Eileen L. Thacker Dept. of Veterinary Microbiology & Preventive Medicine Associate Professor Extracted from The Pig Journal, Vol 53 Order your copy now View Vol. 53 here

The Pig Journal Immunology series comprises:

1. Immunology: The innate immune system
   The Pig Journal (2003) Vol. 52 p. 111-123.
2. Lymphocyte development and maturation
   The Pig Journal (2004) Vol. 53, p. 75-91.
3. The battle between the immune system and pathogens
   The Pig Journal (2004) Vol. 54, p 55-69.

Part 2: Lymphocyte development and maturation

The goal of this second of three articles is to review and discuss the mechanisms by which lymphocytes, which are the effector cells of the acquired or adaptive immune system, develop the ability to recognize and destroy foreign particles and pathogens without also destroying self. Understanding how lymphocytes develop receptor specificity, mature, are selected to survive, differentiate and are activated provides veterinarians with information on how the different components of the immune system combat and control disease. However, for every mechanism used by the immune system for pathogen control, successful pathogens have developed strategies to circumvent the immune system. Knowledge about the normal and optimal immune responses are critical for understanding the pathogenesis of the most problematic of the pathogens, those that either infect cells of the immune system, use the immune response to cause disease, or modulate or evade the immune system for survival. While this section of the three articles is probably the least interesting to practitioners, reviewing how the immune system develops and matures provides the backdrop for the last section on how the immune system responds to infection and, more importantly, the mechanisms used by pathogens for survival and persistence in the host.

As discussed in Part 1, the adaptive immune and the innate immune system are intimately bound together to ensure that an effective immune response occurs in response to microbial invasion. Much of our knowledge arises from people or animals that are either naturally deficient or have been genetically manipulated to lack an integral portion of the immune system. The loss of any component of the immune system, innate or acquired, usually results in overwhelming infection and death. This critical fact demonstrates that the immune system must be viewed as a whole and dissecting it down into bits and pieces, while necessary for learning their function, does not reflect the reality of the system. While an immune response to microbes is critical for the maintenance of life and health of the host, an uncontrolled immune response frequently results in clinical disease and death. Many of the manifestations of disease are due to an over-exuberant or inappropriate response by the immune response to a pathogen. Thus, the immune system has tight checks and balances to ensure inactivation of an immune response following activation.

While the cells of the innate immune system are fairly efficient in preventing infection and disease by microbes, most pathogens have developed mechanisms to evade this response. Those mechanisms will be covered in more detail in Part 3, which will review the immune system's response to pathogens and its role in disease. Because the cells of the innate system rely primarily on surface markers to recognize pathogens, it is not unexpected that many pathogens have developed mechanisms to prevent recognition by the innate immune system. In addition, viruses carry no molecular structures similar to bacteria and, as such, are rarely recognized directly by the phagocytic cells of the innate immune system. While it is possible for some viruses and many of the pathogenic bacteria to be taken up by dendritic cells, other mechanisms have been developed to recognize pathogens that lack the appropriate markers for recognition. Lymphocytes overcome the constraints placed on the innate immune system by recognizing an almost infinite diversity of antigens, thus targeting each pathogen specifically.

Lymphocyte Development

As described in Part 1, lymphocytes are generated in the bone marrow from a single progenitor cell line. These progenitor cells differentiate into the various myeloid cells, including B cells, that when activated develop into antibody- producing plasma cells and T cells of which there are 2 primary classes in the adaptive immune system. One class of T cells upon activation differentiates into cytotoxic T cells (CTLs), whereas the second class of T cells is known as helper T cells (Th) and differentiate into cells that activate other cells such as B cells, macrophages and CTLs. The B lymphocytes mature in the bone marrow, while T lymphocytes migrate to the thymus to mature. Thus, the bone marrow and thymus are considered primary lymphoid organs, in contrast to the secondary lymphoid organs where the adaptive immune responses are initiated and maintained.

In contrast to the cells of the innate immune system, which recognize similar molecular patterns and structures present on the surface of many micro-organisms, lymphocytes have antigen receptors with a single specificity. This specificity is determined by the re-arrangement and recombination of DNA during lymphocyte development in the bone marrow or thymus. This process results in the production of lymphocytes with millions of different variants of the genes coding the receptor molecules. Each individual lymphocyte has a unique specificity, resulting in a repertoire of millions of lymphocytes with millions of different antigen receptors with differing specificity. However, as with every other aspect of the immune system, this repertoire of antigen receptors is tightly controlled and only those lymphocytes that encounter their specific antigen and do not recognize "self" are allowed to become activated and proliferate into armed effector cells. All others are doomed to failure and death.

Each naive lymphocyte undergoes clonal expansion following recognition and activation by their antigen. Clonal expansion and selection is one of the most important concepts of the immune system. This explains how, upon activation, a single lymphocyte divides to produce many identical progeny responding to a single antigen, known as clones. In the case of B cells, all clones produce identical antibodies resulting in the antibody response to an antigen veterinarians commonly measure to assess exposure to pathogens. The T cell progeny upon clonal expansion each have identical T cell receptors that recognize the identical antigen in an identical fashion. In a similar fashion, all lymphocytes originating from a clone considered to be dangerous to the host are eliminated by clonal deletion. Clonal expansion allows a single lymphocyte to produce the majority of lymphocytes that respond specifically to a pathogen, a fascinating and yet intimidating concept in terms of disease control.

B and T Antigen Receptors

The unique antigen receptors developed by each lymphocyte, whether B or T cells, are generated by gene re-arrangement. Gene segments are irreversibly joined by DNA recombination to form a complete receptor. Because each gene set has a number of segments, each lymphocyte receptor has a unique DNA sequence. Once the recombination of gene segments has successfully produced a functional receptor, further gene re-arrangement is prohibited. This ensures that the receptor on each lymphocyte is single and specific. The production of unique antigen receptors by gene re-arrangement has three consequences. First, it enables a limited number of gene segments to combine in unique and individual ways, resulting in large numbers of receptors with differing genetic codes. Second, each cell is unique due to the assembly of these different sets of gene segments. Third, because the re-arrangement is performed at the DNA level, the exact same genetic sequence will be present in all progeny cells ensuring the same receptor specificity. This system of genetic re-arrangement of antigen receptors occurs for both B and T cells antigen receptors, even though they recognize their antigens in extremely different ways.

The diversity of lymphocyte receptors resulting from this genetic re-arrangement is enormous. A few hundred gene segments can combine to generate thousands of different receptors. This diversity is further increased by junctional diversity during which nucleotides are added or removed during the process of gene re-arrangement. In addition, each receptor is produced from 2 variable chains, each encoding a distinct set of genes. The complete sequence of combinations of different genes, the addition or subtraction of nucleotides and the combination of gene chains results in a staggering number of antigen receptors. However, only a subset of cells with these randomly generated receptor specificities will survive the rigorous selection process and survive to become mature lymphocytes. In spite of the rigours of the selection process, lymphocytes with at least 108 different antigen specificities can be present in an individual at any one time, providing the lymphocytes that will undergo clonal expansion into functioning effector T or B lymphocytes following recognition of a foreign invader.

B Cell Receptor (BCR) Gene Re-arrangement

Antibodies are the secreted portion of the B-cell antigen receptor (BCR). They are produced in large quantities by plasma cells in response to antigens. The BCR and antibodies are made up of 2 distinct regions, the region that binds to antigen or the variable (V) region and the portion that is responsible for the effector function or the constant (C) region. Each antibody consists of 2 identical heavy chains and 2 identical light chains. Heavy and light chains each have V and C regions. The V regions of the heavy and light chains combine to form the antigen-binding site. This increases the repertoire of antibodies which, in humans, has been measured to be over 1011. The number of antibody specificities present within an individual is based on both the number of B cells present, as well as the antigen encounters that have occurred over the life of the individual.

While the number of V gene regions in the light chains of humans is 70, the number in pigs is currently unknown. It has been suggested that there may only be one V region in the heavy chain of swine in contrast to 65 in humans. The light chain V region has 2 gene segments classified as λ and κ, which are located on different chromosomes. These numerous gene segments allow for much of the diversity of antigen receptors observed in the variable region. The selected V region then joins with a second gene segment known as the J or junctional segment. It is imperative that the gene re-arrangements in all functional antibodies are consistent and proper. Any alteration in the process results in the lymphocyte either attempting to combine a new set of gene segments or undergoing apoptosis. Once the V and J segments are successfully combined, they join with a C or constant gene segment, forming a complete light chain. In a similar fashion, the heavy chain of the antibody consists of different V, J and C gene segments. However, in the case of the heavy chain, there is an additional gene segment known as D. Thus, in heavy chain gene re-arrangement, the D and J segments join, followed by joining with the V segment. The VDJ segment then combines with a C gene segment. In addition to the joining of the major V, D, J, and C gene segments, additional nucleotides are added or subtracted in both the heavy and light chains, resulting in further genetic diversity for the antigen receptor.

Antibody diversity is further increased by the combining of light and heavy chains, each with a different genetic makeup. The combining of the unique heavy and light chains exponentially increase the diversity of the resulting antigen receptor. While less is known about the number of each of the gene segments in the pig, the same broad diversity of antibodies occurs. The recombination events described above for each lymphocyte are all antigen independent and, in the case of B lymphocytes, occur prior to leaving the bone marrow.

Further diversification of the B cell receptor occurs following exposure to antigen. This occurs in the peripheral or secondary lymphoid organs following exposure to antigens. This process, known as somatic hypermutation, introduces point mutations into specific regions of the variable regions of the heavy and light chains at a greater than normal rate. Some of these point mutations will result in increased binding of antigen by the receptors. The increased binding by these B cells results in their preferential selection for maturation into antibody producing cells. This gives rise to the phenomenon known as affinity maturation and allows an increasing specificity to foreign antigens as the immune response occurs. Thus, the original antibody specificity may be broad to begin controlling the pathogen, but quickly over time, B cells producing antibodies that bind with greater affinity to the antigens are selected. This results in a change of the antibody profile and specificity over the course of an infection with the final antibodies being specific for neutralizing and destroying the pathogen. An example of affinity maturation could be the generation of neutralizing antibodies to PRRSV that can take 3-4 weeks to occur following infection, even though an antibody response as measured by the IDEXX ELISA occurs quickly.

To recognize and fight the wide range of pathogens encountered by the host, the lymphocytes of the adaptive immune system have evolved to recognize a great variety of different antigens from bacteria, viruses, and other disease causing organisms. In the adaptive immune response, antigen is recognized by the distinctly different receptors of the B and T cells. The BCRs are secreted by mature B cells and become immunoglobulins. The genetic diversity of the V region allows immunoglobulins to recognize different antigens. The region of the antibody that engages the effector function consists of the constant region and is responsible for the different types of immunoglobulins, IgG, IgA, IgE, and IgM. This is the region that will determine the type of immune response induced by the antibody and can change over the course of exposure to a pathogen undergoing isotype switching. Naïve B cells express IgM as their BCR. Upon stimulation, IgM is the first antibody produced in response to antigen. Depending on the stimulation and the cytokines present in the environment, IgM antibodies will switch to one of the other types. Isotype switching from IgM is irreversible. However, further antibody isotype switching can occur with IgG antibodies further changing to the IgA or IgE isotypes. As with the switch from IgM, further isotype switching by IgG is irreversible. Antibodies generally recognize only a small region on the surface of a large molecule such as a protein or polysaccharide. Antibodies can recognize conformational or discontinuous epitopes that are brought together by protein folding or linear or continuous epitope consisting of a single segment of a polypeptide. Typically, both types of antibodies will be produced in response to antigen stimulation.

T Cell Receptor (TCR) Gene Re-arrangement

Diversity of T cell receptors (TCR) are generated in a similar fashion as the process of gene re-arrangement described for B cell receptors. While the B cell receptor consists of heavy and light chains, the T cell receptor (TCR) is made up of two chains also, with lymphocytes bearing the α and β chains being the predominant type associated with the adaptive immune response. Similar to the heavy and light chains of B cells, the α and β chains are made up of multiple gene segments. The α chain has V, J and C segments that correspond to the B cell light chain and the β chain of the T cell consists of V, J, D and C segments like the heavy chain. The addition and subtraction of nucleotides at the junctions of each gene re-arrangement also occur, increasing the resulting diversity of the TCR. Thus, the TCR genes re-arrange in an identical manner as described above for B cells to form TCRs with unique specificity. The re-arrangement of T cell gene segments occur in the thymus as the T cells mature. In contrast to B cells, TCRs do not undergo further diversification in the peripheral lymphoid organs following exposure to antigen.

In addition to TCRs consisting of α and β chains, a subset of T cells has different gene segments made up of gene segments termed γ and δ chains. These gene segments are mixed in among the α and β gene segments, thus any T cell has the potential to become either a T cell bearing αβ or γδ types of genes. Both CTLs and Th cells, the predominate populations of T cells of the adaptive immune system, consist of TCRs made up of αβ gene segments. The T cells bearing the γδ gene segments are a distinct population of T cells with a completely different function than T cells with α and β chains, which make up the CTL and Th cells of the adaptive immune system. Less is known about γδ T cells. However, they appear to be more consistent with the innate immune system and play an important role in swine immunology and control of mucosal pathogens. Their function will be further discussed in Part 3.

Major Histocompatibility Complex (MHC) and Antigen Presentation to T Cells

In contrast to antibodies, which interact with pathogens and their products in extracellular spaces, T cells only recognize foreign antigens displayed on the surface of the cells and bound to the Major Histocompatibility Complex (MHC). The TCR remains membrane bound and signals the T cell to become activated and differentiate after binding to its specific antigen. T cells of the adaptive immune system are made up of 2 specific classes determined by co-stimulatory cell-surface proteins known as CD4 and CD8. Each of these proteins binds to a specific site on the MHC molecules, with CD4 binding to MHC class II molecules and CD8 binding to MHC class I. The binding of CD4 or CD8 to their appropriate MHC molecule signals the T cell to become an armed effector cell. Binding of CD4 or CD8 to the MHC increases the sensitivity of the T cell to antigen by approximately 100-fold.

The MHC molecules have different cell distribution and function. The two types of MHC molecules, class I and II, deliver peptides to the cell surface using two distinct pathways. The class of MHC involved, and thus the pathway of peptide delivery, varies depending on the pathogenesis and location of the microbe. Cells that are infected with an intracellular pathogen have antigens delivered to their surface by MHC class I molecules which trigger the recognition of the infected cell by CD8+ CTLs, resulting in their ultimate destruction. All nucleated cells have MHC class I molecules on their surface. This allows constant immune surveillance and control of intracellular invaders, especially viruses, which can infect a wide range of cell types depending on the viral tropism. In contrast, microbes in the extracellular space that are phagocytized by antigen presenting cells, including macrophages, B cells and dendritic cells have antigens presented on MHC class II molecules activating CD4+ Th cells. The Th cells then activate macrophages, CTLs and induce antibody production. Only the professional antigen presenting cells, most of which are associated with the innate immune system and discussed in Part 1, carry MHC class II molecules which specifically activate CD4+ helper T cells.

There are a number of different MHC class I and II genes allowing the expression of a number of different molecules on each cell surfaces with different peptide binding specificities. The MHC genes are extremely variable or polymorphic, resulting in multiple variants of each gene to be expressed within the population. The MHC genes are the most diverse genes present in the genome. This allows the host to present a wide range of proteins to the immune system. The highly polymorphic nature of the MHC has functional consequences that suggest that it remains critical in the evolution of the immune response and its diversity is beneficial to the host.

Peptide: MHC Binding

The two classes of MHC have distinct structures, but are similar in their three dimensional structure. Both molecules consist of two polypeptide chains that form a cleft for peptide binding. The structure of the cleft differs between the two MHC molecules with the binding area of MHC class II being open, while class I molecules have a closed groove. Thus, the peptides that bind in the class I groove are shorter, consisting of 8-10 amino acids (aa), and are buried in the cleft of the molecule. The peptides bind to specific anchor sites at either end of the class I cleft, along with several other peptides within the structure. This results in the peptide laying in an elongated confirmation within the groove and variations in peptide length occur by kinking in the peptide backbone and bowing of the peptide out of the groove.

Peptides binding to the class II molecules can be longer as the cleft is open at both ends. Peptides binding to MHC class II molecules are at least 13 aa long and can be much longer. It appears that if the peptides in the groove are too long, they are trimmed back to a length of 13-17 aa. In a similar fashion, as with class I, specific aa within the groove serve as anchors, although in contrast to class I, they are on the sides and not at the ends of the groove. In addition, the binding regions or pockets are more permissive in the class II molecule, allowing a broader diversity of peptides to bind.

The antigen pieces or peptides presented by MHC class I molecules originate from proteins produced in the cytoplasm of cells. These proteins can be either foreign or from the host and are degraded to short peptides in a large protease complex called a proteosome. The peptides are then translocated into the endoplasmic reticulum by proteins known as transporters associated with antigen binding (TAP). In addition to proteins present in the cytoplasm, membrane proteins and secreted proteins are also engulfed by the cell, degraded in the cytoplasm and transported into the endoplasmic reticulum. It is in the endoplasmic reticulum that the peptides bind to the newly formed MHC class I molecules. Once the MHC molecule contains a peptide it is then exported to the surface of the cell where it can then bind to a T cell with the appropriate TCR.

In the case of MHC class II, proteins that enter cells through phagocytosis or endocytosis are enclosed in endosomes, which become acidic as the travel into the cell interior. The endosomes fuse with lysosomes containing acid proteases, which then degrade the proteins into shorter peptide strands. In contrast to class I where the peptides are translocated into the endoplasmic reticulum, the MHC class II molecules are translocated from the endoplasmic reticulum to the acidic endosomes where peptide loading occurs followed by exportation to the cell surface. Again like MHC class I molecules, the peptides loaded onto the class II molecules consist of both foreign and host proteins.

Lymphocyte Selection

In addition to the generation of millions of antigen specific receptors on lymphocytes, the shaping and maintenance of this repertoire during lymphocyte development is tightly regulated and controlled. Selection of the most useful receptors and how the numbers of lymphocytes are maintained for optimal homeostasis is regulated through signals received by the antigen receptors on each cell. Lymphocytes must constantly receive signals through their antigen receptors in order to survive. Loss of that signal results in the elimination of the cells from the body. This constant communication within the cells ensures that the host has an adequate number of lymphocytes with functional receptors while regulating the number and types of lymphocyte present at any given moment. The majority of these signals are produced by non-lymphoid stromal cells in the bone marrow and thymus for developing lymphocytes, or in the peripheral lymphoid organs for the maintenance and survival of mature lymphocytes. Lymphocytes that fail to receive the signals undergo apoptosis or programmed cell death. Apoptosis of lymphocytes occurs at a fairly constant rate. The bone marrow produces millions of lymphocytes on a daily basis and this production must be balanced by an equal loss of cells. The loss of these cells is also tightly regulated and cells undergoing apoptosis are taken up by specialized macrophages in, primarily, the liver and spleen. Control of lymphocyte numbers is tightly regulated as loss of a lymphocyte can result in the loss of that antigen receptor specificity and newly developed lymphocytes will have a different specificity. Receptor signaling and prevention of apoptosis play an important role in regulating and maintaining the lymphocyte repertoire.

As lymphocytes differentiate from their progenitor cells, they go through stages marked by the re-arrangement of their receptor gene segments. As each chain of the receptor undergoes gene re-arrangement and production of its respective protein, the cell is signaled to go to the next step in its development. For B cells, the heavy chain gene segments undergo re-arrangement and, if successful, heavy chain gene re-arrangement ceases. The resulting B cell proliferates and light chain gene re-arrangement commences. There are several opportunities for the light chain genes to re-arrange successfully and once completed, B cell development proceeds. If the re-arrangement of either the heavy or the light chain is unsuccessful, as commonly occurs, the cell undergoes apoptosis and dies. Over half of the B cells that begin gene re-arrangement are unsuccessful and are eliminated. In addition to gene re-arrangement for the heavy and light chain, the B cells undergo selection, so that cells with antigen receptors that bind to self antigens are also programmed to die. Stromal epithelial cells within the bone marrow play an important role in differentiating cells that recognize self. Loss of these stromal cells and the environment they provide within the bone marrow prevents the normal development of B cells.

In a similar fashion, T cells that have migrated from the bone marrow to the thymus also mature through progressive stages as their α, β, γ, and δ chains re-arrange to produce successful TCRs. Most TCRs are made up with α and β chains. However, in swine, a significant number of γ, and δ T cells are also produced. In addition to the T cell gene re-arrangement that occurs, the CD4 and CD8 surface proteins are also active throughout the developmental stages of the T lymphocytes and at different stages in T cell development either one or both of the proteins are expressed

Once the cells have successfully developed antigen receptors, they undergo the selection process. Cells that recognize self proteins too strongly are clonally deleted by negative selection. Both B and T cells undergo negative selection within the bone marrow and thymus respectively. In addition to negative selection, T cells also undergo a process of positive selection based on recognition of self MHC:self peptide complexes. Positive selection ensures that T cells are capable of responding to foreign peptides presented by self MHC molecules and ensures the functional matching of receptor, co-receptor and class of MHC molecule recognized. The ability of T cells to successfully pass through both positive and negative selection processes remains somewhat a mystery. Both selection procedures are rigorous and few T cells are successful in surviving. How any T cells survive remains unknown. The importance of the selection process is demonstrated by the fact that failure of either process results in either autoimmunity or failure to recognize foreign antigens and, potentially, death of the host as the immune system destroys itself and the host.

Lymphocyte Survival

Once the lymphocytes have left the central lymphoid tissues and migrate into the periphery, their organization and survival is determined by interactions between lymphocytes and other cells within the secondary lymphoid organs. A lymphocyte circulates throughout the blood, lymph, and various secondary lymphoid organs until either it encounters the appropriate antigen or dies.

Each of the different populations of lymphocytes migrates to specific peripheral lymphoid organs and areas within those organs. The daily output of B cells is roughly 5-10% of the total B lymphocyte population. The size of the B cell pool remains constant with cells continually entering the system and an equivalent number of B cells dying by apoptosis. The majority of B cells are long- lived (approximately 90%) and only 1-2% of these cells die each day. Most of the B cells that die are immature B cells in the peripheral circulation, of which more than 50% die each day. Much of the success or failure of a B cell to survive depends upon its ability to access the follicles in the peripheral lymphoid tissues. The follicles, which contain follicular dendritic cells, provide essential survival and maturation signals for naïve B cells. If the newly made B cells are unsuccessful in gaining access to the follicles, they die. The population dynamics are such that access to follicles is favoured by mature B cells, although the mechanism for this is unknown. Thus, the survival of a naïve, newly generated B cell often appears to be a matter of chance.

In contrast to B cells, only a very small number of T cells are exported from the thymus each day. The success of naïve T cells to remain in the periphery is determined by ongoing contact with self peptide:self MHC complexes similar to those in the thymus. The T cells in the periphery appear to encounter these complexes on the follicular dendritic cells in the T cell zone of the peripheral lymphoid organs. These follicular dendritic cells are different from those in the B cell zone and appear to be identical to the dendritic cells that migrate to the secondary lymphoid organs with antigens. As with many aspects of immunology, the exact mechanism by which the dendritic cells interact with T cells remains unknown.

Once the T cells have completed their development in the thymus, they enter the circulation and migrate through the peripheral lymphoid tissues until they encounter their specific antigen. Once the T cells encounter their antigen, they are stimulated to proliferate and differentiate into armed effector cells capable of destroying the foreign invader in the peripheral lymphoid tissues. Naïve Th cells will respond to antigen only when a professional antigen-presenting cell presents both the antigen to the T cell receptor and a secondary co-stimulatory molecule. Activation of either B cells or CTLs to their antigen requires co-activation of the appropriate Th cells. This process further reduces the chance of an inappropriate immune response to self antigens. Expression of co-stimulatory molecules on the antigen presenting cells is triggered in response to signaling from their receptors that they have foreign antigens on their MHC molecules. The differentiation of T cells and their progeny into the different populations of armed effector cells is dependent on the production of cytokines within the environment. The specific cytokines that activate T cells will be covered in Part 3 of this series, as the immune response to pathogens is discussed. Effector T cells can consist of CD8+ CTLs capable of killing infected cells or Th1 cells that activate macrophages. Together, these two populations of effector T cells are active participants in cell- mediated immunity. In addition, however, the activation of B cells by both Th2 and Th1 cells to produce different classes of antibody drive the humoral immune response.

Summary

Understanding the development and maturation of lymphocytes, the rigorous selection process they undergo and the method of antigen recognition and presentation are important in dissecting out the mechanisms by which pathogens cause disease. This was an extremely broad overview of an extremely complex system used by the immune system to allow recognition of foreign antigens to which the host may be exposed, while preventing autoimmunity. Without understanding how the immune response develops in response to pathogens, our ability to develop intervention strategies to prevent infection and disease is limited. All successful pathogens have developed mechanisms to circumvent the immune system. Understanding the immune response allows us to identifying the mechanisms used by pathogens to survive and cause disease. This knowledge becomes crucial in developing long-term methodologies to control microbes. Manipulation of the immune system through vaccination and immunomodulators, as well as recognizing alterations in the immune response induced by pathogens, is critical for the successful production of pigs.

Glossary

Affinity maturation: Genetic point mutations resulting in antibodies with increased affinity for antigen produced by the humoral immune response over time. Antibodies with mutations resulting in higher affinity for antigens are selected for as the immune response matures.

Antigen receptor: The variable regions of both the B and T cell receptors that bind specific antigens.

Apoptosis: Programmed cell death during which the cell activates an internal death program. It is characterized by nuclear DNA degradation and nuclear degeneration. This is in contrast to cellular necrosis, or death from without.

B cells: Upon activation by antigen, B cells differentiate into plasma cells that produce antibodies. The B cell receptor binds antigens on the surface of B cells and when is secreted are known as immunoglobulins.

Clonal deletion: The elimination of complete populations of genetically identical lymphocytes. This is an important role of the selection process when all lymphocytes that bind to self antigens are eliminated to prevent autoimmunity.

Clonal expansion: The proliferation of antigen-specific lymphocytes in response to antigen stimulation followed by differentiation into effector cells. This process allows a single antigen specific cell to increase in number so they can effectively combat pathogens. This is important in controlling the numbers of lymphocytes that are activated in response to antigens.

Clonal selection: The central paradigm of the adaptive or acquired immune system. It is based on the theory that an adaptive immune response is derived from individual antigen-specific lymphocytes that do not recognize self. The antigen-specific lymphocytes proliferate in response to antigen and differentiate into antigen-specific effector cells that eliminate the pathogen and result in memory cells that sustain immunity.

Clusters of differentiation (CD): Cell surface molecules recognized by a group of monoclonal antibodies.

Combinatorial diversity: The many different combinations of the V, D, J and C DNA segments of either immunoglobulin or T cell receptor loci that result in the generation of large numbers of antigen receptors with differing specificity from a limited number of gene segments.

Cytotoxic T lymphocytes (CTLs): T lymphocytes with the CD8+ signaling molecule that recognize antigens presented by MHC class I molecules. These cells are important in destroying cells displaying abnormal antigens on their surfaces, as occurs with intracellular pathogens.

Dendritic cells: Interdigitating reticular cells found in the T-cell areas of lymphoid tissues. Dendritic cells are important stimulators of T-cell immune responses. Nonlymphoid tissues also have dendritic cells. However, they are unable to stimulate T cells until they are activated and migrate to lymphoid tissues. Dendritic cells originate in the bone marrow and are distinct from follicular dendritic cells that present antigen to B cells.

Effector lymphocytes: Lymphocytes (both B and T cells) that differentiate from naïve lymphocytes in response to their specific antigen. Effector lymphocytes mediate the removal of pathogens without requiring further differentiation.

Endosomes: The acidified vesicles present in cells that fuse with the cell membranes following phagocytosis of antigens. Protein antigens entering by this route are presented on the surface of the phagocytic cells by MHC class II molecules.

Follicular dendritic cells: Follicular dendritic cells are located in the B cell zones of lymphoid follicles and are of uncertain origin. They initiate contact with different B cells and hold antigen:antibody complexes on their surfaces. These cells are crucial in the selection process for B cells. Follicular dendritic cells are different from the dendritic cells in the T cell zone described above.

Helper T cells (Th): Lymphocytes with the CD4 signaling molecule that can help B cells make antibodies in response to antigenic challenge. They also provide the cytokines that help activate CD8+ CTLs. A single population of CD4 cells can differentiate into either Th1 or Th2 cells according to the cytokines present at the time of antigen exposure.

Junctional diversity: The diversity in the antibody and T cell receptor sequences due to the random addition or removal of nucleotide sequences at junctions between the V, D, and J segments. This further increases the number of antigen specificities possible from a limited number of gene segments.

Lymphocyte repertoire: The total population made up of lymphocyte with unique antigen receptors (both B and T cell receptors) resulting in an extremely diverse population of cells capable of responding to the various antigens and pathogens to which the host is exposed.

Major histocompatibility complex (MHC): A cluster of genes that encode a set of membrane glycoproteins. The MHC class I molecules present peptides generated in the cytoplasm to CD8+ CTLs and the MHC class II molecules present peptides degraded in intracellular vesicles to CD4+ Th cells.

MHC-restriction: A given T cell will recognize an antigen only when it is bound to a particular MHC molecule. This allows T cells to only recognize foreign proteins in the context of "self."

Naïve lymphocytes: Lymphocytes that have never encountered their specific antigen. All lymphocytes leaving the central lymphoid organs are naïve lymphocytes.

Primary lymphoid organs: The sites of lymphocyte development. Lymphocytes originate in the bone marrow. In pigs, B cells mature in the bone marrow and T lymphocytes mature in the thymus.

Secondary lymphoid organs: The lymph nodes, spleen and mucosal-associated lymphoid tissues where the adaptive immune response is induced. Secondary lymphoid organs are also known as peripheral lymphoid organs.

Self antigens: The antigens produced by normal cells in the body. Lymphocytes are screened as they mature for reactivity with these antigens. Lymphocytes that recognize self antigens undergo clonal deletion and apoptosis.

Somatic hypermutation: The generation of variant antibodies due to mutations in the variable region. Some of these variants bind with a higher affinity to the antigens and are selected for. This is the mechanism of affinity maturation by an antibody response, resulting in an increase in the specificity of the immune response to a pathogen.

T cell receptor (TCR): The TCR of 2 different chains expressed on the cell membrane of T cells that binds to the appropriate MHC molecule containing a antigen recognized by the TCR. Binding of the MHC:peptide:TCR signals the T cell that antigen has been encountered, resulting in the proliferation and differentiation of the T cell into an effector cell. The predominate type of TCR chains in both CTLs and Th lymphocytes are called α and β. However, a small number of specialized T cells are made up of γ and δ chains.

T lymphocytes: A subset of lymphocytes defined by their development in the thymus and by the heterodimeric receptors making up the T cell receptors.

Transporters associated with antigen processing (TAP) proteins: These proteins are involved with transporting short peptides from the cytoplasm into the endoplasmic reticulum where they associate with MHC class I molecules.

Source: Pig Journal Vol 53 - May 2004