Summary of the last lecture

• Features of T cell antigen recognition– APC– Ag processed– MHC restrictions– TCR ligand: Peptide + MHC

• Basis for Ag presentation by MHC– Genetic basis

– Polygeny– Polymorphism

– Structural basis– MHC polymorphic binding pockets– Peptide anchor residues

• Types of APC– Target cell– Professional APC

Student Q1: What cells express MHC molecules:

Class I, Class II or both, & why?

• Virtually all nucleated cells express MHC Class I • Potentially infectable (e.g. by virus) as target cells (for Tc)

• Thymic stromal cells express both • T cell selection –thymic education

• Key immune cells express both• B cells: present Ag to Th to receive help for Ab responses• MQ: first line of defense, once activated ‘professional’ APC• DC: initiation of immune responses ‘true professional’ APC

Pathways of Ag processing,Ag presentation & co-stimulations

• Types of pathogens & Ags

• Two classical pathways for Ag processing

• Ag processing, presentation, & their clinical relevance

• Cell interactions & co-operation

Question:

How is a TCR ligand generated?

Ag processing & presentation

APC

T

Types of pathogens

cytosolER

Vesicles Endocytic vesicles

“exogenous”“endogenous”

1 2 3

Types of pathogens or antigens:

• Cytosolic - “endogenous” - viruses, intracellular bacteria

• Endocytic/vesicular - “exogenous”- Intra-vesicular bacteria, parasite etc.- Extra-cellular pathogens, toxins

Two major subsets of T cells whose recognition of antigens are restricted by

two different classes of MHC:

• CD8+ T cell (cytotoxic T)- MHC Class I restricted

• CD4+ T cell (T helper)- MHC Class II restricted

Ag processing & presentation

APC

Tc Exogenous

Endogenous/CytosolicI II

Intro-vesicular

TAP

TAP: Transporters associated with Ag processing

TH

Two classical pathways of Ag processing:

• Endogenous (cytosolic) pathway “MHC Class I pathway” Tc cells

• Exogenous (endocytic) pathway “MHC Class II pathway” Th cells

Proteasome & subunit

(TAP)

The Class I pathway

1 2

Empty Class I molecules are unstable under physiological temperature

Exogenous peptides

370C 19 - 330C

Normal TAP deficient (RMA-s)

(370C)

Empty MHC class I molecule come out in the

cold

By Ljunggren HG et al.

Nature (1990) 346:476-80

3

The Class II pathway

CLIP

Ii

Functions of the Invariant Chain (Ii)

• Block MHC Class II molecules from binding of peptides derived from endogenous antigens

• Direct MHC Class II molecule to cellular vesicles where exogenous peptides are generated

HLA-DM

CLIP

Cytosolicproteins

proteasome

peptides

TAP

Ag processing, presentation & their clinical relevance

• Pathogen strategies for immune evasion:- prevent TAP function (HSV)- inhibit endosomal acidification (Helicobacter pylori)- retention of MHC molecules (Retroviruses)

• Vaccine design: Types of immune responses:- Humoral (B) endocytic pathway - Cell-mediated (CTL) cytosolic pathway

• MHC deficiencies: - Congenital MHC class II deficiency - Ir gene defects

Heterozygotes

Inbred

Homozygotes

A B

A1 B1 A1 B1

B1 B1A1 A1

A E

1 2 1 2

A E

A1

B1

B1

B1

MHC deficiency

Student Q2:

‘Self’ MHC restriction?

T cells recognize “self” MHC

The environment in which the T cells mature determines the MHC restriction of the mature T cell receptor repertoire

• Cell adhesion molecules

• Cytokines and cytokine receptors

Cell interactions & co-operations

Question:

Chances of the specific T cell:APC interactions ?

(1)

(2)(3)

+ DC

GC

(B + FDC)F

T: T cell areaB: B cell area

F: B cell follicleGC: germinal centre

Organizeddistribution

Question

How do cells know where to go and act ?

Adhesion molecules:

SELECTINS - e.g. L-selectin, P-selectin, E-selectin MUCIN-LIKE MOLECULES - (Addressins) e.g. CD34 INTEGRINS - e.g. LFA-1, VLA-4 IMMUNOGLOBULIN SUPERFAMILY - e.g. CD2 (LFA-2), ICAM-1, 2, 3

T cell-endothelium interactions

T T

Lymph node cortex

HEV

LFA-1L-Selectin

CD34 Glycam 1 ICAMs

Endothelium

(1) (2)

(3)

HEV: high endothelial venue

T cell:APC interactions

Armed effector T cells are guided to sites of infection by newly expressed adhesion molecules

Cytokines

• Lymphokines: produced by lymphocytes

• Interleukins: interleukin 1 - 26 (IL-1 – IL-26)

interferons, TNF etc.

• Monokines: produced by monocytic/phagocytes

• Chemokines: CXC (IL-8), CC (DC-CK, MDC), CX3C (Fractalkine)

- Small pharmacologically active products of cells

- Nomenclature & classification

Chemokines & Chemokine receptors

Chemokine Group Examples Target cells Receptors

CXC ELR+ IL-8 neutrophils CXCR1, 2ELR- Mig, IP-10 activated T CXCR3

CC MIP-3 naïve T CCR7

DC-CK1, naïve T ? MDC DC, T, NK CCR4 C & CX3C Fractalkine T, mono., neutr. CX3CR1

6-Cysteine CC 6Ckine T, B, mesangial ?

*ELR: Cysteine residues ‘Glu-Leu-Arg’

IL-2R (CD25)- Cell activation marker

(high affinity)

T cell activation and proliferation

Summary 1

Two types of pathogens/Ags:

• Cytosolic - “endogenous” - viruses, intracellular bacteria

• Endocytic/vesicular - “exogenous”- Intra-vesicular bacteria, parasite etc.- Extra-cellular pathogens, toxins

Summary 2

Two classical pathways for Ag processing

• “MHC class I pathway” CD8+ T cells (Endogenous/cytosolic, TAP-dependent pathway)

• “MHC class II pathway” CD4+ T cells(Exogenous/endocytic, TAP-independent pathway)

Summary 3

• A defect or defects in Ag processing or presentation may result in severe immunological consequences

• Pathogens may evade host immune system by interfering with the mechanisms of Ag processing and presentation

• Cells of the immune system interact in a complex network

• Cell interactions and re-circulation are mediated by adhesion molecules, cytokines and cytokine receptors

Summary 4 Cytokines - principles of action

• Local & systemic effects

• Unique receptor for each cytokine

• Pleiotropic

• Synergistic & autocrine fashion

• Complex network – a single cell can secrete, or be susceptible to, more than one cytokine