Immunology Notes - Medical Physiology

Overview of Innate and Specific Immunity

First Lecture

Objectives:

1. Identify key characteristics of immune responses
2. Compare innate and specific immunity
3. Explain how immunity is acquired

ESSENTIAL ABBREVIATIONS/DEFINITIONS

Ag:

antigen

Ab:

antibody

Cytokine:

"immunologic hormones" acting as communication molecules between cells of the immune response (as well as a wide range of other cell types ).

APC:

Ag presenting cell, cells needed to help initiate T cell responses

Leukocyte:

a white blood cell, whereas lymphocytes (ie B and T cells) are subpopulations of leukocytes.

Key themes:

1. inducing and maintaining specific immunity (ie immunization programs) and
2. coping with immune disorders (ie allergic disease, autoimmunity, cancer).

Attempts to manipulate the immune response for practical purposes (ie transplantation, prevention of Rh disease) are also important for some subsets of the population.

Overview:

• Phagocytosis and digestion of pathogen (ie. by neutrophil; monocyte/macrophage,eosinophil)
• Increased production or activation of Ag non-specific soluble proteins such as acute phase reactants, complement cascade, interferon, nitric oxide or lysozyme
• Natural killer cells and T cells (cytotoxic, but Ag non-specific) make cytokines but exhibit very little variability in their receptor for Ag.

• Humoral (Ab: IgG, IgA, IgM, IgE) made by B cells/plasma cells
• Cell mediated: Cytotoxic T cells, helper T cells.

The specific response exhibits a wide diversity of different effector mechanisms (~20) aimed at destruction or localization of pathogens. All of these share the characteristics of (i) recognizing each Ag with great specificity and (ii) memory.

Generation of the cells responsible for the immune response involves a process of self vs. non-self discrimination, where Ags considered "self" are not attacked (except inappropriately, such as in autoimmunity). ANY molecule that is "non-self" triggers an immune response, regardless of whether it is a pathogen or not.

• use of a random genetic mechanism to generate phenomenal diversity during lymphocyte development (>10¹⁰ different receptors).
• When a cell with a complementary receptor encounters its Ag, it is activated to multiply (clonal expansion), and participate in the development of an Ag specific primary (ie first time) immune response. Over the following weeks, when as Ag is removed by the immune response, ~95% of those protective lymphocytes die. The remaining cells (memory cells) give the host the ability to remember prior exposure to Ag, and thereby mount, faster, stronger, more protective responses if the same (or very closely related) Ag is re-encountered later in life, up to several decades later.

1. Abs bind to granulocytes to confer specificity on Ag non-specific cells in their killing (ie eosinophils).
2. Cytokine production generated during the innate response help determine the type of specific response that develops (enhancing Ab production vs stimulating more cytotoxic cells)
3. inflammation brings Ag specific cells to the site, promoting expansion of the Ag-specific component of the response.

• Exposure to Ag/potential pathogens
• Skin, Gut and other physical barriers to entry, mechanical defenses (coughing,etc)
• Upon Ag entry: Innate immune response (phagocytosis, soluble proteins, NK cells depending on the Ag in question) localizes Ag, attempts to lyse it and/or phagocytose it. This leads to,
• Generation of inflammatory response. More intense innate immunity due to recruitment of more monocytes, polymorphs and
• Activation of specific immunity: Interactions of T lymphocytes with "Ag presenting cells" and B lymphocytes, leading to induction of specific immune responses.
• These include, T cell activation (hence cytokine synthesis, cytotoxicity) and Ab formation by B cells/plasma cells.

The primary immune response takes about a week to ten days to develop, and typically lasts a few weeks before fading, having resolved the infection. It leaves memory T and B cells that provide the host with enhanced capability to withstand subsequent exposure to that pathogen or closely related pathogens (but not unrelated Ags). Secondary T cell and Ab responses are commonly evident within a day or two of Ag re-exposure.

Active vs. Passive immunity

Anatomy of the Immune Response

Objectives

1. Discuss the identification of main cells of the immune system and their origins
2. Relate lymphocyte subsets to functions and defence against infections

1. The cells express in a orderly fashion certain "markers" i.e. cell surface proteins. You should remember at least four markers: CD3 (associated with the T cell receptor) CD2 (on all T cells), CD4 and CD8 (on subsets of mature T cells in circulation). The CD4⁺ cell regulates all immune responses while the CD8⁺ becomes a "killer" or cytotoxic cell.
2. The genes of the T cell receptor (TCR) rearrange and the TCR is expressed (together with the CD3) on the cell surface. Two major subsets are found: those that express alpha beta TCR (with great Ag specificity) and a smaller one expressing gamma delta TCR (with very limited diversity).
3. The T cells are selected so that those with a TCR specific against foreign antigens survives and expands (positive selection) while those with TCR against "self" antigens are eliminated (negative selection). When T cells become mature they migrate to secondary lymphoid organs and mucosal surfaces (ie the gut)

Antibodies, T cells and Lymphocyte activation

Objectives:

Antibody isotypes

• Five main types of Ab made, each with different functions in vivo.
• All differ in the Fc end, all can express the same Ag binding end.
• All built from same basic structural unit: 2 heavy chains/ 2 light chains to form a subunit with an Ag binding pocket.

Neutralizing antibodies (in serum or secretions) prevent attachment of pathogens or of toxins to host cells.

Antigen-antibody complexes are eliminated by effector mechanisms such as phagocytosis. Ag:Ab complexes allow activation of complement hence (i) increased phagocytosis by cells with receptors for that isotype of Ab or (ii) lysis via complement activation.

Antigen binding to IgE that is on mast cells or basophils triggers them mast cells to release substances that cause the typical symptoms of allergy.

How does B cell activation occur?

Key points

Following initial B cell activation (which requires Ag and CD4 T cell help), B cells generally change the isotype of antibody produced from IgM to IgG, IgA or, less commonly, to IgE. This process is directed by T cell derived cytokines, with different cytokines directing the B cell to switch to different Ab isotypes.

Different stimuli (and different genetic backgrounds) predispose for dominance of different patterns of cytokines. These different cytokine responses steer the direction of the developing immune response (both B and T cell) to different types of effector responses (ie more CTL or delayed hypersensitivity vs. more antibody production). The significance of this is that effective clearance of different pathogens (virus, vs. bacteria vs. parasites vs. normal food antigens vs. grass pollen, etc) requires induction of quite different immune responses, hence different forms of lymphocyte activation. Generating the wrong type of immune response can be much worse than induction of no response at all (ie allergy, parasite infection, vaccine reactions).

Developing a strong immune response of the right type (ie. in a vaccine) remains an empirical process. Optimal strategies, identified in clinical trials, elicit responses in the great majority of (though usually not all) subjects. In the weeks that follow immunization (natural or physician mediated), the response evolves in important ways: The type of Ab that is produced changes (IgM to IgG in serum, and partially to IgE in allergics, to secretory IgA for mucosal exposure), T cell immunity becomes stronger and more efficient.

External factors influence this capacity to develop or to recall an immune response. These include nutritional status, age, pregnancy, drug use, other ongoing infections/conditions (ie TB, HIV).

1. both require interaction with an Ag complementary to their single Ag-specific receptor (ie. on Ag they can physically bind in their receptor)
2. both require other cell surface mediated signals to be activated,
3. the function of both populations is influenced by cytokines that are locally expressed both during ontogeny and during Ag-driven activation,
4. both exhibit "memory" allowing for improved secondary responses upon subsequent exposure to the same (or very closely related) Ag.

Both B and T cells can also be non-specifically activated by "polyclonal activators" that act in an Ag independent manner (ie bacterially derived lipopolysaccharide, LPS, or products termed superantigens such as toxic shock syndrome toxin, ie. from tampons).

Activation of mature lymphocytes leads to: cell proliferation, cytokine synthesis, maturation of B cells (ie switching from IgM to IgG etc), then cell death. These are controlled through distinct intracellular signalling pathways that may provide selective therapeutic targets in the future.

Following antigen stimulation, most activated lymphocytes (>95%) die within a few weeks, having completed their defensive task. At the same time, a small proportion of memory cells develop that are resting, long lived cells which maintain the stronger, higher affinity responses, to ready that person for subsequent exposure to that Ag.

OVERHEADS

Overhead 1: Module Objectives:

• Overview of key characteristics of immune responses: Innate vs Acquired immunity.
• Who are the players? Different cell types that are critical in immune responses.
• How are they activated? How are they turned down, off or redirected to a more appropriate immune response?
• Applied systems of direct relevance to pharmacists.

Overhead 2

1. specificity
2. self / non-self discrimination
3. memory.

• antibody production leading to direct neutralization of toxins or viruses, opsonization (enhanced phagocytosis) or complement activation ... or to allergy)
• cell mediated cytotoxicity (killing of virus/parasite infected cells or tumor cells)
• delayed hypersensitivity (useful against TB, not against poison ivy)
• cytokine production (determining the intensity and the type of immune response that best deals with that pathogen).

Overhead 3 - How is an immune response generated? Case study.

You are hiking in Jasper up a mountain, you have a big drink from a cool mountain stream and unfortunately, a beaver upstream has some digestive difficulties which include Giardia.

• Exposure to Ag / potential pathogen
• Skin, Gut and other physical barriers to entry, mechanical defenses (coughing,etc)
• Upon Ag entry: Triggering of Innate immune response. This causes,

• Generation of inflammatory response. More intense innate immunity due to recruitment of more monocytes, polymorphs and lymphocytes leading to
• Activation of specific immunity: Interactions of T lymphocytes with "Ag presenting cells, (which are specialized cells required to make Ag visible to T cells) and B lymphocytes, the cells that lead to Ab production (overview of process in third lecture)
• Causes induction of specific immune responses. Many are potentially available including (i) T cell activation (hence cytokine synthesis, cytotoxicity) and (ii) Ab formation by B cells/plasma cells. Lymphocyte activation discussed later.

The primary immune response takes several days to a week to develop, and typically lasts a few weeks before fading, having resolved the infection.

It leaves memory T and B cells that provide the host with improved capability to withstand subsequent exposure to that pathogen (but no benefit for unrelated Ags).

Secondary T cell and Ab responses are commonly evident within a day or two of Ag re-exposure. They are characterized by faster, more intense, more diverse responses.

Overhead 4 - Who are the main players in an immune response?

Overhead 5 - Antibody isotypes

Overhead 6 - T lymphocytes

1. alpha/beta T cells. This is the dominant type of T cell in circulation. They play key roles in the specific acquired immune response. They are highly Ag-specific (like Ab). Unlike Ab, the T cell receptor is not secreted, rather it acts as a signaling molecule allowing selective activation of only those T cells specific for a given Ag. Most alpha/beta T cells express either the (i) CD4 marker (helper T cells) whose main role is regulating the intensity and type of immune response that develops by secreting a variety of different cytokines OR (ii) the CD8 marker (cytotoxic T cells) whose main function is identifying virus, parasite or tumor infected cells and killing them by forming holes in their membrane.
2. gamma/delta T cells. This is the dominant T cell type at mucosal surfaces (gut, lung...). Its T cell receptor has very little variability, because rather than specifically recognizing 10⁸ specific antigens (as do alpha/beta T cells or Ab), these cells recognize molecules that are widely conserved "danger signals" present in prokaryotes but not in eukaryotes. Gamma/delta T cells are key members of the innate immune response, although recent research indicates that they also influence the subsequent development of the acquired immune response by the cytokines they make. Gamma/Delta cells can express CD4 and CD8 (or both or neither), but the activity of these molecules role in the function of these cells remains controversial.

Overhead 7 - Chronic Inflammatory Diseases

• Immunological trigger (a normal or an innocuous Ag triggers an IR that normally would be self limiting)
• Development of a prolonged inflammatory response (Rheumatoid arthritis, Asthma, inflammatory bowel disease, solid organ transplant)
• Destruction of healthy tissue as a result of chronic immune activation.

Development of allergy: A Multi-step process

1. enhances disease process (by activating naive T cells and B cells in an environment where there is lots of IL-4)
2. triggers symptoms immediately, hence "immediate hypersensitivity" (Crosslinking of cell surface bound IgE). Entire process begins within a few minutes and lasts up to ~½ Hour (symptoms longer).
3. triggers "late phase reaction". Begins 4-8h post Ag exposure, lasts ~"12h". Eosinophils

Therapeutic approaches for managing allergic disease

2. Pharmacotherapy : Common Strategies

3. Immunotherapy:

Rheumatoid arthritis

Other types of hypersensitivity diseases

Drug hypersensitivity

Delayed hypersensitivity

Overhead - Typical acute infection

Fig. 10.1 The course of a typical acute infection. The level of infectious agent increases as the pathogen replicates. When numbers of the pathogen exceed the threshold dose of antigen required for an adaptive response, the response is initiated; the pathogen continues to grow, retarded only by the innate and nonadaptive responses. At this stage, immunological memory also starts to be induced. After 4-5 days, effector cells and molecules of the adaptive response start to clear the infection. When the infection is cleared and the dose of antigen falls below the response threshold, the response ceases, but antibody, residual effector cells, and also immunological memory provide lasting protection against reinfection in most cases. Author: Charles A. Janeway, Paul Travers, Mark Walport, Mark Shlomchik.   Publisher: Garland Publishing.   Copyright: 2001

Overhead - Ag-specific immune responses

Sample questions for practice - 2001

1. Which one statement of the following is NOT correct? Upon binding to its specific antigen, a B lymphocyte can now
1. differentiate into a memory cell
2. turn into a T cell
3. act as a killer cell
4. undergo apoptosis and die
5. produce antibody


2. The main isotype of antibody present in serum is :
1. IgM
2. IgG
3. IgA
4. IgD
5. IgE


3. Inflammation
1. is almost always a highly undesirable process that can be controlled using steroids.
2. is characterized by increased heat, pain and swelling, but without redness or strong antibody production
3. is characterized by production of cytokines that bring other immune cells (both Ag specific and non-specific) to the inflammatory site and can lead induction of antigen specific responses.
4. is rarely a problem in the clinical management of asthma or autoimmunity
5. represents the most advanced evolutionary form of antigen specific immune response
.


4. Mast cells and their products are of particular importance in Canada in the immune response to:
1. vaccines
2. bacteria
3. environmental toxins
4. allergens
5. tumor causing viruses


5. Clonal selection refers to
1. a current beer commercial involving people at a boardroom table
2. a means of activating complement
3. a means of neutrophil activation in response to bacterial exposure
4. a means of determining which lymphocytes will respond to a given Ag
5. a series of cell surface Ags also known as CD antigens