Differential Expression of Interleukin-8 by Polymorphonuclear Leukocytes of Two Closely Related Species,Ovis canadensisandOvis aries, in Response toMannheimia haemolyticaInfection

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Abstract
The pneumonic lesions and mortality caused byMannheimia haemolyticain bighorn sheep (BHS;Ovis canadensis) are more severe than those in the related species, domestic sheep (DS;Ovis aries), under both natural and experimental conditions. Leukotoxin (Lkt) and lipopolysaccharide (LPS) are the most important virulence factors of this organism. One hallmark of pathogenesis of pneumonia is the influx of polymorphonuclear leukocytes (PMNs) into the lungs. Lkt-induced cytolysis of PMNs results in the release of cytotoxic compounds capable of damaging lung tissue. Interleukin-8 (IL-8) is a potent PMN chemoattractant. The objective of the present study was to determine if there is differential expression of IL-8 by the macrophages and PMNs of BHS and DS in response toM. haemolytica. Macrophages and PMNs of BHS and DS were stimulated with heat-killedM. haemolyticaor LPS. IL-8 expression by the cells was measured by enzyme-linked immunosorbent assays and real-time reverse transcription-PCR (RT-PCR). The PMNs of BHS expressed severalfold higher levels of IL-8 than those of DS upon stimulation. Lesional lung tissue ofM. haemolytica-infected BHS contained significantly higher levels of IL-8 than nonlesional tissue. The bronchoalveolar lavage (BAL) fluid of infected BHS also contained higher levels of IL-8 than that of infected DS. Depletion of IL-8 reduced migration of PMNs toward BAL fluid by approximately 50%, indicating that IL-8 is integral to PMN recruitment to the lung duringM. haemolyticainfection. Excessive production of IL-8, enhanced recruitment of PMNs, and PMN lysis by Lkt are likely responsible for the severity of the lung lesions inM. haemolytica-infected BHS.

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