Cells, Tissues, & Organs of The Immune System Pin Ling ( 凌 斌 ), Ph.D. ext 5632; References: 1. Abbas, A, K. et.al, Cellular and Molecular Immunology (6th ed., 2007), Chapter 3 2. Male D., J. Brostoff, D. B Roth, and I. Roitt Immunology (7th ed., 2006), Chapter 2

Questions 1. Why is it necessary to vaccinate against tetanus only every 10 years, though antibodies against the toxoid disappear from the circulation within a year? 2. Why is the vaccine against tetanus always effective, whereas the vaccine against influenza protects on some epidemics but not others? => Memory lymphocytes are still existed and reactivated instantly. => Toxoid is more stable but influenza changes frequently.

Outline Cells of the Immune SystemCells of the Immune System Anatomy & Functions of Lymphoid Tissues & Organs Pathways of Lymphocyte Activation, Recirculation & Homing Summary & Question

Key concepts about immune cells 1. The principle cells of the immune system: Antigen-presenting cells  Lymphocytes => Effector cells 2. All immune cells are derived from “Hematopoietic stem cells” in Bone Marrow (BM) (& Fetal liver during fetus). 3. Immune cells are divided into two major lineages: => Lymphoid & Myeloid => Multiple cell types => express distinct “Surface molecules (markers)” => Classification 4. Development and differentiation of different cell types depend on “Cell Interactions and Cytokines”.

Figure 1-3 Cells of the Immune System-I 1. Originated from Bone Marrow (BM) 2. Two major lineages: - Lymphoid - Myeloid 3. NK cells - Large Granular Lymphocytes (LGL) NK => LGL

Cells of the Immune System-II T lymphocytes are first derived from BM and further developed in Thymus for maturation.

Cells of Adaptive immunity 1. Lymphocytes are the key players of the adaptive immunity. 2. Lymphocytes are able to specifically recognize and respond to diverse antigens. => Antigen (Ag) receptors on lymphocytes (T & B cells) => Specificity (Clone), Diversity (Repertoire: total number of Ag-specific lymphocytes in an individual; ), & Memory 3. Lymphocytes are classified into three major groups: - B cells => Antibodies => Humoral immunity - T cells => Multiple subtypes => Cell-mediated immunity & Regulate other immune cells - NK cells => Innate immunity

Classes of Lymphocytes 1. B cells => Plasma cells => Abs 2. T cells: - T helper cells - T cytotoxic cells - T regulatory cells => suppress immune responses 3. NK cells 4. Subtypes in each class

Identification of cell populations 1. Molecules on or in cells => Identification & Classification => CD (cell differentiation) molecules 2. Methods: (1) Immunofluorecence (2) Flow Cytometry & Sorting (3) ELISPOT

Lymphocytes Small lymphocyte Large lymphocyte 1. Small lymphocytes (T & B) are at the resting state (No Ag stimulation) - Large nucleus w/o active organelles - Die after 1-3 months w/o Ag recognition 2. Large lymphocytes are activated by specific Ags - More cytoplasm & active organelles - Become “Effector Lymphocytes” to secret Abs (Plasma cells), Cytokines, Enzymes => Most of them die shortly - Some become long-lasting “Memory Lymphocytes” w/ different surface proteins

Plasma Cells-Ab Secretion

NK cells - Appear as Large lymphocytes - Contain many cytoplasmic granules

Cells of innate immunity 1. Innate immune cells are derived from Bone Marrow. 2. Their primary functions are (1) to identify and kill microbes, and (2) Antigen-presenting cells (APCs) additionally function to present Ag to and activate lymphocytes. 3. Innate immune cells recognize the common microbial structures PAMPs through Pattern-Recognition Receptors (PRRs) -limited diversity.

Interaction between innate and & adaptive immunity 1. Innate immunity => Ag presentation (by Dendritic cells) 2. Adaptive immunity => Ag recognition (by T & B lymphocytes)

Innate immune cells are classified as following: - Monocyte/Macrophage - Dendritic cell (DC) - Polymorphonuclear granulocyte (PMN; Neutrophil, Eosinophil, Basophil) - Mast cell - NK cells (lymphocyte) => Killing virus-infected cells & tumors Classes of innate immune cells (most abundant)

Maturation of Macrophages Activated Macro Monocyte

Dendritic cells (DCs) as Ag-presenting cells (APCs) DCs link the innate & adaptive immune systems. FDC:Follicular DC

Dendritic cells as Ag-presenting cells-II

Outline Cells of the Immune System Anatomy & Functions of Lymphoid Tissues & OrgansAnatomy & Functions of Lymphoid Tissues & Organs Pathways of Lymphocyte Activation, Recirculation & Homing Summary & Question

Primary lymphoid organs: BM & Thymus => Lymphocyte development, selection, & maturation. 2nd lymphoid organs: Lymphoid nodes & Spleen => Lymphocyte activation & effector functions BM  Other immune cells e.g., DCs, Macrophages, PMNs

The Lymphatic System Link to the blood circulation system

Bone Marrow 1. The site of generation of all immune and blood cells <= Hematopoietic Stem Cell 2. Provides Cell-cell interactions and Cytokines for the development of all immune cells. <= Stromal reticular cells & other cells adipocytes Bony trabeculae

B cell development in the Bone Marrow

Thymus-I 1. The site of T cell maturation => Thymus-dependent (T) lymphocytes or T cells => Thymocytes: developing T cells in thymus 2. Upper anterior thorax (above the heart). 3. Multiple lobules => Each has Outer Cortex => Dense T cells Inner Medulla => Sparse T cells Other cells: epithelial cells, DCs, Macrophages => Cell-cell interactions and Cytokines

Thymus-II Positive selection:  Self MHC Negative selection:  Self-Ag (Autoantigen)

2nd Lymphoid Organ-Lymph Node (LN) LNs => Small nodular organ => Body => Lymphocyte activation Lymphocytes => Segregated in the distinct regions of LN => Chemokines The outer cortex => B cell zone (follicle) => Germinal Center (2nd follicle)=>B cell activation The inner region => T cell zone + DCs

Lymph Node (LN)-II C => Cortex => B cell zone P => Paracortex => T cell zone M => Medulla

Spleen 1. The site of immune responses to blood Ags => A filter of blood 2. White pulp => T cell & B cell zones Marginal zone (MZ) Red pulp (RP) 3. T cells => periarteriolar lymphoid sheaths B cells => follicle => marginal zone

Outline Cells of the Immune System Anatomy & Functions of Lymphoid Tissues & Organs Mechanisms of Lymphocyte Activation, Recirculation & HomingMechanisms of Lymphocyte Activation, Recirculation & Homing Summary & Question

Overview of Lymphocyte Generation & Activation -I

Overview of Lymphocyte Generation & Activation -II

Activation of Lymphocytes In general, lymphocyte activation requires “Two Signals” (from Antigen presenting cells (APCs)) Signal 1: Ag  Ag receptor on lymphocytes (T & B) Signal 2: Molecules (innate response)  Costimulatory receptors on lymphocytes

Migration of Lymphocytes-I 1. In LN, naïve lymphocytes  HEVs Rolling => Adhesion => Transmigration Adhesion molecules for cell-cell interactions 2. In peripheral infection sites, effector lymphocytes  blood vessels

High Endothelial Venules (HEVs)

Migration of Lymphocytes II - Recruitment of Lymphocytes to the infection site

Outline Cells of the Immune System Anatomy & Functions of Lymphoid Tissues & Organs Mechanisms of Lymphocyte Activation, Recirculation & Homing Summary & QuestionSummary & Question

SUMMARY 1. Immune cells are derived from “Bone Marrow (BM)”. T-lineage progenitor cells are first derived from BM and move to “Thymus” for maturation. 2. Key cells in innate immune responses include - Macrophage & DCs => Ag presentation - Granulocytes (PMNs) - NK cells 3. Key cells in Adaptive immune responses include - B lymphocytes => Ab => Extracellular bacteria - Several T cell types => Fight intracellular microbes & Regulate the program of an immune response 4. Primary lymphoid organs (BM & Thymus)=> Immune cell development 2nd lyphoid organs (LN & Spleen)=> Concentrate Ag from tissues or blood for lymphocyte activation.

Question What happened to the host immunity against infection when the thymus is removed (thymectomy)?

Questions 1. Why is it necessary to vaccinate against tetanus only every 10 years, though antibodies against the toxoid disappear from the circulation within a year? 2. Why is the vaccine against tetanus always effective, whereas the vaccine against influenza protects on some occasions but not others? => Memory lymphocytes are still existed and reactivated instantly. => Toxoid is more stable but influenza changes frequently.