Cell-Mediated Immunity: The Other Adaptive Immune Response
CCBC Professional Development Conference, 2102
Gary E. Kaiser, Ph. D.
Professor, Biology Department, CCBC Catonsville

The online version of this handout can be found on my website at
http://faculty.ccbcmd.edu/~gkaiser/index.html

 

Introduction to Adaptive Immunity

Adaptive (acquired) immunity refers to antigen-specific defense mechanisms that take several days to become protective and are designed to react with and remove a specific antigen (def). This is the immunity one develops throughout life.

An antigen is defined as a substance that reacts with antigen receptors on lymphocytes and with antibody molecules. An immunogen is an antigen that is recognized by the body as non-self and stimulates an adaptive immune response. For simplicity we will use the term antigen when referring to both antigens and immunogens. The actual portions or fragments of an antigen that react with antibodies and lymphocyte receptors are called epitopes (def). The size of an epitope is generally thought to be equivalent to 5-15 amino acids in the case of protein antigens; 3-4 sugar residues in the case of polysaccharide antigens.

The body recognizes an antigen as foreign when epitopes of that antigen bind to epitope-specific receptor molecules on the surface of B-lymphocytes (def) and/or T-lymphocytes (def). The epitope receptor on the surface of a B-lymphocyte is called a B-cell receptor (BCR) and is actually an antibody molecule (def). The receptor on a T-lymphocyte is called a T-cell receptor (TCR).

 It is estimated that the human body has the ability to recognize 107 or more different epitopes, each with a unique specificity. In order to recognize this immense number of different epitopes, the body produces 107 or more distinct clones of both B-lymphocytes and T-lymphocytes, each with a unique B-cell receptor or T-cell receptor. Among this large variety of B-cell receptors and T-cell receptors there is bound to be at least one that has an epitope-binding site able to fit, at least to some degree, any antigen the immune system eventually encounters. With the adaptive immune responses, the body is able to recognize any conceivable antigen it may eventually encounter.

The downside to the specificity of adaptive immunity is that only a few B-lymphocytes and T-lymphocytes in the entire body recognize any one epitope. These few cells that recognize the epitope must then rapidly proliferate in order to produce enough cells to mount an effective immune response against that particular epitope, and then differentiate into effector lymphocytes (def). This typically takes several days. During this time the pathogen could be causing considerable harm. (That is why the innate immune defenses that can become immediately activated are so essential to body defense.)

There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity.

1. Humoral immunity (def): humoral immunity involves the production of antibody molecules in response to an antigen (def) and is mediated by B-lymphocytes.

2. Cell-mediated immunity (def): Cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines in response to an antigen (def) and is mediated by T-lymphocytes.

 

An Overview of the Major Cells Involved in Cell-Mediated Immunity

1. Dendritic Cells

The primary role of dendritic cells is to activate naiveT-lymphocytes (def).

Most dendritic cells are derived from monocytes and are referred to as myeloid dendritic cells. They are located under the surface epithelium of the skin and the surface epithelium of the mucous membranes of the respiratory tract, genitourinary tract, and the gastrointestinal tract. They are also found throughout the body's lymphoid tissues and in most solid organs.

The primary function of dendritic cells, then, is to capture and present protein antigens (def) to naive T-lymphocytes (def). (Naive lymphocytes are those that have not yet encountered an antigen that fits their specific T-cell receptor.) They are among a select group of cells called antigen-presenting cells or APCs.

Interactions with dendritic cells enable naive T4-lymphocytes and naive T8-lymphocytes to become activated, proliferate, and differentiate into effector cells (def). (Effector lymphocytes are lymphocytes that have encountered an antigen, become activated, proliferated, and matured into a form capable of actively carrying out immune defenses.)

How dendritic cells function as APCs will be discussed below.

 

2. T4-Lymphocytes (T4-Helper Cells, CD4+ Cells) (def)

The primary role of T4-lymphocytes is to regulate the body's immune responses.

T-lymphocytes refer to lymphocytes that are produced in the bone marrow but require interaction with the thymus for their maturation. T4-lymphocytes are T-lymphocytes displaying CD4 (def) molecules on their surface. Also present on their surface are antigen receptors called T-cell receptors (def) or TCRs. The TCRs, in cooperation with the CD4 molecules (def), have molecular shapes capable of recognizing peptides from exogenous antigens (def) bound to MHC-II (def) molecules displayed on the surface of antigen-presenting cells (def) (APCs) such as dendritic cells (see Fig. 1), macrophages, and B-lymphocytes. The TCR recognizes the peptide while the CD4 molecule recognizes the MHC-II molecule.

Once naive T4-lymphocytes are activated by dendritic cells, they proliferate and differentiate into T4-effector lymphocytes that regulate the immune responses by way of the cytokines they produce (def). (Cytokines are low molecular weight, soluble proteins that function as chemical messengers for regulating the innate and adaptive immune systems.)

Functionally, there are probably many different types or subpopulations of effector T4-lymphocytes based on the cytokines they produce. The most studied and characterized subtypes are TH1 cells, TH2 cells, TH17 cells, Treg cells, and TFH cells.

T4-lymphocytes are discussed in further detail below.

 

3. T8-lymphocytes (T8-Cells; CD8+Cells; Cytotoxic T-Lymphocytes) (def)

The primary role of T8-lymphocytes is to kill infected cells and tumor cells.

T8-lymphocytes are T-lymphocytes displaying CD8 (def) molecules on their surface. Also present on their surface are antigen receptors called T-cell receptors or TCRs (def), similar to those on T4-lymphocytes. The TCRs on T8-lymphocytes, in cooperation with the CD8 molecules, bind peptides from endogenous antigens (def) bound to MHC-I molecules (def) on the surface of dendritic cells (see Fig. 2), infected cells, and tumor cells.

Once naive T8-lymphocytes are activated by dendritic cells, they proliferate and differentiate into effector cells (def) called cytotoxic T-lymphocytes or CTLs (def).

CTLs are then, by way of their TCRs and CD8 molecules, able to recognize infected cells and tumor cells displaying MHC-I molecules with bound peptides on their surface and destroy them through apoptosis (def), a programmed cell suicide.

T8-lymphocytes are discussed in further detail below.

 

4. Macrophages

When monocytes (def) leave the blood and enter the tissue, they become activated and differentiate into macrophages. Those that have recently left the blood during inflammation and move to the site of infection through positive chemotaxis (def) are sometimes referred to as wandering macrophages.

In addition, the body has macrophages already stationed throughout the tissues and organs of the body, including lymphoid tissue, connective tissue, and body cavities. These are sometimes referred to as fixed macrophages. Many fixed macrophages are part of the mononuclear phagocytic (reticuloendothelial) system. They, along with B-lymphocytes (def) and T-lymphocytes (def), are found supported by reticular fibers in lymph nodules, lymph nodes, and the spleen where they filter out and phagocytose foreign matter such as microbes. Similar cells derived from stem cells, monocytes, or macrophages are also found in the liver (Kupffer cells), the kidneys (mesangial cells), the brain (microglia), the bones (osteoclasts), the lungs (alveolar macrophages), and the gastrointestinal tract (peritoneal macrophages).

Macrophages have a number of very important functions in body defense including:

1. Ingesting and killing microbes, infected cells, and cancer cells. They are also important scavengers of dead and dying cells.

2. Processing antigens so they can be recognized by effector T-lymphocytes (def) during the adaptive immune responses (def). Certain effector T-lymphocytes are able to activate macrophages for more effective phagocytosis.

 

5. Natural Killer (NK) Cells

NK cells are important in innate immunity because they are able to recognize infected cells, cancer cells, and stressed cells and kill them by inducing apoptosis (def). In addition, they produce a variety of cytokines (def), including proinflammatory cytokines (def), chemokines (def), colony-stimulating factors (def), and other cytokines that function as regulators of body defenses. For example, through cytokine production NK cells also suppress and/or activate macrophages (def) , suppress and/or activate the antigen-presenting capabilities of dendritic cells (def), and suppress and/or activate T-lymphocyte (def) responses.

While they play a critical role in innate immunity by killing certain infected cells and tumor cells, NK cells also play a role in adaptive humoral immune responses. NK cells are capable of antibody-dependent cellular cytotoxicity or ADCC where they kill cells to which antibody molecules have bound.

 

The Role of the MHC-I and MHC-II Systems in Cell-Mediated Immunity

Major Histocompatibility Complex (MHC) Molecules

MHC molecules enable T-lymphocytes (def) to recognize epitopes (def) of antigens (def) and discriminate self from non-self. Unlike B-cell receptors on B-lymphocytes that are able to directly bind epitopes on antigens, the T-cell receptors (TCRs) of T-lymphocytes can only recognize epitopes - typically short chains of amino acids called peptides - after they are bound to MHC molecules (see Fig. 11).

The MHC genes are the most polymorphic genes (def) in the human genome, possessing many alleles for each gene. The MHC genes are co-dominantly expressed so that an individual expresses the alleles inherited from each parent. In this way, the number of MHC molecules that bind peptide for presentation to T-lymphocytes is maximized. In addition, each MHC molecule is able to bind a wide variety of different peptides (def), both self-peptides and foreign peptides.

There are two classes of MHC molecules: MHC-I and MHC-II.

The expression of MHC molecules is increased by cytokines (def) produced during both innate immune responses and adaptive immune responses. Cytokines such as interferon-alpha (IFN-α), interferon-beta (IFN-β), interferon-gamma (IFN-γ), and tumor necrosis factor (TNF) increase the expression of MHC-I molecules, while IFN-γ is the main cytokine to increase the expression of MHC-II molecules.

a. MHC-I molecules

MHC-I molecules are designed to enable the body to recognize infected cells and tumor cells and destroy them with cytotoxic T-lymphocytes or CTLs (def). CTLs are effector (def) defense cells derived from naive T8-lymphocytes (def).

Characteristics of MHC-I molecules:

Endogenous antigens are proteins found within the cytosol (def) of human cells. Examples of endogenous antigens include:

During the replication of viruses and intracellular bacteria within their host cell, as well as during the replication of tumor cells, viral, bacterial, or tumor proteins in the cytosol are degraded into a variety of short peptide epitopes by cylindrical organelles called proteasomes (def). The body's own cytosolic proteins are also degraded into peptides by proteasomes.

These peptide epitopes (def) are then attached to a groove of MHC-I molecules that are being produced in the endoplasmic reticulum. From here they are transported to the surface of that cell (see Fig. 3 and Fig. 4) where they can be recognized by a complementary-shaped T-cell receptor (TCR) and CD8 molecule (def) on the surface of either a naive T8-lymphocyte (see Fig. 2) or a cytotoxic T-lymphocyte (CTL) (see Fig. 5). TCRs, however, will not recognize self-peptides bound to MHC-I. As a result, normal cells are not attacked and killed.

MHC-I molecules are coded for by three major MHC-I genes, HLA-A, HLA-B, and HLA-C. As mentioned above, however, there are many different alleles for each gene that a person inherits. This maximizes the number of MHC-I molecules available to bind peptides for presentation to T-8 lymphocytes.

 

 

b. MHC-II molecules

MHC-II molecules are designed to enable T4-lymphocytes to recognize epitopes (def) of exogenous antigens (def) and discriminate self from non-self.

Characteristics of MHC-II molecules:

Exogenous antigens (def) are antigens that enter from outside the body, such as bacteria, fungi, protozoa, and free viruses. These exogenous antigens enter macrophages, dendritic cells, and B-lymphocytes through phagocytosis. The microbes are engulfed and placed in a phagosome (def) which then fuses with lysosomes (def). Following this fusion, the phagolysosome becomes acidified. Acidification, in turn, activates the proteases within the phagolysosome enabling protein antigens from the microbe to be degraded into a series of short peptides (def). These peptide epitopes (def) are then attached to MHC-II molecules and are then transported to the surface of the APC (see Fig. 6). (Certain dendritic cells, as discussed later, can also cross-present endogenous antigens to MHC-II molecules.)

Some pathogens, such as Mycobacterium tuberculosis, Mycobacterium leprae, and Leishmania, are able to grow in the endocytic vesicles of macrophages without being killed by lysosomes. These macrophages can, however, become activated by T4-effector lymphocytes called TH1 cells and subsequently use intravesicular proteases to degrade the proteins from these pathogens into peptides for presentation to MHC-II molecules that pass through on their way to the cell surface.

Here the MHC-II molecules with bound peptides can be recognized by a complementary-shaped T-cell receptor and CD4 molecule (def) on the surface of a T4-lymphocyte (see Fig. 1) . T4-lymphocytes are the cells the body uses to regulate both humoral immunity (def) and cell-mediated immunity (def).

MHC-II molecules are coded for by three MHC-II genes, HLA-DR, HLA-DP, and HLA-DQ.

 

 

The Role of Antigen-Presenting Dendritic Cells in Cell-Mediated Immunity

Antigen-presenting cells (def) or APCs include dendritic cells, macrophages, and B-lymphocytes. APCs express both MHC-I (def) and MHC-II molecules (def) and serve two major functions during adaptive immunity:

1. they capture and process antigens (def) for presentation to T-lymphocytes (def), and

2. they produce signals required for the proliferation and differentiation of lymphocytes.

We will now look at how antigen-presenting dendritic cells function to activate naive T-lymphocytes.

Most dendritic cells (def) are derived from monocytes and are referred to as myeloid dendritic cells. They are located under the surface epithelium of the skin and the surface epithelium of the mucous membranes of the respiratory tract, genitourinary tract, and the gastrointestinal tract. They are also found throughout the body's lymphoid tissues and in most solid organs.

In these locations, in their immature form, they are attached by long cytoplasmic processes. Upon capturing antigens through pinocytosis and phagocytosis and becoming activated by inflammatory cytokines, the dendritic cells detach from their initial site, enter lymph vessels, and are carried to regional lymph nodes. Activation of the dendritic cell promotes its expression chemokine receptor CCR7 that enables the dendritic cell to migrate towards the chemokine CCL21 produced by lymphoid tissues. By the time the dendritic cells enter the lymph nodes, they have matured and are now able to present antigen epitopes to the ever-changing populations of naive T8-lymphocytes and naive T4-lymphocytes (def) located in the cortex of the lymph nodes.

The primary function of dendritic cells, then, is to capture and present protein antigens (def) to naive T-lymphocytes (def). (Naive lymphocytes are those that have not yet encountered an antigen.) Since dendritic cells are able to express both MHC-I and MHC-II molecules, they are able to present antigens to both naive T8-lymphocytes and naive T4-lymphocytes (See Fig. 1 and Fig. 2). Because the amino acid sequences recognized by TCRs are typically buried within protein antigens, processing of that protein by dendritic cells is required in order to unwind the protein and chop it up into the short peptide chains that will be able to bind to MHC molecules for presentation to naive T-lymphocytes.

You Tube movie of a dendritic cell engulfing melanoma cells (red).

These interactions enable the naiveT4-lymphocyte or T8-lymphocyte to become activated, proliferate, and differentiate into effector cells (def). (Effector lymphocytes are lymphocytes that have encountered an antigen, been activated, have proliferated, and matured into a form capable of actively carrying out immune defenses.)

1. MHC-II presentation of protein antigens to naive T4-lymphocytes (def)

a. MHC-II presentation of exogenous antigens (def) to naive T4-lymphocytes

Immature dendritic cells take in protein antigens for attachment to MHC-II molecules and subsequent presentation to naive T4-lymphocytes by:

1. Receptor-mediated phagocytosis, e.g., PAMPs binding to endocytic PRRs, IgG or C3b attachment of microbes to phagocytes during opsonization (see Fig. 7).

2. Macropinocytosis, a process where large volumes of surrounding fluid containing microbes are engulfed. This also enables dendritic cells to take in some encapsulated bacteria that might resist classical phagocytosis (see Fig. 8).

The binding of microbial PAMPs to the PRRs of the immature dendritic cell activates that dendritic cell and promotes production of the chemokine receptor CCR7 that directs the dendritic cell into local lymphoid tissue. Following maturation, the dendritic cell can now present protein epitopes bound to MHC molecules to all the various naive T-lymphocytes passing through the lymphoid system (See Fig. 9 and Fig. 10).

The MHC-II molecules (def) bind peptide epitopes (def) from exogenous antigens (def) and place them on the surface of the dendritic cell (see Fig. 6). Here the MHC-II/peptide complexes can be recognized by complementary shaped TCRs and CD4 molecules (def) on naive T4-lymphocytes (see Fig. 1).

 

b. MHC-II cross-presentation (def) of endogenous antigens (def) to naive T4-lymphocytes (def)

While most dendritic cells present exogenous antigens to naive T4-lymphocytes, certain dendritic cells are capable of cross-presentation of endogenous antigens to naive T4-lymphocytes. In this way, T4-lymphocytes can play a role in defending against both exogenous and endogenous antigens. This is done via autophagy, the cellular process whereby the cell's own cytoplasm is taken into specialized vesicles called autophagosomes (See Fig. 12A). The autophagosomes subsequently fuse with lysosomes containing proteases that will degrade the proteins in the autophagosome into peptides. From here, the peptides are transported into the vesicles containing MHC-II molecules where they can bind to the MHC-II groove, be transported to the surface of the denritic cell, and interact with the TCRs and CD4 molecules of naive T4-lymphocytes (See Fig. 12A).

 

2. MHC-I presentation of protein antigens to naive T8-lymphocytes (def)

Immature dendritic cells take in protein antigens for attachment to MHC-I molecules and subsequent presentation to naive T8-lymphocytes.

a. MHC-I cross-presentation (def) of exogenous antigens (def) to naive T8-lymphocytes (def)

While most dendritic cells present endogenous antigens to naive T8-lymphocytes, certain dendritic cells are capable of cross-presentation of exogenous antigens to naive T8-lymphocytes. In this way, T8-lymphocytes can play a role in defending against both exogenous and endogenous antigens. There are two proposed mechanisms for cross-presentation of exogenous antigens to T8-lymphocytes:

1. The dendritic cell engulfs the exogenous antigen and places it in a phagosome which then fuses with a lysome to form a phagolysosome. The antigen is partially degraded in the phagolysosome where proteins are translocated into the cytoplasm where they are processed into peptides by proteasomes, enter the endoplasmic reticulum, and are bound to MHC-I molecules (see Fig. 12B).

2. The dendritic cell engulfs the exogenous antigen and places it in a phagosome which then fuses with a lysome to form a phagolysosome. The protein antigens are degraded into peptides within the phagolysome which then directly fuses with vesicles containing MHC-I molecules to which the peptides subsequently bind (see Fig. 12C).

In addition, dendritic cells are very susceptible to infection by many different viruses. Once inside the cell, the viruses become endogenous antigens in the cytosol.

The binding of microbial PAMPs to the PRRs of the immature dendritic cell activates that dendritic cell and promotes production of the chemokine receptor CCR7 that directs the dendritic cell into local lymphoid tissue. Following maturation, the dendritic cell can now present protein epitopes bound to MHC molecules to all the various naive T-lymphocytes passing through the lymphoid system.

The MHC-I molecules (def) produced by dendritic cells bind peptide epitopes (def) from exogenous and endogenous antigens (def) and place them on the surface of the dendritic cells (see Fig. 1). Here the MHC-I/peptide complexes can be recognized by complementary shaped T-cell receptors (TCRs) and CD8 molecules (def) on naive T8-lymphocytes (see Fig. 2).

 

 

 

For a Summary of Key Surface Molecules and Cellular Interactions of Antigen-Presenting Dendritic Cells, see Fig. 14.

 

 

The Role of T4-Lymphocytes (T4-Helper Cells, CD4+ Cells) (def) in Cell-Mediated Immunity

The primary role of T4-lymphocytes is to regulate the body's immune responses. Once naive T4-lymphocytes are activated by dendritic cells, they proliferate and differentiate into T4-effector lymphocytes that regulate the immune responses by way of the cytokines they produce (def).

T4-lymphocytes are T-lymphocytes displaying CD4 (def) molecules on their surface. Also on their surface are epitope receptors called T-cell receptors (def) or TCRs. The TCRs on the T4-lymphocytes, in cooperation with the CD4 molecules, have a shape capable of recognizing peptides epitopes from exogenous antigens (def) bound to MHC-II (def) molecules on the surface of antigen-presenting cells (def) (APCs) such as dendritic cells (see Fig. 1), macrophages, and B-lymphocytes. The TCR recognizes the peptide while the CD4 molecule recognizes the MHC-II molecule. (See Fig. 6 for an illustration showing a dendritic cell processing exogenous antigens and binding peptide epitopes to MHC-II molecules.)

During its development, each T4-lymphocyte becomes genetically programmed by gene-splicing reactions to produce a T-cell receptor or TCR (def) with a unique specificity. Identical molecules of that TCR are placed on its surface where they are able to bind an epitope/MHC-II complex on an APC (def) such as a dendritic cell, a macrophage, or a B-lymphocyte (def) with a corresponding shape. It is estimated that the human body has the ability to recognize 107 or more different epitopes (def). In order to recognize this immense number of different epitopes, the body produces 107 or more distinct clones of T-lymphocytes, each with a unique T-cell receptor. In this variety of T-cell receptors there is bound to be at least one that has an epitope-binding site able to fit, at least to some degree, peptides of any antigen the immune system eventually encounters.

Naive T-lymphocytes circulate in the blood. In response to chemokines produced by lymphoid tissues, they leave the vascular endothelium in regions called high endothelial venules and enter lymph nodes or other lymphoid tissues, a process called diapedesis (def).

As naive T4-lymphocytes (def) migrate through the cortical region of lymph nodes, they use surface cell adhesion molecules such as LFA-1 and CD2 to bind transiently to corresponding receptors such as ICAM-1, ICAM-2 and CD58 on the surface of dentritic cells. This transient binding allows time for the TCRs on the T4-lymphocyte to sample large numbers of MHC-II/peptide complexes on the antigen-presenting dendritic cells (see Fig. 1A).

Those naive T4-lymphocytes not activated by epitopes of antigens on the dendritic cells exit the lymph node (or other lymphoid tissue) and eventually re-enter the bloodstream. However, if a TCR and CD4 molecule of the naive T4-lymphocyte detects a corresponding MHC-II/peptide complex on a mature dendritic cell, this will send a first signal for the activation of that naive T-lymphocyte. Next, a second signal that promotes survival of that T-lymphocyte is sent when co-stimulatory molecules such as B7.1 and B7.2 on the dendritic cell bind to CD28 molecules on the T4-lymphocyte. Finally, the dendritic cell produces cytokines such as interleukin-6 (IL-6), IL-4, IL-12, and T-cell growth factor-beta (TGF-β) that contribute to proliferation of the T4-lymphocytes and their differentiation into effector T4-lymphocytes (def), the cells the body uses to regulate both humoral immunity and cell-mediated immunity through the cytokines they produce. (Activated T4-lymphocytes remain in the lymph node as they proliferate (clonal expansion) and only leave the lymphoid tissues and re-enter the bloodstream after they have differentiated into effector T4-lymphocytes.)

CD28-dependent co-stimulation of the T4-lymphocyte also stimulates it to synthesize the cytokine interleukin-2 (IL-2) as well as a high-affinity IL-2 receptor. The binding of IL-2 to its high affinity receptor allows for cell proliferation and formation of a clone of thousands of identical T4-lymphocytes after several days. IL-2 also contributes to survival of those activated T4-lymphocytes and their differentiation into effector cells.

 

 

Functionally, there are many different types or subpopulations of effector T4-lymphocytes based on the cytokines they produce. Immune reactions are typically dominated by five primary types: TH1 cells, TH2 cells, TH17 cells, Treg cells, and TFH cells.

1. CD4 TH1 cells: Coordinate immunity against intracellular bacteria and promote opsonization. They:

2. CD4 TH2 cells: Coordinate immunity against helminths and microbes that colonize mucous membranes 3. CD4 TH17 cells: Promote a local inflammatory response to stimulate a strong neutrophil response and promote the integrity of the skin and mucous membranes

4. CD4 Treg cells: Suppress immune responses

5. TFH cells: Promote humoral immunity by stimulating antibody production and antibody isotype switching by B-lymphocytes

 

For a Summary of Key Surface Molecules and Cellular Interactions of Naive T4-Lymphocytes, see Fig. 19.

 

The Role of T8-lymphocytes (T8-Cells; CD8+Cells; Cytotoxic T-Lymphocytes) in Cell-Mediated Immunity (def)

The primary role of T8-lymphocytes is to kill infected cells and tumor cells by inducing apoptosis (def) of those cells. T8-lymphocytes are T-lymphocytes displaying CD8 (def) molecules on their surface. Also on their surface are T-cell receptors or TCRs (def). The TCRs on T8-lymphocytes, in cooperation with the CD8 molecules, bind peptides from endogenous antigens (def) bound to MHC-I (def) molecules on the surface of dendritic cells, infected cells, and tumor cells (see Fig. 2). The TCR recognizes the peptide while the CD8 molecule recognizes the MHC-I molecule. (See Fig. 3 for an illustration showing a dendritic cell processing endogenous antigens and binding peptide epitopes to MHC-I molecules.)

During its development, each T8-lymphocyte becomes genetically programmed, by gene-splicing reactions similar to those in B-lymphocytes and T4-lymphocytes, to produce a TCR with a unique shape capable of binding epitope/MHC-I complex with a corresponding shape. It is estimated that the human body has the ability to recognize 107 or more different epitopes (def). In order to recognize this immense number of different epitopes, the body produces 107 or more distinct clones of T-lymphocytes, each with a unique T-cell receptor. In this variety of T-cell receptors there is bound to be at least one that has an epitope-binding site able to fit, at least to some degree, peptides of any antigen the immune system eventually encounters.

Naive T-lymphocytes circulate in the blood. In response to chemokines produced by lymphoid tissues, they leave the vascular endothelium in regions called high endothelial venules and enter lymph nodes or other lymphoid tissues, a process called diapedesis (def).

As naive T8-lymphocytes migrate through the cortical region of lymph nodes, they use surface cell adhesion molecules such as LFA-1 and CD2 to bind transiently to corresponding receptors such as ICAM-1, ICAM-2 and CD58 on the surface of dentritic cells. This transient binding allows time for the TCRs on the T8-lymphocyte to sample large numbers of MHC-I/peptide complexes on the antigen-presenting dendritic cells (see Fig. 2A).

Those naive T8-lymphocytes not activated by epitopes of antigens on the dendritic cells exit the lymph node (or other lymphoid tissue) and eventually re-enter the bloodstream. However, if a TCR and CD8 molecule of the naive T8-lymphocyte detects a corresponding MHC-I/peptide complex on a mature dendritic cell, this will send a first signal for the activation of that naive T-lymphocyte. Next, a second signal that promotes survival of that T-lymphocyte is sent when co-stimulatory molecules such as B7.1 and B7.2 on the dendritic cell bind to CD28 molecules on the T8-lymphocyte. Finally, the dendritic cell produces cytokines such as interleukin-6 (IL-6), IL-4, IL-12, and T-cell growth factor-beta (TGF-β) that contribute to proliferation of the T8-lymphocytes and their differentiation into effector T8-lymphocytes (def) called cytotoxic T-lymphocytes (CTLs) that are able to bind to and kill infected cells and tumor cells displaying the same peptide/MHC-I complex on their surface. (Activated T8-lymphocytes remain in the lymph node as they proliferate (clonal expansion) and only leave the lymphoid tissues and re-enter the bloodstream after they have differentiated into CTLs.)

While activated T8-lymphocytes produce interleukin-2 (IL-2) as well as a high-affinity IL-2 receptor themselves, in most cases it is the IL-2 produced by effector T4-lymphocytes that enables cell proliferation and formation of a clone of thousands of identical T8-lymphocytes after several days. IL-2 also contributes to survival of those activated T8-lymphocytes and their differentiation into effector cells.

One of the body's major defenses against viruses, intracellular bacteria, and cancers is the destruction of infected cells and tumor cells by cytotoxic T-lymphocytes or CTLs (def). These CTLs are effector cells derived from naive T8-lymphocytes during cell-mediated immunity. However, in order to become CTLs, naive T8-lymphocytes (def) must become activated by cytokines (def) produced by antigen-presenting dendritic cells (def) as shown in Fig. 3 and Fig. 2.

 

Flash animation of MHC-I molecules binding epitopes from endogenous antigens by an antigen-presenting dendritic cell.
Flash animation of the activation of a naive T8-lymphocyte recognizing epitopes bound to MHC-I molecules on an antigen-presenting dendritic cell.
YouTube animation illustrating the MHC-I system marking an infected cell for destruction and its subsequent killing by CTLs. Howard Hughes Medical Institute.

 

CTLs are, by way of their TCRs and CD8 molecules, able to recognize infected cells and tumor cells displaying MHC-I molecules with bound peptides on their surface (see Fig. 13) and destroy them through apoptosis (def), a programmed cell suicide.

Apoptosis involves a complex of intracellular granules. This complex of granules in a protected state including:

1.  Pore-forming proteins called perforins (def);
2.  Proteolytic enzymes called granzymes (def); and
3. A proteoglycan called granulysin.

When the TCR and CD8 of the CTL binds to the MHC-I/epitope on the surface of the virus-infected cell or tumor cell (see Fig. 13), this sends a signal through a CD3 molecule which triggers the release of the perforins/granzymes/granulysin complexes from the CTL.

The exact mechanism of entry of the granzymes into the infected cell or tumor cell is still debated. It is, however, dependent on perforins. Possibilities include:

Killing of the infected cell or tumor cell by apoptosis involves a variety of mechanisms:

 

For a Summary of Key Surface Molecules and Cellular Interactions of Naive T8-Lymphocytes, see Fig. 21 .

 

The Role of Macrophages in Cell-Mediated Immunity

When monocytes (def) leave the blood and enter the tissue, they become activated and differentiate into macrophages. Those that have recently left the blood during inflammation and move to the site of infection through positive chemotaxis (def) are sometimes referred to as wandering macrophages.

In addition, the body has macrophages already stationed throughout the tissues and organs of the body, including lymphoid tissue, connective tissue, and body cavities. These are sometimes referred to as fixed macrophages. Many fixed macrophages are part of the mononuclear phagocytic (reticuloendothelial) system. They, along with B-lymphocytes (def) and T-lymphocytes (def), are found supported by reticular fibers in lymph nodules, lymph nodes, and the spleen where they filter out and phagocytose foreign matter such as microbes. Similar cells derived from stem cells, monocytes, or macrophages are also found in the liver (Kupffer cells), the kidneys (mesangial cells), the brain (microglia), the bones (osteoclasts), the lungs (alveolar macrophages), and the gastrointestinal tract (peritoneal macrophages).

Macrophages have a number of very important functions in body defense including:

1. Ingesting and killing microbes, infected cells, and cancer cells. They are also important scavengers of dead and dying cells.

2. Processing antigens so they can be recognized by effector T-lymphocytes (def) during the adaptive immune responses (def). Certain effector T-lymphocytes are able to activate macrophages for more effective phagocytosis.

3. Producing inflammatory cytokines (def) such as tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), IL-6, and IL-8.

Effector T4-lymphocytes called TH1 cells coordinate immunity against intracellular bacteria and promote opsonization by macrophages.

For a summary of the key surface molecules and cellular interactions of macrophages, see Fig. 23.