Role of immunodeficiency inAcinetobacter baumanniiassociated pneumonia in mice

Background Acinetobacter baumannii(A. baumannii) has become one of the most important opportunistic pathogens inducing nosocomial pneumonia and increasing mortality in critically ill patients recently. The interaction betweenA. baumanniiinfection and immune response can influence the prognosis ofA. baumanniirelated pneumonia. The target of the present study was to investigate the role of immunodeficiency inA. baumanniiinduced pneumonia. Methods Male BALB/c mice were randomly divided into the normal immunity control (NIC) group, normal immunity infection (NIA) group, immune compromised control (CIC) group, and immune compromised infection (CIA) group (n = 15 for each group). Intraperitoneal injection of cyclophosphamide and intranasal instillation ofA. baumanniisolution were used to induce compromised immunity and murine pneumonia, respectively. The mice were sacrificed at 6 and 24 h later and the specimens were collected for further tests. Seven-day mortality of mice was also assessed. Results AfterA. baumanniistimulation, the recruitment of neutrophils in mice with normal immunity increased sharply (P = 0.030 at 6 h), while there was no significant raise of neutrophil counts in mice with compromised immune condition (P = 0.092 at 6 h,P = 0.772 at 24 h). The Th cell polarization presented with pulmonary interleukin (IL)-4 and interferon (IFN)-gamma level in response to theA. baumanniiin CIA group were significantly depressed in comparison with in NIA group (IFN-gamma:P = 0.003 at 6 h;P = 0.001 at 24 h; IL-4:P < 0.001 at 6 h;P < 0.001 at 24 h). The pulmonary conventional dendritic cell accumulation was even found to be inhibited afterA. baumanniiinfection in immunocompromised mice (P = 0.033). Correspondingly,A. baumanniiassociated pneumonia in mice with compromised immunity caused more early stage death, more severe histopathological impairment in lung. Conclusion A. baumanniicould frustrate the immune response in immunocompromised conditions, and this reduced immune response is related to more severe lung injury and worse outcome inA. baumanniiinduced pneumonia.