Dr. William Bowers 

 

 Edited and illustrated by Dr Richard Hunt


SUGGESTED READING: Roitt, Brostoff, Male, 6th Edition, Mosby, 2002  Chapter 9, pp.157-161; chapter 10 pp. 163-171

  IMMUNOLOGY - CHAPTER   THIRTEEN 

  IMMUNOREGULATION 

 

TEACHING OBJECTIVES

Subpopulations of helper T cells: Th1 and Th2
Cytokines and class (isotype) switching
Cytokine activation of macrophages and functions
Maturation and mechanism of killing by cytolytic T lymphocytes (CTL)
Characteristics of killing mechanisms of other cytolytic cells
Immunoregulatory processes

I. CENTRAL ROLE OF HELPER T CELLS IN CELL-MEDIATED IMMUNITY

After a helper T cell recognizes specific antigen, it can initiate several key immune processes.

1. Selection of effector mechanisms
2. Induction of proliferation in appropriate effector cell types.
3. Enhancement of functional activities of phagocytes and other effector cells.

These activities are depicted in Figure 1.

II. SUBPOPULATIONS OF HELPER T CELLS: TH1 AND TH2

When a naive CD4+ T cell (Th cell) responds to antigen in secondary lymphoid tissues, it is capable of differentiating into an inflammatory Th1 cell or a helper Th2 cell, which release distinctive patterns of cytokines, as shown in Figure 2.

Functionally these subpopulations, when activated, affect different cells, as shown in Figure 3.


central.jpg (61815 bytes) Figure 1
Th cells are at the center of cell-mediated immunity. The antigen-presenting cells present antigen to the T helper (Th) cell. The Th cell recognises specific epitopes which are selected as target epitopes. Appropriate effector mechanisms are now determined. For example, Th cells help the B cells to make antibody and also activate other cells. The activation signals produced by Th cells are cytokines (lymphokines) but similar cytokines made by macrophages and other cells also participate in this process
diff-2.jpg (41861 bytes)  Figure 2 
Differentiation of murine Th cells. 
Mouse Th cells differentiate into subsets that synthesize different patterns of lymphokines. This also occurs in humans 
select-3.jpg (44925 bytes) Figure 3
Selection of effector mechanisms by Th1 and Th2 cells. In addition to determining various effector pathways by virtue of their  lymphokine production, Th1 cells switch off Th2 cells and vice versa
 
 

Th1 cells produce IFN-gamma that activates macrophages.

Th2 cells produce IL-4 and IL-5 that increases production of eosinophils and mast cells and enhances production of antibody, especially IgE.

Equally important, each subpopulation can exert inhibitory influences on the other:

IFN gamma produced by Th1 cells inhibits proliferation of Th2 cells.

IL-10 produced by Th2 cells inhibits production of IFN gamma and, although not shown, IL-4 inhibits the production of Th1 cells

 

III. CYTOKINES AND CLASS (ISOTYPE) SWITCHING

Recall that antibodies:

1. have effector functions in different anatomical locations, and
2. have a constant (Fc) portion that determines the effector function

Because the specificity of antigen binding is determined by the variable portion of an antibody molecule, class (isotype) switching is an important mechanism that allows a host to produce the class (subclass) of antibody that will be most effective.

The molecular mechanism for class switching was discussed in chapter 8. Although the details are not completely understood, cytokines acting alone or in combination from Th1 and Th2 cells regulate which class or subclass of antibody is produced.

Cytokine

IgG1

IgG2a

IgG2b

IgG3

IgA

IgE

IgM

IL-4

Induce

Inhibit

  

Inhibit

  

Induce

Inhibit

IL-5

  

  

  

  

Augment
production

  

  

IFN-gamma

Inhibit

Induce

  

Induce

  

Inhibit

Inhibit

TGF-beta

  

  

Induce

Inhibit

Induce

  

Inhibit

Isotype regulation by murine T cell cytokines.
Certain cytokines either induce (green) or inhibit (pink) the production of certain antibody isotypes. Inhibition mostly results from switch to the different isotype

Figure 4

 

macro-5.jpg (45039 bytes) Figure 5
Macrophages play a central role in the immune system. before T and B-cell immunity starts. Macrophages process antigens and present them to T cells which then release lymphokines which activate the macrophages to perform various other functions including the production of more cytokines
IV. MACROPHAGE ACTIVATION

Macrophages play a central role in the immune system. As shown in Figure 5, macrophages are involved in:

1. initial defense
2. antigen presentation
3. effector functions

Figure 6 presents a more detailed picture of the roles played by macrophages in immunity and inflammation. These are:

1. inflammation and fever
2. lymphocyte activation
3. tissue reorganization
4. tissue damage
5. microbicidal activity
6. tumoricidal activity

 

Inflammation - Fever

Production of: 
IL-6, TNF alpha, IL-1 – act as pyrogen

Damage to tissues

Hydrolases
Hydrogen peroxide production
Complement C3a
TNF alpha production

Immunity

Selection of lymphocytes to be activated:
IL-12 results in Th1 activation
IL-10 results in Th2 activation

Activation of lymphocytes:
Production of IL-1
Processing and presentation of antigen

Antimicrobial action

Oxygen –dependent production of:
  
hydrogen peroxide
    superoxide
    hydroxyl radical
    hypochlorous acid

Oxygen-independent production of:
   acid hydrolases
    cationic proteins
    lysozyme

Reorganization of tissues

Secretion of a variety of factors:
Degradative enzymes (elastase, hyaluronidase,collagenase)
Fibroblast stimulation factors
Stimulation of angiogenesis

Anti-tumor activity

Toxic factors
Hydrogen peroxide
Complement C3a
Proteases
Arginase
Nitric oxide
TNF alpha

Figure 6

 

A number of these functions are performed by activated macrophages. Macrophage activation can be defined as quantitative alterations in the expression of various gene products that endow the activated macrophage to perform some function that cannot be performed by the resting monocyte.

Macrophage activation is the most important function of Th1 cells, which release IFN gamma, one of two signals required to activate a macrophage. Lipopolysaccharide (LPS) from bacteria can deliver the second signal, as well as TNF alpha. See Figure 7.

One example of the importance of macrophage activation by Th1 cells is the following: Pneumocystis carinii, an extracellular pathogen, is controlled in normal individuals by activated macrophages; it is, however, a common cause of death in AIDS patients because they are deficient in Th1 cells. 

 

macro-7.jpg (40710 bytes) Figure 7
Macrophage activation results from the interaction of multiple cytokines and other factors.
In pathway 1, TNF-alpha is released from macrophages as a result of activation by interferon gamma and interaction with bacterial components that trigger cytokine production. An example of such a triggering component is bacterial lipopolysaccharide. The TNF-alpha from pathway 1 leads to the production of nitric oxide by the interferon-activated macrophage in pathway 2.
ctl-8.jpg (76712 bytes) Figure 8
CTL cells must differentiate in response to antigen. In order to differentiate into functional cytotoxic T lymphocytes, pre-CD8+ CTLs must receive two different signals. First, they must recognize antigen presented by MHC-I expressing cells (the stimulator cells) and, second, they must be stimulated by cytokines.  IL-2, interferon-gamma and others are made by CD4+ helper T cells as a result of their interaction with class II MHC-expressing antigen presenting cells. As a result of these two signals, the pre-CTL differentiates into an active CTL that can then lyse target cells that bear the same antigen.  Adapted from Abbas, et. al. Cellular and Molecular Immunology. 3rd Ed., p. 292.

V. CYTOLYTIC T LYMPHOCYTES (CTL)

Cytolytic T lymphocytes are not fully mature when they exit the thymus. They have a functional TCR that recognizes antigen, but they cannot lyse a target cell. They must differentiate.

A. Differentiation of CTL

They differentiate from a "pre-CTL" in response to two signals: 1) specific antigen associated with class I MHC, and 2) cytokines, especially IL-2, and IFN-gamma. This is shown in Figure 8.

B. Features of CTL-mediated lysis

1. CTL killing is antigen-specific. To be killed by a CTL, the target cell must bear the same class I MHC-associated antigen that triggered pre-CTL differentiation.

2. CTL killing requires cell contact. CTL are triggered to kill when they recognize the target antigen associated with a cell surface MHC molecule. Adjacent cells lacking the appropriate target MHC-antigen are not affected.

3. CTLs are not injured when they lyse target cells. Each CTL is capable of killing sequentially numerous target cells.

 

ctl-9.jpg (63199 bytes) Figure 9
Steps in cytotoxic T cell (CTL)-mediated lysis of a target cell. Step1 also requires interaction of specific molecules on the CTL cell (e.g. CD8) with their specific ligands on the target cell

C. Steps in CTL-mediated lysis (These are indicated in Figure 9)

1. Recognition of antigen
2. Activation
3. Delivery of lethal hit
4. Release of CTL from target cell
5. Death of target cell

VI. OTHER CYTOLYTIC CELLS

A. Natural killer (NK) cells

1. Characteristics

a. Derived from bone marrow

b. Lack most markers for T and B cells (no TCR or CD3)

c. Do not undergo thymic maturation

d. Express CD56, a specific NK marker

e. Express a low-affinity receptor for Fc portion of IgG, called FcRIII (CD16), also expressed on granulocytes and macrophages

f. Cytokines, especially IL-2, promote further differentiation into lymphokine-activated killer (LAK) cells

2. Effector mechanisms

a. Similar to CTL

b. Not MHC-restricted

c. Kill a variety of virus-infected cells and tumor cells, but not all. Susceptibility to killing by NK cells is inversely correlated to expression of class I MHC. Killer inhibitory receptors (KIRs) on human NK cells that recognize class I MHC prevent killing. Some virally-infected cells and tumors down-regulate the expression of class I MHC and they can be killed by NK cells. Because such class I MHC-deficient cells might escape being killed by cytotoxic T cells, NK cells afford another type of protection to the host

d. IgG-coated target cells recognized by CD16 are killed by antibody-dependent cell mediated cytotoxicity, (ADCC)

e. LAK kill broader range of cells - including some normal cells - than NK cells

f. LAK cells predominate in lesions in graft-vs-host disease in recipients of bone marrow transplants


Figure 10 Mechanisms for the CTL destruction of target cells

ctl-10a.jpg (37761 bytes) 1. CTL degranulates and releases perforin monomers into the surroundings. Enzymes that polymerize perforin to form polyperforin channels are also released and these along with Ca++ catalyze channel formation in the membrane of the target cell

ctl-10b.jpg (39466 bytes)   2. The CTL may also release degradative enzymes and toxins which travel through the perforin chanels and damage the target cell

ctl-10c.jpg (37919 bytes)  3. Cytokines such as TNF alpha and TNF beta are released from the CTL or nearby macrophages. Interferon gamma may also be released from the CTLs or from other nearby lymphoid cells. These bind to receptors on the target cell and trigger apoptosis

B. Macrophages

1. Effector mechanisms

The following have been shown to participate in the killing of tumor cells by activated macrophages:

a. TNFalpha (even more potent when it acts synergistically with IFN gamma)

b. nitric oxide and other Reactive Nitrogen Intermediates (RNI)

c. Reactive Oxygen Intermediates (ROI)

d. cationic proteins

e. hydrolytic enzymes

 

Some of the effector mechanisms used by cytotoxic cells are shown in Figure 10.

VII. IMMUNOREGULATION

A. An immune response has the following features:

1. Its magnitude is determined by a balance between lymphocyte activation and tolerance induced by an antigen

2. Its nature is determined by the specificities and functional classes of lymphocytes that are activated by that antigen

3. Regulatory mechanisms may act at the recognition, activation, and effector phases of an immune response

Some of the factors that regulate immune responses are given in Figure 11.

Figure 11 - Factors that determine the nature and magnitude of immune responses

  Factors that favor:
stimulation of immune response inhibition of immune response
Cognitive phase
Lymphocyte repertoire Diversity of lymphocyte receptors for foreign antigens Absence of MHC molecules capable of binding specific antigenic determinants
Antigen presentation Presence of MHC molcules capable of binding processed antigen Absence of MHC molecules capable of binding certain antigenic determinants
Induction and activation phase
Features of Antigen    

Nature

Immunogenic forms Tolerogenic forms

Amount

Optimal doses vary for different antigens High doses favor tolerance

Portal of entry

Subcutaneous, intradermal Intravenous, oral

Adjuvants

Recruitment and activation of accessory cells, induction of co-stimulators Antigens with adjuvants are non-immunogenic or tolerogenic
Accessory cells Presence of co-stimulators fro T cells Absence of co-stimulators
Antigen-specific T cells Helper T cells Suppressor T cells
Anti-idiotype immune responses Stimulatory or inhibitory Stimulatory or inhibitory
Antibodies Enhance antigen uptake and presentation by macrophages Antibody feedback
Cytokines Positive amplification loops Antagonistic effects of various cytokines. Immunosuppressive effects
Adapted from Abbas et al. Cellular and Molecular Immunology. Second edition, 1994, p. 209

 

 

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