VIETNAMESE

IMMUNOLOGY - CHAPTER TWELVE 

CELL-MEDIATED IMMUNITY: 
Cell-cell interactions in specific immune responses 

Dr Gene Mayer
Professor
Department of Pathology, Microbiology and Immunology
University of South Carolina School of Medicine

and

Dr Jennifer Nyland
Assistant Professor
Department of Pathology, Microbiology and Immunology
University of South Carolina School of Medicine

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Edited and illustrated by Dr Richard Hunt

TEACHING OBJECTIVES
To discuss the central role of Th cells in immune responses

To describe the cell-cell interactions which occur in (i) antibody responses to T-dependent antigens, (ii) generation of cytotoxic T cells, and (iii) activation of macrophages and NK cells

To discuss the mechanisms of killing by cytotoxic T cells and NK cells
To discuss responses to T-independent antigens.

Central role of Th cells in immune responses

      As depicted in Figure 1, after Th cells recognize specific antigen presented by an antigen-presenting cell (APC), they can initiate several key immune processes.  These include:

• Selection of appropriate effector mechanisms ( e.g., B cell activation or Tc generation);
   

• Induction of proliferation of appropriate effector cells

• Enhancement of the functional activities of other cells (e.g., granulocytes, macrophages, NK cells).

      There are four subpopulations of Th cells: Th0, Th1, Th2 and Th17 cells.  When naïve Th0 cells encounter antigen in secondary lymphoid tissues, they are capable of differentiating into inflammatory Th1 cells, helper Th2 cells or pathogenic T17 cells, which are distinguished by the cytokines they produce (Figure 2).  Whether a Th0 cells becomes a Th1, a Th2 or a T17 cell depends upon the cytokines in the environment, which is influenced by antigen.  For example some antigens stimulate IL-4 production which favors the generation of Th2 cells while other antigens stimulate IL-12 production, which favors the generation of Th1 cells.  Th1, Th2 and Th17 cells affect different cells and influence the type of an immune response, as shown in Figure 3 for Th1 and Th2 cells. 

Cytokines produced by Th1 cells activate macrophages and participate in the generation of cytoxic lymphocytes (CTL), resulting in a cell-mediated immune response.  In contrast cytokines produced by Th2 cells help to activate B cells, resulting in antibody production. 

In a relatively recent discovery, Th17 cells (designated as such by their production of IL-17) differentiate (in humans) in response to IL-1, IL-6, and IL-23. TGF-β is important for Th17 differentiation in mice, but not in humans. IL-17 enhances the severity of some autoimmune diseases including multiple sclerosis, inflammatory bowel disease, and rheumatoid arthritis. Equally important, each subpopulation can exert inhibitory influences on the other.  IFN-γ produced by Th1 cells inhibits proliferation of Th2 cells and differentiation of Th17 cells and IL-10 produced by Th2 cells inhibits production of IFN-γ by Th1 cells.  In addition, although not shown, IL-4 inhibits production of Th1 cells and differentiation of Th17 cells.  Thus, the immune response is directed to the type of response that is required to deal with the pathogen encountered – cell-mediated responses for intracellular pathogens or antibody responses for extracellular pathogens.

KEY WORDS

Th1 cells
Th2 cells
Hapten-carrier model
CD28
B7
CD40
CD40 ligand
CD5
B1 cells
B2 cells
CTL
Fas ligand
Perforin
Granzymes
Caspases
IFN-γ
Activation

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

  Figure 2 
Differentiation of murine Th cells.   Mouse Th cells differentiate into subsets that synthesize different patterns of lymphokines. This also occurs in humans 

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

Figure 4

Molecules involved in the interactions of B and TH cells

Antigen is processed by B cell. Co-stimulators are expressed. The processed antigen peptide is presented in association with MHC class II antigens. The T cell recognizes the peptide along with the MHC antigen and the co-stimulators. The T cell expresses CD40 ligand. The latter binds to CD40 antigen on the B cell and the B cells divide and differentiate. Antibodies are produced by the B cell 

Figure 5
Cooperation of cells in the immune response

Antigen-presenting cells (e.g. dendritic cells) present processed antigen to virgin T cells, thereby priming them. B cells also process the antigen and present it to the T cells. They then receive signals from the T cells that cause them to divide and differentiate. Some B cells form antibody-forming cells while a few form B memory cells

Hapten-carrier model

Historically one of the major findings in immunology was that both T cells and B cells were required for antibody production to a complex protein. A major contribution to our understanding of this process came from studies on the formation of anti-hapten antibodies. Studies with hapten-carrier conjugates established that:

• Th2 cells recognized the carrier determinants and B cells recognized haptenic determinants

• Interactions between hapten-specific B cells and carrier-specific Th cells was self MHC restricted

• B cells can function both in antigen recognition and in antigen presentation

B cells occupy a unique position in immune responses because they express immunoglobulin and class II MHC molecules on their cell surface. They therefore are capable of producing antibody having the same specificity as that expressed by their immunoglobulin receptor; in addition they can function as an antigen presenting cell. In terms of the hapten-carrier conjugate model, the mechanism is thought to be the following: The hapten is recognized by the immunoglobulin receptor, the hapten-carrier is brought into the B cell, processed, and peptide fragments of the carrier protein are presented to a helper T cell. Activation of the T cell results in the production of cytokines that enable the hapten-specific B cell to become activated to produce soluble anti-hapten antibodies. Figure 4 summarizes the B cell-T cell interactions that occur.

Note that there are multiple signals delivered to the B cells in this model of Th2 cell-B cell interaction. As was the case for activation of T cells where the signal derived from the TCR recognition of a peptide-MHC molecule was by itself insufficient for T cell activation, so too for the B cell. Binding of an antigen to the immunoglobulin receptor delivers one signal to the B cell, but that is insufficient. Second signals delivered by co-stimulatory molecules are required; the most important of these is CD40L on the T cell that binds to CD40 on the B cell to initiate delivery of a second signal.
 

Cell-cell interactions in the primary antibody response

B cells are not the best antigen presenting cell in a primary antibody response; dendritic cells or macrophages are more efficient. Nevertheless, with some minor modifications the hapten-carrier model of cell-cell interactions described above also applies to interactions in a primary antibody response (Figure 5). In a primary response the Th2 cell first encounters antigen presented by dendritic cells or macrophages. The “primed” Th2 cell can then interact with B cells that have encountered antigen and are presenting antigenic peptides in association with class II MHC molecules. The B cells still require two signals for activation – one signal is the binding of antigen to the surface immunoglobulin and the second signal comes from CD40/CD40 ligand engagement during Th2/B cell-cell interaction. In addition, cytokines produced by the Th2 cells help B cells proliferate and differentiate into antibody secreting plasma cells.

Cell-cell interactions in the secondary antibody responses

As a consequence of a primary response, many memory T and B cells are produced. Memory B cells have a high affinity immunoglobulin receptor (due to affinity maturation), which allows them to bind and present antigen at much lower concentrations than that required for macrophages or dendritic cells. In addition, memory T cells are more easily activated than naïve T cells. Thus, B/Th cell interactions are sufficient to generate secondary antibody responses. It is not necessary (although it can occur) to “prime” memory Th cells with antigen presented by dendritic cells or macrophages.

Cytokines and class switching

Cytokines produced by activated Th2 cells not only stimulate proliferation and differentiation of B cells, they also help regulate the class of antibody produced. Different cytokines influence the switch to different classes of antibodies with different effort functions. In this way the antibody response is tailored to suit the pathogen encountered (e.g. IgE antibodies for parasitic worm infections). Table 1 shows the effects of different cytokines on the class of antibody produced.

Cell-cell interactions in antibody responses to exogenous T-independent antigens

Antibody responses to T-independent antigens do not require cell-cell interactions.  The polymeric nature of these antigens allows cross-linking of antigen receptors on B cells resulting in activation.  No secondary responses, affinity maturation or class switching occurs.  Responses to T-independent antigens are due to the activation of a subpopulation of B cells called CD5+ B cells (also called B1 cells), which distinguishes them from conventional B cells that are CD5- (also called B2 cells).

CD5+ (B1) cells

CD5+ cells are the first B cells to appear in ontogeny.  They express surface IgM but little or no IgD and they produce primarily IgM antibodies from minimally somatically mutated germ line genes.  Antibodies produced by these cells are of low affinity and are often polyreactive (bind multiple antigens).  Most of the IgM in serum is derived from CD5+ B cells.  CD5+ B cells do not give rise to memory cells.   An important characteristic of these cells is that they are self-renewing, unlike conventional B cells which must be replaced from the bone marrow.  CD5+ B cells are found in peripheral tissues and are the predominant B cell in the peritoneal cavity.  B1 cells are a major defense against many bacterial pathogens that characteristically have polysaccharides in their cell walls.  The importance of these cells in immunity is illustrated by the fact that many individuals with T cell defects are still able to resist many bacterial pathogens.

Figure 6
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.

Figure 7
Fas- and TNF-mediated killing of target cells by CTLs

Cell-cell interactions in cell-mediated immunity - generation of Tc cells in response to endogenous antigens in the cytosol

      Cytotoxic 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 into fully functional effector Tc cells.  Cytotoxic cells differentiate from a "pre-CTL" in response to two signals:

• Specific antigen in the context of class I MHC, on a stimulator cell

• Cytokines produced by Th1 cells, especially IL-2, and IFN-gamma. This is shown in Figure 6.

Features of CTL-mediated lysis

• 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.

• 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.

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

Mechanisms of CTL-mediated killing

CTLs utilize several mechanisms to kill target cells, some of which require direct cell-cell contact and others that result from the production of certain cytokines.  In all cases death of the target cells is a result of apoptosis.

• Fas- and TNF-mediated killing (Figure 7)
  Once generated CTLs express Fas ligand on their surface, which binds to Fas receptors on target cells.  In addition, TNF-α secreted by CTLs can bind to TNF receptors on target cells.  The Fas and TNF receptors are a closely related family of receptors, which when they encounter their ligands, for trimers of the receptors.  These receptors also contain death domains in the cytoplasmic portion of the receptor, which after trimerization can activate caspases that induce apoptosis in the target cell.

•  Granule-mediated killing (Figure 8)
  Fully differentiated CTLs have numerous granules that contain perforin and granzymes.  Upon contact with target cells, perforin is released and it polymerizes to form channels in the target cell membrane.  Granzymes, which are serine proteases, enter the target cell through the channels and activate caspases and nucleases in the target cell resulting in apoptosis.

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

   2. The CTL may also release degradative enzymes and toxins which travel through the perforin chanels and damage the target cell

  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

Figure 9
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

Figure 10
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.

Cell-cell interactions in cell-mediated immunity - activation of macrophages in response to endogenous antigens in vesicles

Macrophages play a central role in the immune system. As shown in Figure 9, macrophages are involved in:

• Initial defense as part of the innate immune system

• Antigen presentation to Th cells

• Various effector functions (e.g., cytokine production, bactericidal and tumoricidal activities). 

Indeed macrophages play an important role not only in immunity but also in reorganization of tissues.  However, because of their potent activities, macrophage can also do damage to tissues.  Table 2 summarizes the many functions of macrophages in immunity and inflammation.

Many of these macrophage functions can only be performed by activated macrophages. Macrophage activation can be defined as quantitative alterations in the expression of various gene products that enable the activated macrophage to perform some function that cannot be performed by the resting macrophage.

Macrophage activation is an important function of Th1 cells.  When Th1 cells get activated by an APC such as a macrophage, they releases IFN-γ, which is one of two signals required to activate a macrophage.  Lipopolysaccharide (LPS) from bacteria or TNF-α produced by macrophages exposed to bacterial products deliver the second signal (Figure 10).

Effector mechanisms employed by macrophages include production of:

• TNF-α, which can induce apoptosis

• Nitric oxide and other reactive nitrogen intermediates

• Reactive oxygen intermediates

• Cationic proteins and hydrolytic enzymes

Macrophage activation by Th1 cells is very important in protection against many different pathogens  For example, 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.  Similarly, Mycobacterium tuberculosis, an intracellular pathogen that resides in vesicles, is not efficiently killed by macrophages unless they are activated; hence this infection is a problem in AIDS patients.

Cell-cell interactions in cell-mediated immunity - activation of NK cells

Cytokines produced by activated Th1 cells, particularly Il-2 and IFN-γ, also activate NK cells to become lymphokine activated killer cells (LAK cells).  LAK cells are able to kill virus infected and tumor cells in a non-MHC-restricted manner.  Indeed, susceptibility of target cells to killing by NK and LAK cells is inversely proportional to the expression of MHC class I molecules (see lecture on innate immunity).  The effector mechanisms used by NK and LAK cells to kill target cells is similar to those used by CTLs (e.g., perforin and granzymes).  NK and LAK cells are also able to kill antibody coated target cells by ADCC.

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This page last changed on Monday, September 18, 2017
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