Chapter 17

Antigen-Presenting Cells

Antigen presenting cells (APC) are cells that contain MHC II molecules and present antigens to activate T cells

Antibodies (Ig)

Different isotypes exist which differ in their heavy- chain C regions. *Look at image on slide. All isotypes exist as monomers except IgA (monomers and dimers) and IgM (pentamer) *IgG (main antibody that fights pathogens), IgA (gut antibody)

Principles of Adaptive Immunity

Immunoglobulins and T-cell receptors are highly variable recognition molecules of adaptive immunity Immunoglobulins are expressed on the surface of B cells where they can bind to pathogens. Effector B cells, called plasma cells, secrete a soluble form of immunoglobulins, called antibodies. T-cell receptors are only expressed as cell-surface recognition molecules.

Processing of Antigens by MHC molecules

Look at image on slide

Activation of Naïve CD4 T Cells

Once activated, CD4 T cells can differentiate into one of two main pathways: 1. TH1: cytokines secreted by TH1 T cells lead to macrophage activation, inflammation, and the production of AB that facilitate phagocytosis 2. TH2: mainly activate B cells and the production of neutralizing antibodies • The mechanisms that determine which pathway a CD4 T cell will differentiate are not well understood • Although most adaptive immune responses consist of both a TH1 and TH2 response, it is usually biased towards one or the other. • TH1 biased: cell-mediated immune response dominated by effector cells of the immune system • TH2: humoral immune response dominated by he presence of antibodies

Antigen processing and presentation (continued)

Pathogen-derived proteins must be degraded into peptides and assemble into a peptide: MHC molecule complex, and displayed on the cell's surface

Follicular T cells

See images on slides

Adaptive immune response starts with T-cell activation

The innate immune responses develops locally at the site of infection through: • the action of plasma proteins • the stimulation of macrophages • the recruitment of effector cells • the establishment of a state of infection • If the innate immune systems fails to clear a response, then an extensive process in which pathogen-specific lymphocyte clones are selected, expanded, and differentiated. • All this takes place in the secondary lymphoid tissues over the course of several days • The start of the adaptive immune response begins when antigen-laden or infected dendritic cells migrate from the infected tissue to the draining lymph node and activate T cells.

B cell activation by CD4 TH2 cells

The primary follicle changes its morphology to a secondary follicle which now contains a germinal center (GC)

Different Antibody Functions

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Antibody isotypes

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T cells first encounter with antigen

• A T cell that has never seen antigen is called a naïve T cell • Naïve T cells enter the lymph nodes through the high endothelial venules (HEV) via the blood. The naïve T cells are attracted to the DCs and start scanning them for a peptide:MHC complex to which it's T cell receptor binds to • If the T cell does not find a peptide:MHC complex on a DC, then it leaves the LN through the efferent vessel to recirculate • If the T cell does encounter a peptide:MHC complex, then the T cell is triggered to proliferate and differentiate into effector T cells

Antibodies

• A mature B cell has membrane bound immunoglobulin (Ig) of a single antigen specificity • When a foreign antigen binds to the Ig, the B cell is stimulated to proliferate. • Its progeny differentiate into plasma cells that secrete antibody of the same specificity as the membrane bound Ig The middle of the heavy chain forms a flexible hinge region at which the molecule can be cleaved to produce defined antibody fragment. * check cleavage by papain and pepsin picture

Germinal Centers

• Affinity maturation: Those centrocytes with the highest affinity for antigen are selected for survival and further differentiation into antibody producing plasma cells or long-lived memory B cells. • AB isotype switching also takes place in in GCs, principally from IgM to IgG antibodies • Some plasma cells remain in the LN to produce Abs, others migrate to the BM which becomes the main site of AB production • Memory B cells will recirculate for long periods of time and only require minimal activity to induce a rapid response upon re-exposure to the antigen

Antibodies (continued)

• Antibodies that binds to parts of a molecule that are adjacent in the linear sequences are called linear epitopes • Epitopes that are formed by parts of a protein that are separated in their amino acid sequence, but are brought together in the folded protein, are called discontinuous epitopes •Binding of antibodies is by non-covalent forces (ie hydrogen bonds) • Small differences in antibodies structure can give several different antibodies with specificity for the same epitope, different binding strengths, or affinities.

B cell activation by CD4 TH2 cells

• Antigen-primed B cells then move to the B-T cell border of the lymph node and wait to encounter its cognate-T cell • When B cells interact with their cognate T-cell, they receive signals to undergo proliferation • The TCR of TH2 cell bind to the peptide: MHC II complex on B cell • Co-stimulatory signals must be sent through CD40 receptor along with additional cytokines • These proliferating B cells (and their cognate T cells??) then move into primary follicles ( B cells zone of the LN)

Surface immunoglobulins and T-cell Receptors

• Antigen: is any molecule, macromolecule, virus particle, or cell that contains a structure recognized and bound by an immunoglobulin or T-cell receptor. • Thus, immunoglobulins and T-cell receptors are antigen receptors • The particular part of the antigen bound by the immunoglobulin or T-cell receptor is called the antigenic determinant or epitope • immunoglobulins orT-cell receptors are specific to the antigens they bind

Dendritic Cells (DCs)

• Dendritic cells and macrophages are present in all the body's tissues • Upon infection, both cell types become active by uptake of the pathogen and the processing and presentation of their antigen by MHC class I and II molecules. • Unlike macrophages, which have a range of functions, the only known function of DCs is to trigger T-cell responses. • DCs are migratory: they carry their load of antigen from the site of infection to the nearest secondary lymph node, which is organized so they can meet T cells. • DCs that reside in the tissues are called immature dendritic cells, whereas those that reside in the lymph nodes are called mature or activated dendritic cells

Generation of Ig diversity

• In all cell, except B cells, the Ig genes are in a fragmented form that cannot be expressed • For an Ig gene to be expressed, individual gene segments must first be rearranged to assemble a functional gene, a process that occurs only in developing B cells in the bone marrow.

T-cell Receptors

• Like B cells, T-cell have antigen-specific receptors, calledT-cell receptors • T-cells have different and more diverse functions than B cells, but use similar means of recognizing antigens. • Like B cells, each clone of T cells expresses a single species of antigen receptor • Unlike B cells, which bind to a wide range of antigens, T cells only recognize antigens presented to them on the surface of another human cell • T-cell receptors are used only as receptors to recognize antigen, whereas Ig are both receptors and effector molecules

Two classes of MHC molecules present antigen to CD4 and CD8 T cells

• MHC I molecules: present antigens of intracellular origin to CD8 T cells and are present on all cell types • MHC II molecules: present antigens of extracellular origin to CD4 T cells which are present only on specific immune cells called antigen- presenting cells. • The CD8 co-receptor binds to the α3 domain of the MHCI molecule • The CD4 co-receptor binds to the β2 domain of the MHCII molecule.

TH1 CD4 Cells activate Macrophages

• Macrophages phagocytize, destroy, and degrade pathogens and then display pathogen-derived peptides by MHC II molecules, which in turn can activate T cells. • TH1 act back on macrophages to increase their phagocytic ability and capacity to kill ingested microorganisms. This enhancement is called macrophage activation. • Requires interaction of peptide:MHC II interaction with T-cell receptor, co-receptor CD40 interaction, and IFN-λ • Causes phagosomes to more efficiently fuse with lysosomes • Increases the synthesis of reactive molecules: oxygen radical, nitric oxide, proteases, etc... • Increased expression of MHC II

B cell activation by CD4 TH2 cells

• Mature B cells circulate through secondary lymphoid organs about every 24 hours congregating in the follicles in between. The B cell must pause in the follicle where it received survival signals from follicular dendritic cells (FDCs) • In the absence of antigen, the half-life is only a few days • B cells can encounter antigen either from free circulating antigen that has migrated to the LN, or by antigen that has accumulated on the surface FDCs -FDCs pick up antigen but do not internalize it • Antigen bind to the surface IgM molecules on B cells, which causes internal signaling, antigen is internalized and presented by MHC II on surface of cell. - B cell is not activate yet

Importance of TH1/TH2 bias

• Mycobacterium leprae is a bacterium that grows within the vesicular system of macrophages • In patients that are infected, the bias towards either TH1 or TH2 profoundly influences their disease progression • TH1 response enable macrophages to suppress their growth • TH2 response, however, favor the production of antibodies which can't access the bacterium inside the macrophages • They are so dissimilar the conditions are give a different names: tuberculoid leprosy (TH1) and lepromatous leprosy (TH2)

Germinal Centers: SHM

• Proliferating centroblasts in the GC undergo somatic hypermutation (SHM) • After SHM the surface immunoglobulin expressed by an individual centrocyte can have an affinity for its specific antigen that is higher, lower, or the same as that of the unmutated Ig • The centrocyte is programmed to die by apoptosis within a short period if it is not bound by antigen (provided by FDCs) and receive a signal from a follicular T cell.

Recap

• Surface Ig, Antibodies, T-cell receptors • MHC I: on all cells for intracellular pathogens • MHC II: on Antigen Presenting Cells • APCs: dendritic cells, macrophages, and B cell T Cells: • CD8 T cells: cytotoxic cells • CD4 T cells: Helper T cells • TH1: activate macrophages and secrete cytokines • TH2: stimulate B cells to produce antibodies

T Cell receptor

• T-cell receptors posses a single antigen binding site • T-cell receptors also undergo germ-line DNA rearrangement T-cell receptors alone are not able to recognize an antigen, but forms a functional T-cell receptor complex four other proteins collectively called the CD3 complex (CD3γ, CD3δ, CD3ε, and CD3ζ)

Germinal Centers

• The activated B cells become large proliferating cells called centroblasts • Dark zone section of the GC that stains dark in histology sections due to the closely packed and dividing centroblasts • The centroblasts give rise to non- dividing centrocytes which leave the close-packed lymphocytes to interact with FDC in the light zone • Mantle zone is the naïve, non-specific B cells of the primary follicle that got pushed to the outside of the GC

Antibodies (continued)

• The function of antibodies is to bind to microorganisms and facilitate their destruction • The part of the antigen to which an antibody binds is called the epitope, which are usually either carbohydrates or proteins on the surface of pathogens • Complex macromolecules will contain several different epitopes, each of which can be bound by a different antibody. • Any antigen that contains more than one epitope, or more than one copy of the same epitope, is known as a a multivalent antigen.

TH1 CD4 Cells activate Macrophages

• The microbial substances produced by activated macrophages are also harmful to human tissues, therefore activation is under strict control • Cytokines secreted by CD4 TH2 cells (TGF-β,IL-4, IL-10, and IL-13) inhibit macrophage activation. • Example of how TH2 cells control a TH1 response

Two Types of T Cells

• There are two classes of T cells that are defined by the express of either the CD4 or CD8 glycoprotein on the cell surface that serve as co-receptors in antigen recognition. • CD8 T cells: are cytotoxic and their main function is to kill cells that have been infected with a virus or intracellular pathogen • CD4 T cells: (a.k.a. Helper T cells) help other cells of the immune system to respond to extracellular pathogens. Two subsets of CD4 T cells: Helper T cell type 1 (TH1): activate tissue macrophages to phagocytose and kill extracellular pathogens and to secret cytokines Helper T cells type 2 (TH2): involved mainly with stimulating B cells to make antibodies

Antigen processing and presentation

• Unlike Ig, T-cell receptor complex can only recognize antigen presented by another human cell by an MHC molecule. • MHC (major histocompatibility complex) molecules are special antigen-presenting glycoproteins present on almost all cells of the body. • There are large number of diverse genetic variants of MHC molecule in the human population and is the primary cause of graft rejection or graft-versus-host disease.

CD4 TH2 cells activate B cells

• When T cells are activated in the T cell zone of the LN by DCs, some of the progeny will differentiate into TH2 cell. • These will activate B cells to produce antibodies

Activation of Naïve CD8 T Cells

• While CD4 T cells can be activated by any APC, CD8 T cells can only be activated by DCs • They require more stimulation to be activated than CD4 cells • CD8 T cells are so damaging to the tissues, that they are only activated in the clear presence of an overwhelming intracellular infection • Once activated, CD8 cells start producing specialized lytic granules that contain cytotoxins. • Effector CD8 cells then migrate to the site of infection, where they recognize specific peptide: MHC I complexes presented by infected cells, and then release their cytotoxic granules.