Summary Of The Last Section  Types of pathogens & antigens  Cytosolic – “endogenous”  Endocytic/vesicular – “exogenous”  Two classical pathways for Ag processing  Endogenous  MHC Class I pathway  CD8+ T cells  Exogenous  MHC Class II pathway  CD4+ T cells  Ag processing, presentation & clinical relevance  MHC deficiencies  Mechanisms used by pathogens to evade host immunity  Considerations in vaccine design  Cell interactions & co-operation  Lymphocyte re-circulation & homing  Cell interactions: membrane-bound & soluble molecules

Dendritic cells (DC) & the initiation of immune responses Lymphocyte activation –Ag recognition ( Signal 1 ) –Co-stimulations ( Signal 2 ) T-B cell cooperation –The original model –The modified model –The contemporary model DC – the TRUE professional APC –Activator of naïve T cells –Special Ag processing machine –Ag cross-presentation Immune initiation & the ‘Danger model’

Ag recognition alone (signal 1) does not initiate an immune response  “signal 2” is needed Question 1: How is an (adaptive) immune response initiated?

Importance of thymus in immune responses – Miller & Good (1961) Phenomenon of T-B cell co-operation – AJS Davies (1964) – Claman & Mitchell (1967) ‘Linked recognition’ – the ‘hapten-carrier effect’ – N. A. Mitchison (1970) T-B cell cooperation

The “original” 2-signal model (Bretscher & cohn, 1970) THTH B 2 nd signal (‘co-stimulation’) (1) (2) Activated

Finding: T cell Ag recognition is MHC-restricted Question 2 How could T and B cells recognize the same Ag if T cells recognized only processed Ag presented by MHC?

Linked recognition - the ‘carrier-priming’ experiment (N. A. Mitchison) TNP-C1 C2 TNP-C2 B THTH Anti-TNP response TNP: A hapten of trinitrophenyl group

The “modified” model THTH B Cytokines etc. (1) (2) Activated TNP C2

Another problem: Resting B cells do not activate naïve T cells Question 3 How can naive T cells be activated?

Naïve lymphocytes Cells that have never encountered specific antigen Armed effector lymphocytes Activated & differentiated cells that may respond to antigen binding alone to produce effector functions Memory lymphocytes Cells that have experienced specific antigen previously but need to be triggered to differentiate again to become effector cells

Dendritic cell (DC) -The activator of naïve T cells R. M. Steinman (Rockefeller) Photograph copyright: G.G. MacPherson (Oxford, 1997)

Dendritic cells & follicular dendritic cells are two very different cell types DCFDC Origin: Bone marrow-derived? Haematopoietic Where: T areas,B follicles & peripheral tissues Nature: endocytic & migratoryresident Life-span: daysyears Present Ag: as peptide to T cellsas IC to B cells by MHCby Ig-Fc

DC Biology Sentinel (constant surveillance) –Distribution throughout peripheral tissues Endocytic (Ag uptake) –Phagocytosis –Micropinocytosis –Macropinocytosis Migratory (Ag transport) –from peripheral tissues to secondary lymphoid organs

AL : Afferent Lymphatics SCS : Subcapsular Sinus PCV : Post-Capillary Venule ( HEV : High Endothelial Venule) The site of lymphocyte activation

+ DC GC (B + FDC) F T: T cell area B: B cell area F: B cell follicle GC: germinal centre Organized distribution

DC: An unique Ag processing machine (immature DC) Attenuated lysosomal potential for Ag degradation –Ag sequestered from lysosome for extended period Regulated cathepsin S activity –delayed cleavage of MHC II-associated I i chain favouring MHC II transport to lysosome (Mellman I & Steinman RM, Cell. 2001; 106:255-8)

The invariant chain CLIP Ii

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)

Ag ‘cross-presentation’ - DC breaking the rules Ag “cross-presentation”: –Endocytic/exogenous Ag  Class I pathway (T C ) (proteasomal proteolysis, TAP-dependent) “Indirect” Ag presentation: –Cytosolic/endogenous Ags  Class II pathway (T H )

Cytosolic/ Endogenous Ag Endocytic/ Exogenous Ag CD8 + T MHC Class I CD4 + T MHC Class II Cross-presentation Indirect presentation Proteasome (TAP-dependent) Lysosome

CRAIG RR. Nature 425, (2003)

Antigens cross-presented Virus-infected apoptotic cells Apoptotic tumor cells Other cell death due to normal cell turnover Transplantation Ags Endocytosed Ag: small fragments (3-12 KD)

DC – the TRUE professional APC Sentinel position Endocytic Migratory Unique location in the secondary lymphoid organs Special Ag processing machine High MHC Class I, Class II (Ag presentation) Constitutive expression of B7 (co-stimulation)

Co-stimulations - cell interactions other than Ag specific stimulation  Adhesion molecules  Cytokines & cytokine receptors  B7/CD28, CTLA-4: - B7:CD28 interaction delivers a positive signal to T cells - B7:CTLA-4 interaction delivers a negative signal to T cells  CD40/CD40L: - crucial for B cell growth & differentiation

The contemporary model Naïve T H Cytokines etc (1) (2) DC Activated (1) (2) CD28 B7 Co-stimulations B

T-B cell cooperation Effector T H B CD40L:CD40 (1) (2) Abs: IgM IgG1, IgE IgA IgG2a, IgG3 (isotype switch) Cytokines: IL-4 IL-5 IFNg

Do DCs need to be activated? What then activates DCs? Vaccination: why adjuvant? Immune system turned on by “non-self” or by “Danger” ? Further questions: Do DCs need to be activated? What then activates DCs? Vaccination: why adjuvant? Immune system turned on by “non-self” or by “Danger” ?