short and long-term side effects of the IONPs become even more essential. There are several important criteria (e.g. size and size-distribution, charge, coating molecules, and plasma protein adsorption) that can be effectively tuned to control the in vivo pharmacokinetics and biodistribution of the IONPs. This paper reviews these crucial parameters, in light of biological barriers in the body, and the latest IONPs design strategies used to overcome them. A careful review of the long-term biodistribution and side effects of the IONPs in relation to nanoparticle design is also given. While the discussions presented in this review are specific to IONPs, some of the information can be readily applied to other nanoparticle systems, such as gold, silver, silica, calcium phosphates and various polymers.

Macrophages and kidney transplantation

Macrophages are present within the transplanted kidney in varying numbers throughout its lifespan. Because of their prominence during acute rejection episodes, macrophages traditionally have been viewed as contributors to T-cell-directed graft injury. With growing appreciation of macrophage biology, it has become evident that different types of macrophages exist within the kidney, subserving a range of functions that include promotion or attenuation of inflammation, participation in innate and adaptive immune responses, and mediation of tissue injury and fibrosis, as well as tissue repair. A deeper understanding of how macrophages accumulate within the kidney and of what factors control their differentiation and function may identify novel therapeutic targets in transplantation.

Cellular and Functional Imaging of Cardiac Transplant Rejection

Heart transplantation is now an established treatment for patients suffering from end-stage heart diseases. With the advances in immunosuppressive treatment, the survival rate for transplant patients has improved greatly. However, allograft rejection, both acute and chronic, after heart transplantation is still a limitation leading to morbidity and mortality. The current clinical gold standard for screening rejection is endomyocardial biopsy (EMB), which is not only invasive, but also error-prone, due to the limited sample size and the site location of sampling. It would be highly desirable to have reliable and noninvasive alternatives for EMB in monitoring cardiac allograft rejection. The objective of this review is to highlight how cardiovascular imaging can contribute to noninvasively detecting and to evaluating both acute and chronic allograft rejection after heart transplantation, in particular, cardiovascular MRI (CMRI); and how CMRI can assess both immune cell infiltration at the rejecting organ, and the cardiac dysfunctions resulting from allograft rejection.

Allogeneic renal graft rejection in a rat model: in vivo MR imaging of the homing trait of macrophages

PURPOSE

To evaluate the feasibility of MR imaging to depict the in vivo recruitment of superparamagnetic iron oxide (SPIO)-labeled macrophages and to aid diagnosis of graft rejection in kidney transplantation.

MATERIALS AND METHODS

This study was approved by the institution's committee on animal research. Eighteen male Lewis rats received a kidney transplant; 12 had an F344 rat donor and six had a Lewis rat donor. Peritoneal macrophages were harvested from thioglycollate-treated Lewis rats, cultured, and labeled with SPIO. After resuspension of macrophages in a concentration of 1 x 10(7) cells per milliliter of Hanks balanced salt solution, 5 x 10(6) of SPIO-labeled macrophages was administered through the tail vein 2 or 5 days after transplantation in each group. The transplanted kidneys were imaged on a 4.7-T MR imager 24 hours after macrophage administration. The Wilcoxon signed rank test was performed for evaluating the differences between the relative signal intensity (SI) before and after SPIO-labeled macrophage administration.

RESULTS

A low-SI zone was predominantly noted in the medulla of the transplanted kidneys, and the relative SI decreased significantly from 1.40 to 0.53 (P < .001) in the allogeneic transplants following SPIO-labeled macrophage administration 5 days after the allogeneic transplantation. In the syngeneic group, the lower-SI zone was not noted in the grafts. At histopathologic examination, the lower-SI zone corresponded to the distribution of the SPIO-labeled macrophages.

CONCLUSION

This study demonstrates that the homing of intravenously administered SPIO-labeled macrophages can be monitored in the allograft rejection model on in vivo MR images.

Molecular magnetic resonance imaging of the genitourinary tract: recent results and future directions

This article focuses on preclinical and early clinical applications of renal cell MR imaging, on new developments in MR control of intrarenal gene therapy, and on several potential applications of molecular imaging techniques, mainly targeting cell receptors and enzyme activity, which could find exciting applications within the genitourinary tract.

In vivo mouse imaging and spectroscopy in drug discovery

Imaging modalities such as micro-computed tomography (micro-CT), micro-positron emission tomography (micro-PET), high-resolution MRI, optical imaging, and high-resolution ultrasound have become invaluable tools in preclinical pharmaceutical research. They can be used to non-invasively investigate, in vivo, rodent biology and metabolism, disease models, and pharmacokinetics and pharmacodynamics of drugs. The advantages and limitations of each approach usually determine its application, and therefore a small-rodent imaging laboratory in a pharmaceutical environment should ideally provide access to several techniques. In this paper we aim to illustrate how these techniques may be used to obtain meaningful information for the phenotyping of transgenic mice and for the analysis of compounds in murine models of disease.