Antibody-Mediated Cytotoxicity(Type II)Hypersensitivity3 Mechanisms

Video clips

• IgE mediated type I allergy (http://www.healthcentral.com/allergy/video-44016-47.html)

• MAC formation http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit2/bacpath/mac_flash.html

Immunreaktionen der Haut

Type II Hypersensitivity• Type II hypersensitivity involves IgG or IgM

induced damage to self cells (Cell-surface or Matrix Antigen)

• Either IgG or IgM is made – against normal self antigens- failure in

immune tolerance– or a foreign antigen resembling some

molecule on the surface of host cells enters the body and IgG or IgM made against that antigen then cross reacts with the host cell

– Antibodies against drugs• Immune Processes involved:

– Classical Complement Pathway– Phagocytosis via FcR and Complement

receptor– ADCC via NK cells or eosinophils

• Many autoimmune diseases result from type II hypersensitivity generated by autoantibodies

Ab can activate the complement system, creating pores in the membrane of a foreign cell

Activation of the classical complement pathway causing MAC lysis of the cells

Ab can serve as an opsonin, enabling phagocytic cells with Fc or C3b receptors to phagocytose

Opsonization of the host cells whereby phagocytes stick to host cells by way of IgG, C3b, or C4b and discharge their lysosomes

It can mediate cell destruction by antibody dependent cell-mediated cytotoxicity (ADCC)

whereby NK cells attach to the Fc portion of the antibodiesThe NK cell then release pore-forming proteins called perforins and proteolytic enzymes called granzymesGranzymes pass through the pores and activate the enzymes that lead to apoptosis of the infected cell by means of destruction of its structural cytoskeleton proteins and by chromosomal degradation

Examples of type II hypersensitivityAB and Rh blood group reactionsautoimmune diseases such as:

• rheumatic fever where antibodies result in joint and heart valve damage

• idiopathic thrombocytopenia purpura where antibodies result in the destruction of platelets

• myasthenia gravis where antibodies bind to the acetylcholine receptors on muscle cells causing faulty enervation of muscles

• Goodpasture's syndrome where antibodies lead to destruction of cells in the kidney

• multiple sclerosis where antibodies are made against the oligodendroglial cells that make myelin, the protein that forms the myelin sheath that insulates the nerve fiber of neurons in the brain and spinal cord; and some drug reactions

Type II hypersensitivity also participates in early transplant rejections

Transfusion Reactions Are Type II Reactions

• Proteins and glycoproteins on the membrane of RBCs are encoded by different genes (with different alleles)

• One allelic form of a blood-group Ag can recognize other allelic forms as foreign and mount an antibody response

• Ab ( Isohaemagglutinins) IgM can be induced by natural exposure to similar antigenic determinants on microorganisms ( normal flora) of gut

• This is the case with the ABO blood-group Ags

Blood Transfusion Reactions

Clinical manifestations in association with hemolytic anemia linked to blood transfusions

• May occur due to massive hemolysis due to antibody and complement system– Immediate (ABO) IgM– Late IgG (2-6 days)

• Symptoms include fever, low hemoglobin, increased bilirubin, mild jaundice, and anemia

• Free hemoglobin is usually not detected in the plasma or urine in these reactions because RBC destruction occurs in extravascular sites

Antibodies to other blood-group AgsIgG class

• May result from repeated blood transfusions because of minor allelic differences in these antigens

• Delayed hemolytic transfusion reaction which may develop in 2 and 6 days reflecting the secondary nature of these reactions

• The transfused blood induces clonal selection and production of IgG against a variety of blood-group membrane antigens

• most commonly Rh, Kidd, Kell, and Duffy • The predominant isotype involved in these reactions is IgG- less

effective than IgM in activating complement• many of the transfused cells are destroyed at extravascular sites

– by agglutination– opsonization– and subsequent phagocytosis by macrophages–

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Type II HypersensitivityAntibody-Complement Dependent Mediated Lysis

Example: Autoimmune Hemolytic Anemia

Reactions – immediate(hrs) with ABO incompatibilities

• Complement- mediated lysis triggered by IgM

• Free Hb in plasma; filtered through the kidneys, hemoglobinuria

• Some Hb bilirubin, (toxic at high levels)

• fever, chills, nausea, clotting within blood vessels, pain in the lower back,

• Treatment- Stop transfusion , maintain urine flow with a diuretic, otherwise Hb in kidney acute tubular necrosis

• Fetus- Rh+; Mother Rh-• Maternal IgG specific for fetal blood-group Ags cross the placenta

and destroy fetal RBCs . CF minor, serious, or lethal..• During 1st pregnancy, fetal RBCs separated from the mother’s by

trophoblast. • Delivery, separation of placenta from uterine wall fetal blood

mother’s circulation. Rh-specific plasma cells and memory B cells in the mother.

• Secreted IgM clears Rh+ fetal red cells from mother’s circulation, but memory cells remain, a threat to any subsequent pregnancy with an Rh+ fetus.

• Activation of these memory cells in a subsequent pregnancy results in the formation of IgG anti-Rh antibodies, which cross the placenta and damage the fetal RBC .

Erythroblastosis fetalis,

…Cont

• Severe anemia and brain damage due to lipid- soluble bilirubin

• Rhogam antibodies given 24-48 hours after the birth, bind with fetal RBCs

Hemolytic Disease of the Newborn.

ABO blood-group incompatibilityIn Newborn

• (65%) of hemolytic disease of the newborn have minor consequences; caused by ABO blood-group incompatibility

• Type A or B fetuses with Type O mothers IgG to A or B - natural exposure or through exposure to fetal blood-group in successive pregnancies.

• Usually fetal anemia is mild; Slight elevation of bilirubin,• Blood-exchange transfusion may be required in infants.• Low levels of UV light is enough to break down bilirubin.

Diagnosis

• Rising titer of Ab to Rh Ag in mother in pregnancy • Maternal IgG on the surface of fetal RBC are detected by

Coombs test.

Treatment

• Severe reaction- intrauterine blood-exchange transfusion.

• Less severe cases- blood-exchange transfusion after birth, UV light

• Mother Tt during pregnancy by plasmapheresis

Type II Response With Antibiotics

Drug such as:

Pencillin

Cephaliosporin

Streptomycin

May act as hapten- protein complexes(Ag) adsorb on RBCs Ab attaches to Ag (adsorbed drug) induces complement activation lysis progressive anemia

• Complexing of Ag with Ab facilitates the clearance of antigen by phagocytic cells.

• Large amounts of immune complexes can lead to tissue-damaging type III hypersensitive reactions;

• The severity may be dependent on the quantity & distribution of immune complexes – Localized inflammation if at the site of antigen entry– Widespread inflammation where ever complexes

deposits through blood

Immune Complex–Mediated(Type III) Hypersensitivity

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Type III Hypersensitivity

• “Immune complex disease”

• Soluble Ag/IgG or IgM– high titers of each required

• Immune processes involved:– classical complement pathway– phagocytic cells

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Type-III Hypersensitivity: Immune Complex

Animation: Large quantities of soluble antigen-antibody complexes form in the blood and are not completely removed by macrophages. These antigen-antibody complexes lodge in the capillaries between the endothelial cells and the basement membrane. The antigen-antibody complexes activate the classical complement pathway and complement proteins and antigen-antibody complexes attract leukocytes to the area. The leukocytes then discharge their killing agents and promote massive inflammation. This leads to tissue death and hemorrhage

Pathogenesis• Ag – Ab complexes + C3b as opsonin recruitment of

neutrophils (R for C3b)- tissue injury as granules release

• C3a, C4a, and C5a (complement split products) – anaphylatoxin localized mast-cell degranulation– C3a, C5a and C5b67 are also chemotactic factor for neutrophils

• Damage caused due to innocent bystander lysis

• Activation of C aggregation of platelets release of clotting factors formation of microthrombi

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Sites of Complex Deposition

Site Outcome

glomeruli glomerulonephritis

blood vessel wall arteritis

synovial membrane arthritis

skin rash

Note: Ab responsible for immune complexes may be generate at a site distant from the point of deposition.

Arthus Reaction- localized type III hypersensitivity

• Inj of Ag ID or S/C who has high levels of circulating Ab specific for that Ag localized immune complexes acute Arthus reaction within 4–8 h

• Microscopic- neutrophils adhering to the vascular endothelium migrate to tissues towards complexes

• Insect bite( sensitive individual) rapid localized type I reaction 4–8 h later Arthus reaction with pronounced erythema and edema

• Intrapulmonary Arthus-type reactions induced by bacterial spores, fungi, or dried fecal proteins pneumonitis or alveolitis

• Disease names reflecting source of Ag – “farmer’s lung” inhalation of thermophilic actinomycetes from moldy hay,

– “pigeon fancier’s disease” from inhalation of serum protein in dust derived from dried pigeon feces

Arthus Reaction

•As the reaction develops, localized tissue and vascular damage results in an accumulation of fluid (edema) and red blood cells (erythema) at the site•The severity of the reaction can vary from mild swelling and redness to tissue necrosis

• Large amounts of Ag enter bloodstream & bind to Ab circulating immune complexes

• If Ag in excess, small complexes form; not easily cleared by the phagocytic cells tissue-damaging at various sites of deposition

• Serum sickness - antitoxins containing foreign serum• E.g. horse antitetanus and antidiptheria serum • Symptoms appear within days and weaks after exposure• These symptoms include fever, weakness, generalized vasculitis

(rashes) with edema and erythema, lymphadenopathy, arthritis, and sometimes glomerulonephritis

Type III Generalized Reactions -Serum Sickness

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Serum sickness

Serum Sickness

• The precise manifestations of serum sickness depend on– Quantity of immune complexes– Size of complexes ( determine the site of deposition)

• Complexes accumulate in tissues where filtration of plasma occurs. This explains the high incidence of – glomerulonephritis (complex deposition in the

kidney)– vasculitis (deposition in the arteries) and – arthritis (deposition in the synovial joints)

Other diseases caused due to the formation of Immune Complexes

• 1. Autoimmunity (SLE, RA, Goodpasture’s )

• 2. Drug Reactions (Allergy to penicillin etc )

• 3. Infectious Diseases(Poststreptococcal glomerulonephritis, Meningitis, Hepatitis, Mononucleosis, Malaria, Trypanosomiasis

Type IV or Delayed-TypeHypersensitivity (DTH)

• Subpopulations of activated TH cells encounter Ag, secrete cytokines localized inflammatory reaction called (DTH)

• Characterized by large influxes of nonspecific inflammatory cells, in particular, macrophages

• 1890 by Robert Koch, “tuberculin reaction” Later named DTH

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Examples of Type IV Hypersensitivity

TH1 Influence of Immune Response

• The hallmarks of a type IV reaction are – the delay in time

required for the reaction to develop

– recruitment of macrophages as opposed to neutrophils (type III reaction)

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TH1-mediated Type IV Hypersensitivity

• Initial sensitization phase is 1-2 weeks after the primary contact

• Primary APCs are:– Macrophages– Langerhans cells– MHCII+- Vascular

endothelial cells

• During this phase – Activation of TH

cells (TH1 and CTLs)

– Clonal expansion of TH cells 35

Pathways of Cytotoxicity utilized by CTL’s

• Effector phase:– Starts after 24hrs, peaks at 48-

72 hrs– Initiates due to recruitment of

non-specific immune cells due to TH1 (5%) mediated cytokines

– Denovo synthesis of macrophages from blood monocytes

• DTH response is important for defense against intracellular parasites and bacteria (phagocytic cells, lytic enzymes)

• However prolonged DTH can be damaging to the host

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Contact Dermatitis

DTH due to either TH1 or CTL mediated hypersensitivity

• DTH is not always detrimental• Granuloma formation (fusion of

continuously activated macrophages lead to giant multinucleated cells)

• These giant cells displace the normal tissue cells, forming palpable nodules

• granuloma-type lesion called a tubercle

• e.g. M. Tuberculosis, activated macrophages wall off bacterium in lungs

• and release high concentrations of lytic enzymes, which destroy bacterium and perhaps the surrounding tissue - extensive tissue necrosis

• Development of delayed-type hypersensitivity reaction

• after a second exposure to poison oak

• Cytokines such as IFN-, macrophage-chemotactic factor (MCF), and migration-inhibition factor (MIF) released from sensitized TH1 cells mediate this reaction

• Tissue damage results from lytic enzymes released from activated macrophages

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Patch testThis test is used to diagnose delayed allergic reactions such as Contact Dermatitis. It involves taping traces of various known contact allergens on the skin and keeping them there for 48 hours

It can test for allergy to Rubber, Nickel, Lanolin, dyes, cosmetics, solvents, preservatives, and medication.

• Neutrophils – first to appear, peaking 6h

• The monocyte- 24 -48 h after antigen exposure

• MCAF (monocyte chemotactic and activating factor)- recruitment of macrophages

• MIF (mirgration inhibitory factor)- inhibits migration of macrophages beyond DTH

IFN Importance

• knockout mice for IFN when infected with an attenuated strain of Mycobacterium bovis known as BCG (Bacille Calmette Guérin), nearly all the animals died within 60 days, whereas wild-type mice survived

• Macrophages from the IFN- knockout mice were shown – to have reduced levels of class II MHC molecules – nd of bactericidal metabolites such as

• nitric oxide • superoxide anion

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter22/animation__delayed__type_iv__hypersensitivity.html

Summary

Hypersensitive reactions on the basis of effector molecules rather than antigens

Contribution of environmental factors on allergy susceptibility

Environmental factors may interact with genetic susceptibility to cause allergic disease

• environmental factors and genetic variation each account for about 50%

• The prevalence of atopic allergic diseases- is increasing in economically advanced regions

• Environmental changes- involves exposure to infectious diseases in early childhood

• Change from traditional rural society meant less exposure to microbes

• Changes in intestinal microbiota• idea that exposure to

microorganisms is associated with allergy was first mooted in 1989- hygiene hypothesis

Hygiene hypothesis to counter- regulation hypothesis

• The original proposition was that less hygienic environments → infections early in childhood → help to protect against the development of atopy and allergic asthma

• mechanisms that skewed immune responses towards TH1 rather than TH2 cells & their associated cytokines → IgE production

• A study in Venezuela showed that children treated for a prolonged period with antihelminthic agents had a higher prevalence of atopy than did untreated and heavily parasitized children

• This proposes that all types of infection might protect against the development of atopy

• the production of cytokines such as IL-10 and transforming growth factor (TGF)-β, which down regulate both TH1 and TH2 responses

• mucosal surfaces (respiratory or intestinal)- evolved mechanisms of regulating responses to commensal flora and environmental antigens

• Less early exposure to microbes- less regulatory T cells- increased risk of allergy

Hygiene hypothesis to counter- regulation hypothesis

Break of Tolerance- Autoimmunity

autoimmune diseases•Systemic

•Organ-specific

History• 1960- it was believed that all self-reactive lymphocytes were

eliminated during their development

• 1970s- experimental evidence countered that belief revealing that not all self-reactive lymphocytes are deleted

• normal healthy individuals have been shown to possess mature, recirculating, self-reactive lymphocytes- not causing autoimmune reactions

• their activity must be regulated in normal individuals through – clonal anergy – clonal suppression

Organ-Specific AutoimmuneDiseases

• the immune response is directed to a target antigen unique to a single organ or gland, so that the manifestations are largely limited to that organ

• Autoimmune diseases involving direct cellular damage occur

• when lymphocytes or antibodies bind to cell-membrane antigens causing cellular lysis and/or an inflammatory response in the affected organ

• Gradually, the damaged cellular structure is replaced by connective tissue (scar tissue), and the function of the organ declines

Some Autoimmune Diseases AreMediated by Direct Cellular Damage

HASHIMOTO’S THYROIDITIS

• The DTH response common in middle-aged women

• Manifested by the visible enlargement of the thyroid gland, goiter- a physiological response to hypothyroidism

• characterized by an intense infiltration of the thyroid gland by lymphocytes, macrophages, and plasma cells → lymphocytic follicles and germinal centers

• auto-antibodies and sensitized TH1 cells specific for thyroid antigens

• Antibodies are formed– to a number of thyroid proteins, including

thyroglobulin– thyroid peroxidase- both of which are

involved in the uptake of iodine

• Binding of the auto-antibodies to these proteins interferes with iodine uptake → hypothyroidism