Pulmonary monocyte kinetics in healthy volunteers and subjects with inflammatory arthritis.

Monocytes are central to innate and adaptive immunity, regulating tissue homeostasis and inflammation. However, data on their steady-state biodistribution in healthy humans are limited. We developed a non-invasive method to track radiolabelled autologous monocytes. Monocytes showed distinct intravascular transit kinetics vs neutrophils, with 45 min recovery of 47% for neutrophils vs 7% for monocytes (p=0.008), indicating predominant distribution in marginated pools. In patients with psoriatic and rheumatoid arthritis but no documented lung disease, we observed preferential early retention of monocytes in the lungs (transit half-life 25.2 min vs 14.2 min; p=0.008) compared with healthy volunteers, with reduced intravascular recovery in patients with rheumatoid arthritis (4.3% vs 12%; p=0.0043). These findings provide the first direct evidence of human monocyte pulmonary transit kinetics, systemic biodistribution and fate in health and inflammatory disease, advancing understanding of in vivo monocyte biology.