Concept of Immune Regulation

• Immune responses are tightly regulated complex Immune responses are tightly regulated complex interaction of cells & mediators, and by mechanisms interaction of cells & mediators, and by mechanisms to prevent anti-self reactivityto prevent anti-self reactivity

• Failure of regulatory control can occur…Failure of regulatory control can occur…– Enhancement of immune responses or infection can Enhancement of immune responses or infection can

generate autoimmune reactions (loss of self–tolerance)generate autoimmune reactions (loss of self–tolerance)– Decrease of immune responses may lead to an Decrease of immune responses may lead to an

immunodeficiency stateimmunodeficiency state– Shift in immune responses can lead to allergyShift in immune responses can lead to allergy

Immunological ToleranceImmunological Tolerance

• History - Ehrlich, Owen, Burnet, –Billingham, Brent and Medawar

Burnet’s Clonal Selection Model:Central Tolerance

Anti-selfLymphocyte

Self Ag

ClonalDeletion

Anti-non-selfLymphocyte

Activation

Foreign Ag + second signal

DEVELOPMENT MATURITY

Differentiation

Medawar’s experiment demonstrating Medawar’s experiment demonstrating neonatal tolerance induction (Nobel Prize)neonatal tolerance induction (Nobel Prize)

x

Immunological ToleranceImmunological Tolerance

• Definition and Properties

– Specific unresponsive state induced by Specific unresponsive state induced by exposure to antigenic epitopesexposure to antigenic epitopes

– Tolerance to self is initially induced during embryonic life, and is maintained by antigen

– Tolerance occurs in both T and B cellsTolerance occurs in both T and B cells

– Multiple mechanisms of tolerance existMultiple mechanisms of tolerance exist

Central Tolerance

Mechanisms of Immunological Tolerance - Overview

• Central Tolerance through Clonal Deletion

– Clones of cells that have receptors for self-antigens are deleted during development

• Peripheral Tolerance

– Clonal Anergy-failure of APC to deliver a second signal during antigen presentation (example: B7-CD28 interaction)

– Suppression of responses may occur by production of regulatory T cells that inhibit immune response to self-antigen (example: TGF-, IL10 and Th1 vs. Th2 cytokines)

– Ignorance to some self antigens may also exist

Generation of immune repertoires

Central Tolerance

Peripheral Tolerance

Autoimmune Diseases

TherapiesGlobal Selective

Tolerance failsWrong environment

(viral infection?)Wrong genes or mutations

Bone Marrow Thymus

Self-reactive lymphocytes Deleted by negative selection

Leakage of self-reactive lymphocytes controlled

Tolerance: Establishment and Failure

Tolerance Exists in Both T and B CellsTolerance Exists in Both T and B Cells

However, the Kinetics and Waning of Tolerance Induction However, the Kinetics and Waning of Tolerance Induction Differs in T and B LymphocytesDiffers in T and B Lymphocytes

Fas

FasL

cytokines

Apoptosis

Inhibition of proliferation &effector action

Activated T cells

NormalResponse

CD28 B7

Proliferation & differentiation

Antigen Recognitionwithout co-stimulation

Anergy

CTLA4 B7

FunctionallyUnresponsiveCTL4-B7 interaction

Fas-FasL interaction

Cytokine-mediated suppression

Activation induced cell

death

Cytokine regulation

Pathways to Peripheral Tolerance

The Two Signal Hypothesis for T-cell Activation

Mature Mature Dendritic Dendritic

cellcellAPCAPC

TTHH cellcell

CD28CD28B7 B7

MHC IIMHC II TCR TCR

Signal 2Signal 2

Signal 1Signal 1

Activated Activated TTHH cell cell

Hypothetical mechanism of tolerance in mature T cells

CD28CD28

RestingRestingB-cellB-cellAPCAPC

TTH0H0 cell cell

Tolerance (anergy or apoptosis) Tolerance (anergy or apoptosis) from lack of signal 2from lack of signal 2

Signal 1Signal 1

TolerantTolerant T cellT cell

NormalResponse

CD28 B7

Proliferation & differentiation

Summary: Lack of co-stimulation can lead to tolerance (anergy)

Antigen Recognitionwithout co-stimulation

Anergy

Regulation by CTLA-4

CTLA4

B7

FunctionallyUnresponsive (Anergic) T cell

CTLA4-B7 interaction

Activated T cell

Regulatory T cells

FunctionallyUnresponsive T cell

Production of IL-10 or TGF-

RegulatoryT cell

Pathways to Peripheral Tolerance

Inhibition by Antibody Feedback

• Passively administered antibody can prevent an antibody response

• Antibody produced during an immune responses leads to elimination of antigen (stimulus)–Less antigen available to stimulate specific cells–Immune complexes can bind to inhibitory receptors

Application: RhoGam for Erythroblastosis Fetalis

Major Immune Inhibitory Receptors

• B cells– FcRII

• T cells– CTLA4

• NK cells– KIR (killer cell Ig-like receptors),

Anti-Idiotypes and Immune Regulation

• Definition

– anti-idiotype response-antibody produced against immunoglobulin or TCR idiotypes that serve to down-regulate immune response

– The epitope for an responsive anti-idiotype molecule (antibody, BCR, or TCR) is the internal image formed by the CDR region of the respective epitopes antigen receptor

Idiotype/Anti-idiotype network

Cytokines and Immune Regulation

• Definition – Soluble mediators – Made by a variety of cells– Multifunctional proteins and peptides

• Involved in initiating immune response • Involved in turning off immune response• Some serve as direct effector molecules (e.g., TNF)

Cytokine Regulation via TH1 – TH2 Balance

Th1

Th2

IL-4 IL-12

IFN-g

IL-18

IL-10 & TGF-

Low affinityBetween TCR and

APC

Low [Antigen]

High affinityBetween TCR and

APC

High [Antigen]

Th1 versus Th2 BalanceDisease Th1 Th2

Experimental Cure Progression Leishmaniasis

Experimental autoimmune Progression Preventionencephalomyelitis

Tuberculosis Cure/Prevention Progression

Atopy Prevention? Progression

Type 1 Diabetes (NOD) Progression Prevention

CNS–Immune System InteractionsCNS

Hypothalamus

Pituitary

Adrenal gland

Activated Immune cells

Sympathetic nervous system

NE

Cytokines

Cytokines

Antibodies

Immune SystemNE

2AR

APC B cell Th1 Th2 CTL

Immunosuppression (adapted from Roitt)

G0 G0 G1 S G2/M G1/0

IL2R

Anti-TCR, -CD3, CD4/8, CD45RB, LFA-1, ICAM-1 Anti-IL2

Steroid

CTLA-4-Fc-fusion peptide

UV

Cyclosporin

FK506

Steroid

Rapamycin Azathoprine

Methotrexate

etc

X-rays

Cyclophosph.-amide

And now for a clinical case….

• 6 year old male, ER with unexplained bruising associated with minor trauma

• Patient has minimal clotting activity• FVIII levels <1% of normal

• Patient given i.v. FVIII concentrate i.v. and released but returns in two weeks with same problem• Repeated FVIII treatment

• However, FVIII is ineffective.

Patient Presentation

Issues• Coagulation factor inhibitors (anti-FVIII activity)

• Basis?•Lack of tolerance. Why?

• Prevalence/impact•20-30% FVIII, less FIX

• Treatment/problems•FVIII concentrate or rFVIII

• Inhibitors develop that neutralize FVIII• Therapy?

• Porcine FVIII with less cross-reactivity • Tolerance (high dose)• Gene therapy

What are Inhibitors?

• IgG; commonly subclass 4, mixed 1 & 4• Occur in

• Congenital factor deficiency = alloimmune

• Previously unaffected = autoimmune • Associated with pregnancy, autoimmunity,

malignancy, multi-transfusion, advanced age etc.