Introduction and Objectives Eosinophils are major cellular effectors of allergic inflammation and represent an important therapeutic target in asthma. While much is understood about the generation and activation of eosinophils, little is known about their intravascular kinetics and physiological fate. The purpose of this study was to image sites of eosinophil distribution and measure eosinophil kinetics in healthy individuals using autologous 111-Indium-labelled eosinophils.

Methods To determine “gold standard” kinetics of minimally-manipulated eosinophils, mixed leucocytes were isolated from the blood of healthy volunteers, labelled with 111-Indium-tropolonate and re-injected. Blood was sampled 0.75–72 h post-injection. Neutrophils and eosinophils were isolated in parallel, and cell-associated radioactivity was measured. To image sites of eosinophil margination/uptake eosinophils were purified using plasma-Percoll gradients and anti-CD16 immunomagnetic beads, labelled with 111-Indium-tropolonate and re-injected. The distribution of eosinophils was recorded by γ camera dynamic imaging (0–40 min) followed by static imaging up to 72 h.

Results Using minimally manipulated granulocytes we found that the 45min neutrophil recovery was 57±10% (n=7) and the intravascular lifespan was 10.3±0.1 h, in agreement with previous studies. By contrast, the 45min recovery of eosinophils was 15±2% (n=7) and eosinophil lifespan was 25.2±3.8 h. Moreover, eosinophils appeared to re-circulate at ∼4 h and 9 h before monoexponential removal. Using 111-Indium-eosinophils and γ camera imaging, we demonstrated initial retention of cells in the lungs, clearing to baseline by 40 min, with some early accumulation in the liver and progressive accumulation in the spleen (n=3). Simultaneous blood sampling showed that the 45 min recovery and intravascular lifespan of purified labelled eosinophils were 11±2% and 29.3±2.1 h, respectively, comparable to minimally manipulated cells. Purified cells also exhibited recirculation at ∼6 h and 12 h. Of note, the disappearance of eosinophils from the liver at 6 h and 9 h coincided with their re-appearance in circulating blood, suggesting the liver as a possible site of transient eosinophil sequestration.

Conclusions This work provides the first in vivo measurements of eosinophil kinetics in healthy volunteers. Our data suggest that 111-In-labelled-eosinophils can be used to monitor the organ distribution and fate of eosinophils non-invasively. This technique may have an important role in assessing the therapeutic effects of eosinophil-targeted drugs.