Mannheimia haemolytica is an important etiological agent of
pneumonia in domestic sheep (DS, Ovis aries).
Leukotoxin (Lkt) produced by this organism is the principal
virulence factor responsible for the acute
inflammation and
lung injury characteristic of M. haemolytica caused disease. Previously, we have shown that the leukocyte-specific
integrins, beta(2)
integrins, serve as the receptor for Lkt. Although it is certain that CD18, the beta subunit of beta(2)
integrins, mediates Lkt-induced cytolysis of leukocytes, it is not clear whether CD18 of all three beta(2)
integrins,
LFA-1, Mac-1 and CR4, mediates Lkt-induced cytolysis of DS leukocytes. Since polymorphonuclear leukocytes, which express all three beta(2)
integrins, are the leukocyte subset that is most susceptible to Lkt, we hypothesized that all three beta(2)
integrins serve as the receptor for Lkt. The objective of this study was to determine whether DS
LFA-1 serves as a receptor for M. haemolytica Lkt. We cloned the
cDNA for DS CD11a, the alpha subunit of
LFA-1, and co-transfected it along with the previously cloned
cDNA for DS CD18, into a Lkt-non-suceptible cell line. Transfectants stably expressing DS
LFA-1 were bound by Lkt. More importantly, Lkt lysed the DS
LFA-1 transfectants in a concentration-dependent manner. Pre-incubation of Lkt with a Lkt-neutralizing
monoclonal antibody (MAb), or pre-incubation of transfectants with MAbs specific for DS CD11a or CD18, inhibited Lkt-induced cytolysis of the transfectants. Exposure of
LFA-1 transfectants to low concentrations of Lkt resulted in elevation of intracellular [Ca(2+)](i). Taken together, these results indicate that DS
LFA-1 serves as a receptor for M. haemolytica Lkt.