The role of inhaled endotoxin in the aetiopathogenesis of equine heaves

Soluble lipopolysaccharide (LPS) inhalation challenge induced a dose-dependent bronchoalveolar lavage fluid (BALF) neutrophilia in both heaves-susceptible and control horses, and significant lung dysfunction in the heaves group. The response thresholds were lower for the heaves group, yet were markedly greater than airborne endotoxin exposure during the 5h dusty hay/straw challenge. In addition, there was no significant difference in BALF neutrophil numbers between the 2 groups following challenge with the middle and high LPS dose. There was a significant difference in the airway inflammatory response of the heaves group to 2 separate hay/straw exposures. This response was not related to the level of airborne endotoxin exposure. These findings indicated that inhaled endotoxin is not solely responsible for the induction of naturally occurring heaves. Inhalation challenge of the heaves group with 3 incremental doses of soluble A. fumigatus extract resulted in an increase in a BALF neutrophilia and lung dysfunction, which plateaued following inhalation of the middle dose. Inhalation challenge with LPS-depleted A. fumigatus extract resulted in a significant reduction in airway neutrophil numbers, of a magnitude that was greater than predicted by extrapolation from soluble LPS dose response inhalation experiments. These findings indicated that inhaled endotoxin may act synergistically with mould antigens, and contribute to the pulmonary inflammation observed in heaves. Inhalation challenge with hay dust suspensions (FIDS), prepared from fine hay dust particles, induced an airway neutrophilia, airway dysfunction and mucus hyper¬ secretion in the heaves group only. Inhalation challenge of the heaves group with the soluble fraction of HDS (SUP) failed to induce the magnitude of response measured following HDS challenge, despite containing almost all of the endotoxin activity of the HDS. These findings supported the involvement of HDS components, other than endotoxin, in the aetiopathogenesis of heaves. Inhalation challenge of the heaves group with the particulate fraction of FIDS (WP) induced only a mild BALF neutrophilia, however a combined challenge with SUP and WP induced a neutrophilic response approaching the magnitude of that following HDS challenge. These findings indicated a synergistic action between the soluble and particulate fractions of HDS. Inhalation challenge of the heaves group with LPS-depleted HDS resulted in a significant reduction in BALF neutrophil numbers, of a magnitude that was greater than predicted by extrapolation from soluble LPS dose response inhalation experiments. Replacement of the depleted LPS resulted in the re-establishment of the original level of BALF neutrophilia. Inhalation challenge of the heaves group with WP reconstituted in LPS solution (containing an equivalent LPS activity to SUP) resulted in a BALF neutrophilia that was not significantly different from that following challenge with combination of WP and SUP. These findings indicated that the endotoxin content of HDS acts synergistically with other HDS components, most notably the particulate fraction.