Previous Article | Next Article ![]()
Infection and Immunity, September 2005, p. 5809-5816, Vol. 73, No. 9
0019-9567/05/$08.00+0 doi:10.1128/IAI.73.9.5809-5816.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Microbiology, Immunology and Pathology, Colorado State University, Fort Collins, Colorado,1 Aeras Foundation, Bethesda, Maryland,2 Infectious Disease Research Institute, Seattle, Washington,3 Corixa Corporation, Seattle, Washington4
Received 17 November 2004/ Returned for modification 27 January 2005/ Accepted 18 April 2005
|
|
|---|
|
|
|---|
One new candidate that has undergone substantial evaluation is the fusion or polyprotein designated Mtb72F, derived from the M. tuberculosis proteins Mtb32 and Mtb39 (5, 37). Previous studies have shown that mice immunized with Mtb72F, either as a protein and adjuvant formulation or in the form of a DNA vaccine, were protected against aerosol challenge with M. tuberculosis H37Rv at a level comparable to that of BCG (37). Furthermore, when used as a boost in guinea pigs primed with BCG, survival of these animals to aerosol challenge was doubled from 1 to 2 years compared to animals given BCG only (5). These animals had substantial evidence of lesion resolution and very low bacterial numbers in the lungs.
Although the nature of protective immunity to TB is not completely understood (4, 25, 30), CD4 T cells have been shown to be essential mediators of protection (7, 33, 35). Production of gamma interferon (IFN-
) is essential, as shown by increased susceptibility in IFN-
or IFN-
receptor gene-disrupted animal models, as well as in humans possessing a rare mutation in the IFN-
receptor (9, 15, 20). In addition to this, however, there has been a growing realization over the past decade or more that CD8 T cells also contribute significantly to protective immunity. Adoptive transfer of immune CD8 T cells can confer protection in mice (27, 29), and such cells accumulate in the lungs of aerosol-infected mice, where they secrete IFN-
(12, 36). Antibody depletion of CD8 T cells during infection, as well as the use of mouse models possessing gene disruptions that result in significantly reduced numbers of CD8 T cells, also supports a role for CD8 T cells in protective immunity (2, 16, 19, 26, 38, 41).
The vaccine candidate Mtb72F contains an Mtb32-derived major histocompatibility complex (MHC) class I epitope, GAPINSATAM, which is strongly recognized by cytolytic and IFN-
-secreting CD8 T cells (37). Since the precise role that CD8 T cells play during infection with M. tuberculosis has not been completely defined, we decided to examine the Mtb32 antigen-specific CD8-T-cell response to this peptide epitope in the lungs of mice. Using an MHC class I tetramer reagent containing the GAPINSATAM peptide, we were able to track CD8 T cells recognizing this epitope in the lungs of mice vaccinated with a DNA vaccine encoding Mtb72F and/or challenged by aerosol exposure to M. tuberculosis. The results of this study show that significant numbers of tetramer-positive CD8 T cells accumulate in the lungs of vaccinated mice and expand further in response to challenge infection. These cells expressed markers of cellular activation, and a subpopulation up-regulated expression of a recently identified early memory cell marker (22), the interleukin-7 receptor alpha chain (IL-7R
). Approximately one-third of the CD8 T cells in the lungs expressed IL-7R
when Mtb72F protein was delivered in a novel liposomal formulation aimed at inducing CD8-T-cell responses. These data show that antigen-specific CD8 T cells traffic to the lungs after vaccination and respond to subsequent infection with M. tuberculosis.
|
|
|---|
Bacterial strains. M. tuberculosis strain H37Rv, originally obtained from the Trudeau Mycobacteria Collection (TMCC 102), was grown from low-passage seed lots in Proskauer-Beck liquid medium containing 0.05% Tween 80 to mid-log phase, and 1.5-ml aliquots were frozen at 70°C until use.
Immunizations and aerosol infection.
Mice were immunized three times at 3-week intervals with either 100 µg of Mtb72F DNA or phosphate-buffered saline (PBS) via the intramuscular (tibialis) route. For each immunization, mice were injected with a total volume of 100 µl/mouse using 50 µl/leg. Three weeks following the last immunization, mice were infected via the aerosol route using a Glas-Col aerosol generator (Glas-Col, Terre Haute, IN), such that
100 viable bacteria were deposited in the lungs of each animal. The number of viable bacteria in the lungs was determined at specific time points by plating 10-fold serial dilutions of individual partial organ homogenates on nutrient Middlebrook 7H11 agar and counting bacterial colonies after 21 days of incubation at 37°C.
Preparation of cationic liposome-DNA complexes. Liposomes were prepared by dissolving the cationic lipid DOTIM [octadecenolyoxy(ethyl-2-heptadecenyl-3 hydroxyethyl) imidazolinium chloride; Sigma-Aldrich, St. Louis, MO] and cholesterol (Avanti Polar Lipids, Alabaster, AL) in chloroform and adding equimolar concentrations to round-bottom 15-ml glass tubes. The solution was then dried overnight in a vacuum desiccator to a thin film. The lipids were rehydrated in 5% dextrose in water to form liposomes at a final concentration of 20 mM. Liposomes were incubated at 50°C for 50 min, followed by incubation for 2 h at room temperature. The liposomes were then extruded through a series of 1-µm, 0.45-µm, and 0.20-µm filters to form the final liposomes, as described previously (40). Noncoding plasmid DNA was prepared under very-low-endotoxin conditions (<0.25 endotoxin units/mg; Althea Technologies). Complexes of liposomes and noncoding plasmid DNA were prepared by first dissolving liposomes in 5% dextrose in water at a concentration of 100 µl liposomes per 1 ml dextrose solution. Next, plasmid DNA was added to the liposome solution at a final concentration of 100 µg DNA per ml liposome solution and complexes were formed spontaneously by gentle pipetting. To prepare vaccines, recombinant Mtb72F protein (kindly provided by the Corixa Corporation and GlaxoSmithKline) was added to preformed liposome-DNA complexes and mixed by gentle pipetting. After preparation, vaccines were kept at room temperature and administered within 30 min. For immunizations using cationic liposome-DNA complexes, mice were immunized two times, 14 days apart, via the intraperitoneal route using 10 µg/mouse of either Mtb72F protein or ova albumin (Sigma-Aldrich) in a total volume of 200 µl of liposome formulation.
Single-cell suspension preparation. Animals were sacrificed by carbon dioxide asphyxiation and were perfused through the heart with 10 ml cold PBS containing 3 U/ml heparin (Sigma-Aldrich). The lungs were removed from the pulmonary cavity and placed in cold Dulbecco's minimal essential medium (DMEM; Cellgro, Herndon, VA). The lung tissue was disrupted using sterile razor blades and incubated for 30 min at 37°C in a final volume of 2 ml DMEM containing 0.7 mg/ml collagenase D (Roche Applied Science, Indianapolis, IN) and 30 µg/ml DNase (Sigma-Aldrich). Following incubation, the digested lung tissue was gently passed through a sterile 70-µm nylon screen; rinsed using 8 ml cold DMEM supplemented with 10% heat-inactivated endotoxin low fetal bovine serum (Atlas Biologicals, Fort Collins, CO), 10 mM HEPES buffer (Sigma-Aldrich), 2 mM L-glutamine (Sigma-Aldrich), 2% MEM-nonessential amino acids (100x; Sigma-Aldrich), and 50 µM 2-mercaptoethanol (Sigma-Aldrich); and centrifuged at 200 x g. Red blood cells were lysed using Gey's solution (0.15 M NH4Cl, 10 mM KHCO3), and the samples were centrifuged and resuspended in DMEM plus supplements. Cells were counted using a hemacytometer, and aliquots were taken for flow cytometric analysis.
Flow cytometry. Single-cell suspensions from individual mice were resuspended in PBS containing 0.1% sodium azide (Fisher Chemicals, Fairlawn, NJ). Cells were incubated in the dark for 20 min at 4°C with specific antibody (directly conjugated to fluorescein isothiocyanate [FITC], phycoerythrin [PE], peridinin chlorophyll a protein [PerCP], or allophycocyanin [APC] at 3 µg/ml), followed by 2 washes using PBS with sodium azide. Cells were analyzed immediately using a FACSCalibur dual-laser flow cytometer (BD Biosciences, Mountain View, CA) and the data were analyzed using CellQwest software (BD Biosciences, San Jose, CA) and Summit software (DakoCytomation, Fort Collins, CO). Viable lymphocytes were gated based upon their forward- and side-scatter characteristics, and cell populations were analyzed by expression of fluorochrome-labeled markers. All analyses were performed with a minimum acquisition of 200,000 events.
Cell surface markers were analyzed with the following specific antibodies: FITC anti-CD44 (clone IM7), anti-CD3
(clone 145-2C11), anti-CD62L (clone MEL-14), anti-CD45RB (clone 16A), PerCP anti-CD8
(clone 53-6.7), APC anti-CD3
(clone 145-2C11), and anti-CD127 (clone A7R34; eBiosciences, San Diego, CA). Cell suspensions were also stained with a PE-Mtb32 H-2Db MHC class I tetramer (Beckman Coulter Immunomics, San Diego, CA) containing the peptide GAPINSATAM.
For intracellular antibody staining, single-cell suspensions were resuspended in DMEM with supplements and stimulated with 0.1 µg/ml anti-CD3
(clone 145-2C11) and 1 µg/ml anti-CD28 (clone 37.51), in the presence of 3 µM monensin for 4 h at 37°C at 5% CO2. Cells were stained with antibodies for cell surface molecules as described above, before permeabilization with BD Cytoperm/Cytofix (BD PharMingen, San Diego, CA) according to the manufacturer's instructions. Cells were incubated with FITC-conjugated anti-IFN-
(clone XMG1.2) or APC-conjugated anti-granzyme B (clone GB12; Caltag Laboratories, Burlingame, CA) for 20 min at 4°C, washed twice, and resuspended in PBS with sodium azide before analysis. Appropriate isotype controls were prepared simultaneously and were included in each analysis. Unless otherwise noted, all antibodies were purchased from BD PharMingen.
|
|
|---|
11% of CD8 T cells in the lungs recognized the Mtb72F epitope GAPINSATAM. The number of these cells also slowly declined over time.
![]() View larger version (64K): [in a new window] |
FIG. 1. Kinetic analysis of the Mtb32 antigen-specific CD8-T-cell response in nonimmunized mice and in mice immunized with Mtb72F followed by low-dose aerosol (LDA) infection with M. tuberculosis. C57BL/6 mice were immunized three times, at 3-week intervals, via the intramuscular route. Three weeks following the last immunization, mice were challenged. Time points represent the number of days after infection. Lung cells were stained with the Mtb32 H-2Db tetramer and antibodies specific for CD3, CD8, and CD44. Lymphocytes were gated based upon characteristic size and granularity and further gated based upon concurrent CD3 and CD8 expression. The data are from one mouse (representative of n = 4 mice). Percentages represent the number of CD44hi tetramer-positive cells as compared to the total number of CD3/CD8-positive cells. In all cases, background tetramer staining in naïve mice was <0.15%.
|
![]() View larger version (13K): [in a new window] |
FIG. 2. Immunization with Mtb72F resulted in greater numbers of antigen-specific CD8 T cells in the lungs of mice following infection. Data are expressed as the means ± the standard errors of the means and are from four mice per group per time point. Triangles, Mtb72F-immunized mice; squares, nonimmunized mice. Statistical significance based upon a comparison of Mtb72F-immunized and nonimmunized groups was determined using Student's t test and is indicated by an asterisk (P < 0.005).
|
or granzyme B. As shown in Fig. 3, a large percentage of tetramer-positive cells were also positive for IFN-
, whereas only a very small percentage were positive for the cytocidal molecule granzyme B.
![]() View larger version (37K): [in a new window] |
FIG. 3. Intracellular cytokine staining of lung-derived cells for granzyme B and IFN- at 30 days following aerogenic infection suggests that the tetramer-positive cells are primarily IFN- producing rather than cytolytic. The data are from one mouse (representative of n = 4 mice). Percentages represent the number of antigen-specific IFN- -positive or granzyme B-positive cells as compared to the total number of tetramer-positive cells.
|
(22). To determine if expression of this receptor could be detected on CD8 T cells entering the lungs, cells were further analyzed for tetramer staining and staining for IL-7R
. As shown in Fig. 4, cells expressing this memory marker could be detected in nonimmunized infected mice by day 30 and the numbers increased steadily through day 100. In mice vaccinated with Mtb72F prior to challenge, more than half of the antigen-specific CD8 T cells expressed IL-7R
by day 16 and this declined slowly through day 100. Relative total cell numbers are shown in Fig. 5 and illustrate the large numbers of such cells present in the lungs over the initial stage of the infection where vaccinated mice showed approximately a 1-log10 reduction in the lung bacterial load (data not shown).
![]() View larger version (41K): [in a new window] |
FIG. 4. Mtb32 tetramer-positive CD8 T cells in the lung express high levels of IL-7R following immunization with Mtb72F and subsequent aerogenic challenge. At multiple time points, lung cells were stained with the H-2Db tetramer and antibodies specific for CD3, CD8, and IL-7R . Lymphocytes were gated based upon characteristic size and granularity and further gated based upon concurrent CD3 and CD8 expression. The data are from one mouse (representative of n = 4 mice). Percentages represent the number of IL-7R hi tetramer-positive cells as compared to the total number of tetramer-positive cells. In all cases, background tetramer staining in naïve mice was <0.15%.
|
![]() View larger version (13K): [in a new window] |
FIG. 5. Greater numbers of memory precursor cells were present in the lungs of Mtb72F-immunized mice prior to day 60 as compared to nonimmunized mice following M. tuberculosis infection. Data are expressed as the means ± the standard errors of the means (n = 4 mice/group/time point). Triangles, Mtb72F-immunized mice; squares, nonimmunized mice. Statistical significance based upon a comparison of Mtb72F-immunized and nonimmunized groups was determined using Student's t test and is indicated by asterisks (P < 0.05).
|
initially, but by day 28 nearly a third possessed this phenotype.
![]() View larger version (9K): [in a new window] |
FIG. 6. Immunization with Mtb72F protein in conjunction with cationic liposome complexes and noncoding DNA results in a strong pulmonary antigen-specific CD8-T-cell response in the absence of infection. C57BL/6 mice (n = 4 mice/group/time point) were immunized two times, 14 days apart, via the intraperitoneal route. Five days and 28 days following the second immunization, lung cells were isolated and stained with the H-2Db tetramer and antibodies specific for CD3, CD8, and CD44. Lymphocytes were gated based upon characteristic size and granularity and further gated based upon concurrent CD3 and CD8 expression. Data are expressed as the means ± the standard errors of the means.
|
![]() View larger version (44K): [in a new window] |
FIG. 7. Mtb32-specific CD8 T cells isolated from lungs of mice following immunization possess an activated phenotype. Mice (n = 4 mice/group/time point) were immunized two times, 14 days apart via the intraperitoneal route with cationic liposomes plus Mtb72F protein. Five days and 28 days following the second immunization, lung cells were isolated and stained with the H-2Db tetramer and antibodies specific for CD3, CD8, and surface activation markers. Lymphocytes were gated based upon characteristic size and granularity and further gated based upon concurrent CD3 and CD8 expression. Representative dot plots are shown where the percentages represent the number of tetramer-positive cells falling within the indicated region as compared to the total number of tetramer-positive cells. In all cases, background tetramer staining in naïve mice was <0.15%.
|
|
|
|---|
The data further showed that the antigen-specific CD8 T cells in the lungs of infected mice were capable of producing IFN-
, which is known to be essential for protection following aerosol challenge with M. tuberculosis. In contrast, the majority of these cells did not produce granzyme B early during infection, suggesting that CD8 T cells entering the sites of infection in the lungs at early time points are more likely to control M. tuberculosis infection indirectly through activation of local macrophages via IFN-
secretion, rather than to directly lyse infected target cells by secretion of perforin and granzyme B. This is in keeping with the observation that perforin gene knockout mice are not less resistant to aerosol challenge (10, 24). These results, however, do not preclude cytolytic activity at time points later during the infectious process, as observed by others (23, 34). Further experiments utilizing an in vivo cytotoxic assay will delineate the lytic function of these antigen-specific CD8 T cells late in infection. That antigen-specific CD8 T cells capable of producing IFN-
preferentially accumulate in the lungs of mice following aerosol infection confirms the findings of a recent paper by Kamath et al., which examined the response to two vaccine candidates, CFP10 and Mtb10.4 (23).
Using IL-7R
as a marker, we found that a subset of M. tuberculosis-specific CD8 T cells that accumulated in the lungs of infected mice gradually assumed a memory precursor phenotype over the course of the infection. In contrast to the nonimmunized mice, if the mice were first vaccinated with Mtb72F, then memory precursor CD8 T cells comprised a major component of the early cellular response in the lungs after challenge. The actual numbers (and percentages) of these cells then declined for unknown reasons, but might indicate transition of these cells to an effector phenotype.
Whether the antigen-specific cells entered the lungs following infection or were already there prior to challenge was not addressed in the Mtb72F DNA vaccination experiments. However, pilot studies using a BCG infection model indicated a substantial establishment of activated protective T cells (A. Kipnis and I. Orme, unpublished data) similar to those demonstrated in chronic infection (21). Furthermore, recruitment of activated T cells to the lungs following immunization was demonstrated here by studies in which we examined the lungs of mice vaccinated with Mtb72F protein delivered in a CD8-directed liposomal formulation. This study showed that a strong antigen-specific CD8-T-cell response could be elicited in the lungs of mice in the absence of infection and that 1 month after two immunizations, about one-third of the antigen-specific CD8 T cells in the lungs expressed the IL-7R
. Moreover, by this time most of these cells had begun to reduce the expression of CD45RB from high to moderate, which is a further suggestion of transition to a memory state (18).
The data presented here demonstrate preferential recruitment and accumulation of activated, antigen-specific CD8 T cells into the lungs of mice following aerosol challenge with M. tuberculosis. Furthermore, that these cells were capable of producing IFN-
upon restimulation suggests that they could be actively participating in the control of bacterial replication. It was also shown that vaccination with the Mtb72F DNA vaccine significantly increased the number of these cells in the lungs following infection, which indicates that the CD8-T-cell component of adaptive immunity can be specifically targeted by vaccination. Further studies currently in progress will quantify the amount of protection conferred by this population of Mtb32-specific CD8 T cells, to further define the relevance of the CD8-T-cell response to M. tuberculosis infection. These experiments, as well as others in progress, will ultimately determine whether CD8-T-cell epitopes should be included in future M. tuberculosis candidate vaccines.
We are very grateful to Yves Lobet (GlaxoSmithKline) for providing the Mtb72F recombinant protein.
|
|
|---|
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»