Attenuated Coxiella burnetii Phase II Causes a Febrile Response in Gamma Interferon Knockout and Toll-Like Receptor 2 Knockout Mice and Protects against Reinfection

C. burnetii preparation and mouse infections. C. burnetii phase II organisms of the NMII (RSA439, clone 4) strain were cultured in African green monkey kidney (Vero) fibroblasts (CCL-81; American Type Culture Collection), as previously described (9).

Breeder pairs of TLR2^−/− mice on a C57BL/6 background were obtained from the Jackson Laboratory. IFN-γ^−/− mice on a BALB/c background were obtained as previously described (46). Pathogen-free female BALB/c and C57BL/6N mice were obtained from the National Cancer Institute, Frederick Cancer Research Facility, Frederick, MD. All mice were maintained at the Montana State University Animal Resources Center under pathogen-free conditions in individual ventilated cages under HEPA-filtered barrier conditions and were fed sterile food and water ad libitum.

All animal care and procedures were in accordance with institutional policies for animal health and well-being. Seven-week-old female TLR2^−/−, C57BL/6, IFN-γ^−/−, and BALB/c mice were infected intraperitoneally (i.p.) (five mice/group) with 1 × 10⁸ bacteria or left uninfected, and rectal temperatures were measured daily. For the reinfection experiments, mice were i.p. infected with 1 × 10⁸ CFU 30 days after the primary infection. Rectal temperatures were measured daily for another 10 days, after which lymphoid tissues were harvested for splenic weights and enumeration of genomic DNA copies.

Rectal temperatures.Rectal temperatures of C. burnetii NMII-infected and uninfected mice were measured using an electronic thermometer (Physitemp-Model BAT-12, Clifton, NJ) provided with a rectal probe (Physitemp-Model RET-3). The probe was washed with 70% ethanol and sterile phosphate-buffered saline (PBS) between interanimal measurements. Normal body temperatures of 35.5°C to 36.5°C were measured and were consistent with previously reported normal temperatures (47).

Antibody ELISA.Antibody titers in serum samples were determined by an enzyme-linked immunosorbent assay (ELISA) adapted from previously described methods (31). Briefly, heat-killed Coxiella (5 × 10⁹ bacteria/ml) in sterile PBS was used to coat Maxisorp Immunoplate II microtiter plates (Nunc, Roskilde, Denmark) at 100 μl/well, and the plates were incubated overnight at room temperature. Various dilutions of serum samples were prepared in ELISA buffer (PBS, 0.5% bovine serum albumin, 0.05% Tween 20) and incubated overnight at 4°C. Specific reactivities against Coxiella were determined with horseradish peroxidase conjugates of the following detecting antibodies (1.0 μg/ml): goat anti-mouse immunoglobulin (IgG), IgG1, IgG2a, and IgG2b antibodies (Southern Biotechnology Associates, Birmingham, AL). Following 90 min of incubation at 37°C and a washing step, the specific reactivity was determined by the addition of the ABTS [2,2′-azinobis(3-ethylbenzthiazolinesulfonic acid] enzyme substrate (Moss, Inc., Pasadena, CA) at 100 μl/well, and the absorbance was measured at 415 nm on a Bio-Tek Instruments ELx808 microtiter plate reader (Winooski, VT). End point titers were expressed at the reciprocal log₂ of the last sample dilution giving an absorbance of ≥0.1 optical density unit above negative control values after 1 h of incubation, similar to that previously described (49).

Peritoneal macrophage isolation.Peritoneal macrophages from C57BL/6, TLR2^−/−, BALB/c, and IFN-γ^−/− mice were induced with a single i.p. injection of 1.0 ml of aged thioglycolate medium (Difco, Detroit, MI) as previously described (31), and 3 days later the peritoneum of each mouse was washed with sterile RPMI 1640 medium (Gibco BRL-Life Technologies, Grand Island, NY) containing 2% fetal calf serum (Atlanta Biologicals, Lawrenceville, GA) without antibiotics. Peritoneal cells were washed twice in the same medium without antibiotics.

Infection of macrophages with C. burnetii NMII.RAW264.7 (American Type Culture Collection, Manassas, VA) and peritoneal macrophages from C57BL/6, TLR2^−/−, BALB/c, and IFN-γ^−/− mice were evaluated for cytokine production after infection with C. burnetii NMII. Macrophages were added at 1.25 × 10⁶ cells/well in RPMI 1640 medium containing 2% fetal calf serum (31) without antibiotics and were allowed to adhere to plastic in 24-well microtiter dishes (BD Labware, Franklin Lakes, NJ) for 3 h at 37°C. The wells were washed to remove nonadherent cells. The nonadherent cells were collected and counted to determine the number of cells that remained adherent to the plastic. After overnight culture at 37°C in complete medium without antibiotics, cells were infected at various C. burnetii NMII-to-macrophage ratios (1:1 to 100:1) for 8 h or 24 h at 37°C. Supernatants were collected and frozen until analysis by cytokine ELISA.

Cytokine ELISAs.Cytokine secretion of infected macrophages was detected using cytokine-specific ELISAs for IFN-γ, IL-1α, IL-1β, TNF-α, IL-6, IL-10, IL-12p40, IL-12p70, GM-CSF, and transforming growth factor β (TGF-β). The ELISA protocols used were identical to those assays previously described for each cytokine (29, 31). For development, horseradish peroxidase-conjugated antibodies were used (29, 31) and developed with ABTS peroxidase substrate (Moss). Microtiter wells were read using a Bio-Tek Instruments ELx808 microtiter plate reader. To determine the amount of cytokine present in the test samples, various dilutions of recombinant murine cytokines (R&D Systems, Minneapolis, MN) were used to establish standard curves from which values for the test samples could be extrapolated.

To assess T-cell-derived cytokines, spleens were aseptically removed 10 days after infection of mice i.p. with 1 × 10⁸C. burnetii NMII organisms. Lymphocytes were prepared as previously described (29) and resuspended in a complete medium consisting of RPMI 1640 medium supplemented with 1 mM sodium pyruvate, 1 mM nonessential amino acids, penicillin-streptomycin (10 U/ml), and 10% fetal bovine serum (Atlanta Biologicals). Lymphocytes were cultured in 24-well tissue plates at 5 × 10⁶ cells/ml in complete medium alone or in the presence of heat-killed Coxiella NMII at various C. burnetii-to-lymphocytes ratios (1:1 to 100:1) in a total volume of 1 ml for 72 h at 37°C. The supernatants were collected by centrifugation and stored at −80°C. Capture ELISA was employed to quantify, on duplicate sets of samples, the levels of IFN-γ, TNF-α, IL-6, IL-10, IL-4, IL-13, and TGF-β produced by lymphocytes, as previously described (29, 31).

Adoptive transfer studies.Six weeks after i.p. infection of BALB/c, IFN-γ^−/−, C57BL/6, and TLR2^−/− mice with C. burnetii NMII, T cells (T-cell isolation kit; Miltenyi Biotec, Auburn, CA) and B cells (mouse B-cell negative isolation kit; Dynal Biotec ASA, Oslo, Norway) were obtained and combined at ratios of 1:1. The purity of lymphocyte populations was ≥98% for B cells and 96% for T cells as determined by fluorescence-activated cell sorter analysis. Immune or naive T and B cells (3 × 10⁶ cells) were intravenously (i.v.) injected into naive recipient BALB/c, IFN-γ^−/−, C57BL/6, and TLR2^−/− mice. One day later, separate groups of mice were i.p. infected with 1 × 10⁸C. burnetii NMII, and rectal temperatures were monitored daily. At 3 and 10 days postchallenge, spleens were harvested, weighted, and evaluated for the number of C. burnetii genomic copies.

Tissue detection of C. burnetii by quantitative PCR. C. burnetii replication in spleens of mice during infection and reinfection was quantified by using TaqMan quantitative PCR of genome equivalents. Briefly, spleens were aseptically removed, and DNA was extracted by using a DNeasy tissue kit (QIAGEN, Hilden, Germany). The tissues were weighed and homogenized in detergent lysis buffer and proteinase K for 3 h at 55°C according to the manufacturer's instructions.

The primers (QrpoS-F [5′-CGCGTTCGTCAAATCCAAATA-3′] and QrpoS-R [5′-GACGCCTTCCATTTCCAAAA-3′]) for the stationary-phase sigma factor (rpoS) were used, and the probe (5′-ACGCTCTGCAGCAATTACGCCA-3′) sequences were designed with PrimerExpress software (Applied Biosystems, Foster City, CA). Purified C. burnetii (NMII strain) DNA was used as the template to generate standard curves, as previously described (9). The concentration and purity were determined by measuring the A₂₆₀ and A₂₈₀ (9). Quantitative PCR was performed in 96-well plates with a final volume of 25 μl consisting of 12.5 μl of TaqMan universal master mix (PE Applied Biosystems, Foster City, CA), 1 μl of rpoS primer (100 nM) (0.5 μl of forward primer and 0.5 μl of reverse primer), 1 μl of DNA template, and 10.5 μl of distilled water per well, using an ABI 7500 real-time PCR system (Applied Biosystems), with one cycle of 95°C for 5 min to activate AmpliTaq Gold DNA polymerase (Applied Biosystems) and then 40 cycles of 95°C for 15 s and 60°C for 1 min to amplify the target DNA. The threshold cycle values obtained were extrapolated into the standard curves, and the DNA copies per ml of sample and then per spleen were determined.

Statistical analysis.Analysis of variance followed by a post hoc Tukey test was applied to show differences in body temperatures between immunocompetent and immunodeficient mice after infection with C. burnetii NMII. The Student t test was used to evaluate differences between variations in the averages of spleen weights, differences in cytokine production levels, and differences in the DNA copies of C. burnetii NMII in the quantitative PCR analysis.

Infection with C. burnetii NMII induces fever in immunodeficient mice.Previous studies have shown that control of C. burnetii infections requires IFN-γ (10, 11, 27, 41) and recognition by the innate immune system via TLR2 (28, 51). In addition, the role of TLR4 in the recognition of C. burnetii was also described (17). Thus, we questioned whether disease could be induced in mice deficient in IFN-γ or TLR2 using the avirulent NMII. To test this possibility, daily rectal temperatures were measured in infected and uninfected mice. Measurement of febrile responses has been previously employed to follow disease after infection with various pathogens (5, 36, 39, 42, 44, 47). Infected BALB/c mice showed no significant changes in their rectal temperatures after infection with C. burnetii NMII compared to uninfected mice. In contrast, IFN-γ^−/− mice showed significant increases in their rectal temperatures (P < 0.001) compared to BALB/c mice (Fig. 1A and 2A and B). Likewise, TLR2^−/− mice showed immediate increases in their body temperatures following infection (Fig. 1C and 3A and B). Immunocompetent C57BL/6 mice also showed significant increases in their temperatures (P < 0.001) after infection with NMII compared to uninfected mice (Fig. 3B), although these increases were significantly less than in TLR2^−/− mice (Fig. 1C) (P < 0.001). These results show that the infection with C. burnetii NMII can cause a fever response in immunodeficient mice but not in immunocompetent BALB/c mice and only a mild fever for a few days in C57BL/6 mice.

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FIG. 1.

Infection with C. burnetii NMII provokes greater febrile responses in TLR2^−/− and IFN-γ^−/− mice than in C57BL/6 or BALB/c mice. (A and C) BALB/c and IFN-γ^−/− mice (A) and C57BL/6 and TLR2^−/− (C) mice were infected i.p. with 1 × 10⁸Coxiella burnetii NMII organisms. Daily measurements of rectal temperatures were performed, showing that infected IFN-γ^−/− and TLR2^−/− mice had statistically higher temperatures than the respective normal strain. *, P < 0.001 for IFN-γ^−/− versus BALB/c and for TLR2^−/− versus C57BL/6 mice. (B and D) Splenic weights of individual mice (five/group/time point) were evaluated at weekly intervals after challenge. At 1 week postinfection, significant increases (*, P < 0.001) in the IFN-γ^−/− and TLR2^−/− splenic weights were observed compared to BALB/c and C57BL/6 mice, respectively. Only BALB/c mice showed a slight increase in their splenic weights at 1 week postinfection. Error bars indicate standard deviations of the means.

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FIG. 2.

Reinfection with C. burnetii NMII provokes less severe fever in IFN-γ^−/− mice and no increases in their splenic weights. (A and B) IFN-γ^−/− (A) and BALB/c (B) mice were infected i.p. with 1 × 10⁸C. burnetii NMII organisms (5/group) or PBS (10/group). Rectal temperatures were measured daily. Thirty days after primary infection, mice were rechallenged with same dose of C. burnetii NMII or PBS, and rectal temperatures were measured. Half of the PBS groups were infected with C. burnetii NMII; the other half were left uninfected. The normal body temperature used as a control to compare febrile responses was between 35.5 and 36.5°C. *, P < 0.001 for Coxiella/Coxiella-reinfected group and for PBS/Coxiella-infected group versus PBS/PBS mice. †, P < 0.001 for PBS/Coxiella- versus Coxiella/Coxiella-reinfected mice. (C) Mice were sacrificed at 10 days after the second challenge and splenic weights measured. Reinfection of IFN-γ^−/− mice provoked increases in rectal temperatures, but these were less severe than after the primary infection. There were no increases in splenic weights of IFN-γ^−/− mice after the second infection. *, P < 0.001 for PBS/Coxiella mice versus PBS/PBS mice and Coxiella/Coxiella-reinfected mice. Data represent the means for individual mice from two experiments ± standard deviations of the means. n.s., not significant.

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FIG. 3.

Reinfection with Coxiella burnetii NMII provokes less severe fever in TLR2^−/− mice and no increases in splenic weights. (A and B) TLR2^−/− (A) and C57BL/6 (B) mice were infected i.p. with 1 × 10⁸C. burnetii NMII organisms (5/group) or PBS (10/group). Rectal temperatures were measured daily. Thirty days after primary infection, mice were rechallenged with same dose of C. burnetii NMII or PBS, and rectal temperatures were measured. Half of the PBS group were infected with C. burnetii NMII, and the other half were left uninfected. The normal body temperature used as a control to compare febrile responses was between 35.5 and 36.5°C. *, P < 0.001 for Coxiella/Coxiella-reinfected versus PBS/PBS mice and for PBS/Coxiella-infected group versus PBS/PBS mice. †, P < 0.001 for PBS/Coxiella- versus Coxiella/Coxiella-reinfected mice. (C) Mice were sacrificed 10 days after the second infection, and splenic weights were measured. As observed with IFN-γ^−/− mice, reinfection of TLR2^−/− mice provoked increases in rectal temperatures, but they were less severe than after the primary infection. No increases in the splenic weights of TLR2^−/− mice were observed after the second challenge. *, P < 0.001 for PBS/Coxiella mice versus PBS/PBS mice and Coxiella/Coxiella-reinfected mice. Data represent the means for individual mice from two experiments ± standard deviations of the means. n.s., not significant.

Infection with C. burnetii NMII provokes splenomegaly in immunodeficient mice.Spleens of infected immunocompetent and immunodeficient mice were harvested and weighed at weekly intervals for up to 3 weeks postinfection (Fig. 1B and D). Infection of IFN-γ^−/− mice showed significant increases in the splenic weights at 7 days after infection (Fig. 1B) (P < 0.001). At 2 weeks after the infection, the IFN-γ^−/− splenic weights were reduced, but they were still significantly greater than splenic weights of infected BALB/c mice (P < 0.001). TLR2^−/− mice also showed significant increases in their splenic weights by 7 days postinfection compared to C57BL/6-infected mice (Fig. 1D) (P < 0.001). However, in TLR2^−/− mice, this increase in size lasted only 1 week, and by 2 weeks postinfection, no significant differences in splenic weights were observed between TLR2^−/− and C57BL/6 mice. Slight increases in splenic weights of BALB/c mice were observed at 1 week postinfection, but this was not evident with the C57BL/6 mice.

Previous exposure to C. burnetii NMII reduces fever in immunodeficient mice after a reinfection.To assess whether prior exposure to C. burnetii NMII could protect immunodeficient mice from fever, IFN-γ^−/− and TLR2^−/− mice, as well as their respective immunocompetent controls, were infected i.p., as before, monitored for primary fever responses, and then 30 days later reinfected with the same dose of C. burnetii NMII. For each group, a PBS-dosed group was used as a primary fever response control for the twice-infected mice, or some mice were left uninfected to serve as a normal temperature control group. Primary infection of IFN-γ^−/− mice provoked a fever response that persisted for 2 weeks (Fig. 2A), and beyond 2 weeks, the IFN-γ^−/− mice recovered with no subsequent fever episodes until the second infection. Infected BALB/c mice showed no significant incidence of a fever response (Fig. 2B). When infected IFN-γ^−/− mice were reinfected with C. burnetii NMII, the magnitude of the induced febrile response was as potent as after the primary infection, but it was of shorter duration, lasting only 3 to 4 days (Fig. 2A). IFN-γ^−/− mice given a primary infection on day 30 of this experiment reproduced the long-term febrile response. Twice-infected BALB/c mice again showed no febrile responses (Fig. 2B).

Infection of TLR2^−/− mice induced a rapid febrile response that lasted 1 week, followed by a persistently low-grade febrile episode on days 8 to 16 and another one on days 27 to 30 (Fig. 3A). When these mice were twice infected with the same dose of C. burnetii NMII, a delay of 4 days in the febrile response was observed, and the magnitude and duration of this response were less than those after the primary infection. TLR2^−/− mice given a primary infection on day 30 of this experiment also reproduced the long-term febrile responses. C57BL/6 mice developed febrile responses soon after primary infection and recovered by day 8, with no subsequent episodes (Fig. 3B). The magnitude of these responses was also less than that observed for the TLR2^−/− mice. When these C57BL/6 mice were reinfected with C. burnetii NMII, the magnitude and duration of the second febrile responses were similar to those obtained following the primary infection (Fig. 3B), but these febrile responses were similar in magnitude to those obtained in the twice-infected TLR2^−/− mice (Fig. 3A). These results show that IFN-γ^−/− mice and TLR2^−/− mice are susceptible to infections with C. burnetii NMII resulting in disease.

Prior exposure to C. burnetii NMII protects against subsequent colonization in immunodeficient mice.To ascertain whether the initial exposure to C. burnetii could protect immunodeficient mice against colonization by a second infection, mice were evaluated at 10 days after secondary infection for their splenic weights and C. burnetii genomic copies. Once- or twice-infected BALB/c mice showed no measurable inflammation of their spleens compared to age-matched spleens from uninfected mice (Fig. 2C). On the other hand, C57BL/6 mice did show increased splenic weights after a single infection or two infections (Fig. 3C). However, infection of IFN-γ^−/− mice and TLR2^−/− mice protected against subsequent inflammation following the second infection (Fig. 2C and 3C). The splenic weights in twice-infected IFN-γ^−/− mice and TLR2^−/− mice were not significantly different from those in PBS control groups, suggesting that the primary infection protected these mice against subsequent infection. However, during a primary infection, the splenic weights in IFN-γ^−/− mice and TLR2^−/− mice increased 1.5-fold and 3-fold, respectively, compared to splenic weights in twice-infected mice (P < 0.001) or in the PBS control groups (P < 0.001).

Quantitative PCR to determine genomic DNA copy number showed a significant reduction (P < 0.001) in genomic DNA copies of Coxiella present in the spleens of twice-infected IFN-γ^−/− mice compared to the spleens of these mice 10 days after a single infection with C. burnetii (Fig. 4A). No significant differences were observed between Coxiella genomic DNA copies detected in BALB/c spleens from once- or twice-infected mice (Fig. 4A). There were significant differences between once- or twice-infected BALB/c and IFN-γ^−/− mice (Fig. 4A). There was a difference in Coxiella genomic DNA copies in spleens from once- or twice-infected C57BL/6 mice, but this difference was greater in TLR2^−/− mice, in which spleens from twice-infected mice showed a significant reduction (P < 0.001) in Coxiella genomic DNA copies (Fig. 4B). Moreover, this level of detection for twice-infected TLR2^−/− mice was at the threshold of sensitivity for this assay (Fig. 4B). Singly infected TLR2^−/− mice showed a significant enhancement (P < 0.001) of Coxiella genomic DNA copies in their spleens compared to the spleens from similarly NMII-infected C57BL/6 mice, but spleens from twice-infected TLR2^−/− mice showed significantly fewer Coxiella genomic DNA copies than those from twice-infected C57BL/6 mice (Fig. 4B).

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FIG. 4.

Coxiella burnetii NMII is cleared from spleens of reinfected IFN-γ^−/− and TLR2^−/− mice. Quantitative PCR was performed for C. burnetii detection in spleens from the IFN-γ^−/− and BALB/c mice (A) and TLR2^−/− and C57BL/6 mice (B) depicted in Fig. 2 and 3. C. burnetii NMII in mice was detected by quantitative PCR 10 at days postinfection. The results show that the presence of NMII in spleens of immunodeficient mice infected and reinfected with C. burnetii was reduced significantly (*, P < 0.001 for PBS/Coxiella versus Coxiella/Coxiella groups and for PBS/Coxiella versus PBS/PBS groups). No significant differences were observed in genomic DNA copies in the spleens of once- or twice-infected BALB/c mice; reinfection of C57BL/6 mice did show an increase in genomic DNA copies. †, P < 0.001 for differences between once- or twice-infected BALB/c versus IFN-γ^−/− mice and for once-or twice-infected C57BL/6 versus TLR2^−/− mice. Data represent the means for individual mice from two experiments ± standard deviations of the means.

Subdued inflammatory responses by C. burnetii NMII-infected macrophages from IFN-γ^−/− and TLR2^−/− mice.To evaluate what proinflammatory cytokines may be induced after infection with C. burnetii NMII, in vitro infections of RAW264.7 and peritoneal macrophages were performed at various bacterium-to-macrophage ratios. Cytokines were measured from the collected supernatants at 8 h and 24 h postinfection. Infection of RAW264.7 cells for 8 h showed increased production of TNF-α and IL-6 and, to a lesser extent, IFN-γ and GM-CSF, but no production of IL-1α, IL-1β, IL-12p40, IL-12p70, or TGF-β was detected (Fig. 5). Peritoneal macrophages from BALB/c and C57BL/6 mice were also infected for 24 h with C. burnetii NMII at various bacillus-to-macrophage ratios and evaluated for their cytokine production. Cytokine production was dependent on the number of bacilli used for infection, but results with the 100:1 infection ratio are reported here (Fig. 6). Significant increases in IFN-γ and TNF-α were observed, but neither was produced by infected IFN-γ^−/− and TLR2^−/− peritoneal macrophages (P < 0.001) (Fig. 6). In fact, the TLR2^−/− peritoneal macrophages did not produce detectable levels of IFN-γ, TNF-α, IL-6, IL-1α, IL-1β, or TGF-β, and only a slight amount of IL-10 was detected, which was significantly less (P < 0.001) than that produced by C57BL/6 peritoneal macrophages. However, TLR2^−/− peritoneal macrophages produced GM-CSF and IL-12p40 (at levels not significantly different from those detected in C57BL/6 macrophages), but significant levels of IL-12p70 were produced compared to C57BL/6 macrophages. IFN-γ^−/− murine peritoneal macrophages produced the greatest amounts of IL-6 and IL-12p40 (P < 0.001) compared to that produced by BALB/c macrophages and substantively more than C57BL/6 macrophages (Fig. 6). IFN-γ^−/− macrophages also produced significantly increased levels of IL-12p70 compared to BALB/c and C57BL/6 macrophages. Interestingly, no IL-12p70 was detected from macrophages from immunocompetent BALB/c or C57BL/6 mice. GM-CSF production was significantly enhanced in BALB/c peritoneal macrophages compared to the levels detected in IFN-γ^−/− macrophages. As with the RAW264.7 cells, IL-1α, IL-1β, and TGF-β were not produced by any of the strains of mice (Fig. 6).

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FIG. 5.

RAW264.7 cells produce proinflammatory cytokines in response to NMII. RAW 264.7 macrophages were infected with C. burnetii NMII at various bacterium-to-macrophage ratios of between 1:1 and 100:1. Macrophages were allowed to adhere overnight, infected, and cultured for 8 h, and supernatants were harvested for cytokine ELISAs. RAW264.7 cells produced increased levels of TNF-α and IL-6 and, to a lesser extent, IFN-γ and GM-CSF. A slight increase in IL-10 was measured, and no production of IL-1α, IL-1β, IL-12p40, IL-12p70, or TGF-β was observed. Samples from 24 h of infection showed a similar trend. Depicted are the means from two experiments ± standard deviations of the means.

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FIG. 6.

Peritoneal macrophages from IFN-γ^−/−, but not TLR2^−/−, mice produce proinflammatory IL-6 cytokine. Peritoneal macrophages isolated from C57BL/6, TLR2^−/−, BALB/c, and IFN-γ^−/− mice were infected with C. burnetii NMII at various bacterium-to-macrophage ratios of between 1:1 and 100:1. Data from two individual experiments for each mouse strain infected at a 100:1 ratio are shown. Adherent macrophages were allowed to adhere overnight, infected, and then cultured for 24 h, and supernatants were harvested for cytokine ELISAs. Peritoneal macrophages from C57BL/6 and BALB/c mice infected with NMII produced elevated IFN-γ, TNF-α, GM-CSF, and IL-12p40, with minimal to no IL-6 or IL-12p70. C57BL/6, but not BALB/c, macrophages produced IL-10. IFN-γ^−/− peritoneal macrophages produced significantly increased levels of IL-6, IL-10, IL-12p40, and IL-12p70 compared to BALB/c macrophages. TLR2^−/− macrophages produced only GM-CSF, IL-12p40, IL-12p70, and some IL-10. None of the infected macrophages produced IL-1α, IL-1 β, or TGF-β. Statistical analyses were performed for each described cytokine between infected C57BL/6 versus TLR2^−/− macrophages and between infected BALB/c versus IFN-γ^−/− macrophages. *, P < 0.001. Cytokines were not detected in uninfected macrophages. Error bars indicate standard deviations of the means.

IgG subclass titers are reduced in TLR2^−/− mice.In order to ascertain the nature of the protection observed in immunodeficient mice after infection with NMII, heat-killed Coxiella-specific IgG, IgG1, IgG2a, and IgG2b antibody responses after one or two infections with C. burnetii NMII were measured by ELISA. Remarkably, no IgG1 responses were detected in any of the serum samples from immunocompetent or immunodeficient mice after one or two infections with NMII. In BALB/c and IFN-γ^−/− mice, a second infection with NMII caused significant increases in the serum IgG and IgG2a antibody responses (Fig. 7A). In IFN-γ^−/− mice, IgG2b levels were also significantly higher (P < 0.001) in mice infected twice with NMII than in mice infected once. IgG and IgG2a antibody responses were significantly greater (P < 0.001) in BALB/c mice than in IFN-γ^−/− mice when mice were infected once or twice with NMII. No significant differences were observed in serum IgG2b antibody responses by BALB/c and IFN-γ^−/− mice.

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FIG. 7.

Heat-killed Coxiella-specific IgG, IgG1, IgG2a, and IgG2b responses by immunocompetent or immunodeficient mice infected once or twice with C. burnetii NMII. No IgG1 responses were detected in any of the sera of immunocompetent or immunodeficient mice. (A) IgG and IgG2a responses were significantly greater (P < 0.001) in BALB/c mice and IFN-γ^−/− mice infected twice than after a single infection. In IFN-γ^−/− mice, serum levels of IgG2b were significantly greater (P < 0.001) in mice challenged twice with NMII than in mice infected once. IgG and IgG2a antibody responses were significantly greater (P < 0.001) in BALB/c mice than in IFN-γ^−/− mice when mice were infected once or twice with NMII. No significant differences were observed in IgG2b serum levels. (B) IgG, IgG2a, and IgG2b responses were significantly greater (P < 0.001) in C57BL/6 mice infected twice than in those infected once. In TLR2^−/− mice, significant increases in IgG, IgG2a, and IgG2b levels were observed after two infections compared to mice infected once with NMII. IgG levels were significantly reduced compared to those in C57BL/6 mice after a single infection. IgG2a levels in TLR2^−/− mice infected twice with NMII were significantly higher (P < 0.001) than those in C57BL/6 mice infected and subsequently reinfected. By contrast, IgG2b levels in C57BL/6 mice infected twice were significantly greater than those in TLR2^−/− mice infected and reinfected with NMII. Depicted are the means ± standard deviations of the serum IgG end point titers (log₂). *, P < 0.001, for log₂ end point titers of twice- versus once-infected mice. †, P < 0.001 for BALB/c versus IFN-γ^−/− mice and for C57BL/6 versus TLR2^−/− mice.

In twice-infected C57BL/6 mice, serum IgG, IgG2a, and IgG2b antibody responses were significantly more elevated than those in singly infected mice (Fig. 7B) (P < 0.001). In TLR2^−/− mice, significant increases (P < 0.001) in IgG, IgG2a, and IgG2b levels were observed after two NMII infections than after one infection. IgG antibody responses were significantly lower in TLR2^−/− mice infected once with NMII than in C57BL/6 mice, but no differences were observed after two infections. IgG2a production was enhanced in TLR2^−/− mice infected twice compared to twice-infected C57BL/6 mice. By contrast, IgG2b levels in twice-infected TLR2^−/− mice were significantly reduced (P < 0.001) compared to those in twice-infected C57BL/6 mice.

Th cell cytokine responses following antigen restimulation.The lack of IgG1 antibodies in serum and the high production of IgG2a antibodies regardless of the mouse strain tested suggest that Th1-type immune responses are induced. Thus, antigen restimulation assays were conducted to ascertain the cytokine profiles in response to Coxiella antigens. BALB/c, IFN-γ^−/−, C57BL/6, and TLR2^−/− mice were infected i.p. with C. burnetii NMII, and at 10 days postinfection splenic lymphocytes were isolated and restimulated with heat-killed Coxiella (Table 1). Elevations in IFN-γ were detected in BALB/c and C57BL/6 lymphocyte cultures but to a lesser extent in TLR2^−/− lymphocyte cultures. Interestingly, IFN-γ^−/− lymphocytes produced significantly higher levels of IL-10 (P < 0.001) in the absence of IFN-γ pressure than lymphocytes from BALB/c or C57BL/6 mice (Table 1). There were no differences detected in the production of IL-6 in BALB/c and IFN-γ^−/− mice, but it was elevated in C57BL/6 mice. No TNF-α, IL-4, IL-13, or TGF-β was detected in any of the restimulation lymphocyte cultures from any of the infected mice. These results might offer a possible explanation for the reduction of fever and inflammation observed in immunodeficient mice after rechallenge with NMII. The protection observed in IFN-γ^−/− mice after a subsequent rechallenge may be IL-10 dependent.

Immune T and B cells confer long-term protection against fever.In our previous experiments, the results showed that a prior exposure to C. burnetii NMII in immunodeficient mice induced protection against a subsequent challenge 4 weeks later. In order to ascertain whether protection was due to the activation of the innate immunity or to the development of acquired immunity, adoptive transfer studies using immune or naive T and B cells were performed. Immunocompetent and immunodeficient mice were infected i.p. with C. burnetii NMII, and 6 weeks later T and B cells were purified, combined at 1:1 ratios, and adoptively transferred (3 × 10⁶ cells/mouse) into naive recipient mice. One day after the adoptive transfer, mice were challenged with NMII, and rectal temperatures were monitored daily. Spleens were harvested at 3 or 10 days after infection and weighed, and quantitative PCR analysis was performed to detect C. burnetii genomic copies (see Fig. 10). Naive immunocompromised or immunocompetent mice adoptively transferred with immune T and B cells from immunocompetent mice exposed to NMII 6 weeks earlier were protected against fever after subsequent challenge with NMII.

When immune T and B cells from BALB/c mice were transferred into naive IFN-γ^−/− recipients, no fever episodes were observed in these mice following challenge with NMII, as opposed to the case for IFN-γ^−/− mice treated with PBS, which suffered significant increases (P < 0.001) in their body temperatures (Fig. 8C). When immune T and B cells obtained from infected IFN-γ^−/− mice were transferred into naive IFN-γ^−/− recipients, a significant reduction of fever was observed for only 2 days (days 3 and 4 postchallenge), as occurred with naive IFN-γ^−/− mice that received naive BALB/c T and B cells. Splenic weight measurements at 3 and 10 days postchallenge showed a significant reduction (P < 0.001) in inflammation after infection in IFN-γ^−/− recipients that received infected IFN-γ^−/− or infected BALB/c T and B cells compared to IFN-γ^−/− recipients receiving PBS or naive BALB/c mice cells (Fig. 8D). However, when naive BALB/c mice received naive IFN-γ^−/− or infected IFN-γ^−/− T and B cells, significant increases in their body temperatures (P < 0.001) were observed for 2 days following challenge with NMII (Fig. 8A) compared to PBS-treated BALB/c mice. In agreement with the data in Fig. 1 and 2A, no significant increases in the temperatures were observed when naive PBS-treated BALB/c mice were infected with NMII. Interestingly, when infected BALB/c cells were transferred into BALB/c mice, slight increases (not significant) in the temperatures were observed compared to the PBS group. Splenic weights of BALB/c recipients given immune T and B cells from infected BALB/c mice showed significant reductions (P < 0.001) at 3 and 10 days after challenge with NMII (Fig. 8B). BALB/c recipients given immune lymphocytes from infected IFN-γ^−/− mice showed a reduction in splenic weights only on day 3 postchallenge (Fig. 8B).

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FIG. 8.

Adoptive transfer of immune lymphocytes prevents fever after infection with NMII in IFN-γ^−/− mice. Immune splenic T and B cells were purified from BALB/c and IFN-γ^−/− mice at 6 weeks after their i.p. infection with C. burnetii NMII or from naive BALB/c and IFN-γ^−/− mice and were adoptively transferred (i.v.) into naive BALB/c or IFN-γ^−/− mice. One day later, mice were challenged with 1 × 10⁸C. burnetii organisms, and rectal temperatures were measured daily. At 3 and 10 days postchallenge, spleens were harvested and weighed. (A and B) Temperatures (A) and splenic weights (B) on day 3 or 10 postchallenge of naive BALB/c recipients given PBS, lymphocytes from once-infected BALB/c mice, or lymphocytes from naive BALB/c or IFN-γ^−/− mice. (C and D) Temperature (C) and splenic weights (D) on day 3 or 10 postchallenge of naive IFN-γ^−/− recipients given PBS, lymphocytes from once-infected BALB/c or IFN-γ^−/− mice, or lymphocytes from naive IFN-γ^−/− mice. (A and C) *, P < 0.001 for body temperatures of naive IFN-γ^−/− cell- or immunized IFN-γ^−/−cell-treated mice versus PBS-treated mice. (B and D) *, P < 0.001 for splenic weights of PBS-, naive BALB/c cell-, or naive IFN-γ^−/− cell-treated mice versus infected BALB/c or immunized IFN-γ^−/− cell-treated mice at 3 days postchallenge. †, P < 0.001, 10 days postchallenge. Error bars indicate standard deviations of the means.

Infection of C57BL/6 mice with NMII showed the expected slight increases in the body temperatures (Fig. 9A), as previously observed (Fig. 1C and 3B). When immune T and B cells obtained from infected C57BL/6, infected TLR2^−/−, or naive TLR2^−/−mice were adoptively transferred into naive C57BL/6 recipients, no significant reductions in fever were observed following infection with NMII (Fig. 9A). Moreover, no significant differences in the splenic weights of these mice were observed (Fig. 9B). However, very remarkable differences were observed in the body temperatures of TLR2^−/− recipients given immune T and B cells and subsequently challenged with NMII (Fig. 9C). When immune T and B cells from infected C57BL/6 donors were transferred into naive TLR2^−/− recipients, basically no fever was observed (P < 0.001) compared to PBS-treated TLR2^−/− mice infected with NMII, which had elevated body temperatures that lasted for more than 1 week. The reduction of fever was less apparent, although statistically significant (P < 0.001), in TLR2^−/− recipients that received immune T and B cells from infected TLR2^−/− mice compared to the PBS-treated TLR2^−/− mice. When cells from naive C57BL/6 mice were transferred into TLR2^−/− recipients, no significant differences in the temperatures were observed compared to PBS-treated mice, but significant increases (Fig. 9C) (P < 0.001) were observed compared to the body temperatures of naive TLR2^−/− recipients that received immune T and B cells from infected C57BL/6 cells. Splenic weights of naive TLR2^−/− recipients that received immune T and B cells from infected TLR2^−/− or C57BL/6 mice were significantly lower (P < 0.001) than the splenic weights from PBS-treated TLR2^−/− mice or those given naive C57BL/6 cells at 3 days after challenge (Fig. 9D). At 10 days postchallenge, a significant reduction (P < 0.001) in splenic weights of TLR2^−/− recipients given immune T and B cells from infected C57BL/6 mice was observed compared to TLR2^−/− recipients given PBS or T and B cells from naive C57BL/6 mice or from infected TLR2^−/− mice.

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FIG. 9.

Adoptive transfer of immune lymphocytes prevents fever after infection with NMII in TLR2^−/− mice. Immune splenic T and B cells were purified from C57BL/6 and TLR2^−/− mice 6 weeks after their i.p. infection with C. burnetii NMII or from naive C57BL/6 and TLR2^−/− mice and adoptively transferred (i.v.) into naive C57BL/6 or TLR2^−/− mice. One day later, mice were challenged with 1 × 10⁸C. burnetii organisms, and rectal temperatures were measured daily. At 3 and 10 days postchallenge, spleens were harvested and weighed. (A and B) Temperatures (A) and splenic weights (B) on day 3 or 10 postchallenge of naive C57BL/6 recipients given PBS, lymphocytes from once-infected C57BL/6 mice, or lymphocytes from naive C57BL/6 or TLR2^−/− mice. (C and D) Temperature (C) and splenic weights (D) on day 3 or 10 postchallenge of naive TLR2^−/− recipients given PBS, lymphocytes from once-infected C57BL/6 or TLR2^−/− mice, or lymphocytes from naive TLR2^−/−. (A and C) *, P < 0.001 for body temperatures of immunized C57BL/6 cell-, naive C57BL/6 cell-, or immunized TLR2^−/−cell-treated mice versus PBS-treated mice. (B and D) *, P < 0.001 for splenic weights of PBS-, naive C57BL/6 cell-, or naive TLR2^−/− cell-treated mice versus immunized C57BL/6 cell- or immunized TLR2^−/− cell-treated mice at 3 days postchallenge. †, P < 0.001, 10 days postchallenge. Error bars indicate standard deviations of the means.

Quantitative PCR analysis was performed on spleens at 3 and 10 days after challenge with C. burnetii NMII, and the data revealed that the adoptive transfer of immune T and B cells obtained from BALB/c and C57BL/6 mice prevented the colonization by Coxiella NMII (Fig. 10). The genomic DNA copy numbers for NMII in spleens of IFN-γ^−/− mice treated with immune T and B cells from infected BALB/c or IFN-γ^−/− mice were significantly reduced (P < 0.001) compared to those in PBS-treated or naive BALB/c mice (Fig. 10C) at 3 and 10 days after challenge. BALB/c mice treated with lymphocytes from immunized BALB/c or IFN-γ^−/− mice showed a significant reduction in the genomic copies at 10 days postchallenge (Fig. 10A). No reductions in the NMII genomic copies were observed in C57BL/6 recipient mice (Fig. 10B). By contrast, TLR2^−/− mice that received immune T and B cells from infected C57BL/6 or TLR2^−/− mice showed a significant reduction (P < 0.001) in NMII colonization at 3 and 10 days after challenge. These results demonstrate that the immune response induced in immunocompetent mice after a prior exposure with NMII possesses an acquired component that lasts for at least 6 weeks and is able to protect against a subsequent challenge with C. burnetii NMII.

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FIG. 10.

Adoptive transfer of immune lymphocytes prevents splenic colonization by Coxiella NMII. Quantitative PCR was performed for C. burnetii detection in spleens of recipient mice used in the adoptive transfer and challenge studies as described for Fig. 8 and 9. Immune or naive splenic T and B cells from BALB/c or IFN-γ^−/− mice were adoptively transferred (i.v.) into naive BALB/c (A) or IFN-γ^−/− (C) recipients; immune or naive T and B cells from C57BL/6 or TLR2^−/− mice were adoptively transferred into naive C57BL/6 (B) or TLR2^−/− (D) recipients. Detection of DNA genomic copies was performed at 3 and 10 days postinfection. (A) BALB/c mice treated with immunized BALB/c or IFN-γ^−/− cells showed a significant reduction in the DNA genomic copies at 10 days postchallenge. (C) Detection in IFN-γ^−/− mice that received immune T and B cells from infected BALB/c or IFN-γ^−/− mice was reduced significantly (P < 0.001) compared to that in PBS- or naive BALB/c cell-treated mice at 3 and 10 days after challenge. (B) No reductions in the Coxiella genomic copies were observed in C57BL/6 recipient mice. (D) In contrast, TLR2^−/− mice that received immune T and B cells from infected C57BL/6 or TLR2^−/− mice showed a significant reduction (P < 0.001) in the presence of Coxiella at 3 and 10 days after challenge. *, P < 0.001 for splenic Coxiella DNA genomic copies in recipient mice given PBS or naive lymphocytes versus recipients given immune lymphocytes at 3 days postchallenge; †, P < 0.001, the same for 10 days postchallenge. Data represent the mean for five mice per time point ± standard deviations of the means.