Adeno-Associated Viral Vectors Penetrate Human Solid Tumor In Vivo

HUMAN GENE THERAPY 13:1115–1125 (June 10, 2002)
© Mary Ann Liebert, Inc.
Adeno-Associated Viral Vectors Penetrate Human Solid Tumor
Tissue In Vivo More Effectively than Adenoviral Vectors
PER ØYVIND ENGER,1 FRITS THORSEN, 1 PER EYSTEIN LØNNING, 2,3 ROLF BJERKVIG,1–3
and FRANK HOOVER 3
ABSTRACT
The transduction efficiencies of adeno-associated viral vectors (AAV, serotype 2) and adenovirus vectors (ADV,
serotype 5) were examined in three different models of cancer. First, we used flow cytometry to quantitate
AAV-GFP or ADV-GFP transduction in 13 cell lines derived from malignant tissue (6 gliomas, 6 mammary
cancers, and 1 leukemia). These experiments showed variable transduction efficiency (0%–81%) between the
cell lines, with ADV being more effective compared to AAV in 9 of 13 cell lines. Second, spheroids prepared
from human glioblastomas were infected with ADV or AAV expressing GFP or lacZ cassettes, and after 2
weeks, uniform reporter gene expression was observed on the spheroid. Whereas AAV produced consistent
transduction throughout the spheroids, ADV infection was mainly limited to the outer cell layers of the spheroids, suggesting that AAV were more efficient at penetrating solid tumor tissue. Third, human biopsies from
glioblastoma multiforme patients were xenografted into nude rats and grown for 4 weeks followed by viral
vector injection. Combined use of high-resolution magnetic resonance imaging (MRI) and histologic analysis
allowed the identification of transduced cells and their spatial distribution within the tumors. AAV-mediated
transgene expression was observed in cell clusters through the entire tumor, while ADV-mediated transduction was restricted to cells at the tumor periphery. Thus, while AAV and ADV vectors may infect tumor-derived cell lines to a similar degree, AAV penetrated glioblastoma spheroids and xenografts more efficiently
compared to ADV vectors. These results suggest that AAV may be suitable for therapeutic gene delivery to
malignant tumors.
OVERVIEW SUMMARY
Sustained therapeutic gene expression is a major limitation
to cancer gene therapy. Thus, there is a need to identify efficient delivery systems. Here we demonstrate that both
adeno-associated vectors (AAV, serotype 2) and adenovirus
vectors (ADV, serotype 5) can transduce a broad range of
glioma and mammary cancer cell lines grown as monolayers.
We show that AAV effectively penetrated human glioma biopsy spheroids, as indicated by the presence of reporter gene
positive cells throughout the tissue. In contrast, ADV-transduced cells were limited to the outer layers of the spheroids.
To examine transduction in human material in vivo, we injected the vectors expressing GFP and lacZ into human
glioblastomas xenografted into the nude rat brain. Fluores-
cence microscopy and lacZ staining revealed clusters of AAVtransduced cells distributed within the tumors, while ADVmediated transduction was restricted to the tumor periphery. Taken together, these data suggest a positive potential
for AAV vector systems in cancer gene therapy.
INTRODUCTION
G
represents a novel and attractive alternative
to conventional cancer therapies. Although in its infancy,
gene therapy has already begun to provide encouraging results
in human clinical trials (Somia and Verma, 2000). A major limitation to this technology has been sufficient and stable gene
transfer to the target cells in solid tissue (Somia and Verma,
1Department
ENE THERAPY
of Anatomy and Cell Biology, University of Bergen, Bergen, Norway.
of Oncology, Haukeland Hospital, Bergen, Norway.
3Gene Therapy Program, Bergen, Norway.
2Department
1115
1116
2000; Rainov and Kramm, 2001). Therefore, much of the current effort in the field is spent on identifying and developing
suitable gene delivery systems of both viral and nonviral origin. Most clinical trials in cancer gene therapy have favored
retroviral and adenoviral vectors for gene transfer (www.
wiley.co.uk/genmed-2001). While these systems each have distinct advantages, current evidence suggests that there are significant limitations regarding their transduction efficiency in
solid tumor tissue (Benedetti et al., 1997; Thorsen et al., 1997;
Puumalainen et al., 1998; Enger et al., 1999; Sandmair et al.,
1999; Kuriyama et al., 2001).
Recombinant adeno-associated viruses (AAV) have been gaining favor as a reliable gene transfer system (Fisher et al., 1997;
Monahan and Samulski, 2000a,b; Smith-Arica and Bartlett, 2001).
These vectors are derived from parvoviruses and have small capsids (25 nm) containing single-stranded genomes, and have never
been associated with any disease conditions in humans (Muzyczka et al., 1984; Snyder, 1999). Clinical trials in humans have
demonstrated a safe toxicity profile, and additional studies are in
progress (Kay et al., 2000; Flotte et al., 1996).
Despite having many hallmarks necessary for cancer gene
therapy, few studies have evaluated AAV for use in the oncologic setting. These studies have been limited to cell lines and
cell lines transplanted into animals (Qazilbash et al., 1997;
Kunke et al., 2000; Veldwijk et al., 2000). We have hypothesized that AAV’s small capsid size and innate antioncogenic
properties (reviewed in Ponnazhagan, et al. [2001]) could augment its potential for transducing solid tumor tissue and overcome some of the additional difficulties displayed by other vector systems (Somia and Verma, 2000). The elucidation of this
information would be important to establish AAV’s future role
in cancer therapy. To begin to address this issue, we have compared the transduction efficiencies of AAV (serotype 2) and
ADV (serotype 5) in leukemia, glioma, and mammary cancers.
These cancer subtypes were chosen because they display a variety of tumor biology and clinical behavior. Gliomas show a
local invasive growth but rarely spread outside of the central
nervous system (CNS), whereas mammary cancer is characterized by local invasive growth as well as metastasis. In contrast
to these two diseases, leukemia is a nonsolid systemic disease
from the outset.
In this study, we designed experiments to examine transduction efficiency in three different oncologic models using increased levels of complexity that mimic the pathologic state in
situ. First, we quantitatively assessed the transduction efficiency
of ADV and AAV in 13 different cancer cell lines by flow cytometry to assess the targetable range of the cancers. Second, we
compared the efficiency of these vector systems in human tumor
spheroids to examine penetration into solid tumor tissue. Third,
we generated a unique in vivo model system in which the human
glioblastoma tumor tissue was grown in nude rats followed by
vector injection. To identify transduced cells and their spatial distribution within human tumor tissue in vivo, we performed highresolution magnetic resonance imaging (MRI) and histologic
analyses. Collectively, we observed that AAV and ADV vectors
could transduce a large range of malignant cells, however, AAV
expressed distinct superiority in penetrating solid tumor tissue.
These results clearly demonstrate that AAV have potential to be
an effective delivery system for cancer gene therapy.
ENGER ET AL.
MATERIALS AND METHODS
Viral vectors
Recombinant replication-defective ADV shuttle vectors were
obtained from Quantum Biotechnology Inc. (Montreal, Canada)
and modified by oliogonucleotides to expand the multiple
cloning site and to insert the EGFP gene (F.H. Hoover, unpublished data). Recombinant replication-defective ADV vectors (E1/E3-deficient) were generated by homologous recombination in a human embryonic kidney cell HEK 293. The ADV
lacZ virus was propagated from stocks obtained from Quantum
Biotechnologies. Virus was purified from cells according to
published procedures (Graham et al., 1995). Viral titers were
routinely 1 3 1012 virus particles per milliliter and were determined according to the QBI protocol and protein coat determination with UV adsorption at 260 nm. Recombinant AAV expressing the GFP gene and lacZ genes were gifts from Avigen,
Inc. (Alameda, CA) that were prepared in an adenovirus-free
system using a triple transfection technique with the pHLP19
and pLadeno5 helper vectors (Matsushita et al., 1998; Burton
et al., 1999). AAV titers were 5.6 3 1011 and 8.2 3 1011 particles per milliliter for AAV-GFP and AAV-lacZ, respectively.
Cell culture
All cell cultures were kept at 37°C in a standard tissue-culture incubator (100% relative humidity, 5% CO2), except for
the cell line MDA-MB-468, which was cultured in a CO2 2free
incubator. The human glioma cell lines A172 and U373 were
obtained from the American Type Culture Collection (ATCC,
Rockville, MD). The human glioma cell line D-37 was kindly
supplied by Dr. D.D. Bigner (Duke University Medical Center,
Durham, NC). HF-66 is a cell line derived from a patient with
glioblastoma multiforme (established at Henry Ford Midwest
Neuro-Oncology Center, Detroit, MI). The GaMg cell line was
obtained from a 42-year-old female, and histologically identified as a glioblastoma (Akslen et al., 1988). The BT4 C tumor
cell line is an ethylnitrosurea-induced rat glioma, established in
syngeneic BD-IX rats (Lærum et al., 1977). All glioma cell
lines, as well as the biopsy tissue were grown in Dulbecco’s
modified Eagle’s medium (DMEM) supplemented with penicillin (100 IU/ml), streptomycin (100 g/ml), 2% L -glutamine,
10% heat-inactivated newborn calf serum, and 4 times the prescribed concentration of nonessential amino acids, hereafter
called complete medium (newborn calf serum: Sigma, Steinheim, Germany, all other reagents: Bio Whittaker, Verviers,
Belgium). The human breast carcinoma cell lines MCF7,
AU565, MDA-MB-468, HCC1395, HCC1569, and T-47D were
all obtained from ATCC. The MCF7 cell line was cultured in
DMEM supplemented with 10% fetal calf serum (BioWhittaker). The AU565, HCC1395, HCC1569 cells were grown in
RPMI 1640 supplemented with 10% fetal calf serum (FCS).
MDA-MB-468 cells were grown in L15 medium supplemented
with 10% FCS. T-47D cells were grown in RPMI 1640 supplemented with 10% FCS and 200 U/ml of insulin (Sigma, St.
Louis, MO). The leukemia cell line KG-1a was grown as a suspension culture in DMEM supplemented with 20% FCS and
was obtained from ATCC. A brief description of these cell lines
is provided in Table 1.
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AAV TRANSDUCTION IN HUMAN CARCINOMAS
TABLE 1. ATTRIBUTES
Cell
line
OF
CANCER CELL LINES
Histologic type
A172
Human
glioblastoma
BT4C
Rat glioblastoma
D-37
Human
glioblastoma
GaMg
Human
glioblastoma
HF-66
Human
glioblastoma
U373
Human
glioblastoma
Au565
Adenocarcinoma
breast, human
HCC1395
Ductal
carcinoma,
breast, human
Meta-plastic,
breast, human
HCC1569
MCF7
MDA-MB-468
T-47D
KG-1a
Adenocarcinoma,
breast, human
Adenocarcinoma,
breast, human
Ductal
carcinoma,
breast, human
Myeloblast cell
line, human
Patient 1
Glioblastoma
Patient 2
Glioblastoma
Patient 3
Glioblastoma
Patient 4
Glioblastoma
AND
HUMAN GLIOBLASTOMA TUMOR SPHEROIDS
Characteristics
Nontumorogenic, hypertriploid, EGFR
positive, and a3,vb1,4,5 integrin
positive (Knott et al., 1998).
Tumorogenic in BD IX rats, expresses
N-CAM (Lærum et al., 1977).
Forms spheroids, invades brain
aggregates in vitro, EGFR, a2,3,4,5,6,v
and b1,3,4,5 integrin positive
(Haugland et al., 1997).
Forms spheroids, invades brain
aggregates in vitro, EGFR, a2,3,4,5,6,v
and b1,3,4,5 integrin positive
(Haugland et al., 1997).
Forms spheroids, invades rat brain
aggregates in vitro, a3,4,6,v and
b1,4,5 integrin positive (Knott
et al., 1998).
Tumorogenic in nude mice, EGFR
a3,4,6,v and b1,4,5 integrin
positive (Knott et al., 1998).
EGFR-positive, estrogen-receptor
negative, overexpresses HER-2/neu
and erbB-2 gene, HER-3, HER-4,
and p53 positive (ATCC).
Estrogen receptor positive,
progesterone receptor negative, p53positive (ATCC).
Estrogen and progesterone receptor
negative, HER-2/neu positive, p53
negative (ATCC).
Estrogen receptor positive, wnt7h1
(ATCC).
EGFR positive, tumorogenic in nude
mice (ATCC).
Estrogen and progesterone receptor
positive, wnt3, wnt7h positive
(ATCC).
Patient with erythroleukemia in
myeloblastic relapse. Expresses CD
7, 29, 34, 44, 49d, 49e, 56 antigens
(ATCC).
Female, 35 years old, no previous
treatment.
Male, 71 years old, no previous
treatment.
Male, 69 years old, no previous
treatment.
Female, 47 years old, no previous
treatment.
EGFR, epidermal growth factor receptor. Source or relevant reference is provided in parentheses.
Preparation of spheroids
Spheroids composed of glioma cells were prepared as described previously (Bjerkvig et al., 1990). Briefly, tissue for the
generation of spheroids and xenograft models were obtained
from glioblastoma multiforme tumors at surgery. All patients
gave their verbal informed consent, and the study was approved
by the local ethical committee. Biopsies were minced into 0.5mm fragments and placed into agar-coated tissue flasks (Nunc,
Roskilde, Denmark) with complete medium. After 5–10 days
of culture, tumor cells rounded up, aggregated, and formed multicellular spheroids of various sizes. Spheroids of 250 mm in
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diameter were selected with a pasteur pipette under a Leica
stereomicroscope (Leica, Heidelberg, Germany) and cultured
separately in agar-coated multiwell dishes (Nunc) in complete
medium.
Generation of xenograft models
Sixteen male and female nude rats (Rowett, Aberdeen, Scotland) weighing between 50 and 100 g were kept on a standard
pellet diet, given unlimited access to water, and caged at constant temperature and humidity. The rats were anesthetized subcutaneously with midazolam, 0.2 g per 100 g of body weight;
fentanyl citrate, 0.0126 g per 100 g of body weight; and fluanizone, 0.4 g per 100 g of body weight, before mounting the
animals in a stereotactic frame (David Kopf Instruments, model
900, Tujunga, CA). After a midsagittal incision, a burrhole was
made 1 mm posterior to the bregma suture and 3 mm to the
right of the midline suture. Ten tumor spheroids (250 mm in diameter each) were implanted with a syringe in the brain 2.5 mm
under the surface. The spheroids were derived from two of the
human glioblastoma specimens, the burrhole was closed with
bone wax, and the wound closed with polyamide thread suture.
Animals were returned to their cages and observed daily. Routine monitoring by MRI (see below) 4 weeks after implantation
in each case revealed that the tumor was growing. Typically,
animals harboring tumors survived nonsymptomatic for up to
3 months (depending on the aggressive nature of the tumor).
Animals were killed at the onset of tumor burden-induced
symptoms such as overt weight loss or loss of appetite. MRI
was performed in a Siemens (Erlangen, Germany) Magnetom
Vision Plus 1.5T scanner and a small loop finger coil. Anesthetized rats were fixed in a polystyrene-immobilizing tube.
Coronal T1 (TR 400 ms, TE 14 ms, slice thickness 2.0 mm,
slice center distance 2.0 mm, total, 13 coronal slices covering
the forebrain) presubcutaneous and postsubcutaneous injection
of contrast agent (1.0 ml of gadolinium; 0.5 mmol/ml) and coronal T2 (TR 4000 ms, TE 96 ms, slice thickness 2.0 mm, slice
center distance 2.0 mm, total, 19 coronal slices covering the
forebrain) were obtained. After MRI, the brains were removed
and prepared for histologic analyses (as described below).
Viral transduction
To determine if the culture medium affected the transduction efficiency, pilot experiments were performed assessing
AAV transduction in spheroids and monolayers in three different media preparations: (1) complete medium with 10% FCS;
(2) Optimem (serum-free media; BRL Life Technologies, Oslo,
Norway), and (3) PBS. The different media were prewarmed
and 500 ml of each was added to separate wells, containing 8
glioblastoma spheroids of 250 mm in diameter. To each well,
1 3 108 AAV-GFP particles were added and eight spheroids
were kept as controls. After a 15-hr incubation, the medium
containing virus was removed and replaced with complete
medium, and incubated for 15 days. Monolayer cultures were
processed similarly, but were only incubated for 4 days. These
preliminary experiments were analyzed by confocal fluorescence laser microscopy (CLSM) and showed that different media preparations did not significantly affect transduction in solid
tumor tissue or monolayer culture (data not shown). For further
experiments, we therefore used complete medium. AAV-GFP
and ADV-GFP particles were added at various multiplicity of
ENGER ET AL.
infections (100–500 MOI) in media to all the cell lines at subconfluency. After incubation for 4 days, we assayed for green
fluorescent protein (GFP) activity using CLSM.
Glioblastoma biopsy spheroids (250 mm in diameter) from
four patients were kept individually in 1 ml of complete medium
in 24 wells. ADV- and AAV-lacZ particles (1 3 108) were each
added to eight spheroids and eight spheroids were used as controls for every patient. For two patients, we also added 1 3 107,
5 3 108, and 1 3 109 of AAV- and ADV-lacZ particles to the
spheroids. To study transduction by confocal fluorescence microscopy, we also added 1 3 108 particles of AAV- or ADVGFP to spheroids from two patients.
Xenografts were injected with viral vector 2–4 weeks after implantation using Hamilton syringes connected to the stereotactic
frame. Vector was administered slowly using hand control. MRI
was performed to verify the presence of tumor and to establish
the location of the tumor for injection. ADV and AAV expressing lacZ or GFP was injected stereotactically in the same coordinates by reopening the original incision under anesthesia. To aid
in transduction, 10 ml of 10% mannitol containing 4 3 108 particles were injected in the rats as described elsewhere (Mastakov
et al., 2001). The rats were killed 1, 2, 3, and 6 weeks after viral
injection, and the brains were removed, frozen, cryosectioned, and
stained with lacZ. In rats injected with vectors carrying GFP, the
whole brain was taken out and examined immediately using
CLSM with a low-power objective. By using the transmission detector on the confocal microscope, the fluorescence distribution
was related to the anatomic landmarks. This resulted in a detailed
distribution map of GFP-transduced cells at the brain surface. All
experiments were approved by the Norwegian Animal Research
Authority (Oslo, Norway) in accordance with the Animal (Scientific Procedures) Act 1986.
Flow cytometric analysis
Quantification of viral transduction efficiency in monolayer
cultures was determined by flow cytometric analysis of cells
grown in 6-well plates (Nunc). After 4 days, the cultures were
trypsinized with 3 ml of 0.025% trypsin (BioWhittaker), and
resuspended in 4 ml of growth medium. The cells were then
centrifuged at 100g for 4 min, and resuspended in Dulbecco’s
phosphate-buffered saline (PBS; Sigma) with 0.5% D -Glucose
(Merck, Darmstadt, Germany). The transduction efficiencies
were determined using a FACSort flow cytometer (Becton
Dickinson, Palo Alto, CA). The GFP fluorescence intensities
were obtained by gating a two-parameter forward- and sidescatter cytogram to a one-parameter green fluorescence intensity plot. Both MilliQ-water and PBS were used as negative controls, while fluorescein isothiocyanate conjugated
(FITC) fluorescent CaliBRITE beads (Becton Dickinson)
were used as positive controls and to calibrate the instrument.
Two regions M1 and M2 were chosen to determine the number of untransduced and transduced cells, respectively. The
percentage of gated cells into these regions was determined,
as well as the mean fluorescence intensity in each region.
Three parallels were performed for every flow cytometric experiment.
Confocal Fluorescence Laser Microscopy (CLSM)
GFP-mediated transduction of cells in multicellular tumor
spheroids was assessed by a Leica NT CLSM with an argon-
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AAV TRANSDUCTION IN HUMAN CARCINOMAS
FIG. 1. Increasing multiplicity of infection (MOI) results in increased number of transduced cells. Adeno-associated viral vectors (AAV) or adenovirus vectors (ADV) mediated transduction in AU565 and MCF7 breast cancer and A172 and HF-66 glioma
cell lines at MOI 100, 500, and 1000. Cells were infected with recombinant virus and cultured for 4 days prior to performing
flow cytometry. ADV displayed higher transduction rates than AAV at similar MOIs.
krypton laser using FITC filter optics (Leica). After 8–21 days,
infected spheroids were transferred to an object glass in 1 drop
of PBS, and examined under the microscope. Sixteen optical
sections covering a total of 160 mm were recorded from each
spheroid. All sections were recorded with identical gain settings
on the CLSM. The sections were superimposed into a single
image, and the mean fluorescence intensity was then determined
within a central region of each spheroid. Nontransduced (no
virus medium) spheroids were used as negative controls.
In rats harboring xenografts and injected with vectors carrying GFP, after being sacrificed (1, 2, 3, and 6 weeks), the whole
brain was taken out and examined immediately using CLSM
with a low power objective. By using the transmission detector on the confocal microscope, the fluorescence distribution
was related to the anatomic landmarks.
b-Galactosidase staining
Sections from xenografts, spheroids, or monolayer cultures
were fixed for 20 min in 0.2% glutaraldehyde and 2% paraformaldehyde in PBS. Thereafter, they were washed 3 3 5
min in PBS and stained for b-galactosidase activity with
5-bromo-4-chloro-3-indolyl-b-D -galactopyranoside (X-Gal,
Sigma) as previously described (Thorsen et al., 1997). Stained
spheroids were fixed in 2% glutaraldehyde in 0.1 M sucroseadjusted cacodylate buffer for 24 hr, prior to dehydration in
ethanol and embedding in a mixture of 1:1 of Epon (epoxy
resin, Agar 100) (Agar Scientific, Stensteed, Essex, UK) and
propylenoxide. Polymerization was carried out at 60°C for 48
hr. Xenografts and spheroids were cut using a microtome
(Reichert, Wetzler, Germany) in 20-mm thick sections collected onto Super Frost Slides (Kebo Labs, Bergen, Norway)
and processed as above. Sections were examined under a light
microscope, and pictures were taken at 203 magnification using Nikon digital camera (Nikon Coolpix 990, Nikon Corporation, Tokyo, Japan).
RESULTS
Quantitative analysis of ADV and AAV transduction
efficiency in monolayer cell cultures
We infected six glioma and six mammary cancer cell lines
in monolayer cultures (Table 1) with ADV-GFP or AAV-GFP
at an MOI (virus particles per cell) between 300 and 500 to
identify any qualitative differences in transduction. Four days
postinfection, we confirmed by fluorescence microscopy that
both AAV and ADV vectors were able to transduce cells of different morphology within the same cell line (data not shown).
To quantitate differences in transduction efficiency, ADVGFP and AAV-GFP vectors were added to all cell lines including a leukemia suspension cell line, KG-1a, at an MOI of
100. Flow cytometry was performed on day 4. All cell lines,
with the exception of the leukemia cell line, displayed positive
but varying levels of GFP. Table 2 shows that transduction rates
of the different cell lines ranged between 1.5% and 50%, but
with the majority having a transduction rate over 20% at an
MOI of 100. In general, the cell lines were more permissive to
transduction by the ADV vectors compared to the AAV. The
experiments using ADV and AAV were performed on the cell
lines simultaneously.
To examine the relationship between dose and transduction,
AAV-GFP and ADV-GFP were added at MOIs of 100, 500, and
1000 to two glioma and two mammary cancer cell lines (Fig. 1).
The results were quantitated by flow cytometry 4 days after infection. For all cell lines, the transduction rates increased with
increasing MOI. At all MOIs, transduction rates were higher for
ADV, and were above 80% for all cell lines at 1000 MOI.
Penetration of ADV and AAV in human
biopsy spheroids
Several studies have concluded that gene transfer to solid tumors is a limiting factor (Benedetti et al., 1997; Thorsen et al.,
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ENGER ET AL.
TABLE 2. CANCER CELL LINE PERMISSIBILITY
Glioma
cell line
A172
BT4C
D-37
GaMG
HF-66
U373
AAV
% transduction
ADV
% transduction
Breast
cancer
cell line
13 (12.5–14)
1.5 (1.5–2.0)
22 (20–23)
44.5 (44–45.5)
16 (15.5–16.5)
38 (37.5–38)
18 (16.5–19)
1.5 (1.5–1.5)
51.5 (50–52.5)
81 (80–82)
20 (19–20.5)
81 (78–84.5)
AU565
MCF7
MDA-MB-468
HCC1395
HCC1569
T-47D
AAV
% transduction
43
26
27
51
26
7
(41–45.5)
(25–27)
(26–27.5)
(49–52.5)
(25.5–27)
(6–7.5)
ADV
% transduction
55 (54.5–5.6)
59 (57.5–60)
14 (13.5–14)
23.5 (23–24.5)
46 (43.5–47.5)
26 (24.5–27)
AAV-GFP or ADV-GFP were added to malignant cell lines at a multiplicity of infection of 100. After 4 days, cells were analyzed for GFP expression using flow cytometry. The results represent the mean transduction efficiency from 3 parallel experiments, each based on 5000 cells analyzed by flow cytometry. Range of transduction efficiency is given in parenthesis.
AAV, adeno-associated vector; ADV, adenovirus vector; GFP, green fluorescent protein.
1997; Puumalainen et al., 1998; Enger et al., 1999; Sandmair
et al., 1999; Kuriyama et al., 2001; Rainov and Kramm, 2001).
We generated a three-dimensional model comprising multicellular tumor spheroids established from primary glioblastoma
specimens, and infected with 1 3 108 viral vector particles.
CLSM showed that spheroids transduced with AAV-GFP or
ADV-GFP were evenly fluorescent at the spheroid surface (Fig.
2). Negative controls were largely devoid of positive signal, although some cells displayed autofluorescence. We observed
only minor differences in the fluorescence intensity between
groups of spheroids from different patients (data not shown).
Spheroids were sectioned to compare viral penetration and
transduction of AAV-lacZ and ADV-lacZ within the central areas. Because biopsy spheroids are heterogenous with regard to
cellular composition, we examined at least eight spheroids for
each vector system for all four patients (Fig. 3). Microscopy
showed that transduction varied little between different spheroids from the same patient, or between groups of spheroids obtained from different patients. However, a remarkably different
transduction profile was observed between the two vector systems. While AAV-transduced cells were evenly distributed
throughout the spheroids, ADV-transduced cells formed a halo
at the periphery with few cells transduced in the center of the
spheroid (Fig. 3). Immunohistochemical experiments showed
that these spheroids consisted mostly of glial-derived tumor
cells and endothelial cells; homogenous cellularity through the
nontransduced regions was confirmed using toluidine blue
staining (data not shown). Additional transduction experiments
FIG. 2. Human biopsy spheroids are effectively transduced by adeno-associated viral vectors (AAV) and adenovirus vectors
(ADV). AAV-GFP or ADV-GFP vectors were added to media containing human glioblastoma spheroids. Confocal laser scanning microscopy was performed 8 days after adding vector. Examination revealed an even distribution of green fluorescent protein (GFP) reporter gene expression across the surface of the spheroid (A, shown for AAV) compared to control spheroids (B).
Spheroids from several patients were examined and all were similar. ADV yielded similar results (not shown). Scale bar 5 100
mm.
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AAV TRANSDUCTION IN HUMAN CARCINOMAS
on smaller spheroids (100 mm) gave similar results. Furthermore, comparable observations were recorded when the number of ADV and AAV particles was increased and decreased
by a factor of 10 (data not shown).
ADV and AAV transduction in vivo using human
biopsy xenografts
Genotypic heterogeneity and phenotypic diversity are wellestablished characteristics of human tumor tissue. In order to
examine the efficiency of vector systems for cancer gene transfer in vivo, these parameters need to be considered, thus cell
lines xenotransplanted into animals in vivo are limited by this
contention. Therefore we used animal models that in every way
examined recapitulated human tumor tissue in situ. Nude rats
received tumor spheroids sterotactically placed intracerebrally.
During tumor growth, examination of the brain sections with
MRI indicated that the spheroids developed into diffuse and invading tumors reminiscent and characteristic of the glioma phenotype in situ. After 4 weeks of growth, ADV and AAV (4 3
108 particles) containing GFP or lacZ transgenes were injected
slowly and directly into the center of the growing tumor as assessed by MRI. Following sacrifice (1–6 weeks) and using
CLSM on the fresh rat brain, we identified transduced cells by
GFP expression from both vectors. However, we observed a
striking difference between the two vectors with respect to the
transduction profile. Whereas AAV transduction produced confluent areas with GFP expression up to 2–3 mm from the injection site, ADV-transduced cells were more scattered. In fact,
after injection of ADV-GFP, single cells expressing GFP were
detected around vessels even in the contralateral hemisphere
(Fig. 4A and 4B).
To determine more precisely the domains of transduced cells,
we used AAV-lacZ and ADV lacZ. AAV- lacZ transduced cells
were observed centrally in the tumor, and at distances up to 3
mm from the injection site. In general, clusters of positive cells
(10–15) were separated by a distance of 1–2 mm (Fig. 4E and
4F). After injection of ADV-lacZ, positive cells were consistently observed at the periphery of the tumor where they formed
a characteristic rim at the margin. In contrast, no positive cells
were observed in the central regions of the tumors (Fig. 4G and
4H), consistent with the observations from the spheroids. The
transduction profile was similar in rats sacrificed at different
time points, and control sections of rat brain stained for the lacZ
gene product showed no positive cells (data not shown).
from human tissue in contrast to ADV, which only demarcate
the periphery; and (3) in animal tumor models prepared from
human biopsy material, AAV-mediated transduction is observed in the central regions of the growing tumor, while ADVmediated transduction is restricted to the tumor periphery.
Cellular tropism of ADV and AAV
Human tissue is a natural host for ADV5 and AAV2 vectors. Cellular entry is mediated by several combinations of integrins and the CAR protein for ADV (Bergelson et al., 1997;
Nemerow et al., 1999; Li et al., 2001), whereas heparan sulfate
DISCUSSION
Sufficient and stable gene delivery is a critical prerequisite
for the development of successful gene transfer strategies. Here,
we have assessed the ability of recombinant ADV5 and AAV2
vectors to transduce and penetrate various cancer cell lines and
human biopsy material in vitro. Furthermore, we have evaluated these vectors in a unique in vivo model system using human biopsy material displaying biological characteristics resembling the mother tumor in situ. Our salient findings are: (1)
ADV and AAV are capable of transducing a wide range of
breast cancer and glioma cell lines; (2) AAV efficiently and
completely transduce entire glioblastoma spheroids prepared
FIG. 3. Sections from the center of human spheroids after
transduction with adeno-associated viral vectors (AAV)-lacZ or
adenovirus vector (ADV)-lacZ vectors. Spheroids were selected
from several specimens from four patients and were similar in
diameter. After a period of up to 2 weeks postinfection, the
spheroids were processed for lacZ histochemistry and sectioned.
Spheroids transduced with AAV (A, C, E, G) showed even
transduction through mid-sections, while ADV vectors (B, D,
F, H) transduced few cells in the interior of the spheroid. Increased particles of ADV did not result in any major differences in transduction efficiency. These were the same batches
of ADV and AAV as used in flow cytometry experiments where
ADV yielded higher transduction rates. Scale bar 5 100 mm.
1122
proteoglycan, fibroblast growth factor receptor I, and the avb5
integrin appear to be responsible for AAV infection (Summerford and Samulski, 1998; Qiu et al., 1999, 2000; Qing et al.,
1999; Qiu and Brown, 1999; Summerford et al., 1999). These
cell surface markers are widely expressed, which is consistent
with our findings that both the ADV5 and AAV2 vectors in this
study were capable of transducing a wide range of human breast
and glioma cell lines. Flow cytometry experiments showed that
the quantities of cells transduced by AAV or ADV varied substantially, and this may be accounted for by a differential expression of the receptor components on these cells. Our experiments included cell lines expressing a variety of different
phenotypes including wild-type as well as mutant p53 (Table
1), indicating little correlation between phenotype and permissibility. We observed that the human leukemia cell line, KG1a, and the rat glioma cell line , BT4C, were completely or relatively nonpermissive for AAV2 and ADV5 vectors. In other
studies, leukemia (Itou et al., 1998), rat brain tissue and glioma
cell lines have been transduced (Okada et al., 1996; Rosenfeld
et al., 1997; Mizuno and Yoshida, 1998; Mizuno et al., 1998;
Cunningham et al., 2000; Tenenbaum et al., 2000). Accordingly, the explanation for the low receptivity of AAV and ADV
transduction does not seem to be a general feature of cell lines
derived from particular tissues. An alternative explanation is
that the cytomegalovirus (CMV) promoter common to these
vectors or other cis-acting elements were downregulated by certain factors in these cell lines. An increase in MOI resulted in
a increase in the number of transduced cells. This indicates that
cell surface receptors were not a limiting component of transduction.
Penetration of AAV and ADV particles in human
glioblastoma spheroids
In order to study transduction efficiency in solid tumor tissue, we generated a three-dimensional model comprising multicellular tumor spheroids established from primary glioblastoma specimens. Previous studies have shown that in vitro,
these spheroids contain glioma cells, astrocytes, neurons, and
blood vessels, and express cell ploidy and growth kinetics similar to the mother tumor (Sutherland, 1988; Bjerkvig et al.,
1990). Using this model system, we observed an even transduction over the surface of the spheroids with ADV- or AAVGFP, whereas only AAV were able to transduce the lacZ re-
ENGER ET AL.
porter gene into cells in the central regions of the spheroids.
This is in contrast to and could not have been predicted based
on the results observed in the cell lines. These findings were
consistently observed in spheroids from different tumors and
with use of increased and decreased viral particle numbers. Notably, the same AAV and ADV preparations were used in the
monolayer cultures and spheroid experiments, and typically
more cells were transduced by ADV vectors in the cell line
model. Two explanations can account for these observations
that are not mutually exclusive. First, it is possible that the low
or differential levels of CAR protein in glioma spheroids could
restrict transduction (Grill et al., 2001). Second, the smaller size
of the AAV-capsid (20 nm, compared to adenovirus [100 nm]),
may facilitate AAV movement through solid tumor tissue.
Therefore, based on the available evidence (Benedetti et al.,
1997; Thorsen et al., 1997; Puumalainen et al., 1998; Enger et
al., 1999; Sandmair et al., 1999; Kuriyama et al., 2001), it can
be concluded that AAV are able to penetrate solid tumor tissue
more effectively than either adenoviral or retroviral vectors.
Comparison of AAV and ADV in animal models
Transplantation of human biopsy material into animal models represents a model different from xenografts derived from
immortalized cell lines (Engebraaten et al., 1990, 1999). This
model displays diverse phenotypes consistent with the biologic
heterogeneity of glioblastoma multiforme in situ (Horten et al.,
1981; Stromblad et al., 1982; Engebraaten et al., 1990, 1999;
Peterson et al., 1994). Furthermore, this model has the physical barriers to gene delivery associated with the extracellular
matrix.
Comparing AAV and ADV vectors in this in vivo model revealed several important observations and differences not
recorded in cell culture experiments. Most notably, AAV transduction was seen scattered throughout central regions of the tumor, while ADV transduction demarcated cells in a halo configuration. These observations are consistent with those
observed in the spheroid model, and suggest that AAV can diffuse more effectively compared to ADV. Additionally, the
AAV-transduced cells in vivo had a more clustered appearance
than the vast spread of lacZ-positive cells observed in the spheroids. This may be explained by the dynamic microenvironment
of the tumor in vivo where spread of the vector is mediated by
flow of cerebrospinal fluid and pressure gradients within the tu-
FIG. 4. Adeno-associated viral vectors (AAV) and adenovirus vectors (ADV) transduction in human glioblastoma xenografts.
Human biopsy spheroids were sterotactically injected into the hemispheres of young nude rats. After 4 weeks of tumor development, AAV and ADV vectors carrying lacZ or green fluorescent protein (GFP) were injected into the center of the tumor. After a period of up to 6 weeks, the brains were analyzed for reporter gene expression. Shown in (A) is a confluent area with fluorescence representing clusters of GFP-positive cells in the tumor after AAV injection. In the lower right corner at higher
magnification, is a picture of the area where the tumor infiltrates the surface illustrating a close correlation with the area of fluorescence. In contrast, cells transduced with ADV are distributed in a more scattered pattern (B, arrows indicating transduced
cells). Fluorescence was also detected in the contralateral hemisphere suggesting convection based spread of the vector (blue arrow). The pictures were obtained by imposing a fluorescence picture of the brain surface on a digital photo image of the same
region (See materials and methods for details). (C) and (D) are MRI scans of rat brains harboring tumors that were injected with
AAV and ADV, respectively. Blue spots indicates the area of transduction in a region of 3 mm anterior and posterior to the plane
of the needle tract (red line). After injection of AAV-lacZ, transduced cells are observed in the central regions of the tumor (E).
Transduced cells are also seen at the periphery of the tumor, although these are few in number (F). After injection with ADV,
transduced cells were virtually absent from the central regions of all tumors (G), wherease a strong band of positive cells was
observed at the periphery of the tumor (H).
AAV TRANSDUCTION IN HUMAN CARCINOMAS
1123
1124
ENGER ET AL.
mor. Also, the in vivo tumors grow and expand to be larger than
the spheroids, such that AAV may not be available to tumor
cells born later. In contrast, cells transduced by ADV were observed at the tumor periphery and not the injection site. One
explanation could be that ADV initially transduced cells at the
injection site, but as the tumor expanded cells remained in the
same spatial configuration, such that at the time of analysis cells
were positioned at the tumor periphery. Furthermore, the presence of ADV-mediated GFP-positive cells in the perivascular
space of capillaries outside the tumor region, suggests that the
spread of this vector to some degree is convection-based.
In summary, we have generated a rational and graded comparison between AAV and ADV using models with increasing
complexity: (1) cell line data from highly standardized material; (2) three-dimensional models in vitro from human biopsy
spheroids that are less standardized, and (3) in vivo models in
which the experimental conditions are not standardized and thus
are more representative of the pathologic state in situ. Our interpretation of the current data, is that even though ADV and
AAV are comparable in their tropism for cancer cell types,
AAV are better gene transfer vectors for cancer gene therapy
because they penetrate solid tumor tissue in vivo more effectively. Taken together, the evidence provided here suggests that
AAV could play a role in achieving therapeutic efficacy for
cancer gene therapy.
ACKNOWLEDGMENTS
This work was supported by grants from the Norwegian
Health Ministry and the Norwegian Cancer Society. We thank
Dr. Kenneth Chahine and the team at Avigen Inc. (Alameda,
CA) for their kind gift of high-titer AAV-EGFP and AAV-lacZ
particles. We are grateful to Iren Sefland (University of Oslo),
Tove Johansen, and Beatrice Probst for technical assistance.
Tore-Jacob Raa and Aina Johannessen are gratefully acknowledged for their animal husbandry. We thank Drs. Karl-Henning
Kalland, Stig-Ove Bøe, and Kjersti Lønning for critical discussions.
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Address reprint requests to:
Frank Hoover
Haukeland Hospital
Department of Oncology
Oncology Research Laboratory
5021 Bergen
Norway
E-mail: frank.hoover@vir.uib.no
Received for publication January 9, 2002; accepted after revision April 22, 2002.
Published online: May 30, 2002.