Demonstration of respiratory sensitization in guinea pigs by a protein and a low-molecular-weight sensitizer using a biochemical assay of pulmonary eosinophilia

Respiratory sensitization resulting from exposure to proteins and certain low-molecular-weight chemicals has been identified as the cause of widespread respiratory disease, but there are currently no well-validated or widely applied methods for determining which chemicals may cause respiratory sensitization (asthma). An important and characteristic factor in respiratory hypersensitivity reactions is a marked pulmonary eosinophilia. This eosinophil influx is thought to play a major role in the so-called late asthmatic response. This study investigated the use of a recently described biochemical assay for eosinophil peroxidase (EPO) enzyme to quantify the eosinophilia associated with inhalation exposure of presensitized guinea pigs to respiratory sensitizers. An experimental protocol was developed that could be useful as a screen for potential sensitizing chemicals. Immune responses were induced by intraperitoneal injection followed by inhalation challenge exposures. Approximately 17 h after inhalation exposure, samples of blood-free lung parenchyma were homogenized and subjected to the EPO assay. Using this technique it was possible to detect a substantial increase in pulmonary EPO activity following induction and challenge with both a protein sensitizer (ovalbumin) and a low-molecular-weight sensitizer (trimellitic anhydride). Similar increases were not seen when inhalation exposure was conducted without presensitization, and presensitization with ovalbumin followed by inhalation challenge with a different protein (bovine serum albumin) also produced no increase in EPO activity, confirming a specific immunologically mediated mechanism. The magnitude of the increased EPO activity in lung tissue from sensitized animals was such that the assay appears to be highly sensitive and equally effective with either low-molecular-weight or protein sensitizers. Furthermore, the results are not confounded by respiratory irritant effects, which is a major disadvantage of certain other proposed methods for identifying respiratory hypersensitivity. Inhalation exposure concentrations are not, therefore, limited to nonirritant levels.