Key Role of Toll-Like Receptor 2 in the Inflammatory Response and Major Histocompatibility Complex Class II Downregulation in Brucella abortus-Infected Alveolar Macrophages

Abstract

c Alveolar macrophages (AM) seem to constitute the main cellular target of inhaled brucellae. Here, we show that Brucella abortus invades and replicates in murine AM without inducing cytotoxicity. B. abortus infection induced a statistically significant increase of tumor necrosis factor alpha (TNF-␣), CXCL1 or keratinocyte chemoattractant (KC), interleukin-1␤ (IL-1␤), IL-6, and IL-12 in AM from C57BL/6 mice and BALB/c mice, but these responses were generally weaker and/or delayed compared to those elicited in peritoneal macrophages. Studies using knockout mice for TLR2, TLR4, and TLR9 revealed that TNF-␣ and KC responses were mediated by TLR2 recognition. Brucella infection reduced in a multiplicity of infection-dependent manner the expression of major histocompatibility complex class II (MHC-II) molecules induced by gamma interferon (IFN-␥) in AM. The same phenomenon was induced by incubation with heat-killed B. abortus (HKBA) or the lipidated form of the 19-kDa outer membrane protein of Brucella (L-Omp19), and it was shown to be mediated by TLR2 recognition. In contrast, no significant downregulation of MHC-II was induced by either unlipidated Omp19 or Brucella LPS. In a functional assay, treatment of AM with either L-Omp19 or HKBA reduced the MHC-II-restricted presentation of OVA peptides to specific T cells. One week after intratracheal infection, viable B. abortus was detected in AM from both wild-type and TLR2 KO mice, but CFU counts were higher in the latter. These results suggest that B. abortus survives in AM after inhalatory infection in spite of a certain degree of immune control exerted by the TLR2-mediated inflammatory response. Both the modest nature of the latter and the modulation of MHC-II expression by the bacterium may contribute to such survival. Brucellosis is zoonotic disease caused by Brucella species, which is distributed worldwide and affects over 500,000 people annually (1) and for which there is no approved efficacious human vaccine available. Brucella melitensis, B. suis, and B. abortus are the most pathogenic species for humans and are responsible for the vast majority of human cases (2). The infection can be transmitted to humans by several ways, among which inhalation of infected aerosols is one of the most frequent. The easy aerosolization and airborne transmission of Brucella species has contributed to their consideration as potential biological weapons (1) and their classification by the CDC and NIAID as category B bioterrorism agents. Outbreaks of human brucellosis due to airborne transmission have been reported in different settings, including abattoirs, vaccine production laboratories, and rural areas (3-5). Notably, aerosols have been implicated in most cases of laboratory-acquired brucellosis, which is considered the commonest laboratory-acquired infection (6).Lung epithelial cells and alveolar macrophages (AM) are the first cells to be contacted by inhaled microorganisms. AM constitute the first line of pulmonary defense and are capable of initiating a local immune respons...

Identification of Immune Effectors Essential to the Control of Primary and Secondary Intranasal Infection with Brucella melitensis in Mice

“…Furthermore, during the late phase of control, only TNF-a deficiency strongly affected CFU counts of Brucella in the organs. In keeping with this observation, Brucella was recently shown to induce weak IL-1b, IL-6, and TNF-a production in lung alveolar macrophages compared with peritoneal macrophages (58). The chemokine receptor CCR2 has been implicated in the recruitment of inflammatory monocytes during infection (40).…”

TLR9 is required for MAPK/NF-κB activation but does not cooperate with TLR2 or TLR6 to induce host resistance to Brucella abortus

“…The usual mouse model of B. abortus infection through an intraperitoneal route demonstrated that TLR2 does not contribute to host control of brucellosis in vivo [21]. However, TLR2 showed a role controlling B. abortus infection 1 wk after intratracheal infection [22]. In addition, our group demonstrated that TLR6 is required for full production of proinflammatory cytokines and MAPK activation in DCs, resulting in resistance to B. abortus infection [23].…”

Immune response triggered by Brucella abortus following infection or vaccination

“…Regarding resistance to B. abortus infection in mice, there is no consensus in the literature on the participation of TLR4, whereas TLR2 was reportedly to be not involved [18,49,53]. Even though possibly not involved in resistance to infection, it is well established that the recognition of B. abortus PAMPs by both TLR2 and TLR4 induce the secretion of TNF-␣, IL-12 and IL-6 by murine cells [18,42,[48][49][50][53][54][55]. Interestingly, TLR4-linked signaling interacting with Janus kinase 2 (TLR4-JAK2) is also involved in B. abortus internalization by murine macrophages [56].…”