title: Acylation Enhances, but Is Not Required for, the Cytotoxic Activity of Mannheimia haemolytica Leukotoxin in Bighorn Sheep.
abstract: Mannheimia haemolytica causes pneumonia in domestic and wild ruminants. Leukotoxin (Lkt) is the most important virulence factor of the bacterium. It is encoded within the four-gene lktCABD operon: lktA encodes the structural protoxin, and lktC encodes a trans-acylase that adds fatty acid chains to internal lysine residues in the protoxin, which is then secreted from the cell by a type 1 secretion system apparatus encoded by lktB and lktD. It has been reported that LktC-mediated acylation is necessary for the biological effects of the toxin. However, an LktC mutant that we developed previously was only partially attenuated in its virulence for cattle. The objective of this study was to elucidate the role of LktC-mediated acylation in Lkt-induced cytotoxicity. We performed this study in bighorn sheep (Ovis canadensis) (BHS), since they are highly susceptible to M. haemolytica infection. The LktC mutant caused fatal pneumonia in 40% of inoculated BHS. On necropsy, a large number of necrotic polymorphonuclear leukocytes (PMNs) were observed in the lungs. Lkt from the mutant was cytotoxic to BHS PMNs in an in vitro cytotoxicity assay. Flow cytometric analysis of mutant Lkt-treated PMNs revealed the induction of necrosis. Scanning electron microscopic analysis revealed the presence of pores and blebs on mutant-Lkt-treated PMNs. Mass spectrometric analysis confirmed that the mutant secreted an unacylated Lkt. Taken together, these results suggest that acylation is not necessary for the cytotoxic activity of M. haemolytica Lkt but that it enhances the potency of the toxin.
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA.
authors.forename: Sai A
authors.initials: SA
authors.lastname: Batra
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA.
authors.forename: Sudarvili
authors.initials: S
authors.lastname: Shanthalingam
authors.affiliation: Molecular Biology and Genomics Core, Washington State University, Pullman, Washington, USA.
authors.forename: Gerhard R
authors.initials: GR
authors.lastname: Munske
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA.
authors.forename: Bindu
authors.initials: B
authors.lastname: Raghavan
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA.
authors.forename: Abirami
authors.initials: A
authors.lastname: Kugadas
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA.
authors.forename: Jegarubee
authors.initials: J
authors.lastname: Bavanthasivam
authors.affiliation: J. Craig Venter Institute, San Diego, California, USA.
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Highlander
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA ssrikumaran@vetmed.wsu.edu.
authors.forename: Subramaniam
authors.initials: S
authors.lastname: Srikumaran
mesh_terms: D000215:Acylation; D000818:Animals; D005098:Exotoxins / Q000378:metabolism / Q000633:toxicity*; D005434:Flow Cytometry; D008168:Lung / Q000276:immunology / Q000382:microbiology; D016978:Mannheimia haemolytica / Q000378:metabolism*; D009504:Neutrophils / Q000276:immunology; D012766:Pasteurellosis, Pneumonic / Q000276:immunology / Q000382:microbiology*; D012756:Sheep; D012757:Sheep Diseases / Q000276:immunology / Q000382:microbiology*; D034581:Sheep, Bighorn
chemical_list: D005098:Exotoxins; C018579:leukotoxin