Pharm-Immuno 9-11Cell Mediated Immunity

Dr. Saber Hussein

Learning ObjectivesCell-mediated immune responses

• Activation of T lymphocytes by cell-associated microbes• Phases of T cell responses • Antigen recognition and costimulation • Recognition of MHC-associated peptides • Role of adhesion molecules in T cell activation • Role of costimulation in T cell activation • Responses of T lymphocytes to antigens and costimulation • Secretion of cytokines and expression of cytokine receptors • Clonal selection and expansion • Differentiation of naïve T cells into effector cells • Development of memory T lymphocytes • Biochemical pathways of T cell activation

Effector mechanisms of cell-mediated immunity

• Types of cell-mediated immunity• Migration of effector T lymphocytes to sites of

infection • Effector functions of CD4+ T lymphocytes • T cell-mediated macrophage activation • Elimination of microbes by activated macrophages • Role of TH2 cells in cell-mediated immunity • Effector functions of CD8+ cytolytic lymphocytes• Resistance of pathogenic microbes to cell-mediated

immunity

CD4+ TH1 Cellular immunity

against intracellular microbes

• Effector T cells of the CD4+ TH1 subset recognize the antigens of microbes ingested by phagocytes and activate the phagocytes to kill the microbes and induce inflammation

• Phagocyte activation and inflammation are responses to cytokines produced by the T cells

Fig 6-1Fig 6-1

CD8+ CTLs Cell-mediated immunity against intracellular

microbes• CD8+ T cells produce cytokines

that elicit inflammatory and M reactions

• CD8+ T cells recognize microbial antigens in the cytoplasm of infected cells

• CD8+ CTLs kill infected cells with microbes in the cytoplasm

[CTLs: cytolytic T lymphocytes]

Fig 6-1

The induction and effector phases of cell-mediated immunity

• Induction of response: CD4+ T cells & CD8+ T cells recognize peptides that are derived from protein antigens and presented by professional APCs in peripheral lymphoid organs

• The T lymphocytes are stimulated to proliferate and differentiate, and effector cells enter the circulation

• Migration of effector T cells and other leukocytes to the site of antigen: – Effector T cells and other leukocytes

migrate through blood vessels in peripheral tissues by

• binding to endothelial cells that have been activated by cytokines produced in response to infection in these tissues.

Fig 6-2

Migration of naive and effector T cellsA. Naive T lymphocytes home to lymph nodes as a result

of L-selectin binding to its ligand on high endothelial venules (HEVs), which are present only in lymph nodes

• Activated T lymphocytes, including effector cells, home to sites of infection in peripheral tissues, and this migration is facilitated by:– E-selectin Ligand E-selectin– P-selectin Ligand P-selectin– Integrins:

• LFA1 ICAM-1 (ligand)• VLA4 VCAM-1 (Ligand)

• Chemokines that are produced in lymph nodes and sites of infection also participate in the recruitment of T cells to these sites (see next, Fig 6-3)

Migration of naive and effector T cellsFig 6-3

HEV

Migration of naive and effector T cells

•B. The functions of the principal T cell homing receptors and their ligands

Fig 6-3

E- & P-S ligandLFA-1

Immunity to the intracellular bacterium, Listeria monocytogenes is Cell-mediated

• Lymphocytes or serum (a source of antibodies) was taken from a mouse that had previously been exposed to a sublethal dose of Listeria bacteria (immune mouse) and

Transferred to a normal (naive) mouseThe recipient of the "adoptive transfer" was challenged with the

bacteria • The numbers of bacteria were measured in the spleen of the

recipient mouse to determine if the transfer had conferred immunity• Manifestations of listeriosis include

1. Septicemia 2. Meningitis (or meningoencephalitis)3. Encephalitis 4. Corneal ulcer 5. Pneumonia6. Intrauterine or cervical infections in pregnant women, which

may result in spontaneous abortion (2nd/3rd trimester) or stillbirth

(see next, Fig 6-5)

Gram-positiveL. monocytogenes

Immunity to the intracellular bacterium, Listeria monocytogenes is Cell-mediated

A. Protection against bacterial challenge was induced by the transfer of immune T cells

– seen by reduced recovery of live bacteria

B. No protection was conferred by the transfer of serum

Fig 6-5

Cell-mediated immunity to Listeria monocytogenes

• The bacteria were killed in vitro by activated macrophages but

• No killing by T cells

Protection is dependent on antigen-specific T lymphocytes

• Bacterial killing is the function of activated macrophages

Fig 6-5C:

Activation of macrophages by T cells

• Effector T (TH1?) lymphocytes recognize the antigens of ingested microbes on macrophages

• In response to this recognition, the T lymphocytes express CD40L, which engages CD40 on the macrophages, and the T cells secrete IFN-, which binds to IFN- receptors on the macrophages

• This combination of signals activates the macrophages to – Produce microbicidal substances that kill the ingested microbes– Secrete cytokines that

• induce inflammation (TNF, IL-1, chemokines) • activate T cells (IL-12)

– Express more MHC molecules and costimulators, which • enhance T cell responses

(see Fig 6-9, next)

Activation of macrophages by T cells

The illustration shows a CD4+ T cell recognizing class II MHC-associated peptides& activating the M, but the same reaction may be elicited by a CD8+ T cell that

recognizes class I MHC-displayed peptides derived from cytoplasmic microbial Ags

Fig 6-7

Fig 6-8: Cytokine-mediated interactions between T

lymphocytes and macrophages in cell-mediated immunity

• Macrophages that encounter microbes secrete the cytokine IL-12

• IL-12 stimulates naive CD4+ T cells to:– Differentiate into

IFN--secreting TH1 cells and

– Enhances IFN- production.

• IFN- activates the macrophages to kill ingested microbes

The balance between TH1 and TH2 cell

activation determines the outcome of intracellular infections

• Naive CD4+ T lymphocytes may differentiate into

• TH1 cells, which activate phagocytes to kill ingested

microbes, and

• TH2 cells, which inhibit macrophage activation

• The balance between these two subsets may influence the outcome of infections, as illustrated by Leishmania (parasite) infection in mice & leprosy (Mycobacterium leprae) in humans

• Both agents are intracellular need cell-mediated immunity (see Fig 6-9, next)

Fig 6-9:TH1/TH2

Mechanisms of killing of infected cells by CD8+ CTLs

• CTLs recognize MHC-I-associated peptides of cytoplasmic microbes in infected cells

• CTLs form tight adhesions conjugates • Adhesion molecules, such as integrins,

stabilize the binding of the CTLs to infected cells

• The CTLs are activated to release ("exocytose") their granule contents toward the infected cell (referred to as "targets" of CTL killing)

• The granule contents include:– Perforin, which forms pores in the target

cell membrane, and – Granzymes, which enter the target cell

through these pores (or by receptor-mediated endocytosis) and induce apoptosis.

Fig 6-10

Granule-associated killing mechanisms

Granzymes

Cooperation between CD4+ and CD8+ T cells

in the eradication of intracellular infections

• In a macrophage infected by an intracellular bacterium, some of the bacteria are sequestered in vesicles (phagosomes) and others may escape into the cytoplasm

• CD4+ T cells recognize antigens derived from the vesicular microbes and activate the macrophage to kill the microbes in the vesicles

• CD8+ T cells recognize antigens derived from the cytoplasmic bacteria and are needed to kill the infected cell, thus eliminating the reservoir of infection

Fig 6-11

Evasion of cell-mediated immunity by microbes• Different bacteria & viruses resist the effector mechanisms

of cell-mediated immunity by different mechanisms:– Inhibition of phagosome-lysosome fusion (Mycobacterium

tuberculosis)

– Inhibition of Ag presentation

• HSV peptide interference with TAP transporter

• Inhibition of proteasomal activity (CMV, EBV)

• Removal of MHC I from ER (CMV)

Fig 6-12

Evasion of cell-mediated immunity by microbes

Fig 6-12

Evasion of cell-mediated immunity by microbes

– IL-10 production inhibition of M activation (EBV)

– Inhibition of effector cell activation by soluble cytokine receptor (Pox virus)

Fig. 6-12

Phases of humoral immune responses• Naive B lymphocytes

recognize antigens • Under the influence of

helper T cells and other stimuli the B cells are activated to proliferate

• This gives rise to: – clonal expansion,

and – differentiation into

antibody-secreting effector cells

• Some of the activated B cells undergo: – heavy chain class

switching and – affinity maturation

• Some become long-lived memory cells

Fig.7-1

Features of primary and secondary Ab responses

• Primary and secondary antibody responses differ in several respects.

• In a primary response– naive B cells in peripheral

lymphoid tissues are activated to proliferate and differentiate into antibody-secreting cells and memory cells

– Some antibody-secreting plasma cells may migrate to and survive in the bone marrow for long periods

• In a secondary response, – memory B cells are

activated to produce larger amounts of antibodies, often with more heavy chain class switching and affinity maturation

Fig.7-2

Features of primary and secondary Ab responses

•Features of secondary responses such as:•heavy chain class switching and •affinity maturation

•are seen mainly in responses to protein antigens, because •these changes in B cells are stimulated by helper T cells •and only proteins activate T cells. The kinetics of the responses may vary with different antigens and types of immunization.

Fig.7-2

The role of the complement protein C3d in B cell activation

• Activation of complement by microbes leads to the binding of a complement breakdown product, C3d, to the microbes.

• The B cell Ig receptor recognizes a microbial antigen and the CR2 receptor recognizes bound C3d

• CR2 is attached to a complex of proteins (CD19, CD81) that are involved in delivering activating signals to the B cell

Fig7-4

Complementreceptor

Affinity maturation in antibody responses• Analysis of several individual antibodies produced by

different clones of B cells against one antigen at different stages of primary, secondary, and tertiary immune responses shows that:– with time and repeated immunization the antibodies

that are produced contain increasing numbers of mutations in their antigen-binding regions (the complementarity-determining regions [CDRs])

• The antibodies also show increasing affinities for the antigen, as revealed by the lower dissociation constants (Kd)

• These results imply that the mutations are responsible for the increased affinities of the antibodies for the immunizing antigen

• Secondary and tertiary responses refer to responses to the second and third immunizations with the same antigen

Affinity maturation in antibody responses

Fig7-11

Selection of high-affinity B cells in germinal centers

• Some of the B cells that are activated by antigen with help from T cells – Migrate into follicles to form germinal centers – There, they undergo rapid proliferation and – Accumulate mutations in their Ig V genes

• The mutations generate B cells with different affinities for the antigen

• Follicular dendritic cells (FDCs) display the antigen

• Only B cells that recognize the antigen are selected to survive

Selection of high-affinity B cells in germinal centers

• FDCs display antigens: – by binding immune complexes

to Fc receptors or – by binding immune complexes

with attached C3b and C3d complement proteins to C3 receptors

• As more antibody is produced, the amount of available antigen decreases, so the B cells that are selected have to express receptors with higher affinities to bind the antigen

• FDCs express CD40L, and germinal centers contain a few T cells that also express CD40L

• CD40L may be the molecule that delivers survival signals to the B cells that recognize antigen on the FDCs

Fig 7-12

Fig 7-14

Fig 7-14

The mechanism of Ab feedback

• Secreted IgG Abs form immune complexes (Ag-Ab complexes) with residual antigen

• The complexes interact with B cells specific for the Ag in 2 ways: 1. BCR-Ig recognizes & binds the Ag

2. Fc receptor (Fc RII) recognizes the Ab of the Ag-Ab complexes

• The Fc receptors block activating signals from the Ag receptor thus terminate B cell activation

• The cytoplasmic domain of B cell Fc RII contains an immunoreceptor tyrosine-based inhibition motif (ITIM) that binds enzymes that inhibit antigen receptor-mediated B cell activation.

Fig 7-15

Fig 8-1 AEffector functions

ofAntibodies

The effector functions of antibodies

•Abs are produced by the activation of B Lymphocytes by Ags and other signals

•Abs of different heavy chain classes(isotypes) perform different effector functions

Fig 8-1B

Neutralization of microbes and toxins by antibodies

A. Antibodies prevent the binding of microbes to cells and thus block the ability of the microbes

to infect host cells

Fig8-2A

Neutralization of microbes and toxins by antibodies

B. Antibodies inhibit the spread of microbes from an infected cell to an adjacent uninfected cell.

Fig8-2B

Neutralization of microbes and toxins by antibodies

C. Antibodies block the binding of toxins to cells and thus inhibit the pathologic effects of the toxins

Fig8-2C

Antibody-mediated opsonization and phagocytosis of microbes

A. Antibodies of certain IgG subclasses bind to microbes and are then recognized by Fc receptors on phagocytes

Signals from the Fc receptors promote the phagocytosis of the opsonized microbes and activate the phagocytes to destroy

these microbes

Fig 8-3A

Antibody-mediated opsonization and phagocytosis of microbes

B. The different types of human Fc receptors, and their cellular distribution and functions, are listed

Fig 8-3B

Splenectomy, phagocytosis & infections

• The spleen contains large number of phagocytes

• It is important for phagocytic clearance of opsonized bacteria

Splenectomy makes patient susceptible to disseminated infections by encapsulated bacteria

• Encapsulated bacteria are cleared by antibody-mediated phagocytosis

• Without opsonization bacteria with capsule are able to escape phagocytosis

Antibody-dependent cellular cytotoxicity (ADCC)

Antibodies of certain IgG subclasses bind to cells (e.g., infected cells), and the Fc regions of the bound antibodies are

recognized by an Fc receptor on NK cellsThe NK cells are activated and kill the antibody-coated cells

Fig 8-4A

Antibody-dependent cellular cytotoxicity (ADCC)

IgE antibodies bind to helminthic parasites, and the Fc regions of the bound antibodies are recognized by

Fc receptors on eosinophils. The eosinophils are activated to release their granule

contents, which kill the parasites

Fig 8-4B

Transport of IgA through epithelium• In the mucosa of the gastrointestinal and respiratory tracts,

– IgA is produced by plasma cells in the lamina propria and is actively transported through epithelial cells by an IgA-specific Fc receptor (called the poly-Ig receptor because it recognizes IgM as well).

– On the luminal surface, the IgA with a portion of the bound receptor is released

• Here the Ab recognizes ingested or inhaled microbes and blocks their entry through the epithelium

Fig8-9

Evasion of humoral immunity by microbes The principal mechanisms by which microbes evade humoral immunity

are listed, with illustrative examples

Vaccination strategies• Different

types of vaccines induce different protective immune responses