The association between blood eosinophil percent and bacterial infection in acute exacerbation of chronic obstructive pulmonary disease

Choi, Juwhan; Oh, Jee Youn; Lee, Young Seok; Hur, Gyu Young; Lee, Sung Yong; Shim, Jae Jeong; Kang, Kyung Ho; Min, Kyung Hoon

This study was the first to analyze whether bacterial infections can be differentiated based on the eosinophil percent of 2% in AECOPD patients in Korea. Our results showed that only bacterial pathogen identification events and bacterial-viral pathogen coidentification events are significantly more frequent in groups with eosinophil counts of less than 2%. Since some studies used eosinophil percent of 2% as a cut-off value to predict the prognosis and treatment response in COPD, we also used the same cut-off value. For example, some studies analyzed the prognosis of steroid inhaler response and treatment in COPD based on the 2% value, 14, 15 while some analyzed the pneumonia risk and stability in stable COPD based on the 2% value. 11, 16 Eosinophils, which make up 1-6% of the total WBCs, have several receptors on their cell surface and secrete various cytokines and mediators. 17 Research in eosinophils was initially focused on helminth infection and allergic disorders. 18 As research progressed, it was found that eosinophils performed various functions in the human body such as regulation of innate and adaptive immunity and responses to infection and inflammation. 19, 20 Since animal studies showed that eosinophil count decreased with acute bacterial infection, eosinophils have been studied as a marker of bacterial infections. 12, 21 The reason for the decrease in peripheral eosinophils count in acute bacterial infection was the accumulation of eosinophils at the inflammatory site and inhibition of egress from the bone marrow. 22 Additionally, bacterial infection in lungs affects the cytokine and chemokine release from the airway smooth muscle cells (ASMC). 23 Bacterial infections activate human ASMC to release C-X-C motif chemokine (CXCL)-8 that increases neutrophil recruitment. 24 On the other hand, bacterial infections inhibit the release of eotaxin-1 that is proven to induce blood eosinophilia in studies on mice. 25 Many studies have been conducted on the relationship between eosinophil and COPD and AECOPD. In COPD analysis, it was thought that sputum eosinophilia could be indirectly predicted by the peripheral blood eosinophils, 26 and blood or sputum eosinophil count were inversely related to the bacterial load. 10 In AECOPD analysis, eosinopenia is predicted to have a poor prognosis. Eosinopenia group based on eosinophil count of 40/mm 3 is associated with long hospital day and high mortality. 27 Our results suggested that the eosinopenia group showed an increase in the mortality rate and duration of hospitalization because the rate of bacterial infection was higher than the eosinophilia group.