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

Outline Cells of the Immune SystemCells of the Immune System Anatomy & Functions of Lymphoid Tissues & Organs 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 educated in Thymus for maturation.

Cells of innate immunity-I 1. Innate immune cells are derived from Bone Marrow. 2. Their primary function is to identify and kill microbes. 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.

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

Development of monocytes & granulocytes 1. Myelopoiesis – Development of myeloid cells. 2. Cytokines from stromal cells, myeloid, & lymphoid cells are involved in this process.

Identification of cell populations 1. Molecules on or in cells => Identification & Classification e.g. CD3, CD4, CD8…. 2. Methods: (1) Immunofluorecence (2) Flow Cytometry & Sorting (3) ELISPOT

Identification & isolation of cell populations

Maturation of Macrophages Activated Macro

Key concepts of Monocytes/Macrophages 1. Relatively long-lived and distributed throughout the whole body. 2. Engulf and kill pathogens by phagocytosis. 3. Express various receptors to recognize different pathogens, e.g. PRR, Scavenger receptor,….etc. 4. Trigger inflammation and Antigen presentation

Phagocytosis by innate immunity

Ag-presenting cells (APCs) link the innate & adaptive immune systems

Polymorphonuclear Granulocytes: Neutrophils 1. Comprise over 95% of granulocytes 2. Short-lived (2-3 days) 3. Multi-lobed nucleus 4. Kill pathogens by phagocytosis & releasing cytotoxic substances.

Basophils & mast cells: critical for immunity against parasites

Eosinophils

Dendritic cells (DCs) as Ag-presenting cells (APCs) FDC:Follicular DC

Key concepts of Dendritic Cells (DCs) 1. Most effective Ag-presenting cells linking innate and adaptive immunity 2. High MHC-II expression on DCs 3. Several types of DCs: (1) Myeloid DCs (also known as conventional DCs) (2) Lymphoid DCs => Plasmacytoid DCs (3) Lymphoid organ-specific DCs: Follicular DCs in Lymph nodes (lacking MHC-II) Interdigitating Dcs in Lymph nodes & Thymus

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 => 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

T cells vs B cells

Lymphocytes Small Lymphocytes Large Granular Lymphocytes

Plasma Cells-Ab Secretion

Outline Cells of the Immune System Anatomy & Functions of Lymphoid Tissues & OrgansAnatomy & Functions of Lymphoid Tissues & Organs 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 Eg. 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

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: thymic epithelial cells (TECs), DCs, Macrophages => Cell-cell interactions and Cytokines

Thymus-II Positive selection Negative selection

T cell differentiation in the Thymus

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

Lymph Node (LN) Section

Lymph Node (LN) Section-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

Mucosa-Associated Lymphoid Organs (MALT)

Outline Cells of the Immune System Anatomy & Functions of Lymphoid Tissues & Organs 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 effect would you expect the thymus removal (thymectomy) to have on the ability of host immunity against infection?