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Title: Subdominant outer membrane antigens in anaplasma marginale: conservation, antigenicity, and protective capacity using recombinant protein

item DUCKEN, DEIRDRE - Washington State University
item BROWN, WENDY - Washington State University
item ALPERIN, DEBRA - Washington State University
item BRAYTON, KELLY - Washington State University
item Reif, Kathryn
item TURSE, JOSHUA - Washington State University
item PALMER, GUY - Washington State University
item Noh, Susan

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/2015
Publication Date: 6/6/2015
Citation: Ducken, D.R., Brown, W.C., Alperin, D.C., Brayton, K.A., Reif, K.E., Turse, J.E., Palmer, G.H., Noh, S.M. 2015. Subdominant outer membrane antigens in anaplasma marginale: conservation, antigenicity, and protective capacity using recombinant protein. PLoS One. doi: 1371/journal.pone.0129309.

Interpretive Summary: Vector borne bacterial and protozoal pathogens, such as Plasmodium spp., Babesia spp., Ehrlichia spp, and Anaplasma spp, generally establish persistent infection in the mammalian host through, in large part, antigenic variation. The development of effective vaccines to prevent disease caused by these pathogens has proven difficult. While comprehensive lists of vaccine candidates are readily available, one of the major limitations is prioritizing these candidates for live animal testing, which is difficult, time consuming and expensive. In this work, we report an approach to prioritize antigens and then test those antigens for their protective capacity using cattle, which are the natural host of A. marginale. While we failed to induce protective immunity with our best candidates, this work represents incremental progress in vaccine development.

Technical Abstract: Anaplasma marginale is a tick-borne rickettsial pathogen of cattle with a worldwide distribution. Currently a safe and efficacious vaccine is unavailable. Outer membrane protein (OMP) extracts or a well- defined surface protein complex reproducibly induce protective immunity. However, there are several knowledge gaps limiting progress in vaccine development. First, are these OMPs conserved among the diversity of A. marginale strains circulating in endemic regions? Second, are the most highly conserved outer membrane proteins in the immunogens recognized by immunized and protected animals? Lastly, can this subset of OMPs recognized by antibody from protected vaccinates and conserved among strains recapitulate the protection of outer membrane vaccines? To address the first goal, genes encoding OMPs AM202, AM368, AM854, AM936, AM1041, and AM1096 (Oma87), major subdominant components of the outer membrane immunogens, were cloned and sequenced from geographically diverse strains and isolates. AM202, AM936, AM854, and AM1096 share 99.9 to 100% amino acid identity while AM1041 and AM368 had minor variation. While all four of the most highly conserved OMPs were recognized by IgG from animals immunized with outer membranes, linked surface protein complexes, or unlinked surface protein complexes and shown to be protected from challenge, the highest titers and consistent recognition among vaccinates were to AM854 and AM936. Consequently, animals were immunized with recombinant AM854 and AM936 and challenged. Recombinant vaccinates and purified outer membrane vaccinates had similar total IgG and IgG2 responses to AM854 and AM936. However, the recombinant vaccinates developed higher bacteremia after challenge as compared to adjuvant-only controls, while outer membrane vaccinates were significantly protected. These results provide the first indication that vaccination with specific antigens may exacerbate disease and indicate that progressing from the protective capacity of outer membrane formulations to recombinant vaccines requires testing of additional subdominant antigens and a better understanding of the protective immune response.