Differential susceptibility of bighorn sheep (Ovis Canadensis) and domestic sheep (Ovis Aries) neutrophils to Mannheimia Haemolytica Leukotoxin is not due to differential expression of cell surface CD18

Authors: Dassanayake, Rohana; SHANTHALINGAM, SUDARVILI - WASHINGTON STATE UNIVERSITY; LIU, WEIGUO - WASHINGTON STATE UNIVERSITY; Casas, Eduardo; SRIKUMARAN, SUBRAMANIAM - WASHINGTON STATE UNIVERSITY

Submitted to: Journal of Wildlife Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2017
Publication Date: 3/21/2017
Publication URL: https://handle.nal.usda.gov/10113/5763059
Citation: Dassanayake, R.P., Shanthalingam, S., Liu, W., Casas, E., Srikumaran, S. 2017. Differential susceptibility of bighorn sheep (Ovis Canadensis) and domestic sheep (Ovis Aries) neutrophils to Mannheimia Haemolytica Leukotoxin is not due to differential expression of cell surface CD18. Journal of Wildlife Diseases. 53(3):625-629. doi: 10.7589/2016-11-244.

Interpretive Summary: Mannheimia haemolytica is known to cause severe pneumonia in cattle, bighorn sheep (BHS), domestic sheep (DS), and goats. However, BHS are much more susceptible to M. haemolytica caused pneumonia than other ruminants, including DS. M. haemolytica produces several virulence factors, of which leukotoxin has been identified as the major determinant contributing to lung tissue damage and subsequent development of pneumonia. The leukotoxin mediates killing of all types of ruminants white blood cells, but neutrophils has been identified as the most sensitive subset. Furthermore, BHS neutrophils are 4-8 times more susceptible to leukotoxin-induced cytolysis (or killing) than those of DS. Surface CD18 molecules in ruminant white blood cells including neutrophils has been identified as the receptor for leukotoxin. However, it was not clear whether higher susceptibility of BHS to leukotoxin mediated cytolysis was related to the higher expression of leukotoxin receptor CD18 on neutrophil surface compared to DS neutrophils. In this study, neutrophils isolated from BHS and DS blood samples were used to determine the relative expression levels of cell surface CD18. The results revealed that DS have more CD18 expression on neutrophils than those of BHS. Therefore, these findings suggest that higher sensitivity of BHS neutrophils (compared to the DS) was not related to CD18 expression levels.

Technical Abstract: Bighorn sheep (BHS) are more susceptible to pneumonia caused by Mannheimia haemolytica than domestic sheep (DS). Leukotoxin produced by M. haemolytica is the principal virulence factor involved in pneumonia pathogenesis. Although leukotoxin is cytolytic to all subsets of ruminant leukocytes, neutrophils are the most susceptible subset. BHS neutrophils are 4-8 times more susceptible to leukotoxin than DS neutrophils. We hypothesized that the higher susceptibility of BHS neutrophils, in comparison to DS neutrophils, is due to higher expression of CD18, the receptor for leukotoxin on leukocytes. The objective of this study, therefore, was to quantify CD18 expression on neutrophils of BHS and DS. Cell-surface CD18 expression on BHS and DS neutrophils was measured as antibody binding capacity of cells (ABC) by flow cytometric analysis with two fluorochrome-conjugated anti-CD18 mAbs (BAQ30A and HUH82A) and microspheres. Contrary to our expectation, significantly higher (P < 0.0001) CD18 expression was observed with mAb BAQ30A on DS neutrophils (98207 ± 3010 ABC) in comparison to BHS neutrophils (71487 ± 3010 ABC). Similarly, significantly higher (P < 0.0002) CD18 expression was observed with mAb HUH80A on DS neutrophils (120,289 ± 4207 ABC) in comparison to BHS neutrophils (89173 ± 4207 ABC). These findings suggest that the higher susceptibility of BHS neutrophils to leukotoxin, in comparison to DS neutrophils, is not due to higher expression of the leukotoxin receptor CD18 on BHS neutrophils.