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1 From the Schepens Eye Research Institute, 2 Brigham and Womens Hospital and 3 Committee on Immunology, 4 Harvard Medical School, Boston, Massachusetts; 5 Santen Inc., Napa, California.
| Abstract |
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METHODS. A murine model of allergic conjunctivitis involving SWR/J mice sensitized and challenged with short ragweed was used to test immunostimulatory DNA sequences for therapeutic potential. ISS-ODN or control ODN (0.1 mg/mouse) was administered intraperitoneally or topically to the conjunctiva 3 days before final allergen challenge. Multiple parameters of clinical symptoms evident during the acute-phase reaction and the cellular components of the late-phase reaction were evaluated in both groups of mice.
RESULTS. All parameters of clinical symptoms were markedly inhibited after intraperitoneal injection of ISS-ODN, whereas topical application to the conjunctiva did not inhibit clinical symptoms significantly. Remarkably, a single topical treatment with ISS-ODN (as well as by intraperitoneal injection) completely inhibited both eosinophilia and neutrophilia in the late-phase reaction.
CONCLUSIONS. Systemic or conjunctival administration of ISS-ODN was shown to significantly inhibit allergic responses in this mouse model. This indicates that ISS-ODN may be an effective form of immunotherapy for this class of allergic disease.
| Introduction |
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Ocular allergies are most commonly initiated in response to airborne allergens,1 2 3 with short ragweed (SRW) pollen being the major cause of late summer rhinoconjunctivitis in North America.4 In Japan, allergic responses to cedar pollen constitute a problem of even greater relative magnitude. Antihistamines or steroids are commonly used to treat ocular allergies; however, they only provide temporary relief for a subset of patients. Moreover, use of steroids can increase the incidence of serious complications such as the development of glaucoma or cataracts.
For these reasons, immunotherapy may be preferable for providing long-term relief without medication. Conventional immunotherapy through desensitization by repeated exposure of low-dose allergen has achieved some success, although the majority of patients remain unresponsive. Recently, considerable interest has materialized in the potential use of immunostimulatory sequence oligodeoxynucleotides (ISS-ODN) in the treatment of allergic diseases.5 6 7 8 This field developed from the finding by Tokunaga et al.9 that bacterial DNA has immunostimulatory effects. After years of searching for the responsible sequences, Krieg et al.10 reported that the unmethylated Cytidine-phosphate-Guanosine (CpG) dinucleotide motif has immunostimulatory activity. These ISS-containing CpG motifs have now been shown to bias the immune response toward the development of an antigenspecific T helper (Th) cell type 1 response.11 12 Indeed, ISS-ODNs have been strong therapeutic agents in murine models of airway inflammation.12 13 14
As is the case for allergic reactions in other mucosal tissues, conjunctival allergy results from a prevalent Th2 response to allergen (as determined by the analysis of conjunctiva-derived T-cell clones from patients with SAC).15 16 Thus, skewing of the Th1Th2 response to an allergen in patients with SAC and VC could have significant therapeutic value, as has been shown in studies of airway hyperresponsiveness and allergic responses in the skin.
We show in this study that intraperitoneal injection of ISS-ODN prevents clinical effects of SAC in a mouse model, and effectively suppresses the late-phase reaction in the conjunctiva. Moreover, although a single topical administration of ISS-ODN does not significantly affect the acute-phase reaction in this model, it completely inhibits inflammatory cell recruitment in late-phase reaction. Together with a recent report, these data indicate that ISS-ODN treatment of conjunctival allergy may be a highly promising future immunotherapy for this major human allergy.17
| Materials and Methods |
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Allergen Challenge Protocol
Mice were sensitized to SRW pollen according to the protocol
originally reported by Magone et al.18
A suspension of 50
µg of SRW (ICN, Aurora OH) and 1 mg of aluminum hydroxide (Sigma, St.
Louis, MO) were injected into the left hind footpad under anesthesia.
On day 14, conjunctivitis was induced by topical application of 1.5 mg
SRW suspended in 10 µl of phosphate-buffered saline (PBS; Fig. 1
).
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Clinical Evaluation
Mice were observed in a double-blind study for clinical symptoms
of immediate hypersensitivity response 20 minutes after the topical
challenge with SRW. Chemosis, conjunctival redness, lid edema and
redness, and tearing and discharge were scored 0 to 4, after evaluation
by slit lamp, according to the modified criteria described by Magone et
al.18
For assessment of severity of the symptoms,
representative photographs of the mouse eyes were used. The cumulative
clinical score was calculated as the sum of the scores of each of these
four parameters.
Histology
To evaluate the cellular infiltrates in the conjunctiva during the
late-phase reaction, eyes were enucleated with the attached lids and
conjunctiva intact (24 hours after SRW challenge) and immediately fixed
in 4% paraformaldehyde. The tissue was then embedded in resin
(Historesin; Leica Instruments, Heidelberg, Germany) and serially
sectioned (3 µm thick). The sagittal sections were stained with
Giemsa or hematoxylin and eosin. Five conjunctival tissue sections from
each eye were examined and counted by a masked observer under a
microscope, using a x400 high-powered field.
Statistical Analysis
Data were analyzed using MannWhitney test or ANOVA, as
appropriate, and differences were considered significant at
P < 0.05.
| Results |
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Inhibition of Late-Phase Reaction by Both Systemic and Topical
ISS-ODN Treatment
The late-phase reaction of antigen-challenged conjunctival tissue
is characterized by a profound inflammatory cell recruitment consisting
of eosinophils and neutrophils. This chronic inflammation is pronounced
in cases of severe SAC and VC. We examined whether systemic or topical
ISS-ODN treatment could inhibit the late-phase reaction in this mouse
model. Conjunctival eosinophilia was evaluated by counting eosinophils
attracted to the forniceal region of the conjunctiva. SRW challenge to
the sensitized mice (control animals and those treated with control
ODN) resulted in a massive eosinophil recruitment into the conjunctival
fornix, as expected. Intraperitoneal injection of ISS-ODN abolished the
conjunctival eosinophilia, consistent with that observed in lung
eosinophilia. Surprisingly, a single topical application of ISS-ODN
also abolished eosinophil recruitment, although the effect on clinical
symptoms was insufficient (Fig. 3)
.
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| Discussion |
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These experiments also raise interesting questions about the mechanism of action of ISS-ODN. As discussed previously, bacterial DNA was initially shown to have immune stimulatory properties9 and the responsible element was found to be the CpG motif now known as ISS-ODN. ISS-ODN are known to affect both the innate immune system (by dendritic cells and macrophages) and the acquired immune system (by activation of B cells). The dendritic cells and macrophages have two major roles in priming the immune responses. First, antigen-presenting cells (APCs) process antigens and present antigenic peptides to T cells through class II major histocompatability complex (MHC) molecules. Second, APCs secrete proinflammatory cytokines, leading to T- and B-cell activation. Because polarization of activated T-helper cells into Th1 or Th2 phenotypes is observed in ISS-ODNtreated animals, the APCs are thought to be a major target of ISS-ODN with respect to the deviation toward the Th1 phenotype.
It is now known that ISS-ODN induce the expression of various cytokines
that in total contribute to the skewing of the T-cell response toward
the Th1 phenotype systemically.14
Using in vivo
administration of ISS-ODN, splenic antigen-presenting cells (APCs)
efficiently activate naïve T cells and bias their
differentiation toward a Th1 phenotype.20
Importantly,
ISS-ODN stimulate APCs to upregulate the Th1-like cytokines interleukin
(IL)-12 and IL-18.7
In addition, ISS-ODN induces IL-2
receptor and interferon-
receptor expression on macrophages and B
cells.21
Systemically, ISS-ODN administration is known to
promote high IgG2a titers and lower IgG1 and IgE titers and reduce
serum IL-4 and IL-5 levels, also dampening the allergic
response.6
22
Using murine allergic airway models, three research groups have shown
that ISS-ODN treatment can effectively inhibit airway
hyperresponsiveness and inflammation.12
13
14
Although
interferon
and IL-12 usually oppose Th2-mediated inflammation,
Kline et al.22
also showed that neither of these cytokines
is absolutely required, by using allergen-sensitized knockout mice,
which suggests an IL-12/interferon-
independent mechanism of
ISS-ODN action. While preparing this manuscript, we have identified an
abstract by Magone et al., 17
which concurs with this
articles conclusion about the therapeutic potential of ISS-ODN for
allergic conjunctivitis. Also using a mouse model, they report that
systemic ISS-ODN treatment can block early- and late-phase reactions
after antigen challenge, agreeing completely with our findings. Because
the protective effect was abolished by systemic anti-IL-12 treatment,
they concluded that ISS-ODN action is mediated by IL-12. This is
consistent with the data of Kline et al.,22
who also
showed, in the absence of either IL-12 or interferon-
, that smaller
amounts of ODN do not provide protection against airway eosinophilia.
The treatment protocol is likely to have an important impact on the
effectiveness of this new form of therapy. Kobayashi et
al.19
reported that ISS-ODN have prepriming effects that
last for up to 2 weeks with an optimal effect observed when animals are
treated between 3 and 7 days before challenge. The ISS-ODN prepriming
induces Th1 bias, characterized by interferon-
and IL-12 production
by splenocyte and elevated serum IgG2a levels and cytotoxic T
lymphocyte (CTL) responses. The activated interferon-
production by
splenocytes peaks when prepriming is conducted 3 days before antigen
challenge. In addition, other successful treatment protocols for
disease models support that ISS-ODN triggers an immunostimulatory
cascade that matures over a period of several days.12
23
For all these reasons, we treated the animals with ISS-ODN 3 days
before the final challenge.
For clinical use, we believe that ISS-OSN treatment in the form of eyedrops may provide significant benefits. However, as was shown in Figure 2 , we could not achieve sufficient inhibition of clinical symptoms, despite a clear inhibition of late-phase reaction. To exclude the possibility that the prepriming period was not optimal, we also treated the animals from day 0 to day 3. However, the outcome of inhibitory effect on clinical symptoms corresponded to Figure 2 , and any inhibitory effect was subtle at best (data not shown). These data indicate that there are limitations for ISS-ODN eyedrop treatment of allergic conjunctivitis. However, we cannot exclude the possibility that other drug delivery methods may permit topical ISS-ODN administration to inhibit both phases of the allergic reaction in the eye.
The molecular mechanisms of immune stimulation by ISS-ODN also remain
unknown. For the signaling pathway of ISS-ODN, downstream signaling
steps including the nuclear factor (NF)
B and mitogen-activated
protein kinase (MAPK) pathways.24
25
26
To transmit the
signals, ISS-ODN must be internalized into the cell by endocytosis.
Intracellular CpG receptors have not been found. To understand the
signaling pathway, it is essential to identify such intracellular
binding proteins. Following the signaling pathway of ISS-ODN, multiple
early-response genes, proto-oncogenes, and cytokine genes are
induced.27
For cytokine gene expression, in vitro studies
have shown that ISS-ODN induces IL-12, tumor necrosis factor (TNF)-
,
IL-6, IL-1ß, IL-1RA, macrophage inflammatory protein (MIP)-1ß,
monocyte chemotactic protein (MCP)-1, interferon-
/ß/
, and
IL-18.27
However, there is little knowledge concerning the
mechanisms by which these genes are activated by ISS-ODN. In the case
of conjunctival allergy, virtually nothing is known. Because the
pathophysiology of SAC and atopic asthma are considered to be distinct,
targeted research on the mechanism of ISS-ODN action in both clinical
entities is required.13
28
It is difficult to attribute the impressive therapeutic effect of
topical ISS-ODN treatment on conjunctival eosinophilia and neutrophilia
solely to the known mechanism by which ISS-ODN skews the Th response.
Other mechanisms, including induced expression of cell surface MHC,
costimulatory and adhesion molecules, and cytokines and Fc
receptors,20
may play significant roles in the
conjunctiva. In the lung models, inhibition of eosinophilia seems to be
derived in large part from systemic effects, which is more difficult to
envision in the current experiments. Thus, the cytokine milieu of the
target tissue must be considered in the ISS-ODN response, because this
has a significant impact on the outcome of ISS-ODN
treatment.29
30
In the conjunctiva, the well known
abundance of dendritic cells may substantially affect or deviate the
outcome of the treatment. Although the relative importance of each
target cell in vivo is not yet well understood, dendritic cells and
macrophages seem to have a crucial role in the release of inflammatory
mediators, including IL-12, IL-18, and interferon-
, which then
induce interferon-
from natural killer cells.22
In this
Th1-like environment, T cells are skewed toward the Th1 type, whereas
no clear evidence has been obtained that ISS-ODN directly skews Th cell
subsets.
In conclusion, these studies indicate that ISS-ODN treatment (together with other novel approaches) may be useful in the treatment of SAC and VC, the most common allergic diseases of humans.31 32 Acting by distinct pathways from existing therapies, ISS-ODN has the potential to significantly enhance our ability to treat these diseases.
| Acknowledgements |
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| Footnotes |
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Submitted for publication March 1, 2000; revised May 15, 2000; accepted July 5, 2000.
Commercial relationships policy: F, C (SJO); N (all others).
Corresponding author: Santa Jeremy Ono, Schepens Eye Research Institute, Harvard Medical School, 20 Staniford Street, Boston, MA 02114. sjono{at}vision.eri.harvard.edu
| References |
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