What is Type IV or Delayed-Type Hypersensitivity (DTH)?
- When some subpopulations of activated TH cells meet specific antigens, they produce cytokines that induce a delayed-type hypersensitivity-like inflammatory response (DTH).
- Large influxes of nonspecific inflammatory cells, particularly macrophages, define the reaction.
- Robert Koch observed in 1890 that persons infected with Mycobacterium TB who had an intradermal injection of a mycobacterial culture filtrate developed a localised inflammatory response.
- This localised skin reaction was termed a “tuberculin reaction” by him. Later, as it became recognised that a range of additional antigens may trigger similar response, its name was altered to delayed-type or type IV hypersensitivity to reflect the delayed beginning of the reaction and the frequently occurring tissue damage (hypersensitivity).
- The word hypersensitivity is somewhat deceptive because it implies that a DTH response is always negative. Although in certain situations a DTH response causes severe tissue damage and is pathologic in and of itself, in the majority of cases tissue damage is limited, and the response serves a crucial role in defence against intracellular infections and contact antigens.
- Type IV reactions are distinguished by the length of time necessary for the reaction to develop and the recruitment of macrophages as opposed to neutrophils, which are characteristic of type III reactions.
- Macrophages are the predominant component of the infiltrate surrounding an inflammatory site.
Mechanism of DTH
- TH1 subtype CD4 cells are activated during the sensitization phase, which occurs 1–2 weeks after the initial interaction with an antigen.
- Langerhans cells and macrophages, among other antigen-presenting cells (APCs), have been implicated in the activation of a DTH response. It is believed that these cells transfer antigens that enter the body through the epidermis to regional lymph nodes, where they activate T cells.
- Antigen-presenting cells (APCs) display antigens complexed in the groove of major histocompatibility complex (MHC) molecules on their cell surface.
- Antigens coupled to MHC class II alleles, human leukocyte antigen (HLA)-DR, HLA-DP, and HLA-DQ, are presented to CD4 T cells for the majority of protein antigens or haptens related with cutaneous DTH. Certain MHC class II alleles have been identified as causing increased immunological activation in response to antigens.
- Subsequent exposure stimulates the effector phase. The TH1 cells secrete several cytokines that recruit and activate macrophages and other nonspecific inflammatory cells.
- The response is not noticeable until 2–3 days following the second exposure. In most cases, the infection is eliminated swiftly with minimal tissue harm. Nonetheless, in rare instances, particularly if the antigen is not quickly eliminated, a protracted DTH response can be harmful to the host, as the severe inflammatory response evolves into a noticeable granulomatous reaction.
Types of DTH Reactions
There are two forms of DTH reactions: contact hypersensitivity and tuberculin-type hypersensitivity.
1. Contact hypersensitivity
- Contact hypersensitivity is a sign of DTH resulting from substance-specific sensitization.
- These include medications like sulfamides and neomycin, plant products like poison ivy and poison oak, chemicals like formaldehyde and nickel, as well as cosmetics, soaps, and other compounds.
- This reaction occurs when chemicals acting as haptens penetrate the skin and combine with body proteins to form full antigens to which a person develops a hypersensitivity.
- On second exposure to the same antigen, the immune system responds by launching an onslaught of cytotoxic T cells, which mostly cause skin damage.
- Within 12–48 hours of the second encounter, the syndrome presents as irritation, erythema, vesicles, eczema, or skin necrosis.
2. Tuberculin-type hypersensitivity reaction
Tuberculin reaction is a common instance of delayed hypersensitivity to microbial antigens, which is utilised to diagnose the condition.
Tuberculin skin test
- This test is performed to identify whether or not an individual has previously been exposed to Mycobacterium tuberculosis.
- Tuberculin (PPD), a protein generated from the cell wall of M. tuberculosis, is injected intradermally in this test.
- Positive test results are indicated by the development of a red, slightly enlarged, and firm lesion at the injection site within 48 to 72 hours.
- A positive test result indicates that the individual has been infected with the bacteria, but it does not prove the diagnosis of tuberculosis.
- However, if a patient with a negative tuberculin skin test develops positive, this implies that the patient was infected recently. However, the skin test may also be negative if:
- Infected individuals getting immunosuppressive medicines (such as corticosteroids and anticancer drugs).
- In patients with illnesses linked with reduced cell-mediated immunity, immunosuppression is observed (such as AIDS, sarcoidosis, lymphoma, post measles vaccination, etc.).
- The response to Mycobacterium TB demonstrates that while processes involved in DTH are needed for defence against the infection, they are also responsible for long-term tissue damage. Cytokines (such as TNF and IFN-), which are created to activate macrophages and so control an infection, also initiate additional cascades that ultimately result in substantial tissue damage.
- Other skin tests are also utilised to diagnose DTH. Included are numerous skin tests for bacterial, fungal, viral, and helminthic illnesses. Lepromin is an effective test for leprosy.
- A positive lepromin test implies tuberculosis with intact cell-mediated immunity. A negative lepromin test, on the other hand, shows the presence of lepromatous leprosy with compromised cell-mediated immunity.
- Exposure to the fungi is indicated by positive skin tests in coccidioidomycosis, paracoccidioidomycosis, and other fungal illnesses.
- In both viral and parasite illnesses, skin tests are less specific and less valuable for diagnosis than serological tests.
Chronic DTH Reactions
- DTH reactions are typically triggered by antigens originating from intracellular infections that are resistant to standard immune response strategies. For instance, chronic Mycobacterium tuberculosis is eventually compartmentalised by the formation of a granuloma.
- Granuloma development is dependent on activated Th1 cells and hyperactivated macrophages, as we have observed. The hypersensitivity occurs when these cells do not “retreat” after the virus has been contained.
- Persistent production of pro-inflammatory cytokines and substances causing granuloma development begins to compromise the surrounding healthy tissue. The DTH response then develops into a chronic type IV HS reaction.
- This kind of HS develops more slowly than antibody-mediated varieties due to the time required for T cell activation and differentiation, cytokine and chemokine production, and macrophage accumulation at the site of exposure.
- The site of antigen entrenchment affects the clinical manifestations of a DTH reaction. 2–3 days after exposure, the skin above the location of exposure may seem unnaturally red.
- Calcification, caseation necrosis (tissue deterioration that results in a cheese-like appearance), and cavity formation are frequent clinical findings.
- DTH responses associated with infectious disorders (tuberculosis, leprosy, leishmaniasis), reactions to non-infectious agents (silicosis, berylliosis), and reactions to unknown agents (Crohn’s disease, sarcoidosis) are characterised by infiltrating mononuclear leukocytes.
- When a granuloma is finally encased in a layer of fibroblasts and connective tissue, it can form a lesion that may impair the function of an organ such as the liver or lung.
- Consequences may include chronic liver disease or respiratory failure. Corticosteroids can be utilised to inhibit cytokine production by effector T cells during type IV HS responses.
- If the allergic reaction is localised, topical treatments may be utilised. More severe tissue involvement may necessitate systemic corticosteroid therapy.
- As described in Chapter 22, DTH reactions have been utilised to determine whether an individual has been exposed to a disease in the past.
- An example of such a test is the skin prick test for TB, in which redness and swelling at the injection site of a little quantity of M. tuberculosis antigen indicate that the subject has been infected with the bacteria in the past.
- Comparable tests can be done to determine a previous infection with germs that cause diphtheria or brucellosis. The DTH reaction can also be used to evaluate the functioning of T cells in an individual.
- Candida albicans is so common in our environment that almost everyone has had at least one infection as a youngster.
- Therefore, a DTH test with Candida antigen should induce redness and swelling at the testing site in the vast majority of patients.
- A patient who fails to mount a DTH response during this test generally has impaired T cell function and may have acquired immunodeficiency.
Hypersensitivity Pneumonitis (Hp)
- Hypersensitivity pneumonitis, also known as extrinsic allergic alveolitis, is a type IV HS lung reaction produced by continuous inhalation of an antigen.
- HP can be triggered by a wide variety of antigens, but the resultant lung response often follows the same pattern and occurs in three distinct clinical phases: the acute phase, the subacute phase, and the chronic phase.
- When a sensitised individual inhales the offending antigen, macrophages in the lungs are activated either because these cells ingest the antigenic particles directly or because the complement system is engaged.
- Within 48 hours, macrophage-secreted chemokines (IL-8, MIP-1, and RANTES) induce an influx of neutrophils and T cells. CD4 Th0 cells initiate development into cytokine-secreting Th1 effectors.
- At this stage, the infected individual is likely to develop influenza-like symptoms, including fever, chills, muscle discomfort, a nonproductive cough, and difficulty breathing. Typically, if future exposure to the causal antigen is avoided, symptoms diminish rapidly.
- In the event of a formal diagnosis, a brief course of corticosteroids may be administered to assist reduce the inflammatory reaction. However, if the acute phase of HP is unrecognised and antigen exposure persists or reoccurs, the sub-acute phase ensues, during which hyperactivated macrophages commence granuloma development.
- For several days or weeks, there are few obvious symptoms at this phase. However, weariness and cough then recur and develop gradually but steadily.
- Again, corticosteroids can be of tremendous value to a patient with HP at this stage. In contrast, if no action is taken and exposure persists, the chronic phase of HP develops and leads to fibrosis of lung tissue via the same processes outlined for chronic DTH responses.
- Activated alveolar macrophages produce substantial amounts of TGF, hence promoting fibrosis. HP patients may potentially develop particular antibodies that contribute to the reported disease via type II and type III HS reactions.
- For instance, antibodies may bind to organs in which the inhaled antigen has taken up residence, or they may produce ICs whose deposition promotes lung inflammation.
- In the last stages of chronic HP, patients exhibit progressively difficult breathing and inexorable weight loss. Unfortunately, this phase of the disease is often confused with other lung diseases involving fibrosis, and despite the use of corticosteroids, the damage to the lungs may be irreparable and finally deadly.
- HP is linked to a variety of illnesses that are often designated for their distinguishing antigen. Typically, these antigens are proteins originating from microorganisms, fungi, plants, or animals.
- In some cases, the inducing agent is an inorganic chemical molecule that can activate specific cells only after reacting with lung tissue. In some instances, the induction of type IV HS in HP resembles a lung CHS event.
- Historically, exposure to trigger antigens happened mostly in the workplace, resulting in diseases such as “farmer’s lung” and “cheese washer’s lung.” Nonetheless, when the nature of these illnesses became clearer, work exposure was regulated, so that the majority of HP cases currently result from residential or recreational contact to inhaled antigens.
- Thus, the terms “sauna-disease,” taker’s “hot-tub lung,” and “bird-lung” fancier’s are more common. Studies indicate that only a small percentage of persons exposed to a certain antigen develop HP.
- In farming areas in North America, less than 6% of exposed individuals develop the disease. Disease-free individuals appear to mount a harmless IgG response to inhaled antigens without cellular involvement.
- Similar to IgE-mediated type I HS, the determinants of HP susceptibility are poorly understood. In some cases, persons with HP recall having an acute respiratory infection prior to the onset of symptoms, suggesting that infection may be a risk factor.
- Our explanation of allergy and hypersensitivity concludes here. We will now address autoimmunity, one of the greatest immunological mysteries.
Examples of Type IV (Cell Mediated) Hypersensitivity
- The tuberculin reaction (Mantoux test): This is a “recall” response to pure mycobacterial antigens and serves as the foundation for a diagnostic skin test for a tuberculosis immune response.
- Granuloma formation: Inability of macrophages to eliminate intracellular infections frequently results in persistent activation of pathogen-specific T cells, which leads to granuloma development. Granulomas are generated as a result of macrophages harbouring persistent antigens being ‘walled off’ by the cytokines released.
- Allergic contact dermatitis: Environmental pollutants, metals, or topical drugs that cause epidermal necrosis, inflammation, skin rash, and blisters; allergic contact dermatitis.
- Type-1 diabetes: Insulin insufficiency caused by the destruction of the pancreatic islet cells by cytotoxic T lymphocytes.
Numerous Cytokines Participate in the DTH Reaction
- TH1 cells produce a multitude of cytokines that attract and activate macrophages at the site of an infection.
- IL-3 and GM-CSF cause localised granulocyte-monocyte lineage hematopoiesis. IFN-γ and TNF-β (together with macrophage-derived TNF-α and IL-1) induce a number of alterations in adjacent endothelial cells that promote the extravasation of monocytes and other nonspecific inflammatory cells.
- Neutrophils and monocytes in circulation attach to adhesion molecules expressed by vascular endothelial cells and extravasate into tissue spaces.
- Neutrophils develop early in the reaction, reach a peak at approximately 6 hours, and subsequently decrease in number. After antigen exposure, monocyte infiltration occurs between 24 and 48 hours later.
- As monocytes penetrate tissues to become macrophages, chemokines such as monocyte chemotactic and activating factor attract them chemotactically to the location of the DTH response (MCAF).
- Migration-inhibition factor (MIF), an additional chemokine, prevents macrophages from moving beyond the site of a DTH reaction. As macrophages aggregate at the site of a DTH reaction, they are triggered by cytokines generated by TH1 cells, specifically IFN-γ and membranebound TNF-β.
- Activation increases the efficiency of macrophages as antigen-presenting cells, as mentioned previously. Thus, active macrophages can easily facilitate the activation of other T cells, which then release additional cytokines that attract and activate additional macrophages.
- This self-sustaining reaction, however, is a double-edged sword, with a thin line separating a good, protective response and a harmful response marked by significant tissue damage.
- Experiments with mutant mice incapable of producing IFN-γ revealed the significance of this cytokine in the DTH response.
- When these knockout mice were infected with an attenuated form of Mycobacterium bovis known as BCG (Bacillus Calmette-Guérin), almost all of them perished within 60 days, whereas wild-type mice survived.
- It was discovered that macrophages from IFN—γ deficient animals have lower concentrations of class II MHC molecules and bactericidal metabolites such as nitric oxide and superoxide anion.
A misled individual has touched poison ivy (1), causing urushiol to attach to self-proteins in the skin (2) and generate a neoantigen. The neoantigen causes skin cells to upregulate the expression of adhesion molecules and produce a large number of chemokines and cytokines (3), which attract immune system cells from the circulation (4). Neo-antigen shed by skin cells is picked up by Langerhans cells (LC), from which peptides are cross-presented to memory Tc cells (5). IFN is produced alongside CTL effectors that directly harm host skin cells by degranulation and/or Fas-mediated apoptosis (6). A high concentration of IFN in the surrounding environment induces the degranulation of mast cells and basophils (7) and the release of lytic mediators that lead to host cell destruction (8).