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 10 8 lymphocytes specific for a particular antigen. Blood circulation WP spleen Blood circulation Efferent lymph Recirculation facilitates the encounter of lymphocyte with cognate antigen. Pathogen in Tissue fluid or Mucous surface Pathogen in blood
Lymphocytes ender secondary tissues through High Endothelial Venules (HEVs). Naïve lymphocytes In blood circulation Lymphocytes migrate into Lymph node through the HEV. Lymphocytes exit through efferent lymph
Lymphocytes extravasation Naïve T cell CCL21 CCR7 CCL19 HEV T cell Chemokine signaling B cell follicles T cell area
T and B cells are localized in distinct regions in secondary lymphoid tissues. White pulp of spleen Lymph node The survival of T cells (several months) depends on low-affinity TCR recognition Of 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 Monocyte M 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 infection and 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 low Levels 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 cell co-stimulation molecules and migrate to the lymph node. Recognition of pathogen by PRRs and 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 interaction With T cells Facilitate T cell activation 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, macrophages cannot 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 expression of MHC.
Pathogens enter mucous associated lymphoid tissues 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 cells Marophages B cells M cell Dendritic cells Macrophages T cells MALT Blood circulation Dendritic cells Marophages B cells B 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 interaction Allows TCR to recognize MHC-antigen. TCR signaling stabilizes the interaction.
T cell activation requires both TCR-antigen/MHC and co-stimulator interactions. B7.1 (CD80), B7.2 (CD86) are expressed by activated DCs, M s and B cells. The requirement for B7 explains why only APCs can activate T cells. Ig superfamily members
The requirement for co-stimulatory molecules is another measure to prevent T cell response to self-antigens APC Activation through PRRs-PAMPs interaction pathogen T cell activation MHC-antigen B7 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 clustering of antigen receptors. APC Antigen/MHC TCR Co-receptor kinase Signal transduction Immunological synapse Antigen receptor clustering bring together many signaling molecules such as kinases with 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 TCR Complex by Lck. ITAM: immunoreceptor tyrosin-based activation motifs. Y XX[L/I]X 6-9 Y XX[L/I] Lck is a src-family tyrosine kinase. Phosphorylated ITAM is binding site for SH2 domain in ZAP-70 (tyrosin kinase).
Phosphorylated ZAP-70 activates many downstream Signaling pathways. ZAP-70 PLC- GEF PIP 2 IP 3 DAG Ca 2+ 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-6 TNF- 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 inhibits Co-stimulator functions and terminate T cell activaiton.
Activated T cells differentiate into effector cells. Naïve CD4 + T cell Uncommitted effector T cell (T H 0) T H 1 T H 2 Naïve CD8 + T cell Cytotoxic T lymphocytes (CTL) T H 1 cytokines T H 2 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 cells Marophages B cells M cell Dendritic cells Macrophages T cells MALT Blood circulation Dendritic cells Marophages B cells B 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 phosphorylation cascade. Srk family kinases Syk is activated by transphosphorylation.
Phosphorylated Syk activates many downstream Signaling pathways. Syk PLC- GEF PIP 2 IP 3 DAG Ca 2+ 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, some of which are specific to B or T cells. (Combinatorial regulation of gene expression) T cell specific gene NF-kB T1 T2 B cell specific gene T cell T cell specific gene NF-kB B1 B2 B cell specific gene B cell
BCR also internalizes antigens for presentation to T cells. BCR Ag Ag Receptor-mediated endocytosis Ag endosome Degradation of Ag Golgi complex Invariant 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 activates The T cell to express CD40L (CD154, TNF family member). CD40L interacts with CD40 to stimulate B cell proliferation. T H cells secret IL-4, IL-5, which facilitate B cell proliferation and differentiation. Activated B cells undergo additional diversification at the Ig loci and differentiate into plasma cells or memory cells.
The secreted cytokines are confined to the B-T contact region.
Relevant parts in book T cell activation: page 229-235 B cell activation: page 254-261