Download Pathogenic Mechanisms of Uveitis

Pathogenic Mechanisms of Uveitis
Brian C. Gilger, DVM, MS, Dipl. ACVO, Professor of Ophthalmology
Department of Clinical Sciences, College of Veterinary Medicine
North Carolina State University, Raleigh, NC
General Features of Ocular Immunology and Uveitis
The two components of the anterior uvea, the iris and the ciliary body, contain heavily
pigmented connective, vascular, and muscle tissue. The iris functions as a shutter that
responds to prevailing light conditions, and the ciliary body produces aqueous humor
through active secretion and ultrafiltration of plasma. Both the iris and ciliary body have
many blood vessels within their connective tissue, and the inner aspect of both
structures is lined by a double layer of epithelium. The layer of epithelium closest to the
connective tissue is pigmented, and the boundary layer closest to the vitreous is
nonpigmented. The choroid, or posterior part of the uvea, functions as the primary
vascular supply of the horse retina. It lies between the sclera and the retina and
contains the tapetum, the fibrous reflective layer.
The uveal tract contains most of the blood supply of the eye and is in direct contact with
peripheral vasculature. Therefore diseases of the systemic circulation (e.g., septicemia
and bacteremia) will also affect the uveal blood circulation. There is a barrier between
this blood circulation and the internal aspects of the eye, called the blood-ocular barrier.
The blood-ocular barrier consists of the blood-aqueous barrier (i.e., tight junctions
between the nonpigmented epithelial cells of the ciliary body and
nonfenestratediridalblood vessels) and the blood-retinal barrier (i.e., tight junctions
between the cells of the retinal pigmented epithelium [RPE] and nonfenestrated retinal
vessels). These semipermeable barriers normally prevent large molecules and cells
from entering the eye and help the intraocular fluids remain clear. The blood-ocular
barrier also limits the immune response to the internal aspects of the eye, causing the
eye to be considered an immune-privileged site. In cases of trauma or inflammation,
these barriers can be disrupted, allowing blood products and cells to enter the eye.
Flare, cell accumulations, and haze in the aqueous or vitreous are clinically observable
signs of the disruption of the blood-ocular barrier that occurs in uveitis. Disruption of the
barrier enables activation of various host immune responses, including production of
antibodies to self-antigens not normally recognized by the animals's own immune
system, as well as production of antibodies to foreign antigens inside the eye.
The inner eye is devoid of immune cells, and the eye maintains an immunosuppressive
environment through, for example, factors expressed in the vitreous such as
transforming growth factor β (TGF-β). The eye has long been recognized as an
immune-privileged organ. Immune privilege in the eye was originally ascribed to its
separation from the systemic immune system by the blood-ocular barrier, lack of
lymphatics, and the presence of limited numbers of resident leukocytes. More recent
work, however, has demonstrated that ocular privilege is a far more active process than
this.
In uveitis, leucocytes are able to enter the inner eye due to a bread-down of the blood
ocular barrier, and cause damage to the inner ocular tissues.
From: Caspi R. Immune mechanisms of uveitis, NIH
In equine recurrent uveitis,analysis of the infiltrating cells revealed that the majority of
cells are lymphocytes. These cells are predominantly CD4-positive T cells that secrete
proinflammatory cytokines such as interleukin 2 (IL-2) and interferon γ (IFN-γ). The IFNγ-producing phenotype is named TH1 helper cell. TH cells are involved in activating and
directing other immune cells and are particularly important in the immune system. They
are essential in determining B-cell antibody class switching, in the activation and growth
of cytotoxic T cells, and in maximizing activity of macrophages. Until recently, this cell
type of IFN-γ-producing TH1 effector cells was considered the main pathogenic
pathway in autoimmune diseases. TH17 cells are a newly identified subset of CD4+ Thelper cells producing IL-17. They are found at interfaces between the external
environment and the internal environment, such as the skin and lining of the GI tract.
Numerous immune regulatory functions have been reported for the IL-17 family of
cytokines, presumably due to their induction of many immune signaling molecules. Most
notably, IL-17 is involved in inducing and mediating proinflammatory responses. IL-17
induces the production of many other cytokines, chemokines, and prostaglandins from
many cell types. The increased expression of chemokines attracts other cells, including
neutrophils but not eosinophils. To date, it is not known if a certain percentage of ERU
cases are caused through TH17 cells.
The efferent phase of Uveitis: the mechanisms of tissue damage
It is likely that specific and the nonspecific cells penetrate the eye at random in small
numbers; however, only autoantigen specific cells (see below) - probably upon
recognition of their specific antigen in situ - are able to subsequently cause recruitment
of massive numbers of inflammatory host leukocytes and induce uveitis.
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The phenomenon of "nonspecific" inflammatory cell recruitment is a critical step in
uveitis development. Lymphocytes, monocytes and neutrophils are recruited to the site,
with mononuclear cells accounting for the majority of the infiltrate. Cell recruitment is
dependent on local production of lymphokines and chemokines by the first infiltrating T
cells, that result in induction of adhesion molecules on retinal vascular endothelium and
further production of chemoattractants from the tissue, and establish a chemotactic
gradient for leukocytes. The recruited cells then amplify this process by contributing
their own products, thus fueling an escalating inflammatory cascade. Degranulation of
mast cells that occurs at the time of disease onset helps to break down the blood-retinal
barrier. This facilitates the entry of cells into the eye, and the exit of tissue breakdown
products and soluble mediators into the circulation, thus fueling the progression of the
autoimmune process.
An important part of the tissue damage in the eye is mediated by active oxygen
products, such as nitric oxide, peroxide, and other products. The oxidative and other
tissue damage mechanisms are redundant, and no single pathway by itself is critical for
pathogenesis.
Autoantigens and Uveitis
Uveitis is a clinically heterogeneous disease. Although the antigenic triggers of
autoimmune uveitis are still under discussion, there is a large body of evidence
implicating responses to retinal antigens in the etiology and/or progression of the
disease.
Current concepts to explain the origin and perpetuation of autoimmune diseases include
molecular mimicry, bystander activation, and epitope spreading. These mechanisms do
not exclude each other but could appear together and even interact. Epitope spreading
is defined as the diversification of epitope specificity from the initial focused, dominant,
epitope-specific immune response, directed against a self or foreign protein to cryptic
epitopes on that protein (intramolecular spreading) or other proteins (intermolecular
spreading).
The immune response consists of an initial magnification phase and a later
downregulatory phase to return the immune system to homeostasis. In most
autoimmune diseases, several autoantigens participate in the pathogenesis, and
epitope spreading is accountable for disease induction, progression, and inflammatory
relapses. The shifts in immunoreactivity could account for the remitting/relapsing
character of ERU. Different target antigens may be important for a given individual,
depending on the MHC background. Genetic background and antigens encountered
influence the direction and extent of epitope reactivity and probably play an important
role in the heterogeneous clinical manifestations of disease. In many autoimmune
diseases, epitope spreading is suspected to occur as a result of an immune response
against endogenous target antigens first and secondary to the release of self-antigen
during the chronic autoimmune response. The formation of new antibodies may
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determine a different clinical picture. For example, the transformation of anterior uveitis
to posterior uveitis may be an excellent example of the effects of epitope spreading.
Through tissue damage, cryptic or hidden epitopes on the same molecule will be
suddenly presented to the immune system. The end result is that every target antigen
generally contains several epitopes, each of which reacts with a T cell or antibody
response of different specificity and affinity. Thus, epitope spreading in autoimmune
diseases results in the detection of an increasing array of autoantibodies against
various target antigens. Studies characterizing the shifting immune response of ERUdiseased horses are currently underway, but initial studies have confirmed epitope
spreading in a high percentage of cases.
Ocular Immunotherapy and Immunomodulation
Immunotherapy is a hot topic in ophthalmology because of the recent advancements in
cellular and intracytoplasmic mechanisms that control inflammation and immunology.
There are many suspected ocular surface and intraocular inflammatory diseases that
are thought to be mainly immune-mediated in origin or involved in the disease
pathogenesis. Steroidal medications are the most frequently used “immunotherapy”
medication in veterinary medicine. However, this drug is truly taking the “sledge
hammer” approach to therapy, with lots of peripheral damage (i.e., side effects).
Steroidal and non-steroidal therapy will be reviewed in other lectures. The newer
medications are designed to be more specific in their treatment approach and thus,
have fewer side effects. Most of these medications are systemically administered,
although local injections, including intraocular injections, are being evaluated in humans
and experimentally. The ocular penetration, both topically and via the systemic route
have not been studied extensively for many drugs, but one would assume that with the
blood-aqueous barrier disrupted, most blood-bourne medications should enter the eye
easily. The purpose of these notes and lectures is to review existing and emerging
immunomodulation drugs to better our understanding of immunotherapy of ocular
disease.
Corticosteroids are used as first-line treatment for many ocular inflammatory conditions.
The risk of adverse effects, however, necessitates conversion to steroid-sparing
immunomodulatory therapy (IMT) for disease that is recurrent, chronic, or poorly
responsive to treatment. Immunomodulatory agents include the broad categories of
antimetabolites (azathioprine, methotrexate, mycophenolatemofetil), alkylating agents
(cyclophosphamide, chlorambucil), T-cell inhibitors (cyclosporine, tacrolimus, sirolimus),
cytokine or cytokine receptor inhibitors (Etanercept, infliximab, adalimumab, and
anakinra), immunosuppressive cytokines (IL-10, TGF), and oral tolerance.
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TABLE OF IMMUNOMODULATING DRUGS
Drug
Trade name
MolWt
Use in Vet
ophthalmology
Dose*
Azasan®
Imuran®
277.264
NGE, uveitis,
chorioretinitis
Methotrexate
Rheumatrex®
454.45
Chemotherapy
Mycophenolatemofetil
CellCept®
433.50
NYR
Dog: 2 mg/kg PO qd x
2 wks; then 1 mg/kg
qod, then 1 mg/kg once
weekly for 30d.49
Dog: 2.5 mg/m2 PO qd
Cat: 2.5 mg/m2 PO 2-3x
weekly.49
Various
Alkylating Agents
Cyclophosphamide
Cytoxan®
279.1
Uveitis
Chlorambucil
Leukeran®
304.2
Chemotherapy
Neoral®,
Atopia®,
Optimmune®,
Restasis®
FK-506;
Fujimycin;
Prograf®)
1202.61
KCS, uveitis,
VKH, IMMK
See text
804.024
KCS
Rapamycin
804.024
NYR
Topical: 0.02%
Human: 0.050.1mg/kg/day IV; 0.50.75 mg/kg/day
P)(maintain trough
blood concentrations @
10-20 ng/ml)
Various
Antimetabolites
Azathioprine
T-cell Inhibitors
Cyclosporine
Tacrolimus
Sirolimus
Cytokine or Cytokine Receptor Inhibitors
Etanercept
Enbrel®
75 kDa
NYR
Infliximab
Adalimumab
Remicade®
Humira®
149 kDa
148 kDa
NYR
NYR
Anakinra
Kineret®
17.3 kDa
NYR
18.5 kDa
Various
NYR
NYR
Immunosuppressive Cytokines
IL-10
Not commercial
Not commercial
TGF
Dog: 2.2 mg/kg PO or
IV for 4 consecutive
days each wk.
Cat: 2.5 mg PO 4
consecutive days each
wk.49
Dog: 0.1-0.2 mg/kg PO
qd or qod
Cat: 0.w mg/kg PO
q24-48 hours.49
Human: 25-50mg SQ 12 times/ week
Human: IV q8 weeks
Human: 40 mg SQ q 2
weeks
Human: 1-2 mg/kg/day
SQ
Unknown
Unknown
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Oral Tolerance
S-antigen; IRBP
Not commercial
NA
NYR
Unknown
*Please check accuracy of all doses prior to therapy.
NYR – Not yet reported.
References
1. English RE, Gilger BC. Ocular Immunology. In: Gelatt KN, Gilger BC, Kern TJ.
Veterinary Ophthalmology 5th Edition, Blackwell (in press).
2. Gilger BC, Deeg CA. Equine recurrent uveitis. Ed.: Gilger BC. Equine Ophthalmology
Second Edition, Elsevier, Philadelphia. 2010.
3. Gilger BC, Dywer A. Equine Recurrent Uveitis. In: Gilger BC. Equine Ophthalmology,
Elsevier, Philadelphia. 2005
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