Figure 1 The principle components of the immune system are listed, indicating which cells

26 February 8, 2002 OT

Table 2: Examples of cells of the mononuclear phagocytic system and their respective locations

Blood stream

Alveolar macrophages

Lungs

Sinus macrophages

Lymph nodes and spleen

Kupffer cells

Liver

Phagocytosis - the process

Phagocytosis is the process by which cells engulf microorganisms and particles ((FFiig gu urree 3 3)). Firstly, the phagocyte must move towards the microbe under the influence of chemotactic signals, e.g. complement (see later). For the process to continue, the phagocyte must attach to the microbe either by recognition of the microbial sugar residues (e.g. mannose) on its surface or complement/antibody, which is bound to the pathogen. Following attachment, the phagocyte’s cell surface invaginates and the microbe becomes internalised into a phagosome. The resultant p ph haag go osso om mee fuses with

multiple vesicles containing O 2 free radicals and

other toxic proteins known as lysosomes to form

a phagolysosome. The microbe is subsequently destroyed.

Figure 3

Phagocytes arrive at a site of inflammation

Dendritic cells

Opsonisation (“to make tasty” - Greek)

Dendritic cells consist of Langerhans’ and Op O psso on niin nss are molecules, which enhance the

by chemotaxis. They may then attach to

interdigitating cells and form an important efficiency of the phagocytic process by coating

microorganisms via their non-specific cell

bridge between innate and adaptive immunity, the microbe and effectively marking them for

surface receptors. Alternatively, if the

as the cells present the antigenic peptide to the their destruction. Important opsonins are the

organism is opsonised with a fragment of

T helper cell (adaptive immunity). Such cells are complement component C3b and antibodies.

the third complement component (C3b),

attachment will be through the phagocyte’s

therefore known as professional antigen

receptors for C3b. If the phagocyte

presenting cells ((A AP PCCss)). TTaab bllee 3 3 illustrates the

Natural killer (NK) cells

various types of dendritic cells together with an N NK K cells are also known as “large granular

membrane now becomes activated by the

example of their location. lymphocytes” (LGLs) and are mainly found in the

infectious agent, it is taken into a phagosome by pseudopodia extending

around it. Once inside, lysosomes fuse with

circulation. They comprise between 5-11% of the

the phagosome to form a phagolysosome

Eosinophils

total lymphocyte fraction. In addition to

Eosinophils (so called because their granules possessing receptors for immunoglobulin type G

and the infectious agent is killed. Undigested

stain with eosin – FFiig gu urree 4 4) are granulocytes (IgG), they contain two unique cell surface

microbial products may be released to the

that possess phagocytic properties. Despite the receptors known as killer activation receptor and

outside

fact that they represent only 2-5 % of the total killer inhibition receptor. Activation of the

leukocyte population, they are instrumental in former initiates cytokine (“communication”)

the fight against parasites that are too big to be molecules from the cell whilst activation of the

stain with a basic dye. Unlike mast cells, which

are present in close proximity to blood vessels in

phagocytosed.

latter inhibits the aforesaid action.

connective tissue, basophils reside in the

NK cells serve an important role in attacking

circulation.

virally-infected cells in addition to certain

Both cell types are instrumental in initiating

tumour cells. Destruction of infected cells is

the acute inflammatory response. Degranulation

achieved through the release of perforins and

is achieved either by binding to components of

granyzymes from its granules, which induce

the complement system or by cross-linking of

apoptosis (programmed cell death). NK cells are

the IgE antibody which results in the release of

also able to secrete interferon-γ (IFN-γ). This

pro-inflammatory mediators including histamine

interferon serves two purposes: first, to prevent

and various cytokines. The former induces

healthy host cells from becoming infected by a

vasodilation and augments vascular permeability

virus; and second, to augment the T cell

whilst the latter are important in attracting

response to other virally infected cells

both neutrophils and eosinophils.

(see later).

Table 3: Dendric cells and location

Mast cells and basophils

Cells

Location

Figure 4 Morphology of the eosinophil. The very similar in that both contain electron dense

Morphologically, mast cells and basophils are

Langerhans cell

Limbus, skin

multilobed nucleus is stained blue and the granules in the cytoplasm. Basophils are

Interdigitating cell

T cell areas in

cytoplasmic granules are stained red. so-called owing to the fact that their granules

lymph nodes

Leishman stain, x 1800

Molecules of the innate

Figure 5

immune system

When host cells become infected There are many molecules, which work in concert

by virus, they may produce with the cells of the innate immune system and

interferon. Different cell types which also foster close functional links with their

produce interferon-α (IFN-α ) or adaptive counterpart. The three major molecules

interferon-β (IFN-β); interferon-γ are:

(IFN-γ) is produced by some types • Complement

of lymphocyte (T H ) after activation • Acute phase proteins (APP)

by antigen. Interferons act on other • Interferons (IFNs)

host cells to induce a state of resistance to viral infection. IFN-γ

Complement

has many other effects as well The complement system represents a large group of independent proteins (denoted by the letter C

Table 4:

and followed by a number), secreted by both

Primary and secondary lymphoid organs

hepatocytes (liver cells) and monocytes. Although these proteins maybe activated by both

Secondary lymphoid organs the adaptive immune system (ccllaassssiiccaall p paatth hw waayy)

Primary lymphoid organs

or innate immune system (aalltteerrn naattiivvee p paatth hw waayy),

Bone marrow

Lymph nodes

the nomenclature is derived from the fact that MALT - mucosa associated lymphoid the proteins help (“complement”) the antibody

tissue (includes bronchus, gut, nasal and response.

conjunctival associated mucosal tissues) Activation of complement via the microbe

Spleen

itself is known as the alternative pathway. The classical pathway requires the interaction of antibody with specific antigen. The C3 component is the pivotal serum protein of the

proteins on the cell surface adjacent to special complement system. Binding of the antigen to

for the successful eradication of an invading

host proteins called major histocompatibility C3 results in two possible sequelae. In either

virus by the innate immune system.

complex (MHC) class II molecules. As discussed, case, C3 component becomes enzymatically

Type II IFN, IFN-γ, is produced by T Helper

antigen presenting cells which express MHC class converted to C3b. The bacterial cell wall can

cells and NK cells and is able to augment both

II molecules include dendritic cells and either remain bound to C3b and become

the antigen presenting properties together with

macrophages. This “afferent” phase must occur opsonised (since phagocytes have receptors for

the phagocytic properties of the APCs (e.g.

in order for the T cell to recognise the antigen. C3b) or act as a focus for other complement

macrophages and dentritic cells).

The “efferent” phase occurs when activated proteins (namely C5, 6, 7, 8 and 9). The latter

lymphocytes enter the tissue and meet antigen form the membrane attack complex (MAC), which

again. This results in multiplication and secretion induces cellular lysis.

Adaptive immunity

of cytokines or immunoglobins in order to The functions of the complement system may

As mentioned previously, there is a great deal of

synergy between the adaptive immune system

destroy the antigen.

be summarised as follows:

and its innate counterpart. The adaptive immune

• Opsonisation

system comprises two main types of leukocyte

T cells

• Lysis (destruction of cells through damage/

T cells can be broadly divided into both T helper rupture of plasma membrane)

known as B and T lymphocytes. Before describing

(T H ) and cytotoxic T cells (T c ). Furthermore, T H • Chemotaxis (directed migration of immune

these important cell types, it is necessary to

cells may be sub-divided into T H1 and T H2 . The cells)

acquaint the reader with both the primary and

former are pro-inflammatory T cells and • Initiation of active inflammation via direct

secondary lymphoid organs and tissues in the

body. These are summarised in TTaab bllee 4 4. stimulate macrophages whilst the latter activation of mast cells

The bone marrow represents the dominant site

orchestrate B cell differentiation and maturation

and hence are involved in the production of It is important that complement is regulated to

for haemopoiesis (production of blood cells and

humoral immunity (antibody mediated). T cells protect host cells from damage and/or their total

platelets). Although most of the haemopoietic

express cell surface proteins, described by cluster destruction. This is achieved by a series of

cells maturate in this region, T lymphocytes do

determination (CD) numbers. T H cells express CD4 regulatory proteins, which are expressed on the

so in the thymus. In the thymus, premature T

molecules on their cell surface, which enable the host cells themselves.

cells undergo a process of positive and negative

selection whereby the former are allowed to

lymphocyte to bind to a MHC class II molecule.

progress to maturity whilst the latter are marked

The T cell receptor is unique in that it is only

Acute phase proteins

able to identify antigen when it is associated These serum proteins are synthesised by

for termination via apoptosis (see central

with a MHC molecule on the surface of the cell. hepatocytes and are produced in high numbers

tolerance).

Cytotoxic T cells are primarily involved in the in response to cytokines released from

destruction of infected cells, notably viruses. macrophages.

Lymphocytes

Morphologically, there are three types of

Unlike T H cells, cytotoxic cells possess CD8 cell

lymphocytes: T, B and NK cells. However, only T

surface markers, which bind to antigenic

Interferons (IFNs)

peptides expressed on MHC class I molecules. IFNs are a group of molecules, which limit the

and B lymphocytes exhibit memory and

specificity and, as such, are responsible for the

spread of viral infections ((FFiig gu urree 5 5)). There are

unique quality of the adaptive immune system.

B cells and antibodies

two categories of IFNs, namely type I and type II.

Resting B lymphocytes are able to react with

(immunoglobulins - Ig)

Type I IFNs maybe sub-divided further into IFN-α

B cells are lymphocytes that produce antibodies and β. IFN-γ is the sole type II interferon. Type I

free antigen directly when it binds to their cell

(immunoglobulins) and can recognise free IFNs are induced by viruses, pro-inflammatory

surface immunoglobins which act as receptors. T

antigen directly. They are produced in the bone cytokines and endotoxins from gram negative

lymphocytes do not react with free antigen and

marrow and migrate to secondary lymphoid bacterial cell walls. Their presence remains vital

instead make use of APCs to phagocytose the

antigen and then to express its component

organs. B cells are responsible for the

28 February 8, 2002 OT

Sponsored by

Module 4 Part 2

Figure 6

When a microorganism lacks the

development of antibody mediated immunity

inherent ability to activate

known as h hu um mo orraall m meed diiaatteed mm d iim mu un niittyy.

complement or bind to

When activated by foreign antigen, B cells

phagocytes, the body provides

undergo proliferation and mature into antibody

antibodies as flexible adaptor

secreting p pllaassm maa cceellllss. The latter are rich in

molecules. The body can make

organelles such as rough endoplasmic reticulum

several million different antibodies

and mitochondria, which confer their ability to

able to recognise a wide variety

secrete soluble proteins (antibodies). Not all

of infectious agents. Thus the

proliferating B cells develop into plasma cells.

antibody illustrated binds microbe

Indeed, a significant proportion remain as

1, but not microbe 2, by its

memory B cells through a process known as

‘antigen-binding protein’ (Fab).

clonal selection. This process is vital in

The ‘Fc portion’ may activate

eliminating the antigen should the body become

complement or bind to Fc

re-exposed to it in the future. T cells are also

receptors on host cells,

clonally selected and this confers to the

particularly phagocytes.

production of T memory cells. Although T and B cells behave differently, both are able to recirculate around the body

Table 5:

migrating from blood to tissue and vice versa. Antibody isotypes and corresponding functions

The ability to recirculate obviously increases the efficiency with which cells of the immune system

Antibody Characteristics

can home onto the invading antigen. IgG

Crosses placenta thus providing newborn with useful humoral immunity High affinity

Antibodies

Predominant antibody in blood and tissue fluid Antibodies have two roles to play - the first is to bind antigen and the second is to interact with

IgM Large pentameric structure in circulation host tissues and effector systems in order to Present in monometric form on B cell surface

ensure removal of the antigen Secreted form is predominant antibody in early immune

((FFiig gu urree 6 6)).

response against antigen There are five different types (known as Reaches 75% of adult levels at 12 months of age

iisso ottyyp peess) of antibody in the human immune IgA

system - namely IgM, IgG, IgA, IgE and IgD. In Exists in both a monometric and dimeric form

addition, there are four sub classes of IgG Secretory IgA (dimeric form) represents 1st line of defence against

(IgG1-4). The basic antibody unit consists of a microbes invading the mucosal surface, e.g. tears

glycosylated protein consisting of two heavy and IgE

Low levels in circulation two light, polypeptide chains. The region which Increased levels in worm infections

binds to the antigen is known as the FFaab b region, Fc region has high affinity for mast cell thus involved in allergy

while the constant region, FFcc, not only determines the isotype but is the region

IgD Antigen receptor on B cells responsible for evoking effector systems, e.g. Absent from memory cells

mast cell activation. The term immune complex refers to the combination of antigen and

Table 6:

antibody and will be discussed later in the article Various cytokines, their sources and functions

(see type III hypersensitivity). The antibody isotypes together with their

Cytokine

corresponding function are illustrated in TTaab bllee 5 5.. IL-1

1. T, B cell activation

2. Mobilisation of PMNs

MHC

3. Induction of acute phase proteins

Major histocompatability complex (MHC) are cell surface proteins classified as class I (also termed

IL-2

human leucocytic antigen [HLA] A, B and C), Il-4

T cells

Proliferation of T and NK cells

Th2 cells

B cell activation

found on all nucleated cells and class II (termed

Mast cells

IgE response

HLA, DP, DQ and DR), found on all antigen presenting cells (APCs). MHC molecules are the

IL-8

Macrophages

Chemotaxis of PMNs

sine qua non of T cell induced immunity.

T cells

Clinically, there is a strong association

Fibroblasts

between HLA and certain systemic and ocular

diseases (see later). IL-10

Keratinocytes

TH2 cells

B cell activation

Macrophages

Suppress macrophages

Cytokines

Cytokines (also termed interleukins [IL] meaning IL-12

B cells

Stimulate T H1 “between white blood cells”) are small molecules Inhibit T H2 that act as a signal between cells and have a

variety of roles including chemotaxis, cellular TGF β

growth and cytotoxicity. Owing to their ability to TNF α (tumour necrosis factor)

(transforming growth factor)

T cells

Inhibits other cytokines

Macrophages

Inflammation

control immune activity, they have been described as the “hormones” of the immune system.

TTaab bllee 6 6 summarises some of the cytokines

endothelial cells and the presence of junctional

4 and their progenitors. Since interferons have amongst ophthalmologists and immunologists cells . alike. It appears such suppression is achieved by been discussed earlier in the article, they have

complexes linking retinal pigment epithelial

various factors present in the aqueous humour been omitted from the table.

Lymphatic role

(e.g. transforming growth factor - β).

The fact that skin allografts were not rejected

Central and peripheral

following lymph node removal 5 led investigators

Anterior chamber associated immune

tolerance: nature’s way of

to hypothesise that immune privilege was solely

deviation (ACAID)

containing the immune

due to the absence of the same said system at a

As a result of experiments with rats,

investigators discovered that antigens placed in Since various cells of the immune system are

response

particular anatomical site. However, although

6 capable of reacting with self-antigens, it is the anterior chamber resulted in systemic lymphatic drainage, others such as the testes therefore essential that the human body has

certain immune privileged sites do indeed lack

inhibition of delayed type hypersensitivity (DTH

or type IV hypersensitivity) reactions to the mechanisms to suppress/eliminate autoreactive

and eye 7

do possess such a system. It appears

same said antigens 17 . This phenomenon has been cells. Failure to do so, can, in some cases, lead

that a proportion of the aqueous humour drains

coined anterior chamber associated immune to the development of autoimmune diseases

via the uveoscleral pathway into the lymphatic

deviation (ACAID). The anterior chamber is thus (see later).

vessels in the head and neck.

able to suppress delayed type hypersensitivity

The eye, APCs & MHCs

reactions and inhibit the production of

Central tolerance

complement fixing antibodies 18,19 . However, it Central tolerance refers to the process whereby

As mentioned previously, APCs, through their

has no inhibitory effect on cytotoxic T cell both immature B and T cell lymphocytes, which

ability to express MHC class II molecules, are

activity and has a minimal influence on the react against normal, healthy cells

potent progenitors of the immune response.

production of non-complement fixing (self-antigens), are eliminated via apoptosis.

Moreover, such cells are capable of activating T

cells within the tissue itself. It is therefore not

antibodies.

unreasonable to assume that a paucity of APCs

With respect to the endothelium, two

adaptations prevent it from immunological This involves the removal of mature lymphocytes,

Peripheral tolerance

may play an important role in immune

injury: first, avoidance of cytotoxic T lymphocyte which are not tolerant to healthy cells.

privilege. In addition, failure to express MHC

class I molecule would make a tissue immune

(CTL)-mediated lysis; and second, inhibition of

against the lytic action of the cytotoxic T cells.

DTH responses in the anterior chamber 22 . It

Ocular immune privilege

achieves the former through the cells inability There are numerous sites in the body whereby

Although the aforementioned mechanisms

to express MHC class I molecules 21 . The corollary tissue may be grafted with minimal risk of

are theoretically plausible, cells expressing both

of this, however, is that virally infected cells rejection. Such regions include, inter alia, the

MHC class I and II molecules have been

may persist in this region. The râison d’etre of testis, thyroid lens, anterior chamber, cornea,

detected in the eye. TTaab bllee 7 7 illustrates the

ACAID is to protect the eye from the DTH iris and ciliary body 1,2.

relationship between histocompatability class

response to pernicious antigens. As a result of It is important that immune privilege is not

and ocular cell type.

its location in relation to the anterior chamber, simple interpreted as the host’s inability to

It is noteworthy that the epithelial cells of

14 initiate an immune response to a transplanted the corneal endothelium seems well placed to expression and that the Langerhans’ cells reap the benefits of ACAID. tissue. Rather, it is an area of the body in which

the crystalline lens are devoid of class I

ACAID is beneficial in reducing the incidence there exists a paucity of various elements of the

(class II expression) are absent from the central

of stromal keratitis in herpes simplex virus human immune system in response to an

cornea 15 .

infection. It therefore seems reasonable to antigen.

It is interesting that not all cells, which

express MHC class II act as professional APCs in

assume that such an unwanted corneal side-

the eye. Indeed, it has been shown that such

effect occurs as a result of a DTH response

Factors

rather than the toxic effect of the virus per se 22 . The factors purported by investigators that

cells reside in the iris and ciliary body and not

Corneal graft rejection may be due, in part, contribute to the phenomenon of ocular immune

only fail to present alloantigens to T cells, but

to failure to invoke ACAID. Streilein at al 23 privilege include:

have the ability to suppress mixed lymphocyte

not

only discovered that the immunosuppressive • Isolation from a vascular supply

reactions 16

effects of the cornea were abolished in corneas • Isolation from a lymphatic supply • Presence of a vascular barrier

The failure to incite the inflammatory

Table 7:

• Ability to suppress the immune response

MHC and ocular cell type 8-13

• Anterior chamber associated immune deviation (ACAID)

Ocular cell type

MHC class I

MHC class II

Corneal epithelium

Vascular supply

Corneal stroma

The healthy cornea is a good example of an ocular site devoid of a vascular network. The

Corneal endothelium

evidence to support the role a vascular network

Trabecular meshwork

has to play in the mechanism of graft rejection

Pigmented and non pigmented

is unequivocal since the risk of failure correlates

cells of ciliary body

positively with the degree of host vascularisation 3 .

Anterior iris

RPE cells

Vascular barrier

• There is a plethora of evidence in the

Corneal limbus

• ophthalmic literature to support the existence of

Iris and ciliary body

• said barrier encompasses different elements

a blood-ocular barrier. Furthermore, the same

Uveoscleral pathway

• including tight junctions between retinal

Ora serrata

February 8, 2002 OT

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that had ACAID removed via cauterisation or

limbal region. The latter antibody is restricted keratoplasty but also abolished in corneas that

the microorganism enables the tears to wash

from the central cornea owing to its large size. had been denervated.

them away. IgA possesses other functions

including opsonisation and inactivation of

Antibodies in the cornea are found in the

various bacterial enzymes and toxins. In

stroma since they are cationically charged. Due

Ocular immunology

to their positive ionic charge, they bind to Anterior segment immunology may be

addition, it may be involved in antibody

proteoglycans and the anionic sub-divided into the following aspects:

dependent cell-mediated cytotoxicity. It must be

glycosaminoglycans 38 . • Tear film

emphasised, however, that IgA is unable to bind

Antigen/antibody complexes may sometimes • Corneal immunology

to complement and, as such, is not involved in

be observed in the corneal stroma in a variety • Conjunctival immunology

the classical pathway 27 .

of pathological conditions (e.g. herpes simplex • Scleral immunology

keratitis), and because of their ringed shaped • Uveal immunology

4. Miscellaneous proteins

β-Lysin, a bacteriocidal cationic protein, is

appearance are known as “Wessley rings”.

present both in the tears and aqueous humour 28 .

Tear film

Its bacteriocidal properties are conferred

Complement

The elements of complement found in the This layer is produced both by the conjunctival

a) Mucous layer

through their ability to disrupt the micro-

cornea include C1-7 39 in addition to the goblet and epithelial cells. The glycocalyx

organisms’ walls. Various components of

regulatory proteins H and I and C1 inhibitor 40 . synthesised by the corneal epithelial cells serves

complement are present in varying degrees

Interestingly, C1 is present in high to attach the mucous layer and in doing so binds

depending on whether the eye is closed or not.

concentrations in the limbus and is restricted

from the central cornea. This finding is suggested that the latter immunological sign may

to immunoglobulin in the aqueous 24 . It has been

Closed eye vs open eye

significant since the limbal region is susceptible have antiviral effects. The evidence to support

to ulceration following complement activation this is that certain intestinal mucosa, which have

Open eye

and immune complex deposition. It has been similar binding properties to those seen in the

In addition to the components of complement

suggested that C1 is produced by corneal eye, are able to exert an inhibitory action against

mentioned above, the tears are rich in lysozyme,

fibroblasts whereas the remaining components viral replication 25 .

lactoferrin and lipocalin (tear pre albumin)

together with low levels of IgA 29,30 .

detected in the cornea appear to be derived from the plasma via linked vessels.

b) Aqueous layer

Closed eye

The aqueous layer contains the following

In the closed eye environment, levels of IgA and

Conjunctival immunology

antimicrobial factors:

complement components are increased. In

The conjunctival associated lymphoid tissue

1. Lactoferrin

addition, neutrophils are also recruited. The eye

(CALT) is part of the more general mucosa

2. Lysozyme

can be interpreted as being in a state of

associated lymphoid tissue (MALT). Langerhans’

3. IgA

subclinical inflammation. However, the eye is

cells and lymphocytes exist within the

4. Miscellaneous proteins

protected from damage induced by either

conjunctival epithelial layer. In the substantia

complement or neutrophils by DAF (Decayed

propria, neutrophils, lymphocytes, IgA and IgG,

dendritic cells and mast cells all reside. It is Lactoferrin is produced via the acinar cells of the

1. Lactoferrin

Accelerating Factor [an inhibitory component of

noteworthy that eosinophils and basophils are lacrimal gland. Its main function is to bind iron,

the complement system]) 31 and α 1 –antitrypsin 33 .

not present in the healthy conjunctiva. which is required for bacterial growth. It

Langerhans’ cells and dendritic cells present therefore possesses both bacteriostatic and

Corneal immunology

the antigenic peptide to the conjunctival T bacteriocidal properties. Furthermore, it is able

Paradoxically, the cornea, an immune privileged

helper cells. Following antigenic presentation, to enhance the effects of certain

site, is capable of evoking an immunological

the T cells secrete the cytokine IFN-γ which immunoglobulins.

response evidenced by the presence of

subepithelial infiltrates, keratic precipitates,

serves to promote antigen elimination by

epithelial and stromal keratitis under certain

macrophages. This is the delayed-type

hypersensitivity (DTH) response (see later) and Produced by type A cells in the lacrimal gland,

2. Lyszome

clinical conditions.

is characteristic of conjunctival pathology such lysozome constitutes up to 25% of the total tear

Cytokine release

as phlyctenulosis.

protein. It is surprising somewhat that despite

Numerous cytokines are synthesised in the

being effective at lysing gram positive bacterial

cornea including IL-1, IL-6, IL-8, TNF-α, IFN-γ,

Scleral immunology

cell walls, staphylococcus aureus appears

There exists only a small number of immune recalcitrant to its action 26 . However, it is

C5a and prostaglandins 33 . Phagocytosis of certain

cells in the sclera compared to the conjunctiva instrumental in enhancing IgA against gram

antigens via corneal epithelial cells initiate the

since it is relatively avascular. In its resting negative bacteria.

release of IL-1 which, in turn, gives rise to the

recruitment of Langerhans’ cells from the limbus

state, IgG appears to be present in large

to the central cornea. It is worthy of note that

amounts 41 . However, a sclera under stress, may

become immunologically active as a result of This is the predominant isotype found in the

3. IgA

the same said APC may be recruited centrally in

migrating cells from the overlying episcleral and human tears in the non-inflamed eye. Very little

response to chemical or localised damage 34,35 .

underlying choroidal vasculature 42,43 . is present in serum with the majority secreted

Investigators have discovered that stromal

fibroblasts synthesise

through epithelial cells of structures such as the

IL-8 in response to infection by the herpes

Uveal immunology

The uveal tract is an important site from an present in the tears is produced by plasma cells

lacrimal gland and the lactating breast. The IgA

simplex virus 36 . Furthermore, it seems plausible

immunological perspective for two reasons: situated underneath the secretory cells lining the

that the latter cytokine release may be

first, it is highly vascular in nature; and second, acini in the lacrimal gland.

responsible for the neutrophilic infiltration

the majority of vessels are highly fenestrated IgA is very effective at binding microbes and,

observed in cases of herpetic keratitis.

which facilitates the recruitment of leukocytes as such, prevents the microbe from adhering to

during the inflammatory cascade. The uvea the mucosal surface. The antibody achieves this

Immunoglobulins

contains numerous cellular components of the either by interfering with the binding site

The three isotypes detected in the cornea are

immune system including macrophages, mast directly or by agglutination. Successful binding of

IgM, G and A. IgG predominates in the central

cornea 37 . By contrast, IgM predominates in the

cells, lymphocytes and plasma cells in addition

Table 8:

Hypersensitivity reactions

Hypersensitivity Appearance

Mediators

Mechanism

type time

I. Immediate 2-30 mins

IgE

Mast cell response (enhance acute inflammation)

II. Cytotoxic 5-8 hours

IgM and IgG

Antibody and complement

III. Immune 2-8 hours

IgM and IgG

Antibody/antigen

complex

immune complexes

complexes

IV. Delayed type 24-72 hours

CD4 and CD8 T cells, T cell mediated

APCs

Macrophages activated Include granulomatous reactions

Figure 7:

V. Stimulatory

Autoantibodies

Autoantibodies against hormone

Corneal thinning and vascularisation in AKC, excess mucous is present

to an appreciable number of immune factors.

diffuses throughout the body until it comes into

Although IgG and IgM have been detected in

contact with mast cells and basophils. Both

both the choroid and iris, they exist in greater

these cell types have receptors for IgE antibody.

numbers in the former structure. The reason for

Although the patient experiences no symptoms

such disparate levels is the dearth of anionic

after the initial binding, reintroduction of the

antibody binding sites on the iris surface 44 . By

antigen/allergen induces the production of

contrast, the ciliary body harbours tremendous

more IgE and, furthermore, increase the

amounts of IgG by virtue of its anionic tissue

likelihood of cross-linking with existing

sites.

antibodies on the mast cell surface. Such cross- linking induces the mast cell to degranulate and

Immunopathology and the eye

release a host of inflammatory mediators such

Immunopathology encompasses both pathology

as histamine, prostaglandins and bradykinin.

Figure 8:

as a result of an over-active immune system

Epithelial macroerosion in VKC with that acquired through an individuals

Histamine characteristically causes the itchy

(h hyyp peerrsseen nssiittiivviittyy and aau utto oiim mm mu un niittyy) together

symptoms experienced by patients and as a

result of binding to H1 receptors in the eye,

inability to fight off infection – namely

induces vasodilation and enhances mucous

immunodeficiency. Unfortunately, it is far

secretion by the goblet cells. Bradykinin

beyond the scope of this article to describe the

augments vascular permeability, decreases

latter and its ophthalmic correlates.

blood pressure and contracts smooth muscle.

In this section, the classification system

Prostaglandins are also powerful inflammatory

pertaining to hypersensitivity reactions will be

mediators.

described and each subtype with an anterior segment manifestation will be discussed. The

Ocular correlates:

association between HLA and systemic and

The following anterior segment conditions are

ophthalmic disease will be illustrated together

due, if not only in part, to type I

with a brief overview of the concepts pertaining

hypersensitivity:

to autoimmunity.

• Seasonal allergic conjunctivitis (type I) • Giant papillary conjunctivitis (types I and IV)

Hypersensitivity

• Vernal keratoconjunctivitis (VKC)

Figure 9:

The term hypersensitivity refers to the process

Corneal plaque in VKC whereby the adaptive immune response over-

(type I and IV)

• Atopic keratoconjunctivitis (AKC)

reacts to a variety of infectious and inert

giant cobblestone papillae, or in a limbal form antigens resulting in damage to the host tissue.

(types I and IV) ((FFiig gu urree 7 7))

with gelatinous deposits known as Trantas’-dots, Five types of hypersensitivity reactions exist –

which represent degenerating epithelial cells and all of which vary in their timing following

Seasonal allergic conjunctivitis can be easily

eosinophils. The first corneal change is a contact with the antigen ((TTaab bllee 8 8)).

recognised by chemosis and hyperaemia of the

conjunctiva, lid swelling and excessive

punctate epithelial keratitis, which if left

lacrimation. The cornea is not affected.

unchecked develops into a macroerosion

• Type I hypersensitivity: allergy

((FFiig gu urree 8 8)) and finally a corneal plaque develops Allergies may affect approximately 17% of the

GPC is well known to optometrists as an

((FFiig gu urree 9 9)). The conjunctiva itself is population 45 . The term atopic is used to describe

allergic response to contact lenses, prosthetic

characteristically oedematous and contains an those individuals who possess a genetic

lenses, protruding corneal sutures and scleral

array of immunological cells including predisposition to allergy. Allergies may occur to

buckles. Histological examination of eyes

lymphocytes and mast cells. Evidence of type I otherwise innocuous antigens (known as

suffering from GPC have revealed degranulated

involvement includes the histological detection aalllleerrg geen nss) and infectious agents, e.g. worms.

mast cells (type I hypersensitivity) 46 and CD4+ T

of, inter alia, degranulated mast cells, Type I hypersensitivity exists in two phases, the

cells (type IV) 47 , thus corroborating the theory

eosinophils and increased levels of IgE in sensitisation and effector phases.

that such a condition is mediated by both types

affected eyes 49 . The detection of CD4+ T cells and Firstly, a harmless allergen causes production

of hypersensitivity.

macrophages is indicative of a delayed of IgE antibody on first exposure. This IgE

Vernal conjunctivitis can present either in a

palpebral form characteristically exhibiting

inflammatory component 49 .

32

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Module 4 Part 2 a

Type II hypersensitivity: cytotoxic

This classification of hypersensitivity involves either IgG or IgM antibodies, which may induce cellular lysis due to the involvement of the classical complement pathway (as seen in blood transfusion reactions) or recruit and activate inflammatory cells via complement. The components of complement include the C5a, which serves to attract inflammatory cells to the site of interest. The hypersensitivity reaction is a result of the excessive amount of extracellular mediators released by the inflammatory cells to antigens that are too big to be completely phagocytosed.

Antibodies to self-antigens, such as the acetylcholine receptor in myasthenia gravis, is not only another example of type II hypersensitivity but also an example of autoimmune disease.

Ocular manifestations

• Mooren’s ulcer • Cicatrical pemphigoid

Mooren’s ulcer is a rare peripheral ulcerative keratitis that exists either as a unilateral, non-progressive form which has a predilection for elderly patients or as a more severe, progressive form affecting both eyes of relatively young individuals. The signs range from a small patch of grey infiltrate near the margin to frank ulceration involving the entire corneal circumference and, in some cases, the central region as well. The healing process results in a thin, vascularised, opaque cornea. Investigators have identified a significant number of

lymphocytes, neutrophils and plasma cells 50 in

the cornea, thus providing unequivocal evidence to support the theory that this condition has an immunopathological aetiology.

Cicatrical pemphigoid is a chronic, blistering disease, which has a predilection for both the ocular and oral mucous membranes. Unlike its self-limiting counterpart, pemphigus vulgaris, cicatrical pemphigoid rarely affects the skin. Ocular cicatrical pemphigoid is a serious, bilateral condition that represents effective shrinkage of the conjunctiva. Although the initial presentation may be subacute and non-specific, it frequently progresses to symblepharon, entropion with secondary trichiasis, dry eye, ankyloblepharon and conjunctival fornix shortening. There is evidence to support the presence of IgG antibodies directed against self-antigen in the basement

membrane of both the skin and eye 51 . Binding of

the aforementioned antibodies may activate complement with subsequent recruitment of inflammatory cells into the area. The process of cicatrisation is achieved through the secretion of collagen via fibroblasts as a result of stimulation via cytokines released from the invading inflammatory cells.

Type III hypersensitivity: immune complex

Large pathogens with multiple antigenic sites have several antibodies bound to them forming

immune complexes. Normally, these complexes are removed by the mononuclear phagocytes in the liver and spleen with no adverse sequelae. However, persistence of immune complexes does occur in certain individuals leading to their deposition in tissue. As a consequence of the latter action, complement may be activated thus paving the way for inflammatory cells to enter the deposition site. Since blood vessels (which filter plasma at high pressure and exhibit a great deal of tortuosity) are more susceptible for immune complex deposition, the ciliary body is particularly vulnerable to this type of hypersensitivity reaction.

Ocular manifestations

• Uveitis (Crohn’s disease) • Peripheral corneal lesions associated with

rheumatoid arthritis • Stevens Johnson syndrome • Sjögren’s syndrome

The signs and symptoms of the above will be described in future articles in the series.

Type IV hypersensitivity: delayed-type hypersensitivity (DTH)

The term DTH has been used to describe such a reaction owing to its prolonged time-scale relative to the other hypersensitivity types. Although DTH can be transferred by T cells that have been previously sensitised by an antigen, it cannot be transferred in serum.

The sequence of events leading to DTH begins with initial presentation of the antigen peptide to T cells by APCs (e.g. Langerhans’ cells). The primed T cell migrates to the site of antigenic entry whereby it releases pro-inflammatory mediators such as TNF. The release of these cytokines facilitates blood flow and extravasation of plasma contents to the area. The activation of CD4 T helper and CD8 cells results in the release of IFN-γ and, as a consequence, enhances macrophage activity in that area. Resolution of DTH is dependent on the efficacy with which such phagocytes can remove the offending antigen.

Recalcitrant infectious agents result in a chronic DTH that causes the chronically activated macrophages to fuse together and form multinucleated giant cells. In an attempt to contain the infectious agent, macrophages may undergo further inter connections to resemble an epithelial layer. Owing to the similarity to this layer they are referred to as epitheloid cells. Both epitheloid and multinucleated giant cells secrete factors that induce fibrosis resulting in granuloma formation. Thus granulomata are the hallmark of chronic inflammation. Damage and loss of function of the neighbouring tissues frequently ensues until the agent is removed either chemically or surgically.

Ocular manifestations

• Ocular allergies (VKC, AKC GPC) • Idiopathic uveitis • Sympathetic ophthalmia • Phlyctenulosis

Type V hypersensitivity

This relatively new category encompasses the concept of autoantibodies binding to hormone receptors that mimic the hormone itself. This results in stimulation of the target cells. Examples include thyrotoxicosis.

Autoimmunity

The ability to react against self-antigens is known as autoimmunity. However, a significant number of people exist who harbour auto- antibodies and yet remain asymptomatic. The corollary of this is that the presence of autoreactive cells per se is not sufficient to trigger autoimmune disease. In fact, autoimmune disease is a result of breakdown of one of the immunoregulatory mechanisms. Furthermore, autoimmune disease may be classified as either being organ specific (e.g. insulin dependent diabetes mellitus, Grave’s disease) or non-organ specific (e.g. Sjögren’s syndrome, ankylosing spondylitis).

It is important to realise that the causes of autoimmune diseases are multifactorial. The main predisposing factors are age, gender, infection and genetics. However, one of the most important factors of interest to immunologists and clinicians alike is the association between HLA and autoimmune disease. When determining the likelihood of contracting a disease both epidemiologists and clinicians refer to the relative risk. In the case of HLA antigen, the relative risk compares the chance of a person who has a particular HLA antigen acquiring a disease to those individuals who do not have such an antigen. The association is exemplified by the relative risk of suffering from ankylosing spondylitis (AS) in individuals who possess HLA-B27. In patients suffering from AS, the prevalence of HLA-B27 is 90% and this figure rises to 95% in patients who suffer both with the disease and acute iritis. Indeed, in the UK approximately 45% of patients who present with acute iritis will

harbour HLA-B27 52 . It is therefore important that patients who present with anterior uveitis are screened for HLA-B27 because although a positive result may not necessarily be diagnostic, its presence will certainly improve the sensitivity of further radiological tests.

TTaab bllee 9 9 compares the HLA associations with both ophthalmic disorders together with those systemic disorders relevant to the ophthalmic practitioner.

Conclusion

This article has only broached the fascinating subject of ocular immunology. A basic understanding of immunology is required if practitioners are to therapeutically manage their patients. Further articles in the series will help to reinforce the concepts of this challenging subject.

Acknowledgement

The author would like to thank Professor Roger Buckley for his permission to use FFiig gu urreess 7 7 tto o 9 9.

Relative risk 54 Figures 1 to 6 reprinted from Ocular

Immunology, Third Edition, Roitt, Brostoff and Male, 1993 by

90 Anterior uveitis permission of the publisher

Ankylosing spondylitis

B27

33 Mucopurulent conjunctivitis Mosby.

Reiter’s disease

B27

Anterior uveitis Keratitis

About the author

7 Keratoconjunctivitis sicca Gregory Heath is an optometrist

working part-time in private

Scleritis

practice. He was recently awarded

10 Keratoconjunctivitis sicca the diploma in clinical optometry

Primary Sjögrens’ syndrome

DR3, DR5

Anterior uveitis (acute and chronic) at City University and is currently

Sarcoidosis

DR3

Not known

Dacryoadentis reading medicine at the Royal

Retinal vasculitis, Free and University College,

neovascularisation, London, Medical School.

Optic nerve granulomata

Cicatrical pemphigoid

DQw7

Not known

Shrinkage of conjunctiva

References

References are available upon

B5 3 Anterior uveitis request. Please fax 01252-816176

Behcet’s disease

Retinitis, periphlebitis, retinal oedema

or email info@optometry.co.uk.

Sympathetic

DR4, A11,

Not known

Systemic lupus

DR2, DR3

3 Punctate epithelial keratopathy

erythematosus

Keratopathy Necrotising scleritis

Retinal lesions (cotton wool spots) HLA and ophthalmic disease Autoimmune optic neuropathy

Table 9:

Multiple choice questions - Basic immunology

Please note there is only one correct answer

1 1.. W Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng no g is n ott p paarrtt o off tth hee

9 9.. IIn n aa p paattiieen ntt ssu uffffeerriin ng om g ffrro m vveerrn naall cco on njju un nccttiivviittiiss,, iin nn naattee iim mm mu un nee ssyysstteem m??

5.. W 5 Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

cco orrrreecctt rreeg gaarrd diin ng g TT cceellllss??

wh w hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

a. Mast cells

a. T cells can be subdivided into TH 1 and TH 2 cco orrrreecctt??

b. Complement

subtypes only

a. Trantas’-dots represent infiltrating T cells

c. Phagocytes

b. T cells alone can identify any type of antigen

b. Affected eyes have increased levels of IgD

d. T cells

c. T cells express cell surface proteins denoted by

c. Histologically, mast cells, lymphocytes and

cluster determinant (CD) numbers

macrophages have been identified

2 2.. W Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

d. Is due to type III Hypersensitivity cco orrrreecctt rreeg gaarrd diin ng hee iin g tth nn naattee iim mm mu un nee ssyysstteem m??

d. All T cells are involved in initiating the

inflammatory response

a. It is specific

6 6.. W Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

1 10 0.. W Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

b. It evokes a more potent response on

iin ncco orrrreecctt??

iin ncco orrrreecctt??

secondary exposure

a. B cells have antibodies as their cell surface

a. HLA-B27 is a risk factor for both anterior uveitis

c. It represents the first line of defence

receptor

and ankylosing spondylitis

b. Granulomas are present in type IV subsequent exposures

d. It is able to memorise pathogens on

b. There are five types of antibody

c. IgE is an important antibody in allergies

hypersensitivity reactions

d. All B cells differentiate into plasma cells

c. Histamine is an important vasoconstrictor

3 3.. W Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

d. IgE mediated hypersensitivity is of rapid onset cco orrrreecctt rreeg gaarrd diin ng hee cceellllss o g tth off tth hee iin nn naattee

7 7.. W Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

ssyysstteem m??

cco orrrreecctt rreeg gaarrd diin ng occu ullaarr iim g o mm mu un nee p prriivviilleeg gee??

a. There is an absence of Langerhans’ cells in the

1 11 1.. W Wh haatt p prro op po orrttiio on n o off p paattiieen nttss w wiitth uttee iritis w h aaccu wiillll

a. Basophils are important phagocytes haarrb h bo ou urr H HLLA A--B B2 27 7??

b. During phagocytosis the pathogen becomes

central cornea

initially internalised as a phago-lysosome

b. Aqueous humour has no role

a. 15%

b. 25%

c. Eosinophils play an important role in

c. Abundant vascular supply is vital

c. 45%

combating virally-infected cells

d. All ocular cells express MHC class II

d. 65%

d. Langerhans’ cells form a bridge between

8.. W 8 Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss

innate and adaptive immunity

cco on ncceerrn niin ng occu ullaarr iim g o mm mu un no ollo og gyy is iin ncco orrrreecctt??

12 1 2.. W Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is cco orrrreecctt

rreeg gaarrd diin ng g aa ttyyp pee II h hyyp peerrsseen nssiittiivviittyy rreeaaccttiio on n?? 4.. W 4 Wh hiicch on nee o h o off tth hee ffo ollllo ow wiin ng g ssttaatteem meen nttss is

a. Levels of IgA and complement

a. It always occurs in isolation cco orrrreecctt rreeg gaarrd diin ng hee aad g tth daap pttiivvee iim mm mu un nee

increase when the eyes are closed

b. It is characterised by the presence of ssyysstteem m??

b. IgA is the predominant antibody

in blood and tissue fluid

macrophages

a. It consists of all types of lymphocytes

c. IgM, IgG and IgA antibody isotypes have been

c. It is associated with myasthenia gravis

b. T cells produce antibodies

identified in the cornea

d. It involves the degranulation of mast cells

c. T cells maturate in the thymus

d. The sclera contains a smaller number of

following the cross-linking of IgE bound to its

d. B cells are produced in the spleen

immune cells than the conjunctiva

cell surface