Immunobiology - I
SLIDE 1 We are constantly being exposed to infectious agents. The main function of the immune system is to enable us to resist infections. The ability to distinguish between self and non-self is necessary to protect the organism from invading pathogens and to eliminate modified or altered cells (e.g. malignant cells). Since pathogens may replicate intracellularly (viruses and some bacteria and other parasites) or extracellularly (most bacteria, fungi and other parasites), different components of the immune system have evolved to protect against these different types of pathogens. It is important to remember that infection with an organism does not necessarily mean diseases, since the immune system in most cases will be able to eliminate the infection before disease occurs. Disease occurs only when the bolus of infection is high, when the virulence of the invading organism is great or when immunity is compromised. Although the immune system, for the most part, has beneficial effects, there can be detrimental effects as well. During inflammation, which is the response to an invading organism, there may be local discomfort and collateral damage to healthy tissue as a result of the toxic products produced by the immune response. In addition, in some cases the immune response can be directed toward self tissues resulting in autoimmune disease (e.g. rheuamatoid arthritis).
SLIDES 2-4 The two subdivisions of the immune system
SLIDE 2The immune system is composed of two major subdivisions, the innate (or non-specific) immune system and the adaptive (or specific) immune system. The innate immune system is our first line of defense against invading organisms while the adaptive immune system acts as a second line of defense and also affords protection against re-exposure to the same pathogen. Each of the major subdivisions of the immune system has both cellular and humoral components by which they carry out their protective function. In addition, the innate immune system also has anatomical features that function as barriers to infection.
SLIDE 3 Although these two arms of the immune system have distinct functions, there is interplay between these systems (i.e., components of the innate immune system influence the adaptive immune system and vice versa). Innate immune responses are activated directly by pathogens and defend all multicellular organisms against infection. In vertebrates, pathogens, together with the innate immune responses they activate, stimulate adaptive immune responses, which then work together with innate immune responses to help fight the infection.
SLIDE 4 Although the innate and adaptive immune systems both function to protect against invading organisms, they differ in a number of ways. The adaptive immune system requires some time to react to an invading organism, whereas the innate immune system includes defenses that, for the most part, are constitutively present and ready to be mobilized upon infection. Second, the adaptive immune system is antigen specific and reacts only with the organism that induced the response. In contrast, the innate system is not antigen specific and reacts equally well to a variety of organisms. Finally, the adaptive immune system demonstrates immunological memory. It “remembers” that it has encountered an invading organism and reacts more rapidly on subsequent exposure to the same organism. In contrast, the innate immune system does not demonstrate immunological memory.
SLIDE 5 The cells of immune system All cells of the immune system have their origin in the bone marrow and they include (1) myeloid cells and (2) lymphoid cells, which differentiate along distinct pathways. The myeloid progenitor cell in the bone marrow gives rise to erythrocytes, platelets, neutrophils, monocytes/macrophages and dendritic cells whereas the lymphoid progenitor cell gives rise to the natural killer (NK) cells, T cells and B cells.
INNATE IMMUNE SYSTEM (non-specific immunity)
SLIDE 6 The elements of the innate (non-specific) immune system include anatomical barriers, secretory molecules and cellular components. Among the mechanical anatomical barriers are the skin and internal epithelial layers, the movement of the intestines and the oscillation of broncho-pulmonary cilia. Associated with these protective surfaces are chemical and biological agents.
A. Anatomical barriers
1. Mechanical factors The epithelial surfaces form a physical barrier that is very impermeable to most infectious agents. Thus, the skin acts as our first line of defense against invading organisms. The desquamation of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces. Movement due to cilia or peristalsis helps to keep air passages and the gastrointestinal tract free from microorganisms. The flushing action of tears and saliva helps prevent infection of the eyes and mouth. The trapping effect of mucus that lines the respiratory and gastrointestinal tract helps protect the lungs and digestive systems from infection.
2. Chemical factors Fatty acids in sweat inhibit the growth of bacteria. Lysozyme and phospholipase found in tears, saliva and nasal secretions can breakdown the cell wall of bacteria and destabilize bacterial membranes. The low pH of sweat and gastric secretions prevents growth of bacteria. Defensins (low molecular weight proteins) found in the lung and gastrointestinal tract have antimicrobial activity. Surfactants in the lung act as opsonins (substances that promote phagocytosis of particles by phagocytic cells).
3. Biological factors The normal flora of the skin and in the gastrointestinal tract can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces.
B. Humoral barriers
The anatomical barriers are very effective in preventing colonization of tissues by microorganisms. However, when there is damage to tissues the anatomical barriers are breached and infection may occur. Once infectious agents have penetrated tissues, another innate defense mechanism comes into play, namely acute inflammation. Humoral factors play an important role in inflammation, which is characterized by edema and the recruitment of phagocytic cells These humoral factors are found in serum or they are formed at the site of infection.
1. The complement system is the major humoral non-specific defense mechanism. Once activated complement can lead to increased vascular permeability, recruitment of phagocytic cells, and lysis and opsonization of bacteria. The complement system helps or “complements” the ability of antibodies and phagocytic cells to clear pathogens from an organism. It is not adaptable and does not change over the course of an individual's lifetime. The complement system is the part of innate immune system; however, it can be recruited and brought into action by the adaptive immune system. The complement system consists of a number of small proteins found in the blood, generally synthesized by the liver, and normally circulating as inactive precursors. When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavages. The activation of complement involves the sequential proteolysis of proteins to generate enzymes with catalytic activities. The end-result of this activation cascade is massive amplification of the response and activation of the cell-killing membrane attack complex. Over 25 proteins and protein fragments make up the complement system.
SLIDE 7 Members of the herpesvirus, orthopoxvirus and retrovirus families mimic or interact with complement regulatory proteins to block complement activation and neutralization of virus particles.
2. Coagulation system Depending on the severity of the tissue injury, the coagulation system may or may not be activated. Some products of the coagulation system can contribute to the non-specific defenses because of their ability to increase vascular permeability and act as chemotactic agents for phagocytic cells. In addition, some of the products of the coagulation system are directly antimicrobial. For example, beta-lysin, a protein produced by platelets during coagulation can lyse many Gram positive bacteria by acting as a cationic detergent.
3. Lactoferrin and transferrin By binding iron, an essential nutrient for bacteria, these proteins limit bacerial growth.
4. Interferons are proteins that can limit virus replication in cells.
5. Lysozyme breaks down the cell wall of bacteria.
6. Interleukin-1 induces fever and the production of acute phase proteins, some of which are antimicrobial because they can opsonize bacteria.
C. Cellular barriers
Part of the inflammatory response is the recruitment of polymorphonuclear eosinopiles and macrophages to sites of infection. These cells are the main line of defense in the non-specific immune system.
1. Neutrophils are recruited to the site of infection where they phagocytose invading organisms and kill them intracellularly. In addition, these cells contribute to collateral tissue damage that occurs during inflammation.
2. Macrophages Tissue macrophages and newly recruited monocytes, which differentiate into macrophages, also function in phagocytosis and intracellular killing of microorganisms. In addition, macrophages are capable of extracellular killing of infected or altered self target cells. Furthermore, macrophages contribute to tissue repair and act as antigen-presenting cells, which are required for the induction of specific immune responses.
3. Natural killer (NK) and lymphokine activated killer (LAK) cells can nonspecifically kill virus infected and tumor cells. These cells are not part of the inflammatory response but they are important in nonspecific immunity to viral infections and tumor surveillance.
4. Eosinophils have proteins in granules that are effective in killing certain parasites.
SLIDE X Inflammation See text on the SLIDES
ADAPTIVE IMMUNE SYSTEM (specific immunity)
SLIDE 8 Invertebrate animals use simple defense strategies that rely on protective barriers, toxic molecules, and phagocytic cells that ingest and break up invading microorganisms. Vertebrate animals also depend on such innate immune responses as a first line of defense, but they can also mount much more sophisticated defenses, called adaptive immune responses. In vertebrates, the innate responses call the adaptive immune responses into play, and both work together to eliminate the pathogens. Whereas the innate immune responses are general defense reactions, the adaptive responses are highly specific to the particular pathogen that induced them. Any substance capable of eliciting an adaptive immune response is referred to as an antigen (antibody generator). The adaptive immune system recognizes the fine molecular details of macromolecules: it can distinguish between two proteins that differ in only a single amino acid. Adaptive immune responses are carried out by white blood cells called lymphocytes. There are two classes of such responses, (1) Humoral immune response (antibody response, B cell-mediated response) and (2) Cellular immune response (T-cell-mediated immune response), which are carriet out by different classes of lymphocytes, called B cells and T cells, respectively. In antibody responses, B cells are activated to secrete antibodies, which are proteins called immunoglobulins (Igs). Binding of antibody inactivates viruses and microbial toxins (such as tetanus toxin or diphtheria toxin) by blocking their ability to bind to receptors on host cells. Antibody binding also marks invading pathogens for destruction, mainly by making it easier for phagocytic cells of the innate immune system to ingest them. In T-cell-mediated immune responses, the second class of adaptive immune responses, activated T cells react directly against a foreign antigen that is presented to them on the surface of a host cell, which is therefore referred to as an antigen-presenting cell. T cells can detect microbes hiding inside host cells and either kill the infected cells or help the infected cells or other cells to eliminate the microbes. The T cell, for example, might kill a virus-infected host cell that has viral antigens on its surface, thereby eliminating the infected cell before the virus has had a chance to replicate. In other cases, the T cell produces signal molecules that either activate macrophages to destroy the microbes that they have phagocytosed or help activate B cells to make antibodies against the microbes.
SLIDE Y Humoral and cellular immune response See text on the SLIDES
II/A. LYMPHOCYTES AND THE CELLULAR BASIS OF ADAPTIVE IMMUNITY
SLIDE 9 Human lymphoid organs Lymphocytes occur in large numbers in the blood and lymph. They are also concentrated in lymphoid organs, such as the thymus, lymph nodes (also called lymph glands), spleen, and appendix. There are about 2 x 1012 lymphocytes in the human body, making the immune system comparable in cell mass to the liver or the brain.
SLIDE 10, 11 The innate and adaptive immune systems work together Lymphocytes usually respond to foreign antigens only if the innate immune system is first activated. The rapid innate immune responses to an infection depend largely on pattern recognition receptors made by cells of the innate immune system. These receptors recognize microbe-associated molecules (pathogen-associated molecular patterns; PAMPs) that are not present in the host organism. (1) Some of the pattern recognition receptors are present on the surface of professional phagocytic cells (phagocytes) such as macrophages and neutrophils, where they mediate the uptake of pathogens, which are then delivered to lysosomes for destruction. (2) Others are secreted and bind to the surface of pathogens, marking them for destruction by either phagocytes or a system of blood proteins collectively called the complement system. (3) Still others, including the Toll-like receptors (TLRs), activate intracellular signaling pathways that lead to the secretion of extracellular signal molecules that promote inflammation and help activate adaptive immune responses. The cells of the vertebrate innate immune system that respond to antigens and activate adaptive immune responses most efficiently are dendritic cells. Present in most tissues, dendritic cells express high levels of TLRs and other pattern recognition receptors, and they function by presenting microbial antigens to T cells in peripheral lymphoid organs. In most cases, they recognize and phagocytose invading microbes or their products or fragments of infected cells at a site of infection and then migrate with their prey to a nearby lymph node; in other cases, they pick up microbes or their products directly in a peripheral lymphoid organ such as the spleen. In either case, the microbial antigens activate the dendritic cells so that they, in turn, can directly activate the T cells in peripheral lymphoid organs to respond to the microbial antigens displayed on the dendritic cell surface. Once activated, some of the T cells then migrate to the site of infection, where they help destroy the microbes. Other activated T cells remain in the lymphoid organ, where they help keep the dendritic cells active, help activate other T cells, and help activate B cells to make antibodies against the microbial antigens. Thus, innate immune responses are activated mainly at sites of infection (or injury), whereas adaptive immune responses are activated mainly in peripheral lymphoid organs such as lymph nodes and spleen.
SLIDE 12 The development of T and B cells T cells and B cells derive their names from the organs in which they develop. T cells develop in the thymus, and B cells, in mammals, develop in the bone marrow in adults or the liver in fetuses. Both T and B cells are thought to develop from the same common lymphoid progenitor cells. The common lymphoid progenitor cells themselves derive from multipotential hematopoietic stem cells, which give rise to all of the blood cells, including red blood cells, white blood cells, and platelets. These stem cells are located primarily in hematopoietic tissues - mainly the liver in fetuses and the bone marrow in adults. T cells develop in the thymus from common lymphoid progenitor cells that migrate there from the hematopoietic tissues via the blood. In most mammals, including humans, B cells develop from common lymphoid progenitor cells in the hematopoietic tissues themselves. Because they are sites where lymphocytes develop from precursor cells, the thymus and hematopoietic tissues are referred to as central (primary) lymphoid organs. Most lymphocytes die in the central lymphoid organs soon after they develop, without ever functioning. Others, however, mature and migrate via the blood to the peripheral (secondary) lymphoid organs - mainly, the lymph nodes, spleen, and epithelium-associated lymphoid tissues in the gastrointestinal tract, respiratory tract, and skin. It is in these peripheral lymphoid organs that foreign antigens activate T and B cells. T and B cells become morphologically distinguishable from each other only after they have been activated by antigen. Resting T and B cells look very similar. After activation by an antigen, both proliferate and mature into effector cells. Effector B cells secrete antibodies. In their most mature form, called plasma cells, they are filled with an extensive rough endoplasmic reticulum that is busily making antibodies. In contrast, effector T cells contain very little endoplasmic reticulum and do not secrete antibodies; instead, they secrete a variety of signal proteins called cytokines, which act as local mediators. There are three main classes of T cells: (1) cytotoxic T cells, (2) helper T cells, and (3) regulatory (suppressor) T cells. Cytotoxic T cells directly kill infected host cells. Helper T cells help activate macrophages, dendritic cells, B cells, and cytotoxic T cells by secreting a variety of cytokines and displaying a variety of co-stimulatory proteins on their surface. Regulatory T cells are thought to use similar strategies to inhibit the function of helper T cells, cytotoxic T cells, and dendritic cells. Thus, whereas B cells can act over long distances by secreting antibodies that are widely distributed by the bloodstream, T cells can migrate to distant sites, but, once there, they act only locally on neighboring cells.
SLIDE 13 Hematopoesis Hematopeietic stem cells (HSCs) reside in the medulla of the bone (bone marrow) and have the unique ability to give rise to all of the different mature blood cell types. HSCs are self renewing: when they proliferate, at least some of their daughter cells remain as HSCs, so the pool of stem cells does not become depleted. The other daughters of HSCs (myeloid and lymphoid progenitor cells), however can each commit to any of the alternative differentiation pathways that lead to the production of one or more specific types of blood cells, but cannot self-renew. This is one of the vital processes in the body. All blood cells are divided into three lineages.
Erythroid cells are the oxygen carrying red blood cells. Both reticulocytes and erythrocytes are functional and are released into the blood. In fact, a reticulocyte count estimates the rate of erythropoiesis.
Lymphocytes are the cornerstone of the adaptive immune system. They are derived from common lymphoid progenitors. The lymphoid lineage is primarily composed of T-cells and B-cells (types of white blood cells). This is lymphopoiesis.
Myelocytes, which include granulocytes, megakaryocytes and macrophages and are derived from common myeloid progenitors, are involved in such diverse roles as innate immunity, adaptive immunity, and blood clotting. This is myelopoiesis.
Granulopoiesis (or granulocytopoiesis) is haematopoiesis of granulocytes. Megakaryocytopoiesis is haematopoiesis of megakaryocytes.
Theories Cell determination appears to be dictated by the location of differentiation. For instance, the thymus provides an ideal environment for thymocytes to differentiate into a variety of different functional T cells. For the stem cells and other undifferentiated blood cells in the bone marrow, the determination is generally explained by the (1) determinism theory of hematopoiesis, saying that colony stimulating factors and other factors of the hematopoietic microenvironment determine the cells to follow a certain path of cell differentiation. This is the classical way of describing hematopoiesis. In fact, however, it is not really true. The ability of the bone marrow to regulate the quantity of different cell types to be produced is more accurately explained by a (2) stochastic theory, which claims that undifferentiated blood cells are determined to specific cell types by randomness. The hematopoietic microenvironment prevails upon some of the cells to survive and some, on the other hand, to perform apoptosis and die. By regulating this balance between different cell types, the bone marrow can alter the quantity of different cells to ultimately be produced.
Hematopoetic growth factors Red and white blood cell production is regulated with great precision in healthy humans, and the production of granulocytes is rapidly increased during infection. The proliferation and self-renewal of these cells depend on stem cell factor (SCF). Glycoprotein growth factors regulate the proliferation and maturation of the cells that enter the blood from the marrow, and cause cells in one or more committed cell lines to proliferate and mature. Three more factors that stimulate the production of committed stem cells are called colony-stimulating factors (CSFs) and include granulocyte.macrophage CSF (GM-CSF), granulocyte CSF (G-CSF) and macrophage CSF (M-CSF). These stimulate much granulocyte formation and are active on either progenotor cells or end product cells. Erythropoetin is required for a myeloid progenitor cell to become an erythrocyte. On the other hand, thrombopoietin makes myeloid progenitor cells differentiate to megakaryocytes (thrombocyte-forming cells). Examples of cytokines and the blood cells they give rise to, is shown in the picture to the right.