Lecture 1

Lecture 1

Cells and Organs of the Immune System

Introduction

• The many cells, organs and tissues of the immune system are found throughout the body.

• They are functionally classified into two main groups.1. Primary lymphoid organs: Bone marrow and thymusProvide appropriate microenvironment for the development and

maturation of lymphocytes. 2. Secondary lymphoid organs: Trap antigen, generally from nearby

tissues or vascular spaces and are sites where mature lymphocytes can interact effectively with antigen.

Secondary lymphoid organs include the:lymph nodes, spleen, mucosa-associated lymphoid tissue (MALT) which also include gutassociated lymphoid tissue (GALT) (e.g., Peyer’s patches) and bronchus associated lymphoid tissue (BALT) e.g., tonsils, appendix)

Figure 2-13

Human Lymphatic System

• Consists of lymph glands found in:– Neck– Armpits– Groin

• Lymphatics – small vessels

• Lymph - watery fluid

Introduction

• 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 cells of the immune system.

• Only the antigen-specific lymphocytes posses the attributes of diversity, specificity, memory and self-nonself recognition, the hallmarks of an adaptive immune response.

• Other leukocytes also play important roles, some as antigen presenting cells and others participating as effector cells in the elimination of antigen by phagocytosis or the secretion of 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 coordination and regulation of immune responses.

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, maintaining 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.

• Yolk sac stem cells differentiate into primitive erythroid cells that contain embryonic hemoglobin.

• By the third month of gestation, hematopoietic stem cells have migrated from the yolk sac to the fetal liver and subsequently colinize the spleen; theses 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.

Hematopoiesis

• In bone marrow, hematopoietic cells and their descendants grow, differentiate, and mature on a mesh-like scaffold of stromal cells, which 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.

x40 magnification of bone marrow

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Hematopoiesis is Regulated At the Genetic Level

Hematopoietic HomeostasisInvolves Many Factors

• Hematopoiesis is a steady-state process in which mature blood cells are produced at the same rate at which they are lost.

• The principle cause of blood cell loss is 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 day , for

neutrophils, to as long as 20 to 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.• This massive system is regulated by complex mechanisms that

affect all of the individual cell types, and 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.

Programmed Cell Death is an Essential Homeostatic Mechanism

• Programmed cell death is a critical factor in the homeostatic regulation of 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.

• These changes include: A pronounced decrease in cell volume. Modification of the cytoskeleton, which results in membrane blebbing. A condensation of the of the chromatin and degradation of the DNA into

fragments.• Following these morphologic changes, an apoptotic cell sheds tiny membrane-

bound apoptotic bodies containing intact organelles.• Macrophages 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.

• Apoptosis does not induce a local inflammatory response

Comparison of Morphologic Changes that Occur in Apoptosis & Necrosis

Genes Regulate Apoptosis

• The expression of several genes accompanies apoptosis in leukocytes and other cell types.

• Some of the proteins specified by these genes induce apoptosis, others are critical as apoptosis progresses, and still others inhibit apoptosis.

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Peripheral Blood Cells

Morphology and Staining of Blood Cells

Lymphoid Cells

• Lymphocytes: Constitute 20% to 40% of the bodies white blood cells and 99% of the

cells in the lymph. There are approximately a trillion (1012) lymphocytes in the human

body. Circulate continuously in the blood and lymph and are capable of

migrating into the tissue spaces and lymphoid organs, serving thereby as a bridge between parts of the immune system.

Broadly subdivided into three major populations – B cells, T cells, and natural killer cells – on the basis of function and cell membrane components.

Key cells of adaptive immunity, B cells and T cells each bear their own distinctive antigen receptors.

Natural killer (NK) cells are large granular lymphocytes (granular refers to their grainy appearance under a microscope) that are the part of the innate immune system and do not express the set of surface markers that characterize B or T cells.

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Fate of Antigen-Activated Small Lymphocyte

Natural Killer (NK) Cells

Monocyte & Macrophage

Phagocytosis

Mononuclear Phagocyte System

Granulocytic Cells

The granulocytes are classified as neutrophils, eosinophils, or basophils on the basis of cellular morphology and cytoplasmic-staining characteristics

Different Kinds of Dendritic Cells &

Their Origins

Bacterial Structures that are Recognized by PRRs

• Peptidoglycans in bacterial cell wall • Mannans, bacterial cell surface polysaccharides• Gram negative bacteria (such as E. coli, Pseudomonas, Salmonella etc)

all make Lipopolysaccharide (LPS/endotoxin), which is made up Lipid A and carbohydrates.

• Teichoic acids and lipoteichoic acids, not found in vertebrate cells. Teichoic acids are phosphate linked polymers of ribitol or glycerol

• The N-terminal amino acid of most bacterial proteins is formyl-methionine.

• Bacterial lipoproteins (BLPs) contain a unique N-terminal lipo-amino acid, N-acyl S-diacylglyceryl cysteine.

• Bacterial DNA contains specific unmethylated CpG repeats that are not found in vertebrate DNA

Toll Like Receptors (TLRs)

• PRRs (Pattern Recognition Receptors) that activate phagocytes and DCs

Dendritic cells initiate immune responsesll

• Immature dendritic ces constantly internalize and process proteins, debris, and microbes, when present.

• Binding of microbial components to Toll-Like Receptors (TLRs) activates the maturation of the DC so that it ceases to internalize any new material, moves to the lymph node, up-regulates MHC II, B7 and B7.1 molecules for antigen presentation, and produces cytokines to activate T cells.

• Release of IL-6 inhibits release of TGF B; and IL-10 by T regulatory cells. The cytokines produced by DC and its interaction with TH0 cells initiate immune responses. IL-12 and IL-2 promote TH1 responses while IL-4 promotes TH2 responses. Most of the T cells divide to enlarge the response, but some remain as memory cells.

• Memory cells can be activated by DC, macrophage, or B cell presentation of antigen for a secondary response

Cells of the Immune Response

Cells of the Immune Response

Cells of the Immune Response

Cells of the Immune Response

Cells of the Immune Response

Cells of the Immune Response

*Monocyte/macrophage lineage.APCs, antigen-presenting cells; CNS, central nervous system; DTH, delayed-type hypersensitivity; IFN, interferon; Ig, immunoglobulin; IL, interleukin; LT, lymphotoxin; MHC, major histocompatibility complex; TNF, tumor necrosis factor.

Major Cytokine-Producing Cells

• Innate (acute phase responses) Dendritic cells and macrophages: IL-1, TNF-α, TNF-β, IL-6, IL-12, GM-CSF,

chemokines, interferons α,β.• Immune: T cells (CD4 and CD8)

TH1 cells: IL-2, IL-3, GM-CSF, interferon-γ, TNF-α, TNF-β. TH2 cells: IL-4, IL-5, IL-6, IL-10, IL-3, IL-9, IL-13, GM-CSF, TNF-α.

Thymus

• Derived from the third and fourth pharyngeal pouches during the embryonic life and attracts (by chemoattractive molecules) circulating T- cell precursors derived from HSC in the bone marrow.

• Site of T-cell development and maturation.• Flat, bilobed organ situated behind the sternum, above and in front of the

heart.• Each lobe surrounded by a capsule and divided into lobules, which are

separated from each other by strands of connective tissue called trabeculae.

• Each lobule is organized into two compartments.1. Cortex: The outer compartment, is densely packed with immature T

cells, called thymocytes.2. Medulla: The inner compartment, is sparsely populated with

thymocytes.

Thymus

• Both the cortex and the medulla are criss-crossed by a three dimensional stromal cell network composed of epithelial cells, dentritic cells, and macrophages, which make up the framework of the organ and contribute to the growth and maturation of the thymocytes.

• The accessory cells are important in the differentiation of the immigrating T cell precursors and their education (positive and negative selection), prior to their migration into the secondary lymphoid tissues.

• Thymic epithelial cells produces the hormones thymosin and thymopoietin and in concert with cytokines such as IL-7 are probably important for the development and maturation of thymocytes into mature cells.

• The thymic cortex is the major site of activity and thymocyte proliferation, with a complete turnover of cells approximately every 72 hours.

Thymus• These thymocytes then move into the medulla, where they undergo further

differentiation and selection and finally migrate via circulation to the secondary lymphoid organs/ tissues where they are able to respond to microbial antigens.

• Most (95%) of the thymocytes generated each day in the thymus die by apoptosis with less than 5% surrviving.

• Molecules important to T cell function such as CD4, CD8 and T cell receptor develop at different stages during the differentiation process.

• The main functions of the thymus as a primary lymphoid organ are:1. To produce sufficient numbers (millions) of different T cells each

expressing unique T cell receptors such that, within this group, there are at least some cells potentially specific for huge number of microbial antigens in our environment (generation of diversity).

2. To select for survival those T cells which bind weakly to self MHC molecules (positive selection), but then to eliminate those which bind too strongly to these same self MHC molecules (negative selection) so that the chance for an autoimmune response is minimized.

Changes in the Thymus with Age• Thymic function is known to decline with age.• The thymus reaches its maximal size at puberty and then atrophies, with a

significant decrease in both cortical and medullary cells and an increase in the total fat content of the organ.

• The average weight of the thymus in human infants is 30 grams and only 3 grams in the elderly.

• The age dependent loss in mass is accompanied by a decline in T-cell output.• By age 35, the thymic generation of T cells has dropped to 20% of production

in newborns, and by age 65, the output has fallen to only 2% of the newborn rate.

Thymus

Diagrammatic cross section ofa portion of the thymus, showing several lobules separated by connective tissue strands (trabeculae). The densely populated outer cortex contains many immature thymocytes (blue), which undergo rapid proliferation coupled with an enormous rate of cell death. The medulla is sparsely populated and contains thymocytes that are more mature. During their stay within the thymus, thymocytes interact with various stromal cells, including cortical epithelial cells (light red), medullary epithelial cells (tan), dendritic cells (purple), and macrophages (yellow). These cells produce regulatory factors and express high levels of class I and class II MHC molecules. Hassall’s corpuscles, found in the medulla, contain concentric layers of degenerating epithelial cells. [Adapted with permission from W.van Ewijk, 1991, Annual Review of Immunology 9:591 by Annual Reviews.]

Structure of a Lymph Node

The three layers of a lymph node support distinct microenvironments.

Structure of a Lymph Node

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The left side depicts the arrangement of reticulum and lymphocytes within the various regions of a lymph node. Macrophages and dendritic cells, which trap antigen, are present in the cortex and paracortex. THcells are concentrated in the paracortex; B cells are primarily in the cortex, within follicles and germinal centers. The medulla is populated largely by antibody-producing plasma cells. Lymphocytes circulating in the lymph are carried into the node by afferent lymphatic vessels, they either enter the reticular matrix of the node or pass through it and leave by the efferent lymphatic vessel.

The right side depicts the lymphatic artery and vein and the postcapillary venules. Lymphocytes in the circulation can pass into the node from the postcapillary venules by a process called extravasation (inset)

Structure of the Spleen

The spleen, which is about 5 inches long in the adult, is the largest lymphoid organ. It is specialized for trapping blood-borne antigens.

Diagrammatic cross section of the spleen. The splenic artery pierces the capsule and divides into progressively smaller arterioles, ending in vascular sinusoides that drain back into the splenic vein. The erythrocyte-filled red pulp surrounds the sinusoids. The white pulp forms a sleeve, the periarteriolar lymphoid sheath (PALS), around the arterioles; this sheath contains numerous T cells. Closely associated with PALS is the marginal zone, an area rich in B cells that contains lymphoid follicles that can develop into secondary follicles containing germinal centers.

Mucosa Associated Lymphoid Tissue (MALT)

Cross-sectional diagram of the mucous membrane ling the intestine, showing a Peyer’s patch lymphoid nodule in the submucosa. The intestinal lamina contains loose clusters of lymphoid cells and diffuse follicles.

Mucosa Associated Lymphoid Tissue (MALT)

Antigen transported across the epithelial layer by M cells at an inductive site activates B cells in the underlying lymphoid follicles. The activated B cells differentiate into IgA-producing plasma cells, which migrate along the submucosa. The outer mucosal epithelial layer contains intraepithelial lymphocytes, of which are T cells.

Cutaneous Associated Lymphoid Tissue (CALT)

The skin is the largest organ in the body and plays an important role in nonspecific (innate ) defences. The epidermal (outer) layer of the skin is composed of specialized cells called keratinocytes. These cells secrete a number of cytokines that may function in local inflammatory reaction. Scattered among the epithelial-cell matrix of the epidermis are Langerhann’s cells, atype of dendritic cell, which internalize antigen by phagocytosis or endocytosis. They undergo maturation and migrate from the epidermis to regional lymph nodes, where they function as potent activators of naïve TH cells. In addition to Langerhans cells, the epidermis also contaions so-called intraepidermal lymphocytes, which are mostly T cells. The underlying dermal layer of the skin also contains scattered T cells and macrophages. Most of these dermal cells appear to be either previously activated cells or memory cells.

Bronchus Associated Lymphoid Tissue (BALT)

Summary• The humoral (antibody) and cell mediated responses of the immune system

result from the coordinated activities of many cell types of cells, organs, and tissues found throughout the body.

• Many of the body’s cells, tissues, and organs arise from different stem cell populations. Leukocytes develop from a pluripotent hematopoietic stem cell during a highly regulated process called hematopoiesis.

• Apoptosis, a type of programmed cell death, is a key factor in regulating the levels of hematopoietic and other cell populations.

• There are three types of lymphoid cells: B cells, T cells, and natural killer (NK) cells. Only B and T cells are members of cloned populations distinguished by antigen receptors of unique specificity. B cells synthesize and display membrane antibody, and T cells synthesize and display T-cell receptors. Most NK cells do not synthesize antigen-specific receptors, although a small subpopulation of this group, NK-T cells, do synthesize and display a T cell receptor.

• Macrophages and neutrophils are specialized for the phagocytosis and degradation of antigens. Macrophages also have the capacity to present antigen to T cells.

Summary

• Immature forms of dendritic cells have the capacity to capture antigen in one location, undergo maturatiuon , and migrate to another location, where they present antigen to TH cells. Dendritic cells are the major population of antigen presenting cells.

• Primary lymphoid organs are the sites where lymphocytes develop and mature. T cells arise in the bone marrow and develop in the thymus; in humans and mice, B cells arise and develop in bone marrow.

• Secondary lymphoid organs provide sites where lymphocytes encounter antigen, become activated, and undergo clonal expansion and differentiation into effector cells.

• Vertebrate orders differ greatly in the kinds of lymphoid organs, tissues, and cells they posses. The most primitive, the jawless fish, lack B and T cells and cannot mount adaptive immune responses; jawed vertebrates have T and B cells, have adaptive immunity, and display an increasing variety of lymphoid tissues.