Dendritic Cells Flashcards
Describe what Antigen Presenting cells are and what their function is?
APCs are specialised cells that capture and present antigens to effector T cells (mostly CD4+ cells)
Examples of these APCs include dendritic cells, macrophages and B cells
APCs serve2 major functions:
- Capture and process antigens for presentation to T cells
- Produce signals required for proliferation and differentiation of lymphocytes (Co-stimulation)
What are dendritic cells and what are some of their main functions?
Dendritic cells (DCs) are a special type of leukocyte and APC which are actively involved in initiating innate and adaptive responses against pathogens
They are fundamental in deciding the type and efficiency of the adaptive immune response and they are the only APC which can activate naive T cells. They are mainly located near the skin, mucosa or in tissues, particularly lymphoid tissue
Their main functions:
- Capture microbes.
- Co-stimulation by other substances, Cytokines released, Receptors for migration
- Transport microbes from tissues to draining lymph nodes or spleen (the naïve B cells are found here)
- Process microbes forming antigens
- Present antigens to naïve T cells and activate them
Briefly describe the life cycle of a dendritic cell
- DCs start as haematopoietic stem cells in the bone marrow, particularly from CD34+ progenitors which have lots of intracellular MHC molecules
- These stem cells differentiate into immature DCs under the control of growth factors e.g. GM-CSF, IL-3 or FLT3L
- Immature DCs then move into circulation to home to tissues and searching for PAMPs using their PRRs and chemokine
- When they come into contact with invading pathogens (antigens), they become activated and start to mature
- Mature DCs then migrate via chemokine to the lymph node , up regulate T cell coreceptors to activate lymphocytes
How do Immature DCs home to sites of infection to capture antigens?
Immature cells are widely distributed in all tissues particularly throughout epithelium of the skin, the respiratory tract, and the gastrointestinal tract. They are recruited very quickly to sites of inflammation and infection in peripheral tissues via chemokines.
Immature DCs express chemokine receptors and hone to sites with corresponding chemokines:
Examples include (Receptor: Ligand)
- CCR1: MIP-1A (CCL3)/RANTES (CCL5)
- CCR4: TARC/MFC
- CXCR4: SDF-1
Immature DCs are very efficient at antigen capture. What methods do they use to engulf pathogens
Receptor (clathrin-) mediated endocytosis:
- clathrin binds to transmembrane part of antigen receptor and coats the vesicle
- this causes inward budding of the cell membrane which eventually forms vesicles within the DC
Receptors include:
• C-type lectin receptors e.g. mannose receptor,
• Fc receptor types I (CD64) and types II (CD32)
• CD91 α2-macroglobulin receptor (hsp)
Phagocytosis
- engulfing antigenic material including apoptotic and necrotic cell fragments ect.
- It involves different receptors including FcgRs, Complement receptors and mannose receptors (pathogen associated sugars)
Macropinocytosis
- a type of pinocytosis where small particles suspended in extracellular fluid are brought into the cell via invagination into vesicles. This is done via actin filaments and receptors (EGFR) on the plasma membrane which enclose over the antigenic exosomes. This requires PIP3 and active Ras which direct the actin reorganisation The vesicles then fuse with endosomes to hydrolyse the particles
Aswell as antigenic uptake, immature DCs need other signals to be able to mature. What are these signals?
Firstly there needs to be a balance of pro-inflammatory and anti-inflammatory cytokines in the local environment. This response occurs when the DC recognises a PAMP single from the pathogen using its PRR e.g. a TLR. This causes NF-kB activation which causes pro-inflammatory cytokines release.
Secondly, if the tissue is under stress due to injury or infection, it will produce danger signals (DAMPS). DAMPS are usually normal intracellular molecules which are not normally found in the extracellular space. These could be genetic material, heat shock proteins (HSP) or cytokines (IFN-A). DAMPS are recognised by TLRS on the DC surface and trigger different effects
What effect does TLR signalling have on DC cells?
TLRs triggers different effects on DC cells including survival, chemokine receptor expression and chemokine secretion, DC migration, cytoskeletal and shape changes. This drives the DC through maturity and effect the DC:T cell interaction.
It also increases expression of costimualtion molecules such as CD40 and B7.1/.2 and promote release of certain cytokines contributing to clonal expansion and differentiation of T cells.
There are 9 type of TLRS each responding to different ligands in different pathogens and either on the surface the DC or inside the DC on the phagolysosome.
Immature DCs have to mature to have an effective immune response. What are the 2 pathways of maturation for DCs ?
Dendritic cells need to receive a maturation stimulus to trigger their transition from immature antigen capturing cells to mature antigen presenting cells.
There are 2 pathways of maturation:
1.Immunity – via DAMP- induced migration which activate the DC
This signals to TH1 and TH2 cells for presentation
- Tolerance – via Steady state migration, DCs which can spontaneously mature
This pathway produces quienscent DCs reacting to self-antigens for MHC 2 presentation to CD4 or MHC1 cross presentation to CD8 cells. This leads to Treg deletion.
What are the consequences of DC maturation?
- Down regulation of receptors for inflammatory chemokines
- Down regulation of antigen capture -> Loss of endocytic and phagocytic receptors e.g. mannose or Fc receptors.
• Change in morphology
– Loss of adhesive structures
– Cytoskeleton remodelling
– Acquisition of high cellular motility
- Up regulation of receptors for homing to lymphoid tissue ->CCR7
- Up regulation of antigen presentation
- Up regulation of co-stimulatory molecules -> CD40, CD58 (LFA-3 cell adhesion), CD80, CD86 (B7.1/.2)
How do DCs migrate to the lymphoid tissue?
Migration of Dendritic cells to the lymphoid tissues is regulated by chemokine - chemokine receptor interactions and CCR7 upregulation upon maturation.
DCs also become responsive to CCL19/MIP-3β and CCL21
• CCL19 is expressed in the afferent lymph ducts
• CCL21 is expressed in high endothelial venules of lymph nodes and in the T-cell areas of spleen and lymph nodes
• CCL19 and CCL21 guide DCs from the tissue to the T cell areas of lymph nodes
What are lymph nodes and how are they involved in the DC-T cell interaction
Lymph nodes are kidney bean shaped with afferent and efferent lymphatic ducts. They have different zones which contain different APCs and lymphoid cells and it is where they meet and interact.
Example: in the follicles and B cell areas on the outside is where the follicular DCs are compared to T cell areas in the middle (between follicle and medulla) is where interdigitating DCs lie.
Mature DCs migrate to the different zones and stimulate quiescent, naïve and memory T and B lymphocytes. There is then selection and priming of rare CD4+ and CD8+ T cells and B cell clones to induce an immune reaction
– Antigens presented in the context of Class II prime T helper cells
– Antigens presented in the context of Class I prime Cytotoxic T cells
Give some examples of DC:T cell interactions in the lymph node.
• DC-SIGN stands for DC-specific intercellular adhesion molecule (ICAM)-3 grabbing non-integrin). This is expressed on naïve T cells and promotes transient clustering between a DC and a T cell, this allows a DC to screen numerous T cells for a matched TCR
• Dectin-1 (a.k.a C-type lectin domain family 7 member A (CLEC7A)
– DC specific type II C-type lectin which binds T cells for proliferation promotion
• CD80 and CD86
Co-stimulatory molecules expressed on mature DCs to regulate T cell activity
• CD40 – CD40L
- T cells can activate DCs via CD40L > 2 way signal for B cell co-stimulation
DCs also interact with NK cells. How do they do this?
This occurs at site of infection rather than in the lymph nodes so it is non-MHC dependent
• Pathogen activated DCs can activate NK cells through cell-cell contact and soluble signals
– IFN, IFN-, IL-2, IL-12, IL-15 and IL-18 (last 3 promoye NK proliferation and survival)
– Leads to NK cell secretion of IFN and cytolytic activity (TNF release) > can mature other DCs
Antigens are presented by DCs to T cells by MHC so what is cross presentation?
Cross-presentation is the ability of certain antigen-presenting cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells (cytotoxic T cells). Cross-priming, the result of this process, describes the stimulation of naive cytotoxic CD8+ T cells into activated cytotoxic CD8+ T cells.
This process is necessary for immunity against most tumors and viruses that do not readily infect APCs but infect peripheral tissue cells.
Cross presentation is also required for the induction of cytotoxic immunity by vaccination with protein antigens, for example, tumour vaccination.
Cross-presentation is important, because it permits the presentation of exogenous antigens, which are normally presented by MHC II on the surface of dendritic cells, to also be presented through the MHC I pathway
What is cross dressing?
Cross-dressing involves the transfer of preformed functional peptide-MHC complexes from the surface of donor cells to recipient cells, such as dendritic cells (DCs). These cross-dressed cells might eventually present the intact, unprocessed peptide-MHC complexes to T cells
