Mature T cells circulate among secondary lymphoid tissues.

Lymphocyte (B and T cells)

Other secondary lymphoid tissues(lymph node, Peyer’s patch, etc)

Less than 1 in 108 lymphocytes specific for a particular antigen.

Blood circulation

WP

spleen Blood circulationEfferent lymph

Recirculation facilitates the encounter of lymphocyte with cognate antigen.

Pathogen in Tissue fluid or Mucous surface

Pathogen in blood

Lymphocytes ender secondary tissues throughHigh Endothelial Venules (HEVs).

Naïve lymphocytesIn blood circulation

Lymphocytes migrate intoLymph node through the HEV.

Lymphocytes exit through efferent lymph

Lymphocytes extravasation

Naïve T cell

CCL21CCR7

CCL19

HEVT cell

Chemokine signaling

B cell follicles

T cell area

T and B cells are localized in distinct regionsin secondary lymphoid tissues.

White pulp of spleenLymph node

The survival of T cells (several months) depends on low-affinity TCR recognitionOf self-peptide/MHC interaction in the periphery.

How do antigens encounter T and B cells in Secondary lymphoid tissues?

Antigens are presented to T cells by antigen presenting cells (Dendritic cells, M, and B cells).

HSC

CMP CLP T

B

Neutrophil

MonocyteM

Dendritic cell

Present in many tissues(Secondary lymphoid tissues, skin,Interstitial space)

Different subsets (Myeloid dendritic cells,Lymphoid dendritic cells, plasmacytoid dendritic cells, Langerhans cells,Interstitial dendritic cells)

Dendritic cells are the most effective APC.

Dendritic cells take up antigen at the site of infectionand migrate to lymph nodes.

(Macropinocytosis: large volumes of surrounding fluid are engulfed.)

Before taking up antigen, the dendritic cells are called immature dendritic cells. These immature dentritic cells are effective in taking up antigens, but express lowLevels of MHC and T cell co-stimulation molecules (B7).

Dendritic cells can capature antigen by phagocytosis and macropinocytosis.

Immature DC

Mature DC

Mature DC loses the ability to take up antigen,expresses high levels of MHC/antigen, T cellco-stimulation molecules and migrate to the lymph node.

Recognition of pathogen by PRRsand inflammatory cytokines (IL-1, TNF-)activate DCs.

Green: Class II MHC

Red: lysosomal protein

Activated DCs express high levels of:

MHC/peptide

T cell costimlatory Molecule (B7)

Adhesion molecule

Chemokine (CCL18) attracts T cells

Stabilize interactionWith T cells

Facilitate T cellactivation

Antigen presentation

Macrophages also present antigens to T cells.

Resting macrophage expresses low levels of MHC class II molecules and no B7.

Recognition of pathogens by PRRs and phagocytosis activate macrohages to express MHC class II/peptide and B7.

Macrophage cannot migrate from sites of infection to lymph node and unable to present antigen from peripheral tissues.

Macrophages in secondary lymphoid tissues phagocytose pathogens and present antigens to surrounding T cells.

The phagocytosis by macrophage in secondary lymph nodes also prevents the pathogens from entering blood circulation.

B cell can present antigens that bind to BCR.

B cells can internalize soluble antigens through antigen receptor. (DCs can also internalize soluble antigens by macropinocytosis. However, macrophagescannot phagocytose soluble antigens)

B cells can only interact with antigens that enter into secondary lymphoid tissues.

B cell can only present antigens that bind to antigen receptor.(DCs and macrophages can present many kinds of antigens)

BCR-antigen interaction activates the expression of B7 and increases the expressionof MHC.

Pathogens enter mucous associated lymphoidtissues through M (multifold) cells.

transcytosis

Antigen presentation in secondary lymphoid tissues

Skin and other connective tissue

Dendritic cells

antigen

Lymph node

T cells

Mucous surface Blood

antigen antigen

Afferent lymphatic vessel

Dendritic cellsMarophagesB cells

M cell

Dendritic cellsMacrophages

T cells

MALT

Blood circulation

Dendritic cellsMarophagesB cellsB cells

T cells

spleen

T cells interact with APCs in secondary lymphoid tissues.

T cells interact with APCs through cell adhesion molecules.

Dendritic cells

Cell adhesion molecules initiate transient interaction.

CD2, CD58, ICAM-1,2,3: Ig superfamily members

LFA-1: integrin

The initial transient interactionAllows TCR to recognize MHC-antigen.

TCR signaling stabilizesthe interaction.

T cell activation requires both TCR-antigen/MHCand co-stimulator interactions.

B7.1 (CD80), B7.2 (CD86) are expressed by activated DCs, Ms and B cells.

The requirement for B7 explains why only APCs can activate T cells.

Ig superfamilymembers

The requirement for co-stimulatory molecules is another measure to prevent T cell response to self-antigens

APC

Activation throughPRRs-PAMPs interaction

pathogen

T cell activation

MHC-antigenB7

Class I MHC-selfantigen

Non-APC or unactivated APC

T cell with cognate antigen receptor

T cell anergy

T cell activation is initiated by the clusteringof antigen receptors.

APC

Antigen/MHC

TCRCo-receptor

kinase

Signal transduction

Immunological synapse

Antigen receptor clustering bring together many signaling molecules such as kinaseswith their substrates.

Clustered antigen receptors tend to be associated with lipid rafts.Lipid rafts are membrane regions enriched for saturated lipids and cholesterol. Many signaling molecules are associated with lipid raft through covalently attached lipids.

Lipid raft

Lipid anchored kinases

T cell

Receptor clustering leads to the phosphorylation of TCRComplex by Lck.

ITAM: immunoreceptor tyrosin-based activation motifs.

YXX[L/I]X6-9YXX[L/I]

Lck is a src-family tyrosine kinase.

Phosphorylated ITAM is binding site for SH2 domainin ZAP-70 (tyrosin kinase).

Phosphorylated ZAP-70 activates many downstreamSignaling pathways.

ZAP-70

PLC- GEF

PIP2

IP3 DAG

Ca2+ PKC

NFAT NF-B

Ras

MAP kinase cascade

AP-1

Expression of genes involved in proliferation and T cell effector functions

Abbreviations can be found on page 232.

Phosphorylation (kinases/phosphatases)

transcription

Activated T cells produce IL-2 and IL-2R.

IL-2R: CD25

IL-2

IL-8

IL-1, IL-6TNF-

IL-1, TNF-

Co-stimulators (B7-CD28) augment the production of IL-2 by stabilizing IL-2 mRNA and upregulating transcription of IL-2 gene.

Immunosuppressive drugs (cyclosporin A and FK506) inhibit IL-2 production.Rapamycin inhibits signaling through IL-2R.

CTLA-4 is homologous to CD28, and is induced by T cell activation. CTLA-4 inhibitsCo-stimulator functions and terminate T cell activaiton.

Activated T cells differentiate into effector cells.

Naïve CD4+ T cell

Uncommitted effector T cell (TH0)

TH1 TH2

Naïve CD8+ T cell

Cytotoxic T lymphocytes (CTL)

TH1 cytokines TH2 cytokines

Kill cells infected cells

Activate macrophage Activate B cells

B cells encounter antigens in secondary lymphoid tissues.

Skin and other connective tissue

Dendritic cells

antigen

Lymph node

T cells

Mucous surface Blood

antigen antigen

Afferent lymphatic vessel

Dendritic cellsMarophagesB cells

M cell

Dendritic cellsMacrophages

T cells

MALT

Blood circulation

Dendritic cellsMarophagesB cellsB cells

T cells

spleen

Clustering of antigen receptor initiates B cell activation.

Lipid raft

pathogen

antigen

BCR

Ig/

Membrane anchored kinases

Signal transduction

Antigen receptor clustering leads to a phosphorylationcascade.

Srk family kinases

Syk is activated by transphosphorylation.

Phosphorylated Syk activates many downstreamSignaling pathways.

Syk

PLC- GEF

PIP2

IP3 DAG

Ca2+ PKC

NFAT NF-B

Ras

MAP kinase cascade

AP-1

Expression of genes involved in proliferation and B cell effector functions

Abbreviations can be found on page 232.

Phosphorylation (kinases/phosphatases)

transcription

B cell activation is enhanced by co-receptors.

(cleavage product of C3b)

Co-receptor can stimulate B cell activation by 1000-10,000 fold.

CD19 knockout severely reduces B cell activation.

B cell activation

Why do the same transcription factors activate the expression of different genes in B and T cells?

Transcriptional activation involves a combination of many transcription factors, someof which are specific to B or T cells. (Combinatorial regulation of gene expression)

T cell specific geneNF-kBT1 T2

B cell specific geneT cell

T cell specific gene

NF-kBB1

B2B cell specific gene

B cell

BCR also internalizes antigens for presentationto T cells.

BCRAg

Ag

Receptor-mediated endocytosis

Ag endosome

Degradation of Ag

Golgi complexInvariant chain

MHC II

Clip

MHC II

Ag-BCR interaction provides activation signal I,Which induces the expression of MHC II and B7.

Internalization of Ag by BCR leads to antigen-presentation with MHC II.

Interaction with cognate CD4 T cell activatesThe T cell to express CD40L (CD154, TNF familymember).

CD40L interacts with CD40 to stimulate B cellproliferation.

TH cells secret IL-4, IL-5, which facilitateB cell proliferation and differentiation.

Activated B cells undergo additional diversificationat the Ig loci and differentiate into plasmacells or memory cells.

The secreted cytokines are confined to theB-T contact region.

Relevant parts in book

T cell activation: page 229-235B cell activation: page 254-261