Basic Immunology from the Dermatologic point of view Cont.

Invading microbes (pathogens) External defenses -1ST Line Skin Mucous membranes Secretions INNATE IMMUNITY Rapid responses to a broad range of microbes ADAPTIVE IMMUNITY Slower responses to specific microbes Internal defenses - 2nd Line Phagocytic cells Inflammatory response Humoral response (antibodies)Antimicrobial peptides Natural killer cells Cell-mediated response (cytotoxic lymphocytes)

The innate immune system effectively prevent free growth of bacteria within the body. however, many pathogens have evolved mechanisms allowing them to bypass the innate immune system and generates a threshold level of antigen which triggers the adaptive immune system . Adaptive immunity

1. The RECOGNITION of specific non-self antigens in the presence of self, during the process of ANTIGEN PRESENTATION. 2. The generation of TAILORED RESPONSES to eliminate specific pathogens. 3. The development of IMMUNOLOGIC MEMORY in which each pathogen is remembered by a signature antibody. These memory cells can be called upon to quickly eliminate a pathogen on subsequent infections due to enhancement with each successive antigen encounter owing to the accumulation of memory . Functions of the adaptive immune system

The lymphocytes of the adaptive immune system are: T cells mature in the thymus B cells mature in the bone marrow The process starts by antigen presentation. Adaptive immune system

It has two separate but overlapping arms: I. Humoral, or antibody-mediated (B Cell) immunity II. Cellular, or cell-mediated (T Cell) immunity Adaptive immune system

1. Foreign substances Mainly proteins, often microorganisms and their toxins 2. Human cells that have been transformed May be tumor cells, or cells infected with viruses 3. Human tissue Organ transplants, tissue grafts, incompatible blood types during a transfusion 4. Autoimmune diseases Tissue from the persons own body becomes an antigen Antigens

With the exception of non-nucleated cells all cells are capable of presenting antigen and of activating the adaptive response. - depending on how and where the antigen first encounters cells of the immune system. - Some cells are specially equipped to present antigen, and to prime naive T cells and are termed professional (APC). Dendritic cells: Langerhans cells (LCs) are key APCs. B-cells Macrophages Neutrophils Antigen presenting cells (APCs)

Defined as professional APCs that display an extraordinary capacity to stimulate naive T cells and initiate a primary immune response. Dendritic cells (DC)

Dendritic Cells of the epidermis. Derived from the bone marrow EXPRESSES: 1. Birbeck granules 2. Langerin 3. MHC class II. 4. CD1, useful marker for LCs, since within the epidermis (normal or inflamed) it is exclusively expressed on LCs. 5. S100 ptn 6. Vimentin 7. FcRI Derived from the bone marrow from CD34 precursor cells. LNGERHANS CELLS

LCs cannot be identified in routinely fixed and stained histologic sections; their recognition requires electron microscopy or histochemical analysis. Numbers of LCs are reduced in following: 1. The palms and soles, genitalia and buccal mucosa. 2. With age. 3. Chronically UV-exposed skin. LNGERHANS CELLS

Langerhans cells can be visualized by staining using an antibody against MHC class II molecules. Note the dendritic shape of Langerhans cells.

Electron microscopic picture of a Langerhans cell. Arrows indicate The Birbeck granules, rod-shaped organelles specific for Langerhans cells. They are said to resemble tennis rackets

Resident Langerhans cell engulfs the exogenous antigen or express the endogenous one Starts emigration to the lymph nodes to meet the T cells. During this trip it develops some changes to become similar to mature Dendritic cell. LC: Antigen presentation

: a) Molecules involved in antigen uptake as Birbeck granules, Fc receptors b) Molecules mediating the attachment to neighboring keratinocytes ( E-Cadherins). : a) Expression of receptors involved in tissue homing at the lymph nodes as CD44. b) Surface molecules necessary for antigen presentation and T cell priming as MHC class I, MHC class II, CD40, CD54, CD58, CD80, CD86. c) Type IV collagenase enable their penetration through the basement membrane. d) Their dendricity becomes more pronounced. Changes of LCs during migration

APC to B cells and T cells is not the same. T cells only identify the antigen when processed into peptides bound to specific surface molecules on APC. B cells can identify the whole antigen by antibodies on their surface Ag Presentation to T-Cells

T-cells identify the processed antigen bound to MHC on the surface of Dendritic cells. T helper CD4 T cells identify antigens bound to MHC II while; Cytotoxic CD8 T cells identify antigen T cells bound to MHC I Exogenous and endogenous antigen presentation. Ag Presentation to T-Cells

The MHC complex is divided into three subgroups called MHC class I, MHC class II and MHC class III. MHC class I is present on all nucleated cells (except RBCs). MHC class II is present on antigen presenting cells. MHC

1. Exogenous antigens: are engulfed by the APC, processed and presented in association with MHC II. 2. Endogenous antigens: (VIRUS AND TUMOURS) are processed and presented in association with MHC I TYPES OF AP to T- cells

lymphocytes T cells undergo thymic education through positive and negative selection. They are taught the difference between self and non-self molecules in their school to achieve Immunologic tolerance.

T cells develop and mature in the Thymus after migration of the stem cells from the bone marrow. At the thymus only T cells that can recognize foreign and not self antigen in the MHC complex get a survival signal (positive selection) and pass to the circulation and lymph nodes. Those who fail have affinity to self antigens receive signals for apoptosis (negative selection) thus no auto attack. Positive and negative selection allow the survival of just those T cells that recognize foreign (but not self) peptides in the context of self MHC molecules and thus are useful for immune defense without causing auto-attack T cells

1. Immature T cells: Express both CD4 and CD8 molecules. 2. Mature T cells: Later with the development of the T-cell receptor (TCR), they either express: a)CD4 and become T helper cell that binds antigens in MHCII b)CD8 molecule and becomes T cytotoxic cell that binds antigens on MHCI. Types of T cells

It is the part responsible for recognition of the specific antigen and the further T cell response. TCR are transmembrane molecules that are mainly of the / type while only 10% are of the / type in body and skin. The T-CELL RECEPTOR (TCR)

Do not follow the classic way of antigen recognition . May play a role in innate immunity. Increase in the skin in leprosy and lieshmaniasis. / T-cells

Can recognize a huge number of antigens encoded by more than 400 genes that are modified and rearranged to cover an endless number of antigens by recombination activation genes when defective ----combined immune deficiency. TCR

CD3 is an important part of TCR responsible for transmission of the signal to the cell that encodes for the cytokine needed to stimulate the required response for that particular antigen. TCR SIGNALLING

Signaling through the TCR complex alone does not suffice to activate T cells. The presence of costimulatory signals is needed for T cells to undergo antigen-specific clonal expansion. Development of a productive T-cell immune response requires exposure of these cells to at least two types of stimuli. The first signal is the interaction of the TCR with peptideMHC complexes presented by APCs, which determines the specificity of the immune response. The second signal involves surface molecules and cytokines, which determine the clonal expansion of specific T cells and their differentiation into effector and memory cells. Costimulatory molecules

B7 FAMILY e.g. B7-1 (CD80) and B7-2 (CD86) induced by cytokines (TNF, IL-1) or by various TLR ligands Cytokines, especially inflammatory mediators like IL-1, IL-6 and TNF-, also provide costimulatory signals by themselves and, in addition, upregulate costimulatory molecules. Are very important for completion of the T-cell response other wise ANERGY (non-reactivity) and failure of T-cell stimulation occurs. Costimulatory molecules

After proper antigen presentation and costimulation T cell becomes activated division occurs. Memory T cells develop CLONAL EXPANSION

Majority are: In the dermis. CD4 OR CD8. / TCR. memory phenotype CD45RO+/CD45RA- Skin homing receptor CLA(cutaneous lymphocyte associated antigen). Criteria of Skin T cells

1. CENTRAL MEMORY T CELLS [TCM]: express lymph node homing receptors and thus stay in the lymph nodes. CD45RO+CD45RA- CCR7+ Have no effector function. They stimulate dendritic cells to produce IL-12 upon secondary stimulation and differentiate into CCR7- cells. 2. EFFECTOR MEMORY T CELLS [TEM]: CD45RO+CD45RA- CCR7- develop receptors to migrate to the inflamed tissues (e.g. CLA in the skin). express receptors for migration to inflamed tissues and have immediate effector function. Memory T cells

After recognition of the antigen CD4: T helper cells (Th): activate the immune system to combat the antigen including both T and B cells. CD8: T cytotoxic cells (Tc): Antiviral and anti-tumor responses Effector T cell function

According to the type of antigen recognized by the Th cells they secrete different cytokine patterns that will further stimulate different parts of the immune system. Pre-activated precursor Th cells (Th0) secretes a wide variety if cytokines which then develops into: Th1 or Th2 with more restricted type of cytokine secretion. T helper response

Th1 cells Th 2 cells Differentiation from Th0 mediated by IL-12 IL-4 Secretes a) IL-2: stimulates both Th and Tc proliferation. b) IFN GAMMA: activates macrophages and NK cells and IL-12. a) IL-4: Stimulates B cells to produce IgE and stimulates further Th 2 response and inhibits Th1 response. b) IL-5: promotes eosinophil growth. c) IL-10: inhibits Th1 response Mediates CELL MEDIATED RESPONSE HUMORAL IMMUNITY Th1 cells versus Th2 cells

1. Depends mainly upon the type of antigen presented. 2. The cytokines it stimulates. 3. The Dendritic cells. 4. the toll like receptor. 5. Dose of antigen. 6. Genetic background of the host. 7. The APC and its cytokines. 8. The co-stimulatory molecules. Th1/Th2 decision

Transforming growth factor B . Helps IgA production. Suppresses both Th1 and Th2 responses. Th3 cells

CD4 + T cells. Secretes large amounts of IL-10. Suppressor effect on both immune responses. Produced by immature inactive dendritic cells. Important for TOLERANCE towards self antigens and regulating inflammation. Regulatory T cells (Tregs)

Function of regulatory T cells

Direct killing of the organism or the abnormal or infected cells. TC1 and TC2 in cytokine patterns (functional roles still remain to be determined. Even). Viral and anti-tumor activities. Cytotoxic T cells with CD8 surface protein are called CD8+ T cells. Three different pathways of killing: a) Perforin which forms pores in the target cell's plasma membrane this causes ions and water to flow into the target cell, making it expand and eventually lyse then Granzyme that can enter target cells via the perforin-formed pore and induce apoptosis. b) Tc cells activate the death receptor Fas on the target cell by expressing the cognate death ligand FasL. The activated Fas also triggers apoptosis. c) Cytokines, including TNF- and IFN-, which are released as long as TCR stimulation continues. These mediators can affect distant cells as well as the target cell Cytotoxic T cells (CD8+ T cells)

B cells function to protect the host by producing antibodies that identify unique antigen and neutralize specific pathogens. B Cells are the major cells involved in the creation of humoral immunity. Like the T cell receptor, B cells express a unique B cell receptor (BCR), in this case, an immobilized antibody molecule. The BCR recognizes and binds to only one particular antigen. Differentiates into an effector cell, known as a PLASMA CELL The B lymphocyte (B cells)

B-cell response

T cell B cell Antigens reorganization Processed form in the context of an MHC Native form Activation signals Th1 Th2 B cell Vs. T cell ACTIVATION

Short lived cells (2-3 days) which secrete antibodies that bind to antigens, making them easier targets for phagocytes, and trigger the complement cascade. About 10% of plasma cells will survive to become long-lived antigen specific memory B cells primed to produce specific antibodies and respond quickly if the same pathogen re-infects the host; while the host experiences few, if any, symptoms. Plasma cells

Primary immune response cellular differentiation and proliferation, which occurs on the first exposure to a specific antigen Lag period: 3 to 6 days after antigen challenge. Peak levels of plasma antibody are achieved in 10 days. Antibody levels then decline. Immunological Memory

Secondary immune response re-exposure to the same antigen Sensitized memory cells respond within hours. Antibody levels peak in 2 to 3 days at much higher levels than in the primary response. Antibodies bind with greater affinity, and their levels in the blood can remain high for weeks to months. Immunological Memory

Primary and Secondary Humoral Responses

Antibodies (or immunoglobulin, Ig), are large Y- shaped proteins used by the immune system to identify and neutralize foreign objects. In mammals there are five types of antibody: IgA, IgD, IgE, IgG, and IgM, differing in biological properties, each has evolved to handle different kinds of antigens. Heavy chains: The heavy chains of a given antibody molecule determine the class of that antibody: IgM( ), IgG( ), IgA( ), IgD( ), or IgE( ). Antibodies

IgG is monomer Ig & the most abundant immunoglobulin, accounting for approximately 75% of the total amount of serum immunoglobulin. The major immunoglobulin of the secondary immune response. Four subclasses (IgG1, IgG2, IgG3, IgG4) are defined by the amino acid sequences of their constant region. Most of the autoimmune dermatoses caused by autoantibodies are mediated by IgG, most often IgG4. Crosses the placenta and confers passive immunity. Immunoglobulin G

The largest immunoglobulin. IgM molecules are pentamers that (in addition to light and heavy chains) contain one J chain. IgM is the major immunoglobulin produced in the primary immune response. Upon binding to its antigen, IgM induces agglutination and activates the classical complement pathway. Immunoglobulin M

IgA is the predominant immunoglobulin present in mucosal surfaces prevent attachment of pathogens to epithelial cell surfaces. IgA can activate the complement system via the alternative (but not the classical) pathway. Two subclasses of IgA exist, IgA1 and IgA2. IgA molecules can be joined by a J chain; this dimer form is mostly found in secretions, while in the serum, IgA circulates primarily as a monomer. IgA molecules can be involved in the pathogenesis of bullous autoimmune diseases. Immunoglobulin A

IgE monomer is the classic anaphylactic antibody that mediates most immediate allergic and anaphylactic reactions. Mast cells and basophils express high-affinity receptors for the Fc portion of IgE (FcRI). Antigens, anti-IgE antibodies or other substances that crosslink at least two IgE molecules bound on mast cells induce the release of mediators. Also LCs, dermal DCs and peripheral blood DCs as well as monocytes from atopic individuals can bind monomeric IgE via the high-affinity FcRI. The second IgE receptor, FcRII exhibits weaker binding affinity and is expressed on macrophages, eosinophils, platelets, and particular subtypes of T and B cells. Immunoglobulin E

The function of IgD still remains mysterious. Recent evidence suggests that IgD participates in respiratory immune defense. It binds to basophils and mast cells, stimulating their production of antimicrobial factors. IgD may also exert proinflammatory functions, as illustrated by the hyperimmunoglobulinemia D with periodic fever syndrome (HIDS). Immunoglobulin D

Mechanisms of Antibody Action

Functions of antibodies

INNATE IMMUNITY ADAPTIVE IMMUNITY Trigger PAMP (Pathogen-associated molecular pattern) Specific antigens Action Min to hours Days to weeks Receptors PRR (Pattern recognition receptor) as TLR TCR, BCR Memory No Yes Communication Cytokines Effectors Complement Antigen presentation Phagocytosis Complement Antigen presentation Antibodies Cytotoxicity

Hypersensitivity An allergic reaction. An exaggerated response. Tissue destruction occurs as a result of the immune response. Four main types.

Type I Hypersensitivity Immediate (Anaphylactic type) The reaction occurs within minutes after exposure to an antigen. Plasma cells produce IgE. IgE causes mast cells to release histamine, causing increased dilation and permeability of blood vessels and constricting smooth muscle in bronchioles of the lungs. The reaction may range from hay fever to asthma and life-threatening anaphylaxis.

When a specific antigen binds to mast cell-bound IgE, the FcRI becomes activated, which leads to degranulation and release of preformed mediators, including: 1. Histamine 2. Bradykinin 3. Serotonin. 4. Prostaglandins 5. Leukotrienes (B4, C4, D4 and E4), 6. Platelet activating factor They enhance i. vascular permeability ii. bronchoconstriction iii. induction of an inflammatory response Type I Hypersensitivity

Acute Allergic Response

Type II Hypersensitivity Cytotoxic type (Sub acute type) Antibody combines with an antigen bound to the surface of tissue cells, usually a circulating RBC. Activated complement components, IgG and IgM antibodies in blood, participate in this type of hypersensitivity reaction. This destroys the tissue that has the antigens on the surface of its cells (e.g., Rh incompatibility).

Type III Hypersensitivity Immune complex type (serum sickness, SLE) (Also a Sub acute type) Immune complexes are formed between microorganisms and antibody in circulating blood. These complexes leave the blood and are deposited in body tissues, where they cause an acute inflammatory response. Tissue destruction occurs following phagocytosis by neutrophils.

Type IV Hypersensitivity Cell-mediated type (delayed) T lymphocytes that previously have been introduced to an antigen cause damage to tissue cells or recruit other cells. Responsible for the rejection of tissue grafts and transplanted organs Allergic contact dermatitis.

The body produces auto-antibodies and sensitized TC cells that destroy its own tissues Examples include: systemic lupus erythematosus (SLE) Pemphigus Autoimmune Diseases

Dr Samia Esmat Professor of Dermatology Cairo University Bolognia: Dermatology, 2nd &3rd ed. Immense Immunology Insight Immunity and the immune system Dr. Angelo Smith WHPL References