4. Primary immunodeficiency 2 Flashcards
Causes of SCID
> 20 possible pathways identified, e.g. deficiency of cytokine receptors, deficiency of signalling molecules, metabolic defects. (Effect on different lymphocyte subsets (T, B and NK) depend on the exact mutation.)
How common is X-linked SCID?
Most common form of SCID. 45% of all severe combined immunodeficiency
What mutation causes SCID?
Mutation of common gamma chain on chromosome Xq13.1.
How common is ADA deficiency
16.5% of all SCID?
What protects the SCID neonate in first 3 months of life
Active transport of maternal IgG across placenta
What is the clinical phenotype of SCID?
Unwell by 3 months of age, infections of all types, failure to thrive, persistent diarrhoea, unusual skin disease, colonisation of infant’s empty bone marrow by maternal lymphocytes - this is sort of like graft-versus-host disease because the maternal lymphocytes are in the baby’s bone marrow, family history of early death
What is the mutation in Di George syndrome?
22q11.2 deletion syndrome
DiGeorge syndrome features
- Characterised by developmental defect of the pharyngeal pouch.
CATCH - Cardiac abnormalities/congenital HD (ToF)
- Abnormal facies: high forehead, low set abnormally folded ears, cleft palate, small mouth and jaw.
- Thymic aplasia (± oesophageal atresia)
- Cleft palate
- Hypocalcaemia (underdeveloped parathyroid gland), hypoparathyroidism
(Consequences of underdeveloped thymus: Normal B cells, low T cells, homeostatic proliferation with age, immune function is mildly impaired and tends to improve with age.)
Bare Lymphocyte Syndrome features
Unwell by 3 months of age, infections of all types, failure to thrive, family history of early infant death
What causes bare lymphocyte syndrome type 1?
Similar to type 2, condition - caused by failure of expression of MHC Class I (HLA molecules)
Clinical features of T lymphocyte deficiencies
Viral infections (e.g. cytomegalovirus), fungal infections (e.g. PCP, cryptosporidium), some bacterial infections (especially intracellular organisms, e.g. TB, salmonella), early malignancy
Ix of T cell deficiencies
- Total white cell count and differential (IMPORTANT: lymphocyte counts in children are much HIGHER than in adults),
- lymphocyte subsets (quantify CD4, CD8, B cell and NK cells),
- immunoglobulins (IgM present but NO IgG or IgA),
- functional tests of T cell activation and proliferation (useful if signalling or activation defects are suspected),
- HIV test
SCID Ix results
Normal: –
Low: CD4, CD8, maybe B cell, maybe IgM, IgG
DiGeorge Ix results
Normal: B cell, IgM
Low: CD4, CD8 and maybe low IgG
Bare lymphocyte syndrome type 2 Ix results
Normal: CD8, B cell, IgM
Low: CD4 and IgG
Severe recurrent infections from 3 months, T cells absent, B cells present, Igs low. Normal facial features and cardiac echo
X-linked SCID
Recurrent childhood infections, abnormal facial features, congenital heart disease, normal B cells, reduced T cells, Low IgA, Low IgG
22q11.2 deletion syndrome (Di George)
Bruton’s X linked agammaglobulinaemia clinical phenotype
- Boys present in the first few years of life (they are fine for the first few months)
- Recurrent bacterial infections e.g. otitis media, sinusitis, pneumonia, osteomyelitis, septic arthritis, gastroenteritis
- Viral, fungal and parasitic infections e.g. enterovirus, pneumocystis
- Failure to thrive
What causes Hyper IgM syndrome?
- This condition blocks the maturation of IgM B cells through germinal centres into B cells that produce other classes of immunoglobulin.
- The IgM B cells are unable to enter a germinal centre reaction
- This is due to a mutation in CD40 ligand gene (CD40L, CD154) -> technically a T cell problem
- This mutation means CD4+ T cells CANNOT signal to B cells and help them during the germinal centre reaction.
- CD40L is a member of the TNF receptor family
- It is encoded on Xq26
(Cannot undergo class switching)
Common variable immune deficiency is defined by:
- Marked reduction in IgG, and low IgA and/or IgE
- poor/absent response to immunisation
- Absence of other defined immunodeficiency
Selective IgA deficiency - what is the cause and how does it present?
Genetic component, but cause yet unknown. 2/3rd asymptomatic, 1/3rd have recurrent respiratory tract infections. (And GI symptoms (IgA found in both GI and resp mucosa). A lot of people with IgA deficiency will not be aware of it.
Investigations of b cell deficiencies
- Total white cell count and differential,
- Lymphocyte subset,
- Serum immunoglobulins and protein electrophoresis
- Functional tests of B cell function
SCID Ix results
Low cd4, cd8, IgG, IgA and maybe low B cell, IgM
Brutons X linked Ix results
Low b cell, IgM, IgG, IgA, normal CD4 and CD8 T cells
Hyper IgM X-linked results
high IgM, low IgG and IgA, normal CD4, CD8 and B cell
Selective IgA deficiency results
Low IgA, normal CD4, CD8, B cell, IgM, IgG
CVID results
Low IgG, low IgA (or IgE), normal CD4, CD8, B cell, IgM
1 year old boy. Recurrent bacterial infections. CD4 and CD8 T cells present. B cells absent. IgG, IgA, IgM all absent.
Bruton’s X-Linked hypogammaglobulinaemia
Recurrent bacterial infections as a child, episode of pneumocystis pneumonia, high IgM, absent IgA, absent IgG
X-linked Hyper IgM syndrome - mutation of CD40 ligand
Adult with bronchiectasis, recurrent sinusitis and development of atypical SLE
Common variable immunodeficiency
What are the components of the adaptive immune system?
T lymphocytes (CD4 T cells, CD8 T cells), B lymphocytes (B cell, plasma cells, antibodies), soluble components (cytokines and chemokines)
What is the definition of primary lymphoid organs?
Sites of development of lymphocytes
What happens in the bone marrow in terms of lymphocyte development?
T and B cells are derived from haematopoietic stem cells in the bone marrow. B cells will mature in the bone marrow
What happens in the thymus in terms of lymphocyte development?
Site of T cell maturation. Most active in the foetal and neonatal period, involutes after puberty
Summarise lymphoid development?
- Lymphocytes are derived from stem cells in the bone marrow
- Some will progress along the T cell lineage - they will leave the bone marrow and undergo thymic selection before exiting as mature CD4 and CD8 T cells
- On the other hand, the cells can leave the bone marrow as Pro B cells or Pre B cells which then become IgM B cells
- These can then undergo germinal centre reactions and mature into memory B cells and plasma cells
What is reticular dysgenesis the most severe form of?
Most severe form of severe combined immunodeficiency (SCID). It is called severe COMBINED because it affects both B and T lymphocytes
What mutation has occured in reticular dysgenesis?
Mutation in mitochondrial energy metabolism enzyme adenylate kinase 2 (AK2)
In reticular dysgenesis, what is there a failure of production in?
Lymphocytes, neutrophils, monocyte/macrophages, platelets
Why is the mutation of common gamma chain on chromosome Xq13.1 in X-linked SCID significant?
This is a component of many cytokine receptors including IL2, IL4, IL7, IL9, IL15 and IL21. The inability to respond to cytokines causes early arrest of T cell and NK cell development and the production of immature B cells.
What is the phenotype of X-linked SCID?
Very low or absent T cell numbers; normal or increased B cell numbers (but low immunoglobulin); very low or absent NK cell numbers
How is ADA deficiency caused?
Caused by adenosine deaminase deficiency. Enzyme required by lymphocytes for cell metabolism. So, if you are ADA deficient, you have a failure of maturation along any of the lineages
What is the phenotype of ADA deficiency?
Very low or absent T cell numbers. Very low or absent B cell numbers. Very low or absent NK cell numbers.
Briefly describe T cell maturation:
- Arise from haematopoietic stem cells.
- Exported as immature cells to the thymus where they undergo selection.
- Mature T lymphocytes enter the circulation and reside in secondary lymphoid organs
Which T cells recognise which HLA molecules?
- T cells will have a receptor that will recognise a peptide presented by the HLA molecule
- CD8+ T cells will recognise peptides from HLA Class I - these peptides are usually intracellular
- CD4+ T cells will recognise peptides from HLA Class II - these peptides are usually from extracellular pathogens
How does selection and central tolerance of T cells occur?
- If the T cell has very low affinity for HLA then it is useless and so will not be selected
- If they have very high affinity then they are dangerous because they can give rise to autoreactivity - so these are also negatively selected and die
- So, you are left with the T cells that have intermediate affinity for HLA (about 10%)
- They will then differentiate further depending on which type of HLA molecule they show intermediate affinity to
CD8+ Cytotoxic T Cells are specialised cytotoxic cells. What peptides do they recognise?
Recognise peptides derived from intracellular proteins in association with HLA Class I e.g. HLA-A, HLA-B, HLA-C
How do CD8+ cytotoxic cells work?
- Kill cells directly: perforin (pore forming) and granzymes, and through expression of Fas ligand (which binds to Fas and induces apoptosis)
- Secrete cytokines (e.g. IFN-gamma, TNFalpha)
- Particularly important in defence against viral infections and tumours
What do CD4+ Helper T cells recognise?
Peptides derived from extracellular proteins. Presented on HLA Class II molecules: HLA-DR, HLA-DP, HLA-DQ
CD4+ Helper T cells also have immunoregulatory functions via cell: cell interactions and expresion of cytokines. What does this help with?
Provide help for development of a full B cell response. Provide help for development of some CD8+ T cell responses
CD4+ T cells can differentiate into further subtypes of CD4+ T cell. What leads to this?
Certain polarising factors will lead to the development of subtypes of T cell with different effector profiles
What are the subtypes of CD4+ T cells?
Th1, Th17, Treg, TFh, Th2
What inflammatory conditions is Th17 important in?
Psoriasis, ankylosing spondylitis
What do TFh cells help B cells in?
TFh cells help the B cells in the germinal centre reactions (e.g. allowing them to become memory cells)
What developmental defect occurs in DiGeorge Syndrome?
The thymus does NOT fully develop
and the condition is characterised by developmental defect of the pharyngeal pouch
What is homeostatic proliferation in DiGeorge Syndrome?
(Consequence of underdeveloped thymus). This means that if T cells are in an empty compartment they will just keep proliferating, so, in the end, T cell numbers tend to increase with age
How does bare lymphocyte syndrome type 2 occur?
- CD4+ T cells selected in thymus based on their ability to recognise MHC Class II
- This can occur if you have a defect in one of the regulatory proteins involved in the expression of Class II genes: Regulatory factor X or Class II transactivator
- Mutations in these genes leads to absent MHC Class II.
- CD4+ T cells will NOT be selected, and so, you will have a profound CD4+ T cell deficiency
In bare lymphocyte syndrome type 2, what are the levels of CD4+, CD8+, B cells, and antibody levels?
Profound deficiency of CD4+. CD8+ count is usually NORMAL. B cell count is NORMAL. LOW IgG or IgA antibody levels due to lack of CD4+ T cell help.
What are disorders of T cell effector function?
Cytokine production (e.g. IFN), cytokine receptors (e.g. IL2 receptor), T-B cell communication
Failure of thymic development:
22q11.2 syndromes - e.g. DiGeorge syndrome
Failure of lymphocyte precursors:
Severe combined immune deficiency (X-linked SCID)
Failure of expression of HLA molecules:
Bare lymphocyte syndromes
Failure of signalling, cytokine production and effector function:
IFNgamma or receptor deficiency, IL12 or receptor deficiency
Management of immunodeficiency involving T cells:
- Aggressive prophylaxis/treatment of infection
- Haematopoietic stem cell transplantation (to replace abnormal populations in SCID and to replace abnormal cells (class II deficient APCs in BLS))
- Enzyme replacement therapy i.e. PEG-ADA for ADA SCID
- Gene therapy e.g. stem cells are treated ex vivo with viral vectors containing missing components. Transduced cells have a survival advantage in vivo.
- Thymic Transplantation, to promote T cell development in Di George syndrome. Cultured donor thymic tissue transplanted into quadriceps muscle.
Lymphocyte deficiency stereotypes, what could this be?: young adult with chronic infection with Mycobacterium marinum
IFN-gamma receptor deficiency
Lymphocyte deficiency stereotypes, what could this be?: 6-month old baby with two recent serious bacterial infections. T cells present - but only CD8+. B cells present. IgM present but IgG low.
Bare lymphocyte syndrome type II
How does central tolerance occur in B cells?
If they recognise self in the bone marrow then they will die to avoid autereactivity. No recognition of self in bone marrow means the B cell will survive
What happens first when there is a B cell antigen encounter?
Early IgM response. When B cells encounter antigens, the early response tends to be an IgM response (this is NOT T cell=dependent). They will differentiate to form IgM memory cells and Ab secreting plasma cells.
Apart from an early IgM response, what else happens when there is a B cell antigen encounter? This process results in the production of much higher affinity memory cells and plasma cells.
T-cell dependent germinal centre reaction.
- Dendritic cell will prime the CD4+ T cells
- Then, these CD4+ T cells will help B cell differentiation. This interaction requires CD40 ligand expression on T cells, and CD40 expression on B cells
- Once they have received this help from CD4+ T cells, the B cells can proliferate and undergo somatic hypermutation (to refine their receptor to develop high affinity) and they can isotype switch (to IgA, IgG and IgE)
Immunoglobulins are soluble proteins made up of …
Two heavy chains and two light chains. The HEAVY CHAIN determines the antibody class (GAMED). There are also subclasses of IgG and IgA.
Which part of the immunoglobulin is the antigen recognised by?
the antigen binding region (Fab) of both the heavy and light chains
Which part of the immunoglubulin determines the effector function?
Effector function is determined by the constant region of the heavy chain (Fc)
Describe the antibody function of immunoglobulins:
- Identification of pathogens and toxins (Fab-mediated), particularly important against extracellular pathogens
- Interacts with other components of the immune response to remove pathogens (Fc-mediated) i.e. complement, phagocytes and NK cells.
- Important in defence against bacteria of all kinds
How is Bruton’s X-linked hypogammaglobulinaemia/ agammaglobulinaemia caused?
This affects B cells at the point at which they emerge from the bone marrow.
- In this condition, you do NOT have B cells maturing and, as such, you do NOT have any antibodies being produced
- Caused by an abnormal B cell tyrosine kinase (BTK) gene
- This means that Pre-B cells cannot develop into mature B cells
- Therefore, there is an absence of mature B cells
- Once maternal IgG is out of the system (around 3 months), they will have NO ANTIBODIES
What are the consequences of Hyper IgM syndrome?
- Normal number of circulating B cells
- Normal number of T cells (but activated cells do NOT express CD40 ligand)
- NO germinal centre development within lymph nodes and spleen
- Failure of isotype switching
- Elevated serum IgM
- Undetectable serum IgG, IgA and IgE
What is the clinical phenotype of Hyper IgM syndrome?
- Boys present in the first few years of life.
- Recurrent infections (bacterial).
- Subtle abnormality in T cell function predisposes to Pneumocystis jirovecii infection, autoimmune disease and malignancy
- Failure to thrive
What is the cause of common variable immunodeficiency?
Cause is unknown
Who does common variable immunodeficiency present in?
It can present in adolescents and in adult life
Common variable deficiency is a heterogenous group of disorders. There are many genetic defects involved but most are unidentified.
What is common variable deficiency due to?
Due to some form of failure of differentiation/function of B lymphocytes
What is the clinical phenotype of common variable deficiency?
- May be adults or children
- Recurrent bacterial infections, often with severe end-organ damage (e.g. pneumonia, persistent sinusitis, gastroenteritis)
- Pulmonary disease (e.g. obstructive airways disease, interstitial lung disease, granulomatous interstitial lung disease (also lymph nodes and spleen))
- Gastrointestinal disease (e.g. IBD-like disease, sprue like illness, bacterial overgrowth)
- Autoimmune disease (e.g. AIHA or thrombocytopaenia, rheumatoid arthritis, pernicious anaemia, thyroiditis, vitiligo
- Malignancy, non-Hodgkin’s Lymphoma)
How common is selective IgA deficiency?
Quite common. Prevalence = 1:600
2/3rd individuals asymptomatic, 1/3rd have reccurent respiratory tract infections.
Failure of lymphocyte precursors:
SCID
Failure of T cell stimulation:
X-linked hyper IgM syndrome
Failure of production of IgG antibodies:
Common variable immune deficiency, selective antibody deficiency
Failure of B cell maturation:
Bruton’s X-linked agammaglobulinaemia
Failure of IgA production:
Selective IgA deficiency
What are the clinical features of antibody deficiency (or CD4+ T cell deficiency)?
Bacterial infections (e.g. Staphylococcus, Streptococcus), toxins (e.g. tetanus, diptheria), some viral infections (e.g. enterovirus)
How can functional tests of B cell function be used to investigate B cell deficiency?
- Specific antibody responses to known pathogens
- Measure IgG antibodies against tetanus, H. influenzae B and S. pneumoniae
- If the specific antibody levels are LOW, immunise with the appropriate killed vaccine and repeat antibody measurement 6-8 weeks later
- Functional tests have generally superseded IgG subclass quantification
How can serum immunoglobulins and protein electrophoresis be used to investigate B cell deficiency?
Production of IgG is a surrogate marker for CD4+ T cell helper function
How to know if a patient is antibody deficient through protein electrophoresis?
- When you run your proteins through the gel you will get a big peak for albumin
- Then you will also get alpha-1, alpha-2 and beta peaks which are showing the presence of various other proteins
- At the end, you get the gamma peak - this contains ALL THE ANTIBODIES
- A patient who is antibody deficient will have NO gamma peak
Management of immunodeficiency involving B cells
- Aggressive prophylaxis/treatment of infection
- Immunoglobulin replacement if required
- Immunisation (only in selective IgA deficiency)
Why is immunoglobulin placement (if required) used in the management of immunodeficiency involving B cells?
- Derived from pooled plasma from thousands of donors.
- Contains IgG antibodies to a variety of common organisms.
- Aim to maintain trough IgG levels within the normal range.
- Treatment is lifelong
Why is immunisation only worthwhile in selective IgA deficiency and not other immunodeficiencies involving B cells?
NOT worthwhile in other immundeficiences because they will NOT be able to produce any antibodies. UNLESS it is a selective IgA deficiency
Recurrent respiratory tract infections, absent IgA, normal IgM and normal IgG
IgA deficiency
What are the consequences of underdeveloped thymus in DiGeorge syndrome?
Consequences of underdeveloped thymus: Normal B cells, low T cells, homeostatic proliferation with age, immune function is mildly impaired and tends to improve with age. Homeostatic proliferation with age - this means that if T cells are in an empty compartment, they will just keep proliferating, so, in the end, T cell numbers tend to increase with age.
