Immune System Cells and Organs

Objectives

1. To describe the anatomy of the immune system, including its organs and cells.

2. To learn the major cellular players in immunology, including their differentiation and maturation pathways.

Keys to an Effective Immune System

• An effective immune system is achieved through a network of cells with specialized functions and very restrictive programs of differentiation and maturation.

The Immune System

• It is a complex system of cells and molecules that interact to maintain homeostasis.

• Its components have redundant features that work together and communicate to generate a complete immune response.

• This response includes both activation and resolution phases.

Sentinel Cells

• Cells present (often resident) within tissues with immunologic function.

• The specific "resident" cells change by location.

• Functions vary by cell types and include:

  • Direct detection and elimination of pathogens.

  • Recruitment of other immune cells and molecules through the release of cytokines, including chemokines.

  • Early polarization of immune responses.

  • Phagocytosis (eating) of pathogens.

  • Antigen presentation.

Circulating/Ready-Made Effector Cells

• Cells present in lymphoid tissues or circulation in the absence of infection.

• Functions vary by cell types and include:

  • Direct detection and elimination of pathogens.

  • Recruitment of other immune cells and molecules through the release of cytokines, including chemokines.

  • Polarization of immune responses.

  • Antigen uptake, such as phagocytosis (eating) and micropinocytosis (sipping).

  • Antigen presentation.

  • Memory.

  • Antibody production.

Hematopoiesis

• Hematopoiesis is the formation and development of both red blood cells (erythrocytes) and white blood cells (leukocytes).

• EVERY leukocyte derives from self-renewing, pluripotent hematopoietic stem cells (HSC).

• There are three lineages/groups of cells made from the HSC:

  1. White blood cells (leukocytes) - 3 groups:

     • Granulocytes: via granulopoiesis (neutrophils, eosinophils, basophils, mast cells).

     • Monocytes: via monopoiesis (monocytes, macrophages, dendritic cells).

     • Lymphocytes: via lymphopoiesis (T cells, B cells, natural killer (NK) cells, ILC, NKT cells).

  2. Red blood cells: via erythropoiesis.

  3. Platelets: via thrombopoiesis.

Sites of Hematopoiesis

• Site according to age:

  • Fetus:

    • 0-2 months: yolk sac.

    • 2-7 months: liver and spleen.

    • 5-9 months: bone marrow.

  • Infants: bone marrow in practically all bones.

  • Adults: vertebrae, ribs, sternum, skull, sacrum, pelvis, and the ends of femurs.

The Bone Marrow

• A primary lymphoid tissue involved in the production of leukocytes.

• Major components of the bone marrow microenvironment (niche):

  • Extracellular matrix: Provides attachment of stem cells and hematopoietic progenitor cells via adhesion molecules.

  • Stromal cells (endothelial, perivascular, nerves, macrophages, and osteoblasts):

    • Offer physical support (scaffold) for the growth of hematopoietic cells.

    • Provide nutrients, produce hematopoietic growth factors, and express adhesion molecules that influence differentiation.

  • Hemopoietic growth factors: Produced by stromal cells and presented to immobilized stem cells.

Leukocyte Differentiation and Development

1. Leukocytes differentiate from hematopoietic stem cells in the bone marrow into two main lineages: myeloid or lymphoid.

2. Within the myeloid lineage are granulocytes (neutrophils, eosinophils, basophils & mast cells) and monocytes (monocytes, macrophages, dendritic cells).

3. T lymphocytes migrate to the thymus for further maturation.

4. Additional differentiation, maturation, and activation-induced differentiation occur in the periphery and in response to stimulation/growth/tissue-resident factors.

How a Developing Cell Knows What to Become

• This process involves tightly-regulated transcription factors that progressively specialize cell types through chromatin modification.

1. Environmental factors trigger transcription factors to turn on or off within cells developing from the HSC; redundancy allows fine-tuning of the system.

2. Important transcription factors to know:

   • Ikaros: Drives differentiation of lymphocytes (shuts down myeloid lineage fate).

   • PU.1: Low levels promote lymphoid lineage; high levels promote myeloid lineage.

3. Maintenance of pluripotency in the HSC is controlled by an array of factors.

Granulopoiesis 

• Key regulatory factors for granulopoiesis:

  • Interleukin (IL)-3

  • Granulocyte-macrophage-colony stimulating factor (GM-CSF)

  • For neutrophils: G-CSF

  • For basophils: IL-4

  • For eosinophils: IL-4

• Maturation characteristics:

  • Nuclear segmentation.

  • Acquisition of primary, then secondary, granules.

• Negative feedback inhibition by mature forms occurs.

• Rate of formation: 1-2 \times 10^9 granulocytes/kg/day.

White Blood Cells - Group 1: Granulocytes (General Features)

• Very early responders to infection (an array of extracellular pathogens, including bacteria and parasites) - immediate: minutes to 48 hours.

• Responses are not antigen-specific, allowing a quick, generic response to quickly contain infection.

• Derived from the myeloid lineage.

• Classified based on morphology and granule staining patterns.

• Release products to recruit other immune system cells, including those capable of more specific responses.

Neutrophils (Polymorphonuclear Leukocyte, PMN)

• Most abundant leukocyte (50-70\% in circulation).

• Circulate for 7-10 hours before migrating into tissues, where they survive for only ~$48$ hours.

• Morphology:

  • Segmented nucleus (most have 3-4 nuclear segments) connected by tapering chromatin strands.

  • Primary granules (larger, denser; contain toxic anti-microbial mediators; formed first).

  • Secondary granules (smaller, contain complement activators and enzymes).

  • Tertiary granules (phosphatases and metalloproteinases).

• Highly responsive to infection - usually first responders, swarming to sites of infection in response to inflammatory molecules called chemokines.

• Increases in circulating PMN (leukocytosis) indicate infection (clinical measurement).

• Functions:

  • Phagocytosis (engulf) bacteria and debris.

  • Release of chromatin to trap microbes - NETosis.

  • Secrete proteins to kill bacteria and signal for tissue remodeling.

  • Assist in shaping the adaptive immune response, when required.

  • Dead neutrophils accumulate as the major cell type in pus.

Neutrophil Function Cont.: Killing of Ingested Bacteria

• 2 major approaches:

  1. Oxygen-dependent (mitochondria):

     • Reactive oxygen intermediates:

       • superoxide anion (O_2^-)

       • hydroxyl radical (OH)

       • hydrogen peroxide (H2O2)

       • hypochlorite anion (ClO^-)

     • Reactive nitrogen intermediates:

       • nitric oxide (NO)

  2. Oxygen-independent (granules):

     • Defensins

     • Lysozyme

     • Hydrolytic enzymes (e.g., collagenase)

     • Tumor necrosis factor

Eosinophils

• Frequency in circulation: 1-3\% of leukocytes.

• Circulate through the blood and into the tissues.

• Especially prevalent in the small intestine.

• Morphology:

  • Segmented nucleus (2 nuclear lobes).

  • Spherical granules (eosinophilic granules).

• Size: approximately 12-17 \mu m in diameter.

• Function:

  • Involved in anti-parasitic function and also allergy and asthma.

  • Release cytokines to instruct adaptive immune responses; coordinate immune responses, especially against multicellular parasites (i.e., worms).

  • Use lysosomal enzymes and oxygen-radicals like neutrophils.

  • Contain an anti-parasite protein called eosinophil cationic protein (ECP).

Basophils

• Frequency in circulation: <1\% of leukocytes.

• Important for killing extracellular parasites, including multicellular worms.

• Morphology:

  • Segmented nucleus (2 nuclear lobes).

  • Spherical purple granules (basophilic granules).

• Size: approximately 12-17 \mu m in diameter.

• Bind circulating antibody/antigen complexes (i.e., tagged pathogens) and release granule contents (i.e., histamines, leukotrienes, prostaglandins, and cytokines).

Mast Cells

• Produced in the bone marrow and released into blood as immature precursors that differentiate upon entering tissues, usually at the interface between the body and the environment (i.e., skin, mucosa).

• Are distinct from basophils, but they have similar functions.

• Characteristically found surrounding blood vessels and nerves.

• At least 2 types: connective tissue mast cells and mucosal mast cells.

• Important for anti-parasite function and involved in allergic reactions.

• Important immune-polarizing cells.

• Like basophils, they release granule contents (i.e., histamines, leukotrienes, prostaglandins, and cytokines).

Monopoiesis

• Key factor: Monocyte-colony stimulating factor (M-CSF).

• Maturation characteristics:

  • Gradual nuclear folding.

  • Acquisition of cytoplasmic granules.

• Stages in development:

  • Monoblast

  • Promonocyte

  • Mature monocyte

• Further differentiation (in tissues) into:

  • Dendritic cells (stimulated by Granulocyte-monocyte colony stimulating factor: GM-CSF, IL-4).

  • Macrophages (stimulated by M-CSF).

White Blood Cells - Group 2: Monocytes & Macrophages

• Sentinel function: To detect infections and signal immune responses.

• Monocytes differentiate into macrophages in tissues.

• Phagocytosis of microorganisms (for immediate and direct immune control).

• Phagocytosis and recycling of apoptotic (dead) cells.

• Killing of ingested microorganisms:

  • Oxygen-dependent.

  • Oxygen-independent.

• Recruitment of immune system cells into inflammatory site by secreting cytokines and chemokines.

• Present antigen to T cells.

Monocyte Differentiation into Macrophages

• Changes involved in differentiation:

  • Increase in size (5-10 fold).

  • Increased numbers and complexity of organelles.

  • Increased phagocytic activity.

  • Increased levels of hydrolytic enzymes.

Macrophages – Types and Functions

• Macrophages are dispersed throughout the body, traveling by amoeboid movement throughout tissues.

• Named according to location:

  • Alveolar: lung.

  • Histiocytes: connective tissues.

  • Kupffer cells: liver.

  • Mesangial cells: kidney.

  • Microglial cells: brain.

  • Osteoclasts: bone.

• Macrophages are activated by:

  • Phagocytosis.

  • Inflammatory Th1 cytokines (e.g., IFN-\gamma).

  • Inflammatory mediators.

  • Bacterial components.

• After activation, macrophages:

  • Exhibit enhanced phagocytic activity.

  • Increased killing ability.

  • Increased secretion of inflammatory mediators.

  • Increased migration.

  • Increased ability to activate T cells via antigen-presentation.

Dendritic Cells

• Many subtypes with different specialized functions and locations.

• Morphology: Spread out with many dendrites (finger-like projections) upon differentiation in tissues.

• Major Functions:

  • Most potent antigen-presenting cell (APC) with ready function.

  • Uniquely, subsets can arise from different lineages (myeloid and lymphoid).

  • Covered with multiple long membrane extensions like dendrites on nerve cells.

  • Take up potential antigens by phagocytosis (large eating), pinocytosis (sipping), and receptor-mediated endocytosis.

  • Reside in the tissues and capture antigens from invading pathogens and load the antigen into MHC II and MHC I molecules.

  • Dendritic cells then migrate to lymph nodes and present these antigens to T cells and co-stimulate T cell activation.

  • Produce cytokines to polarize immune responses.

Lymphopoiesis

• T cells, B cells, NKT cells, NK cells, and innate-like lymphocytes arise from the same stem cell.

• Key hematopoietic growth factors (learn these): IL-3, IL-7, IL-2, IL-4, IL-15.

• Stages of maturation are defined by surface antigen expression (CD antigens) rather than morphologic features.

• B cells mature from their progenitors within the bone marrow.

• NK (natural killer) cells mature from their progenitors and further in the periphery and thymus (one subset).

• T and NKT cells develop in the bone marrow but mature in the thymus.

• Innate lymphoid cells (ILCs) develop in the bone marrow and migrate to tissues; some subsets require thymic input.

White Blood Cells – Group 3: Lymphocytes

• Frequency in circulation: 20-40\% of leukocytes.

• Morphology:

  • Naïve (quiescent) T and B lymphocytes:

    • Round nucleus (size of red cells).

    • Small rim of blue cytoplasm.

    • Size: 9 \mu m in diameter.

  • Large granular lymphocytes; natural killer (NK) cell, cytotoxic T lymphocyte (CTL), plasma cell (antibody secretion):

    • Larger cell.

    • More abundant cytoplasm.

    • Large granules containing perforin and granzyme (not plasma cells).

Lymphocytes

1. T cells:

   • Represent 70-80\% of lymphocytes (CD3+).

   • Further divided into CD4+ and CD8+ T cells.

   • Includes CD4+ helper T and regulatory T cells, and CD8+ CTL.

   • CD4:CD8 ratio is 2:1 in healthy blood.

   • Each exists as naïve, activated, and memory cells based on experience and timing with antigen.

   • Develop in the bone marrow, complete development in the thymus.

   • Express an antigen receptor called the T cell receptor (TCR).

   • CD4+ helper T cells:

     • Considered the "Generals" of the immune response.

     • Help to activate CD8+ T cells, B cells, macrophages, and other immune cells; also regulate immune responses.

     • Function by producing a range of different cytokines.

     • Subsets of helper T cells polarize the type of immune response.

   • CD8+ cytotoxic T cells (CTL):

     • Kill virus-infected cells and cancer cells.

     • Source of cytokines like interferon-\gamma (IFN-\gamma).

   • Regulatory T cells:

     • Mostly CD4+.

     • Control immune responses, generally by regulating T cell reactivity.

2. B cells:

   • Represent 10-20\% of lymphocytes (CD19+).

   • Naïve B cells are antigen-inexperienced and express membrane-bound immunoglobulin (antibodies).

   • Plasma B cells are activated and act as "antibody factories."

   • Memory B cells are maintained after challenge to form memory responses.

   • Develop in the bone marrow.

   • Function as APCs in secondary immune responses.

3. Innate-like lymphocytes:

   • Innate lymphoid cells (ILCs):

     • Subsets (ILC1, 2, and 3) mirror those of CD4+ T cells with polarized function.

     • Primarily tissue-resident.

     • Major function is cytokine production to polarize immune function.

     • Develop primarily in the bone marrow with thymic input for some subsets.

   • NK cells (CD56+):

     • Discriminate "self" from non-self.

     • Secrete granzyme/perforin to kill target cells.

     • Kill via death receptors.

     • Secrete cytokines to polarize immune responses.

   • NKT cells:

     • Express functional T cell receptors that interact with CD1 and conserved glycolipid moieties.

     • Develop in the bone marrow and mature in the thymus.

     • Kill virus-infected and cancer cells.

     • Produce pro-inflammatory cytokines to direct other immune cells.

Summary: Major Leukocyte Cell Types and Their Functions

Leukocyte Abundance in the Blood

Lymphoid Organs and Tissues

• Primary lymphoid tissues: Sites of lymphocyte DEVELOPMENT/MATURATION.

  • The thymus (for T and NKT cells) and bone marrow (for B and ILCs) are the primary (or central) lymphoid organs. This is where the maturation of lymphocytes takes place.

• Secondary lymphoid tissues: Sites of lymphocyte ACTIVATION.

  • Lymph nodes, spleen, and various mucosal-associated lymphoid tissues (MALT), such as gut-associated lymphoid tissue (GALT). This is where antigen is trapped, providing an opportunity for interaction with mature lymphocytes and antigen-dependent maturation of T and B cells.

  • Contain high endothelial venules (HEV) that permit transit of immune cells.

• Tertiary lymphoid tissues: Sites of lymphocyte ACTIVITY.

  • Normally contain fewer lymphoid cells than secondary lymphoid organs but can import lymphoid cells during an inflammatory response; e.g., cutaneous-associated lymphoid tissue (CALT).

Thymus

• Site of T, NKT, and some ILC cell maturation.

• The thymus is a bi-lobed encapsulated organ with lobules, separated by connective tissue strands (trabeculae).

• The outermost compartment (cortex) is densely packed with immature, proliferating thymocytes.

• The inner compartment (medulla) is only sparsely populated with more mature thymocytes (due to selection).

• During maturation, thymocytes interact with cortical epithelial cells, medullary epithelial cells, dendritic cells, and macrophages. This interaction increases T cell maturation.

Lymph Nodes

• Site of generation of T cell and B cell antibody responses to specific antigens.

• Provides a site where lymphocytes can interact with antigens and antigen-presenting cells, especially interdigitating dendritic cells.

• Phagocytosis in the lymph node of particulate matter and microorganisms that enter lymph prevents their entry into the bloodstream.

• Lymph Node Structure:

  • Bean-shaped capsule.

  • Blood supply: Sinuses (subcapsular, cortical, medullary), and blood vessels (arterioles, venules, post capillary venules).

  • Afferent and efferent lymphatic vessels.

  • Parenchyma (functional tissue): Cortex (lymphoid follicles), Paracortex, Medulla (medullary cords).

Lymphocyte Localization Within the Lymph Node

• Cortex:

  • Primary follicles (naïve B cells, follicular dendritic cells, macrophages).

  • Secondary follicles (activated B cells in germinal centers).

• Paracortex: T cells, interdigitating dendritic cells.

• Medulla: Phagocytic macrophages, antibody-secreting plasma cells, some activated/memory T cells and B cells moving into the efferent lymph.

• Reticular network (stroma): Composed of extracellular matrix, reticular fibers, and fibroblastic reticular cells; provides structural support for lymphocyte compartments.

Lymphatic System

• A network of vessels that collect fluid and lymphocytes that have filtered from the capillaries, through tissues, and ultimately return them to the bloodstream.

• In the process, leukocytes and antigens are transported and concentrated in lymph nodes to enable their interactions.

• The fluid component of blood (plasma) that leaks out of capillaries is interstitial fluid.

• Most interstitial fluid returns to the blood through the capillaries, but the remaining fluid, called lymph, is collected by tiny open lymphatic capillaries.

• Lymphatic capillaries join and become progressively larger lymphatic vessels, ultimately draining via the thoracic duct into the left subclavian vein back to the blood to return fluid and activated lymphocytes.

Lymphatic Vessels

• Afferent lymphatic vessels: Bring in lymph fluid containing antigen-carrying dendritic cells, particulate antigen, and a few lymphocytes from tissues to regional lymph nodes.

• Efferent lymphatic vessels: Take lymph fluid away from lymph nodes via the thoracic duct into venous circulation; carry antibodies secreted by plasma cells and activated/memory T cells and B cells, distributing effector cells and antibodies throughout the body to fight off the infection.

Spleen Function and Structure

• A large ovoid organ situated in the upper left quadrant of the abdominal cavity; the spleen is surrounded by a capsule.

• Consists of white pulp and red pulp separated by a marginal zone.

• Overall role: The location where immune responses are mounted against antigens in the blood and where old/defective red blood cells are phagocytosed and recycled.

• Specific components:

  • White pulp: Generation of T cell and B cell responses (antibodies) against blood-borne antigens.

  • Marginal zone: Interdigitating dendritic cells trap blood-borne antigens and transport them to the white pulp.

  • Red pulp: Defective/old red blood cells and blood-borne pathogens are phagocytosed by macrophages.