Anaplasma marginale Type IV Secretion System Proteins VirB2, VirB7, VirB11, and VirD4 Are Immunogenic Components of a Protective Bacterial Membrane Vaccine

Anaplasma and related Ehrlichia spp. are important tick-borne, Gram-negative bacterial pathogens of livestock and humans that cause acute infection and disease and can persist. Immunization of cattle with an Anaplasma marginale fraction enriched in outer membranes (OM) can provide complete protection against disease and persistent infection. Serological responses of OM vaccinees to the OM proteome previously identified over 20 antigenic proteins, including three type IV secretion system (T4SS) proteins, VirB9-1, VirB9-2, and VirB10. Subsequent studies showed that these three proteins also stimulated CD4(+) T-cell responses in OM vaccinees. The T4SS, composed of a complex of proteins spanning the inner and outer membranes of certain bacteria, is an important virulence factor but is relatively unexplored as a vaccine target. The goal of this study was to determine if additional T4SS proteins are immunogenic for animals immunized with the protective OM fraction of A. marginale. T4SS proteins expressed by in vitro transcription and translation were screened for stimulating proliferation of T cells from OM vaccinees, and immunogenic proteins were expressed as recombinant proteins in Escherichia coli and their immunogenicity was verified. VirB2, a putative VirB7, VirB11, and VirD4 were immunogenic for OM vaccinees expressing several common major histocompatibility complex (MHC) class II haplotypes. VirB2 is encoded by multiple genes that share a conserved central region, and epitope mapping revealed T-cell epitopes in this region. The discovery of novel immunogenic T4SS proteins recognized by outbred individuals with common MHC haplotypes further justifies evaluating the T4SS as a potential vaccine candidate for pathogenic bacteria.