PROFESSIONAL ANTIGEN PRESENTING CELLS Express MHC class I and class II molecules Express co-stimulatory molecules (CD40, B7) Take up extracellular antigens B cells – soluble proteins, toxins ADAPTIVE – Ag specific Macrophages – extracellular pathogens (bacteria, yeast) Dendritic cells – viruses, apoptotic cells INNATE

THE ROLE OF PROFESSIONAL ANTIGEN PRESENTING CELLS IN THE IMMUNE RESPONSE Infectious diseases Tissue transplantation Elimination of tumors Autoimmune diseases Gatekeeper function Sensing pathogens Priming adaptive immune responses Maintenance of self tolerance to self structures

Dendritic cells are sensors, gatekeepers and messengers Activation induces a phenotype essential for the initiation of the adaptive immune response In these images of dendritic cells in peripheral tissue (top panels), lymph (middle panels) and lymph node (bottom panels), the left panels are fluorescent micrographs of several cells with MHC class II molecules stained green and a lysosomal protein stained red, and the right panels are scanning electron micrographs of single cells. The dendritic cell body is clearly seen in the top right panel, but in the top left panel the cell bodies are difficult to discern. The observed yellow staining shows endocytic vesicles within the dendrites that contain both MHC class II molecules and lysosomal proteins: the combination of the red and green stains gives the yellow color. On activation and passage to the lymph the morphology of the dendritic cell changes, as shown in the middle panels. Phagocytosis has now stopped, a change indicated in the middle left panel by a partial separation of the green MHC class II staining from the red lysosomal protein staining. This shows that peptide-loaded MHC class II molecules are moving out of the endocytic vesicles and onto the cell surface. On reaching the T-cell area of a lymph node (bottom panels), the now mature dendritic cells concentrate on antigen presentation and T-cell stimulation instead of the uptake and processing of antigens. Here the MHC class II molecules, which are at high density on the surface of the numerous dendrites, are completely separated from the lysosomal protein in the intracellular vesicles

CONTACT OF DENDRITIC CELLS AND T - LYMPHOCYTES IN LYMPHOID ORGANS Activated dendritic cells act as professional antigen presenting cells MHC-peptide complexes 1. signal STRANGER Co-stimulatory molecule 2. signal AMPLIFICATION Cytokines 3. signal DANGER They are in close contact with specific T lymphocytes

INTERDIGITATING RETICULAR (MATURE DENDRITIC) CELL IN T CELL AREAS OF LYMPH NODES NUCLEUS T CELL T CELL CYTOPLASM

Cell-surface molecules of the immunoglobulin superfamily initiate lymphocyte adhesion to professional antigen-presenting cells B. Transient interactions are stabilized by Ag-binding A. Initial contact A A) In the initial encounter of T cells with antigen-presenting dendritic cells, CD2, binding to LFA-3 on the antigen-presenting cell, synergizes with LFA-1 binding to ICAM-1 and ICAM-2. An interaction that seems to be exclusive to the interaction of naive T cells with dendritic cells is that between ICAM-3 on the naive T cell and DC-SIGN, a C-type lectin specific to dendritic cells, which binds ICAM-3 with high affinity. B) When a T cell binds to its specific ligand on an antigen-presenting dendritic cell, intracellular signaling through the T-cell receptor (TCR) induces a conformational change in LFA-1 that causes it to bind with higher affinity to ICAMs on the antigen-presenting cell. The T cell shown here is a CD4 T cell.

Monocyte CHANGES OF TISSUE ENVIRONMENT INDUCES THE ACTIVATION OF MACROPHAGES AND DENDRITIC CELLS Activated macrophage Phagocytosis and degradation of backteria (LPS, TLR) DANGER SIGNAL Macrophage Activated dendritic cell Virus, extracellular pathogens, inflammatory cytokines (LPS, TLR) DANGER SIGNAL Dendritic cell BLOOD TISSUE LYMPHOID TISSUE

ACTIVATION AND MIGRATION OF DENDRITIC CELLS TISSUE LYMPH NODE TISSUE Lymphatics Effector and memory T cells Activated DC Inflammation Pathogen CIRCULATION Naive T cells Tissue DC DC AND T CELLS ENCOUNTER T CELL ACTIVATION ANTIGEN

Huang et al Immunity 2004 Rapid DC Migration in the Subcapsular Space Capture of an Ag-Specific T Cell by an Ag-Bearing DC Bone-marrow derived DCs (either 5 µM CFSE, green) or (50 µM Cell Tracker Blue, blue) were injected into the footpad of a C57BL/6 mouse, followed 18 hours later by intravenous injection of freshly isolated polyclonal CD4+ T cells (5 µM SNARF, red) and CD8+ T cells (5 µM CFSE and 5 µM SNARF, yellow). The draining LN was removed 6 hours after injection Bone-marrow derived DCs (yellow) were pulsed with 1 µM Ova 4 peptide and 10 µM Ova for 1 hour at 37oC, then injected into the footpad of a C57BL/6 recipient. This was followed 6 hours later by i.v. co-injection of OT-I CD8+ T cells (5 µM CFSE, green) and OT-II CD4+ T cells (5 µM SNARF, red). Huang et al Immunity 2004

Morphology of plasmacytoid dendritic cells IPC/DC2 pDC monocyte Scanning EM Transmission EM

Plasmacytoid DCs control the function of many immunocytes IFNα is impotant in SLE pathology HIV infects PDC Role in immune response and in the pathogenesis of autoimmune diseases and cancer

PLASMACYTOID DENDRITIC CELLS AS PROFESSIONAL TYPE I INTERFERON SECRETING CELLS Enhanced NK cell cytotoxic activity TLR4 Vírus infection TRAM TRIF TLR7 TLR8 TLR9 TLR3 TRIF MyD88 TANK IRAK-1 Activation of  and γδ T cells TRAF-6 RIG-1 IKKε TBK1 IFN-β IFN-α1 Cross-presentation by conventional dendritic cells is enhanced IRF-3 IRF-5 IRF-7 IRF-7 Ig production by B cells is induced Type I interferon receptor

Migration Pathways of PDC/IPC versus mDC into a lymph node mDC: afferent lymphatics IPC: HEV Both migrate into the T-cell rich areas