Differentiation and activation of eosinophils in the human bone marrow at steady state and during experimental acute systemic inflammation
Immunological mechanisms (English)Background and objective: Little is known on eosinophil differentiation and maturation in situ in human bone marrow in homeostasis and in response to acute inflammation.Acute systemic inflammation such as evoked by experimental endotoxemia leads to a rapid and transient eosinopenia. It is not clear whether this is caused by decreased mobilization/production of eosinophils in the bone marrow (BM) or merely a result of increased homing from the blood to the tissues.
The objective of this study was to investigate the differentiation, maturation and activation status of eosinophils in the BM and blood during homeostasis and during an acute innate immune challenge in humans.
Methods: BM and blood were obtained from 12 healthy male volunteers in the age range between 20 and 30 years old before and during experimental human endotoxemia (a model of systemic inflammation evoked by intravenous challenge with 2 ng/kg endotoxin).Promyelocytes, myelocytes, metamyelocytes and mature eosinophils were identified and FACS sorted according to their CD11b/CD62L expression. Maturation and activation status were assessed using antibodies against known membrane markers on eosinophils: LAIR1 (CD305), Alpha-4 (CD49d), CCR3 (CD193), FcγRI (CD64), CR1 (CD35), CEACAM- 8 (CD66b), IL-5Rα (CD125) and IL-3Rα (CD123). In addition, cytokines were analyzed in plasma and BM supernatant under both conditions.
Results: Eosinophil promyelocytes were characterized by a unique phenotype of surface markers:CD193dim, CD64dim, CD11b- and CD62L-. During maturation into myelocytes the cells changed to a CD193dim, CD64-, CD11bbright and CD62L-profile. Metamyelocytes were characterized as CD193bright, CD64-, CD11bbright and CD62Ldim cells. Finally, mature eosinophils displayed CD193bright, CD64-, CD11bbright and CD62Lbright expression.
Four hours after endotoxin administration, the percentage of mature eosinophils in the blood and in the BM declined (from 3.0 ± 1.9%% to 0.2 ± 0.11% and 1.3% ± 0.5% to 0.7 ± 0.47% respectively) whereas the number of eosinophil progenitors did not change. The remaining eosinophils in the circulation failed to show signs of activation or degranulation despite significantly increased circulating eotaxin-1 levels after LPS administration (p= 0.02). However, the expression of CD49d and IL-5Rα on circulatory eosinophils was lower after LPS administration compared to baseline (both p= 0.03).
Conclusion: The maturation of eosinophils in the bone marrow can be followed by staining cells in the granulocyte gate with a limited number of antibodies: CD64, CD193, CD11b and CD62L. A systemic innate immune challenge caused mature eosinophil mobilization from the BM without obvious signs of activation nor accelerated maturation of their progenitors. Circulatory eosinopenia evoked by systemic innate immune response is most likely the result of (CD49d) mediated homing to the tissues.