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Infection and Immunity, April 2004, p. 2429-2433, Vol. 72, No. 4
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.4.2429-2433.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Different Responses of Macrophages to Smooth and Rough Brucella spp.: Relationship to Virulence

María P. Jiménez de Bagüés,1* Annie Terraza,2 Antoine Gross,2 and Jacques Dornand2

Unidad de Sanidad Animal, Servicio de Investigación Agroalimentaria, Diputación General de Aragón, 50080 Zaragoza, Spain,1 INSERM U431, Université de Montpellier II, 34095 Montpellier, France2

Received 3 November 2003/ Returned for modification 14 November 2003/ Accepted 11 December 2003


   ABSTRACT
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By comparing smooth wild-type Brucella strains to their rough mutants, we show that the lipopolysaccharide (LPS) O side chain of pathogenic Brucella has a dramatic impact on macrophage activation. It favors the development of virulent Brucella by preventing the synthesis of immune mediators, important for host defense. We conclude that this O chain property is firmly linked to Brucella virulence.


   TEXT
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The genus Brucella is a gram-negative, facultative, intracellular pathogen that produces diseases in different mammals, including humans. As in other gram-bacteria, the lipopolysaccharide (LPS) of Brucella is an important component of the outer membrane (3, 7), but its precise role in the biology of the pathogen is still an unsolved issue. In smooth pathogenic Brucella spp. (B. abortus, B. suis, and B. melitensis), lipopolysaccharide (LPS) has been implicated in bacterial virulence. This proposal is based on the observation that rough mutants derived from these strains are greatly attenuated in animals or isolated macrophages compared to parental Brucella (1, 10, 13, 18, 33, 36, 37, 40). The rough mutants thus appear to act as a putative live vaccine. Their attenuation is mainly explained by the properties of the LPS O side chain, a long linear homopolymer of {alpha}1,2-linked perosamine (5). The O chain appears to be a key molecule for invasion and development (35) and protection from apoptosis (14). It also protects the bacteria from cellular cationic peptides (16, 32), oxygen metabolites (39), and complement-mediated lysis (13, 30). Recently, it was observed that the O chain also impairs cytokine production in infected human macrophages, and it was postulated that this could be a way for the pathogen to control host defense (37). We have analyzed this possibility in a murine model of infection commonly used to compare the levels of virulence of Brucella strains.

B. melitensis B3B2 (18) and R5 (Table 1) and B. suis manb (15) are three rough mutants of wild-type B. melitensis 16M and B. suis 1330, respectively: these mutants are attenuated in BALB/c mice compared to parental Brucella (9) (Table 1). Their ability to infect murine macrophage-like cells was assessed by using J774A.1 cells cultured in 24-well plates (106 cells per well). These cells were incubated at 37°C for 30 min with a bacterial suspension (multiplicity of infection [MOI] = 40) (21, 40). After three washes, the infected macrophages were reincubated in 1 ml of RPMI-10% fetal calf serum (FCS) supplemented with 30 µg of gentamicin/ml for at least 40 min to kill extracellular bacteria. At several intervals postinfection (p.i.), cells were washed and lysed in 0.2% Triton X-100. The number of viable intracellular bacteria (CFU per well) was determined by plating serial 10-fold dilutions onto Trypticase soy agar (TSA) plates. Figure 1 indicates that rough B. melitensis strains R5 and B3B2 were respectively phagocytosed 500- and 100-fold more than smooth B. melitensis strain 16M (P < 0.005 for each mutant versus B. melitensis) and that B. suis manb was internalized 50-fold more than B. suis 1330 (P < 0.003). As reported previously (18, 21, 24), after a short period of decrease, the number of intracellular B. suis and B. melitensis cells significantly increased. At 48 h p.i., there were 100- to 1,000-fold more intracellular smooth bacteria than were found at the onset of infection. In contrast, intracellular rough mutants were eliminated, and depending on the mutant analyzed, there were 102- to 103-fold-fewer intracellular bacteria at 48 h p.i. than after phagocytosis. All of the rough mutants were eliminated, albeit with different kinetics, which can be explained by the genetic background of the mutants. The elevated invasion of the rough mutants was possibly due to the exposure of ligands that are normally hidden by the O chain and the consequent increased capacity of rough Brucella to adhere to macrophages (11, 37). Entry of smooth and rough Brucella strains into the cells through different pathways (35) could also involve receptors with a distinct ability to regulate the levels of phagocytosis. Because rough Brucella strains are efficiently internalized (10, 11, 17, 35), the bacteria could alter the plasma membrane, causing cell damage. Cell toxicity could also have resulted from induction of cell apoptosis, because rough Brucella strains do not protect macrophages from exogenous apoptotic signals (14), unlike smooth Brucella strains (23). However, under our experimental conditions (MOI of 40, presence of serum, no exogenous apoptotic signals, 48 h of culture), the lactate dehydrogenase activities of supernatants (measured as desrcribed in reference 22) were similar in cells infected by rough or smooth Brucella strains (data not shown). This indicates that elimination of the rough mutant did not result from cell toxicity and release of bacteria in the gentamicin-supplemented medium. Therefore, as postulated (37), the observed fate of rough Brucella strains could have resulted from a cellular response triggered by the bacteria. The macrophage response to different Brucella was assessed by measuring the expression of inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNF-{alpha}), interleukin-1 (IL-1), IL-10, IL-12, MIP-2, and KC (CXC chemokine, murine homologue of GRO-alpha) mRNAs in cells infected for 5 h. Expression of mRNAs was determined by reverse transcription-PCR (RT-PCR) under the conditions described in our previous studies (20, 21). The primers, mRNA and cDNA preparations, and procedures used for normalization of amplicon intensities have been described elsewhere in detail (20, 21, 24). The inflammatory cytokine and iNOS transcripts that are not expressed in quiescent cells were strongly induced when the invasive agent was one of the rough bacteria, but not when B. suis or B. melitensis 16M was used. This indicated that rough Brucella strains triggered an activation process that did not occur in smooth Brucella strain-infected cells. The transcripts were expressed at levels comparable to those occurring in quiescent cells stimulated with 100 ng of Escherichia coli LPS per ml. TNF-{alpha} and NO are two products of macrophage response directly involved in killing of intracellular Brucella (2, 9, 21, 42, 43); their production was measured in infected cell supernatants as described in reference 21. A significant accumulation of NO2- (the end product of NO) and TNF-{alpha} was observed in supernatants of all rough Brucella strain-infected cells. The accumulation of NO was time dependent for at least 48 h p.i. and was similar to that induced by the combination of E coli LPS with gamma interferon (IFN-{gamma}) (Table 2) (21). The TNF-{alpha} concentration, optimal at 24 h, varied from 10 to 28 ng/ml, depending on the rough mutant. In accordance with previous reports (12, 21, 41), macrophages invaded by smooth Brucella strains did not express iNOS nor release any NO (Table 2). They produced relatively weak amounts of TNF-{alpha}, the cytokine concentration in supernatants remaining around 0.5 ng/ml (Table 2), which in each case was significantly lower than that induced by each of the rough bacterial strains assessed. Several studies have clearly demonstrated an inverse correlation between TNF-{alpha} release and the invasive capacity of Brucella (reviewed in reference 2). Moreover, NO which is deleterious to Brucella (28), was produced in rough Brucella strain-infected cells but not in smooth Brucella strain-infected cells. Besides TNF-{alpha} and NO, IL-1 is an important mediator of Brucella development (27), and the phagolysosomal destruction of microorganisms commonly corresponds to high levels of cytokine production (34). KC and MIP-2 have antibacterial properties in vivo by attracting neutrophils to the site of infection (24, 34), and IL-12 is a necessary factor for the establishment of the Th1 response, which in vivo determines the elimination of Brucella (44). If IL-10 is detrimental to the cells, it is also an anti-inflammatory molecule that controls the damaging effect of proinflammatory cytokines.


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  		TABLE 1. Brucella strains used in the study

 


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  		FIG. 1. (A) Infection of J774.A1 cells by different smooth and rough strains of Brucella. J774.A1 cells were infected (MOI = 40) with B. suis ({blacksquare}), B. suis manb ({square}), B. melitensis 16M (•), B. melitensis B3B2 ({triangledown}), or B. melitensis R5 ({circ}), and the intracellular fate of the bacteria was evaluated. The data presented are means ± standard deviations of triplicate plate counts and are representative of three different experiments. (B) Cytokine and iNOS mRNA expression in Brucella-infected cells. The gene expression of different cytokines or iNOS was analyzed by RT-PCR performed on mRNAs of J774.A1 cells infected for 5 h and compared to gene expression in control cells (cells) or in cells induced with 100 ng/E. coli LPS/ml. The housekeeping gene coding for ß2-microglobulin was used as a standard (31). Cells were infected with B. melitensis 16M, R5, or B3B2 or B. suis 1330 or manb. Data are representative of three different experiments. The mRNAs and cDNA preparations, primers, and method used to compare amplicon intensities have been described elsewhere in detail (28, 31).

 

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  		TABLE 2. NO2- and TNF-{alpha} production in J774.A1 cells infected with different strains of Brucella or coinfected with B. suis GFP and a Brucella rough mutant

 
Therefore, in rough Brucella strain infection, the high production of TNF-{alpha} and NO concentration and possibly that of other effectors may mount a potent inflammatory response that imbalances the macrophage defense mechanisms to an extent favorable to the host cells. Conversely, smooth Brucella strains could be virulent in part because they induce little or no macrophage response, the LPS O chain limiting this response. To confirm this possibility, J774.A1 macrophages were coinfected with smooth and rough Brucella strains, and the proliferation of the smooth bacteria was measured. J774.A1 cells (106/ml/well) were simultaneously incubated at 37°C with B. suis GFP (an ampicillin-resistant mutant of B. suis 1330 that expresses the green fluorescent protein [GFP] gene) instead of B. suis (MOI = 40) (31) and a rough strain of Brucella (B. suis manb or B. melitensis B3B2) (MOI = 40) for 30 min. The protocol commonly used for infection (Fig. 1) (washing, cultures grown in RPMI-FCS with gentamicin, cell lysis with 0.2% Triton X-100 at different times p.i., and determination of CFU) was then applied. J774. A1 cells were also infected with B. suis GFP alone (MOI = 40) or with one of the rough mutants (MOI = 40). To measure only the development of B. suis GFP, and not that of the rough strain, cell lysates were applied to TSA plates containing 50 µg of ampicillin per ml. B. suis GFP carries a stable resistance gene to this antibiotic (39), while rough Brucella strains did not proliferate on these plates. Figure 2A shows that the phagocytosis of B. suis GFP was not affected by the presence of B. suis manb or B. melitensis B3B2. In parallel, the phagocytosis of the rough strain was not modified in presence of B. suis (data not shown). These observations agreed with the different pathways of entry of smooth and rough Brucella strains (35). At 24 and 48 h p.i., the number of intracellular B. suis GFP organisms was significantly lower within macrophages infected with both strains than within macrophages infected with B. suis GFP alone (Fig. 2A). In coinfected cells, NO2- and TNF-{alpha} accumulated in supernatants of cells simultaneously infected with B. suis GFP and B. suis manb (or B. melitensis B3B2), but not (or poorly for TNF-{alpha}) in B. suis GFP-infected cells. These accumulations were similar to those occurring in supernatants of cells solely infected by rough mutants (P > 0.2) (Table 2). To study whether NO and TNF-{alpha} affect the fate of B. suis GFP, the iNOS inhibitor L-NAME (N-{omega}-nitro-L-arginine methly ester) (21) or a blocking anti-TNF-{alpha} receptor (TNF-{alpha}R) antibody was assessed in coinfection experiments. L-NAME does not affect the infection of J774.A1 cells with only B. suis (21). On the contrary, it reversed the inhibition of the intracellular development of B. suis GFP induced by the rough mutant and favored the growth of the pathogen (Fig. 2C). The anti-TNF-{alpha}R antibody also exerted a similar effect (Fig. 2D). The concomitant measurement of NO2- in cell supernatants confirmed the inhibitory effect of L-NAME on NO production. At 48 h p.i., NO2- concentrations in the presence and absence of L-NAME were 4.25 ± 1 and 37 ± 5 µM, respectively, in coinfections involving B. suis GFP and B. melitensis B3B2. As expected (6, 21), in control cells at 48 h p.i., L-NAME (20-fold) and the anti-TNF-{alpha}R antibody (5-fold) positively affected the development of B. melitensis B3B2 (data not shown). Altogether, the experiments finally indicated that TNF-{alpha} and NO resulting from a response triggered by rough bacteria inhibited the development of B. suis GFP. In the presence of L-NAME or anti-TNF-{alpha}R antibody, the number of B. suis GFP cells in coinfected cells did not differ from that in cells infected with B. suis GFP alone. Therefore, when the NO or TNF-{alpha} effect is neutralized, the intracellular development of B. suis GFP was not affected by the presence of rough bacteria within the cells. Therefore, the inhibition of B. suis GFP development was not due to (i) too large an ingestion of rough bacteria by macrophages that could have damaged the cells, nor (ii) a rerouting of the bacteria towards phagolysosomes, because the level of uptake of B. suis was not significantly affected by the uptake of the rough strain and vice versa, a result in accordance with the different routes of entry of smooth and rough Brucella strains (35). Finally, the impairment of the development of B. suis GFP reversed by L-NAME or anti-TNF-{alpha}R resulted from a direct response of the macrophage to rough Brucella strains. Therefore, smooth Brucella strains might achieve long-term persistence, because they do not initiate the production of NO and TNF-{alpha}, two factors that induce the clearance of rough Brucella. This may also be true for other inflammatory molecules at different stages of infection.



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  		FIG. 2. (A) Intracellular development of B. suis GFP (CFU per well) in macrophages infected with B. suis GFP or coinfected with B. suis GFP and a rough mutant of Brucella. J774.A1 cells (106/well) were infected with B. suis GFP (MOI = 40) (•) or coinfected with B. suis GFP and B. suis manb ({circ}) or B. suis GFP and B. melitensis B3B2 ({triangledown}) (MOI = 40 for each bacteria). Infections were performed in triplicate. At different periods of time, the cells were lysed and the number of intracellular B. suis GFP cells was determined by plating the cell lysates on agar plates supplemented with ampicillin and expressed in CFU per well ± standard deviation. At 48 h p.i., in four different experiments, the number of B. suis GFP cells in macrophages infected with only B. suis GFP was significantly higher than the number of B. suis GFP cells in macrophages coinfected with B. suis GFP and B. suis manb (P < 0.05) or with B. suis GFP and B. melitensis B3B2 (P < 0.01). (B) Effect of NO and TNF-{alpha} on the development of B. suis in J774.A1 cells coinfected with B. suis and B. melitensis B3B2. J774.A1 cells (106/well) were infected with B. suis GFP (MOI = 40) or coinfected with B. suis GFP and B. melitensis B3B2 (MOI = 40, for each bacteria). After the phagocytosis step (i.e., at the same time as gentamicin), L-NAME (3 mM) or the blocking anti-TNF-{alpha}R antibody (5 µg/ml) was added (or not) to the infected cultures. Infections were performed in triplicate. Forty-eight hours later, the intracellular number of B. suis GFP cells was determined for each condition and expressed as CFU per well ± standard deviation. Experiments C and D were performed separately and repeated four times. The numbers of B. suis GFP cells phagocytosed in 30 min were as follows: 1,200 ± 400 (inoculum of B. suis GFP) and 1,500 ± 200 (inoculum of B. suis GFP and B. melitensis B3B2) for experiment C and 2,200 ± 500 (inoculum of B. suis GFP) and 1,900 ± 300 (inoculum of B. suis GFP and B. melitensis B3B2) for experiment D. In four separate experiments, at 48 h p.i., the numbers of intracellular B. suis GFP cells were (i) significantly lower in coinfections than in macrophages infected with only B. suis GFP (P < 0.001 and P < 0.01 for experiments C and D, respectively), (ii) significantly higher in coinfections performed in the presence of L-NAME than in its absence (P < 0.05), and (iii) significantly higher in coinfections performed in the presence of anti-TNF-{alpha}R than in its absence (P < 0.01).

 
B. abortus binds to several different receptors, including LPS receptors (4). However, the LPS from both rough and smooth Brucella strains is a weak activator of macrophages (19, 26). Therefore, the elevated expression of cytokine and iNOS transcripts in rough Brucella-infected macrophages compared to macrophages stimulated by E. coli LPS could mean that the bacterial LPS is not the only molecule involved in macrophage stimulation. Such a possibility is in agreement with observations showing that TLR-2, but not TLR-4 which is involved in LPS response, mediates macrophage activation by heat killed Brucella (25). This should explain how rough Brucella strains are able to trigger the complete cell signaling pathways leading to NO production, while LPS-induced production of NO requires an additional signal given by exogenous IFN-{gamma} (21). In accordance with previous reports comparing B. abortus 2308 and B. abortus RB51 (38), B. melitensis and its whoA mutant (13), or B. suis and its rough mutants (35), observations from our group indicate that many more receptors are engaged in macrophage response to rough Brucella infection than in response to smooth Brucella infection (35, 37). This could explain the potent capacity of rough mutants to stimulate macrophages. The different pathways of phagocytosis of rough and smooth bacteria suggest that these bacteria engage different sets of receptors (35). Therefore, besides the number of receptors engaged, their specificity is certainly of importance. In any case, the slight response associated with smooth Brucella infection indicates a mechanism by which the LPS O chain modulates the host immune response to the pathogen's advantage. Besides its properties mentioned above (13, 14, 16, 30, 32, 35, 39), this O chain characteristic must be taken into account to explain the virulence of pathogenic Brucella. The cell signals by which smooth Brucella avoids macrophage activation at the molecular level are now under investigation.


   ACKNOWLEDGMENTS
 
These studies were supported by grants from INSERM and from the European Community (QLK2-1999-0014). We are grateful for support from the Region Languedoc-Roussillon and Aragón (CTP FDG/CS68 for INSERM and CTPMO1/2002 for Aragón) and the Ministerio de Educación Cultura y Deporte: Programa de Estancias de Investigadores en el Extranjero PR2001-0053 and INSERM (Poste Vert) for exchange grants to María P. Jiménez de Bagüés.

We thank A. Cloeckaert (INRA, France) for providing the LPS monoclonal antibodies and F. Godfroid (Université de Namur, Namur, Belgium) for the B. melitensis B3B2 mutant.

We thank Sue Hagius and E. Caron for correcting and improving our English.


   FOOTNOTES
 
* Corresponding author. Mailing address: Unidad de Sanidad Animal, Servicio de Investigación Agroalimentaria, Diputación General de Aragón, Ap. 727, 50080 Zaragoza, Spain. Phone: 34-976-716-457. Fax: 34-976-716-335. E-mail: mpjimenezdebagues{at}aragob.es. Back

Editor: D. L. Burns


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Infection and Immunity, April 2004, p. 2429-2433, Vol. 72, No. 4
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.4.2429-2433.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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