Studies of an Antiendotoxin Antibody in Preventing the Physiologic Changes of Endotoxemia in Awake Sheep

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Abstract
In sheep, endotoxin (LPS) causes pulmonary hypertension, hypoxemia, leukopenia, exudation of protein-rich lung lymph, reduced dynamic compliance (Cdyn), and increased resistance to airflow (RL), changes similar to those seen in human sepsis and sepsis-induced ARDS. We used well-described methods in the awake sheep-endotoxin model to evaluate the effectiveness of a commercially manufactured antibody to prevent the physiologic changes of endotoxemia. In awake sheep with chronic lung lymph fistulas, we used a whole-body plethysmograph to measure Cdyn, RL, and FRC. Pulmonary artery, left atrial, and systemic arterial pressures were recorded continuously. Arterial blood gases (for calculating AaPO2), leukocyte counts, and lymph samples were collected every 30 min. Animals received a 30-min (2 mg/kg) infusion of antiendotoxin antibody 4 h before LPS (0.75 .mu.g/kg) challenge (n = 4), or were given a mixture of LPS (0.75 .mu.g/kg) and antibody (2 mg/kg) that had been incubated in vitro at 37.degree. C for 30 min before infusion (n = 6). A control group given only 2 mg/kg of antibody (n = 4) showed no change in any measured parameter, whereas control animals receiving LPS alone (n = 6) exhibited a typical endotoxin response. In all animals receiving endotoxin, Cdyn declined by approximately 50% within 30 to 60 min, and RL increased approximately sixfold over a similar time course. Accompanying the abnormalities in lung mechanics were pulmonary hypertension, leukopenia, and widening of the AaPO2. Pretreatment with antiendotoxin antibody or infusion of the in vitro antibody-LPS mixture blunted the early pulmonary artery pressure rise when compared with the LPS control group; however, no other measured variable was significantly improved by the antibody. In several pilot studies similar results were seen when lower doses of LPS (0.075, 0.05, and 0.005 .mu.g/kg) were infused 1 h after antibody administration. In the in vitro limulus lysate assay, incubation of LPS with antibody in molar ratios from 1:103 to 1:106 for 30 min did not show a reduction in LPS-induced limulus activation under serum-free conditions. These studies suggest that the antiendotoxin antibody does not effectively neutralize the Escherichia coli endotoxins tested or that in the sheep, an animal known to be extremely sensitive to the effects of LPS, small amounts of unbound LPS, or the LPS-antibody complexes themselves, are able to produce the endotoxin response.