(99m)Tc-HMPAO (Ceretec) is stored in and released from the granules of eosinophil granulocytes

Direct Cell Radiolabeling for in Vivo Cell Tracking with PET and SPECT Imaging

The arrival of cell-based therapies is a revolution in medicine. However, its safe clinical application in a rational manner depends on reliable, clinically applicable methods for determining the fate and trafficking of therapeutic cells in vivo using medical imaging techniques─known as in vivo cell tracking. Radionuclide imaging using single photon emission computed tomography (SPECT) or positron emission tomography (PET) has several advantages over other imaging modalities for cell tracking because of its high sensitivity (requiring low amounts of probe per cell for imaging) and whole-body quantitative imaging capability using clinically available scanners. For cell tracking with radionuclides, ex vivo direct cell radiolabeling, that is, radiolabeling cells before their administration, is the simplest and most robust method, allowing labeling of any cell type without the need for genetic modification. This Review covers the development and application of direct cell radiolabeling probes utilizing a variety of chemical approaches: organic and inorganic/coordination (radio)chemistry, nanomaterials, and biochemistry. We describe the key early developments and the most recent advances in the field, identifying advantages and disadvantages of the different approaches and informing future development and choice of methods for clinical and preclinical application.

Radiolabelled leucocytes in human pulmonary disease

INTRODUCTION

Radionuclides for leucocyte kinetic studies have progressed from non-gamma emitting cell-labelling radionuclides through gamma emitting nuclides that allow imaging of leucocyte kinetics, to the next goal of positron emission tomography (PET).

SOURCES OF DATA

Mostly the authors' own studies, following on from studies of the early pioneers.

AREAS OF CONTROVERSY

From early imaging studies, it appeared that the majority of the marginated granulocyte pool was located in the lungs. However, later work disputed this by demonstrating the exquisite sensitivity of granulocytes to ex vivo isolation and labelling, and that excessive lung activity is artefactual.

AREAS OF AGREEMENT

Following refinement of labelling techniques, it was shown that the majority of marginated granulocytes are located in the spleen and bone marrow. The majority of leucocytes have a pulmonary vascular transit time only a few seconds longer than erythrocytes. The minority showing slow transit, ~5% in healthy persons, is increased in systemic inflammatory disorders that cause neutrophil priming and loss of deformability. Using a range of imaging techniques, including gamma camera imaging, whole-body counting and single photon-emission computerized tomography, labelled granulocytes were subsequently used to image pulmonary trafficking in lobar pneumonia, bronchiectasis, chronic obstructive pulmonary disease and adult respiratory distress syndrome.

GROWING POINTS

More recently, eosinophils have been separated in pure form using magnetic bead technology for the study of eosinophil trafficking in asthma.

AREAS TIMELY FOR DEVELOPING RESEARCH

These include advancement of eosinophil imaging, development of monocyte labelling, development of cell labelling with PET tracers and the tracking of lymphocytes.

In vivo imaging of hepatic neutrophil migration in severe alcoholic hepatitis with 111In-radiolabelled leucocytes

The study’s aim was to image severe alcoholic hepatitis (SAH) using ¹¹¹In-labelled leucocytes with two objectives in mind: firstly for non-invasive diagnosis and secondly to provide a platform for experimental therapies aiming to inhibit intrahepatic neutrophil migration. ¹¹¹In-leucocyte scintigraphy was performed 30 min and 24 h post-injection in 19 patients with SAH, 14 abstinent patients with alcohol-related cirrhosis and 11 normal controls. Eleven with SAH and seven with cirrhosis also had ^99mTc-nanocolloid scintigraphy. Change in hepatic ¹¹¹In radioactivity was expressed as decay-corrected 24 h:30 min count ratio and, in SAH, compared with histological grading of steatohepatitis and expression of granulocyte marker, CD15. Hepatic microautoradiography on biopsy specimens obtained 24 h post-injection of ¹¹¹In-leucocytes was performed in one patient. Median 24 h:30 min hepatic ¹¹¹In activity ratio was higher in SAH (2.5 (interquartile range (IQR): 1.7–4.0) compared with cirrhotics and normal controls (1.0 (0.8–1.1) and 0.8 (0.7–0.9) respectively, P<0.0001). In SAH, it correlated with CD15 expression (r = 0.62, P=0.023) and was higher in marked compared with mild/moderate steatohepatitis (4.0 (3.0–4.6) compared with 1.8 (1.5–2.6), P=0.006). Hepatic-to-splenic ^99mTc count rate ratio was reduced in SAH (0.5 (0.4–1.4)) compared with cirrhotics (2.3( 0.6–3.0)) and three historic normal controls (4.2 (3.8–5.0); P=0.003), consistent with impaired hepatic reticuloendothelial function. Scintigraphic findings in SAH included prominent lung radioactivity at 30 min, likely the result of neutrophil primimg. Microautoradiography demonstrated cell-associated ¹¹¹In in areas of parenchymal neutrophil infiltration. In conclusion, ¹¹¹In-leucocyte scintigraphy can non-invasively diagnose SAH and could provide a platform for evaluation of novel treatments aiming to inhibit intrahepatic neutrophil migration.

99m-Technetium binding site in bone marrow mononuclear cells

INTRODUCTION

The increasing interest in 99m-technetium ((99m)Tc)-labeled stem cells encouraged us to study the (99m)Tc binding sites in stem cell compartments.

METHODS

Bone marrow mononuclear cells were collected from femurs and tibia of rats. Cells were labeled with (99m)Tc by a direct method, in which reduced molecules react with (99m)Tc with the use of chelating agents, and lysed carefully in an ultrasonic apparatus. The organelles were separated by means of differential centrifugation. At the end of this procedure, supernatants and pellets were counted, and the percentages of radioactivity (in megabecquerels) bound to the different cellular fractions were determined. Percentages were calculated by dividing the radioactivity in each fraction by total radioactivity in the sample. The pellets were separated and characterized by their morphology on electron microscopy.

RESULTS

The labeling procedure did not affect viability of bone marrow mononuclear cells. Radioactivity distributions in bone marrow mononuclear cell organelles, obtained in five independent experiments, were approximately 38.5 % in the nuclei-rich fraction, 5.3 % in the mitochondria-rich fraction, 2.2 % in microsomes, and 54 % in the cytosol. Our results showed that most of the radioactivity remained in the cytosol; therefore, this is an intracellular labeling procedure that has ribosomes unbound to membrane and soluble molecules as targets. However, approximately 39 % of the radioactivity remained bound to the nuclei-rich fraction. To confirm that cell disruption and organelle separation were efficient, transmission electron microscopy assays of all pellets were performed.

CONCLUSIONS

Our results showed that most of the radioactivity was present in the cytosol fraction. More studies to elucidate the mechanisms involved in the cellular uptake of (99m)Tc in bone marrow cells are ongoing.

[(18)F]FDG-6-P as a novel in vivo tool for imaging staphylococcal infections

BACKGROUND

Management of infection is a major clinical problem. Staphylococcus aureus is a Gram-positive bacterium which colonises approximately one third of the adult human population. Staphylococcal infections can be life-threatening and are frequently complicated by multi-antibiotic resistant strains including methicillin-resistant S. aureus (MRSA). Fluorodeoxyglucose ([(18)F]FDG) imaging has been used to identify infection sites; however, it is unable to distinguish between sterile inflammation and bacterial load. We have modified [(18)F]FDG by phosphorylation, producing [(18)F]FDG-6-P to facilitate specific uptake and accumulation by S. aureus through hexose phosphate transporters, which are not present in mammalian cell membranes. This approach leads to the specific uptake of the radiopharmaceutical into the bacteria and not the sites of sterile inflammation.

METHODS

[(18)F]FDG-6-P was synthesised from [(18)F]FDG. Yield, purity and stability were confirmed by RP-HPLC and iTLC. The specificity of [(18)F]FDG-6-P for the bacterial universal hexose phosphate transporter (UHPT) was confirmed with S. aureus and mammalian cell assays in vitro. Whole body biodistribution and accumulation of [(18)F]FDG-6-P at the sites of bioluminescent staphylococcal infection were established in a murine foreign body infection model.

RESULTS

In vitro validation assays demonstrated that [(18)F]FDG-6-P was stable and specifically transported into S. aureus but not mammalian cells. [(18)F]FDG-6-P was elevated at the sites of S. aureus infection in vivo compared to uninfected controls; however, the increase in signal was not significant and unexpectedly, the whole-body biodistribution of [(18)F]FDG-6-P was similar to that of [(18)F]FDG.

CONCLUSIONS

Despite conclusive in vitro validation, [(18)F]FDG-6-P did not behave as predicted in vivo. However at the site of known infection, [(18)F]FDG-6-P levels were elevated compared with uninfected controls, providing a higher signal-to-noise ratio. The bacterial UHPT can transport hexose phosphates other than glucose, and therefore alternative sugars may show differential biodistribution and provide a means for specific bacterial detection.

Imaging Inflammation in Asthma: Real Time, Differential Tracking of Human Neutrophil and Eosinophil Migration in Allergen Challenged, Atopic Asthmatics in Vivo

BACKGROUND

It is important to study differential inflammatory cellular migration, particularly of eosinophils and neutrophils, in asthma and how this is influenced by environmental stimuli such as allergen exposure and the effects of anti asthma therapy.

METHODS

We isolated blood neutrophils and eosinophils from 12 atopic asthmatic human volunteers (Group 1 - four Early Allergic Responders unchallenged (EAR); Group 2 - four Early and Late Allergic Responders (LAR) challenged; Group 3 - four EAR and LAR challenged and treated with systemic corticosteroids) using cGMP CD16 CliniMACS. Cells were isolated prior to allergen challenge where applicable, labelled with (99m)Tc-HMPAO and then re-infused intravenously. The kinetics of cellular influx/efflux into the lungs and other organs were imaged via scintigraphy over 4 h, starting at 5 to 6 h following allergen challenge where applicable.

RESULTS

Neutrophils and eosinophils were isolated to a mean (SD) purity of 98.36% (1.09) and 96.31% (3.0), respectively. Asthmatic neutrophils were activated at baseline, mean (SD) CD11b(High) cells 46 (10.50) %. Isolation and radiolabelling significantly increased their activation to > 98%. Eosinophils were not activated at baseline, CD69(+) cells 1.9 (0.6) %, increasing to 38 (3.46) % following isolation and labelling. Analysis of the kinetics of net eosinophil and neutrophil lung influx/efflux conformed to a net exponential clearance with respective mean half times of clearance 6.98 (2.18) and 14.01 (2.63) minutes for Group 1, 6.03 (0.72) and 16.04 (2.0) minutes for Group 2 and 5.63 (1.20) and 14.56 (3.36) minutes for Group 3. These did not significantly differ between the three asthma groups (p > 0.05).

CONCLUSIONS

Isolation and radiolabelling significantly increased activation of eosinophils (CD69) and completely activated neutrophils (CD11b(High)) in all asthma groups. Net lung neutrophil efflux was significantly slower than that of eosinophils in all asthma study groups. There was a trend for pre-treatment with systemic corticosteroids to reduce lung retention of eosinophils following allergen challenge.

Real-time differential tracking of human neutrophil and eosinophil migration in vivo

BACKGROUND

Hitherto, in vivo studies of human granulocyte migration have been based on indiscriminate labeling of total granulocyte populations. We hypothesized that the kinetics of isolated human neutrophil and eosinophil migration through major organs in vivo are fundamentally different, with the corollary that studying unseparated populations distorts measurement of both.

METHODS

Blood neutrophils and eosinophils were isolated on 2 separate occasions from human volunteers by using Current Good Manufacturing Practice CD16 CliniMACS isolation, labeled with technetium 99m-hexamethylpropyleneamine oxime, and then reinfused intravenously. The kinetics of cellular efflux were imaged over 4 hours.

RESULTS

Neutrophils and eosinophils were isolated to a mean purity of greater than 97% and greater than 95%, respectively. Activation of neutrophils measured as an increase in their CD11b mean fluorescence intensity in whole blood and after isolation and radiolabeling was 25.98 ± 7.59 and 51.82 ± 17.44, respectively, and was not significant (P = .052), but the mean fluorescence intensity of CD69 increased significantly on eosinophils. Analysis of the scintigraphic profile of lung efflux revealed exponential clearance of eosinophils, with a mean half-life of 4.16 ± 0.11 minutes. Neutrophil efflux was at a significantly slower half-life of 13.72 ± 4.14 minutes (P = .009). The migration of neutrophils and eosinophils was significantly different in the spleen at all time points (P = .014), in the liver at 15 minutes (P = .001), and in the bone marrow at 4 hours (P = .003).

CONCLUSIONS

The kinetics of migration of neutrophils and eosinophils through the lung, spleen, and bone marrow of human volunteers are significantly different. Study of mixed populations might be misleading.

Tc-99m HMPAO labeled white blood cell imaging in a child with eosinophilic lung disease

In radiolabeled leukocyte imaging, Tc-99m HMPAO has a significantly higher selectivity for eosinophils than neutrophils, but this may be clinically meaningful in disorders with eosinophilic infiltration. We present the case of a 2-year-old boy with infection who also developed drug-induced eosinophilic lung disease, as established later by bronchoalveolar lavage and discontinuation of the responsible antistaphylococcal agent. In the investigation of sepsis, diffusely increased pulmonary accumulation of Tc-99m HMPAO labeled leukocytes was observed. These findings were consistent with eosinophilic lung infiltration and underline the importance of clinical and laboratory data in the comprehensive interpretation of Tc-99m HMPAO labeled leukocytes scans.

Positively labeled white blood cell scan with eosinophilia and absence of infection

PURPOSE

The authors describe the variability of Tc-99m exametazime-labeled leukocyte distribution as a function of the relative frequency of white cell types in the labeled blood.

MATERIALS AND METHODS

A 76-year-old man who was hospitalized with fever and possible postoperative osteomyelitis underwent scintigraphic imaging with Tc-99m exametazime-labeled leukocytes.

RESULTS

The white cell scan excluded any discrete focus of infection and revealed diffuse involvement of the lymph nodes and skin. The pathologic diagnosis was angioimmunoblastic T-cell lymphoma. The atypical infiltrates seen on the white cell scan can be explained by the severe eosinophilic blood count on the day of leukocyte labeling (total leukocyte count: 8,100 cells/microl with 63% neutrophils, 8.9% lymphocytes, and 22.2% eosinophils).

CONCLUSION

In the labeling of the leukocyte moiety, a higher presence of any leukocyte subpopulation will modify the biodistribution and thus the image interpretation.