Immuno 2 - The immune response to infection/Primary immune deficiencies Part 2 Flashcards
B cells emerge from the bone marrow as Ig_?__ expressing B cells
IgM
Reticular dysgenesis mutation
mitochondrial energy metabolism enzyme adenylate kinase 2 (AK2)
Which features would make you suspicious of SCID
Unwell by 3 months of age* Infections of all types Failure to thrive Persistent diarrhoea o Unusual skin disease Colonisation of infant’s empty BM by maternal lymphocytes graft vs host disease
FHx of early infant death
*(before – protected by IgG from mother across placenta and then colostrum)
• Normal baby will start to produce their own IgG over the course of 6 months
X-linked SCID mutation
Mutation of common γ chain of IL2 on Chr Xq13.1
shared by cytokine receptors IL- 2 7 9 15 21
X-linked SCID
T cells
NK cells
B cells
IgG
o Cells mature but are unable to respond to cytokines
early arrest of T cell + NK cell development
production of immature B cells
- Very low/absent T cells
- Very low/absent NK cells
- Normal or increased B cells (but don’t function well)
- Low IgG
ADA deficiency
T cells
NK cells
B cells
Adenosine Deaminase deficiency
inability to respond to cytokines
- Very low/absent T cells
- Very low/absent NK cells
- Very low/absent B cells
What do CD8 cells vs CD4 cells recognise
CD8
o Peptides derived from intracellular proteins in association with HLA class I (HLA-A, HLA-B, HLA-C)
o Malignant or virus infected cells
CD4 o Peptides derived from extracellular proteins presented on HLA class II molecules (HLA-DR, HLA-DP, HLA-DQ)
Function of
• Treg
• T follicular helper cells (Tfh)
cells
- Treg – important in negatively regulating immune responses, keeping them under control
- T follicular helper cells (Tfh) – important in the development of B cell responses– play an important role in promoting germinal centre reactions and differentiation of B cells into IgG and IgA secreting plasma cells
DiGeorge syndrome features
characteristics
T cells
B cells
Immune function
Defects in T cell maturation/selection in thymus
CATCH22
o Complex congenital heart disease (esp. tetralogy of fallot)
o Abnormal facies (high forehead, low set ears, abnormally folded ear, cleft palate, small mouth, small jaw, oesophageal atresia)
o Thymic aplasia (T cell lymphopenia)
o Cleft palate
o Hypocalcaemia, Hypoparathyroidism
o 22 – chromosome - deletion at 22q11.2
o Reduced T cells
Over time T cells proliferate to fill up the compartment and so their immune function tends to improve with age
o Normal B cells
Low IgA, Low IgG (no T follicular helper cells to help differentiation to IgG or IgA producing B cells in germinal centre reaction)
o Homeostatic proliferation with age
Immune function usually only mildly impaired + improves with age
Bare lymphocyte syndrome
where is the problem T cells B cells IgG IgA
Defects in T cell maturation/selection in thymus
• Defect in one of the regulatory proteins involved in Class II gene expression
o Regulatory factor X
o Class II transactivator
• Absent expression of MHC Class II molecules Profound deficiency of CD4+ cells
o Normal numbers of CD8+
o Low numbers of CD4+ (profound deficiency of CD4+ cells)
o Normal numbers of B cells
o Low IgG or IgA antibodies (lack of CD4+ T cell help, no class switching)
Syndromes due to Defects in T cell maturation/selection in thymus
DiGeorge syndrome - 22q11.2 deletion
Bare lymphocyte syndrome type 2
Causes of disordered T activation/ cell effector function
Failure of
• Cytokine production – IFN
• Cytokine receptors – IL12 receptor
• T-B cell communication
• Cytotoxicity
Clinical features of T cell deficiency
• T cell deficiency clinical features
o Viral infections – CMV
o Fungal infection – pneumocystis, cryptosporidium
o Some bacterial infections, esp. intracellular organisms – TB, Salmonella
o Early malignancy
T cell deficiencies ix
• Total WCC + differential
o Remember that lymphocyte counts are normally much higher in children than in adults
• Lymphocyte subsets
o Quantify CD8 T cells, CD4 T cells, B cells, NK cells
• Immunoglobulins
o If CD4 T cell deficient (because if there is CD4 T cell deficiency, you wont be able to generate your immunoglobulins in the germinal centre reaction)
• Functional tests of T cell activation + proliferation
o Useful if signalling or activation defects are suspected
• HIV test
Management of immunodeficiency involving T cells
- Aggressive prophylaxis/ treatment of infection
- Ig replacement
• Enzyme replacement therapy
o PEG-ADA for ADA SCID
• HSCT
o To replace abnormal populations in SCID
o To replace abnormal cells – class II deficient APCs in BLS
• Thymic transplantation
o To promote T cell differentiation in Di George syndrome
o Cultured donor thymic tissue transplanted in quadriceps muscle
• Gene therapy
o Stem cells treated ex-vivo with viral vectors containing missing components
o Transduced cells have survival advantage in vivo
What are Th1 cells?
Subset of lymphocytes
express CD4
secrete IL-2, IFNγ
Help CD8 cells + macrophages
Describe what happens before and during the germinal centre reaction
o Dendritic cells prime the CD4+ T cells in the lymph nodes
Take up antigens in peripheral tissues, process them into proteolytic peptides, and load these peptides onto major histocompatibility complex (MHC) class I and II molecules.
o CD4+ T cells help B cell differentiation – requires CD40L:CD40 interaction
• CD4+ cells interact w B cells germinal centres (found in secondary lymphoid organs)
in the germinal centre reaction, the B cells undergo
o Somatic hypermutation they edit their receptor on successive ranks of antigen engagement until it becomes very high affinity = stronger and more specific response to the antigen = affinity maturation
o Isotype switching from IgM to IgG, IgA or IgE
o B cell proliferation
Immunoglobulins
What determines the antibody class Antigen is recognised by... Effector function is determined by... IgA is a IgM is a IgE, IgD, IGg are
What determines the antibody class - Heavy chain
Antigen is recognised by antigen binding regions (Fab) on both heavy + light chains
• Antibodies identify pathogens and toxins (Fab mediated)
Effector function is determined by constant region on the heavy chain (Fc)
• Interact with other components of immune response to remove pathogens (Fc mediated)
o Fc binds to Complement, Phagocytes, NK cells to help remove antigens and toxins
IgA is a dimer
IgM is a pentamer
IgE, IgD, IgG are monomers
Response to successive exposures to antigen is dominated by
IgG
Bruton’s x-linked hypogammaglobulinemia/ agammaglobulinemia
What is wrong
When does it present
How does it present
- Abnormal BTK (B cell tyrosine kinase) gene – mutation in B cell tyrosine kinase
- Pre-B cells cannot develop into mature B cell Absence of mature B cells no antibody production
• No circulating IgG 3 months after birth (agammaglobulinemia - not even IgM)
o Boys (X-linked!) present in first few years of life
o Recurrent bacterial infections – otitis media, sinusitis, pneumonia, osteomyelitis, septic arthritis, gastroenteritis
o Viral, fungal, parasitic infections – enterovirus, pneumocystis
o Absent/scanty lymph nodes and tonsils (1o follicles and germinal centres absent)
o Failure to thrive
X-linked hyper IgM syndrome
Where is the problem
Mode of inheritance
What does this problem cause
Clinical phenotype
Features o Number of circulating B cells o Number of T cells o serum IgM o serum IgA, IgE, IgG
o Mutation in CD40 ligand gene (CD40L, CD154) (encoded in Xq26)
• X-linked recessive
- Inability of B cells to class switch causing production of only IgM
- Lack of isotype switch due to a failure of T cell co-stimulation
• B cell maturation defect
o IgM B cells cannot develop into IgM memory and plasma cells
o IgM B cells can’t undergo a germinal centre reaction
• Clinical phenotype
o Boys (X linked!) present in first few years of life
o Recurrent infections – bacterial
o Subtle abnormality in T cell function – predisposition to pneumocystis jiroveci infection, autoimmune disease and malignancy
o Failure to thrive
• Features
o Normal number of circulating B cells
o Normal number of T cells but activated cells do not express CD40L
o No germinal centre development within lymph nodes + spleen
Failure of isotype switching
• Elevated serum IgM
• Undetectable IgA, IgE, IgG
Common variable immune deficiency
Features
o Number of B cells
o serum IgM
o serum IgA, IgE, IgG
Where is the problem
Definition
o Number of B cells - normal
o serum IgM - normal
o serum IgA, IgE, IgG - low
o Failure of normal B cell maturation –> Reduction in immunoglobulins
o Mutation in MHC III
Defined by
o Marked reduction in IgG, with low IgA or IgM
o Poor/absent response to immunisation/vaccination
o Frequent infections
o Absence of other defined immunodeficiency
Associated with NHL, autoimmune diseases
Common variable immune deficiency
Clinical features
• Clinical features – adults and children
o Recurrent bacterial infections
Pneumonia, persistent sinusitis, gastroenteritis
Often with severe end-organ damage
o Pulmonary disease Interstitial lung disease Granulomatous interstitial lung disease (also LN, spleen) Obstructive airways disease Bronchiectasis
o GI disease
Inflammatory bowel like disease
Sprue like illness
Bacterial overgrowth
o Autoimmune disease Autoimmune haemolytic anaemia or thrombocytopenia Rheumatoid arthritis Pernicious anaemia Thyroiditis Vitiligo
o Malignancy
NHL
Selective IgA deficiency
What percentage of people are symptomatic
what kind of infections do they suffer from
- 2/3 individuals asymptomatic
- 1/3 have recurrent respiratory tract infections
- Also GI infections
Failure of B cell maturation
Failure of T cell costimulation
Failure of IgA production
Failure of production of IgG antibodies
Failure of B cell maturation
Bruton’s X-linked agammaglobulinaemia
Failure of T cell costimulation
X-linked hyper IgM syndrome
Failure of IgA production
Selective IgA deficiency
Failure of production of IgG antibodies
Common variable immune deficiency
Selective antibody deficiencY
Investigation of B cell deficiencies
• Total WCC + differentials – do they have lymphocytes?
• Lymphocyte subsets
o B cells, CD4, CD8 T cells, NK cells
• Serum immunoglobulins and protein electrophoresis
o IgG production is dependent on both the B cells and the CD4 T cells Production of IgG is surrogate maker of CD4 T cell helper function
o Therefore serum IgG if CD4 deficient
• Functional tests of B cell function
o Specific antibody responses to known pathogens/immunisations – measure IgG antibodies against tetanus, Haemophilus influenzae B, Strep. Pneumoniae
o If specific antibody levels are low, immunise with appropriate killed vaccine and repeat antibody measurement 6-8 weeks later to see if they have responded
• HIV
Protein electrophoresis for investigation of B cell deficiencies
which band gives you the antibodies?
findings in
Bruton’s X-linked hypogammaglobulinaemia
Monoclonal gammopathy (abnormal production of a single clone of plasma cells/antibodies)
On protein electrophoresis, it’s the γ band that gives you the antibodies – it’s all the antibodies, not just IgG
e.g. Brutons absent γ band
monoclonal gammopathy (abnormal production of a single clone of plasma cells/antibodies) high narrow peak in the γ band
CD4 CD8 Bcell IgM IgG IgA in
SCID Bruton's X-linked HyperIgM X-linked Selective IgA CIVD
Look at table - ix of B cell deficiencies
Management of immunodeficiency involving B cells
• Aggressive prophylaxis/treatment of infection
• Immunoglobulin replacement if required
o Derived from pooled plasma from thousands of donors
o Contains IgG antibodies to a wide variety of common organisms
o Aim of maintaining trough IgG levels within normal range
o Treatment is lifelong, every 3 weeks
• Immunisation
o For selective IgA deficiency (because it will boost the other responses)
o Not otherwise effective because of defect in IgG antibody production
• (for B cell immunodeficiencies you don’t usually do transplantations because what you are missing is the immunoglobulins and you can replace these)
Primary lymphoid organs
Secondary lymphoid organs
Primary lymphoid organs
• Sites of B and T cell development
• BM - B + T cell production, B cell maturation
• Thymus - T cell maturation
Secondary lymphoid organs • Anatomical sites of interaction between naïve lymphocytes and microorganisms • Spleen • Lymph nodes • Mucosal associated lymphoid tissue
Summary
Pre-B cells IgA IgG secreting plasma cells IgM secreting plasma cells Primary lymphoid organs Thoracic duct Thymus Germinal centre
Pre-B cells – exist within the bone marrow and develop from haematopoietic stem cells
IgA – divalent antibody present within mucous which helps provide a constitutive barrier to infection
IgG secreting plasma cells – cell dependent on the presence of CD4 T cell help for generation
IgM secreting plasma cells – are generated rapidly following antigen recognition and are not dependent on CD4 T cell help
Primary lymphoid organs – include both the bone marrow and the thymus; sites of B and T cell development
Thoracic duct – carries lymphocytes from lymph nodes back to the blood circulation
Thymus – site of deletion of T cells with inappropriately high or low affinity for HLA molecules and of maturation of T cells into CD4+ or CD8+ cells
Germinal centre – area within secondary lymphoid tissue where B cells proliferate and undergo affinity maturation and isotype switching
Types of primary immunodeficiency affecting
Phagocytes
Natural Killer cells
Complement
Phagocytes
Kostmann syndrome
Leukocyte adhesion deficiency
Chronic granulomatous disease
Natural Killer cells
Classical NK deficiency
Functional NK deficiency
Complement
Classical pathway deficiencies
Alternative pathway deficiencies
Terminal pathway deficiencies
Types of primary immunodeficiency affecting
Haematopoietic stem
cells
Cytokines
Lymphoid precursors
Haematopoietic stem
cells
Reticular dysgenesis
Cytokines
IL12 and IL12 receptor deficiency
IFNg and IFNg receptor deficiency
Lymphoid precursors
Severe combined immunodeficiency
Types of primary immunodeficiency affecting
T cells
B cells
T cells
22q11.2 deletion syndromes
Bare lymphocyte syndrome
B cells X-linked agammaglobulinaemia X-linked hyperIgM syndrome Common variable immunodeficiency IgA deficiency
Laboratory Tests for Primary Immunodeficiency
White cells
Immunoglobulins
Complement
White cells Full blood count Lymphocyte subsets Special tests for white cell migration/function - Adhesion molecules – eg CD18 - Test for oxidative killing – DHR test
Immunoglobulins
IgM, IgG, IgA
Specific Igs and response to vaccination
Complement
Complement function - CH50 and AP50
Individual complement component
What kind of conditions are seen in T cell deficiency
Intracellular + maligancy
Viral infections (CMV)
Fungal infection (Pneumocystis, Cryptosporidium)
Some bacterial infections (esp. intracellular TB, Salmonella)
Early malignancy
What kind of conditions are seen in B cell deficiency
Bacterial infections (Staph, Strep)
Toxins (Tetanus, Diptheria)
Some viral infections (enterovirus)
