Role of TLRs in Brucella mediated murine DC activation in vitro and clearance of pulmonary infection in vivo

Brucellosis is worldwide zoonoses affecting 500,000 people annually with no approved human vaccines available. Live attenuated Brucella abortus vaccine strain RB51 protects cattle through CD4 and CD8 T-cell mediated responses. However, limited information is known regarding how Brucella stimulate innate immunity. Although the most critical toll like receptors (TLRs) involved in the recognition of Brucella are TLR2, TLR4 and TLR9, it is important to identify the essential TLRs that induce DC activation/function in response to Brucella, to be able to upregulate both vaccine strain RB51-mediated protection, and clearance of pathogenic strain 2308. Furthermore, in spite of the importance of aerosol transmission of Brucella, no published studies have addressed the role of TLRs in the clearance of strain 2308 or strain RB51 from intranasally infected mice. Therefore, we used a (a) bone marrow derived dendritic cell model in TLRKO and control mice to assess the differential role of pathogenic and vaccine strains to induce DC activation and function in vitro, and (b) respiratory model in TLRKO and control mice to assess the critical roles for TLRs in clearance of strains in vivo. In support of the essential TLRs in clearance and protection, we performed challenge experiments to identify if these critical TLRs (as agonists) could enhance vaccine induced protection against pathogenic strain 2308 in a respiratory model. We determined: vaccine strain RB51 induced significant (p ≤ 0.05) DC activation vs. strain 2308 which was not dependent on a specific TLR; strain RB51 induced TNF-α production was TLR2 and TLR9 dependent, and IL-12 production was TLR2 and TLR4 dependent; TLR4 and TLR2 were critical for clearance of vaccine and pathogenic Brucella strains respectively; and TLR2 (p < 0.05), TLR4 (p < 0.05) and TLR9 (p = 0.075) agonists enhanced vaccine strain RB51-mediated protection against respiratory challenge with strain 2308 in the lung.

► The role of TLRs in Brucella mediated respiratory challenge is unknown. ► B. abortus strain RB51 stimulates TLR2, TLR4 and TLR9 dependent DC activation. ► B. abortus mediated DC function was TLR2, TLR4 and TLR9 dependent. ► There was TLR4 and TLR2 dependent clearance for strain RB51 and 2308 respectively. ► TLR agonists upregulated strain RB51-mediated protection to respiratory challenge.