Download IMMUNOCHEMISTRY OF THE EYE

OCULAR IMMUNONOLOGY
Class 12
Dr. Pittler
OVERVIEW:
1. Essential review of molecular & cellular events
2. The compartmentalization of immunoglobulins
3. Ocular immune privilege & its functions
4. The 1st line of defense in the cornea
5. Progressive ocular surface disease
6. Intraocular infection and the loss of immune
7. privilege.
8. Complement reactions.
9. Inflammatory processes.
10. Graves’ disease – autoimmunity.
ESSENTIAL REVIEW:
The immunoglobulins (Igs)
Immunoglobulins are 1st responder
molecules for the immune system. By binding to
antigens they “identify” the substance as
foreign. They initiate immunoreaction
mechanisms and they reinforce (= amplify)
further immune reactions.
Igs that are important on the ocular surface are:
IgG
phagocyte
(attraction)
IgA (secretory)
rapid mover
(tear film)
IgM
limited 1st responder
(complement activation)
THIS TABLE SHOWS THE MAJOR Igs AND THER LOCATIONS:
NOTE:
● Only IgA (sec); IgG and IgM are found in the tear film.
● That only IgA (sec) and IgG are found in the aqueous
● that no Igs at all are found in the lens (nor in any other part of the eye#)
● these values represent the immunologically quiescent eye (nothing is
going on – no infections, inflammation or disease processes).
#except within the blood vessels
OTHER INNATE IMMUNOLOGICAL MOLECULES IN
TEAR FILM:
LYSOZYME – lyses peptidoglycans of Gram+ bacteria.
LACTOFERRIN – the name
comes from the Latin words
for milk and iron. Essentially,
lactoferrin ties up iron atoms and keeps
keeping them from being
used by iron-requiring
bacteria: including both Gm+ and
and Gm- as well as fungi.
The protein weighs 80 kDa.
IRON BINDING SITES
LIPOCALIN – a peptide in tears whose role in immunochemistry is not understood.
CELLS THAT CONTRIBUTE TO ACQUIRED IMMUNE DEFENSE
IN THE EYE (LACRIMAL GLAND)
The upper image shows a plasma cell
(green) and a Th cell (yellow). The lower
HELPER
image shows a Tc cells (blue) attacking
CELL
PLASMA CELL
and killing an infected cell. All three cells
types are normally present in the lacrimal
gland and are released to anterior
segment tissues on demand (i. e., an
immunological threat).
KILLER
CELL
Plasma cells are B cells that are terminally
differentiated to manufacture antibodies.
Helper T cells have a variety of functions
involved in reinforcing the development
and activities of plasma cells, mast cells,
antigen presenting cells, macrophages and killer cells. Killer cells
(cytotoxic killer cells NOT NK cells) will attack infected cells and
cause their destruction.
THE CONCEPT OF IMMUNE PRIVILEGE AND “ACAID”
In order to understand how cells and molecules work together
in the eye’s immune system, it also becomes necessary to
understand the immune privilege of the eye. Basically, immune
privilege is an adaptation of the body’s immune system to
suppress to some degree the normal immune response that
occurs in the body. No one completely understands how it
works, but it is a system that helps the eye avoid massive
reactions (especially in the cornea) that would result in a
disastrous loss of ocular tissue. It also makes possible corneal
transplants without significant rejection of the tissue. The
system allows normally antigenic cells and tissues to be
tolerated in the anterior chamber and has been termed
anterior chamber-associated immune deviation (ACAID).
FLOW OF IMMUNE TRAFFIC WITHOUT IMMUNE PRIVILEGE
NEW Abs
Afferent pathways (
) bring antigenic particles to the lymph node (
) where an
APC sensitizes Th cells. These cells move to the spleen (
) and from there to the
ocular MALT (mucosal associated lymphoid tissue) (
). Here and in the lacrimal
gland, plasma cells produce antibodies to respond to antigenic stimuli (
)
FLOW OF IMMUNE TRAFFIC WITH IMMUNE PRIVILEGE
NEW Abs↓
Cellular and aqueous origin of some immunosuppressants: TGFb,
MSH, and VIP.
FLOW OF IMMUNE TRAFFIC WITH IMMUNE PRIVILEGE
NEW Abs↓
These cytokines migrate to plasma cells and inhibit their production
of antobodies.
The cytokines, such as TGFb, all
operate by the same basic,
hormone-like mechanism. They
TRANSFORMING GROWTH FACTOR
bind to a receptor protein and
transform their signal by means
of a series of protein bindings
(cascade) to a protein that binds
to DNA. The “signal”
communicates to the specific
DNA directed RNA polymerase
whether to upregulate or
downregulate the rate of
protein synthesis (via mRNA).
This signal eventually gets to
Receptor protein
plasma cells concerning the
synthesis of antibodies. It
demonstrates how immune
privilege works in the
DNA binding protein
Cascade proteins
cornea.
Inhibition of Ab protein synthesis
b
WHAT HAPPENS WHEN IMMUNE DEFENSES MUST OVERCOME
PRIVILEGE?
A point is reached when an invading organism can no longer
be tolerated and the ocular surface is threatened. IgA (sec),
IgG and IgM are the 1st defense mechanisms to respond. IgA
(sec) is always present in sufficient concentrations to begin
the response. However, it is IgM that may often be the 1st
responder as can be seen by the table below:
Here the concentration of IgG1 is increased by ~11% and IgG4
by ~700% in an active herpes infection.
As a herpes virus infection progresses, IgM and IgA (sec)
contribute to the defense of the tissue. IgM, in particular, may
begin a complement reaction which is usually limited since
IgM is such as large molecule. However, in the tissue erosions
caused by the damaging virus, the molecule is able to
penetrate to the erosions themselves and
bring the complement reaction to
completion.
On the right are shown the typical
dendritic lesions of a herpes infection
(arrow).
IgG levels can also be considerably more
elevated in other infections. For example,
in acute hemorrhagic conjunctivitis, the levels of IgG can
reach 1300 mg/ mL.
OCULAR COMPLEMENT
Whether complement is fixed by IgG or IgM is irrelevant to the
large molecular weight of the initial C1q complex (410 kD). That
also limits its activity in the corneal stroma. Although there is
evidence for the presence of complement components in the
aqueous, the sequence must still be initiated by an immunoglobulin in the classical pathway. In addition to the destruction
of accessable herpes virus, complement fixation can also
facilitate the removal of Staphylococcus aureus (in conjunction
with lysozyme activity) [Gm+ ]; Neisseria gonorrhoea; and
Haemophilus influenzae [both Gm- ].
It must be remembered that complement causes two effects in
its immune activities: the initiation of inflammation and
membrane destruction of antigenic cells. Complement seems
to be more involved in pore perforation of antigenic cells in the
cornea.
OCULAR INFLAMMATION
Ocular inflammation is not a desirable process in ocular tissues
due to the real possibility of tissue destruction mechanisms that
include: phagocytosis and the invasion and growth of blood
vessels. That is the reason that the eye is immunoprivileged
unless an immune attack overwhelms immune suppression. The
picture on the right shows a phagocyte with pseudopodia reaching out
to engulf bacterial rods. The process
unfortunately causes the production
of highly reactive forms of oxygen
(e. g., O2 – [superoxide] and HOCl
[hypochlorous acid “chlorox”] that
not only destroy offending microorganisms, but also breakdown essential collagen fibers in the
corneal.
GRAVES’ DISEASE AND AUTOIMMUNITY – OCULAR EFFECTS
Graves’ disease is initially an autoimmune pathology that begins
at the thyroid gland. The thyroid
gland (shown on the right)
controls basal metabolism by its
secretion of T3 and T4 hormones.
In the disease (generally in the
3rd or 4th decade of life) individuals
begin to experience an increase
in body temperature; become
hyperactive; have increased
GI activity as well as weight loss
and increased appetite. There are also frequent ocular effects
(called “ophthalmopathy”) in which the eyes become pushed
forward in their sockets.
The reason for the initial effects at the thyroid gland are
related to an invasion of lymphocytes that are responding
to a perceived “antigenic” presence in the gland. These
lymphocytes synthesize a type of antibody known as TSI
(thyroid stimulating immunoglobulin). TSI mimics the ability
of TSH (thryoid
stimulating hormone)
to bind to the TSH
receptor on thyroid
follicular cells. The
Result of this is an
Increase in: the
uptake of iodine
Into the cell, an
Increase in cell
volume and an increase in the release of T3 and T4 into the
body.
This is a typical Graves’ disease patient with ocular effects that is
indicated by the wide-eye stare (upper arrow) due to eyes that
have been pushed forward due to the engorgement of
surrounding fat and
muscle tissue. The
conditions include:
periorbital and lid
edema; lid retraction;
restriction of muscle
movement; proptosis
of the eyes and possible
vsual acuity/ field
defects. Although
some of these characteristics can be attributed to an increase in
circulating T3 and T4, some cannot: i. e., the engorgement of fat
and muscle tissue.
The picture shows the
appearance of fat (green
arrow) and muscle tissues
(red arrow) in normal and
Graves’ disease patients’
eyes at autopsy.
Note the engorgement of
muscles in the periocular
tissues in the Graves’
disease patient. The
engorgement of muscles
is thought to be due to a
separate autoimmune
reaction at these tissues
and not to increased T3
and T4.
STUDY GUIDE FOR OCULAR IMMUNOCHEMISTRY
1. What are the Ig’s that are commonly found in the anterior
segment and what are their roles.
2. What is lactoferrin and how does it work?
3. How does immune privilege (ACAID) work in the cornea?
4. What is the basic mechanism by which immunosuppressant
cytokines work?
5. If IgM and C1q are impeded from entering the corneal
stroma – how is it possible for complement to attack herpes
virus?
6. What are the destructive processes in corneal
inflammation?
7. What is the mechanism in the body by which Graves’
disease operates? How does the disease start?
8. What is the possible immune mechanism in the eye by
which ophthalmopathy is produced in Graves’ disease?