Medical immunology lecture notes

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I M M U N O L O G Y I M M U N O L O G Y CORE NOTES CORE NOTES MEDICAL IMMUNOLOGY 544 FALL 2011 Dr. George A. Gutman SCHOOL OF MEDICINE UNIVERSITY OF CALIFORNIA, IRVINE (Copyright) 2011 Regents of the University of California CHAPTER 1 INTRODUCTION Resistance to infectious diseases relies on both INNATE and ADAPTIVE modes of immunity. While both are effective and significant, a major focus of this course is ADAPTIVE IMMUNITY, that mode of immunity which exhibits SPECIFICITY and MEMORY. Two systems of adaptive immunity protect all vertebrates, namely CELLULAR and HUMORAL immunity. The consequences of both of these classes of immune responses may be harmful as well as beneficial, and are mediated by cells of the highly distributed LYMPHOID SYSTEM. INNATE versus ADAPTIVE IMMUNITY INNATE IMMUNITY The body's first line of defense against pathogenic organisms (including bacteria, fungi and viruses) is the physical barrier provided by the skin, by the epithelium and mucus secretions of the alimentary tract and lungs, etc. This level of protection, however, is relatively non-specific; it distinguishes little, for example, between the bacterial organisms Staphylococcus and Streptococcus, or between the viral agents causing polio and smallpox. A next level of defense is manifested by a variety of cells and serum molecules which may promote ingestion and killing of potentially infectious organisms, cells including macrophages, neutrophils and dendritic cells, and molecules including complement and defensins. These modes of protection are present in all healthy individuals, and are essentially unchanged following repeated challenges by the offending pathogens - that is to say they do not display memory, and are collectively referred to as INNATE IMMUNITY. Mediators of innate immunity contribute to the complex process of development of INFLAMMATION. However, as we will discuss later, the mechanisms of non-specific inflammation overlap and interact extensively with those mediating adaptive immune responses, which will be clearly illustrated in the modes of action of dendritic cells, macrophages and complement. Although relatively non-specific, innate immunity is highly effective and centrally important to our well-being, as evidenced by the consequences of damage to this system by trauma (e.g. wounds which damage epithelium and may get infected, ionizing radiation which can inhibit the inflammatory response) or by disease (e.g. emphysema, which causes greatly increased sensitivity to bacterial infection in diseased lungs). In fact, as we will see, loss of effective innate immunity can have as deadly consequences as any loss of adaptive immune function. ADAPTIVE IMMUNITY What is classically meant when referring to the "immune system", however, is not the non-specific manifestations of innate immunity, but the complex system of immune reactions known as ADAPTIVE IMMUNITY (including both humoral and cellular immunity, defined 3 below), which display the closely related features of SPECIFICITY and MEMORY. This course, and these notes, will attempt to define the nature of, and the molecular basis of both of these features. The discipline of immunology can be approached from two distinct perspectives: 1) Historical/Medical outlook: Resistance to Infectious Disease. The adaptive immune system can confer specific resistance to many infectious diseases, e.g., smallpox. This example illustrates two key features of immune reactions, namely specificity and memory. Having recovered from smallpox (or having been vaccinated) makes one resistant ("immune") to being infected with smallpox later - the immune system exhibits memory. Resistance to smallpox, however, does not make a person resistant to measles, mumps, diphtheria or other diseases caused by unrelated organisms - the immune system thus shows specificity. 2) Biological approach: Recognition of "Self" Versus "Non-self". Quite apart from its importance in resistance to infectious disease, the immune system has been of tremendous interest to biologists interested in the nature and mechanisms of immunological specificity, one aspect of which can be regarded as an organism's ability to distinguish self from non-self. The single-celled amoeba, for instance, ingests food by phagocytosis; how does it distinguish a particle of food from one of its own pseudopods? (Note that the same question may be asked of a macrophage.) "Self" "Non-self" Figure 1-1 For multicellular organisms the problem becomes even more complex they must be capable of recognizing their own diversity of normal cell types as self while at the same time retaining the ability to recognize foreign particles and cells and reacting against them. The biological concept of specificity, particularly in the context of cell surface recognition, extends into many other areas, e. g. that of control of cell proliferation and differentiation. As a result, the problem of immunological specificity has attracted study by many scientists whose basic interests lay in the areas of differentiation and tumor biology. 4 Let’s define the following two terms in the context of adaptive immunity: IMMUNITY - Acquired resistance to infectious disease displaying specificity at the molecular level. We’ve already noted that many factors other than the adaptive immune system contribute to resistance to disease, for instance the barrier to microorganisms provided by our skin and other membranes and phagocytic cells ("innate immunity"). These are not acquired, however, nor do they exhibit the specificity required by this definition, and therefore they are not by themselves considered an expression of the "adaptive immune system". IMMUNE RESPONSE - Reactivity against a target displaying specificity at the molecular level. The targets of such reactivity may be disease-producing organisms, or may be completely harmless substances such as foreign red blood cells or foreign serum proteins. One major criterion for effective reactivity under normal circumstances is that the target be "foreign" to the responding organism, although we shall see that immune responses may be directed against “self” components as well. We shall also learn of many factors that may affect the magnitude of immune responses to various targets. ROLES OF THE IMMUNE SYSTEM Resistance to infectious disease. From a medical or evolutionary standpoint, this is highly beneficial, and obviously a central role of the immune system. Deficiency in the ability to mount effective immune responses leads to increased susceptibility to infection by bacteria, fungi and viruses. (The largely discredited idea that the immune system also effectively seeks out and destroys cells which are undergoing neoplastic transformation, i.e. "immune surveillance", is discussed in Chapter 23.) However, the immune system does not always act in a manner beneficial to the organism; some immune responses result in considerable harmful effects and may be fatal, as illustrated by the following examples: Allergy. Immune responses to food and to plant and animal products in our environment may result in the various manifestations of allergies. Hay fever and allergies to foods and animal products are very common, and while they often are not very serious, they may sometimes be life-threatening, such as in the case of severe asthmatic reactions or anaphylactic shock. Autoimmunity . The normal ability of the immune system to distinguish self and non-self can be disrupted by a variety of influences, resulting in damaging and potentially lethal reactivity to normal "self" components. Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE) are two examples of many such autoimmune reactions. Graft rejection; the rejection of foreign tissues and organ transplants is a “normal” consequence of immunological specificity; however, the ultimate result of immune rejection of a heart or liver transplant, for instance, may be fatal. Much research is stilled aimed at discovering more effective methods to prevent immune rejection of grafts, while at the same time maintaining the recipient's ability to resist infectious organisms. 5 THE LYMPHOID SYSTEM: ORGAN OF IMMUNITY We will discuss later the many different cell types which are directly or indirectly involved in immune responses. One cell type, however, the LYMPHOCYTE, is centrally involved in all adaptive immune responses. No single, localized organ is responsible for immune reactivity, but rather it involves a wide variety of organs which include lymph nodes, spleen, Peyer's patches, tonsils, thymus and bone marrow. These are collectively known as the LYMPHOID SYSTEM, by virtue of the fact that they all contain large numbers of the white blood cells known as lymphocytes. Each has a unique structure and role in immune responses, which we will examine in some detail later (Chapter 16). TWO SYSTEMS OF IMMUNITY PROTECT VERTEBRATES: HUMORAL AND CELL-MEDIATED (“CELLULAR ") IMMUNITY Immune responses can generally be categorized as either humoral or cellular. HUMORAL IMMUNE RESPONSES are those mediated by antibodies in various body fluids ("humors"), including blood, saliva and the mucous secretions of the lungs and intestinal tract (the nature and structure of antibodies will be discussed in Chapters 3 and 4, and the cellular basis for humoral immunity in Chapter 15). In general, the humoral response offers protection from infections caused by organisms which are extracellular, a category which includes most bacteria as well as many of their toxic products (e.g. diphtheria and tetanus toxins). Hay fever and food allergies are examples of humoral responses which are harmful to the host. CELL-MEDIATED IMMUNE RESPONSES are not mediated simply by antibodies, but require the direct participation of immunologically reactive cells (to be discussed in Chapters 11 and 12). Cell-mediated immunity is responsible, in general, for resistance to infectious organisms which are primarily intracellular. This includes resistance to viral infections as well as to certain bacteria (e.g., the Mycobacterium responsible for tuberculosis). Immunity to fungal infections and graft rejection are also largely the responsibility of the cellular immune system. The ability of immune serum to transfer humoral immunity promoted early studies which identified the relevant effector molecules, the ANTIBODIES. In the next few chapters we will examine the structure and function of antibodies, which are the mediators of humoral immune responses, and proceed in later chapters to examine the features of cell-mediated immune responses. 6 CHAPTER 2 ANTIGEN/ANTIBODY INTERACTIONS See APPENDIX (1) THE PRECIPITIN CURVE; (2) LABELING OF ANTIBODIES The defining characteristic of HUMORAL immune responses (which distinguishes them from CELL-MEDIATED responses), is their ability to be transferred by serum, and the proteins within serum which are responsible for such immunity are ANTIBODIES. We can formulate intriguingly circular definitions for antibodies and ANTIGENS, and note that the universal property of antibodies is their ability to specifically bind their cognate antigens. The consequences of such binding, however, can vary considerably, depending on the nature of the particular antigen and antibody involved. We distinguish the PHYSICAL and the BIOLOGICAL PROPERTIES of antibodies, and the properties of ANTIGENICITY versus IMMUNOGENICITY, and introduce the concept of ADJUVANTS, substances which are capable of increasing immunogenicity. We'll begin by defining three important terms: ANTIBODY - The molecule present in serum and other body fluids which mediates humoral immunity, and which can bind specifically to an antigen. Serum which contains antibodies (directed against one or more antigens) is termed an antiserum. ANTIGEN - A molecule which can be specifically bound by an antibody (typically a protein or carbohydrate recognized as "foreign"). EPITOPE (= “antigenic determinant” = "antigenic specificity") - The minimum target structure on an antigen which is bound by a particular antibody molecule. A particular antigen molecule may (and generally does) bear many different epitopes or “determinants”, each of which can be a target for antibody binding. (NOTE: Antibodies themselves can serve as antigens; human antibodies, for instance, are "foreign" to rabbits, and can elicit rabbit antibodies to human antibody molecules. As we will see later, the use of antibodies as antigens has been an extremely powerful tool for understanding antibody structure and genetics.) 9 DEFINING HUMORAL IMMUNITY Experimentally defining a humoral immune response involves demonstrating that such immunity can be transferred by serum (or other fluids). The example below (Fig, 2-1) illustrates some key features of humoral immunity. Live Pneumococcus DIES Naive Killed Live SURVIVES Pneumococcus Pneumococcus wait Immune Naive 2 "Active Immunity" weeks Live SURVIVES Pneumococcus TRANSFER SERUM Immune Naive "Passive Immunity" Figure 2-1 If a mouse is injected with a sufficient dose of live Pneumococcus bacteria, it will die of infection within a few days. If, however, it has previously been injected with killed organisms, not only does it not succumb to infection, but it will survive a subsequent injection of a normally lethal dose of this organism; such a mouse has been immunized, and is therefore said to be immune to Pneumococcus. Although not illustrated here, we can further demonstrate that this resistance is specific – the immune mouse will retain normal susceptibility to some other organism to which it had not previously been exposed. Such specificity establishes that the immunity we see is a result of the mouse’s adaptive immune response. Question: Does this resistance represent humoral immunity? To find out, we take serum from the immune mouse and inject it into a non-immune recipient, then inject a lethal dose of Pneumococcus. We find that this recipient survives this treatment; serum from an immune mouse transfers immunity to a naïve recipient. This demonstrates that immunity to this organism is mediated by humoral immunity. (NOTE: This does not, however, mean that resistance to all bacterial infections is mediated by humoral immunity. As we will see in Chapter 12, transferring serum from a mouse which is immune to another bacterium, Listeria (which is an intracellular pathogen), does not confer resistance to naïve recipients; such immunity is therefore not humoral.) This illustration also serves to define two distinct modes of adaptive immunity, namely ACTIVE IMMUNITY and PASSIVE IMMUNITY. Immunization of the mouse in the second line of Fig. 2-1 results in a state of "active" immunity; the animal's own immune system is responsible for resistance to the subsequent bacterial challenge. On the other hand, transfer of serum, as in line 3 above, results in a state of "passive" immunity in the recipient; such immunity is the result of the presence of transferred antibody (see below). The animal's own immune system does not participate at all, and this immunity lasts only as long as sufficient levels of antibody are present. 10 The substance present in immune serum which is responsible for transferring immunity is antibody. In addition to transferring resistance to infection, these serum antibodies can carry out a variety of other functions. For example, if immune serum is mixed with a suspension of Pneumococcus, the bacteria will be seen to rapidly "clump" together. This effect is known as agglutination, and is one of the many ways in which antibodies can be detected and quantitated. The various effects that antibodies may exhibit can be generally categorized as physical effects, which depend only on the physical nature of the antibody and antigen, or biological effects, which additionally depend on the particular biological properties of the target antigen or other biologically active molecules which are involved. PHYSICAL EFFECTS OF ANTIBODY Agglutination. "Clumping" of a particulate antigen, e.g. bacteria or SRBC (sheep red blood cells). Agglutination of red blood cells is a technique which has been widely used in clinical and basic research as well as in the clinical laboratory, and is called HEMAGGLUTINATION. Many soluble antigens can be made effectively particulate by coating them onto SRBC or latex or other particles; the resulting clumping by antibody is known as passive agglutination. Precipitation. Interaction of antibody with a soluble antigen to form an insoluble complex, e.g., with BSA (bovine serum albumin). In liquid - the precipitate can be recovered by centrifugation and analyzed (see APPENDIX 1, THE PRECIPITIN CURVE). If either the antigen or antibody is radioactively labeled (see APPENDIX 2, LABELLING OF ANTIBODIES), it can be used in a RadioImmunoPrecipitation (RIP) assay, first developed in the 1950s. In agarose - if the antigen-antibody interaction takes place in a semi-solid medium such as agarose, the resulting precipitate can be easily visualized. This is of special significance in a configuration known as Ouchterlony Analysis (see APPENDIX 3, OUCHTERLONY ANALYSIS). Precipitation and agglutination are both consequence of cross-linking of antigens by antibody into large complexes. The ability of antibodies to carry out this process implies that each antibody can bind at least two antigen molecules, and that it can only occur if the antigen molecule has two or more epitopes (“determinants ") which can be recognized by that antibody. Binding. If an antigen is bound to a solid matrix (latex particles or a plastic dish, for example), and if the antibody is labeled in some way (with a visible, radioactive or enzyme molecule), binding of the antibody to its antigen can be easily and sensitively measured. If a radioactive label is used, the assay is called a solid-state RadioImmunoAssay (RIA). With an enzyme-based label, on the other hand, it becomes an Enzyme-Linked ImmunoSorbent Assay (ELISA). These solid state assays (particularly ELISA's) have largely replaced precipitation and agglutination assays in a wide variety of clinical and research applications. 11 BIOLOGICAL EFFECTS OF ANTIBODY Protection from infectious disease. We have already seen in the Pneumococcus example (Figure 2-1) how this manifestation of antibody can be assayed by transferring serum from one animal to another. Immobilization. An antibody directed against components of the flagellae of motile bacteria or protozoa can cause these flagellae to stop moving. This results in the loss of the organisms' ability to move around, and this loss of motility can be detected by microscopic examination. Cytolysis. If the target antigen is an integral component of the membrane of certain sensitive cells, antibodies may cause disruption of the membrane and death of the cell. This requires the participation of a collection of other serum components collectively known as COMPLEMENT (see Chapter 5), and binding of these components to antibodies is referred to as “Complement Fixation”. If the antigen target is a red blood cell, this effect is known as hemolysis, which can be readily detected visually. In the case of a bacterial cell target, the effect is referred to as bacteriolysis. If the target is a nucleated cell the effect is referred to as cytotoxicity, and may be 51 measured by release of a radioactive label incorporated into the cell (such as Cr), exclusion of "vital" dyes such as Trypan Blue, or any of several other measures of cell viability. Opsonization. If the target antigen is particulate (e.g. a bacterium, or an antigen- coated latex particle), bound antibodies may greatly increase the efficiency with which the particles are phagocytosed by macrophages and other "scavenger" cells. This improvement of phagocytosis is known as opsonization, and may be facilitated even further by the presence of complement. As will be discussed later, opsonization is the result of antibodies’ increasing the degree to which antigenic particles will "stick" to phagocytic cells. This phenomenon has therefore been referred to as immune adherence, and depends on the presence in the membranes of white blood cells of specific receptors either for antibody (FcR, or "Fc-receptors") or for complement (CR, or "complement receptors"), both of which will be discussed later (see Chapter 14, for example). ONE COMMON DEFINING PROPERTY OF ANTIBODIES: ALL ANTIBODIES EXHIBIT SPECIFIC BINDING TO ANTIGEN Different antibodies may show various combinations of effects; some antibodies may precipitate but not interact with complement (and therefore not show cytolysis), some may be opsonizing but not be capable of agglutination. The single common feature of all antibodies, however, is that of specific recognition and binding to antigen. All other effects, physical or biological, are secondary consequences of this specific binding. The structure of antibodies and the basis of their ability to specifically bind antigen are the subjects of the next two chapters (Chapters 3 and 4). 12 ANTIGENS, IMMUNOGENS AND HAPTENS We have been discussing "antigens" as molecules (1) which can elicit antibody production upon injection into an appropriate host; and (2) to which these antibodies can then bind. The difference between these two properties is an important one which we will now make explicit by defining two related but distinct terms: IMMUNOGEN. A molecule which can elicit the production of specific antibody upon injection into a suitable host. ANTIGEN. A molecule which can be specifically recognized and bound by an antibody. These definitions imply that an immunogen must be an antigen, but an antigen is not necessarily an immunogen. Let’s illustrate this in the following table: Substance Molecular weight Immunogen? Antigen? 1) BSA "68,000" yes yes 2) DNP 200 no yes 3) DNP -BSA "70,000" DNP - yes yes 10 BSA - yes yes 4) "clarified" BSA 68,000 no (see Note) yes If we take a conventional preparation of purified bovine serum albumin (BSA) and inject it into a mouse (line 1 in the table above), the mouse will produce antibodies which will bind to BSA. BSA is therefore both an immunogen and an antigen. If we take the small organic molecule dinitrophenol (DNP) and inject it into a mouse (line 2), no antibodies will be produced which can bind DNP. DNP is therefore not immunogenic; we will deal with its antigenicity shortly. We can chemically couple DNP molecules to the protein BSA, yielding DNP-BSA. If we inject this material into a mouse (line 3), we see that antibodies to BSA are elicited (as we would expect), but also find antibodies which will bind specifically to the DNP groups on BSA; we can further demonstrate that these anti-DNP antibodies will also bind free DNP (or DNP coupled to any other molecule). Therefore, DNP-BSA is both immunogenic and antigenic (with respect to both the DNP groups and the BSA itself), and the free DNP is also antigenic, even though we have shown it is not immunogenic. DNP is an example of a HAPTEN, a small molecule which is not immunogenic unless it is coupled to a larger immunogenic CARRIER molecule, in this case BSA. (Such a hapten/carrier system will be used in Chapter 14 to illustrate the mechanisms of cell interactions required to generate humoral immune responses). We can further demonstrate that the immunogenicity of BSA depends on the presence of aggregates of BSA molecules. If we take a sample of our BSA and centrifuge it at high speed we can remove any aggregated material, leaving behind only single, monomeric BSA molecules in solution. If we immediately inject this "clarified" BSA into a mouse we find that it does not elicit the production of antibodies (as seen in line 4); this monomeric BSA is 13 therefore not immunogenic (nor can it serve as an effective carrier for a hapten). It is still antigenic, however, which we can show by reacting it with the anti-BSA antibodies which we made against the non-clarified BSA (in line 1, for example). (NOTE: "Clarified" BSA not only fails to induce antibody formation, but can induce a state of TOLERANCE to BSA, defined as the specific inability of the mouse to respond to subsequent injections of normally immunogenic BSA. The mechanism of such tolerance will be discussed in Chapter 18.) REASONS FOR LACK OF IMMUNOGENICITY Substances may lack immunogenicity for a variety of reasons: 1) Molecular weight too low. Haptens, for example, are not immunogenic until they are coupled to a high molecular weight carrier. There is no simple cutoff for required molecular weight, however; we have already seen that even the 68,000 mw of BSA is not sufficient to be immunogenic unless the molecules are aggregated into even larger complexes. 2) Not foreign to host. The adaptive immune system normally responds only to "foreign" substances. A sheep, for instance, will normally not make antibodies against its own red blood cells (SRBC), although SRBC are highly immunogenic in mice. (The basis of normal SELF-TOLERANCE is covered in Chapter 18). 3) Some molecules are intrinsically poor immunogens for reasons which are not well understood. Lipids, in general, are poor immunogens, probably because they do not have a structure rigid enough to be stably bound by antibodies. Nucleic acids are also relatively weak immunogens, although they are nevertheless common targets for antibodies present in various autoimmune diseases (discussed in Chapter 19) HOW TO INCREASE IMMUNOGENICITY: ADJUVANTS (See also CHAPTER 22) An ADJUVANT is any substance which, when administered together with an antigen, increases the immune response to that antigen. One of the most widely used adjuvants (in animals but not in humans) is FREUND'S ADJUVANT, which consists of mineral oil, an emulsifying agent, and killed Mycobacterium (the organism which causes tuberculosis). A solution of the desired antigen in water or saline is homogenized with this oil mixture, resulting in a water-in-oil emulsion which is injected into the recipient. Its powerful adjuvant properties result from several factors: 1) The antigen is released from the emulsion over an extended period of time, causing a continuous and more effective stimulation of the immune system. (Antigen given in soluble form will typically be cleared in a matter of hours or days, whereas it can persist for weeks or months in a depot created by the adjuvant.) 14 2) The Mycobacteria contain substances which non-specifically stimulate the immune system, resulting in a higher level of response to the specific antigen. One of these substances which has been extensively studied is Muramyl Dipeptide (MDP). Although Freund's Adjuvant is not used in humans, other forms of adjuvant can be used, such as alum precipitation of antigen, by which a soluble antigen is precipitated together with aluminum hydroxide, resulting in particles of the salt coated with antigen. A soluble antigen is thus converted to a particulate form, and again is released from the mixture over an extended period of time. Substances such as purified MDP and others are also being used to develop effective adjuvants which are less toxic, and therefore potentially usable in humans (see Chapter 22) CHAPTER 2, STUDY QUESTIONS: 1. Define ANTIBODY, ANTIGEN, IMMUNOGEN and HAPTEN. 2. How would you determine if a particular immune response is a HUMORAL response? 3. Describe assays which could be used to measure AGGLUTINATION, PRECIPITATION, HEMOLYSIS and OPSONIZATION. 4. Describe two antibody assays which require no antibody function other than specific binding to an antigen. 5. Define and distinguish ACTIVE versus PASSIVE immunity. 15 16 CHAPTER 3 ANTIBODY STRUCTURE I See APPENDIX: (3) OUCHTERLONY ANALYSIS; (6), EQUILIBRIUM DIALYSIS; (7) CROSS-REACTIVITY Electrophoretic separation of serum proteins identifies the GAMMA-GLOBULIN fraction as containing the majority of antibodies. Three terms which are often confusingly interchanged are defined and distinguished (GAMMA-GLOBULIN, IMMUNOGLOBULIN, ANTIBODY), as are two terms describing antibody/antigen binding, AFFINITY and AVIDITY. All antibodies are made up of one or more IgG-like subunits, each of which has exactly two antigen-combining sites. The affinity of these sites for their antigen (defined as the K of the binding reaction) is highly heterogeneous in any normal immune eq response. While the avidity of an antibody (its ability to form stable complexes with antigen) does depend on its intrinsic affinity, it also increases dramatically with an increasing number of combining sites per antibody. In order to determine the structure of antibodies, we first must have a way of isolating these molecules in relatively pure form. We’ll begin by describing the general process of serum fractionation, then go on to analyze the nature of antigen-antibody binding. The many components of normal serum can be separated from one another by various means: Salt precipitation. Ammonium sulfate (NH ) SO as well as a variety of other salts 4 2 4 can be used to precipitate serum components; different proteins will precipitate at different concentrations of salt, providing a convenient means of separating them. The fraction containing most of the antibody activity generally precipitates at relatively low salt, at about 30-40% of saturated ammonium sulfate. This is a very widely used experimental method for fractionation of serum components (and proteins in general). Ethanol precipitation. Ethanol can also be used to precipitate serum components, which come out of solution at different concentrations and under different conditions of ionic strength, pH and temperature. This is a more elaborate procedure to carry out than salt fractionation, but is the basis for Cohn Fractionation, which in modified form remains a standard procedure for preparing serum protein fractions for clinical use more than sixty years after its original description in the 1940’s. Electrophoresis. Different serum proteins migrate at varying rates in an electric field, a property which can be used to separate them. While this procedure can be adapted for use on a preparative scale, it is most commonly used for analysis. 17 A typical pattern generated by electrophoresis of a serum sample (e.g. on a filter paper strip) is shown in Figure 3-1. albumin globulins β γ α1 α2 ELECTROPHORESIS OF NORMAL SERUM Figure 3-1 Several important points emerge from this pattern: 1) Most serum proteins carry a negative charge, and therefore tend to migrate from the point of origin (labeled "O") toward the anode, the positively charged electrode. 2) Four major peaks are seen in this example; these are named (from the anodal, or positive, side) the albumin peak (which is by far the largest), followed by four globulin peaks, α1- and α2-globulin, β-globulin and γ-globulin. 3) This pattern is deceptively simple; serum actually contains hundreds of known proteins. Thus, "β-globulin" is not a single protein, but a mixture of many components which all happen to migrate in a particular region on electrophoresis. 4) Most (but not all) antibodies migrate in the γ -globulin region. 5) The γ-globulin peak is markedly broader than the others, reflecting the high degree of heterogeneity of the antibodies it contains. This heterogeneity is so great that some antibody molecules in fact migrate in the positions characteristic of α-globulin or β- globulin. 6) The γ-globulin peak is generally centered near the origin, labeled "O"; this reflects the fact that antibodies as a group are relatively neutral, i.e. less highly charged than most other serum components. 18 Three easily confused terms are all commonly used to refer to antibody molecules, gamma-globulins, immunoglobulins and antibodies. To avoid this confusion let's explicitly define each of them: GAMMA-GLOBULIN Any molecule which migrates in the gamma-globulin peak on electrophoresis. Most, but not all, antibodies are in this category, although the term is often used to refer to antibodies in general. (Other serum components migrate in this region as well; therefore, strictly speaking, not all gamma-globulins are antibodies.) IMMUNOGLOBULIN A family of molecules (to which all antibodies belong) with similar structures and physical properties. We shall see that these involve homologous amino acid sequences, similar "domain" structures and similar quarternary structures (the ways in which different polypeptide chains are joined into a larger functional unit). ANTIBODY A molecule belonging to the Immunoglobulin family, with binding specificity for a particular antigen. While all antibodies are immunoglobulins, most but not all antibodies are gamma-globulins. Note that our definition of "antibody" requires knowledge of the binding specificity of the molecule. If one is dealing with an "antibody" molecule whose specificity is not known, or is irrelevant, it is more accurate to refer to it simply as an "immunoglobulin". (Common usage of these terms varies considerably, however.) ANALYSIS OF THE ANTIBODY COMBINING SITE: VALENCY, AFFINITY AND AVIDITY If we immunize a rabbit with DNP-BSA, we can obtain an antiserum which contains antibodies to both the hapten and the carrier protein. This antiserum will precipitate DNP- BSA in addition to DNP-KLH (Keyhole Limpet Hemocyanin, an unrelated protein carrier). If we attach DNP to SRBC (sheep red blood cells) or to latex particles, we can show that the antiserum is capable of showing agglutination (and possibly hemolysis in the case of SRBC). We can use these antibodies to the DNP hapten in order to learn about antibody structure and function. Specifically, we will ask two questions: 1) How many hapten molecules can a single antibody molecule bind (i.e how many combining sites does it have, or what is its “valency”)? 2) What is the strength of binding of the hapten to its combining site(s) on the antibody molecule (i.e. what is the affinity of the combining site)? We have previously made the prediction that in order for an antibody molecule to be capable of precipitation or agglutination it must have at least two combining sites, in order to permit cross-linking of the antigen into large, insoluble complexes. We can determine the actual number of combining sites of our anti-DNP antibodies, as well as their affinity, by several techniques; one of them, EQUILIBRIUM DIALYSIS, is discussed more fully in APPENDIX 6, and we will use the results of such an analysis as the basis for our discussion below. 19 RABBIT IgG ANTIBODIES HAVE TWO HAPTEN-COMBINING SITES The structure of rabbit IgG antibodies represents the basic structure of all antibodies and we can show by equilibrium dialysis that each anti-DNP antibody molecule can bind exactly two DNP molecules. Thus, our minimum prediction of at least two combining sites is fulfilled. Other kinds of antibodies can be shown to have more than two combining sites (IgM and some IgA), but we will see that such antibodies are always made up of multiple units of the basic "IgG-like" structure, each of which bears precisely two combining sites. CONVENTIONAL ANTIBODIES ARE HETEROGENEOUS WITH RESPECT TO AFFINITY The DNP hapten is bound to each combining site by non-covalent forces, and the strength of this binding is measured by the equilibrium constant of the binding reaction, known as the AFFINITY. The antiserum we describe above contains anti-DNP antibodies 5 10 with many different affinities, typically ranging from 10 to 10 . (Antibodies certainly exist with affinities outside this range, but such values are difficult to determine accurately due to technical limitations.) This antibody heterogeneity is a hallmark of the immune response, and has many practical and theoretical implications (see discussions of Clonal Selection and Affinity Maturation Chapter 7, and Isotype Switching Chapter 9). The broadness of the gamma- globulin peak on serum electrophoresis (which we have already described) is one consequence of this heterogeneity; in fact, a sharp, narrow gamma-globulin peak (representing a homogeneous protein) is a pathological sign of a myeloma or other monoclonal gammopathy. However, homogeneous antibodies known as HYBRIDOMAS, or MONOCLONAL ANTIBODIES can be generated experimentally, and are important in many research and clinical applications (see APPENDIX 13). ANTIBODY AVIDITY: ABILITY TO FORM STABLE COMPLEXES WITH ANTIGEN AFFINITY is a thermodynamically defined term representing the strength of interaction of a single combining site with its hapten. Naturally produced antibodies always have two or more sites, however, so that affinity does not tell the whole story with respect to antigen-binding. A bivalent anti-DNP antibody, for example, can simultaneously bind to two DNP haptens on a single BSA molecule, resulting in a much more stable complex than if it only bound to a single site. AVIDITY, on the other hand, is the term used to describe the ability of an antibody to form stable complexes with its antigen. Avidity, of course, depends partly on affinity; all other things being equal (which they rarely are), one IgG antibody with a higher affinity for DNP than another will also have a higher avidity. However, various other factors also play a role, such as the number and spacing of the epitopes on the antigen, the distance between the combining sites on the antibody, and properties such as the "flexibility" of the particular antibody molecule. Avidity does not have a formal thermodynamic definition, and is most commonly used only in a relative context (by demonstrating that one antiserum may exhibit a higher or 20 lower avidity than another). Nevertheless, in discussing the interaction of an intact antibody (which is at least bivalent) with a conventional antigen (which is almost always highly multivalent), one must almost always think in terms of "avidity" rather than "affinity". This is of particular importance when considering the biological effectiveness of antibodies which have more than two combining sites, such as serum IgM and some IgA. CHAPTER 3, STUDY QUESTIONS: 1. Define the terms ANTIBODY, IMMUNOGLOBULIN and GAMMA-GLOBULIN. 2. How is EQUILIBRIUM DIALYSIS carried out, and what can it measure? 3. Define and distinguish antibody AFFINITY and AVIDITY. 21 22 CHAPTER 4 ANTIBODY STRUCTURE II See APPENDIX: (4) AFFINITY CHROMATOGRAPHY; (5) RADIOIMMUNOASSAY The “IgG-like” subunit of all human antibodies consists of two identical LIGHT CHAINS and two identical HEAVY CHAINS, and proteolytic digestion can be used to establish the different functions of distinct portions of antibody molecules. The nine different CLASSES and subclasses (ISOTYPES) of human immunoglobulins exhibit different physical and biological properties, determined by the amino acid sequence of the CONSTANT REGION of the heavy chains. On the other hand, the VARIABLE REGION of heavy and light chains together form the ANTIGEN-BINDING SITES of antibodies. The DOMAIN organization of antibodies, and an understanding of the COVALENT and NON-COVALENT interactions involved in their structure and in their interaction with antigen, form the basis for understanding their distinct roles in PRIMARY and SECONDARY IMMUNE RESPONSES, and the key features of IMMUNOLOGICAL MEMORY. ANTIBODY HEAVY AND LIGHT CHAINS: PROTEOLYTIC FRAGMENTS In order to examine the structure of antibodies, we will again start with a "typical" IgG molecule, such as the rabbit antibody we discussed at in the last chapter. A schematic diagram of an IgG molecule, together with its proteolytic fragments, is shown below: L L H H PAPAIN Fd L H Fab Fab Fc L H L H L Fd' PEPSIN H Intact IgG (Fc degraded) F(ab') 2 Figure 4-1 23

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