8536d_ ch02024-056 9/6/029: 00 PM Page 24 mac85 Mac 85: 365 smm poldsby et al./Immunology 5e: Cells and Organs of the chapter 2 Immune System HE IMMUNE SYSTEM CONSISTS OF MANY DIFFERENT organs and tissues that are found throughout the body. These organs can be classified functionall nto two main groups. The primary lymphoid organs provide appropriate microenvironments for the development and maturation of lymphocytes. The secondary lymphoid organs trap antigen from defined tissues or vascular spaces and are sites where mature lymphocytes can interact effectively with that antigen. Blood vessels and lymphatic systems connect these organs, uniting them into a functional whole Carried within the blood and lymph and populating the Macrophage Interacting with Bacteria uphold various white blood cells, or leuko- cytes, that participate in the immune response. Of these a Hematopoiesis cells, only the lymphocytes possess the attributes of diversity specificity, memory, and self/nonself recognition, the hall- Cells of the Immune System marks of an adaptive immune response. All the other cells a Organs of the Immune System play accessory roles in adaptive immunity, serving to activate lymphocytes, to increase the effectiveness of antigen clear- a Systemic Function of the Immune System ance by phagocytosis, or to secrete various immune-effector a Lymphoid Cells and Organs-Evolutionary molecules. Some leukocytes, especially T lymphocytes, se- Compariso rete various protein molecules called cytokines. These mol ecules act as immunoregulatory hormones and pla important roles in the regulation of immune responses. Th chapter describes the formation of blood cells, the properties contrast to a unipotent cell, which differentiates into a single of the various immune-system cells, and the functions of the cell type, a hematopoietic stem cell is multipotent, or pluripo- lymphoid organs. ent, able to differentiate in various ways and thereby generate erythrocytes, granulocytes, monocytes, mast cells, lympho- cytes, and megakaryocytes. These stem cells are few, normally Hematopoiesis fewer than one HSC per 5 X 10" cells in the bone marrow The study of hematopoietic stem cells is difficult both be All blood cells arise from a type of cell called the hematopoi- cause of their scarcity and because they are hard to grow in etic stem cell(HSC). Stem cells are cells that can differentiate vitro. As a result, little is known about how their proliferation into other cell types; they are self-renewing-they maintain and differentiation are regulated. By virtue of their capacity heir population level by cell division. In humans, for self-renewal, hematopoietic stem cells are maintained at hematopoiesis, the formation and development of red and stable levels throughout adult life; however, when there is an white blood cells, begins in the embryonic yolk sac during the increased demand for hematopoiesis, HSCs display an enor- first weeks of development. Here, yolk-sac stem cells differen- mous proliferative capacity. This can be demonstrated in tiate into primitive erythroid cells that contain embryonic mice whose hematopoietic systems have been completely de hemoglobin In the third month of gestation, hematopoietic stroyed by a lethal dose of x-rays(950 rads; one rad repre- stem cells migrate from the yolk sac to the fetal liver and then sents the absorption by an irradiated target of an amount of to the spleen; these two organs have major roles in radiation corresponding to 100 ergs/gram of target). Such ir hematopoiesis from the third to the seventh months of gesta- radiated mice will die within 10 days unless they are infused tion. After that, the differentiation of HSCs in the bone mar bone-marrow cells from a syngeneic(genetically row becomes the major factor in hematopoiesis, and by birth identical)mouse. Although a normal mouse has 3 X 10 there is little or no hematopoiesis in the liver and spleen bone-marrow cells, infusion of only 10-10 bone-marrow It is remarkable that every functionally specialized, ma- cells(i. e, 0.01%-0.1% of the normal amount)from a donor ture blood cell is derived from the same type of stem cell In is sufficient to completely restore the hematopoietic system
contrast to a unipotent cell, which differentiates into a single cell type, a hematopoietic stem cell is multipotent, or pluripotent, able to differentiate in various ways and thereby generate erythrocytes, granulocytes, monocytes, mast cells, lymphocytes, and megakaryocytes. These stem cells are few, normally fewer than one HSC per 5 104 cells in the bone marrow. The study of hematopoietic stem cells is difficult both because of their scarcity and because they are hard to grow in vitro. As a result, little is known about how their proliferation and differentiation are regulated. By virtue of their capacity for self-renewal, hematopoietic stem cells are maintained at stable levels throughout adult life; however, when there is an increased demand for hematopoiesis, HSCs display an enormous proliferative capacity. This can be demonstrated in mice whose hematopoietic systems have been completely destroyed by a lethal dose of x-rays (950 rads; one rad represents the absorption by an irradiated target of an amount of radiation corresponding to 100 ergs/gram of target). Such irradiated mice will die within 10 days unless they are infused with normal bone-marrow cells from a syngeneic (genetically identical) mouse. Although a normal mouse has 3 108 bone-marrow cells, infusion of only 104 –105 bone-marrow cells (i.e., 0.01%–0.1% of the normal amount) from a donor is sufficient to completely restore the hematopoietic system, chapter 2 ■ Hematopoiesis ■ Cells of the Immune System ■ Organs of the Immune System ■ Systemic Function of the Immune System ■ Lymphoid Cells and Organs—Evolutionary Comparisons Cells and Organs of the Immune System T organs and tissues that are found throughout the body. These organs can be classified functionally into two main groups. The primary lymphoid organs provide appropriate microenvironments for the development and maturation of lymphocytes. The secondary lymphoid organs trap antigen from defined tissues or vascular spaces and are sites where mature lymphocytes can interact effectively with that antigen. Blood vessels and lymphatic systems connect these organs, uniting them into a functional whole. Carried within the blood and lymph and populating the lymphoid organs are various white blood cells, or leukocytes, that participate in the immune response. Of these cells, only the lymphocytes possess the attributes of diversity, specificity, memory, and self/nonself recognition, the hallmarks of an adaptive immune response. All the other cells play accessory roles in adaptive immunity, serving to activate lymphocytes, to increase the effectiveness of antigen clearance by phagocytosis, or to secrete various immune-effector molecules. Some leukocytes, especially T lymphocytes, secrete various protein molecules called cytokines. These molecules act as immunoregulatory hormones and play important roles in the regulation of immune responses. This chapter describes the formation of blood cells, the properties of the various immune-system cells, and the functions of the lymphoid organs. Hematopoiesis All blood cells arise from a type of cell called the hematopoietic stem cell (HSC). Stem cells are cells that can differentiate into other cell types; they are self-renewing—they maintain their population level by cell division. In humans, hematopoiesis, the formation and development of red and white blood cells, begins in the embryonic yolk sac during the first weeks of development. Here, yolk-sac stem cells differentiate into primitive erythroid cells that contain embryonic hemoglobin. In the third month of gestation, hematopoietic stem cells migrate from the yolk sac to the fetal liver and then to the spleen; these two organs have major roles in hematopoiesis from the third to the seventh months of gestation. After that, the differentiation of HSCs in the bone marrow becomes the major factor in hematopoiesis, and by birth there is little or no hematopoiesis in the liver and spleen. It is remarkable that every functionally specialized, mature blood cell is derived from the same type of stem cell. In Macrophage Interacting with Bacteria 8536d_ch02_024-056 9/6/02 9:00 PM Page 24 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e:
8536d_cho2024-056 8/5/02 4: 02 PM Page 25 mac79 Mac 79: 45_BW: Go dsby et al./Immunology se Cells and Organs of the Immune System CHAPTER 2 which demonstrates the enormous proliferative and differ- progenitor cell( Figure 2-1). The types and amounts of entitative capacity of the stem cell growth factors in the microenvironment of a particular stem Early in hematopoiesis, a multipotent stem cell differenti- cell or progenitor cell control its differentiation. During the ates along one of two pathways, giving rise to either a com- development of the lymphoid and myeloid lineages, stem mon lymphoid progenitor cell or a common myeloid cells differentiate into progenitor cells, which have lost the VISUALIZING CONCEPTS Hematopoictic stem cell g progenitor (NK)cell ② TH helper cell cutrophill T-cell monocyte progenitor 像 B cell Erythrocyte Erythroid progenitor FICURE2-1Hematopoiesis. Self-renewing hematopoietic of the myeloid lineage arise from myeloid progenitors. Note that tem cells give rise to lymphoid and myeloid progenitors. All lym- some dendritic cells come from lymphoid progenitors, others phoid cells descend from lymphoid progenitor cells and all cells from myeloid precursors
Cells and Organs of the Immune System CHAPTER 2 25 which demonstrates the enormous proliferative and differentiative capacity of the stem cells. Early in hematopoiesis, a multipotent stem cell differentiates along one of two pathways, giving rise to either a common lymphoid progenitor cell or a common myeloid progenitor cell (Figure 2-1). The types and amounts of growth factors in the microenvironment of a particular stem cell or progenitor cell control its differentiation. During the development of the lymphoid and myeloid lineages, stem cells differentiate into progenitor cells, which have lost the TH helper cell TC cytotoxic T cell Natural killer (NK) cell Myeloid progenitor Lymphoid progenitor Hematopoietic stem cell Selfrenewing B cell Dendritic cell T-cell progenitor B-cell progenitor Eosinophil Monocyte Neutrophil Basophil Platelets Erythrocyte Erythroid progenitor Megakaryocyte Eosinophil progenitor Granulocytemonocyte progenitor Basophil progenitor Macrophage Dendritic cell VISUALIZING CONCEPTS FIGURE 2-1 Hematopoiesis. Self-renewing hematopoietic stem cells give rise to lymphoid and myeloid progenitors. All lymphoid cells descend from lymphoid progenitor cells and all cells of the myeloid lineage arise from myeloid progenitors. Note that some dendritic cells come from lymphoid progenitors, others from myeloid precursors. 8536d_ch02_024-056 8/5/02 4:02 PM Page 25 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_ch02_024-0568/5/02 4: 02 PM Page 26 mac79 Mac 79: 45_BW: Godsby et al. /Immunology 5e apacity for self-renewal and are committed to a particular cell possible to identify many hematopoietic growth factors. In neage. Common lymphoid progenitor cells give rise to B, T, these in vitro systems, bone-marrow stromal cells are cul- and nk(natural killer)cells and some dendritic cells. Myeloid tured to form a layer of cells that adhere to a petri dish stem cells generate progenitors of red blood cells(erythro- freshly isolated bone-marrow hematopoietic cells placed on ytes), many of the various white blood cells(neutrophils, this layer will grow, divide, and produce large visible colonies eosinophils, basophils, monocytes, mast cells, dendritic cells),(Figure 2-2). If the cells have been cultured in semisolid agar, nd platelets Progenitor commitment depends on the acquisi- their progeny will be immobilized and can be analyzed for tion of responsiveness to particular growth factors and cy- cell types. Colonies that contain stem cells can be replated to tokines. When the appropriate factors and cytokines are produce mixed colonies that contain different cell types, in present, progenitor cells proliferate and differentiate into the cluding progenitor cells of different cell lineages. In contrast, corresponding cell type, either a mature erythrocyte, a partic- progenitor cells, while capable of division, cannot be replated ular type of leukocyte, or a platelet-generating cell(the and produce lineage-restricted colonies. megakaryocyte). Red and white blood cells pass into bone Various growth factors are required for the survival, pro- marrow channels, from which they enter the circulation liferation, differentiation, and maturation of hematopoietic In bone marrow, hematopoietic cells grow and mature on cells in culture. These growth factors, the hematopoietic a meshwork of stromal cells, which are nonhematopoietic cytokines, are identified by their ability to stimulate the for- cells that support the growth and differentiation of hema- mation of hematopoietic cell colonies in bone-marrow topoietic cells Stromal cells include fat cells, endothelial cells, cultures. Among the cytokines detected in this way was a ibroblasts, and macrophages Stromal cells influence the dif- family of acidic glycoproteins, the colony-stimulating fac- ferentiation of hematopoietic stem cells by providing a tors(CSFs), named for their ability to induce the formation hematopoietic-inducing microenvironment(HIM)con- of distinct hematopoietic cell lines. Another importan sisting of a cellular matrix and factors that promote growth hematopoietic cytokine detected by this method was the gly- and differentiation. Many of these hematopoietic growth coprotein erythropoietin(EPO). Produced by the kidney, factors are soluble agents that arrive at their target cells by this cytokine induces the terminal development of erythro diffusion, others are membrane-bound molecules on the cytes and regulates the production of red blood cells. Fur surface of stromal cells that require cell-to-cell contact be- ther studies showed that the ability of a given cytokine to tween the responding cells and the stromal cells. During in- signal growth and differentiation is dependent upon the fection, hematopoiesis is stimulated by the production of presence of a receptor for that cytokine on the surface of the hematopoietic growth factors by activated macrophages and target cell--commitment of a progenitor cell to a particular Tcells differentiation pathway is associated with the expression of membrane receptors that are specific for particular cy Hematopoiesis Can Be Studied In Vitro tokines. Many cytokines and their receptors have since been shown to play essential roles in hematopoiesis. This topic is Cell-culture systems that can support the growth and differ- explored much more fully in the chapter on cytokines entiation of lymphoid and myeloid stem cells have made it Chapter 11) Adherent layer of marrow cells Culture in semioli Visible colonies of bone-marrow cells ∠s8、g 發 FIGURE2-2(a) Experimental scheme for culturing hematopoietic in long-term culture of human bone marrow.[Photograph from ells Adherent bone-marrow stromal cells form a matrix on which M.J. Cline and D. W. Golde, 1979, Nature 277: 180; reprinted by the hematopoietic cells proliferate. Single cells can be transferred permission;@ 1979 Macmillan Magazines Ltd, micrograph cour. to semisolid agar for colony growth and the colonies analyzed for tesy of S. quan J differentiated cell types. (b) Scanning electron micrograph of cells
26 PART I Introduction capacity for self-renewal and are committed to a particular cell lineage. Common lymphoid progenitor cells give rise to B, T, and NK (natural killer) cells and some dendritic cells. Myeloid stem cells generate progenitors of red blood cells (erythrocytes), many of the various white blood cells (neutrophils, eosinophils, basophils, monocytes, mast cells, dendritic cells), and platelets. Progenitor commitment depends on the acquisition of responsiveness to particular growth factors and cytokines. When the appropriate factors and cytokines are present, progenitor cells proliferate and differentiate into the corresponding cell type, either a mature erythrocyte, a particular type of leukocyte, or a platelet-generating cell (the megakaryocyte). Red and white blood cells pass into bonemarrow channels, from which they enter the circulation. In bone marrow, hematopoietic cells grow and mature on a meshwork of stromal cells, which are nonhematopoietic cells that support the growth and differentiation of hematopoietic cells. Stromal cells include fat cells, endothelial cells, fibroblasts, and macrophages. Stromal cells influence the differentiation of hematopoietic stem cells by providing a hematopoietic-inducing microenvironment (HIM) consisting of a cellular matrix and factors that promote growth and differentiation. Many of these hematopoietic growth factors are soluble agents that arrive at their target cells by diffusion, others are membrane-bound molecules on the surface of stromal cells that require cell-to-cell contact between the responding cells and the stromal cells. During infection, hematopoiesis is stimulated by the production of hematopoietic growth factors by activated macrophages and T cells. Hematopoiesis Can Be Studied In Vitro Cell-culture systems that can support the growth and differentiation of lymphoid and myeloid stem cells have made it possible to identify many hematopoietic growth factors. In these in vitro systems, bone-marrow stromal cells are cultured to form a layer of cells that adhere to a petri dish; freshly isolated bone-marrow hematopoietic cells placed on this layer will grow, divide, and produce large visible colonies (Figure 2-2). If the cells have been cultured in semisolid agar, their progeny will be immobilized and can be analyzed for cell types. Colonies that contain stem cells can be replated to produce mixed colonies that contain different cell types, including progenitor cells of different cell lineages. In contrast, progenitor cells, while capable of division, cannot be replated and produce lineage-restricted colonies. Various growth factors are required for the survival, proliferation, differentiation, and maturation of hematopoietic cells in culture. These growth factors, the hematopoietic cytokines, are identified by their ability to stimulate the formation of hematopoietic cell colonies in bone-marrow cultures. Among the cytokines detected in this way was a family of acidic glycoproteins, the colony-stimulating factors (CSFs), named for their ability to induce the formation of distinct hematopoietic cell lines. Another important hematopoietic cytokine detected by this method was the glycoprotein erythropoietin (EPO). Produced by the kidney, this cytokine induces the terminal development of erythrocytes and regulates the production of red blood cells. Further studies showed that the ability of a given cytokine to signal growth and differentiation is dependent upon the presence of a receptor for that cytokine on the surface of the target cell—commitment of a progenitor cell to a particular differentiation pathway is associated with the expression of membrane receptors that are specific for particular cytokines. Many cytokines and their receptors have since been shown to play essential roles in hematopoiesis. This topic is explored much more fully in the chapter on cytokines (Chapter 11). FIGURE 2-2 (a) Experimental scheme for culturing hematopoietic cells. Adherent bone-marrow stromal cells form a matrix on which the hematopoietic cells proliferate. Single cells can be transferred to semisolid agar for colony growth and the colonies analyzed for differentiated cell types. (b) Scanning electron micrograph of cells Add fresh bonemarrow cells Culture in semisolid agar Adherent layer of stromal cells Visible colonies of bone-marrow cells (a) (b) in long-term culture of human bone marrow. [Photograph from M. J. Cline and D. W. Golde, 1979, Nature 277:180; reprinted by permission; © 1979 Macmillan Magazines Ltd., micrograph courtesy of S. Quan.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 26 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_cho2024-056 8/5/02 4: 02 PM Page 27 mac79 Mac 79: 45_BW: Go dsby et al./Immunology se Cells and Organs of the Immune System CHAPTER 2 Hematopoiesis Is Regulated at the numbers of B, T, and NK cells, their production of erythro Genetic Level cytes, granulocytes, and other cells of the myeloid lineage is unimpaired. Ikaros knockout mice survive embryonic devel- The development of pluripotent hematopoietic stem cells opment, but they are severely compromised immunologi into different cell types requires the expression of different cally and die of infections at an early age sets of lineage-determining and lineage-specific genes at ap propriate times and in the correct order. The proteins speci- Hematopoietic Homeostasis Involves fied by these genes are critical components of regulatory Many Factors its descendants. Much of what we know about the depen- Hematopoiesis is a continuous process that generally main- dence of hematopoiesis on a particular gene comes from tains a steady state in which the production of mature blood studies of mice in which a gene has been inactivated or cells equals their loss(principally from aging). The average "knocked out"by targeted disruption, which blocks the pro- erythrocyte has a life span of 120 days before it is phagocytosed duction of the protein that it encodes(see Targeted Disrup- and digested by macrophages in the spleen. The various white tion of Genes, in Chapter 23). If mice fail to produce red cells blood cells have life spans ranging from a few days, for neu or particular white blood cells when a gene is knocked out, trophils, to as long as 20-30 years for some Tlymphocytes. To we conclude that the protein specified by the gene is neces- maintain steady-state levels, the average human being must sary for development of those cells. Knockout technology is produce an estimated 3.7 X 10 white blood cells per day one of the most powerful tools available for determining the Hematopoiesis is regulated by complex mechanisms that roles of particular genes in a broad range of processes and it affect all of the individual cell types. These regulatory mech has made important contributions to the identification of anisms ensure steady-state levels of the various blood cells, many genes that regulate hematopoiesis they have enough built-in flexibility so that production of Although much remains to be done, targeted disruption blood cells can rapidly increase tenfold to twentyfold in re- hematopoiesis.Some of these transcription factors affect cude. o hemorrhage or infection. Steady-state regulation of and other approaches have identified a number of transcrip- sponse to tion factors (Table 2-1)that play important roles in hematopoiesis is accomplished in various ways, which in many different hematopoietic lineages, and others affect only Control of the levels and types of cytokines produced by a single lineage, such as the developmental pathway that leads bone-marrow stromal cells to lymphocytes. One transcription factor that affects multi ple lineages is GATA-2, a member of a family of transcription The production of cytokines with hematopoietic activity factors that recognize the tetranucleotide sequence GATA, a by other cell types, such as activated T cells and nucleotide motif in target genes. A functional GATA-2 gene, which specifies this transcription factor, is essential for the The regulation of the expression of receptors for eages. As might be expected, animals in which this gene is hematopoietically active cytokines in stem cells and cell disrupted die during embryonic development. In contrast to GATA-2, another transcription factor, Ikaros, is required The removal of some cells by the controlled induction of only for the development of cells of the lymphoid lineage cell death though Ikaros knockout mice do not produce significant A failure in one or a combination of these regulatory mecha- nisms can have serious consequences. For example, abnormal- ities in the expression of hematopoietic cytokines or their TABLE 2.1 Some transcription factors essential for hematopoietic lineages e ceptors could lead to unregulated cellular proliferation and may contribute to the development of some leukemias. Ulti- mately, the number of cells in any hematopoietic lineage is set Factor Dependent lineage a balance between the number of cells removed by cell death and the number that arise from division and differentiation GATA-1 Erythroid Any one or a combination of regulatory factors can affect rates GATA-2 Erythroid, myeloid, lymphoid f cell reproduction and differentiation. These factors can also PU. 1 Erythroid (maturational stages), myeloid(later determine whether a hematopoietic cell is induced to die. tages), lymphoid Programmed Cell Death Is an Essential Homeostatic Mechanism B lymphoid (differentiation of B cells into plasma Programmed cell death, an induced and ordered process in which the cell actively participates in bringing about its own demise, is a critical factor in the homeostatic regulation of
Cells and Organs of the Immune System CHAPTER 2 27 Hematopoiesis Is Regulated at the Genetic Level The development of pluripotent hematopoietic stem cells into different cell types requires the expression of different sets of lineage-determining and lineage-specific genes at appropriate times and in the correct order. The proteins specified by these genes are critical components of regulatory networks that direct the differentiation of the stem cell and its descendants. Much of what we know about the dependence of hematopoiesis on a particular gene comes from studies of mice in which a gene has been inactivated or “knocked out” by targeted disruption, which blocks the production of the protein that it encodes (see Targeted Disruption of Genes, in Chapter 23). If mice fail to produce red cells or particular white blood cells when a gene is knocked out, we conclude that the protein specified by the gene is necessary for development of those cells. Knockout technology is one of the most powerful tools available for determining the roles of particular genes in a broad range of processes and it has made important contributions to the identification of many genes that regulate hematopoiesis. Although much remains to be done, targeted disruption and other approaches have identified a number of transcription factors (Table 2-1) that play important roles in hematopoiesis. Some of these transcription factors affect many different hematopoietic lineages, and others affect only a single lineage, such as the developmental pathway that leads to lymphocytes. One transcription factor that affects multiple lineages is GATA-2, a member of a family of transcription factors that recognize the tetranucleotide sequence GATA, a nucleotide motif in target genes. A functional GATA-2 gene, which specifies this transcription factor, is essential for the development of the lymphoid, erythroid, and myeloid lineages. As might be expected, animals in which this gene is disrupted die during embryonic development. In contrast to GATA-2, another transcription factor, Ikaros, is required only for the development of cells of the lymphoid lineage. Although Ikaros knockout mice do not produce significant numbers of B, T, and NK cells, their production of erythrocytes, granulocytes, and other cells of the myeloid lineage is unimpaired. Ikaros knockout mice survive embryonic development, but they are severely compromised immunologically and die of infections at an early age. Hematopoietic Homeostasis Involves Many Factors Hematopoiesis is a continuous process that generally maintains a steady state in which the production of mature blood cells equals their loss (principally from aging). The average erythrocyte has a life span of 120 days before it is phagocytosed and digested by macrophages in the spleen. The various white blood cells have life spans ranging from a few days, for neutrophils, to as long as 20–30 years for some T lymphocytes. To maintain steady-state levels, the average human being must produce an estimated 3.7 1011 white blood cells per day. Hematopoiesis is regulated by complex mechanisms that affect all of the individual cell types. These regulatory mechanisms ensure steady-state levels of the various blood cells, yet they have enough built-in flexibility so that production of blood cells can rapidly increase tenfold to twentyfold in response to hemorrhage or infection. Steady-state regulation of hematopoiesis is accomplished in various ways, which include: ■ Control of the levels and types of cytokines produced by bone-marrow stromal cells ■ The production of cytokines with hematopoietic activity by other cell types, such as activated T cells and macrophages ■ The regulation of the expression of receptors for hematopoietically active cytokines in stem cells and progenitor cells ■ The removal of some cells by the controlled induction of cell death A failure in one or a combination of these regulatory mechanisms can have serious consequences. For example, abnormalities in the expression of hematopoietic cytokines or their receptors could lead to unregulated cellular proliferation and may contribute to the development of some leukemias. Ultimately, the number of cells in any hematopoietic lineage is set by a balance between the number of cells removed by cell death and the number that arise from division and differentiation. Any one or a combination of regulatory factors can affect rates of cell reproduction and differentiation. These factors can also determine whether a hematopoietic cell is induced to die. Programmed Cell Death Is an Essential Homeostatic Mechanism Programmed cell death, an induced and ordered process in which the cell actively participates in bringing about its own demise, is a critical factor in the homeostatic regulation of TABLE 2-1 Some transcription factors essential for hematopoietic lineages Factor Dependent lineage GATA-1 Erythroid GATA-2 Erythroid, myeloid, lymphoid PU.1 Erythroid (maturational stages), myeloid (later stages), lymphoid BM11 Myeloid, lymphoid Ikaros Lymphoid Oct-2 B lymphoid (differentiation of B cells into plasma cells) 8536d_ch02_024-056 8/5/02 4:02 PM Page 27 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_ch02_024-0569/6/02 9: 00 PM Page 28 macas Mac 85: 365_smm pldsby et al./ Immunology Se mema su stem popuauons, Incuding those of tne leasing t creseosand possiBly triggering a damaging in Cells undergoing programmed cell death often exhibit Each of the leukocytes produced by hematopoiesis has distinctive morphologic changes, collectively referred to characteristic life span and then dies by programmed ce as apoptosis(Figures 2-3, 2-4). These changes include a death. In the adult human, for example, there are about pronounced decrease in cell volume, modification of the cy- 5 X 10 neutrophils in the circulation. These cells have a toskeleton that results in membrane blebbing, a condensa- life span of only a few days before programmed cell death tion of the chromatin, and degradation of the DNA into is initiated. This death, along with constant neutrophil smaller fragments. Following these morphologic changes, an production, maintains a stable number of these cells. If apoptotic cell sheds tiny membrane-bounded apoptotic bod- programmed cell death fails to occur, a leukemic state may s containing intact organelles. Macrophages quickly phago- develop Programmed cell death also plays a role in main cytose apoptotic bodies and cells in the advanced stages of taining proper numbers of hematopoietic progenitor cell apoptosis. This ensures that their intracellular contents, in- For example, when colony-stimulating factors are re- cluding proteolytic and other lytic enzymes, cationic pro- moved, progenitor cells undergo apoptosis. Beyond teins, and oxidizing molecules are not released into the hematopoiesis, apoptosis is important in such immuno- urrounding tissue. In this way, apoptosis does not induce a logical processes as tolerance and the killing of target cells local inflammatory response. Apoptosis differs markedly by cytotoxic T cells or natural killer cells. Details of th from necrosis, the changes associated with cell death arising mechanisms underlying apoptosis are emerging; Chapte from injury. In necrosis the injured cell swells and bursts, re- 13 describes them in detail NECROSIS APOPTOSIS Chromatin clumping Mild convolution tion Flocculent mitochondria nd segregation Condensation Blebbing Apoptotic Disintegration Phagocytosis Release of intracellular Phagocytic nflammation FIGURE 2-3 Comparison of morphologic changes that occur in tory response. In contrast, necrosis, the process that leads to death poptosis and necrosis. Apoptosis, which results in the programmed of injured cells, results in release of the cells'contents, which may in- ell death of hematopoietic cells, does not induce a local inflamma- duce a local inflammatory response Gotowww.whfreeman.com/immun② Cell Death
28 PART I Introduction many types of cell populations, including those of the hematopoietic system. Cells undergoing programmed cell death often exhibit distinctive morphologic changes, collectively referred to as apoptosis (Figures 2-3, 2-4). These changes include a pronounced decrease in cell volume, modification of the cytoskeleton that results in membrane blebbing, a condensation of the chromatin, and degradation of the DNA into smaller fragments. Following these morphologic changes, an apoptotic cell sheds tiny membrane-bounded apoptotic bodies containing intact organelles. Macrophages quickly phagocytose apoptotic bodies and cells in the advanced stages of apoptosis. This ensures that their intracellular contents, including proteolytic and other lytic enzymes, cationic proteins, and oxidizing molecules are not released into the surrounding tissue. In this way, apoptosis does not induce a local inflammatory response. Apoptosis differs markedly from necrosis, the changes associated with cell death arising from injury. In necrosis the injured cell swells and bursts, releasing its contents and possibly triggering a damaging inflammatory response. Each of the leukocytes produced by hematopoiesis has a characteristic life span and then dies by programmed cell death. In the adult human, for example, there are about 5 1010 neutrophils in the circulation. These cells have a life span of only a few days before programmed cell death is initiated. This death, along with constant neutrophil production, maintains a stable number of these cells. If programmed cell death fails to occur, a leukemic state may develop. Programmed cell death also plays a role in maintaining proper numbers of hematopoietic progenitor cells. For example, when colony-stimulating factors are removed, progenitor cells undergo apoptosis. Beyond hematopoiesis, apoptosis is important in such immunological processes as tolerance and the killing of target cells by cytotoxic T cells or natural killer cells. Details of the mechanisms underlying apoptosis are emerging; Chapter 13 describes them in detail. NECROSIS APOPTOSIS Chromatin clumping Swollen organelles Flocculent mitochondria Mild convolution Chromatin compaction and segregation Condensation of cytoplasm Nuclear fragmentation Blebbing Apoptotic bodies Phagocytosis Phagocytic cell Apoptotic body Disintegration Release of intracellular contents Inflammation FIGURE 2-3 Comparison of morphologic changes that occur in apoptosis and necrosis. Apoptosis, which results in the programmed cell death of hematopoietic cells, does not induce a local inflammatory response. In contrast, necrosis, the process that leads to death of injured cells, results in release of the cells’ contents, which may induce a local inflammatory response. Go to www.whfreeman.com/immunology Animation Cell Death 8536d_ch02_024-056 9/6/02 9:00 PM Page 28 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e:
8536d_cho2024-056 8/5/02 4: 02 PM Page 29 mac79 Mac 79: 45_BW: Go dsby et al./Immunology se Cells and Organs of the Immune System CHAPTER 2 FIGURE 2-4 Apoptosis. Light micrographs of (a)normal t apoptotic thymocytes. [From B. A Osbome and S Smith, 1997, Jour- cytes( developing T cells in the thymus) and(b)apoptotic nal of NIH Research 9: 35: courtesy B. A. Osborne, University of Mass- cytes. Scanning electron micrographs of (c) normal and (d) achusetts at Amherst. J The expression of several genes accompanies apoptosis scriptional activation of the bcl-2 gene and overproduction in leukocytes and other cell types(Table 2-2). Some of the of the encoded Bcl-2 protein by the lymphoma cells. The proteins specified by these genes induce apoptosis, others resulting high levels of Bcl-2 are thought to help transform are critical during apoptosis, and still others inhibit apop- lymphoid cells into cancerous lymphoma cells by inhibit- tosis. For example, apoptosis can be induced in thymocytes ing the signals that would normally induce apoptotic cell by radiation, but only if the protein p53 is present; many death cell deaths are induced by signals from Fas, a molecule<( gulating the normal life span of various hematopoietic cell Bcl-2 levels have been found to play an important role in sent on the surface of many cells; and proteases known as caspases take part in a cascade of reactions that lead to lineages, including lymphocytes. A normal adult has about apoptosis. On the other hand, members of the bcl-2(B-cell 5 L of blood with about 2000 lymphocytes/mm for a total of lymphoma 2)family of genes, bcl-2 and bcl-XL encode pro- about 10 lymphocytes. During acute infection, the lym tein products that inhibit apoptosis. Interestingly, the first phocyte count increases 4-to 15-fold, giving a total lympho member of this gene family, bcl-2, was found in studies that cyte count of 40-50 X 10. Because the immune system rere concerned not with cell death but with the uncon- cannot sustain such a massive increase in cell numbers for an trolled proliferation of B cells in a type of cancer known as extended period, the system needs a means to eliminate un- B-lymphoma. In this case, the bcl-2 gene was at the break needed activated lymphocytes once the antigenic threat has point of a chromosomal translocation in a human B-cell passed. Activated lymphocytes have been found to express lymphoma. The translocation moved the bcl-2 gene into lower levels of Bcl-2 and therefore are more susceptible to the the immunoglobulin heavy-chain locus, resulting in tran- induction of apoptotic death than are naive lymphocytes or
Cells and Organs of the Immune System CHAPTER 2 29 The expression of several genes accompanies apoptosis in leukocytes and other cell types (Table 2-2). Some of the proteins specified by these genes induce apoptosis, others are critical during apoptosis, and still others inhibit apoptosis. For example, apoptosis can be induced in thymocytes by radiation, but only if the protein p53 is present; many cell deaths are induced by signals from Fas, a molecule present on the surface of many cells; and proteases known as caspases take part in a cascade of reactions that lead to apoptosis. On the other hand, members of the bcl-2 (B-cell lymphoma 2) family of genes, bcl-2 and bcl-XL encode protein products that inhibit apoptosis. Interestingly, the first member of this gene family, bcl-2, was found in studies that were concerned not with cell death but with the uncontrolled proliferation of B cells in a type of cancer known as B-lymphoma. In this case, the bcl-2 gene was at the breakpoint of a chromosomal translocation in a human B-cell lymphoma. The translocation moved the bcl-2 gene into the immunoglobulin heavy-chain locus, resulting in transcriptional activation of the bcl-2 gene and overproduction of the encoded Bcl-2 protein by the lymphoma cells. The resulting high levels of Bcl-2 are thought to help transform lymphoid cells into cancerous lymphoma cells by inhibiting the signals that would normally induce apoptotic cell death. Bcl-2 levels have been found to play an important role in regulating the normal life span of various hematopoietic cell lineages, including lymphocytes. A normal adult has about 5 L of blood with about 2000 lymphocytes/mm3 for a total of about 1010 lymphocytes. During acute infection, the lymphocyte count increases 4- to 15-fold, giving a total lymphocyte count of 40–50 109 . Because the immune system cannot sustain such a massive increase in cell numbers for an extended period, the system needs a means to eliminate unneeded activated lymphocytes once the antigenic threat has passed. Activated lymphocytes have been found to express lower levels of Bcl-2 and therefore are more susceptible to the induction of apoptotic death than are naive lymphocytes or (a) (c) (b) (d) FIGURE 2-4 Apoptosis. Light micrographs of (a) normal thymocytes (developing T cells in the thymus) and (b) apoptotic thymocytes. Scanning electron micrographs of (c) normal and (d) apoptotic thymocytes. [From B. A. Osborne and S. Smith, 1997, Journal of NIH Research 9:35; courtesy B. A. Osborne, University of Massachusetts at Amherst.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 29 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_ch02_024-0568/5/02 4:02 PM Page 30 mac79 Mac 79: 45_BW: Godsby et al./Immunology 5e 30 PaRT I Introduction TABLE 2-2 Genes that regulate apoptosis Hematopoietic Stem Cells Can Be Enriched L. L. Weissman and colleagues developed a novel way of en Gene Function Role in apoptosis riching the concentration of mouse hematopoietic stem cells bc- Prevents apoptosi Inhibits which normally cor e less than 0.05% of all bone marrow cells in mice. Their approach relied on the use of an- Opposes bcl-2 Promotes tibodies specific for molecules known as differentiation bcl-XL(bck-Long) Prevents apoptosi Inhibits antigens, which are expressed only by particular cell types bcl-Xs(bcl-Short) Opposes bcl-X, They exposed bone-marrow samples to antibodies that had Protease Promotes been labeled with a fluorescent compound and were specific different ones) for the differentiation antigens expressed on the surface of fa duces apoptosis Initiates mature red and white blood cells( Figure 2-6). The labeled cells were then removed by flow cytometry with a luorescence- activated cell sorter(see Chapter 6). After each sorting, the remain ing cells were assayed to determine the number needed for memory cells. However, if the lymphocytes continue to be restoration of hematopoiesis in a lethally x-irradiated mouse activated by antigen, then the signals received during activa- As the pluripotent stem cells were becoming relatively more tion block the apoptotic signal. As antigen levels subside, so numerous in the remaining population, fewer and fewer does activation of the block and the lymphocytes begin to die cells were needed to restore hematopoiesis in this system. by apoptosis(Figure 2-5) Because stem cells do not express differentiation antigens Antigen Cytokine Tu cell l Bcl2 Activated b cell Cessation of, or inappropriate, Continued activating signals (e.g,cytokines, TH cells, antigen) Apoptotic cell Plasma cell B memory cell FIGURE2-5Regulation of activated B-cell numbers by apoptosis. making activated B cells more susceptible to programmed cell death Activation of B cells induces increased expression of cytokine recep- than either naive or memory B cells. A reduction in activating signals tors and decreased expression of Bcl-2. Because Bcl-2 prevents apop. quickly leads to destruction of excess activated B cells by apoptosis tosis, its reduced level in activated B cells is an important factor in Similar processes occur in T cells
30 PART I Introduction memory cells. However, if the lymphocytes continue to be activated by antigen, then the signals received during activation block the apoptotic signal. As antigen levels subside, so does activation of the block and the lymphocytes begin to die by apoptosis (Figure 2-5). Hematopoietic Stem Cells Can Be Enriched I. L. Weissman and colleagues developed a novel way of enriching the concentration of mouse hematopoietic stem cells, which normally constitute less than 0.05% of all bonemarrow cells in mice. Their approach relied on the use of antibodies specific for molecules known as differentiation antigens, which are expressed only by particular cell types. They exposed bone-marrow samples to antibodies that had been labeled with a fluorescent compound and were specific for the differentiation antigens expressed on the surface of mature red and white blood cells (Figure 2-6). The labeled cells were then removed by flow cytometry with a fluorescenceactivated cell sorter (see Chapter 6).After each sorting,the remaining cells were assayed to determine the number needed for restoration of hematopoiesis in a lethally x-irradiated mouse. As the pluripotent stem cells were becoming relatively more numerous in the remaining population, fewer and fewer cells were needed to restore hematopoiesis in this system. Because stem cells do not express differentiation antigens TABLE 2-2 Genes that regulate apoptosis Gene Function Role in apoptosis bcl-2 Prevents apoptosis Inhibits bax Opposes bcl-2 Promotes bcl-XL (bcl-Long) Prevents apoptosis Inhibits bcl-XS (bcl-Short) Opposes bcl-XL Promotes caspase (several Protease Promotes different ones) fas Induces apoptosis Initiates FIGURE 2-5 Regulation of activated B-cell numbers by apoptosis. Activation of B cells induces increased expression of cytokine receptors and decreased expression of Bcl-2. Because Bcl-2 prevents apoptosis, its reduced level in activated B cells is an important factor in B cell TH cell Antigen Cytokine receptor ↓ Bcl-2 ↑ Cytokine receptors Cessation of, or inappropriate, activating signals Continued activating signals (e.g., cytokines, TH cells, antigen) Plasma cell B memory cell Activated B cell Apoptotic cell Cytokines making activated B cells more susceptible to programmed cell death than either naive or memory B cells. A reduction in activating signals quickly leads to destruction of excess activated B cells by apoptosis. Similar processes occur in T cells. 8536d_ch02_024-056 8/5/02 4:02 PM Page 30 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_cho2024-056 8/5/02 4: 02 PM Page 31 mac79 Mac 79: 45_BW: Go dsby et al./Immunology se Cells and Organs of the Immune System CHAPTER 2 b) Lethally irradiated mouse (950 rads) 100 enriched cells ully enriched cells enriched cells Unenriched Restore hematopoiesis ANtibodies ouse lives to differentiation 10210310 Number of cells injected into lethally irradiated mouse 1×103 partly enriched cells ko o Differentiated React FIant bodies against Sca-1 FIGURE 2-6 Enrichment of the pluripotent stem cells from bone narrow.(a) Differentiated hematopoietic cells(white) are removed by treatment with fluorescently labeled antibodies( Fl-antibodies) specific for membrane molecules expressed on differentiated lin- ages but absent from the undifferentiated stem cells(S)and prog enitor cells(P). Treatment of the resulting partly enriched preparation fully enriched cells with antibody specific for Sca-1, an early differentiation antigen, re- moved most of the progenitor cells. M= monocyte; B= basopl N= neutrophil; Eo eosinophil; L= lymphocyte; E erythrocyte (b)Enrichment of stem-cell preparations is measured by their ability to restore hematopoiesis in lethally irradiated mice. Only animals in which hematopoiesis occurs survive. Progressive enrichment of cell stem cells is indicated by the decrease in the number of injected cells ore hematopoiesis needed to restore hematopoiesis. a total enrichment of about 1000- se lives fold is possible by this procedure known to be on developing and mature hematopoietic more than 10"nonenriched bone-marrow cells were needed cells, by removing those hematopoietic cells that express for restoration. Using a variation of this approach, H known differentiation antigens, these investigators were able Nakauchi and his colleagues have devised procedures that al to obtain a 50-to 200-fold enrichment of pluripotent stem low them to show that, in l out of 5 lethally irradiated mice, maining cells were incubated with various antibodies raised lymphoid lineages(Table 2-3/ sive rise to both myeloid and against cells likely to be in the early stages of hematopoiesis It has been found that CD34, a marker found on about One of these antibodies recognized a differentiation antigen of hematopoietic cells, while not actually unique to stem called stem-cell antigen 1 ( Sca-1). Treatment with this anti lls, is found on a small population of cells that contains body aided capture of undifferentiated stem cells and yielded stem cells. By exploiting the association of this marker wit a preparation so enriched in pluripotent stem cells that an stem cell populations, it has become possible to routinely quot containing only 30-100 cells routinely restored rich preparations of human stem cells. The administration of matopoiesis in a lethally x-irradiated mouse, whereas human-cell populations suitably enriched for CD34 cells
Cells and Organs of the Immune System CHAPTER 2 31 known to be on developing and mature hematopoietic cells, by removing those hematopoietic cells that express known differentiation antigens, these investigators were able to obtain a 50- to 200-fold enrichment of pluripotent stem cells. To further enrich the pluripotent stem cells, the remaining cells were incubated with various antibodies raised against cells likely to be in the early stages of hematopoiesis. One of these antibodies recognized a differentiation antigen called stem-cell antigen 1 (Sca-1). Treatment with this antibody aided capture of undifferentiated stem cells and yielded a preparation so enriched in pluripotent stem cells that an aliquot containing only 30–100 cells routinely restored hematopoiesis in a lethally x-irradiated mouse, whereas more than 104 nonenriched bone-marrow cells were needed for restoration. Using a variation of this approach, H. Nakauchi and his colleagues have devised procedures that allow them to show that, in 1 out of 5 lethally irradiated mice, a single hematopoietic cell can give rise to both myeloid and lymphoid lineages (Table 2-3). It has been found that CD34, a marker found on about 1% of hematopoietic cells, while not actually unique to stem cells, is found on a small population of cells that contains stem cells. By exploiting the association of this marker with stem cell populations, it has become possible to routinely enrich preparations of human stem cells. The administration of human-cell populations suitably enriched for CD34 cells Restore hematopoiesis, mouse lives E Eo L P L B E N Differentiated cells M N P S P React with Fl-antibodies against Sca-1 Lethally irradiated mouse (950 rads) Restore hematopoiesis, mouse lives 2 × 105 unenriched cells 1 × 103 partly enriched cells 30–100 fully enriched cells (a) E Eo L P L B E N M N P P S React with Fl-antibodies to differentiation antigens S P P Stem cell Progenitor cells P Restore hematopoiesis, mouse lives Survival rate, % 100 101 102 103 104 105 Number of cells injected into lethally irradiated mouse Fully enriched cells Partly enriched cells Unenriched cells (b) FIGURE 2-6 Enrichment of the pluripotent stem cells from bone marrow. (a) Differentiated hematopoietic cells (white) are removed by treatment with fluorescently labeled antibodies (Fl-antibodies) specific for membrane molecules expressed on differentiated lineages but absent from the undifferentiated stem cells (S) and progenitor cells (P). Treatment of the resulting partly enriched preparation with antibody specific for Sca-1, an early differentiation antigen, removed most of the progenitor cells. M = monocyte; B = basophil; N = neutrophil; Eo = eosinophil; L = lymphocyte; E = erythrocyte. (b) Enrichment of stem-cell preparations is measured by their ability to restore hematopoiesis in lethally irradiated mice. Only animals in which hematopoiesis occurs survive. Progressive enrichment of stem cells is indicated by the decrease in the number of injected cells needed to restore hematopoiesis. A total enrichment of about 1000- fold is possible by this procedure. 8536d_ch02_024-056 8/5/02 4:02 PM Page 31 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_ch02_024-0569/6/02 9: 00 PM Page 32 maca Mac 85: 365_smm pldsby et al./ Immunology Se y TABLE 2.3 Reconstitution of hematopoeisis tant roles, engulfing and destroying microorganisms, pre- by HSCs senting antigens, and secreting cytokines. Number of Number of mice hoid Cell enriched HSCs reconstituted(%) Lymphocytes constitute 20%-40% of the body s white blood 9of41(21.9% cells and 99% of the cells in the lymph(Table 2-4). There are pproximately 10(range depending on body size and age 100-102)lymphocytes in the human body. These lym- 9of17(529% phocytes continually circulate in the blood and lymph and 10of11(90.9%) re capable of migrating into the tissue spaces and lymphoid 4of4(100%) organs, thereby integrating the immune system to a hig SOURCE: Adapted from M. Osawa, et al. 1996. Science 273: 242. The lymphocytes can be broadly subdivided into three populations--B cells, T cells, and natural killer cells--on the basis of function and cell-membrane components. Natural killer cells(NK cells) are large, granular lymphocytes that do he+"indicates that the factor is present on the cell mem- not express the set of surface markers typical of B or T cells brane)can reconstitute a patients entire hematopoietic sys- Resting B and T lymphocytes are small, motile, nonphago tem (see Clinical Focus) cytic cells, which cannot be distinguished morphologically. B A major tool in studies to identify and characterize the and T lymphocytes that have not interacted with antigen- human hematopoietic stem cell is the use of SCID(severe referred to as naive, or unprimed--are resting cells in the G ombined immunodeficiency)mice as in vivo assay systems phase of the cell cycle. Known as small lymphocytes, these for the presence and function of HSCs. SCID mice do not cells are only about 6 um in diameter; their cytoplasm forms have B and T lymphocytes and are unable to mount adaptive a barely discernible rim around the nucleus. Small lympho- immune responses such as those that act in the normal rejec ytes have densely packed chromatin, few mitochondria, and tion of foreign cells, tissues, and organs. Consequently, these a poorly developed endoplasmic reticulum and Golgi appa- animals do not reject transplanted human cell populations ratus. The naive lymphocyte is generally thought to have a containing HSCs or tissues such as thymus and bone mar- short life span. Interaction of small lymphocytes with anti ow. It is necessary to use immunodeficient mice as surrogate gen, in the presence of certain cytokines discussed later, in- or alternative hosts in human stem-cell research because duces these cells to enter the cell cycle by progressing from go there is no human equivalent of the irradiated ScId into G and subsequently into S, G2, and M(Figure 2-7a).As nice implanted with fragments of human thymus and bone they progress through the cell cycle, lymphocytes enlarge arrow support the differentiation of human hematopoietic into 15 um-diameter blast cells, called lymphoblasts; these tem cells into mature hematopoietic cells. Different subpop- cells have a higher cytoplasm: nucleus ratio and more or- ulations of CD34* human bone-marrow cells are injected ganellar complexity than small lymphocytes(Figure 2-7b) into these SCID-human mice, and the development of vari- Lymphoblasts proliferate and eventually differentiate into ous lineages of human cells in the bone-marrow fragment is effector cells or into memory cells. Effector cells function in bsequently assessed In the absence of human growth fac- various ways to eliminate antigen. These cells have short life tors,only low numbers of granulocyte-macrophage progen erythropoietin and others are administered along with LTABLE 2.4 Normal adult blood-cell counts CD34 cells, progenitor and mature cells of the myeloid lymphoid, and erythroid lineages develop. This system has Cell type Cells/mm enabled the study of subpopulations of CD34 cells and the Red blood cells 50×105 effect of human growth factors on the differentiation of var- Platelets 25×105 Leukocytes 73×103 Neutrophil Cells of the Immune Systen mphocyte Monocyte 1-6 Lymphocytes are the central cells of the immune system,re- sponsible for adaptive immunity and the immunologic at- tributes of diversity, specificity, memory, and self/nonself Basophil recognition. The other types of white blood cells play impor Gotowww.whfreeman.com/immunology@animation Cells and Organs of the Immune System
32 PART I Introduction (the “” indicates that the factor is present on the cell membrane) can reconstitute a patient’s entire hematopoietic system (see Clinical Focus). A major tool in studies to identify and characterize the human hematopoietic stem cell is the use of SCID (severe combined immunodeficiency) mice as in vivo assay systems for the presence and function of HSCs. SCID mice do not have B and T lymphocytes and are unable to mount adaptive immune responses such as those that act in the normal rejection of foreign cells, tissues, and organs. Consequently, these animals do not reject transplanted human cell populations containing HSCs or tissues such as thymus and bone marrow. It is necessary to use immunodeficient mice as surrogate or alternative hosts in human stem-cell research because there is no human equivalent of the irradiated mouse. SCID mice implanted with fragments of human thymus and bone marrow support the differentiation of human hematopoietic stem cells into mature hematopoietic cells. Different subpopulations of CD34 human bone-marrow cells are injected into these SCID-human mice, and the development of various lineages of human cells in the bone-marrow fragment is subsequently assessed. In the absence of human growth factors, only low numbers of granulocyte-macrophage progenitors develop. However, when appropriate cytokines such as erythropoietin and others are administered along with CD34 cells, progenitor and mature cells of the myeloid, lymphoid, and erythroid lineages develop. This system has enabled the study of subpopulations of CD34 cells and the effect of human growth factors on the differentiation of various hematopoietic lineages. Cells of the Immune System Lymphocytes are the central cells of the immune system, responsible for adaptive immunity and the immunologic attributes of diversity, specificity, memory, and self/nonself recognition. The other types of white blood cells play important roles, engulfing and destroying microorganisms, presenting antigens, and secreting cytokines. Lymphoid Cells Lymphocytes constitute 20%–40% of the body’s white blood cells and 99% of the cells in the lymph (Table 2-4). There are approximately 1011 (range depending on body size and age: ~1010–1012) lymphocytes in the human body. These lymphocytes continually circulate in the blood and lymph and are capable of migrating into the tissue spaces and lymphoid organs, thereby integrating the immune system to a high degree. The lymphocytes can be broadly subdivided into three populations—B cells, T cells, and natural killer cells—on the basis of function and cell-membrane components. Natural killer cells (NK cells) are large, granular lymphocytes that do not express the set of surface markers typical of B or T cells. Resting B and T lymphocytes are small, motile, nonphagocytic cells, which cannot be distinguished morphologically. B and T lymphocytes that have not interacted with antigen— referred to as naive, or unprimed—are resting cells in the G0 phase of the cell cycle. Known as small lymphocytes, these cells are only about 6 m in diameter; their cytoplasm forms a barely discernible rim around the nucleus. Small lymphocytes have densely packed chromatin, few mitochondria, and a poorly developed endoplasmic reticulum and Golgi apparatus. The naive lymphocyte is generally thought to have a short life span. Interaction of small lymphocytes with antigen, in the presence of certain cytokines discussed later, induces these cells to enter the cell cycle by progressing from G0 into G1 and subsequently into S, G2, and M (Figure 2-7a). As they progress through the cell cycle, lymphocytes enlarge into 15 m-diameter blast cells, called lymphoblasts; these cells have a higher cytoplasm:nucleus ratio and more organellar complexity than small lymphocytes (Figure 2-7b). Lymphoblasts proliferate and eventually differentiate into effector cells or into memory cells. Effector cells function in various ways to eliminate antigen. These cells have short life TABLE 2-3 Reconstitution of hematopoeisis by HSCs Number of Number of mice enriched HSCs reconstituted (%) 1 9 of 41 (21.9%) 2 5 of 21 (23.8%) 5 9 of 17 (52.9%) 10 10 of 11 (90.9%) 20 4 of 4 (100%) SOURCE: Adapted from M. Osawa, et al. 1996. Science 273:242. TABLE 2-4 Normal adult blood-cell counts Cell type Cells/mm3 % Red blood cells 5.0 106 Platelets 2.5 105 Leukocytes 7.3 103 Neutrophil 50–70 Lymphocyte 20–40 Monocyte 1–6 Eosinophil 1–3 Basophil 1 Go to www.whfreeman.com/immunology Animation Cells and Organs of the Immune System 8536d_ch02_024-056 9/6/02 9:00 PM Page 32 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e:
8536d_cho2024-056 8/5/02 4:02 PM Page 33 mac79 Mac 79: 45_BW: Go dsby et al./Immunology se Cells and Organs of the Immune System CHAPTER 2 Effector cell Go (i. e, plasma cell) Memory cell G Cycle repeats duces cell cycle entry ell division Small lymphocyte (T or B) Blast cell (T or B) Plasma cell(B) 6 um diameter 15 um diameter m URE2-7Fate of antigen-activated small lymphocytes. (a)a densed chromatin indicative of a resting cell, an enlarged lym- resting (naive or unprimed)lymphocyte resides in the Go phoblast(center) showing decondensed chromatin, and a plasma phase of the cell cycle. At this stage, B and T lymphocytes cannot be cell (night) showing abundant endoplasmic reticulum arranged in stinguished morphologically After antigen activation, a B or T cell concentric circles and a prominent nucleus that has been pushed to enters the cell cycle and enlarges into a lymphoblast, which under- a characteristically eccentric position. The three cells are shown goes several rounds of cell division and, eventually, generates effector different magnifications. [Micrographs courtesy of Dr.J. R. Goodman, cells and memory cells. Shown here are cells of the B-cell lineage. Dept of Pediatrics, University of Califomia at San Francisco. J (b) Electron micrographs of a small lymphocyte(left) showing con-
Cells and Organs of the Immune System CHAPTER 2 33 Lymphoblast S (DNA synthesis) Effector cell G0 (i.e., plasma cell) Memory cell G0 Small, naive B lymphocyte G0 Antigen activation induces cell cycle entry Cycle repeats Cell division M G1 (gene activation) (a) (b) Small lymphocyte (T or B) 6 µm diameter Blast cell (T or B) 15 µm diameter Plasma cell (B) 15 µm diameter G2 FIGURE 2-7 Fate of antigen-activated small lymphocytes. (a) A small resting (naive or unprimed) lymphocyte resides in the G0 phase of the cell cycle. At this stage, B and T lymphocytes cannot be distinguished morphologically. After antigen activation, a B or T cell enters the cell cycle and enlarges into a lymphoblast, which undergoes several rounds of cell division and, eventually, generates effector cells and memory cells. Shown here are cells of the B-cell lineage. (b) Electron micrographs of a small lymphocyte (left) showing condensed chromatin indicative of a resting cell, an enlarged lymphoblast (center) showing decondensed chromatin, and a plasma cell (right) showing abundant endoplasmic reticulum arranged in concentric circles and a prominent nucleus that has been pushed to a characteristically eccentric position. The three cells are shown at different magnifications. [Micrographs courtesy of Dr. J. R. Goodman, Dept. of Pediatrics, University of California at San Francisco.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 33 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
