Submitted to: Infection and Immunity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2008
Publication Date: 5/1/2008
Citation: Noh, S.M., Brayton, K.A., Brown, W.C., Norimine, J., Munske, G.R., Davitt, C.M., Palmer, G.H. 2008. Composition of the Surface Proteome of Anaplasma marginale and Its Role in Protective Immunity Induced by Outer Membrane Immunization. Infection and Immunity. 76(5):2219-2226. Interpretive Summary: Anaplasmosis, caused by Anaplasma marginale, is a tick transmitted disease of cattle, which is characterized by severe anemia, fever, and weight loss, and is a production-limiting disease in many parts of the world. A. marginale is within the family Anaplasmataceae, which includes many pathogens that cause disease in both humans and animals. These pathogens include Anaplasma phagocytophilum, Ehrlichia chaffeensis, E. canis, and E. ruminantium. Currently there are no effective vaccines which protect against disease caused by these pathogens. Additionally, these organisms share remarkable similarities. They are all obligate, intracellular, Gram negative bacteria, which cycle between the tick vector and the mammalian host. Outer membrane proteins mediate many functions necessary to complete this lifecycle. Thus, surface expressed proteins are logical vaccine targets to either induce protective immunity in the mammalian host or prevent tick colonization and thus pathogen transmission. Consequently, characterization of the surface expressed proteins is a critical step in vaccine development. Extracts of A. marginale outer membranes induce protective immunity. However, these extracts are complex and composed of more than 20 proteins, not all of which are surface expressed. Thus, the first objective of this study was to define specific members of protein complexes expressed on the surface of A. marginale, and test whether these complexes induce protective immunity that recapitulates the immunity induced by outer membrane immunization. The second objective of this study was to compare the surface expressed proteins of A. marginale isolated from tick cells and erythrocytes. The results indicate that a surface protein subset of the outer membrane is capable of inducing protective immunity and serves to target vaccine development. Furthermore, the data support that remodeling of the surface proteome accompanies the transition between mammalian and arthropod hosts and identifies novel targets for blocking tick transmission.
Technical Abstract: Surface proteins of tick-borne, intracellular bacterial pathogens mediate functions essential for invasion and colonization. Consequently, the surface proteome of these organisms is specifically relevant from two biological perspectives, induction of protective immunity in the mammalian host and understanding the transition from the mammalian host to the tick vector. In this study, the surface proteome of A. marginale, a tick transmitted bacterial pathogen, was targeted by using surface specific cross-linking to form inter-molecular bonds between adjacent proteins. Liquid chromatography and tandem mass spectroscopy were then employed to characterize the specific protein composition of the resulting complexes. The surface complexes of A. marginale isolated from erythrocytes of the mammalian host were composed of multiple membrane proteins, most of which belong to a protein family, pfam01617, which is widely conserved among bacteria in the genus Anaplasma and the closely related genus Ehrlichia. In contrast, the surface proteome of A. marginale isolated from tick cells was much less complex and contained a novel protein, AM778, not identified within the surface proteome of organisms from the mammalian host. Immunization using the cross-linked surface complex induced protection against high-level bacteremia and anemia upon A. marginale challenge of cattle and effectively recapitulated the protection induced by immunization with whole outer membranes. These results indicate that a surface protein subset of the outer membrane is capable of inducing protective immunity and serves to direct vaccine development. Furthermore, the data support that remodeling of the surface proteome accompanies the transition between mammalian and arthropod hosts and identifies novel targets for blocking transmission.