31 Hypersensitivity IV and V

Dr. M. N. Gupta

1. Objectives

•  To understand the three types of class IV hypersensitivity reaction: contact dermatitis, tuberculin reaction and granulomatous hypersensitivity reaction.

•  To explain how machanistically all the three types follow the same route.

•  To learn how certain diseases caused by infectious agent have involvement of type IV hypersensitivity

•  To understand type V hypersensitivity reactions which occur because of chronic stimulation

2. Concept map

Type IV hypersensitivity is also called delayed type as results are seen from 48 hrs-28 days ! In the three variants of type IV, the delay is of different time periods.

We will also look at the type V hypersensitivity which was introduced as a separate class after Coombs and Geller classification. This hypersensitivity is involved in some autoimmune diseases.

Contact with some substances can cause contact dermatitis.

Injection of some antigens derived from infectious agents can be used as a test for hypersensitivity towards that microbe.

When some microbes persist, it causes formation of granulomes.

All are examples of delayed hypersensitivity.

On the other hand, a variant of type II hypersensitivity when antigen is in fact an antibody against a host antigen causes chronic stimulation. This is classified as type V hypersensitivity by some workers.

Type IV hypersensitivity

In Coombs and Gell classification, all hypersensitivity reactions which take > 12 hrs to develop and are dependant upon cell mediated immunity are classified as Type IV or delayed type hypersensitivity.

Much after this classification, late phase IgE mediated reactions were discovered. These are at their maximum in the time period of 12-24 hours after the exposure to allergen. As we have discussed, these also involve Th2 cells. So, that definition of type IV sensitivity is not adequate.

Type IV sensitivity cannot be transferred by serum but by T-cells. T-cells involved are sensitized T-cells, that is, they had encountered the specific antigen (allergen) before and this time recruit other cells to the site of the reaction.

Obviously, the exposure is through skin which comes into contact with allergen.

In Europe, the most common allergens are:

• Nickel
• Chromate
• Chemicals in rubber

In USA, poison ivy and poison oak are reported to be responsible for maximum reported cases of contact dermatitis.

In poison ivy, the responsible compound is urushiol, an oily mixture ofcatechol derivatives with long hydrocarbon side chains. Urushiol is secreted by leaves of the poison ivy vine and related plants.

Catechol derivatives are easily oxidized to reactive o-quinones which can react with cell surface molecules to generate allergens. The skin pigment melanin and melanoprotein, in fact, arises from similar reaction.

Various metals like Ni and Cr present in jewellery and hooks/zipper of garments chelate with skin proteins to form allergens.

Allergic dermatitis sometimes is observed on pathologist fingers because of constant exposure to formaldehyde – another reactive molecule.

Other common chemicals are picric acid, aniline dyes, organo phosphates, plant resins and oils.

Unless the allergen is liquid, normally hairy parts of the skin are not affected.

2,4 Dichlronitobenzene (DNCB) and similar compounds have been used to induce contact dermatitis in experimental animals.

Most of these compounds which cause contact hypersensitivity act as haptens, react with skin cell surface molecules to form hapten-carrier conjugate to become immunogenic. Contact hypersensitivity is distinguished from other type IV reactions in being a epidermal phenomena. The langerhans cells (antigen presenting cells in the skin) are the first step in the chain which is involved in the hypersensitivity.

Langerhans cells originate from the family of follicular dendritic cells and are derived from bone marrow. These are about 3% of epidermis cells.

With few hours (~ 4hrs) of using DNCB in mice, langerhans cells appear in draining lymph nodes. It is in lymph nodes that these cells act as APC and present antigen to T-lymphocytes.

The first phase is sensitization phase. This consists of uptake, processing and presentation of the allergen by local APC.

Sensitization takes place about 10-14 days in humans. Exposure leads to absorption. The haptens reacts with skin protein and epidermal langerhans cells internalize the antigen.

The hapten containing peptides produced by APC are now ready to be presented. APC, the langerhans cells migrate to paracortical areas of regional lymph nodes and present the allergy to CD4+ lymphocyte. These produce a population of memory T-cells.

In humans, both dose (amount) response as well as concentration (DNCB per unit area) response correlations have been observed. There is a plateau beyond a certain level. DNCB amount per unit area determines the response.

During the second and subsequent exposures, primed (memory) Th1 cells in lymph nodes appear at the skin.

Triggered T-cells secret cytokines such as IFN-γ which stimulate keratinocytes to secret further cytokines and chenokines.

Langerhans cells present the processed antigen in the form of hapten-peptides to the memory CD4+ T-cells. CD3 receptor of the T-cells is involved and T-cells release cytokines which include IL-2, IL-3, IFN-γ and GM-CSF.

The released IL-2 binds to the receptors on T-cells and induce their proliferation.

IFN-γ and TNF induce epidermis keratinocytes to express intercellular adhesion molecule including ICAM-1 in about 24-48 hrs of the exposure to the allergen.

At 48 hrs, kerationcytes express HLA-DR under the influence of IFN-γ.

Lymphocytes and macrophages have integrin LFA-1 on their surface which recognise ICAM-1. The result is that lymphocytes and macrophages come to the site in the skin.

Activated keratinocytes also release IL-1, IL-6 and GM-CSF promoting activation and proliferation of T-cells. GM-CSF also activates langerhans cells similarly.

Endothelial cells in the dermis also express adhesion cells and can get involved in accumulation of lymphocytes to the site.

The reaction begins to fade after 48-72 hours. At this point macrophages and keratinocytes produce prostaglandin E which inhibit IL-1 and IL-2 production. T-cells bind to keratinocytes. The hapten-peptide is degraded by cellular mechanism.

So, essentially, the type IV hypersensitivity is mediated by chemokines and cytokines secreted by Th1 cells.

The clear roles of these and there physiological consequences is shown in Figure 5.

Chronic diseases in humans which involve type IV hypersensitivity

Most are due to infections from mycobacteria, protozoa and fungi. Some have already been mentioned. An illustrative list is:

•  Tuberculosis

•  Leprosy

•  Leishmaniasis

•  Listeriosis

•  Blastomycosis (a deep fungal infection)

•  Schitosomiasis (due to worm infection)

•  Sarcoidosis (presumably an infection disease)

These all are caused as a result of chronic antigenic stimulation.

Leprosy is of three types: tuberculoid, borderline and lepromatous.Both tuberculoide and borderline types involve type IV hypersensitivity

Both boderline leprosy and sarcoidosis show lesions typical of granulomatous hypersensitivity

Tuberculin-Type hypersensitivity

This was first observed by Koch. Koch found that subcutaneous administration of tuberculosis (a lipoprotein antigen from mycobacterium tuberculosis) to TB patients led to the tissue at the site of injection become hard and swelling was also observed.

The “tuberculin reaction” is now the basis of Mantoux test. In this test, a purified antigen which is protein derivative of tuberculin is injected into skin and consequences developed are observed at the site of injection after 72 hours. A positive test consists of red swelling.

Histology reveals that these leukocytes and macrophages infiltrate locally in the skin at the sight of injection.

Soluble antigens from many other organism (which include M. lepare and Leishmania tropica) produce similar responses. This hypersensitivity is also observed with some non microbial antigens such as zirconium and beryllium reacted with skin proteins.

Hence tuberculin reaction is frequently used to test sensitivity to diverse antigens if there has been a previous exposures.

Tuberculin injected into the skin is processed and presented by dendritic cells to T-cells along with along with class II MHC proteins.

These T-cells are memory T-cells which are specific to tuberculin. T-cells produce cytokines which alter endothelial cell adhesion molecules. This allows monocytes to enter the local site and mature into macrophages. It is the synergy of T-cells and macrophages which produce products which induce swelling.

The time course for the different phases of tuberculin reaction is known.

In about 12 hours after intradermal exposure to tuberculin, T-cells migrate out of capillaries and accumulate around appendages like sweat glands. Collagen bundles of the dermis are broken.

By 48 hrs, CD4+ cells out number CD8+ cells by 2:1. CD1+ cells (langerhans-like cells) also begin appearing between 24-48 hrs time periods.

In the final phase of 48-96 hrs, keratinocytes get involved.

The movement of cells between local sites and lymph nodes is very similar to contact dermatitis.

In some cases, tuberculin lesion may develop into granuloma.

Granulomatous hypersentivity

This results from the persistence of some antigens/microorganisms which T-cells are unable to destroy CD4+ T cells control many intracellular microbial infections. However, mycobacteria (M. tuberculosis and M. laprae) have escape mechanism to prevent elimination. These consist of modulation of MHC class II synthesis, trafficking and expression and inhibition of antigen processing for presentation by MHC class II protein.

Hence activation of macrophages by CD4+ T-cells is ineffective. This leads to chronic stimulation of CD4+ T-cells which continues cytokine production.

While immunological granuloma occur, apart from due to M. tuberculosis and M. lapara, schistosoma infection, crohn’s disease, by zirconium and beryllium and in Sarchiodosis, nonimmunological granuloma also occur with talc and a variety of particulate matter. In the latter case, macrophage are unable to degrade the inorganic particles. These granulomas are characterized by the absence of lymphocyte in the lesion.

While the tuberculin reaction also is caused by similar organism, granulomes have a different histological appearance.

Granulomatous reaction sometimes can also result from persistence of immune complexes as in allergic alveolitis.

The continuous production of cytokine leads to macrophages containing the microbe and fibroblast proliferation. Granuloma is a walled off microbe along with epitheloid cells and giant cells.

Epitheloid cell is a large and flattened cell with more endoplasmic reticulum. Epitheloid cells lack phagosomes.

The accumulating macrophage secret TNF which results into differentiation of macrophage into epitheloid cells which fuse into giant cells due to continuous secretion of TNF.

Experimentally it has been shown that TNF is necessary for the development of granulomas.

BCG-injected mice, if injected with anti-TNF antibody do not form granulomas.

Giant cells are multinucleate cells which are multinuclear cells which are sometimes called langerhans’ gaint cells. These have little endoplasmic reticulum and degenerated mitochondria and lysosomes. These perhaps are terminally differentiated cells of monocyte/macrophage line.

An immunological granuloma has epitheloid cells and macrophages at core, sometime alongwith giant cells. In some tdiseases like tuberculosis, this area shows necrosis. Core is surrounded by lymphocytes along with fibrosis (deposition of collagen fibers).

Mitsuda reaction is a granulomatous reaction in case of leprosy and Kveim test is the similar reaction in case of sarcoidosis.

It was mentioned earliar that original classification of Gell and Coombs mentioned four types of hypersensitivity. Later on Vth type called chromic hypersensitivity was added. It is in fact a variation of Type-II. In this case the antibody mediating it is against self antigen. Hence, many books still divide hypersensitivity into four types and cover the diseases arising out of this type V hypersensitivity under autoimmune diseases. We will discuss the frequently cited example of type V hypersensitivity here and revisit the issues in more detail when we discuss autoimmune diseases.

Type V hypersensitivity

It is also called chronic or stimulatory hypersensitivity The autoantibodies to the hormone receptors are generated and stimulate receptor cells just like hormone.

The classical example is Graves diseases where antibodies to the thyroid stimulating hormone (TSH) receptor cause overactivity of the thyroid.

The result is hyperthyroidism:

Enlarge thyroid gland visible as the swelling at the base of the neck and protecting eyeballs are typical appearance of a patient suffering from Graves disease.

TSH is a polypeptide hormone produced by pituitary. Upon binding to the TSH receptors or follicle cells of thyroid, it triggers production of hormones by thyroid.

TSH production is feedback inhibition by thyroid hormones.

In the autoimmune disease, as mentioned antibody to TSH receptor bind the receptor. This also act as a trigger for the thyroid cells to produce thyroid hormones. As in a normal stimulation, the hormones feedback regulate/stop the production of TSH by the pituitary.

However, the autoantibodies to TSH continue stimulating thyroid cells (by binding to TSH receptors) to continue production of thyroid hormones. This leads to hyperthyroidism. This is also the reason the graves disease is an example of chronic hypersensitivity with chronic stimulation as the key mechanism.

So essentially, Graves disease is an autoimmune disease caused by abolition of a feedback regulation which is normal persons keeps the thyroid hormone level in check.

Graves disease is also called autoimmune thyroiditis. In humans, two major forms of autoimmune thyroiditis are known. Apart from Graves disease, there is Hashimoto Thyroiditis.

In Hashimoto thyroiditis autoantibodies activate complement and destroy thyroid cells. Tc cells also are believed to be involved. The two are distinguished by referring to Graves disease as thyrotoxicosis and the other one as Hastimoto thyroiditis.

In Hashtimoto thyroiditis, auto antibodies are not formed against any receptor. The autoantibodies are directed against tissue specific antigen such as thyroid peroxidase and thyroglobulin. These antibodies are found at extremely high levels.

Women with thyrotoxicosis may give birth to babies afflicted with disease due to the antibodies against TSH receptors are passed through the placenta.

Cases are known when people have both disease and said to have developed hashitoxicosis.

The realization that Graves disease is an autoimmune disease came from studies by physiologists who were interested in understanding how pituitary controls thyroid activity. These workers detected in the blood of the patient of Graves disease, a substance which was named LATS (long acting thyroid stimulator). This was later on believed to be similar to IgG directed against the receptor the receptors on the mouse thyroids that mediate TSH action.

However there were patients with Graves disease which had no LATS in their sera.

It turned out that there are two kinds of auto antibodies. One kind (called LATS) stimulates receptors of TSH in both mouse and humans. Another kind is inert against mouse thyroids (LATS-PR implying LATS-protector).

Thus, experiments with mice have played an important role in our understanding of Graves disease. Some other important results came from experiments with chickens, we will briefly look at them when we discuss autoimmune diseases.

Both antibodies which were called LATS and LATS-PR account for 90% of the case of Graves disease.

We have now completed our discussion of all types of hypersensitivity reactions.

The common feature has been the overriding of protecting role of immunity. Tolerance, hypersensitivity, auto immunity and immune deficiency are all different facets of the same immune system.

We saw how our discussion of type V led to some autoimmune diseases. We will now discuss there in some detail.

Summary

Type IV hypersensitivity

•  Contact dermatitis

•  Tuberculin reaction and mantoux test

•  Granulomatous hypersensitivity reaction

Type V hypersensitivity

• Graves disease