Basic Immunology and

Multiple Sclerosis

Ben Thrower M. D.Shepherd MS Institute

Atlanta, GA

Why Immunology?

• MS is an autoimmune disease.• Existing and future therapies work by

modulating or suppressing the immune system.

References

1. Abbas AK, Lichtman AH.Basic Immunology: Functions and Disorders of the Immune System. W.B. Saunders Company. 2001.

2. Sospedra M, Martin R. Annu. Rev. Immunol. 2005; 23: 683-747.

Immunology is like an onion …..

….. not because it’s stinky and makes you want to cry…..

….. but because there are lots of layers.

Overview of the Immune System

Immune System

Innate(Nonspecific)

1o line of defense

Adaptive(Specific)

2o line of defenseProtects/re-exposure

CellularComponents

HumoralComponents

CellularComponents

HumoralComponents

Function of the Immune System(Self/Non-self Discrimination)

• To protect from pathogens• Intracellular (e.g. viruses and some bacteria

and parasites)• Extracellular (e.g. most bacteria, fungi and

parasites)

• To eliminate modified or altered self

• Beneficial:

• Protection from invaders

• Elimination of altered self

• Detrimental:

• Discomfort and collateral damage

(inflammation)

• Damage to self (hypersensitivity or autoimmunity)

Effects of the Immune System

Innate (Nonspecific) Immunity

Innate Host Defenses Against Infection

• Anatomical barriers– Mechanical factors– Chemical factors– Biological factors

• Humoral components– Complement– Coagulation system– Cytokines

• Cellular components– Neutrophils– Monocytes and macrophages– NK cells– Eosinophils

The Adaptive Immune System

• Cell-mediated Immunity (Cytotoxicity)• T cells

• CD4+ (Th1 & Th2)• CD8+

• Humoral Immunity (Antibody production)• B Cells

Innate Immunity Adaptive Immunity

Comparison of Innate and Adaptive Immunity

• No memory

• No time lag

• Not antigen specific

• A lag period

• Antigen specific

• Developmentof memory

Cells of the Immune System

Immune System

Myeloid Cells Lymphoid Cells

Granulocytic Monocytic T cells B cells

NeutrophilsBasophils

Eosinophils

MacrophagesKupffer cells

Dendritic cells

Helper cellsSuppressor cellsCytotoxic cells

Plasma cells

NK cells

Immune Interactions

• Cell Receptors

Immune Interactions

• Cell Receptors

Endothelial Lining

Immune Interactions

• Cytokines – Powerful chemical substances that allow for interactions between cells of the immune system.

Immune Interactions

• Chemokines – Chemicals used by one immune cell to attract another cell to an area of inflammation.

Cells of the Immune System

• Lymphocytes 1) B cells, T cells,

Natural killer (NK) cells.2) Have receptors for specific

antigens.3) Key mediators of adaptive immunity.4) Nomenclature based upon surface

proteins called CD (cluster of determination).

Cells of the Immune System

• Antigen-presenting cells (APC)1) Dendritic cells, macrophages,

B cells, microglia (CNS).2) Capture of antigen for display to

lymphocytes.

• Mediators involved in cellular immunity, including hormone-like glycoproteins released by activated T cells and macrophages.

• Some are co-stimulators of T cells and T cell proliferation.

Cytokines

T-cells

APC NaïveT cell

CD8T cell

CD4T cell

CD4 T Helper Cells

• Activation of macrophages• Activation, proliferation and differentiation

of T and B lymphocytes

CD4+ T Helper Cells

CD4

CD8

TCR

MHC II – Major Histocompatibility Complex*

Immature CD4+/CD8+ T Cell

APC

* MHC = HLA (Human Lymphocyte Antigen)

CD4+ T Helper Cells

CD4

CD8

TCR

Mature CD4+ Cell

CD4 / APC Receptors

CD4 T Helper Cell APC

CD4

TCR

CD3

CD28

LFA-1VLA-4

Class II MHC

B7-1/B7-2 (CD80/CD86)

ICAM -1

The “Immunological Synapse”

• The interaction between the TCR and MHC molecules is not strong

• Accessory molecules stabilize the interaction– CD4/Class II MHC or

CD8/Class I MHC– CD2/LFA-3– LFA-1/ICAM-1

The “Immunological Synapse”

• Specificity for antigen resides solely in the TCR

• The accessory molecules are invariant

• Expression is increased in response to cytokines

The “Immunological Synapse”• Engagement of TCR and

Ag/MHC is one signal needed for activation of T cells

• Second signal comes from costimulatory molecules– CD28 on T cells interacting

with B7-1 (CD80) or B7-2 (CD86)

– Others• Costimulatory molecules

are invariant

CD4 / APC Receptors

CD4 T Helper Cell APC

CD4

TCR

CD3

CD28

LFA-1VLA-4

Class II MHC

B7-1/ B7-2

ICAM -1

Costimulatory Receptor

CD4 / APC Receptors

CD4 T Helper Cell APC

CD4

TCR

CD3

CD28

LFA-1VLA-4

Class II MHC

B7-1/ B7-2

ICAM -1

Adhesion Receptors

Costimulation is Necessary for T Cell Activation

• Engagement of TCR and Ag/MHC in the absence of co-stimulation can lead to anergy

• Engagement of co-stimulatory molecules in the absence of TCR engagement results in no response

• Activation only occurs when both TCR and co-stimulatory molecules are engaged with their respective ligands

• Downregulation occurs if CTLA-4 interacts with B7– CTLA-4 sends an inhibitory

signal

Key Steps in T cell Activation• APC must process and present peptides to T cells• T cells must receive a costimulatory signal

– Usually from CD28/B7• Accessory adhesion molecules help to stabilize binding

of T cell and APC– CD4/MHC-class II or CD8/MHC class I– LFA-1/ICAM-1– CD2/LFA-3

• Signal from cell surface is transmitted to nucleus– Second messengers

• Cytokines produced to help drive cell division– IL-2 and others

CD4 Subsets

CD4T cell

CD4Th1 cell CD4

Th2 cell

CD4Th17 cell

Naïve CD4 T cells differentiate into TH1 and TH2 cells

Th1 Th2

T cell

NaïveCD4

Macrophage

Pro-inflammatory Cytokines

• IL-2• Il-12• IFN-γ• TNF

Anti-inflammatoryCytokines

• IL-4• IL-5• Il-10• IL-13• TGF-β (TH3)

B cell

Reciprocal Inhibition

Yong, W

CD4 Th17 Cells

• Recently described subset of CD4 cells.• Cytokines produced = IL-17 and TNF-

alpha.• Stimulated by IL-23 cytokine.

Bettelli E, Carrier Y, Gao W, et al. Nature 2006; 441(7090):235-238.

T-cells

APC NaïveT cell

CD8T cell

CD4T cell

CD8 T Cells

• Cytotoxic CD8 cells are the only T cells capable of killing other cells by direct contact.

• CD8 cells may also serve a regulatory or suppressive role.

CD8+ T Cells

CD4

CD8

TCR

MHC I – Major Histocompatibility Complex*

Immature CD4+/CD8+ T Cell

APC

* MHC = HLA (Human Lymphocyte Antigen)

CD8+ T Cells

CD4

CD8

TCR

Mature CD8+ Cell

TC

Cytotoxic

TS

Suppressor

CD8 / APC Receptors

CD8 T Cell APC

CD8

TCR

CD3

CD28

LFA-1VLA-4

Class I MHC

B7-1/ B7-2

ICAM -1

• TC cells, or killer T cells, are the only T cells that can directly attack and kill other cells

• They circulate throughout the body in search of body cells that display the antigen to which they have been sensitized

• Their targets include:– Virus-infected cells– Cells with intracellular bacteria or parasites– Cancer cells– Foreign cells from blood transfusions or transplants

Cytotoxic T Cell (Tc)

• In some cases, TC cells:– Bind to the target cell and release perforin into

its membrane• In the presence of Ca2+ perforin causes cell lysis

by creating transmembrane pores• Other TC cells induce cell death by:

– Secreting lymphotoxin, which fragments the target cell’s DNA

– Secreting gamma interferon, which stimulates phagocytosis by macrophages

Mechanisms of Tc Action

B Cells

• B cells may mature into antibody producing plasma cells.

• B cells may function as APC’s.

B Cell – CD4 Th Interactions

B Cell

CD40

MHCII

B7

Cytokine Receptor

CD4 Th Cell

CD 40 Ligand

TCR

CD28

B Cell

CD40

MHCII

B7

CD4 Th Cell

CD 40 Ligand

TCR

CD28

B Cell Proliferation and Differentiation

CD4 Activation

• Also called immunoglobulins – Constitute the gamma globulin portion of

blood proteins– Are soluble proteins secreted by activated B

cells and plasma cells in response to an antigen

– Are capable of binding specifically with that antigen

• There are five classes of antibodies:IgD, IgM, IgG, IgA, and IgE

Antibodies

Natural Killer (NK) Cells

• Bind to and kill antibody-coated cells.• Called “natural killers” because they do not

have to recognize antigen first.

Immunological tolerance

• Definition:– Specific immune unresponsiveness to an antigen that

is induced by exposure of lymphocytes to that antigen (tolerogen vs immunogen)

• Significance:– All individuals should be tolerant of their own

antigens (self-tolerance); breakdown -->autoimmunity– The induction of tolerance could be exploited to treat

autoimmune diseases– Mechanisms of tolerance must first be understood

Mechanisms of unresponsiveness to self antigens

• Central tolerance– Immature self-reactive T lymphocytes that recognize self

antigens in the thymus undergo negative selection (deletion).

• Peripheral tolerance– Mature self-reactive T lymphocytes that escape central

tolerance and recognize self antigens in peripheral tissues can be inactivated (anergy), killed (deletion) or regulated (suppressed).

• “Clonal ignorance”– Mature self-reactive lymphocytes do not respond to self

antigens in non-inflamed settings.

Multiple Sclerosis

What went wrong?

Key Steps in the MS Process

1. A breakdown in the ability to distinguish self from non-self.

2. Upregulation of myelin reactive CD4 Th1cells.

3. Increased permeability of the BBB.4. The role of B cells and CD8 cells.

Requirements for the development of an autoimmune disease

Nature Immunology (9): 759-761 (2001)

HLA Type and MS Risk

• Many autoimmune diseases in humans are linked to particular HLA alleles.

• HLA DR2 and DR4 haplotypes are associated with an increased risk for MS.

Environmental Factors

• Many pathogens have been proposed to have a role in the pathogenesis of MS.

• HHV-6, EBV, Chlamydia pneumonia

Molecular Mimicry

• The pathogen contains similar peptide sequences to peptides found in myelin

(MBP, MAG, MOG others).• The pathogen may have similar three-

dimensional structures that promote cross-reactive immunity.

Immune Dysfunction in MS

• Myelin autoreactive CD4 cells may have less of a need for co-stimulation to become activated in a person with MS.

• Downregulation of CD4 cells via CTLA-4 may be impaired in MS.

Innate and Adaptive Immune Responses

Innate Immune Cell

TLR (Toll Like Receptor)

Pathogen

Cytokines Autoreactive

CD4/CD8

Naïve CD4 T cells differentiate into TH1 and TH2 cells

Th1 Th2

T cell

Naïve

Macrophage

Pro-inflammatory Cytokines

• IL-2• Il-12• IFN-γ• TNF

Anti-inflammatoryCytokines

• IL-4• IL-5• Il-10• IL-13• TGF-β (TH3)

B cell

Reciprocal Inhibition

Yong, W

Th1 / Th2 Imbalance

Th1 Th2

Normal

Th1 / Th2 Imbalance

MS

Increased BBB Permeability

• Some chemokine levels are increased, leading to increased adhesion and attraction of autoreactive T cells into the CNS.

• Increased production of matrix metalloproteinase leads to increased BBB permeability.

The Role of B Cells in MS

• Increased immunoglobulin (Ig) production in the CSF, but not the serum.

• CSF Ig in MS shows an oligoclonal distribution, i.e. a limited number of B cell clones contribute to the Ig production.

How Do B Cells and Ab Contribute to the Pathogenesis in MS?

• B cells may serve as APC’s for autoreactive T cells.

• B cells provide costimulation for autoreactive T cells.

• B cells may help recruit autoreactive T cells into the CNS.

• B cells produce myelin-specific autoantibodies that destroy myelin.

The Role of CD8 Cells in MS

• Cells in the CNS express MHC I much more so than MHC II.

• Myelin-reactive CD8 cells are more common than myelin-reactive CD4 cells in MS brain tissue.

• Myelin-reactive CD8 cells secrete chemokines for myelin-reactive CD4 cells.

Thanks!