Sensitive detection and quantitation of mouse eosinophils in tissues using an enzymatic eosinophil peroxidase assay: its use to rapidly measure pulmonary eosinophilia during experimental respiratory syncytial virus infection of mice.

Eosinophils are granular proinflammatory leukocytes implicated in the pathogenesis of several inflammatory processes including allergy, asthma, and in the development of enhanced disease during respiratory syncytial virus (RSV) infection. Here, we adapted a colorimetric assay to measure eosinophil peroxidase (EPO) activity in inflamed mouse lung tissue in order to quantitate pulmonary eosinophilia during experimental RSV infection. Using the substrate o-phenylenediamine dihydrochloride (OPD) in the presence of bromide ions, we show that this assay is able to quantitate limiting numbers of eosinophils in lung tissue homogenates even in the presence of large numbers of infiltrating neutrophils and macrophages. The ability of the EPO assay to selectively quantitate eosinophils in a complex tissue inflammatory infiltrate demonstrates the usefulness of this enzymatic assay as a rapid and sensitive method to quantitate the influx of eosinophils into inflamed mouse tissue such as the lung during experimental RSV infection without reliance on morphologic criteria to identify murine eosinophils in tissues or bronchial lavage fluid.