9/26 Flashcards
Phases of B Cell Development
- Repertoire Assembly: generate diverse B cells in bone marrow
- Negative Selection: alteration, elimination, or inactivation of B cell receptors that bind components of the human body
- Positive Selection: promotion of a fraction of immature B cells to become mature in the secondary lymphoid tissues
- Searching for infection: recirculation of mature B cells between lymph, blood, and secondary lymphoid tissues
- Found Infection: activation and clonal expansion of B cells by pathogenic antigens in secondary lymphoid tissues
- Attacking infection: differentiation to antibody secreting plasma cells and memory B cells in secondary lymphoid tissue
Recombination-Activating Genes
RAG1 and RAG2, cut and paste DNA back together for B and T cell recombination
Severe Combined Immunodeficiency
No B or T cells develop so no adaptive immunity
Due to several different mutations, Adenosine deaminase deficiency is second most common
Can have mutation in IL-7 or its receptor, prevents differentiation of lymphoid progenitor cells into Immature B cells
B Cell Development
Dependent on interaction with stromal cells in bone marrow, bind to receptors like CAMs and get secreted IL-7 when late Pro-B cell
Stem cell (lymphoid progenitor cell): germline H and L chains
Early Pro-B Cell: D-J Rearrangement of H chain, germline L chain
Late Pro-B Cell: V-DJ rearrangement, germline L chain
Large Pre-B cell: VDJ Rearranged for H chain genes, mu H chain is made
Small Pre-B cell: V-J rearrangement for L chain genes, mu chain is in ER
Immature B cell: VJ rearranged for L chain genes, mu H chain and lambda/kappa L chain, IgM on surface
Checkpoints where apoptosis if no functional heavy/light chain
Burkitt’s Lymphoma
Myc proto-oncogene on chromosome 8 is joined to a immunoglobulin heavy chain gene on chromosome 14, kappa light chain gene on chromosome 2, or lambda light chain gene on chromosome 22
B Cell Negative Selection
If no reaction with self antigen on bone marrow cell with B cell receptor (IgM and CD79 or Igalpha/beta) then move to blood and expresses IgD/IgM
React with self antigen: immature B Cell retained in bone marrow with IgM ligated, can do a few more rounds of light chain (VJ) rearrangement known as receptor editing
Can leave if rearrangement made not self reactive, apoptosis after some rounds if still self reactive
Central vs. Peripheral Tolerance
Central: testing self reactivity in bone marrow for B cells
Peripheral: encounter soluble antigens from tissues while circulating in the blood
If bind then downregulate IgM while keeping normal levels of IgD to make the B cell anergic (functionally unresponsive), enters peripheral circulation but doesn’t survive for long
Immature B Cell going to a Lymph Node
- Chemokine CCL21 attracts immature an Cell to HEV (high Endothelial venule)
- Chemokines CCL21 and CCL19 attract B cells into lymph node (the T Cell area)
- Chemokine CXCL13 attracts B Cell into the primary follicle
- Interaction with follicular dendritic cells and cytokines drive the maturation of immature B cells
- Mature B cells recirculate between lymph, blood, and secondary lymphoid tissues
Immature B cells become mature B cells in primary follicles
B Cell Positive Selection
Some mature B cells that encounter an antigen that binds their BCR will differentiate into plasma cells, in the lymph node
B Cell Life Overview
- Immature B cell with IgM but no IgD leaves bone marrow and enters peripheral circulation
- Immature B cell with high IgM/low IgD does alternative splicing to both delta and mu chains, gains access to primary lymphoid follicle and matures
- Mature naive B cell with low IgM/high IgD enters circulation and binds specific antigen in lymphoid tissue draining infection
- Antigen activated B lymphoblast does alternative splicing to secrete Ig, isotope switching, somatic hypermutation
- Plasma cells secrete antibodies to fight infections, antibodies are replicas of the BCR
- Memory cells can prepare for future infections
B Cell Activation
- First Signal: BCR receptors become cross-linked when their receptors bind to a pathogen, happens in lymph node by site of infection
Require CD19, CR2, and CD81 (deficient antibodies if mutated CD19 or CD81), CR2 binds to C3d Complement protein on pathogens
Set Up: naive CD4 T cells activated when dendritic cells in lymph node present antigens and get trapped in T cell zone, antigen activated B cells meet up with helper at cells
- Second Signal: CD4 T Cell will bind to MHC Class II receptor on B Cell that broke down antigen
CD40L on T Cells binds to receptor on B cell also, B cell divides and proliferates, cytokines made by type of CDT cell determines antibody isotype
T-Independent Antigens: few antigens bind to the BCR with such high affinity that they can induce B cell differentiation into plasma cells without T Cell help, makes IgM antibodies
Primary vs. Secondary focus for expansion of antigen-activated B cells
Primary: medullary cords
Secondary: germinal center, in B Cell area
Somatic Hypermutation
B cells bind to their antigen via interaction with follicular dendritic cells, generates antibodies that have higher affinity for the antigen and help them be more effective at clearing an infection
Affinity Maturation
Process of increasing the affinity of the antigen binding sites of antibodies for antigens that occur during the course of adaptive Immunity
Due to somatic hypermutation d the consequent selection of mature B cells
Compete and low affinity ones die
Five Isotypes of Antibodies
IgG: most abundant, 4 subtypes, high affinity
IgM: first isotype made by plasma cell, low affinity, pentameric structure
IgA: monomeric in tissue, dimeric in mucosal surfaces
IgE: low abundance in circulation, binds to FceRI on mast cell surface, cross-linking IgE causes production of inflammatory mediators associated with allergies
IgD
Hyper IgM syndrome
Mutation in Activation-Induced Cytidine Deaminase, plays a role in somatic hypermutation and Class switch recombination
How Monomeric Antibodies Tissues Enter Tissues from the Blood
Fc (constant) region of antibody like IgG binds to FcRN on endothelial cell surface, does endocytosis
Acidic pH of endocytosis vesicle causes IgG to bind tight to FcRn and not get degraded, basic pH of basolateral surface causes dissociation into extracellular space
Distribution of Antibodies
IgM: blood
Monomeric IgG and IgA: blood and tissues
IgE: connective tissue below epithelial associated with mast cells
Dimeric IgA: mucosal surfaces and breast milk
Baby development: passively transferred maternal IgG in womb, have high IgM at birth and slowly develop IgG
Antibody Neutralization
IgA coats a pathogen at mucosal surface and prevents the pathogen from entering the host’s cells
Pathogen can’t bind to cell receptors with bound antibodies blocking it
Selective IgA Deficiency
Most common antibody deficiency
Person is relatively healthy but increased risk of infection at mucosal sites and increased risk of asthma
Antibody Function
Neutralization so can’t enter cell, IgA on mucosal surface
Can recognize proteins, lipids, carbs, and DNA
C1q binds to two or more IgG molecules and initiates complement activation
Opsonization: most common method to kill bacteria
Antibody binds to bacteria and antibody-coated bacteria bind to Fc receptors on macrophage, endocytosis and kill in phagolysosome
Antibody-Dependent Cell-Mediated Cytotoxicity
Allergic response: IgE binds to FceRI on mast cells, mast cells degranulate and release histamine to promote clearance of parasite
Antibody-Dependent Cell-Mediated Cytotoxicity
Antibodies bind to antigen s on cell surface of virally infected cells or cancer cells, allows NK cells to recognize and destroy the cells
NK cells use Fc receptors, target cell dies by apoptosis
B cell Memory
First exposure: don’t make antibodies until a week later, peak at about 2 weeks, little left after 4 weeks
Second exposure: make right away, peak at a week later, make much more antibodies also, antibody conc. doesn’t die down to near zero after multiple weeks
Retrovirus Basics
Single stranded positive sense RNS with 5’ cap and poly A tail, 2 identical strands in capsid structure inside lipid envelope
Genes of Retrovirus
Simple has four genes-
- Gag: core protein like capsid
- Pol: reverse transcriptase and integrase
- Pro: protease
- Env: envelope
Complex: same as with simple but spliced regulatory genes controlling protein synthesis and replication
Long terminal repeat sequences of Retro viruses
Identical structures found at both ends of the DNA provirus, transcription generates RNA with same terminal organization as the parental virus
Contains signals necessary for gene expression: enhancers, promoters, transcription initiation/termination signals, polyA signal
Host TFs bind to LTRs
Provirus: integrated retrovirus has 5’ LTR for RNA Pol II promoter, 3’ LTR for terminator sequence
Retrovirus Envelope Gene
gp120 binds tonCD4 of lymphocyte
gp41 is the transmembrane glycoprotein, fusion of viral envelope and allows viral entry into cell
Gag Proteins
All cleaved from one long polypeptide by protease, 4 proteins
p24 forms capsid that surrounds RNA
Matrix protein as well
Pol gene of retro virus
3 enzymes that are targets for drugs
RT: binds to RNA in viral capsid, creates RNA/DNA complex and ribonuclease digests RNA portion, duplicates ssDNA next
Protease: cleaves gag polyprotein
Integrase: integrates viral DNA into host cell
Binding and Fusion of HIV
Binding: gp120 binds with high affinity to CD4
Conformational change to bind to coreceptor, CXCR4 in T cells and CCR5 for macrophages
Fusion: gp41 unfolds and pierces cell membrane, folds back on itself and pulls the virus towards the cell to initiate fusion
HIV Nucleic Acid Synthesis
RNA genome becomes cDNA by RT and cleave RNA via ribonuclease, cDNA goes through RT again to make dsDNA that circularizes via LTRs
Preintegration complex: dsDNA and integrase enter nucleus
Integrate cleaves ends of dsDNA to make sticky and then can insert into host genome, proviral DNA
Cellular DNA Pol can make mRNA and proteins, viral protease cleaves polyproteins made, assemble new viruses and bud off cell membrane
CD4 Count for AIDS
200, normal is 1,000
RT Inhibitors
Nucleoside analogues: Zidovudine (AZT) resembles thymine but has azide group instead of OH so mess up transcription
Non-nucleoside analogues: interfere with binding of viral RNA to RT
Entry Inhibitors
Attachment Inhibitors: receptor antagonist, Maraviroc interferes with gp120 binding to CCR5 coreceptor
Fusion Inhibitor:T-20 binds to gp41 to block its unfolding that allows viral envelope/cell membrane fusion
Method for Treating HIV
Combination Therapy of three drugs, need to suppress replication so no new mutations arise
Monotherapy fails due to mutation and subsequent drug resistance
Retrospective Cohort vs. Case-Control
Case-Control: always starts with diseased (case) and not diseased (control), design of choice for rare diseases (less than 10%), try to find exposure
Retrospective Cohort: need past records, helpful for rare exposures, try to find disease
Strengths and Weaknesses of RCTs
Strengths:
Highest level of evidence
Can assess temporality
Minimize bias and confounding variables
Weaknesses: Not always ethical to randomize Costly Inefficient for rare diseases or diseases with a delayed outcome Might not generalize to larger pop.
Cohort Studies Strengths and Weaknesses
Strengths:
Can measure multiple outcomes/exposures
Good for rare exposures
Can infer directionality
Weaknesses: Not good for rare diseases Costly and time consuming Bias due to loss of follow up Being followed may alter participant's behavior
Case-Control Strengths and Weaknesses
Strengths:
Cost-effective
Good for rare diseases
Good for diseases with long latency periods
Weaknesses: Prone to selection/recall bias Can only examine one outcome Cannot estimate incidence Poor for rare exposures Often cannot determine temporality
Cross-sectional Studies Strengths and Weaknesses
Strengths:
Cheap and quick
Can collect data on lots of exposures and outcomes
Good for Hypothesis generation
Weaknesses: Cannot infer temporality Prone to recall bias Not good for rare diseases Cannot estimate incidence
Prevalence
HIV prevalence increases cut o transmission and also more effective anti-retroviral drug use
A proportion, number of infected / total number of people
Snapshot at a given time
Increases by: people acquire disease at a faster rate, live longer with the disease
Decreases by: people acquire the disease at a slower rate, get cured, die
HIV Transmission
Spread by mucocutaneous contact: blood, semen, vaginal secretions
Routes of spread: vaginal/anal sex, contaminated blood products, perinatal transmission including breast feeding
Prevent transmission:treatment, pre-exposure prophylaxis, circumcision
Enhance Transmission: high viral load, disease stages like acute HIV and end-stage AIDS, lack of circumcision, co-infections with other STIs increase transmission/susceptibility
HIV Risk Groups
MSM: big in western/Central Europe and North America, also Latin America and Caribbean, elsewhere MSM have higher risk than general pop.
Injection Drug Users: problem in Eastern Europe and Central Asia, most effective transmission mode, complex risk since overlap with sex work and jail
Sex Workers: higher number of partners and less protection, had highest prevalence early in, hidden pop. since marginalized/criminalizes but gov intervention can be effective, bridge to normal pop.
