Immunology Flashcards
T helper (CD4+) cell differentiation
What are the cells & molecules of the innate immune system?
Myeloid progenitor (except NK cells)
1. Phagocytes - neutrophils, monocytes, macrophages, dendritic cells
2. Mast cells
3. NK cells
Molecules of innate immunity
- Complement
- Coagulation proteins
- Acute phase reactants
- Cytokines & chemokines
NK cells can only kill cells that have MHC expression. True or false
False
NK cells have inhibitory receptors that recognise MHC molecules on target cells -> this inhibits the NK cells.
Therefore, NK cells are only active against cells that have lost MHC expression. Viral infection and malignant transformation often result in loss of MHC expression
CD8 + T cells target extracellular pathogens. True or false?
False.
Intracellular pathogens are degraded by Golgi, then expressed by MHC -1 to CD8 + T cells
Extracellular pathogens are phagocytosed, degraded & presented to CD4 T cells by MHC -2
Where are the different toll-like receptors expressed?
Toll-like receptors are a type of pattern recognition receptor expressed on innate immune cells
- PAMPs and DAMPs bind to TLRs
- there are 9 types of TLRs - TLRs 1-9 - some expressed on cell surface & some intracellularly
- endosomal (intracellular) TLRs 3, 7, 8, 9 identify dsRNA & DNA -> identify intracellular viruses
What cells express MHC 2?
Antigen presenting cells
Dendritic cells
B cells
Macrophages
Molecules involved in migration of innate immune cells to site of infection / injury
- Rolling adhesion
-> S-LeX binds to E-selectin on the endothelium - Tight binding
-> Leucocyte function-associated antigen 1 (LFA1) binds to ICAM-1 (intracellular adhesion molecules [ICAM-1]) expressed on endothelial cells & APCs => tight binding
Defects in MyD88 (myeloid differentiation primary response protein 88) and IRAK4 (interleukin 1 receptor associated kinase 4) cause?
Susceptibility to pyogenic bacterial infections
MyD88 adaptor protein involved in phagocyte function
TLR activation -> MyD88 adaptor protein -> downstream signalling -> NFkappaB activation -> transcription of inflammatory mediators
Defects in UNC93B1, TLR3, toll/IL-1 receptor, TNF receptor associated factor 3 (TRAF), tank-binding kinase (TBK1) mutations
Susceptibility to HSV1 encephalitis, severe influenza pneumonia, encephalopathy, severe COVID19 infection
Cytokines involved in innate immunity
IL-1, IL-6 and TNF alpha involved
- PAMPs / DAMPs bind to TLR
- release of pro-IL-1beta - cleaved caspase-1 => active IL-1 beta
- endothelial cells produce IL-6
IL-1 -> endothelial activation
IL-6 -> increase acute phase response
TNF alpha - drive Th1 differentiation, increase cell migration
Growth factors - GM-CSF, IL-12
Prostaglandins
Chemokines - CXCL8 / IL-8
Suppressive cytokines - TL-10, TGF beta
Liver produces acute phase reactants - CRP, MBL, ferritin, reduced albumin
What cells express MHC 1?
All nucleated cells
What immune defect underlies Familial Mediterranean syndrome (main type of periodic fever syndrome)?
Genetic defects in inflammasome production
Mechanism of action of natalizumab
Binds VLA-4 (alpha 4 beta 1 integrin)
Blocks adhesion of active T cells to blood vessels / blocks crossing through BBB
MS
Mechanism of action fingolimod
Blocks SIP-1 receptor -> normally allows T cells to leave lymph nodes & move into systemic circulation
Induces significant lymphopenia
Complement pathway
What is the role of natural killer cells?
Derived from a common NK / T cell progenitor
- important for killing virally infected cells & malignant cells
- only active against cells that have LOST MHC EXPRESSION (unlike CD8+ cytotoxic cells that can only kill cells that express MHC molecules)
NK cells express activating & inhibitory receptors
- Activating receptors - analogous to PRRs. Recognise changes associated with stress & viral infection
- Inhibitory receptors = killer inhibitory receptors (KIRs). KIRs recognise MHC class I molecules on target cells -> send negative signals to NK cells -> inhibition of activation
- negative NK receptors have the immunoreceptor tyrosine based inhibitory motif (ITIM)
Cells infected by viruses and malignant cells lose MHC expression
CD8+ cytotoxic T cells can only kill cells that have MHC expression
Describe CD4+ T cell activation
Signal 1 - APC activated via TLRs. Extracellular pathogen phagocytosed -> presented via MHC class II. MHC -2 binds to TCR on CD4 T cell
Signal 2
CD40 on APC binds to CD40L on T cell
This induces expression of B7 (i.e. CD80/86) on APC -> which binds to CD28 on T cell
*CTLA-4 can bind to CD28 instead of B7, which prevents ongoing T cell activation
*Signal 3
Cytokines drive CD4 T helper cell differentiation
Describe CD4 + T cell differentiation
- IL-12 drives TH1 differentiation. TH1 secrets IL-2, IFN gamma, LT. Activates macrophages / dendritic cells (esp via IFN gamma) & induces B cells to produce opsonising antibodies (IgG). Important for intracellular bacteria, fungi and viruses.
- IL-4 drives TH2 differentiation. TH2 produces IL-4, IL-13 and IL-5. Target cells are basophils and eosinophils. Also activates B cells to produce neutralising antibodies. Important for parasitic infections.
- IL-23, TGF beta, IL-6, IL-21 drive Th17 differentiation. Target cells are neutrophils. Important for extracellular bacteria & fungi
- IL-6 and IL-21 drive differentiation to Tfh cells
- IL-10 and TGF beta drive differentiation to T regulatory cells
Mediators of adaptive immunity
IL-4, IL-5, IL-2, TGF beta
Main role of TH1 CD4+ T cells?
TH1 cells activate macrophages (via IFN gamma, alongside CD40 / CD40L binding ) leading to macrophage activation & enhanced microbial killing
TH1 activate B cells (via IFN gamma) - leading to production of complement-binding & opsonising antibodies
TH1 activate neutrophils (via LT & TNF)
Main role of TH2 CD4+ T cells?
- activates B cells via CD40/CD40L AND IL-4 to promite production of IgG and IgE. IgE drives mast cell degranulation
- activates eosinophils via IL-5
- activates alternative macrophage via IL4 and IL13, drives wound repair, tissue fibrosis, suppression of inflammation
Key cytokines released by CD8 T cells to fight viral infections
CD8+ T cells create an anti-viral environment by secreting IL-2 and inteferon gamma
What does CTLA4 bind to?
CTLA4 binds to CD80/86 (B7) with higher affinity than CD28 -> inhibits T cell activation
What are the professional antigen presenting cells?
- Macrophages - lymphoid tissue, connective tissue (skin & mucosa); receptor mediated endocytosis, phagocytosis
- Dendritic cells - blood, lymphoid tissue, connective tissue. NOT phagocytes. Receptor mediated endocytosis, macropinocytosis.
- Activated B cells - blood, lymphoid tissue. Receptor (BCR) mediated endocytosis. Naive B cells do not express MHC II.
Above express MHC I and II
Molecules involved in adhesion of dendritic cell to naive T cell
ICAM1 on DC with LFA-1 on T cell
CD58 on DC with CD2 on T cell
DC-SIGN on DC with ICAM3 on T cell
Signals involved in activation of naive T cell via dendritic cell
Signal 1 - binding of MHC-II with TCR. Conformational change in LFA-1 allowing strong adhesion with T cell
Signal 2 - CD80/86 on DC binds to CD28 on T cell
Signal 3 - release of cytokines
TGF beta -> T regulatory cells
IL-12 -> TH1
IL -4 -> TH2
IL-6 -> Th17 and TFH
What is the main determinant of antibody diversity?
Junctional diversity is the greatest source of diversity.
- variations at sites of junction between V, D & J segments due to insertions & deletions performed by TdT (terminal deoxytransferase). Can accidentally cause frame shift or stop codons.
Combinatorial diversity - various combinations of light & heavy chain pairs
BCR gene re-arrangement
Each immunoglobulin has 2 light chains and 2 heavy chains
- each has a Fab domain (antigen binding site) and a Fc domain (mediator function)
- gene re-arrangement mediated by RAG enzymes
- heavy chain - V, D, J, constant
- light chain - V, J, constant
at pro-B cell stage - heavy chain is tested with surrogate light chain. If first heavy chain re-arrangement is sucessful, the other i snot re-arrangement
at pre-B cell stage - light chain is tested
immature B cell is then tested to ensure not self-reactive
Common pathogens that cause infection in patient’s with antibody deficiency
Encapsulated organisms
Streptoccus pneumoniae
E.coli
Haemophilus influenzae
Giardia lambilia
Describe B cell development in the bone marrow
- Multipotent progenitor cell / early lymphoid progenitor expresses Flt3 receptor, which binds to Flt3 ligand on stromal cells
- Common lymphoid progenitor expresses IL-7 receptor. IL-7 binding drives differentiation to pro-B cell
- Pro-B cell expresses cKit and binds stem cell factor (SCF) on a stromal cell. This drives heavy chain rearrangement.
- IL-7 binds to IL-7 R on Pro-B cell, driving differentiation to Pre-B cell
- IL-7 drives maturation from Pre-B cell to immature B cell
- Bruton tyrosine kinase is important in B cell development, transduces signals from Pre-B and B cell receptors and regulates IL-7 responsiveness
What is the process responsible for affinity maturation for Abs?
Somatic hypermutation
- needed for affinity maturation
Describe Ig isotype switching including interleukins that drive certain isotypes. Describe role of different Ig
Involves activation induced deaminase
Isotype switching occurs in secondary lymphoid tissue after Ig encounters Ag.
Dependent on T cell help.
Irreversible gene recombination (cannot switch back to a constant region that has been cut out)
1st response - mostly IgM
Isotype switching allows specialised responses on later exposures
IgM -> IgG -> IgE -> IgA
- IgM - main role is complement activation
- IgG - IgG1, IgG3 (driven by interferon gamma) - opsonisation & phagocytosis, complement activation, neonatal immunity (placental transfer)
- IgE & IgG4 - driven by IL-4. Immunity against helminths & mast cell degranulation (immediate hypersensitivity)
- IgA - driven by TGF beta, APRIL, BAFF. Actively secreted into mucosal lumen by binding of Fc portions of IgA dimer to a polyIg receptor which triggers phagocytosis of IgA
Neutralisation - IgG, IgA
Opsonisation - IgG, some IgA
Antibody-dependent cellular cytotoxicity - IgG
IgG binds NK cell via Fc receptor -> cross linking of Fc receptor triggers NK cell killing
Degranulation - IgE
Complement activation - IgM, IgG
What antibody isotype is responsible for neonatal immunity?
IgG
Placental transfer
Monoclonal Abs targeting T helper cell pathways
Mediators of innate immunity
IL-6, IL-10, TNF, IFN alpha, IFN beta, IFN gamma, IL-12, IL-1
Stimulators of haematopoiesis
Stem cell factor
GM-CSF
IL-3
IL-7
What cytokines are involved in downregulation of the immune response?
IL-10
TGF beta
IL-35
How does C2 deficiency usually present?
Can present as SLE-like autoimmunity
Also recurrent sinopulmonary infections
What is the role of CRP in acute inflammation / innate immunity?
CRP binds to phosphocholine expressed on bacterial cell surfaces. This activates C1q (classical complement pathway) & promotes phagocytosis.
Positive & negative acute phase reactants
What are the cytokines responsible for suppression / release of positive / negative acute phase reactants?
IL-6 (major)
IL-1beta
TNF alpha
IFN - gamma
Allergy / hypersensitivity classification
What is the most common complement deficiency? How does it present? What is the basic management?
Type 1 C2 deficiency
Homozygous 28 base pair deletion
Treated with prophylactic penicillin (typical treatment for complement deficiency)
Vaccinated for meningococcus, haemophilus & pneumococcus
Describe pathophysiology of allergy
- Sensitisation
- allergen exposure -> presented to T helper cells via MHC II on APCs -> Th2 cells release IL-4, IL-5, IL-13.
- IL-4 drives IgE class switching.
- IgE binds to Fc receptor on tissue mast cells -> mast cell degranulation
- IL-5 primes & recruits eosinophils -> non IgE mediated chronic immune response - Challenge
- subsequent exposure to same allergen -> allergen binds to IgE on mast cells -> rapid release of vasoactive amines -> immediate response of wheal & flare
- Histamine, heparin, tryptase, chymase
- Cytokines IL-4, IL-5, IL-13
- Prostaglandins
- 6-8 hours later, there is inflammatory cell infiltration -> oedema, heat
Describe the principles of allergy desensitisation
- High dose exposure to antigen
- Shift from TH2 response to TH1 and T regulatory cells
- Subsequent exposure to allergen -> Th1 response -> release of IFN gamma -> IgG production -> inhibits IgE
T regulatory response -> IL-10 and TGF beta production -> shift from IgE to IgG and IgA production -> inhibits IgE
T regulatory cells inhibit TH2 cells
HLA associations with autoimmune disease
What is the inheritance pattern for SCID?
Largely X-linked and autosomal recessive
What type of primary immune deficiencies are most common?
Antibody defects
Combined T & B cell deficiencies
What is the most common cause of pneumonia in patient’s with antibody deficiency?
Streptococcus pneumonia
Features of phagocyte defects (PID)
Bacterial infections - especially Staphylococcal. Other bacteria - serratia, pneumocytis, klebsiella, E.coli, Salmonella, proteus
Unusual infections - fungal (candida, aspergillus, nocardia)
Sites - deep seated skin infections, osteomyelitis, periodontal disease, lymph node, lung, liver
Delayed separation of cord (LAD - leucocyte adhesion deficiency)
Associated chronic granulomatous disease
How do defects of the early complement pathway typically present?
C1, C2, C4 deficiency usually presents as SLE-like autoimmunity (failure of tolerance to self antigens)
C2 deficiency can be associated with recurrent sinopulmonary infections
How do defects of the alternate complement pathway present?
Defects in the alternate pathway (factor B, D, properdin) result in Neisserial & bacterial infections
How does C3 deficiency manifest clinically?
Cannot be distinguished clinically from antibody deficiency - recurrent or severe infections
Defects in which part of the complement pathway will cause Neisseria meningitidis or gonococcal arthritis?
Late components
C5-C9
What prophylaxis should patients with defects in neutrophil function be on?
Phagocyte defects associated with bacterial (especially Staph) and fungal infections.
Prophylaxis with itraconazole and Bactrim
What features are suggestive of antibody or combined defect?
Recurrent sinopulmonary infections
Infections typically affect mucosal sites - e.g. sinuses, lung, GIT
Lung - Streptococcus pneumoniae, Haemophilus influenzae
GI - Giardia, enterovirus, campylobacter, salmonella, shigella
If Common Variable Immune Deficiency - can be associated with autoimmune cytopenias, chronic diarrhoea due to villous atrophy, exocrine pancreatic insufficiency, lymphocytic enterocolitis
What clinical syndromes are produced by CTLA4 deficiency?
Cytotoxic T lymphocyte antigen 4 (CTLA4) is expressed on T cells - important role in immune regulation and self tolerance
Heterozygous loss of function mutations in CTLA-4 causes CHAI - CTLA-4 haploinsufficiency with autoimmune infiltration. Characterised by multi-organ autoimmune disease
LRBA deficiency causes a secondary loss of CTLA4
LRBA is a chaperone molecule that enables CTLA4 to be transported to the cell surface
This produces LATAIE - LRBA deficiency with autoantibodies, regulatory T cell defects, autoimmune infiltration & enteropathy.
Above can be treated with Abatacept
Abatacept is a selective costimulation modulator. Inhibits T cell activation by binding to C80/86 expressed on APCs, preventing CD80/86 from interacting with CD28 on T cells
What are the features of CVID?
CVID = common variable immune deficiency
Most common clinically significant PID in Australia
Incidence 1: 10,000 - 1:15,000
Heterogenous
Variable age of presentation (typically <20 yrs but often later in life)
Diagnosis of exclusion
Key features
- hypogammaglobulinaemia (IgG severely reduced) +/- IgA +/- IgM
- Recurrent infections
- Absence of antibody production in response to vaccination
- Needs Ig replacement
- Associated with autoimmune conditions, malignancy, lymphoproliferative disorders
What syndrome is GATA2 deficiency associated with?
Mono Mac syndrome
Very frequent monocytopenias and disseminated mycobacterium avium infection
Toll-like receptors are an important component of the innate immune system. What infectious susceptibility does TLR3 deficiency cause?
Herpes Simplex Encephalitis
CARD9 is an adaptor protein that mediates signals from PRRs to regulate cytokine expression (part of the innate immune system). What specific infectious susceptibility if CARD9 deficiency associated with?
Invasive fungal infections
What underlying defects cause mendelian susceptibility to mycobacterial diseases?
Defects in IFN gamma / IL-12 pathway ( TH-1) pathway
- includes mutations in IL-12, IL-23 receptor beta 1, STAT1 deficiency, IFN gamma receptor, autoantibodies to IFN gamma
- Present with severe BCG or non-tuberculous mycobacterial infection
Also susceptible to other intracellular pathogens - Salmonella, certain fungi, viruses
Leucocyte adhesion deficiency (AR) is a phagocyte defect, which causes impaired adhesion of neutrophils to endothelium. Mutations in which genes cause AE?
CD18 or CD15
What is the triad caused by Wiskott - Aldrich syndrome?
X-linked rare disorder
WASP gene defect
Triad of eczema + thrombocytopenia + immune dysfunction
What is the diagnostic criteria for HLH?
HLH = haemophagocytic lymphohistiocytosis
- multi-system syndrome characterised by excessive immune activation & defective NK and T cell cytotoxicity
- especially common in infants & children
- triggered by viruses (especially CMV & EBV), rheumatologic diseases, malignancy
Common clinical features - fever, rash, lymphadenopathy, hepatosplenomegaly, neurologic symptoms
Common biochemical features - high ferritin, cytopenias, LFT abnormalities
Diagnostic criteria - needs 5 of the following
- fever
- splenomegaly
- cytopenias in 2 or more blood lineages
- hight trigs and/or low fibrinogen
- haemophagocytosis in bone marrow, spleen, lymph node, or liver
- ferritin > 500 ng/mL (typically a lot higher)
- low or absent NK cell activity
- elevated soluble CD25
- elevated chemokine 9
**
What syndrome is a defect in FOXP3 associated with?
Causes X-linked immune dysregulation polyendocrinopathy enteropathy (IPEX)
Disorder of T regulatory cells
FOXP3 is critical for development & function of regulatory T cells (CD4+, CD25+)
Severe and early autoimmune enteropathy, T1DM, eczema, hypothyroidism
What are the most common primary immunodeficiencies?
- Antibody defects
- Combined B & T cell defects (i.e. cellular & humoral immunity)
- Phagocyte defects
- Other well defined PIDs
- Complement deficiencies
What test is used to assess classical complement pathway activity?
Functional screening assay - CH50
If a component of classical pathway missing - CH50 will be 0
If a component of classical pathway reduced - CH50 will be reduced
Assess ability of classical complement pathway to lyse RBCs that are already coated with anti-sheep antibodies
Which test is used to assess function of the alternate complement pathway?
AH50
What are the complement regulators?
C1 inhibitor
Factor H
Factor I
What are the tests to assess neutrophil function?
- Neutrophil oxidative burst test
- checks for activity of NADPH oxidase. Required for production of ROS, oxygen-dependent intracellular killing of phagocytosed pathogens
Uses NBT reduction test - if NADPH oxidase is functional - slide turns blue
* NADPH deficiency is a key feature of chronic granulomatous disease - Flow based assay
- neutrophils are stimulated with potent mitogen
- can identify heterozygote female carriers for X linked chronic granulomatous disease (defect in NADPH oxidase)
What is the mode of inheritance for leucocyte adhesion deficiency?
Results in both lymphocyte & phagocyte dysfunction. Primarily disrupts ability of cells to migrate to site of infection / injury. Autosomal recessive inheritance.
Often mutations in ITGB2 gene
3 types of LAD
Type 1 - adhesion of leucoyte to endothelial surfaces defective due to mutations in CD18 gene resulting in defective beta 2 integrin
Type 2 - absence of Sialyl Lewis X of E-selectin
Type 3 - defect in beta integrins 1, 2 and 3.
What types of infections are associated with T cell deficiency / defects?
Loss of cellular immunity
Susceptibility to intracellular organisms
- Viral - progressive infections ith ordinarily “benign” viruses. CMV, EBV, HSV or other herpes vruses.
- Fungal - Candida (mucocutaneous) - Th17, pneumocystitis, cryptococcus
- Listeria
- Mycobacteria
What types of infections do NK cell defects present with?
NK cell defects are rare
Can cause haemophagohistiocytsois
Intracellular pathogens
Can present with severe & fulminant herpes virus infections
What types of infections do myeloid / phagocyte defects cause?
High grade bacterial infections & fungal infections
- Catalse-positive organisms - especially Staph aureus
Recurrent invasive skin & soft tissue infections, especially focal abscesses requiring incision & drainage, osteomyelitis, periodontal disease
- Gram negative bacteria - E.coli, Proteus, Serratia, Pseudomonas
- Invasive fungal infections - invasive aspergillosis, systemic candidasis, nocardia
What complement factors are involved in the membrane attack complex? What types of infections does deficiency in these factors cause?
C5, 6, 7, 8, 9
Disseminated neisseria infections, sepsis, gonococcal arthritis
What is the most common complement deficiency?
Type 1 C2 deficiency secondary to homozygous 28 bp deletion
C2 deficiency typically manifests in a SLE-like autoimmunity (failure to clear self antigen) - but recurrent sinopulmonary / pyogenic infections are also seen in C2 deficiency
Complement deficiencies
What are the infections associated with immunodeficiency secondary to corticosteroids?
Increased risk of common bacterial, fungal, viral and parasitic infections.
Bacterial - Staph aureus
Fungal - Candida
Viral - Herpes especially.
Parasistic infections
Can patients on high dose steroids receive live vaccines?
In patients with 14 days or more of high dose steroids - avoid live vaccines. Wait 4 weeks after cessation of steroids to vaccinate.
Vaccine responses are preserved if on chronic low-mod doses for kidney, pulmonary or rheumatological diseases.