Peripheral-type benzodiazepine receptors in bronchoalveolar lavage cells of patients with interstitial lung disease

Selective Imaging of Lung Macrophages Using [11C]PBR28-Based Positron Emission Tomography

PURPOSE

We tested whether the translocator protein (TSPO)-targeted positron emission tomography (PET) tracer, N-acetyl-N-(2-[¹¹C]methoxybenzyl)-2-phenoxy-5-pyridinamine ([¹¹C]PBR28), could distinguish macrophage dominant from neutrophilic inflammation better than 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) in mouse models of lung inflammation and assessed TSPO association with macrophages in lung tissue from the mouse models and in patients with chronic obstructive pulmonary disease (COPD).

PROCEDURES

MicroPET imaging quantified [¹¹C]PBR28 and [¹⁸F]FDG lung uptake in wild-type (Wt) C57BL/6J or heterozygous transgenic monocyte-deficient Wt/opT mice at 49 days after Sendai virus (SeV) infection, during macrophage-dominant inflammation, and in Wt mice at 3 days after SeV infection or 24 h after endotoxin instillation during neutrophilic inflammation. Immunohistochemical staining for TSPO in macrophages and neutrophils was performed using Mac3 and Ly6G for cell identification in mouse lung sections and CD68 and neutrophil elastase (NE) in human lung sections taken from explanted lungs from patients with COPD undergoing lung transplantation and donor lungs rejected for transplantation. Differences in tracer uptake among SeV-infected, endotoxin-treated, and uninfected/untreated control mice and in TSPO staining between neutrophils and macrophage populations in human lung sections were tested using analysis of variance.

RESULTS

In Wt mice, [¹¹C]PBR28 uptake (% injected dose/ml lung tissue) increased significantly with macrophage-dominant inflammation at 49 days (D49) after SeV infection compared to controls (p = <0.001) but not at 3 days (D49) after SeV infection (p = 0.167). [¹¹C]PBR28 uptake was unchanged at 24 h after endotoxin instillation (p = 0.958). [¹⁸F]FDG uptake increased to a similar degree in D3 and D49 SeV-infected and endotoxin-treated Wt mice compared to controls with no significant difference in the degree of increase among the tested conditions. [¹¹C]PBR28 but not [¹⁸F]FDG lung uptake at D49 post-SeV infection was attenuated in Wt/opT mice compared to Wt mice. TSPO localized predominantly to macrophages in mouse lung tissue by immunostaining, and TSPO staining intensity was significantly higher in CD68+ cells compared to neutrophils in the human lung sections.

CONCLUSIONS

PET imaging with [¹¹C]PBR28 can specifically detect macrophages versus neutrophils during lung inflammation and may be a useful biomarker of macrophage accumulation in lung disease.

Polyacrylic acid-coated iron oxide nanoparticles could be a useful tool for tracking inflammatory monocytes

Aim:

To establish the effect of poly(acrylic acid)-coated iron oxide nanoparticles (PAC-IONs) and later exposure to a magnetic field on the differentiation of mononuclear phagocytes into macrophages.

Methods:

By flow cytometry, cell death was evaluated with DIOC6 and PI, Poly (ADP-ribose) Polymerases (PARP) fragmentation, H2AX phosphorylation and TUNEL assay. Cytokines by Cytokine bead array and the intracellular amount of iron by atomic absorption spectrometry.

Results:

PAC-IONs did not induce apoptosis, modify the cell membrane integrity or alter the mitochondrial membrane potential. They did not affect the cell morphology, the pattern of cytokine accumulation or the activating role of differentiation of mononuclear phagocytes into macrophages on the proliferation of autologous T cells.

Conclusion:

This evidence indicates that the PAC-IONs are safe and biocompatible. Moreover, the selectivity of the PAC-IONs for mononuclear phagocytes, as well as their increased uptake by non-classical monocytes, warrant future research with a view to their use as a contrast agent, a useful tool for in vivo tracking of tissue-infiltrating mononuclear phagocytes.

In the search for materials that allow the study of inflammatory processes when biopsies are not feasible, magnetic nanoparticles have become an alternative tool for use in MRI. This article examined whether supermagnetic iron nanoparticles can affect the basic function of phagocytic cells, with a view to their use in clinical imaging applications.

TSPO PET Imaging: From Microglial Activation to Peripheral Sterile Inflammatory Diseases?

Peripheral sterile inflammatory diseases (PSIDs) are a heterogeneous group of disorders that gathers several chronic insults involving the cardiovascular, respiratory, gastrointestinal, or musculoskeletal system and wherein inflammation is the cornerstone of the pathophysiology. In PSID, timely characterization and localization of inflammatory foci are crucial for an adequate care for patients. In brain diseases, in vivo positron emission tomography (PET) exploration of inflammation has matured over the last 20 years, through the development of radiopharmaceuticals targeting the translocator protein-18 kDa (TSPO) as molecular biomarkers of activated microglia. Recently, TSPO has been introduced as a possible molecular target for PSIDs PET imaging, making this protein a potential biomarker to address disease heterogeneity, to assist in patient stratification, and to contribute to predicting treatment response. In this review, we summarized the major research advances recently made in the field of TSPO PET imaging in PSIDs. Promising preliminary results have been reported in bowel, cardiovascular, and rheumatic inflammatory diseases, consolidated by preclinical studies. Limitations of TSPO PET imaging in PSIDs, regarding both its large expression in healthy peripheral tissues, unlike in central nervous system, and the production of peripheral radiolabeled metabolites, are also discussed, regarding their possible consequences on TSPO PET signal's quantification.

Pro-inflammatory activation of primary microglia and macrophages increases 18 kDa translocator protein expression in rodents but not humans

The 18kDa Translocator Protein (TSPO) is the most commonly used tissue-specific marker of inflammation in positron emission tomography (PET) studies. It is expressed in myeloid cells such as microglia and macrophages, and in rodent myeloid cells expression increases with cellular activation. We assessed the effect of myeloid cell activation on TSPO gene expression in both primary human and rodent microglia and macrophages in vitro, and also measured TSPO radioligand binding with ³H-PBR28 in primary human macrophages. As observed previously, we found that TSPO expression increases (∼9-fold) in rodent-derived macrophages and microglia upon pro-inflammatory stimulation. However, TSPO expression does not increase with classical pro-inflammatory activation in primary human microglia (fold change 0.85 [95% CI 0.58-1.12], p = 0.47). In contrast, pro-inflammatory activation of human monocyte-derived macrophages is associated with a reduction of both TSPO gene expression (fold change 0.60 [95% CI 0.45-0.74], p = 0.02) and TSPO binding site abundance (fold change 0.61 [95% CI 0.49-0.73], p < 0.0001). These findings have important implications for understanding the biology of TSPO in activated macrophages and microglia in humans. They are also clinically relevant for the interpretation of PET studies using TSPO targeting radioligands, as they suggest changes in TSPO expression may reflect microglial and macrophage density rather than activation phenotype.

Differentiation of septic and aseptic loosening by PET with both 11C-PK11195 and 18F-FDG in rat models

PURPOSE

This study aims to determine the value of PET with C-isoquinoline carboxamide (C-PK11195) and F-fluorodeoxyglucose (F-FDG) in assisting the differentiation of aseptic loosening (AL) from septic loosening (SL) in rat models.

PROCEDURES

Initially, the histological profiles of SL and AL (cellular infiltration and the number of CD68 macrophage and PBR cells) were compared. Subsequently, we investigated whether C-PK11195 alone and also in combination with F-FDG increases the sensitivity and specificity of PET imaging for distinguishing SL from AL.

RESULTS

There were distinguishable features between the histological profiles of the SL and AL rat groups. The number of CD68/PBR cells in AL rats was significantly higher than that seen in SL rats (P<0.05). The uptake of C-PK1195 was higher in AL and lower in SL rats. The uptake of F-FDG was higher in SL and lower in AL rats.

CONCLUSION

PET with a C-PK11195 and F-FDG imaging protocol is helpful in the clinical differential diagnosis of AL from SL.

Time course of metabolic activity and cellular infiltration in a murine model of acid-induced lung injury

PURPOSE

This study investigates whether positron emission tomography (PET) can be used to monitor the inflammatory response and its correlation with the later fibroproliferative phase in an experimental model of acute lung injury.

METHODS

Hydrochloric acid (0.1 N, pH 1, 1.5 ml/kg) was instilled into the right bronchus of mice. A group of mice underwent a micro-computed tomography (CT) scan 1 h after lung injury and a series of 2-[(18)F]fluorine-2-deoxy-D: -glucose (FDG)-PET scans (6, 24 and 48 h and 7 days after surgery). After 21 days respiratory static compliance was assessed and lung tissue was collected in order to measure the hydroxy (OH)-proline content. Other groups of mice underwent micro-CT and micro-PET scans at the same time points, and then were immediately killed to assess arterial blood gases and histology.

RESULTS

Histological analysis showed the recruitment of neutrophils and macrophages into the damaged lung, reaching the peak at 24 and 48 h, respectively. The time course of the [(18)F]FDG signal, used as a marker of inflammation, correlated with that of recruited inflammatory cells. In mice killed 21 days after the surgery, a correlation was found between reduced respiratory static compliance and high PET signal 7 days after lung injury. The PET signal also correlated with the OH-proline content.

CONCLUSIONS

This study demonstrated that PET imaging is a valid means of tracking the inflammatory response, also in longitudinal studies. Moreover, a correlation was found between persistence of the inflammatory response and fibrotic evolution of the injury.

Developing new molecular imaging probes for PET

Positron emission tomography (PET) is a fully translational molecular imaging technique that requires specific probes radiolabelled with short-lived positron emitting radionuclides. This review discusses relevant methods which are applied throughout the different steps in the development of new PET probes for in vivo visualization of specific molecular targets related to diagnosis or important for drug development.