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* U.S. Army Medical Research Institute of Infectious Diseases, and
Developmental Therapeutics Program and
Laboratory of Experimental Immunology, Centre for Cancer Research, National Cancer Institute, Frederick, MD 21702
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Abstract |
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, IL-6, and IL-8 in the absence of DC death. This corresponded temporally with enzymatic cleavage of proximal MAPK signaling proteins (MEK-1, MEK-3, and MAP kinase kinase-4) and may indicate activity of anthrax lethal toxin. Taken together, these results suggest that B. anthracis may exploit DCs to facilitate infection. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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For decades, research has focused almost exclusively on macrophages as an anthrax Trojan horse (7) that engulfs spores and carries them out of the lungs. However, mice that were chemically depleted of macrophages and infected with spores by aerosol nevertheless experienced disease (8). Cote et al. (8) demonstrated that macrophages were not essential for initiation of infection and implicated the existence of unidentified cell populations within the lungs that may be exploited by B. anthracis to facilitate the disease process. In particular, the dendritic cell (DC) has qualities that make it a strong candidate for exploitation by B. anthracis to aid in progression of disease. DCs are found in high frequency within the airway epithelium, submucosa, lung parenchyma, and alveolar space (9). Migration of Ag-loaded DCs from these regions into draining lymph nodes is fundamental to initiating acquired immune responses. Activated DCs initiate innate immune responses via contact interactions with other leukocytes and by secretion of proinflammatory chemokines, cytokines, and lipid mediators. During maturation, DCs increase membrane expression of costimulatory proteins, thus allowing them to efficiently present Ags to cognate T lymphocytes, initiating adaptive immune responses. Therefore, DCs are regarded as a critical link between the innate and adaptive immune responses (10) in the respiratory mucosa (9). The central role of DCs in development of immunity and their ability to take up and transport particles from the lungs led us to postulate that B. anthracis might exploit DCs to invade the host and to evade host immune responses.
In the present study, we examined by confocal and electron microscopy the capacity for human DCs to endocytose fully pathogenic Ames and nonpathogenic B. anthracis spores in vitro. We determined that DCs endocytose B. anthracis spores primarily by coiling phagocytosis. Next, we evaluated the impact of infection and B. anthracis virulence factors on DC immunostimulatory activities by infecting DCs with a strain that secretes ET and LT (Sterne) and a strain that lacks the capacity to produce these toxins (difference in Ames nonresponder (
ANR). DC immunostimulatory activities were assessed by examination of RNA and protein levels of chemokine receptors, proinflammatory cytokines, chemokines, and as well as by activation of MAPK proteins and stimulation of allogenic T cells. We show that DCs can be a receptive host for B. anthracis spores and bacilli, which stimulate secretion of TNF-, IL-6, and IL-8 via activation of the MAPK pathway until virulence factors reach sufficient levels to silence MAPK signaling 6 h after infection. Infected DCs mature and develop the capacity for migration to lymph nodes. The study described in this work supports an alternative model for B. anthracis host invasion and dissemination based on DC endocytosis of spores.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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DCs were cultured from peripheral blood, as described previously (11). Briefly, peripheral blood monocytes were cultured for 4 days in RPMI 1640 supplemented with 1% MEM nonessential amino acids, 10% heat-inactivated FBS (henceforth referred to as cRPMI), 100 IU of penicillin, 100 µg/ml streptomicin, 20 ng/ml human IL-4, and 100 ng/ml human GM-CSF (PeproTech). Cultures were >97% CD11c+ cells. Before all experiments, DCs were washed three times to remove antibiotics, and then resuspended in cRPMI. We initially tested a variety of multiplicity of infection (MOI) (1, 5, 10, and 50), but saw little difference in responses (of MAPK signaling, cytokine secretion, and induced cytokine/chemokine receptor RNA) other than slow responses at very low MOI and faster responses at very high MOI. In this study, an MOI of 10 was used because this is a typical MOI used in studies that investigate interactions of B. anthracis and the immune system. DCs were infected with an MOI of 10, unless otherwise noted.
B. anthracis spores
Culture, spore preparation, and engineering of GFP-Sterne and GFP-ANR spores were described previously (12). Briefly, plasmid pAFp8gfp was used in transfections of B. anthracis bacilli to produce bacilli that constitutively express GFP. pAFp8gfp contains a kanamycin resistance gene and a synthetic B. anthracis promoter sequence ligated upstream of gfp. The extrachromosomal plasmid is replicated during proliferation and segregates with progeny cells. During sporulation, GFP is incorporated inside spores (determined by immunoelectron microscopy; data not shown), giving rise to GFP spores. Spores were prepared using endotoxin-free reagents. Ames spores were provided by T. Abshire and J. Ezzel (U.S. Army Medical Research Institute of Infectious Diseases). Viable spores were heat shocked for 45 min at 65°C, centrifuged for 10 min at 16,000 x g, and resuspended in cRPMI immediately before infections. In some experiments, Sterne spores were inactivated by irradiation (4 x 106 rad) on ice.
Fluorescence microscopy
DCs were infected for 1 h, followed by five washes with 50 ml of cRPMI to remove non-cell associated spores. DCs were cultured in chamber slides coated with CC2 (Nalge Nunc International) to promote adherence to the glass slide and fixed in 0.1% paraformaldehyde overnight. DCs infected with Ames strain spores were fixed in 4% paraformaldehyde for 7 days. Images were collected using a Bio-Rad 2000MP confocal microscope. In some experiments, DCs were permeabilized with 0.2% Triton X-100 in PBS before the cells were stained with Texas Red-phalloidin, Hoechst stain (Molecular Probes), or Abs specific for CD11c (BD Biosciences).
Electron microscopy
For transmission electron microscopy (TEM), spore-infected DCs were fixed for 1 h with 2.5% glutaraldehyde prewarmed to 37°C, and were treated with 1% osmium tetroxide in a modified Millonig’s buffer (0.1 M PBS, 0.5% dextrose, pH 7.3) (13) stained en bloc with 0.5% uranyl acetate in ethanol, dehydrated in graded ethanol and propylene oxide, and embedded in Poly/Bed 812 resin (Polysciences). Ultrathin sections were placed on 200-mesh nickel grids and stained with 5% uranyl acetate and 0.2% lead citrate.
RNase protection assay (RPA)
The RiboQuant MultiProbe RPA was performed according to manufacturer’s directions (BD Biosciences) with minor modifications (14). Radiolabeled probe sets hCR5 and hCK2b were used to hybridize with total RNA purified from TRIzol-treated samples. Band density was quantitated using the UN-SCAN-IT gel automated digitizing system (Silk Scientific). The relative mRNA levels were determined by normalization of band densities for each protected probe fragment with that of L32, a housekeeping gene used as an internal loading control (15).
Mixed lymphocyte reaction
Immature DCs were treated with 100 ng/ml LPS (Escherichia coli O11:B4), Sterne spores, or ANR spores for 6 h before cultures were irradiated by 2000 rad. Different numbers of DCs (0–20,000) were cultured with 100,000 allogenic T cells purified by immunomagnetic depletion using anti-CD56, anti-CD19, anti-CD14, anti-HLA-DR Abs (BD Biosciences), and Dynabeads (Dynal Biotech). Cells were cultured in round-bottom 96-well plates in cRPMI supplemented with 10 µg/ml gentamicin to kill bacilli and spores. T cell proliferation was measured by the addition of 1 µCi/well [3H]thymidine (Amersham Biosciences) for the final 18 h of 5-day culture. Liquid scintillation counting was used to measure incorporated [3H]thymidine.
Cell surface and cytokine analysis
For cell surface analysis of protein expression, DCs were treated with LPS or infected with Sterne or ANR spores for 2 h before gentamicin was added to the culture medium. After 24 h of incubation, DCs were incubated with directly conjugated Abs for surface proteins (BD Biosciences) for 30 min before excess Ab was washed away. Then cells were fixed with 5% formalin overnight, and analyzed by flow cytometry. For cytokine analysis, DCs were treated with LPS or infected with Sterne or ANR spores for the times indicated. Culture supernatants were sterile filtered and analyzed for secreted proteins by cytometric bead array (human inflammation and human chemokine kits; BD Biosciences), according to the manufacturer’s directions. All analyses were performed on a FACSCalibur (BD Biosciences).
Western blot
DCs were lysed in buffer containing 50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 2 mM EDTA, 25 mM 
-glycerophosphate, 1% Triton X-100, 10 mM NaF, 1 mM Na3VO4, protease inhibitor mixture, and phosphatase inhibitor mixture I and II (Sigma-Aldrich), which does not lyse B. anthracis spores or bacilli. Cellular proteins (30 µg of protein/sample) were separated by SDS-PAGE, then blotted with the appropriate Abs.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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To investigate whether DCs might play a role in B. anthracis pathogenesis, first we determined whether DCs internalize B. anthracis spores. Immature DCs derived from human monocytes were incubated with fully virulent Ames (pX01+, pX02+) spores for 1 h, and then rinsed to remove excess spores. The fixed cultures were labeled with Abs to the DC marker CD11c and to B. anthracis spores before examination by confocal microscopy. Ames spores were located inside CD11c+ cells (Fig. 1A). Likewise, DCs internalized GFP-Sterne (pX01+, pX02–) spores (Fig. 1B) and GFP-ANR (pX01–, pX02–) spores (data not shown). Sterne and ANR spores that had clearly germinated based on staining of DNA were also found within DCs, although it was uncertain whether these spores were internalized after germination in the culture medium or had germinated within the DCs. Lung fluids alone are not sufficient to promote germination of inhaled spores (4), so phagocytic cells most likely engulf spores in a dormant state. Therefore, we examined the capacity for immature DCs to internalize ungerminated spores. After 1 h infection, fixed DCs were incubated with an Ab specific for B. anthracis bacilli to identify germinated spores (Fig. 1C). DCs were found to have internalized nongerminated GFP spores as indicated by absence of Ab binding. These data were also supported by TEM analysis of DCs that were infected for 15 min (Fig. 1D). An absence of swelling or elongation in the core region of internalized spores indicated they had not germinated.
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100 grids that showed spores associated with DCs, the mechanism of uptake was apparent in 30 grids. Of those, coiling phagocytosis was observed approximately two-thirds of the time. In contrast to spores, latex beads were engulfed by macropinocytosis without any evidence of coiling phagocytosis (Fig. 2C). These data indicate that the coiling mechanism to capture spores was neither a generalized response to particles of that size and shape nor a result of the culture conditions (16) used to generate DCs in vitro.
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Having confirmed that DCs capture B. anthracis spores, we examined the subsequent impact on DCs to determine whether spores promote lymph node homing and maturation. During an infection, anthrax toxins might influence DC function, so we compared responses to two different strains of spores: Sterne strain (pX01+) and ANR strain (pX01–). We had already confirmed that there was no apparent difference in quantities of internalized spores using these two B. anthracis strains (Table I).
DCs were treated with Sterne spores, ANR spores, or the standard activating agent bacterial LPS for 2 h before harvesting samples. RPA revealed that expression of RNA for tissue-retaining chemokine receptors (CCR1, CCR3, CCR5, CCR8, and CCR2ab) was reduced to a similar extent by treatment with Sterne spores, ANR spores, and LPS (Fig. 4). These changes are characteristic of lymph node homing DCs. For cell surface analysis of protein expression, DCs were treated with LPS or infected with Sterne or ANR spores for 2 h before gentamicin was added to the culture medium and incubated 22 h longer. Flow cytometric analysis (Fig. 5) revealed that all three treatments increased expression of CCR7, a chemokine receptor required for DCs to migrate into lymphatic vessels (18) and to T cell zones in the lymph node (19). Like LPS, spores increased the mean fluorescent intensity of CD11c+ DCs, and there was a loss of CCR5 at the membrane surface (Fig. 5). These changes also facilitate migration to lymph nodes. The maturation marker CD83 and proteins that enable efficient activation of cognate T cells (HLA-DR, CD40, CD80, and CD86) were expressed at higher levels (increased mean fluorescent intensity and percentage of positive DCs), whereas CD1a levels were reduced at the membrane surface of spore- and LPS-treated DCs (Fig. 5). These experiments were repeated exactly as before, except gentamicin was added 6 h after infection, and yielded results similar to those shown. These phenotypic changes indicate that spores induce maturation of DCs.
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ANR (p = 0.03 at 1 x 104, p = 0.05 at 2 x 104) than DCs treated with medium alone. The loss of tissue-retaining chemokine receptors, de novo expression of lymph node homing, and costimulatory proteins, as well as enhanced costimulatory function indicate that spore-infected DCs mature and develop the capacity to migrate into draining lymph nodes. These changes could facilitate dissemination of B. anthracis throughout the host.
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The B. anthracis toxins are important factors that can influence proinflammatory activity of mononuclear cells when added as purified proteins (20, 21). Spores stimulated DC maturation and the capacity to migrate into lymph nodes. To our surprise, there was no apparent influence of anthrax toxins on these responses to spore infection (Fig. 5). Therefore, the next question we asked was whether anthrax toxins might facilitate immune evasion by inhibiting proinflammatory responsiveness of DCs. We first tested for activation of the MAPK cascade, which regulates expression of many inflammatory response genes in DCs (22). DCs were treated with LPS, Sterne, ANR, or inactivated Sterne spores, and cellular proteins were harvested for Western analysis of activated (i.e., phosphorylated) MAPKs. Within 30 min of infection, ANR spores and Sterne spores induced weak phosphorylation of ERK1/2 and strong phosphorylation of p38 (Fig. 7A), whereas LPS induced strong phosphorylation of ERK1/2 and p38. Like the live spores, inactivated Sterne spores induced a similar pattern of phosphorylation of ERK and p38 (data not shown), suggesting that the spore coat contains molecules capable of activating DCs. Unlike LPS, Sterne and ANR spores did not activate stress-activated protein kinase (SAPK)/JNK within 30 min of infection (Fig. 7A). DCs treated with ANR spores showed weak phosphorylation of ERK1/2 and p38 as well as SAPK/JNK phosphorylation at 6 h (Fig. 7B), but Sterne-treated DC cultures lacked any detectable phosphorylation. Activation of the MAPKs as long as 6 h may result from ANR bacilli present in abundance after 2 h of infection and from a presence of nongerminated spores.
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ANR bacilli to produce LT suggested that LT may inactivate the MAPKs via cleavage of kinases (MEKs or MAP II kinase kinases (MKKs) immediately upstream of MAPKs (20, 23, 24, 25). Therefore, we examined levels of MEKs that phosphorylate ERK1/2 (MEK-1), p38 (MEK-3), and SAPK/JNK (MKK-4) at 6 h after infection. Protein samples were probed using Abs that detect peptide sequences of MEK-1, MEK-3, and MKK-4 that are detached by enzymatic activity of LT. Partial cleavage of all three proteins was evident by 4 h in Sterne-treated DCs, but not in ANR- or LPS-treated samples (data not shown). Complete cleavage of all MEK-1 and MEK-3 protein and nearly complete cleavage of all MKK-4 protein were observed at 6 h postinfection (Fig. 7B). These data indicate that spores initially activated DCs. As virulence factors such as LT reach a threshold, the toxins silence transduction of further intracellular signals, effectively shutting down the MAPK pathway. Inflammatory response gene expression from DCs infected with B. anthracis spores
Termination of MAPK signaling would be expected to inhibit expression of inflammatory response genes and curtail secretion of proinflammatory cytokines and chemokines, which are critical for activating the innate immune system. To assess this, we examined expression from genes regulated by the MAPK signaling pathway such as TNF-, IL-6, IL-1, MCP-1, IL-8, as well as other mediators of inflammation (22). RPA revealed that IL-1, IL-1, IL-1Ra, and IL-6 mRNA were induced by treatment with LPS and ANR spores at similar levels, and by Sterne spores to a lesser extent, by 2 h (Fig. 8). In LPS and ANR spore-treated samples, the IL-1, IL-1, IL-1Ra, and IL-6 mRNA levels continued to increase by 4 h. By 6 h, LPS and ANR samples continued to show increasing levels of IL-1 and IL-1Ra mRNA. In contrast, Sterne-treated samples showed a uniform plateau in mRNA levels after 4-h infection.
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ANR spores, or LPS. LPS and ANR spores stimulated secretion of TNF-, IL-6, and IL-8 in copious amounts that continued to rise at 8 h posttreatment (Fig. 9A). In contrast, DCs infected with Sterne spores secreted overall lower levels of these inflammatory mediators and did not continue to rise after 4-h infection. Other mediators of inflammation such as IL-12p70 (Fig. 9A) were stimulated by LPS, but not by Sterne or ANR spores, suggesting differential activation of specific genes. Surprisingly, both Sterne and ANR spores poorly stimulated secretion of other chemokines. From spore-treated DCs, monokine induced by IFN-
was not secreted, and only negligible levels of MCP-1 and moderate levels of RANTES were observed 4 h after infection (Fig. 9B). Examination of DCs by trypan blue exclusion (data not shown) indicated that infection did not induce DC death by 8 h after infection. These data demonstrate that human DCs respond to infection with B. anthracis spores by producing RNA and secreting inflammatory response proteins, which modulate inflammation and promote maturation of DCs. Furthermore, B. anthracis virulence factors, most likely those encoded on the pX01 plasmid, later suppress secretion of inflammatory response proteins in part by impairing transduction of signals through the MAPK cascade.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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Our data clearly demonstrate that immature human DCs are capable of internalizing nongerminated B. anthracis spores. Interestingly, DCs engulf spores primarily by means of coiling phagocytosis, a receptor-mediated (29) method predominantly associated with uptake of pathogenic organisms. Coiling phagocytosis is a disorganized form of zipper-type phagocytosis characterized by single folds of plasma membrane that wrap around a microbe in multiple turns (30). Although the significance of coiling phagocytosis for disease pathogenesis has yet to be elucidated, it is a subject that surely warrants further investigation.
B. anthracis might induce its exit out of the lungs within DCs, in part, by activating the MAPK signaling cascade. In our system, we observed weak activation of ERK with strong activation of p38 (Fig. 7) quickly after endocytosis of spores. In DCs, this pattern of MAPK activation stimulates lymph node homing via secretion of proinflammatory cytokines (e.g., TNF-) known to enable DC trafficking between the lungs and mediastinal lymph nodes (31, 32, 33). Although greatly attenuated in the presence of B. anthracis with preserved pX01 virulence factors, cytokine secretion was apparently sufficient to stimulate pathways that promoted maturation, enhanced activation of allogenic T cells, and membrane protein changes essential for DC migration into lymph nodes.
An effect of virulence factors, such as LT, was evident by 6 h postinfection when suppression of MAPK signaling and production of proteins from downstream target genes were observed. In a previous study, Agrawal et al. (34) reported impaired phosphorylation of ERK1/2 and p38 in response to LPS and a reduction in cytokine expression after 24 h preincubation of murine DCs with purified LT. Our data suggest that the levels of LT released by Sterne bacilli are sufficient to achieve these same effects, in part, by cleavage of MEK-1, MEK-3, and MKK-4. Significantly, our study agrees with their MAPK signaling results using a different experimental system, which involved spore infection with permitted outgrowth of B. anthracis bacilli and production of virulence factors. Our study and that of Agrawal et al. (34) contrast with a new study (35) that showed no impact of Sterne infection on cytokine secretion. Although our results did not indicate an impact of specific virulence factors on expression of the costimulatory membrane proteins that we examined, Agrawal et al. (34) reported a suppression of costimulatory molecule expression with overnight LT preincubation and subsequent LPS stimulation. Significantly, these differences could be reconciled by the capacity for spores to stimulate proinflammatory pathways before activity of virulence factors. Also, new immunostimulatory bacterial products may have become accessible to DCs during incubation with gentamicin, and thus provide further stimulation for DCs to mature.
Because Sterne bacilli are capable of producing both ET and LT, there may be a combined effect of these exotoxins on cytokine and chemokine production. Edema factor is a calmodulin-activated adenylate cyclase (36), which binds to heptamers of protective Ag to traverse cell membranes, forming ET. By increasing intracellular cAMP levels, ET inhibits TNF- and IL-6 secretion from monocytes (21). To our knowledge, no studies have investigated the effects of ET on DCs. However, it was shown that elevated cAMP levels can inhibit release of proinflammatory cytokines without affecting the differentiation process of DCs (37).
Overall, our study demonstrates that B. anthracis virulence factors suppress proinflammatory responsiveness of DCs. Because DCs are regarded as a critical link between the innate and adaptive immune responses (10), impaired responsiveness may diminish DC-mediated activation of other leukocytes in the innate immune system and facilitate escape of B. anthracis from detection, thereby enhancing its ability to complete the infection process.
The results of this study are entirely consistent with a hypothesized scenario in which B. anthracis spores may use DCs to facilitate infection. The sequence of events would be initiated by spores stimulating their endocytosis by DCs. DCs would then become transiently activated in a proinflammatory mode and secrete inflammatory cytokines, undergo maturation, and migrate to draining lymph nodes. Further production of anthrax toxins by bacilli would silence the proinflammatory response and delay an immune response to the infection. Although additional work will be required to confirm this sequence of events, the current study provides a sound foundation of plausibility.
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Acknowledgments |
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Disclosures |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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Footnotes |
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1 The research described in this work was partially supported by the Medical Biological Defense Research Program, U.S. Army Medical Research, and Materiel Command Project 0242C012. K.C.B. is the recipient of a National Research Council fellowship. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the U.S. Army. 
2 Address correspondence and reprint requests to Dr. Sina Bavari, U.S. Army Medical Research Institute of Infectious Diseases, 1425 Porter Street, Frederick, MD 21702-5011. E-mail address: bavaris{at}ncifcrf.gov
3 Abbreviations used in this paper: ET, edema toxin; ANR, Ames nonresponder; DC, dendritic cell; LT, lethal toxin; MKK, MAP kinase kinase; MOI, multiplicity of infection; RPA, RNase protection assay; SAPK, stress-activated protein kinase; TEM, transmission electron microscopy.
Received for publication November 24, 2004. Accepted for publication February 15, 2005.
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induces dendritic cell migration to draining lymph nodes, and possibly provides one stimulus for Langerhans’ cell migration. Immunology 75:257.[Medline]
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J. A. Boydston, L. Yue, J. F. Kearney, and C. L. Turnbough Jr. The ExsY Protein Is Required for Complete Formation of the Exosporium of Bacillus anthracis J. Bacteriol., November 1, 2006; 188(21): 7440 - 7448. [Abstract] [Full Text] [PDF]
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W. J. Ribot, R. G. Panchal, K. C. Brittingham, G. Ruthel, T. A. Kenny, D. Lane, B. Curry, T. A. Hoover, A. M. Friedlander, and S. Bavari Anthrax Lethal Toxin Impairs Innate Immune Functions of Alveolar Macrophages and Facilitates Bacillus anthracis Survival Infect. Immun., September 1, 2006; 74(9): 5029 - 5034. [Abstract] [Full Text] [PDF]
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A. K. White, J. A. Hoch, M. Grynberg, A. Godzik, and M. Perego Sensor Domains Encoded in Bacillus anthracis Virulence Plasmids Prevent Sporulation by Hijacking a Sporulation Sensor Histidine Kinase. J. Bacteriol., September 1, 2006; 188(17): 6354 - 6360. [Abstract] [Full Text] [PDF]
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K. Chakrabarty, W. Wu, J. L. Booth, E. S. Duggan, K. M. Coggeshall, and J. P. Metcalf Bacillus anthracis Spores Stimulate Cytokine and Chemokine Innate Immune Responses in Human Alveolar Macrophages through Multiple Mitogen-Activated Protein Kinase Pathways. Infect. Immun., August 1, 2006; 74(8): 4430 - 4438. [Abstract] [Full Text] [PDF]
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A. Piris-Gimenez, M. Paya, G. Lambeau, M. Chignard, M. Mock, L. Touqui, and P. L. Goossens In Vivo Protective Role of Human Group IIA Phospholipase A2 against Experimental Anthrax J. Immunol., November 15, 2005; 175(10): 6786 - 6791. [Abstract] [Full Text] [PDF]
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G. J. A. Rainey, D. J. Wigelsworth, P. L. Ryan, H. M. Scobie, R. J. Collier, and J. A. T. Young Receptor-specific requirements for anthrax toxin delivery into cells PNAS, September 13, 2005; 102(37): 13278 - 13283. [Abstract] [Full Text] [PDF]
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), Sterne spores (
), or 
) for 6 h before cultures were irradiated and incubated at the indicated numbers with 1 x 105 allogenic T cells. [3H]Thymidine incorporation (cpm x 104) by proliferating T cells is displayed as the mean of triplicate determinations. These data are representative of responses from four different donors.
