In vivo cell tracking reveals the pattern of neutrophil tissue distribution at baseline and in response to glucocorticoid treatment

Abstract Neutrophils are the most abundant circulating immune cell in humans and primates, and they play a central role in inflammation and innate immunity. Glucocorticoids (GCs) are the cornerstone of anti-inflammatory and immunosuppressive therapies. While GCs are known to induce neutrophilia, little is known about the underlying kinetics or molecular mechanisms. To better define the kinetics of GC-induced neutrophilia, we studied healthy human volunteers and rhesus macaques, and found that the neutrophilia peaks within 4 hours of GC infusion and lasts beyond 72 hours. Flow cytometry, nuclear morphology, and single-cell RNA-seq confirmed that GC-mobilized neutrophils are mature and do not have the CXCR1 lo/ICAM1 hireverse-transmigration phenotype. To study the pattern of neutrophil tissue distribution in vivo, we are performing live tracking of neutrophils by PET/CT and serial intravascular staining (SIVS), before and after GC in a rhesus model. Initial Zr-89 oxine PET/CT results indicate that re-injected radiolabeled neutrophils transit the lungs quickly and within 24 hours localize to the liver, bone marrow, and spleen, where the signal remains stable for at least 72 hours. GC treatment does not appear to lead to significant re-distribution of tissue neutrophils. Initial SIVS results indicate that most GC-mobilized neutrophils had been in circulation for less than a day and came from the intravascular compartment. In summary, our findings to date indicate that neutrophils have stable and prolonged tissue localization, and that GCs rapidly mobilize a small proportion of the total neutrophil pool, composed mainly of mature cells that were present in the intravascular compartment at the time of treatment. supported by grants from NIH (Intramural Research Program – NIAMS/NIH)