Lymphocyte Development,
Movement, Receptors Diversity and
Central Tolerance
Movement, Receptors Diversity and
Central Tolerance
Describe the T- and B-cell developmental stages (origin, education/development, where they are released).
Immature lymphocyte precursors derive from pluripotent stem cells replicating in the bone marrow. They leave the marrow and move on to the places they mature.
T-Cell development occurs in the thymus.
B-cell development occurs in the bone marrow.
After lymphocytes undergo development and clonal deletion/central tolerance is achieved, they are released into circulation as naive lymphocytes.
Psych! What is PCV and how do you obtain it (what we did in lab)?
PCV=Packed Cell Volume
Fill microhematocrit tube 3/4 full with blood from purple top tube. Add clay up to the point of the blue line to seal the end. Centrifuge. Place tube on hematocrit reader card and adjust so the tube so that the bottom fo the RBCs is at 0%. Slide the tube until the top of the plasma is at the 100% level. Read the PCV% just above the buffy coat.
Where do you find MHC I, MHC II, and MHC III?
MHC I: Most nucleated cells
MHC II: Professional APCs
MHC III: proteins linked to innate immunity --> complement proteins, heat-shock proteins, tumor necrosis factor-α (TNFs)
What is peripheral tolerance?
Receptor binding by itself inactivates lymphocytes. This happens to naïve lymphocytes when their receptors bind to linear proteins (peptides) on cells that are not APCs. They cannot subsequently respond to antigen even when it is presented together with co-stimulating signals AKA "anergic".
RBC production is stimulated by ____, produced in the ____
erythropoietin, kidneys
Explain the concept of clonal selection
The secondary lymphoid tissues are populated by all the cell types required to initiate specific adaptive responses to foreign antigens. Only those that have a TCR or BCR to bind strongly to an antigen get activated and replicate into clones of identical cells. Clonal selection ensures responses that are best suited for specific antigen elimination.
Explain what happens to a host cell displaying foreign antigen on MHC I & II:
Endogenous antigens displayed on MHC class I by all nucleated cells activate _________(CD_) to destroy the host cells harboring that foreign antigen.
________ antigens displayed MHC class II by APCs activate ________ (CD_) which then secrete a profile of _______ that aid in the maturation of B cell clones and cytolytic t cell clones.
cytolytic T-cells (CD8)
Exogenous
T-helper cells (CD4)
cytokines
Discuss the pros and cons of having more MHC gene diversity.
• The more MHC genes, the greater chance of presenting antigen
• The more MHC genes, the greater chance of presenting SELF antigen
Compare TH1 and TH2 and the molecules they secrete:
TH1 - secrete IL-2 and INFγ, important for developing cell-mediated immunity through activating ________ and help in activating ________ __-____.
TH2 - secrete interleukins 4, 5, 9 and 13, important for developing _______ immunity. Th2 help primed B-cells to change into ______ cells to release antibodies.
macrophages
cytotoxic T-Cells
humoral
plasma
A blue cytoplasm may indicate increased/decreased levels of ____ in the RBC. ____ causes RBCs stain orangish-pink.
increased mRNA for hemoglobin and cytoskeletal protein production. Hemoglobin.
Describe lymphocyte re-circulation in the body and relevance to the “clonal selection theory”: Lymphocytes circulate - they move from one organ to the next during different stages of development or stimulated activity. They move between organs via __________ and _________. This “patrolling” process means a lymphocyte can ________ scan the entire body for its corresponding _______ . When a lymphocyte encounters its matching _______, it is selected to activate and undergoes _______ expansion, producing many _______ cells. These differentiate into effector cells (plasma cells or cytotoxic T cells) and memory cells. All other lymphocytes that don’t find their antigen keep recirculating or die. Only the lymphocytes that can
bind the presented antigen will be stimulated into downstream differentiation and _________.
blood vessels
lymphatics
continuously
antigen
antigen
clonal
identical
proliferation
Langerhans’ cells: the _______ cells of the skin. They differ from those found in the lymphoid tissues in two critical aspects:
1. They can _____ antigen
2. They lack co-stimulatory activity
Langerhans’ cells can be triggered by infection to
migrate through the ____ to the lymphoid organs,
where they differentiate into _____ cells that have potent co-stimulatory activity. Their role in physiological immune responses is to transport antigen from sites of _______ to the lymphoid tissues where they activate re-circulating T lymphocytes.
dendritic
ingest
lymph
dedritic
infection
Is it better to be homozygous or heterozygous for an MHC allele? Why would social animals have high MHC class I and II polymorphism?
Heterozygotes twice as likely to have the right fit to any given antigen. Heterozygotes more likely to reproduce. They have more diseases exposure risk. They need to have MHC with more polymorphism to be able to fight these diseases.
Compare the professional Antigen Presenting Cells (APCs):
Macrophages: Macrophages have a variety of receptors that enable them to recognize & _______ microorganisms. Macrophages can only activate T Cells in the presence/absence of a microbial infection.
Dendritic cells: Dendritic cells (concentrated in the lymphoid tissues) express high/low levels of MHC
class I & MHC class II molecules as well as co-stimulatory molecules B7 & adhesion molecules. They are very potent activators of naïve/primed T cells. The lymphoid dendritic cells are non-phagocytic (viruses & cellular toxins have their own means of entering these cells). Dendritic cells (unlike other cells) can be infected by many different viruses. Able to prime both ___ & ___ T Cells in many viral infections.
B-Cells: B cells recognize antigen, internalize it and present it on MHC class II as a _______. CD+4 Helper T-cells will then activate B cells. This type of antibody response and B-cell activation is named _________ as it depends on T-cell
engulf
presence
high
naïve
CD8
CD4
peptide
T-dependent
In what species would you want to blood type before doing a transfusion? Why?
Cats. Type B cats are born with strong Anti-A antibodies.
Explain how T- and B- cells central tolerance is generated and why it is necessary.
Any developing lymphocyte that harbors receptors that bind self antigens strongly are instructed to undergo apoptosis and no longer remain to initiate an adaptive response. The goal of central tolerance is to prevent autoimmunity.
Explain why antigen processing is needed to initiate an adaptive immune response.
Exogenous and endogenous antigens are processed and presented to different subsets of T cells. This is crucial to produce tailored responses to be the most effective at eliminating the foreign material. It is necessary because T cells can only recognize linear peptides that fit into MHC molecules. The APC baby birds the antigen to the T cells.
Psych! This isn't' about MHC. Discuss the difference between the following blood tubes: lavender, light blue, red, red/black tiger or gold, green top, green/black tiger top.
Lavender - EDTA, anticoagulant, used for routine CBCs and blood smears, contains PLASMA.
Light blue - anti-coagulated blood by binding Ca2+, used for hemostasis (coagulation) testing.
Red - used for SERUM chemistries, SPE, serology, no anticoagulant, may contain clot activator.
Red/black tiger or gold - SERUM separator tube, used for serum chemistries, SPE, serology, contains polymer gel to help separate serum from the cells, also contains clot activator.
Green top - contains heparin, used for PLASMA chemistries, blood gas analysis, CBCs for non-mammalian species, heparin stains pink with routine stains, not recommended for routine hematology,
Green/black tiger top - PLASMA separator tube, PLASMA chemistries, serology, uses heparin, contains polymer gel to separate plasma from the cells.
Compare antigen recognition by T-cell and B-cell
B cells can recognize unprocessed antigens directly via BCRs
T cells can only recognize an antigen when it is broken down into linear peptides and presented on an MHC I or MHC II
Tell me something about the bilirubin pathway (heme excretion).
Unconjugated bilirubin bound to albumin (to prevent toxicity) travels in the blood to the liver. Hepatocytes take up bilirubin. Bilirubin is conjugated into bilirubin diglucuronide (conjugated bilirubin) --> now it’s water-soluble. Conjugated bilirubin is actively transported into bile canaliculi → stored in gallbladder → secreted into small intestine during digestion. Gut bacteria act on bilirubin and convert it to urobilinogen. Some urobilinogen is reabsorbed into the blood → excreted in urine as urobilin (yellow color of urine). Most is converted to stercobilin → gives feces their brown color.
An animal body can generate at least one antibody for every antigen in the universe. They achieve this through ___ and ___ diversity.
How to create receptor diversity during development in bone marrow: 1) multiple copies of _, _, _ gene segments, 2) gene segments can recombine in multiple __________ 3) diversity of junction between gene segments.
Somatic _________ and affinity maturation in BCR after activation in _______ lymphoid organs: Point mutations at high rate into V region of heavy and light chains, preferential selection cells expressing Ab with _____ affinity. Fine tuning via cytosine deaminase converts cytosine to _____ which generates variation.
TCR
BCR
V, D, J
combinations
hypermutation
peripheral
higher
uracil
Illustrate the antigen processing pathways of endogenous and exogenous antigen, the MHC molecules involved in each pathway, and the cells responsive to antigen presented by each class of MHC molecules.
Endogenous: Foreign antigen inside the cytosol is tagged, broken down into smaller peptides, and linear peptides are presented on MHC I to decorate the cell surface. Peptides presented on MHC I activate CD8/Cytotoxic T cells.
Exogenous antigen: APC encounters foreign material, engulfs it via endocytosis, and it is broken down into fragments. Linear peptides bind to MHC II and it then moves to be expressed on the surface of the APC. T helper/CD4 cells can recognize such peptide if it fits the TCR.
Discuss the diversity and mechanisms of variability of MHC I and II:
Diversity of MHC genes enable the presentation of more peptides to _-cells. This means it is less likely that the pathogen can express epitopes that could not be found by the MHC molecules. More than one form of ___ can be expressed on the ____ cell surface. A single ___ can bind to a wide range of peptides without the need for ______ like TCR and BCR.
Mechanisms of Variability in MHC I and II:
# of ____ could increase: MHC loci
• Might increase a population’s and an individual’s success
# of possible ________ in the genes could increase: MHC alleles
• An individual could be __________ or homozygous
T-cells
MHC
same
MHC
diversity
genes
variations
heterozygous
Explain the signals provided by APCs that are needed to activate a T-cell to undergo clonal expansion and differentiation (3).
1. Stimulation through receptor
2. Co-stimulatory signal normally delivered by professional antigen presenting cell. (B-cell, Macrophage or Dendritic cell)
3. Proinflammatory cytokines (IL-1, IL-12) secreted by the APC dictate the direction of subsequent T-helper cell differentiation (TH1 vs. TH2).
What are polychromatophils? When would there be an increased in release of polychromatiphils from marrow to blood? What species would this not occur in?
Immature RBCs that appear blue but no longer have a nucleus. Would be increased if there was an increase in hematopoeisis in response to needing more RBCs. Would not occur in horses.