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Departments of
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Medicine,
Neuroscience,
Pathology, and
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Pediatrics, University of California at San Diego, La Jolla, CA 92093;
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Instituto de Ciencias Biomedicas, Immunology Program, School of Medicine, University of Chile, Santiago, Chile; and
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Androclus Therapeutics, Catania, Italy
| Abstract |
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| Introduction |
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It is currently accepted that genetic immunization is based on the in situ transfection of cells whose identity varies in relation to both the method (5, 6) and the site of injection, i.e., the muscle vs the skin (7, 8). Muscle cells are transfected following i.m. injection of DNA (9, 10). Keratinocytes, fibroblasts, and dendritic cells are transfected after intradermal (i.d.)3 immunization (11, 12, 13, 14). Bone marrow-derived APC and keratinocytes are directly transfected after gene gun immunization (15, 16). It has been suggested that the strong and persistent humoral and cellular immune responses that follow i.d. DNA immunization can be related to the important immune surveillance functions of the skin and the skin-associated lymphoid tissues (17).
Cells of the dendritic lineage such as the bone marrow-derived Langerhans cells (LC) constitute about 5% of epidermal cells. Along with skin macrophages, LC are specialized in uptaking foreign Ags for transport to draining lymph nodes, where primary immune responses are initiated (18, 19). LC efficiently phagocyte both naked DNA and proteic Ags and are a major APC population in the immune response that follows i.d. genetic immunization (15, 20). Once transfected, skin-derived dendritic cells convey the uptaken Ag to draining lymph nodes (13) for elaboration of primary immune responses (21). Similarly, transfected macrophages can transport in vivo-phagocytosed Ag to sites distant from injection (22). However, it remains unclear whether DNA uptaken at a site of i.d. injection is transported beyond regional lymph nodes during inflammation. Such information could have significant implications for a better knowledge of the immune responses after DNA vaccination.
In the experiments herein described, we induced two distant sites of inflammation in vivo in mice to study the effect of proinflammatory chemotactic stimuli on the transport of injected DNA-based Ag. The primary site of inflammation was produced by i.d. coinjection in the nape of adjuvant with naked DNA. The second inflammatory site was induced by injection of adjuvant only at a distant peritoneal site. By combining construct-specific PCR with in situ hybridization and FACS, it was possible to follow, at the individual cell level, active cell-mediated transport of plasmid DNA between the two inflammatory sites. In situ hybridization and immunofluorescence studies detected the presence of plasmid DNA at the dermal site of inflammation for up to 6 wk after immunization and in situ expression of the encoded proteic product in cells of the epidermis and dermis. Injected DNA, uptaken at the dermal site of inflammation, could be transported beyond regional lymph nodes and detected in organs distant from the site of injection such as in the spleen. Repetitive administration at the primary inflammatory site led to accumulation of injected DNA at the distant site of inflammation. This transport was mediated by CD11b+ cells and was cumulative during chronic inflammation. The implications of this pathway for genetic immunotherapy and vaccination are discussed.
| Materials and Methods |
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A segment of 426 b.p. (position 29953385) of EBV Balf2 gene was amplified from genomic DNA of EBV (strain B-958) with the primers: 5'-GCCCAAGCTTGGGATGACCACATATGAGATT-3' and 5'-GCGGGATCCCGCTAGACCACGAGTCC-3' in an Ericomp EasyCycler (San Diego, CA). PCR conditions were: 1 min denaturation at 95°C; 1 min 30 s annealing at 49°C for the first 5 cycles, and 1 min at 52°C during the remaining 30 cycles; 2 min amplification at 72°C and 6 min for the last extension cycle. The amplified DNA fragment was subcloned between the HindIII/BamHI sites of the eukaryotic expression vector p290 (a gift of Dr. Tyler Parr, University of Southern California, Los Angeles, CA), which contains a CMV promoter upstream of the polylinker. p290 does not replicate in mammalian cells nor does integrate in their genome. Bacterial methylation of p290:Balf2 construct was assayed by digestion with HpaII and MspI restriction enzyme isoschizomers (23).
Genetic immunization
A total of 100 µg of p290:Balf2 DNA construct dissolved in sterile endotoxin-free water were mixed or not with 100 µg of the DNase-free synthetic adjuvant Adjuprime (Pierce, Rockford, IL) and injected i.d. into the nape of BALB/c mice (Jackson ImmunoResearch, West Grove, PA). Then, 100 µg of Adjuprime only were injected into the same mice i.p. Injections were performed, when required by the protocol, at weekly intervals. Sets of experiments were performed and confirmed at least three times.
Hematoxylin/eosin staining
Hematoxylin/eosin staining of cryosections of skin at the site of injection was performed using standard procedures, as described (24).
Fluorescence in situ hybridization (FISH)
Slides were treated with 30 µl of 100 µg/ml proteinase K (Sigma, St. Louis, MO) for 30 min at 37°C and then fixed in formaldehyde for 5 min at 4°C. A Balf2 gene probe corresponding to positions 31443202 was 3' tailed with TdT (Boehringer Mannheim, Indianapolis, IN) and 0.05 mM digoxigenin-dUTP (Boehringer Mannheim) for 15 min at 37°C, in 5 mM CoCl2 and 0.5 mM dATP. An irrelevant DNA probe of the same length was tailed in the same way and used as a negative control. Reaction mixtures were spread onto glass slides and incubated at 42°C overnight. After washes with SSC and blocking with PBS/2% casein, FITC anti-digoxigenin (Boehringer Mannheim) was added 1:500, at room temperature, for 30 min.
PCR
Total DNA, isolated and purified from PBMC, spleen, and skin by Easy DNA kit (Invitrogen, Carlsbad, CA), was quantified by DNA Dipstick (Invitrogen). Then, 40 ng of total DNAs were used as templates for PCR with the conditions described above. A second round of PCR amplification was performed with identical conditions on one-tenth of the amount of the first PCR product.
For semiquantitative PCR, two independent PCR were performed, each on DNA from 5 x 105 PBMC harvested at the second week of immunization on a Ficoll-Hypaque (Pharmacia, Uppsala, Sweden) gradient. One PCR was conducted with primers for Balf2, and the other PCR with primers for the ampicillin resistance gene (ampr): 5'-GGCTCCAGATTTATCAGCAATAAACCA-3' and 5'-ATACACTATTCTCAGAATGACTTGGT-3'. A single copy of the ampr gene is present in the plasmid construct containing the Balf2 gene. The length of Balf2 and of ampr PCR products are comparable. Starting from 25 ng, 1:2 scalar dilutions of p290 plasmid DNA were used as control templates for the PCR on ampr. Intensity of ampr bands was compared with the intensity of the Balf2:PCR product on a 2% ethidium-bromide-stained agarose gel.
Southern blotting
Total DNA was extracted with the Easy DNA kit (Invitrogen) according to the manufacturers instructions. Balf2-specific PCR products were blotted on Hybond N membrane (Amersham, Buckinghamshire, U.K.). A digoxigenin-labeled synthetic oligonucleotide corresponding to the positions 31443202 of the Balf2 gene was used as a probe. The membrane was hybridized at 42°C overnight, then washed in 0.1% SSC/0.1% SDS, blocked for 30 min with TBS/2% casein (Sigma), and incubated at room temperature for 30 min with AP-conjugated anti-digoxigenin Ab (Boehringer Mannheim) diluted 1:1000. After washes and incubation with the nonradioactive substrate Lumi-Phos (Boehringer Mannheim), the membrane was exposed to x-ray film (Kodak, Rochester, NY).
Confocal microscopy
Cryosections of skin at the site of injection were fixed in 2% paraformaladehyde/PBS for 10 min at room temperature, permeabilized with 0.2% Triton X-100/PBS for 10 min, and then blocked with PBS/0.005% BSA (Sigma). Tissue sections were double stained with phalloidin-FITC (Sigma) and anti-Balf2 rabbit antiserum plus rhodamine anti-rabbit Ab (H+L) Ab (Cappel ICN, Costa Mesa, CA) in 0.005% BSA. Sections were examined with a Bio-Rad MRC 1024 Laser scanning confocal system (Bio-Rad, Hercules, CA) coupled to a Zeiss Axiovert 35 M microscope (Zeiss AG, Oberkochen, Germany). Individual images (1024 x 1024 pixels) were saved to optical disk (Pinnacle Micro, Irvine, CA), converted to PICT format, and merged as pseudocolor images with Adobe Photoshop (Adobe Systems, Mountain View, CA).
Individual-cell PCR/FISH/flow cytometry
PBMC were purified on Ficoll-Hypaque gradient (Pharmacia). Monocytes were further separated by FACS by anti-CD11b Ab (PharMingen, San Diego, CA), fixed in PBS/2% paraformaldehyde, and permeabilized with streptolysin O (Sigma) at 37°C for 30 min. Cells were pelleted and resuspended in PCR mix. PCR was performed as described above. As a positive control, primers for G3PDH were used. A digoxigenin-labeled irrelevant probe was used as a negative control. After cell pelleting and washes in 2x SSC and PBS, FISH was performed on cells in suspension. After incubation in PBS/2% BSA for 30 min at 4°C, FITC anti-digoxigenin Ab (Boehringer Mannheim) was added 1:500 v/v for 30 min at 4°C. After washes, flow cytometry was performed on a FACScan cell sorter (Becton Dickinson, San Jose, CA).
| Results |
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Six BALB/c mice were immunized i.d. in the nape with 100 µg of
naked DNA construct (p290:Balf2) plus 100 µg of the adjuvant
Adjuprime. The presence of the Balf2 gene and the expression of the
encoded protein product were then monitored at the site of injection.
p290:Balf2 construct was uptaken in vivo and could be visualized by
FISH at the dermal site of inflammation (Fig. 1
) for up to 6 wk after DNA immunization
(not shown). Histological examination of the same area showed cellular
patchy infiltrate of the dermis consistent with nonspecific
inflammatory reaction (not shown).
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To follow the fate of the DNA molecules uptaken at the i.d. site
of inflammation, total DNA was extracted from four injected animals
from the skin of the neck at the site of injection, from PBMC, from
spleen, and from the skin of the tail, which is a site distant from DNA
injection. Balf2-specific PCR in individual mice showed the presence of
injected DNA construct in all these tissues, except the skin of the
tail (Fig. 3
). The amount of transported
plasmid construct was about 40 pg/105 PBMC, as
determined by semiquantitative PCR analysis (not shown). This transport
was cell mediated, as indicated by intracellular colocalization of the
construct and of its encoded product and by the fact that PCR on serum
of immunized animals tested negative (not shown).
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Cell-mediated transport of DNA between distant inflammatory sites
To address the possibility that ingested DNA molecules could be
transported between two distant inflammatory sites, two simultaneous
foci of inflammation were generated in the same animal of a group of
six. The primary site of inflammation was induced by immunizing mice
with naked DNA plus adjuvant i.d. in the neck, and a second site was
induced by concomitant injection of adjuvant only at a distant
peritoneal site. Peritoneal exudate cells (PEC) were collected by
lavage 3 days after the third of three i.d. immunizations associated
with i.p. injection of adjuvant performed at weekly intervals. Total
DNA was extracted from PEC and amplified by Balf2-specific PCR.
Southern blotting on PCR products with an internal Balf2 probe
indicated cell-mediated transport of the DNA-based Ag from the dermal
site of injection to the peritoneal site of inflammation (Fig. 4
). Intradermal injection in the nape of
mice with adjuvant only tested negative, whereas lack of inflammation
at the distant site was associated with diminished DNA transport (Fig. 5
).
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To evaluate the effect of repetitive administration of naked DNA
during inflammation, three groups of three mice were immunized i.d. in
the neck with naked DNA construct at weekly intervals. Inflammation was
induced in both neck and peritoneum by concomitant injection of
adjuvant in both sites. One mouse from each group was sacrificed 3 days
after 1 wk of immunization, another mouse 3 days after 2 wk of
immunization, and the last mouse 3 days after 3 wk of immunization.
Total DNA was extracted from peritoneal lavage cells (site of secondary
inflammation), and Balf2-specific PCR on DNA from peritoneal cells was
followed by Southern blot with an internal probe for Balf2. Southern
analysis showed that repeated i.d. administration of DNA at the primary
inflammatory site led to its accumulation at the peritoneal
inflammatory site (Fig. 4
), but not at an unrelated distant site such
as the skin of the tail (not shown).
CD11b+ cells mediate the DNA transport between distant inflammatory sites
Because CD11b+ cells at the primary site of
inflammation associated with uptake of DNA (not shown), we tested
whether CD11b+ cells could transport DNA to
distant sites. To this aim, we used a novel strategy that is
modification of a technique described by Patterson et al.
(26). Briefly, peritoneal lavage cells and PMBC were
harvested from four individual DNA-immunized mice and labeled with
anti-CD11b mAb. After cell sorting by flow cytometry, cells were
fixed and permeabilized. Individual cells positive for the i.d.
injected Ag were visualized by flow cytometry after Balf2-specific
PCR-FISH (Fig. 6
). This analysis,
performed on both CD11b+ and
CD11b- cells, showed that the majority of
peritoneal cells positive for the Ag were CD11b+
(Fig. 6
). Thus, CD11b+ cells can mediate
transport of ingested DNA between distant and unrelated inflammatory
sites.
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| Discussion |
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DNA vaccination induces broad-based, long-lasting, Ag-specific immune
responses (2) and associates with in vivo transfection of
phagocytic cells (10, 11, 12, 13, 14, 15) that migrate to regional
draining lymph nodes (21, 27, 28). Because of the
capability of dendritic cells to uptake DNA and carry it to local lymph
nodes, Condon et al. proposed to use these cells as vehicles for gene
immunization (13). Also, Chattergoon et al. recently
demonstrated the presence of transfected activated macrophages and
dendritic cells in the blood and peripheral tissues of animals
inoculated i.m. with DNA (22). Schubbert et al. found in
mice that ingested foreign DNA passes the intestinal wall and can be
found in distant organs such as the liver and the spleen
(29). We extend the above information by showing that
CD11b+ cells can mediate the transport of plasmid
DNA between distant inflammatory sites after i.d. DNA immunization. The
interpretation of our findings is schematically depicted in Fig. 7
. DNA molecules are uptaken in the
dermis by cells recruited at the inflammatory site. As indicated by
confocal microscopy experiments, the injected DNA leads to relevant Ag
expression in the host cells. CD11b+ cells that
contain plasmid DNA migrate beyond lymph nodes into the general
circulation and can reach distant organs such as the spleen or distant
inflammatory sites, possibly attracted by a gradient of chemotactic
stimuli (30, 31, 32, 33).
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Our data also imply the possibility to manipulate abnormal immune responses at inflammatory sites. Different Ag-targeting strategies have been devised to specifically direct the Ag to APC to promote "site-directed immunogenesis" and obtain an effective immune response to DNA vaccines (34). Also in consideration of the findings of Song et al., who reported the suppression of ongoing inflammation in arthritis after i.m. injection of a plasmid-encoding TGF-ß1 (35), we can hypothesize that, by associating inflammatory stimuli with injection of naked DNA encoding for a given soluble mediator (or for an Ag such as for example a tolerogenic peptide), one might possibly obtain the transport of the relevant molecule to a distant inflammatory site and its in situ expression. Additional studies are required to address this possibility and the relevance of this pathway to chronic inflammation associated with infection.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Antonio La Cava at his current address: The Scripps Research Institute, Department of Immunology-IMM23, 10550 North Torrey Pines Road, La Jolla, CA 92037. E-mail: ![]()
3 Abbreviations used in this paper: i.d., intradermal; LC, Langerhans cells; FISH, fluorescence in situ hybridization; ampr, ampicillin resistance gene; PEC, peritoneal exudate cell. ![]()
Received for publication January 19, 1999. Accepted for publication November 17, 1999.
| References |
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4-integrin supports leukocyte rolling and adhesion in chronically inflamed postcapillary venules in vivo. J. Exp. Med. 183:1995.This article has been cited by other articles:
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