The Attenuated Brucella abortus Strain 19 Invades, Persists in, and Activates Human Dendritic Cells, and Induces the Secretion of IL-12p70 but Not IL-23

Brucella infection and Toll-like receptors

Brucella consists of gram-negative bacteria that have the ability to invade and replicate in professional and non-professional phagocytes, and its prolonged persistence in the host leads to brucellosis, a serious zoonosis. Toll-like receptors (TLRs) are the best-known sensors of microorganisms implicated in the regulation of innate and adaptive immunity. In particular, TLRs are transmembrane proteins with a typical structure of an extracellular leucine-rich repeat (LRR) region and an intracellular Toll/interleukin-1 receptor (TIR) domain. In this review, we discuss Brucella infection and the aspects of host immune responses induced by pathogens. Furthermore, we summarize the roles of TLRs in Brucella infection, with substantial emphasis on the molecular insights into its mechanisms of action.

Characteristics of Brucella abortus vaccine strain A19 reveals its potential mechanism of attenuated virulence

Brucella vaccination is one of the most important strategies for controlling brucellosis in livestock. The A19 strain was the effective vaccine used to control brucellosis in China. However, the characteristics of physiological and attenuated virulence of the A19 strain are not investigated in detail. In this study, we compared the phenotypic characteristics of the A19 to the wild-type strain S2308. Virulence test showed that the A19 was significantly attenuated at chronic infection stage in infected mouse model. In growth analysis, the A19 exhibited a quick growth at exponential phase and premature at stationary phase. The inflammatory response of macrophages infected by the A19 was detected using TaqMan qPCR assay, indicating that the inflammatory level of the A19-infected macrophages was higher than that of the S2308 infection. Cell death analysis showed that the A19 was not cytotoxic for macrophages. Cell infection showed that the A19 reduced its ability to invade, survive and traffic within host cells, and the intracellular A19 hardly excludes lysosome-associated marker LAMP-1, suggesting that the A19 can't escape the lysosome degradation within host cells. In further study, the sensitivity test exhibited that the A19 is more sensitive to stress and bactericidal factors than the S2308 strain, Western blot and silver staining analysis exhibited that the A19 has a different expression pattern of OMPs and reduces LPS O-antigen expression relative to the S2308 strain. Those data give us a more detailed understanding about the A19 vaccine strain, which will be beneficial for improvement of current Brucella vaccine and overcoming its defects.

Interaction of a live attenuated Salmonella Gallinarum vaccine candidate with chicken bone marrow-derived dendritic cells

Salmonella enterica serovar Gallinarum (SG) is a Gram-negative intracellular host-adapted pathogen that causes fowl typhoid. Attenuated strains of SG are proven and widely used vaccine candidates because of advantages like induction of strong humoral and cell-mediated immune responses. In the present study, we investigated the interaction of chicken bone marrow-derived dendritic cells (chBM-DCs) with an attenuated SG (JOL1355) strain that secretes a heat-labile enterotoxin B subunit protein previously shown to successfully vaccinate chickens. ChBM-DCs were isolated and cultured in the presence of recombinant chicken GM-CSF and IL-4 cytokines. The chBM-DCs were infected with JOL1355 at an multiplicity of infection of 10. JOL1355 was able to invade dendritic cells (DCs); however, the survival of JOL1355 in DCs decreased over time. At 24 h post infection, IL-6, IL-10 and IFN-γ transcript levels were significantly increased in JOL1355-infected DCs compared to non-stimulated DCs. Flow cytometry analysis showed an increased proportion of cells producing CD40, CD80, and MHC class II in the JOL1355-infected cultures compared to the non-stimulated control. In addition, JOL1355-stimulated chBM-DCs could induce significant expression of IL-2 in co-culture with autologous CD4+ T cells. Based on these results, we conclude that chBM-DCs are capable of internalizing the live attenuated SG vaccine candidate and the infected chBM-DCs show signs of maturation as evidenced by the upregulated expression of costimulatory molecules and cytokines.

Inflammatory response of TLR4 deficient spleen macrophages (CRL 2471) to Brucella abortus S19 and an isogenic ΔmglA deletion mutant

Brucellosis is a worldwide distributed zoonosis caused by members of the genus Brucella. One of them, Brucella abortus, is the etiological agent of bovine brucellosis. With the attenuated strain B. abortus S19 a vaccine is available. However, both, virulence (safety) and the ability to induce a protective B and T cell response (efficacy) have to be tested in suitable assays before successful use in the field. For this purpose, several macrophage cell lines of various origins have been used while splenic macrophages are the preferred host cells in vivo. We here characterized the in vitro response of the murine splenic macrophage cell line CRL 2471(I-13.35) to B. abortus. This cell line still depends on the presence of colony-stimulating factor 1 (CSF1) and is derived from LPS resistant (TLR4 deficient) C3H/HeJ mice. For infection the vaccine strain B. abortus S19A as well as the formerly described isogenic deletion mutant B. abortus S19A ΔmglA 3.14 were used. While numbers of viable bacteria did not differ significantly between the vaccine strain and the deletion mutant at 6h post infection, a higher bacterial load was measured in case of the mutant at 24h and 48h after infection. This was also true, when IFNγ was used for macrophage activation. A comprehensive gene expression profile of macrophages was analysed 6 and 24h after infection by means of an RT-PCR based gene expression array. The mutant strain B. abortus S19A ΔmglA 3.14 elicited a stronger cellular response of the splenic macrophages as compared to the parental vaccine strain. This was most prominent for the pro-inflammatory cytokines IL-1α, IL-1β, TNF-α and IL6 as well as for the chemokine ligands CXCL1, CXCL2, CXCL10, CCL2, CCL5, CCL7, CCL17 and the co-stimulatory molecules CD40 and ICAM1. While these differences were also present in IFNγ-stimulated macrophages, an addition of IFNγ after infection not only resulted in a dramatic increase of the translation of the afore mentioned genes but also resulted in the translation of IFNß1, IL12ß, MIP1α and β (CCL3, CCL4), NOS2 (and SOD2) and FAS.

CONCLUSION

The TLR4 deficient murine splenic macrophage cell line CRL 2471 was used for the first time for the characterization of macrophage-Brucella interaction to investigate the pre-immune phase of brucellosis in vitro. Typical pro-inflammatory cytokines and certain surface receptors were differentially induced by B. abortus S19 A and an isogenic ΔmglA deletion mutant in vitro. This model may be useful for further studies to characterize the inflammatory response of splenic macrophages to intracellular gram-negative bacteria avoiding cell responses to soluble LPS.