Dynamic Changes Of Host Pulmonary Immune Microenvironment Induced By Acinetobacter Baumannii Infection

The discovery and use of antibiotics have effectively reduced the mortality rate,but at the same time,the problem of bacterial resistance caused by long-term irrational use of antibiotics has become more and more prominent,posing a great challenge to clinical practice.Acinetobacter baumannii is the most common multidrug-resistant bacterium in hospital infections,which can attack multiple parts of the patient and cause infections,among which pulmonary infections are more common.It does not pose a serious threat to normal individuals,but causes infections when the body is immunocompromised,indicating that the host’s normal state immune system has a strong defense function against A.baumannii invasion.There are limited studies on the interaction between A.baumannii and the host,so this study can provide new therapeutic targets and theoretical basis for clinical treatment by exploring the dynamic changes of the host immune microenvironment induced by A.baumannii infection.In this study,the bacterial burden assay in mice showed that the bacterial burden decreased gradually in mice after A.baumannii infection,and all the bacteria were cleared from the mice 96 hours after infection.The changes in the composition and number of immune cells in the lungs of mice were detected by flow cytometry:the composition of immune cells in the lungs of mice changed accordingly with the reduction of bacterial burden in vivo,mainly showing that the proportion and number of alveolar macrophages decreased significantly within 4 hours after infection,while the proportion and number of neutrophils increased significantly;4-6 days after infection,the proportion of alveolar macrophages gradually increased,while the proportion of neutrophils and other immune cells gradually decreased.Further study of the differences in changes in the immune microenvironment in alveoli and lung tissues under A.baumannii infection conditions revealed that:the bacterial burden in alveoli and lung tissues of mice were comparable after 6 h of infection,and the bacterial burden of both decreased sharply with the time of infection;meanwhile the immune cell composition of both underwent similar dynamic changes,a decrease in alveolar macrophages and an increase in neutrophils in the early stage,and an increase in alveolar macrophages and a decrease in neutrophils in the late phase,suggesting that the immune cell composition in the alveoli mimics the changes in immune cell composition in lung tissue under conditions of A.baumannii infection.In order to investigate the mechanism of changes in immune cell composition in the lung after infection with A.baumannii in mice,infections with A.baumannii in Ifnar-/-mice,Nlrp3-Aim2-/-mice and WT mice were performed,and the changes in immune cell composition in the alveoli and lung tissues of mutant mice were found to be comparable to the trend of changes in WT mice,suggesting that the changes in the host lung immune microenvironment induced by A.baumannii infection were not dependent on the type I interferon signaling pathway,NLRP3 and AIM2 inflammatory vesicle signaling pathways.To provide insight into the components and dynamics of the alveolar immune microenvironment induced by A.baumannii infection,single-cell sequencing and analysis of alveolar lavage fluid samples from WT mice uninfected,infected with A.baumannii for 6 hours and 6 days were performed.The results showed that alveolar macrophages were in an immune response state after 6 hours of A.baumannii infection,with a large number of inflammatory factor pathways activated;after 6 days of infection,alveolar macrophages were in a recovery state,reconstituted by self-proliferation,and then gradually returned to the immune microenvironment in the physiological state.In summary,this study revealed the dynamic process and underlying mechanism of host clearance of A.baumannii,and identified the intrinsic relationship between the significant early reduction of alveolar macrophages and the massive recruitment of neutrophils,and the important functions of both in the clearance of A.baumannii in the lung.The single-cell sequencing analysis of the alveolar immune microenvironment elucidated the specific immune cell composition of the alveolar microenvironment and the changes of alveolar macrophage disappearance and remodeling induced by A.baumannii infection.This study not only enriches the understanding of the cellular mechanisms of host immune defense against infection,but also has important theoretical implications for the diagnosis and prevention of respiratory infectious diseases.