The Journal of Immunology, 2001, 166: 5878-5881.
Copyright © 2001 by The American Association of Immunologists
Cutting Edge: Systemic Inhibition of Angiogenesis Underlies Resistance to Tumors During Acute Toxoplasmosis1
Christopher A. Hunter*,
Duonan Yu*,
Michael Gee
,
Cam V. Ngo*,
Cinzia Sevignani*,
Michael Goldschmidt*,
Tatyana V. Golovkina
,
Sydney Evans
,
William F. Lee
and
Andrei Thomas-Tikhonenko2,*
Departments of
*
Pathobiology,
Medicine, and
Clinical Studies, University of Pennsylvania, Philadelphia, PA 19104; and
The Jackson Laboratory, Bar Harbor, ME 04609
 |
Abstract
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The ability of various infections to suppress neoplastic growth
has been well documented. This phenomenon has been traditionally
attributed to infection-induced concomitant, cell-mediated antitumor
immunity. We found that infection with Toxoplasma gondii
effectively blocked neoplastic growth of a nonimmunogenic B16.F10
melanoma. Moreover, this effect was independent of cytotoxic T or NK
cells, production of NO by macrophages, or the function of the
cytokines IL-12 and TNF-
. These findings suggested that antitumor
cytotoxicity was not the primary mechanism of resistance. However,
infection was accompanied by strong, systemic suppression of
angiogenesis, both in a model system and inside the nascent tumor. This
suppression resulted in severe hypoxia and avascular necrosis that are
incompatible with progressive neoplastic growth. Our results identify
the suppression of tumor neovascularization as a novel mechanism
critical for infection-induced resistance to
tumors.
 |
Introduction
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The
first observation that infection interferes with tumor growth was made
by William B. Coley, who noted that streptococcal infection caused
regression of soft tissue sarcomas (reprinted in Ref. 1).
Subsequent research revealed that other bacterial (Listeria
monocytogenes, Corynebacterium parvum, Mycobacterium
bovis) and protozoan (Toxoplasma gondii,
Besnoitia jellisoni) pathogens nonspecifically activated
macrophages to kill tumor cells in vitro (reviewed in Ref.
2) and conferred upon the host resistance to tumors
(3, 4, 5). The discovery of other cytotoxic components
of innate (NK cells) and adaptive (CTLs) immunity has extended the list
of candidates potentially responsible for tumor suppression (reviewed
in Ref. 6). However, the contribution of the immune system
to the infection-induced resistance to tumors remained unclear. We set
out to determine whether stimulation of the immune response underlies
resistance to tumor growth in infected animals using acute
toxoplasmosis as a model.
 |
Materials and Methods
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Tumor load studies
A total of 1 x 106 tumor cells were
injected s.c. in the flanks of syngeneic (C57BL/6J) or
immunocompromised (B6.129P2-Nos2tm1Lau, C.B-17
scid-beige, C57BL/6-Pfptm1Sdz,
B6.129-Tnfrsf1atm1Mak, and
B6.129S1-Il12btm1Jm) animals. Scid-beige mice
were obtained from Taconic Farms (Germantown, NY); all other strains
were obtained from The Jackson Laboratory (Bar Harbor, ME). Acute
infection was initiated by i.p. injection with 15 cysts of the ME49
strain of T. gondii (7).
Matrigel angiogenesis assay
The assay has been described in detail previously
(8). As angiogenic stimuli, basic fibroblast growth factor
(bFGF)3 (500 ng per
pellet; Collaborative Biomedical Products, Bedford, MA) or
p53-deficient colonocytes (250,000500,000 cells per pellet) were
used. No less than six Matrigel (Collaborative Biomedical
Products) pellets were analyzed in each experiments, and each
experiment was repeated at least twice.
Tumor neovascularization and hypoxia studies
To visualize areas of hypoxia and necrosis as well as newly
sprouting blood vessels, 3 h before sacrifice animals were
injected in the tail veins with EF5
(2-[2-nitro-1H-imidazol-1-yl]-N-(2,2,3,3,3-pentafluoropropyl)acetamide;
provided by C. J. Koch, University of Pennsylvania). Frozen
sections of excised tumors were stained with a Cy3-conjugated Ab
against EF5 (9) and a mAb against platelet-endothelial
cell adhesion molecule (PECAM)/CD31 (BD PharMingen, San Diego,
CA). Slides were counterstained with hematoxylin to reveal areas of
viable and necrotic cells. To visualize the architecture of blood
vessels in nascent neoplasms, 15 min before sacrifice animals were
injected into the tail veins with FITC-conjugated tomato lectin (Vector
Laboratories, Burlingame, CA). Then 1-mm-thick sections were examined
using confocal microscopy (10).
 |
Results and Discussion
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To investigate the connection between acute infection and
resistance to tumors, mice were infected with T. gondii and
challenged with the B16.F10 melanoma cells that on their own are very
weakly, if at all, immunogenic (11). When injected s.c. in
syngeneic C57BL/6 mice, these cells form rapidly growing tumors
reaching
0.5 g in 1012 days. However, in animals infected with
T. gondii on the day of tumor implantation, tumor growth was
severely suppressed (Fig. 1
A).
Histopathological analysis revealed that while B16 tumors from
uninfected animals (Fig. 1
B) possessed adequate vasculature
(yellow arrows) and typically exhibit only small areas of focal
necrosis, B16 cell masses from animals infected for 12 days are largely
nonviable (Fig. 1
C). In the experiment depicted in Fig. 1
D, the weights of these abortive tumors did not exceed 15
mg,
10% of the neoplasms from uninfected mice. To demonstrate that
this inhibitory effect is applicable to other neoplastic cells, we
performed the same experiment with murine colonocytes transformed in
vitro by the Ki-Ras2 oncogene (12), which bears the
nonimmunogenic G-to-V mutation (13, 14). These cells are
normally tumorigenic, but in infected mice formed only small masses
whose average weight did not exceed 30 mg (Fig. 1
E). Thus,
infection of mice with T. gondii effectively inhibited
neoplastic growth.

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FIGURE 1. Characterization of the tumors developing during acute infection.
A, Gross appearance of B16 melanomas forming in normal
(top) and T. gondii (Tg)-infected
(bottom) C57BL/6J mice 12 days postimplantation.
B and C, Histopathological (hematoxylin
and eosin) staining of the specimens depicted in A,
top and bottom, respectively. Yellow
arrows in B point at perfused blood vessels.
DJ, Average weights of tumors formed by B16.F10
(D and FJ) and Ras-transformed
colonocytes (E) in control and T. gondii
(Tg)-infected mice. Each panel represents an individual experiment in
which no less than six tumors per group were analyzed. Error bars
represent SEs of the mean. All differences in tumor weights between
infected and control animals are highly statistically significant per
Students t test, with p < 0.0001.
Each experiment has been repeated at least twice, with comparable
results. Data in FJ were obtained using indicated
strains of immunocompromised mice.
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To assess whether cytotoxic functions associated with innate or
adaptive immunity contribute to this inhibition, mice deficient in
different immune components were used for tumor load studies. Infection
with T. gondii has been shown to result in the activation of
macrophages to nonspecifically kill tumor cells in vitro
(2). Macrophages destroy tumor cells using two principal
mediators, NO produced by the enzyme inducible NO synthase and TNF-
(15, 16). Because B16 melanomas cells are sensitive to NO
but not TNF-
(17), mice lacking inducible NO synthase
(18) were chosen for further studies. In addition,
neoplastic cells could be destroyed by CTLs and NK cells; thus we also
used scid-beige mice where CTLs are absent and NK cells are not
cytotoxic, and perforin (Pfp) knockout mice where cytolytic functions
are essentially absent (19). In all three strains, the
ability of T. gondii to suppress tumor growth was evident:
while absolute tumor weights varied in different strains, the ratios of
tumor weights in uninfected and infected animals were equivalent to
that observed in wild-type animals (Fig. 1
, FH). We have
also analyzed strains of mice deficient in two major immunomodulatory
pathways, TNF-
and IL-12, that are required for activation of
macrophages, T cells, and NK cells and have been implicated in immunity
to tumors. Their functions are disrupted in animals lacking the p40
subunit of IL-12 (20) and the type 1 TNF-
receptor
(21). Nonetheless, infection with T. gondii
still suppressed tumor growth in both of these strains (Fig. 1
, I and J). Together, these data indicated that
most of the major cytotoxic mechanisms implicated in tumor surveillance
are not required for infection-induced resistance to tumors. This
implied the existence of an alternative, noncytotoxic mechanism
responsible for tumor suppression.
Histological analysis of abortive tumors from infected animals revealed
that, in addition to being necrotic, these masses conspicuously lacked
blood vessels (Fig. 1
, B and C). Because
insufficient tumor vascularization could be due to suppression of
angiogenesis during infection, in vivo Matrigel neovascularization
experiments were performed. bFGF was used as an angiogenic stimulus in
A/J mice. The assay was quantitated by determining hemoglobin content
of "empty" or bFGF-containing Matrigel pellets. As shown in Fig. 2
A, Matrigel pellets
containing bFGF were vascular and rich in hemoglobin in uninfected mice
but not in animals infected with T. gondii; the latter
contained only baseline amounts of hemoglobin characteristic of
"empty" Matrigel pellets. bFGF is a poor inducer of angiogenesis in
C57BL/6J mice (Ref. 22 and data not shown), and B16 cells
themselves cannot be used for Matrigel experiments because the presence
of melanin interferes with the hemoglobin assay. Therefore, primary
colonocytes from p53-/- C57BL/6J mice were used
as an angiogenic stimulus. When embedded in Matrigel, these
immortalized but nontumorigenic colonocytes stimulated robust
neovascularization, as indicated by high hemoglobin content (Fig. 2
B). Colonocyte-induced neovascularization was largely
suppressed when Matrigel-bearing animals were infected with
T. gondii (Fig. 2
B). Histopathological staining
of excised pellets confirmed that in uninfected but not in T.
gondii-infected animals, Matrigel pellets contain vascular
channels that were well perfused, as judged by the presence of
erythrocytes (Fig. 2
, C and D, yellow arrows).
Together, these results indicate that infection with T.
gondii results in systemic suppression of angiogenesis that
potentially could cause insufficient tumor vascularization and stunt
neoplastic growth.

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FIGURE 2. Neovascularization of Matrigel implants during acute infection.
A, Average hemoglobin contents of bFGF-containing
Matrigel pellets in normal and T. gondii (Tg)-infected
A/J mice. Bars 1 and 2 and the dotted line refer to "empty"
Matrigels, bars 2 and 4 to Matrigels containing 500 ng of basic FGF.
B, Average hemoglobin contents of p53-deficient
colonocytes containing Matrigels pellets in normal and T.
gondii (Tg)-infected mice. C and
D, Histopathological staining of the Matrigels analyzed
for hemoglobin content in B, left and
right, respectively. Yellow arrows in C
point at perfused vascular spaces. Note the presence of acinar
structures (blue arrows) that are formed by differentiating
colonocytes.
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To compare and contrast tumor vascularization in infected and
uninfected mice, we examined small, nascent tumors earlier in
infection, before wide-spread necrosis developed and while they were
still similar in size to tumors in normal animals. Therefore, animals
infected with T. gondii were used on day 7 or 8
postimplantation and control animals were used on day 5 or 6. The use
of different time points ensured that tumors did not differ in size by
>50%. Furthermore, at these time points B16 cell masses were largely
composed of viable, melanin-producing cells both in uninfected and
infected animals (Fig. 3
, A
and B, outside red dashed line). To assess the level of
hypoxia in these size-matched cell masses, animals were injected i.v.
with EF5. EF5 is a compound that is taken up by all perfused tissues
but only in hypoxic cells is it converted into immunohistochemically
detectable adducts (23). Subsequent staining with an Ab
against EF5 adducts revealed extensive areas of hypoxia in tumors from
infected mice (Fig. 3
D), whereas in control tumors no EF5
staining was apparent (Fig. 3
C). Characteristically, all
contiguous areas of hypoxia surrounded areas of necrosis (inside the
dashed lines, Fig. 3
, B and D) that lacked
viable, melanin-producing cells; no such areas were found in tumors
from uninfected mice (Fig. 3
C).

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FIGURE 3. Tumor neovascularization and hypoxia during acute infection. All images
in the left column refer to small B16 tumors formed in
normal mice; all images in the right column refer to
small tumor masses formed in T. gondii-infected mice.
AF represent frozen sections; G and
H represent live tissues. A,
C, and E correspond to the same
microscopic field, as do images B, D, and
F. A and B, Bright field
microscopy of frozen sections of B16 tumors developing in normal and
infected mice, respectively. Brown staining is melanin produced by live
B16 cells. The unstained area in B outlined by the red
dashed line is the area of tumor necrosis, with no viable cells.
C and D, Immunohistochemical staining of
hypoxic region using an Ab against EF5 adducts. The area within the
dashed line in D is identical with the area outlined by
the red dashed line in B. E and
F, Immunohistochemical staining of nascent blood vessel
using an Ab against the CD31/PECAM surface marker of endothelial cells.
G and H, Staining of perfused blood
vessels by FITC-conjugated lectin as revealed by confocal microscopy
and superimposition of up to 25 computer images corresponding to the
same field at focal planes separated by 5 µm. The original green FITC
color has been digitally converted to red. All analyses have been
repeated on four additional tumors, with comparable results obtained in
all experiments.
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Because tumor hypoxia is likely to result from an inadequate blood
supply (24), studies were performed to assess how
infection with T. gondii affects growth of new blood
vessels. Tumor sections from the previous experiment were
counterstained with an anti-PECAM/CD31 mAb, which is reactive with
capillary endothelial cells. While tumors developing in control animals
stained positively for this marker (Fig. 3
E), tumors from
infected animals exhibited only diffuse background staining (Fig. 3
F). To demonstrate that these CD31-positive endothelial
cells are part of a functional vascular system, tumor-bearing animals
were injected i.v. with FITC-conjugated lectin. This compound binds to
endothelial cells in all perfused tissues, allowing for
computer-assisted three-dimensional reconstruction of the vascular
system in nascent neoplasms. In control animals, B16 tumors possessed
elaborate vasculature, with small vessels branching off larger ones
(Fig. 3
G), whereas no contiguous vascular system was
apparent in tumors from infected animals (Fig. 3
H). The
spatial correlation between insufficient neovascularization, hypoxia,
and necrosis strongly suggests that inhibition of angiogenesis during
infection is crucial for resistance to tumors. Interestingly, this
conclusion is in agreement with the data from the 1940s that treatment
with bacterial LPSs negatively affects tumor vascularity
(25).
It has long been assumed that resistance to tumors during infection
could be explained by concomitant, cell-mediated antitumor immunity.
Our results indicate that resistance requires neither the effector
function of cytotoxic cells (macrophages, CTLs, and NK cells) nor the
two major cytokines (TNF-
and IL-12) that activate them.
Nevertheless, besides being able to lyse neoplastic cells, immune cells
also secrete soluble factors that can stunt tumor growth indirectly,
for instance via suppression of angiogenesis. The existence of such
soluble anti-angiogenic factors is supported by the fact that
plasma from infected but not control mice blocks formation of tube-like
structure by endothelial cells in vitro on the surface of Matrigel
(data not shown). Moreover, several cytokines systemically produced
during acute toxoplasmosis have been reported to possess
anti-angiogenic properties, most notably type I and type II IFNs
(26, 27). Although our preliminary data indicate that
separate neutralization of IFN-
and IFN-
does not restore
angiogenesis in infected animals, their combined action might be
responsible for insufficient tumor vascularization. This concept is
illustrated by the ability of TNF-
to suppress neovascularization in
conjunction with IFN-
(28). Interferon-
is also
known to activate IFN-
-inducible protein 10, a chemokine implicated
in both resistance to T. gondii (29) and
suppression of angiogenesis (30).
Identification of an infection-induced soluble factor(s) responsible
for suppression of angiogenesis and, by inference, tumor growth would
have important therapeutic implications. T. gondii is
unlikely to be useful for treatment of cancer. In our experimental
system, after the acute phase of infection is over, the remaining
viable cells resume neoplastic growth. Moreover, in many cancer
patients toxoplasmosis is a life-threatening illness (31).
In contrast, systemic administration of a soluble anti-angiogenic
factor could be maintained indefinitely, might not result in acquired
drug resistance (32), and has proven successful in
treating experimental neoplasms (33, 34, 35). Understanding
the molecular basis for infection-mediated suppression of angiogenesis
might be instrumental in identifying new inhibitors of tumor
vascularization and developing new treatments for cancer.
 |
Footnotes
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1 This work has been supported by grants from the National Institute of Allergy and Infectious Diseases (AI42334, to C.A.H.), National Cancer Institute (CA71881, to A.T.-T.), and from the University of Pennsylvania Cancer Center (to A.T.-T.). 
2 Address correspondence and reprint requests to Dr. Andrei Thomas-Tikhonenko, University of Pennsylvania, 3800 Spruce Street, Philadelphia, PA 19104-6051. 
3 Abbreviations used in this paper: bFGF, basic fibroblast growth factor; PECAM, platelet-endothelial cell adhesion molecule. 
Received for publication December 27, 2000.
Accepted for publication March 16, 2001.
 |
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S. Ehlers, J. Benini, H.-D. Held, C. Roeck, G. Alber, and S. Uhlig
{alpha}{beta} T Cell Receptor-positive Cells and Interferon-{gamma}, but not Inducible Nitric Oxide Synthase, Are Critical for Granuloma Necrosis in a Mouse Model of Mycobacteria-induced Pulmonary Immunopathology
J. Exp. Med.,
December 17, 2001;
194(12):
1847 - 1859.
[Abstract]
[Full Text]
[PDF]
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