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Clinical and Diagnostic Laboratory Immunology, November 2001, p. 1204-1212, Vol. 8, No. 6
1071-412X/01/$04.00+0   DOI: 10.1128/CDLI.8.6.1204-1212.2001

Modulation of Mycobacterium bovis-Specific Responses of Bovine Peripheral Blood Mononuclear Cells by 1,25-Dihydroxyvitamin D3

W. R. Waters,1,* B. J. Nonnecke,2 T. E. Rahner,1 M. V. Palmer,1 D. L. Whipple,1 and R. L. Horst2

Bacterial Diseases of Livestock Research Unit1 and Periparturient Diseases of Livestock Research Unit,2 National Animal Disease Center, Agricultural Research Service, United States Department of Agriculture, Ames, Iowa 50010-0070

Received 8 May 2001/Returned for modification 17 July 2001/Accepted 7 August 2001


    ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Historically, administration of vitamin D has been considered beneficial in the treatment of tuberculosis. The interaction of this vitamin {i.e., 1,25-dihdroxyvitamin D3 [1,25(OH)2D3]} with the antitubercular immune response, however, is not clear. In the present study, in vitro recall responses of peripheral blood mononuclear cells (PBMC) from cattle infected with Mycobacterium bovis were used to study the immune-modulatory effects of 1,25(OH)2D3 on M. bovis-specific responses in vitro. Addition of 1 or 10 nM 1,25(OH)2D3 inhibited M. bovis-specific proliferative responses of PBMC from M. bovis-infected cattle, affecting predominately the CD4+ cell subset. In addition, 1,25(OH)2D3 inhibited M. bovis-specific gamma interferon (IFN-gamma ) production yet enhanced M. bovis-specific nitric oxide (NO) production. Lymphocyte apoptosis, measured by flow cytometry using annexin-V staining, was diminished by addition of 1,25(OH)2D3 to PBMC cultures. These findings support the current hypothesis that 1,25(OH)2D3 enhances mycobacterial killing by increasing NO production, a potent antimicrobial mechanism of activated macrophages, and suggest that 1,25(OH)2D3 limits host damage by decreasing M. bovis-induced IFN-gamma production.


    INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Before the discovery of effective antimycobacterial drugs, vitamin D therapy in the form of cod liver oil and exposure to sunlight (e.g., heliotherapy) were used to treat human tuberculosis (18). Vitamin D is derived from two sources: dietary intake and by the conversion of 7-dehydrocholesterol to cholecalciferol (i.e., pre-vitamin D) in the skin by a reaction catalyzed by UV light. At body temperature cholecalciferol spontaneously converts to vitamin D3. Vitamin D binding protein aids in the transport of vitamin D3 from the skin to the liver, where it is converted to 25-hydroxyvitamin D3 [25(OH)D3], the predominant circulating form of vitamin D. In response to hypocalcemic states, 25(OH)D3 is hydroxylated to form 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], the major mediator of the biological activity of vitamin D. In humans, circulating concentrations of 25(OH)D3 range from 55 to 75 nM, and circulating concentrations of 1,25(OH)2D3 range from 0.062 to 0.082 nM (55). Vitamin D metabolites play a key role in short-term calcium homeostasis in humans and other animals. Experimental evidence suggests that vitamin D metabolites also modulate specific aspects of immune function (52).

Impaired formation of vitamin D3 in the skin often results in measurable reductions in 25(OH)D3 concentrations in plasma. For instance, concentrations of 25(OH)D3 in plasma are lower in Asians living in Great Britain (48) and Zairians living in Belgium (35) compared to control individuals with less pigmented skin, presumably due to diminished synthesis. Tuberculosis, likewise, is common among individuals with heavily pigmented skin that relocate from equatorial regions to higher latitudes, in part due to deficiencies in vitamin D synthesis within the skin (65). In addition, patients with untreated tuberculosis often have lower concentrations of 25(OH)D3 in plasma than do healthy subjects, and tuberculosis tends to occur during the winter when exposure to sunlight is reduced and production of cholecalciferol within the skin is diminished (16). Evidence for a clear correlation between vitamin D deficiency and susceptibility to tuberculosis, however, remains controversial. In vitro studies, however, provide more compelling evidence linking vitamin D status to susceptibility to tuberculosis.

Addition of 1,25(OH)2D3 to monocyte-macrophage cultures infected with Mycobacterium tuberculosis suppresses bacterial growth and viability (17, 53, 54). The mechanism of this suppression is mediated, at least partially, by nitric oxide (NO) (53). Induction of inducible NO synthase of macrophages and subsequent generation of reactive nitrogen intermediates (RNI) toxic to mycobacteria is a potent mechanism of killing (14, 15, 21, 22, 34). Cytokines (e.g., tumor necrosis factor alpha and gamma interferon [IFN-gamma ]) from antigen-specific T cells and/or from macrophages stimulated directly with mycobacterial antigens is responsible for RNI-mediated antimycobacterial defense (24, 60). Production of RNI is crucial for controlling acute as well as latent infections in the mouse model of virulent M. tuberculosis infection (14, 15, 25, 34). The role of RNI in mycobacterial killing within human macrophages is less clear. Alveolar macrophages of tuberculosis patients express high levels of inducible NO synthase, suggesting a role for RNI in disease pathogenesis and/or host defense (42). Nevertheless, recent evidence suggests that human but not mouse macrophages utilize NO-independent mechanisms (e.g., via Toll-like receptors) for intracellular killing of the tubercle bacilli (60). This species-specific difference in mycobacterial killing may reflect coevolutionary pressures between M. tuberculosis and its natural host, humans.

Primary infection of mice with M. tuberculosis results in the generation of highly reactive IFN-gamma -producing, CD4+ cells that provide long-lived immunologic memory (5). This T-cell subset, however, is not sufficient for clearance of the primary infection, as antibiotic therapy is necessary for resolution of the initial infection. Upon reexposure to the pathogen, the recall response of the IFN-gamma -producing CD4+ cells is greatly accelerated and infection is controlled without the use of antibiotics. Immediate production of IFN-gamma by CD4+ cells upon exposure to the bacilli, therefore, appears essential for immune-mediated protection in the secondary response (13). AIDS patients with depressed CD4+ cell counts are remarkably susceptible to tuberculosis, further demonstrating the essential role of CD4+ cells in the host response to infection (7). CD8+ and gamma delta T cells are also involved in the antituberculous immune response in mice, humans, and cattle (28, 29, 30, 32, 50, 57). Mice depleted of CD8+ cells by treatment with monoclonal antibodies to murine CD8 as well as mice genetically deficient in CD8+ cells are more susceptible to M. tuberculosis infection than are mice with intact CD8+ cell populations (23, 36). Mycobacterium-specific CD8+ and gamma delta T-cell clones have been established from infected individuals, demonstrating a potential role for these subsets in the host response to M. tuberculosis (20, 38). gamma delta T cells also respond to various mycobacterial antigens and accumulate at infection sites (6, 9, 27, 59). Production of IFN-gamma by CD4+, CD8+, and/or gamma delta T-cell receptor positive (TCR+) cells leads to activation of macrophages and enhanced killing of intracellular mycobacteria (11, 12, 28, 32, 41). Mycobacterium-specific cytotoxic T cells (both CD4+ and CD8+) may also be important in the clearance of M. bovis (39, 44, 47). Together, these studies demonstrate the complexity and redundancy of the host response during tuberculosis as well as potential sites for immune modulation with compounds such as 1,25(OH)2D3.

Tuberculosis in humans results from infection with any one of the tubercle bacilli included within the M. tuberculosis complex (e.g., M. tuberculosis, M. bovis, M. africanum, and M. microti). M. bovis, unlike M. tuberculosis, has a wide host range and is the species most often isolated from tuberculous cattle. The wide host range of M. bovis has made its eradication difficult due to the presence of wildlife reservoir hosts. An outbreak of M. bovis in 1994 in white-tailed deer in Michigan has seriously threatened M. bovis eradication efforts in the United States, renewing research interests of this zoonotic agent and economically important pathogen of domestic livestock. In addition to the animal health issues of M. bovis infections of cattle, this infection also represents a potentially useful animal model for M. tuberculosis infection of humans. In the present study, the capacity of 1,25(OH)2D3 to modulate recall proliferative, NO, and IFN-gamma responses of peripheral blood mononuclear cells (PBMC) from cattle experimentally infected with M. bovis was investigated.


    MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Animals, bacterial culture, and challenge procedures. Eight Hereford-cross cattle (four males and four females) approximately 6 months old were obtained from herds with no history of tuberculosis and were housed at the National Animal Disease Center, U.S. Department of Agriculture, Agriculture Research Service, Ames, Iowa, according to institutional guidelines for animal care. At the initiation of the study, all animals were tested and confirmed negative for M. bovis exposure using both the comparative cervical test (CCT) (61) for delayed-type hypersensitivity and the Bovigam assay (CSL Limited, Parkville, Victoria, Australia) for detection of IFN-gamma production in response to M. bovis antigen stimulation. Cattle received water ad libitum and a balanced ration consisting of pelleted alfalfa and grain during the study. Infected cattle were housed in temperature- and humidity-controlled rooms (one to two animals/room) within a biosafety level 3 confinement facility with negative airflow exiting the building through high-efficiency particulate air (HEPA) filters. Directional airflow assured that air from animal pens was pulled towards a central corridor and passed through HEPA filters before exiting the building. Airflow velocity was 10.4 air changes/minute. Noninfected control cattle were housed similarly in a separate building. Personnel in contact with M. bovis-infected animals wore full-face, HEPA-filtered respirators.

The strain of M. bovis used for the challenge inoculum (strain 1315) was isolated from a white-tailed deer in Michigan in 1994 (56). The challenge inoculum consisted of ~105 CFU of mid-log-phase M. bovis cultures grown in Middlebrook's 7H9 medium supplemented with 10% oleic acid-albumin-dextrose complex (OADC; Difco, Detroit, Mich.) plus 0.05% Tween 80 (Sigma Chemical Co., St. Louis, Mo.) as previously described (8). To harvest tubercle bacilli from the culture medium, cells were pelleted by centrifugation at 750 × g, washed twice with 1 ml of phosphate-buffered saline solution (0.01 M, pH 7.2) (PBS), and diluted to the appropriate cell density in 2 ml of PBS. Enumeration of bacilli was by serial dilution plate counting on Middlebrook's 7H11 selective medium (Becton Dickinson, Cockeysville, Md.). For intratonsillar inoculation, cattle (n = 2) were restrained and anesthetized with 500 mg of ketamine (Fort Dodge Animal Health, Fort Dodge, Iowa) and 30 mg of xylazine (Bayer Corp., Shawnee Mission, Kans.) given intravenously. Effects of xylazine were reversed by intravenous administration of yohimbine (0.2 mg/kg; Lloyd Laboratories, Shenandoah, Iowa). The challenge inoculum was instilled directly into the tonsillar crypts of anesthetized cattle as previously described for inoculation of white-tailed deer (45). For aerosol inoculation, cattle (n = 3) were restrained and lightly sedated with 5 to 10 mg of xylazine (Bayer Corp.), and the challenge inoculum was delivered by nebulization into a mask covering the animal's nostrils and mouth. The nebulization apparatus consisted of a compressed air tank, jet nebulizer, holding reservoir, and mask (Trudell Medical International, London, Ontario, Canada). Compressed air (25 lb/in2) was used to jet nebulize the challenge inoculum (2-ml volume of ~105 CFU of M. bovis in PBS) directly into the holding reservoir. Upon inspiration, the nebulized inoculum was inhaled through a one-way valve into the mask and directly into the nostrils. A rubber gasket sealed the mask securely to the muzzle, preventing leakage of inoculum around the mask. Expired air exited through one-way valves on the sides of the mask. The nebulization process was continued until all of the inoculum, a 1-ml PBS wash of the inoculum tube, and an additional 2 ml of PBS were delivered (~12 min). Strict biosafety level 3 protocols were followed to protect personnel from exposure to M. bovis. At the conclusion of the experiment, cattle were euthanatized by intravenous administration of sodium pentobarbital (Sleepaway; Fort Dodge Laboratories). Lesions typical of M. bovis infection were detected in M. bovis-inoculated animals, and infection was confirmed by isolation of M. bovis from tissues of M. bovis-inoculated cattle. Pathological and bacteriologic findings will be presented elsewhere (M. V. Palmer, W. R. Waters, and D. L. Whipple, submitted for publication).

Lymphocyte blastogenesis. Mononuclear cells were isolated from buffy coat fractions of peripheral blood collected in 2× acid citrate dextrose (10). Wells of 96-well round-bottom microtiter plates (Falcon, Becton Dickinson; Lincoln Park, N.J.) were preloaded with 1,25(OH)2D3 solubilized in 100% ethanol or with ethanol alone [i.e., no 1,25(OH)2D3] in a 10-µl volume. Ethanol was then allowed to evaporate, leaving the 1,25(OH)2D3 at the desired concentration (i.e., 0, 1, or 10 nM). Wells were then seeded with 2 × 105 mononuclear cells in a total volume of 200 µl per well. Medium was RPMI 1640 supplemented with 25 mM HEPES buffer, penicillin (100 U/ml), streptomycin (0.1 mg/ml), 50 µM 2-mercaptoethanol (Sigma), and 10% (vol/vol) fetal bovine serum (FBS). Wells contained medium plus M. bovis purified protein derivative (PPD) (5 µg/ml; CSL Limited), rESAT-6 (1 µg/ml; kindly provided by F. C. Minion, Iowa State University), M. bovis strain 1315 culture filtrate (CF) (5 µg/ml), pokeweed mitogen (PWM) (2 µg/ml), or medium alone (no stimulation). The CF was from 2-week M. bovis strain 1315 cultures (bacteria were pelleted, and supernatant was harvested and filtered [0.22-µm pore size] twice). Leukocyte cultures were incubated for 5 days at 37°C in 5% CO2 in air. After 5 days, 0.5 µCi of [methyl-3H]thymidine (specific activity, 6.7 Ci mmol-1; Amersham Life Science, Arlington Heights, Ill.) in 50 µl of medium was added to each well, and cells were incubated for an additional 20 h. Well contents were harvested onto fiber filters with a 96-well plate harvester (EG & G Wallac, Gaithersburg, Md.), and the incorporated radioactivity was measured by liquid scintillation counting. Treatments were run in triplicate, and results are presented as mean counts minute-1.

PKH67 proliferation assay. The PKH67 proliferation assay was performed according to manufacturer instructions (Sigma) and as previously described (62). Briefly, 2 × 107 PBMC were centrifuged (10 min, 400 × g), supernatants were aspirated, and cells were resuspended in 1 ml of diluent provided in the PKH67 kit (Sigma). Diluted cells were added to 1 ml of PKH67 green fluorescent dye (2 µM; Sigma) and incubated for 5 min, followed by a 1-min incubation with 2 ml of FBS to stop the reaction. Cells were then washed (10 min, 400 × g) three times with RPMI 1640. Wells of 96-well round-bottom microtiter plates were precoated with 1,25(OH)2D3 as described for the blastogenesis procedure. PKH67-stained cells were then added to wells (2 × 105/well; six replicates per treatment [e.g., no stimulation or PPD]) of 96-well round-bottom microtiter plates in medium (no stimulation) or medium plus M. bovis PPD (5 µg/ml; CSL Limited). Cultures were incubated for 6 days at 37°C in a humidified chamber with 5% CO2.

Flow cytometry. At the conclusion of the incubation period, cells were analyzed by flow cytometry (FACScan; Becton Dickinson, San Jose, Calif.) for PKH67 staining as well as cell surface marker expression. Modfit Proliferation Wizard (Verity Software House Inc., Topsham, Maine) and CellQuest software (Becton Dickinson) were used for cell proliferation and phenotype analyses. Proliferation profiles were determined as the number of cells proliferating in PPD-stimulated wells minus the number of cells proliferating in nonstimulated wells for both gated (i.e., CD4+, CD8+, or gamma delta TCR+) or ungated (total PBMC) populations. Appropriate isotype control antibodies were used for both the nonstimulated and PPD-stimulated wells as a control for nonspecific binding of lymphocyte subset antibodies to activated cells. Data are presented as the mean (± standard error of the mean [SEM]) number of cells that had proliferated per 10,000 PBMC.

Mononuclear cells were analyzed for PKH67 staining (FL1), cell surface antigen expression (FL3), and annexin V staining (FL2) by flow cytometry. Cells (2 × 106/ml) in 100 µl of balanced salt solution with 1% FBS and 0.1% sodium azide (FACS buffer) were stained with 100 µl of primary antibody to leukocyte surface antigens (CACT138A, anti-CD4; CACT80C, anti-CD8alpha ; and BAQ4A, anti-WC1 [VMRD, Pullman, Wash.]). After a 15-min incubation, cells were centrifuged (400 × g, 2 min) and resuspended in 100 µl of peridinin chlorophyll protein-conjugated goat anti-mouse immunoglobulin G1 (Becton Dickinson). Cells were then incubated for an additional 15 min, centrifuged (400 × g, 2 min), resuspended in 200 µl of 1× annexin V binding buffer (Pharmingen, San Diego, Calif.), and stained with 4 µl of annexin V-phycoerythrin (Pharmingen). Cells were then analyzed using a Becton Dickinson FACScan flow cytometer (10,000 events, live gate, three-color analysis, 488-nm laser).

IFN-gamma ELISA. Wells of 96-well round-bottom microtiter plates were preloaded with 1,25(OH)2D3 as described for the blastogenesis procedure. Isolated mononuclear cells were then added to wells (2 × 105/well, six replicates) with PPDb (5 µg/ml), rESAT-6 (1 µg/ml), CF (5 µg/ml), PWM (1 µg/ml), or medium alone. Plates with cells were then incubated at 37°C in a 5% CO2 humidified chamber. Supernatants were harvested after 24, 48, and 72 h of culture and analyzed for IFN-gamma using a commercial enzyme-linked immunosorbent assay (ELISA)-based kit (Bovigam; CSL Limited).

Nitric oxide assay. Nitrite is the stable oxidation product of NO, and the amount of nitrite within culture supernatants is indicative of the amount of NO produced by cells in culture. Nitrite was measured using the Griess reaction (49) performed in 96-well microtiter plates (Immunolon 2; Dynatech Laboratories, Inc., Chantilly, Va.). Culture conditions were as described for the lymphocyte blastogenesis assay. Culture supernatant (100 µl) was mixed with 100 µl of Griess reagent (0.5% sulfanilamide; Sigma) in 2.5% phosphoric acid (Mallinckrodt Chemicals, Inc., Paris, Ky.) and 0.05% N-(1-naphthyl) ethylenediamine dihydrochloride (Sigma). The mixture was incubated at 21°C for 10 min. Absorbances of test and standard samples at 550 nm were measured using an automated ELISA plate reader (Molecular Devices, Menlo Park, Calif.). All dilutions were made using culture medium (RPMI 1640 medium with 2 mM L-glutamine and 10% [vol/vol] FBS). Absorbances of standards, controls, and test samples were converted to nanograms per milliliter of nitrite by comparison with absorbances of sodium nitrite (Fisher Chemicals, Fair Lawn, N.J.) standards within a linear curve fit. NG-Monomethyl-L-arginine (L-NMMA) (Calbiochem, La Jolla, Calif.), a competitive inhibitor of the enzyme NO synthase (NOS), (1.15 mM; equimolar to the amount of L-arginine in the culture medium) was added to parallel cultures to verify that the nitrite produced was due to the activity of NOS.

Statistical analysis. Data were assessed for normality prior to statistical analysis. Arithmetic or log10-transformed data were analyzed as a split plot with repeated measures analysis of variance using Statview software (version 5.0; SAS Institute, Inc., Cary, N.C.). Concentrations of 1,25(OH)2D3 in unstimulated and CF-, rESAT6-, PPD-, and PWM-stimulated cultures and their interactions constituted the main plot, and incubation period (in hours) was the repeated measure or the split plot. Fisher's protected-least-significant-difference test was applied when treatment effects (P <=  0.05) were detected by the model. Pearson's product-moment correlations were computed between IFN-gamma and NO concentrations in supernatants from unstimulated and CF-, rESAT-6-, PPD-, and PWM-stimulated cultures and were considered significant at P < 0.1.


    RESULTS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Effects of 1,25(OH)2D3 on lymphocyte proliferation. Addition of 1 or 10 nM 1,25(OH)2D3 decreased DNA synthesis in PBMC isolated from M. bovis-infected animals. Responses to unstimulated cultures (background proliferation) and cultures stimulated with M. bovis antigens (PPD, rESAT-6, and CF) and PWM are shown in Table 1. Analysis of PBMC proliferation using a flow cytometry-based assay (e.g., the PKH67 assay) demonstrated that addition of 1,25(OH)2D3 decreased proliferation of PBMC from M. bovis-infected animals in response to M. bovis PPD (Table 2) (e.g., the mean proliferative responses of total cells from M. bovis infected animals). Flow cytometry-based proliferation assays are more informative than the blastogenesis assay because they allow simultaneous characterization of proliferative responses of individual lymphocyte subsets and evaluation of proliferation throughout the culture period (not just the terminal 20-h period, as with radiometric techniques) (2). As previously reported (64), cells responding to PPD from M. bovis-infected cattle were predominantly CD4+ and WC1+ (i.e., gamma delta TCR+) cells. Responses of CD8+ cells from M. bovis-infected cattle to PPD were negligible (data not shown). While proliferative responses were detected for both CD4+ and WC1+ T-cell subsets, addition of 1,25(OH)2D3 decreased CD4+ cell proliferation but not WC1+ cell proliferation in samples from infected animals (Table 2).

                              
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  		TABLE 1.   Addition of 1,25(OH)2D3 to PBMC cultures and decrease in lymphocyte blastogenesis as measured by [methyl-3H]thymidine uptakea


                              
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  		TABLE 2.   Effects of 1,25(OH)2D3 on lymphocyte subset proliferation in response to stimulation with M. bovis PPD

Addition of 1,25(OH)2D3 to PPD-stimulated cultures also diminished the number of CD4+ cells that had proliferated through multiple generations. Addition of 1,25(OH)2D3 decreased the percentage of cells in generations of PPD-stimulated cultures that had gone through the most divisions [e.g., 32, 22, and 11% for 0, 1, and 10 nM 1,25(OH)2D3, respectively, for a representative M. bovis-infected animal] (Fig. 1). The generation that had proceeded through the next greatest number of divisions was also decreased in 1,25(OH)2D3-treated cultures compared to nontreated, control cultures [e.g., 46, 32, and 32% for 0, 1, and 10 nM 1,25(OH)2D3, respectively] (Fig. 1). Although this finding was most apparent for the CD4+ cell subpopulation, WC1+ cells within 1,25(OH)2D3-treated cultures also had a decreased percentage of cells within elder generations compared to control cultures (data not shown). The number of WC1+ cells that had proliferated, however, was not lower in the presence of 1,25(OH)2D3 (Table 2).


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  		FIG. 1.   Addition of 1,25(OH)2D3 decreases the percentage of proliferating CD4+ cells within the eldest generations. PBMC were cultured with no stimulation (A to C) or with M. bovis PPD (5 µg/ml) (D to F). In addition, cultures received either 0 (A and D), 1 (B and E), or 10 (C and F) nM 1,25(OH)2D3. After a 6-day incubation, cells were harvested, stained with either CACT138A, anti-CD4; CACT80C, anti-CD8alpha ; or BAQ4A, anti-WC1 and analyzed by flow cytometry for PKH67 intensity and cell surface marker expression. After flow cytometric analysis, data were analyzed by using the Modfit Proliferation Wizard to determine the number of cells that had proliferated (grey peaks). A representative response from a single M. bovis-infected animal is depicted. Gates for this particular sample were set on live (e.g., based upon light scatter properties) and CD4+ cells. Black peaks depict the parent generations (e.g., PKH67 bright), whereas daughter generations are depicted with peaks in various shades of grey.

Effects of 1,25(OH)2D3 on lymphocyte apoptosis. A significant sequela to lymphocyte proliferation and activation is apoptosis (1). To determine the effect of 1,25(OH)2D3 on apoptosis of lymphocytes during an in vitro recall response, PBMC from M. bovis-infected cattle were incubated with medium alone (e.g., nonstimulated) or incubated with PPD for 6 days and analyzed for annexin V staining. Although not statistically significant (P > 0.05), there was a trend for a lower percentage of annexin V-positive cells and a concurrent lower percentage of cells located in the "apoptotic gate" and a higher percentage of cells located in the "live gate" in PPD-stimulated cultures supplemented with 1,25(OH)2D3 compared to nonsupplemented PPD-stimulated cultures (Table 3). This trend was not detected for nonstimulated PBMC. The apoptotic and live gates were based upon forward and side light scatter properties as well as 7-amino-actinomycin and annexin V staining properties (data not shown). Inhibition of apoptosis by 1,25(OH)2D3 was similar for each T-cell subset (CD4+, CD8+, and WC1+ cells) tested.

                              
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  		TABLE 3.   Inhibition of apoptosis by addition of 1,25(OH)2D3 to lymphocyte culturesa

Antigen-specific IFN-gamma and NO responses of M. bovis-infected cattle. Prior to evaluating the effects of 1,25(OH)2D3 on M. bovis-specific IFN-gamma and NO responses, the capacity of PBMC from infected cattle to produce IFN-gamma and NO in response to stimulation with M. bovis antigens was determined. IFN-gamma secretion by PBMC from M. bovis-infected cattle stimulated with either M. bovis CF, PPD, or rESAT-6 exceeded (P < 0.05) IFN-gamma secretion by nonstimulated, autologous PBMC (Fig. 2a). In addition, the IFN-gamma response of PBMC from M. bovis-infected cattle that were stimulated with CF or PPD was greater (P < 0.01) than the response of PBMC from noninfected cattle stimulated with CF or PPD, respectively. Greater (P < 0.05) concentrations of nitrite were also detected in supernatants from CF- or PPD-stimulated samples from M. bovis-infected cattle compared to concentrations of nitrite in supernatants from CF- or PPD-stimulated samples from noninfected cattle or nonstimulated samples from M. bovis-infected cattle (Fig. 2b). Nitrite levels in rESAT-6-stimulated samples from infected cattle were greater (P < 0.01) than levels in nonstimulated samples from infected cattle. IFN-gamma and NO responses of PBMC from control, noninfected cattle to M. bovis antigens (PPD, CF, or rESAT-6) were not greater (P > 0.05) than responses to medium alone (no stimulation). IFN-gamma and NO responses of PWM-stimulated PBMC were always greater (P < 0.05) than corresponding responses of nonstimulated PBMC regardless of infection status.


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  		FIG. 2.   Antigen-specific IFN-gamma and NO responses of M. bovis-infected cattle. PBMC were cultured with no stimulation (NS) or with either M. bovis strain 1315 CF (5 µg/ml), M. bovis PPD (5 µg/ml), ESAT-6 (1 µg/m), or PWM (2 µg/ml). Supernatants were harvested after 72 h for detection of IFN-gamma by ELISA (a) and detection of nitrite by Griess reaction (b). PBMC were obtained from noninfected cattle (n = 3 [closed bars]) and M. bovis-infected cattle (n = 2 [hatched bars]). Addition of L-NMMA (at a concentration equimolar to the amount of L-arginine in the culture medium), a competitive inhibitor of the enzyme NOS, inhibited nitrite production to levels detected in medium alone (e.g., background levels; data not shown). For a specific stimulant, responses of infected cattle differ from responses of controls. Symbols: *, P < 0.01; **, P < 0.05; ***, P < 0.001. Error bars, SEM.

Effect of 1,25(OH)2D3 on M. bovis-specific IFN-gamma and NO responses. Nonstimulated and stimulated (i.e., M. bovis antigens and PWM) PBMC from M. bovis-infected and noninfected cattle were cultured with 0, 1, or 10 nM 1,25(OH)2D3. Addition of 1,25(OH)2D3 enhanced (P < 0.01) nitrite production by PWM-stimulated PBMC from M. bovis-infected cattle (Fig. 3). Addition of 1,25(OH)2D3 also enhanced (P < 0.05) CF-, rESAT-6-, and PPD-specific production of nitrite by PBMC from infected cattle. As determined previously (3, 4), 1,25(OH)2D3 inhibits IFN-gamma production by antigen (ovalbumin)-stimulated PBMC from ovalbumin-sensitized cattle. A similar trend was also detected for M. bovis-specific responses by PBMC from M. bovis-infected cattle, with 1,25(OH)2D3 inhibiting (P < 0.05) IFN-gamma production in response to both CF and PPD (Fig. 4). While 1,25(OH)2D3 enhanced nitrite production in response to rESAT-6 (Fig. 3), significant modulation by 1,25(OH)2D3 of IFN-gamma responses to rESAT-6 stimulation of PBMC from infected cattle was not detected (Fig. 4).


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  		FIG. 3.   Addition of 1,25(OH)2D3 increases M. bovis-specific and PWM-stimulated production of nitrite by PBMC from M. bovis-infected cattle (n = 5). Mononuclear cells were cultured or with either M. bovis strain 1315 CF (5 µg/ml) (a), rESAT-6 (1 µg/ml) (b), M. bovis PPD (5 µg/ml) (c), or PWM (2 µg/ml) (d). To each of these treatments either no vitamin D, 1 nM 1,25(OH)2D3, or 10 nM 1,25(OH)2D3 was added as described in Materials and Methods. Supernatants were harvested after 24, 48, or 72 h for detection of nitrite by the Griess reaction as an indication of NO production. Symbols: *, P < 0.1; **, P < 0.05; ***, P < 0.001 (differs from unsupplemented [no vitamin D] cultures at specific times). Error bars, SEM.


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  		FIG. 4.   Addition of 1,25(OH)2D3 decreases M. bovis-specific production of IFN-gamma by PBMC from M. bovis-infected cattle (n = 5). Mononuclear cells were cultured with either M. bovis strain 1315 CF (5 µg/ml) (a), rESAT-6 (1 µg/ml) (b), M. bovis PPD (5 µg/ml) (c), or PWM (2 µg/ml) (d). To each of these treatments either no vitamin D, 1 nM 1,25(OH)2D3, or 10 nM 1,25(OH)2D3 was added as described in Materials and Methods. Supernatants were harvested after 24, 48, or 72 h for detection of IFN-gamma by ELISA (Bovigam assay; CSL Limited). Symbols: *, P < 0.1; **, P < 0.05 (differs from unsupplemented [no vitamin D] cultures at specific times). Error bars, SEM.

To evaluate relationships between IFN-gamma and NO production in response to M. bovis infection, Pearson's product-moment correlations were determined for responses to M. bovis antigens. The overall correlations, regardless of 1,25(OH)2D3 concentration and time, were positive (for CF, r = 0.452, P < 0.1; for rESAT-6, r = 0.507, P < 0.001; for PPD, r = 0.505, P < 0.001), with increases in IFN-gamma associated with concurrent increases in NO. A strong positive correlation between IFN-gamma and NO production was also detected for PWM-stimulated cells (r = 0.664, P < 0.001). Addition of 1,25(OH)2D3 to cultures diminished this correlation, although statistically significant differences were not detected (data not shown).


    DISCUSSION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Recent evidence suggests that 1,25(OH)2D3-induced NO limits replication of M. tuberculosis within human macrophages (53). Activation of alveolar macrophages by IFN-gamma results in an increased rate of conversion of 25(OH)D3 to 1,25(OH)2D3, the most active, naturally occurring form of the vitamin (54). 1,25(OH)2D3 induces NOS2 expression and the production of NO by human macrophages, necessary for mycobacterial killing (16, 53). In the present study, it was determined that 1,25(OH)2D3 increased NO production and decreased IFN-gamma production by antigen (i.e., PPD, CF, and rESAT-6)-stimulated PBMC from M. bovis-infected cattle. The biologically active form of vitamin D, 1,25(OH)2D3, has been shown to inhibit IFN-gamma production by in vitro-activated PBMC from several different species, including cattle (3, 4, 19, 37). Although speculative, it is plausible that inhibition of IFN-gamma production by 1,25(OH)2D3 functions as a negative feedback mechanism to inhibit tissue damage once the antimycobacterial response (i.e., that mediated by NO) is elicited. Another possibility is that the increased NO produced by macrophages as a result of 1,25(OH)2D3 addition to the cultures inhibits IFN-gamma production by T cells. Production of NO by splenic macrophages from M. tuberculosis-infected mice inhibits CD4+ T-cell mycobacterium-specific proliferative responses; IFN-gamma responses, however, are not affected (40). In the present study, addition of a competitive inhibitor of NOS (L-NMMA) abolished NO production yet did not affect IFN-gamma responses. The production of IFN-gamma in response to antigen stimulation and the suppression elicited by 1,25(OH)2D3 were similar in L-NMMA-treated cultures compared to the cultures that did not receive L-NMMA (data not shown). Thus, the inhibitory effects of 1,25(OH)2D3 on M. bovis-specific IFN-gamma production is most likely a direct effect of 1,25(OH)2D3 and not a secondary response to increased NO production.

Mitogen-induced CD4+ T-cell proliferation is inhibited by 1,25(OH)2D3 in mice, humans, and cattle (31, 43). Likewise, we determined that M. bovis-specific CD4+ cells were the primary target of inhibition by 1,25(OH)2D3. Antigen selection, however, may have biased the observed suppressive effect. Both CD4+ and gamma delta TCR+ cells but not CD8+ cells proliferate in response to M. bovis PPD (64). Although both subsets responded in the present study, 1,25(OH)2D3 inhibited only CD4+ cell proliferation. These findings are in agreement with a previous study in which 1,25(OH)2D3 inhibited mitogen-induced proliferation of CD4+ cells yet had no affect on gamma delta TCR+ cell proliferation (43). Since PPD is composed of a mixture of soluble antigens, these antigens are likely processed and presented in association with major histocompatibility complex class II for CD4+ cells or directly without processing for gamma delta TCR+ cells. Major histocompatibility complex class II restricted CD4+ cells are the predominant cell type responding to PPD-stimulated PBMC from M. bovis-infected white-tailed deer (63). Unlike those of white-tailed deer, gamma delta TCR+ cells of M. bovis-infected cattle do respond to soluble M. bovis antigens (51, 58, 64). One interpretation of the specific effects of 1,25(OH)2D3 on CD4+ cells is that the proliferative response of gamma delta TCR+ cells is dependent upon cytokine production by other cells (e.g., bystander proliferation in response to interleukin 2 [IL-2] produced by antigen-specific CD4+ cells). 1,25(OH)2D3 inhibits IL-2 production by human T cells (37) and IL-2 receptor expression by activated bovine PBMC (43). Rhodes et al. (50, 51), however, have clearly demonstrated that gamma delta TCR+ cells (isolated and enriched by magnetic bead sorting) from M. bovis-infected cattle proliferate in response to soluble M. bovis antigens, including M. bovis PPD and rESAT-6. Thus, it appears that 1,25(OH)2D3 affects CD4+ cells specifically in regards to inhibition of proliferation in response to PPD.

1,25(OH)2D3 also induces apoptosis of mitogen-stimulated human T cells by inhibiting IL-2 production (46). In contrast, results from the present study suggest that 1,25(OH)2D3 inhibits apoptosis of antigen-stimulated cells. This discrepancy may reflect the role of the stimulus (mitogen versus antigen) used to determine the effects of vitamin D on apoptosis. Species differences (cattle versus humans) may also influence the outcome of the response. Additional studies are needed to confirm the inhibition of apoptosis of bovine T cells stimulated with mycobacterial antigens by 1,25(OH)2D3 and to determine the underlying mechanisms. Inhibition of apoptosis of M. bovis-specific T cells by 1,25(OH)2D3 at the site of M. bovis infection would likely be beneficial to the host antitubercular response.

It has been postulated that 1,25(OH)2D3 enhances mycobacterial killing via an NO-dependent mechanism. It is also postulated that production of 1,25(OH)2D3 at the site of infection dampens cell-mediated responses to mycobacterial antigens through antiproliferative and IFN-gamma -inhibitory actions. Our findings that 1,25(OH)2D3 enhances M. bovis-specific NO production, inhibits M. bovis-specific IFN-gamma production, and inhibits M. bovis-specific CD4+ cell proliferation are consistent with these hypotheses. Future studies are planned to further evaluate the relevance of these findings.


    ACKNOWLEDGMENTS

We thank Rebecca Lyon, Jody Mentele, Lori Dethloff, Nancy Eischen, and Darrel Hoy for excellent technical support. We also thank Katherine Lies and Terry Krausman for excellent animal care.


    FOOTNOTES

* Corresponding author. Mailing address: United States Department of Agriculture, Agricultural Research Service, National Animal Disease Center, Bacterial Diseases of Livestock Research Unit, 2300 Dayton Ave., P.O. Box 70, Ames, IA 50010-0070. Phone: (515) 663-7756. Fax: (515) 663-7458. E-mail: rwaters{at}nadc.ars.usda.gov.


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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

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Clinical and Diagnostic Laboratory Immunology, November 2001, p. 1204-1212, Vol. 8, No. 6
1071-412X/01/$04.00+0   DOI: 10.1128/CDLI.8.6.1204-1212.2001


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