An approach to immune deficiency diseases Flashcards
What information have immune deficiencies provided us with?
A lot of information regarding the parts of the immune system
What are the two categories of immunodeficiencies?
Primary
Secondary
Hallmarks of all immunodeficiencies
Recurrent infections
Inflammatory disorders
Increased risk of malignancy
What does the immune system provide us with?
Defense mechanisms to pathogens and malignant cells
Is tolerant to self and does not react to harmless substances
Elicits a controlled response that switches on and off when the pathogen has been destroyed
What are the three disorders of the immune response?
Allergy - eczema, asthma, food
Autoimmunity - lupus, type I diabetes
Immunodeficiency - primary and secondary (HIV, immune-suppressing medications)
Important factor about the disorders of the immune response
They have overlapping features
A patient commonly presents with features of all conditions
What causes primary immunodeficiency disorders?
Caused by mutations in key genes important for immune function
Causes of primary immunodeficiency disorders
Familial
Sporadic
How are primary immunodeficiency disorders clustered together?
> 320 distinct molecular causes
Clustered into main categories depending on what arm of the immune system is affected
Prevalence of PID
1 in 1200
Causes of secondary immunodeficiencies
HIV
Immunosuppressive therapy
Chemotherapy
Characteristics which make us suspicious of PID
Predisposition to infection
Family history
Poor growth in children
Variable presentation
Which infections are PID sufferers predisposed to?
Different infections depending on which part of the immune system is deficient
Common types infections caused by low neutrophil and B cell count
Bacterial infections
Common types infections caused by low T cell counts
Viral infections
Common types infections caused by low T cell and neutrophil count
Fungi
Important cells required for mycobacterial clearance
T cells
Macrophages
Classes of PID clustered depending on what part of the immune system is deficient
Humoral immune defects
T cell defects
Combined immune defects
Neutrophil defects
Other immune defects
Defined syndromes
Examples of humoral immune defects
- XLA
- selective IgA deficiency
- specific antibody deficiency
- transient hypogammaglobulinemia of infancy
- hyper IgM syndromes
Which primary immune defect is most common in adults?
Humoral immune deficiencies
Hallmarks of humoral immune defects
Low level of immunoglobulins in the presence of normal or low B cell count
What mutation leads to X linked agammaglubilinemia?
BTK protein
Required for B cell formation and maturation
Level of antibody in the blood of XLA sufferers
No antibody production
What is selective IgA deficiency an example of?
Some immunodeficiencies which are selective to specific classes of B cell antibody
IgA deficiecy is most common
Presentation of selective IgA deficiency
Normally asymptomatic
Pathogenesis of specific antibody deficiency
Normal amount of antibodies
But the antibodies don’t work well
What is transient hypogammaglobulinemia of infancy?
Developmental delay in the production of antibodies
But the immunoglobulin count returns to normal after 2 years
What is hyper IgM syndrome?
Disease characterised by defects in class switch recombination
Resulting in low IgG and IgD with normal or high IgM
Examples of deficiencies caused by T cell defects
DiGeorge syndrome
Idiopathic T cell lymphopenia
What mutation underlies DiGeorge syndrome?
22q deletion
Presentation of DiGeorge syndrome
Cardiac problems
Intellectual disability
Thymic defects
Low or absent T cell production
What causes idiopathic T cell lymphopenia?
Can’t explain
What are the two main subclasses of combined immune defects?
Severe combined immunodeficiency
Combined T cell and antibody production
What is important regarding SCID?
Important to identify early on
Without diagonsis, the patient dies within the first weeks of life
Types of combined T cell and antibody production defects
Wiskott Aldrich Syndrome
CD40/CD40L deficiency
DOCK8 deficiency
Late onset combined immunodeficiency disorder
Mutation underling Wiskott Aldrich Syndrome
Caused by loss of function WASp protein mutation
Why do mutations to WASp lead to deficient antibody production?
Important in managing cytoskeletal composition of cells
Important in managing B and T cell interactions
= Don’t produce good antibodies
Why does CD40/CD40L deficiencies lead to low B and T cell count?
CD40/CD40L interaction is important for proper class switching
Gives rise to hyper IgM syndrome
What is different between CD40/CD40L and hyper IgM syndrome?
Hyper IgM syndrome affects only the antibody production
CD40/CD40L causes a combined immunodeficiency because both T and B cells are in low numbers
Presentation of DOCK8 deficiency
Recurrent infections
Eczema
Important factor regarding DOCK8 deficiencies
Many are genetically undefined following whole genome and exome sequencing
What is late onset combined immunodeficiency disorder?
SCID-like presentation in older child or adult
No genetic cause found
What causes neutrophil defects?
Increased breakdown of neutrophils in the periphery
Lack of formation of neutrophils
Types of neutrophil defects
Autoimmune neutropenia
Severe congenital neutropenia
Chronic granulomatous disease
Leukocyte adhesion defect
Example of a defined syndrome
Chediak-higashi syndrome
What causes Chediak-higashi syndrome?
Defects in monocytes and dendritic cells
Unique presentation of Chediak-higashi syndrome sufferers
Occulocutaneous albinism
Caused by abnormalities in neutrophil and melanin granules
What other immune defects exist?
Complement defects - gives rise to a range of presentations from autoimmune disease to predisposition to infection
IRAK-4 deficiencies
Rare defects to monocytes and dendritic cells
What tests can be due to measure immune function?
Cell number
Cell function
More complex tests
How can we assess T cell number?
Flow cytometry allows us to see if cells are present and how high their count is
How can we assess B cell number?
Measuring the IgG/IgA/IgM/IgE count
How can we assess T cell function?
CFSe dye becomes incorporated into the T cells
Stimulation with agonist should lead to increased proliferation
Gradually the dye becomes less concentrated
Used to assess T cell proliferation
What more complex tests can be done to assess lymphocyte number?
T cells - check if you have both naive and memory T cells, check if T cells are polyclonal
B cells - look at post-vaccine responses to check for dynamic function of B cells
What extended tests can be carried out if the basic tests show abnormal results?
Can measure protein expression
Carry out functional studies
Look at genetics to identify SNPs that explain the phenotype
Example of protein expression used to diagnose a immunodeficiency
Wiskott Aldrich syndrome is caused by a lack of WASp production
Western blotting results show absent WASp protein
Example of a functional study used to diagnose a immunodeficiency
STAT1 phosphorylation tests
Some conditions are caused by the overstimulation of the immune system
Check for overstimulation using STAT1
Which cell marker is low in XLA?
CD19
Due to low B cell production
Which condition is caused by the same pathway as SCID?
CVID
Characterised by low but detectable B cell count
Marked decrease in IgG and at least one of the isotypes IgM or IgA
Poor vaccine response
What is significant about lymphocyte studies of SCID sufferers?
Low T cell production
No thymic shadow on X-ray
If just B cell count low = XLA
Why can a lot of mutations lead to a SCID-like phenotype?
Because a lot of genes are required for normal T cell function
Which mutations lead to defective T cells?
Cytokine signalling
Antigen presentation
VDJ recombination
T cell receptor signalling
Basic cellular functions
Lymphocyte survival
Describe how the concept of newborn screening can be applied to SCID sufferers
SCID is very severe, so it is a priority to identify the condition early in life
Identification before 3 months of age improves survival
Allows the patients to undergo a bone marrow transplant
What test is specific for the identifiication of immunodeficiencies?
TREC
CREBS
Describe the technology behind TREC
Relies on the fact that a circular protein structure is excluded during VDJ recombination
If this is low = linked to SCID due to low T cell count
Describe the pathogenesis behind CGD
Neutrophils use a transmembrane enzyme called NAPDH oxidase to oxidise NADH and make superoxide ions
Superoxide is used downstream to make bleach using myeloperoxidase
CGD arises due to mutations in the transmembrane enzyme
This leads to abnormal respiratory burst formation
Subunits of the NADPH enzyme
p22
p40
p47
p67
gp91
Which subunit in the NADPH oxidase is the most commonly mutated in CGD?
gp91
Inheritance of CGD mutations
Normally recessive
Why are gp91 mutations the most common causes of CGD?
It is X-linked
The others are recessive
Three ways to assess neutrophil function
DHR
NBT
Gene expression
Explain how a DHR assays are carried out to look for neutrophil function
Stain neutrophils with DHR
This becomes fluorescent following the respiratory burst
Change in fluorescence is detected through peak distribution changes
In patients with CGD, there is no change in the peak distributions since the neutrophils cannot undertake normal respiratory burst
Explain how NBT assays are carried out to look at neutrophil function
Deposition of blue dye to neutrophils following respiratory burst
Does not happen in CGD
Explain how gene expression tests are carried out to test for CGD
Expression of CPG9fox
No expression in CGD
What causes secondary immunodeficiencies?
Either not enough immune cells are produced or an external factor is blocking their function
Causes of secondary immune deficiencies
Malnutrition
Loss of immune components
Other systemic diseases
Tumour and malignancies
Immunosuppressive therapies
Infectious disease
How can immune components be lost leading to secondary immune deficiencies?
Lost passively in the intestine
Proteins lost in the urine
What systemic disease is linked to immune deficiency?
Diabetes
How can cancers lead to immune deficiency?
Cytotoxic therapies cause immune suppression
Describe the complicated nature of immunosuppressive therapy
They are commonly used for inflammatory conditions
But their use can also lead to secondary immune deficiencies
- steroids
- steroid-sparring agents
- biologic agents (monoclonal antibodies)
Which infectious diseases lead to immunodeficiency?
HIV
Malaria
Therapies for PID patients
Prevention and treatment of infections
Replacement of immunoglobulins
Screening for complications
Immunosuppression for inflammatory complications
Targeted therapies
Transplantation
Gene therapy
Examples of targeted therapies used for PID sufferers
Monoclonal antibodies to switch on and off pathways
How can human stem cell therapy be used for PID treatment?
Potentially curative treatment option for severe forms of PID
Involves the restoration of immune cell number and function from HLA-matched healthy stem cell donors
Describe the process of stem cell transplant for PID sufferers
- Collect and process stem cells
- Patient undergoes chemotherapy and radiotherapy to get rid of faulty immune cells
- Donor cells enter the bone marrow and regenerate the immune system
What are the risks of stem cell therapy?
Immunosuppression causes risks like infection
Which forms of PID can gene therapy be a potential therapy for?
ADA
Wisckott Aldrich syndrome
CGD
All caused by faulty proteins
What are the advantages of gene therapy?
Option is the patient is not suited for a bone marrow transplant
Avoids graft vs host disease
Describe the process behind gene therapy to restore function in the immune system
Insert a virus containing DNA coding for the functional protein into the host cell
Implant the host cell into the recipient
The recipient will now start producing the faulty protein
What is the goal of stem cell therapy?
Replace the whole immune system
What is the goal of gene therapy?
Replace the malfunctional protein
