Ted this analysis with a second set of immunizations, a replicate of the outer membrane 15900046 vaccinates (n = 5), and observed the same results: there were significantly greater titers to Msp2 (all .20,000) than to AM779 (median, 3,000; mode, 10,000).Immunization with AM779 overcomes sub-dominance for T cell but not B cell responsesUsing the second set of immunized animals, the AM779-specific T cell responses were measured in both AM779 and outer membrane vaccinates. All outer membrane vaccinates responded, as expected, to the outer membrane antigen; however only 1/5 responded to purified AM779, indicating that AM779 was a very weak immunogen in the context of the membrane complex (Table 2). That this likely reflects relative abundance in the native membrane was supported by the inconsistent response in cells from animals vaccinated with AM779 that were then stimulated with outer membranes (Table 2). In order Ebselen contrast, all AM779 vaccinates responded when stimulated with the autologous antigen. The response was maintained following depletion of CD8+ and cd+ T lymphocytes (to 1 and ,0.5 , respectively), evidence that the response was CD4+ T cell specific (data not shown). All of the animals responded to the positive control clostridial antigen and the cells from adjuvant-only animals wereInfectious challengeThe second set of vaccinates was challenged by adult male Dermacentor andersoni infected with the St. Maries strain of A. marginale [27]. Ticks were infected by acquisition feeding for seven days on an infected calf with a bacteremia of .107 A. marginale per ml. Following one week of incubation at 37uC to allow blood meal digestion and initial replication, 27 ticks were allowed to attach and transmission feed for seven days on each of the vaccinated or control calves. To confirm that transmission was equal among groups, the ticks were then dissected and the number of A. marginale per salivary gland pair determined using msp5 quantitative PCR as previously described [28]. Challenged calves were monitored daily for A. marginale bacteremia by microscopic examination of Giemsastained blood smears. Calves that developed anemia, using a priori established criteria, were treated with 20 mg/kg long-acting tetracycline and removed from the study. All procedures were approved by the Institutional Animal Care and Use Committee at Washington State University (Approval #2732).Results AM779 is a sub-dominant antigen in the context of complex immunogensBoth isolated native A. marginale outer membrane and crosslinked surface protein complexes have been shown to induceFigure 1. Expression and purification of recombinant AM779. Coomassie stained SDS-PAGE of E. coli lysates of uninduced (lane 1) and induced cells (lane 2) expressing AM779, and purified rAM779 (lane 3). The position and size of molecular weight standards is indicated to the left of the image. The arrow designates the position of AM779. doi:10.1371/journal.pone.0046372.gSubdominant Bacterial AntigensFigure 2. Recognition 
of AM779 antigen by IgG2 from protected vaccinates. Equal amount of protein (0.4 mg) of the outer membrane immunogen (OM), recombinant CAL-120 site full-length AM779, and, as negative antigen controls, uninfected erythrocytes (URBC) and recombinant Babesia bovis Rap-1 were electrophoretically separated, immunoblotted and probed for serum IgG2 binding. Sera from A. marginale outer membrane immunogen (A) or cross-linked surface protein complex (B) immunized animals were diluted 1:1000 and tested fo.Ted this analysis with a second set of immunizations, a replicate of the outer membrane 15900046 vaccinates (n = 5), and observed the same results: there were significantly greater titers to Msp2 (all .20,000) than to AM779 (median, 3,000; mode, 10,000).Immunization with AM779 overcomes sub-dominance for T cell but not B cell responsesUsing the second set of immunized animals, the AM779-specific T cell responses were measured in both AM779 and outer membrane vaccinates. All outer membrane vaccinates responded, as expected, to the outer membrane antigen; however only 1/5 responded to purified AM779, indicating that AM779 was a very weak immunogen in the context of the membrane complex (Table 2). That this likely reflects relative abundance in the native membrane was supported by the inconsistent response in cells from animals vaccinated with AM779 that were then stimulated with outer membranes (Table 2). In contrast, all AM779 vaccinates responded when stimulated with the autologous antigen. The response was maintained following depletion of CD8+ and cd+ T lymphocytes (to 1 and ,0.5 , respectively), evidence that 
the response was CD4+ T cell specific (data not shown). All of the animals responded to the positive control clostridial antigen and the cells from adjuvant-only animals wereInfectious challengeThe second set of vaccinates was challenged by adult male Dermacentor andersoni infected with the St. Maries strain of A. marginale [27]. Ticks were infected by acquisition feeding for seven days on an infected calf with a bacteremia of .107 A. marginale per ml. Following one week of incubation at 37uC to allow blood meal digestion and initial replication, 27 ticks were allowed to attach and transmission feed for seven days on each of the vaccinated or control calves. To confirm that transmission was equal among groups, the ticks were then dissected and the number of A. marginale per salivary gland pair determined using msp5 quantitative PCR as previously described [28]. Challenged calves were monitored daily for A. marginale bacteremia by microscopic examination of Giemsastained blood smears. Calves that developed anemia, using a priori established criteria, were treated with 20 mg/kg long-acting tetracycline and removed from the study. All procedures were approved by the Institutional Animal Care and Use Committee at Washington State University (Approval #2732).Results AM779 is a sub-dominant antigen in the context of complex immunogensBoth isolated native A. marginale outer membrane and crosslinked surface protein complexes have been shown to induceFigure 1. Expression and purification of recombinant AM779. Coomassie stained SDS-PAGE of E. coli lysates of uninduced (lane 1) and induced cells (lane 2) expressing AM779, and purified rAM779 (lane 3). The position and size of molecular weight standards is indicated to the left of the image. The arrow designates the position of AM779. doi:10.1371/journal.pone.0046372.gSubdominant Bacterial AntigensFigure 2. Recognition of AM779 antigen by IgG2 from protected vaccinates. Equal amount of protein (0.4 mg) of the outer membrane immunogen (OM), recombinant full-length AM779, and, as negative antigen controls, uninfected erythrocytes (URBC) and recombinant Babesia bovis Rap-1 were electrophoretically separated, immunoblotted and probed for serum IgG2 binding. Sera from A. marginale outer membrane immunogen (A) or cross-linked surface protein complex (B) immunized animals were diluted 1:1000 and tested fo.