Linkage between Anaplasma marginale Outer Membrane Proteins Enhances Immunogenicity but Is Not Required for Protection from Challenge

Abstract

The prevention of bacterial infections via immunization presents particular challenges. While outer membrane extracts are often protective, they are difficult and expensive to isolate and standardize and thus are often impractical for development and implementation in vaccination programs. In contrast, individual proteins, which are easily adapted for use in subunit vaccines, tend to be poorly protective. Consequently, identification of the specific characteristics of outer membrane-based immunogens, in terms of the antigen contents and contexts that are required for protective immunity, represents a major gap in the knowledge needed for bacterial vaccine development. Using as a model Anaplasma marginale, a persistent tick-borne bacterial pathogen of cattle, we tested two sets of immunogens to determine whether membrane context affected immunogenicity and the capacity to induce protection. The first immunogen was composed of a complex of outer membrane proteins linked by covalent bonds and known to be protective. The second immunogen was derived directly from the first one, but the proteins were individualized rather than linked. The antibody response induced by the linked immunogen was much greater than that induced by the unlinked immunogen. However, both immunogens induced protective immunity and an anamnestic response. These findings suggest that individual proteins or combinations of proteins can be successfully tested for the ability to induce protective immunity with less regard for overall membrane context. Once protective antigens are identified, immunogenicity could be enhanced by cross-linking to allow a reduced immunogen dose or fewer booster vaccinations.

Anaplasma marginale: Diversity, Virulence, and Vaccine Landscape through a Genomics Approach

“…In contrast, individual proteins, easily cloned and expressed and adapted for use in subunit vaccines have not shown a significant protection [75]. The effect of the protein complexed as immunogens strongly suggests that new vaccines candidates may work better as a complex instead of as free proteins.…”

Identification of a T-Cell Epitope That Is Globally Conserved among Outer Membrane Proteins (OMPs) OMP7, OMP8, and OMP9 of Anaplasma marginale Strains and with OMP7 from the A. marginale subsp. centrale Vaccine Strain

“…Immunization with individual OMPs has, to date, failed to provide protection against homologous strain challenge in the natural bovine host equivalent to that achieved by immunizing with OM (6,12,(28)(29)(30)(31). In contrast, complexes of cross-linked OM proteins, and a mixture of individual proteins from the complexes were able to provide protection against disease in challenged cattle (18). In addition, immunization with the live A. centrale vaccine strain can provide protection against disease caused by some strains of A. marginale but does not prevent infection (5,11).…”

“…These proteins are considered to be logical vaccine candidate antigens because they are surface exposed, present in protective OM and cross-linked OM proteins, recognized by immune serum IgG2 and T cells in cattle immunized with the protective OM antigen preparation, and recognized by immune serum from cattle immunized with the A. centrale vaccine strain (8,(16)(17)(18). This epitope, together with additional T-cell epitopes that we have shown are present within these proteins, may be important for inclusion in a multivalent vaccine for A. marginale that can provide protection against disease caused by geographically distant bacterial strains in cattle that express common BoLA class II haplotypes DRB3*1201 and *1501.…”

“…In the present study, we test the hypothesis that A. centrale OMP7 and A. marginale OMPs 7 to 9 share at least one T-cell epitope for incorporation in a multivalent subunit vaccine. A vaccine comprised of multiple surface-exposed antigens may recapitulate the protection afforded by vaccination with live A. centrale, A. marginale OM, a complex of OMPs linked by covalent bonds, or the individual proteins within this OM complex (18).…”

A vaccine using Anaplasma marginale subdominant type IV secretion system recombinant proteins was not protective against a virulent challenge

“…Immunization of cattle with the native purified outer membrane (OM) of A. marginale has induced complete protection against infection and clinical disease [4,9,10]. Such protection was correlated with induction of high titers of IgG2 opsonizing antibodies against A. marginale surface epitopes and macrophage activation mediated by CD4+ T cells [4,11]. The capacity of OM native proteins to induce protection has promoted their consideration as vaccine candidates [12,13].…”