The BtaF Adhesin Is Necessary for Full Virulence During Respiratory Infection by Brucella suis and Is a Novel Immunogen for Nasal Vaccination Against Brucella Infection

Abstract

Brucella enters their hosts mostly through mucosae from where it spreads systemically. Adhesion to extracellular matrix (ECM) components or to host cells is important for the infectious process, and is mediated by several adhesins, including the BtaF trimeric autotransporter. Although Th1 responses and gamma interferon (IFN-γ) are important for protection, antibodies able to block adhesions might also contribute to prevent Brucella infection. We evaluated the importance of BtaF for respiratory Brucella infection, and characterized the immune response and protection from mucosal challenge induced by nasal vaccination with recombinant BtaF. While lung CFU numbers did not differ at day 1 p.i. between mice intratracheally inoculated with B. suis M1330 (wild type) and those receiving a Δ btaF mutant, they were reduced in the latter group at 7 and 30 days p.i. For vaccination studies the BtaF passenger domain was engineered and expressed as a soluble trimeric protein. Mice were immunized by the nasal route with BtaF or saline (control group) plus the mucosal adjuvant c-di-AMP. Specific anti-BtaF antibodies (IgG and IgA) were increased in serum, including a mixed IgG2a/IgG1 response. In vitro , these antibodies reduced bacterial adhesion to A549 alveolar epithelial cells. Specific IgA antibodies were also increased in several mucosae. Spleen cells from BtaF immunized mice significantly increased their IL-2, IL-5, IL-17, and IFN-γ secretion upon antigen stimulation. In cervical draining lymph nodes, antigen-experienced CD4+ T cells were maintained mainly as central memory cells. A BtaF-specific delayed-type hypersensitivity response was detected in BtaF immunized mice. Lung cells from the latter produced high levels of IFN-γ upon antigen stimulation. Although nasal immunization with BtaF did not protect mice against B. suis respiratory challenge, it conferred significant protection from intragastric challenge; the splenic load of B. suis was reduced by 3.28 log CFU in immunized mice. This study shows that nasal vaccination with BtaF+c-di-AMP protects against intragastric challenge with B. suis by inducing local and systemic antibody responses, central memory CD4+ T cells and strong Th1 responses. Therefore, although BtaF vaccination did not protect from B. suis respiratory infection, this adhesin constitutes a promising immunogen against mucosal B. suis infection.

Adhesins of Brucella: Their Roles in the Interaction with the Host

“…Brucella infection is frequently acquired through the oral and respiratory routes, and adhesins are anticipated to have a relevant role in these infectious processes. As mentioned above, our results showed that BtaF adhesin from B. suis is necessary for complete virulence of B. suis after both oral and intratracheal infection [ 70 , 71 ]. In line with these results, we assessed the immunogenic and protective potential of recombinant BtaF when administered intranasally with the mucosal adjuvant Bis-(3’,5’)-cyclic dimeric adenosine monophosphate (c-di-AMP) [ 71 ].…”

“…An indirect ELISA assay on sera from healthy and sick pigs infected with B. suis suggested that both adhesins are expressed in vivo in the natural host (swine), supporting the role of these adhesins in the infection process. Recently, it was shown that BtaF is also required for virulence in mice after inoculation via a respiratory (intratracheal administration) route [ 71 ]. In this case, the splenic load of the deletion mutant was significantly reduced at 7-days and 30-days post-infection as compared to the wild-type strain.…”

Development of attenuated live vaccine candidates against swine brucellosis in a non-zoonotic B. suis biovar 2 background

“…The literature shows that the reduction in mouse spleen colonization has been reported previously for several B. suis vaccine candidates. Nasal immunization with a recombinant adhesin (BtaF) in cyclic-di-AMP protected mice against B. suis bv1 administered intragastrically but not when challenged intratracheally [ 61 ]. However, the protocol of immunization and costs indicate that this vaccine when improved with BtaF synergic antigens [ 61 ] would be more suitable for human use.…”

“…Nasal immunization with a recombinant adhesin (BtaF) in cyclic-di-AMP protected mice against B. suis bv1 administered intragastrically but not when challenged intratracheally [ 61 ]. However, the protocol of immunization and costs indicate that this vaccine when improved with BtaF synergic antigens [ 61 ] would be more suitable for human use. Strain VTRS1, an R wboA mutant of B. suis biovar 4 [ 22 ], when administered intravenously (IV), was more efficacious against B. suis bv4 than RB51 administered IP.…”

Immunogenicity of trimeric autotransporter adhesins and their potential as vaccine targets

“…The Brucella suis trimeric autotransporter F (BtaF) and E (BtaE) are described as a promising immunogenic target for vaccination against mucosal B. suis infections [242]. Other research concerning the vaccine development targeting melioidosis and glanders caused by Burkholderia pseudomallei and Burkholderia mallei, respectively, is ongoing and describes various expressed TAAs with immunogenic properties such as BPSL2063 [243] and BimA [244,245].…”